The solution to quantum gravity is ready.

April 21, 2011 11:41 AM

... and there is no dark matter/energy! Dr. Philip Mannheim has succeeded in developing a cosmological and quantum field theoretic consistent PT symmetric theory that contains no kind of dark matter and dark energy. Space is flat in the absence of matter, and even the largest galactic rotation curves are predicted. Perhaps most interestingly, it also handles the cosmological constant and zero-point energy 'problems' simultaneously! (This is the final paper in a long list of publications, but it makes the case such that it's importance is immediately recognized. I leave it to the experts to recognize it's true beauty.) All hail the internets!
posted by quanta and qualia (210 comments total)

The forgotten link. Comprehensive solution doesn't show up on the list of articles, but it's probably the most complete of the published papers. The link is available through Mannheim's publication list.
posted by quanta and qualia at 11:45 AM on April 21, 2011

Forgive the typos... "it's" should clearly read "its". If someone could correct this, I would be much less embarrassed. Heh.
posted by quanta and qualia at 11:48 AM on April 21, 2011

Henceforth to be known as the Steamroller Theory.
posted by It's Raining Florence Henderson at 11:50 AM on April 21, 2011 [14 favorites]

"All hail the internets" I understand. The rest of this, not so much.
posted by finite at 11:51 AM on April 21, 2011 [5 favorites]

The apostrophe I understood well enough.
posted by sneebler at 11:52 AM on April 21, 2011

His analysis of special-relativistic Dirac gamma matrices is a little oversimplified for my taste.
posted by brain_drain at 11:54 AM on April 21, 2011 [24 favorites]

Needs more cowbell.
posted by schmod at 11:55 AM on April 21, 2011 [3 favorites]

He published it in arXiv, and he has tenure at an accredited university in the Northeast, so if he says he has general solutions to quantum gravity, we should just accept that as true prima facie, since most of us lack the expertise to even begin to evaluate his claim.
posted by [citation needed] at 11:57 AM on April 21, 2011 [4 favorites]

we should just accept that as true prima facie, since most of us lack the expertise to even begin to evaluate his claim.

We need a citation for....wait. Damn.
posted by procrastination at 11:59 AM on April 21, 2011 [11 favorites]

I always knew dark matter was bullshit! Seriously. Even though I don't understand it at all, it always seemed to this layman as something invented to satisfy a mathematical formula, not so much something for which there was compelling evidence.

See my publication, "Dark Matter is A Crock of Shit" in the August '99 Quantum Astrophysics Illustrated.

PS If this guy turns out to also be full of hooey, I will still not believe in dark matter or dark energy.
posted by Mister_A at 11:59 AM on April 21, 2011 [24 favorites]

Makes a nice breeze as it sails over my head.
posted by From Bklyn at 12:00 PM on April 21, 2011 [19 favorites]

we should just accept that as true prima facie, since most of us lack the expertise to even begin to evaluate his claim.

Based on past success rates with these kinds of claims, I'd suggest the opposite.
posted by empath at 12:01 PM on April 21, 2011 [7 favorites]

I would love to get a layman's explanation of this.

Not that I know anything, at all, really, but dark matter / dark energy, as far as I have read about it, always seemed a little off. It seems like it was invented to fit a hole in a theory, and then convoluted experiments have been done (and are being done) in order to back up this hole-plugging. My (extraordinarily scientific) gut feeling is that dark matter will turn out to be the phlogiston of our age.
posted by molecicco at 12:01 PM on April 21, 2011 [4 favorites]

Henceforth to be known as the Steamroller Theory.

I can see the headline now!

Mannheim's 'Steamroller' Flattens Space, Launches 'New Age' of Physics
New theory a breath of fresh aire for cosmologists
posted by Ratio at 12:01 PM on April 21, 2011 [51 favorites]

we should just accept that as true prima facie

I'm not a physicist, but I think it's a little early to declare one theory to be true and all the others to be bunk. We're talking one thread of a conversation that has been ongoing for years, and will keep going (I suspect) until the next Einstein comes along and tidies everything up for us.
posted by muddgirl at 12:02 PM on April 21, 2011 [1 favorite]

why aren't these in TeX? I'm suspicious.
posted by oonh at 12:02 PM on April 21, 2011 [18 favorites]

I always knew dark matter was bullshit! Seriously. Even though I don't understand it at all, it always seemed to this layman as something invented to satisfy a mathematical formula, not so much something for which there was compelling evidence.

Nobody knows what dark matter is, so how can it be bullshit? Dark matter is just shorthand for whatever the reason is that galaxies seem to have more mass in them than we can see.

Dark matter is a question, not an answer.
posted by empath at 12:03 PM on April 21, 2011 [27 favorites]

Scott Adams was RIGHT!

Suck it, haters.
posted by The otter lady at 12:03 PM on April 21, 2011 [5 favorites]

Dark matter is a question, not an answer.

Yeah, this. Dark matter doesn't plug a hole, it describes the hole.
posted by muddgirl at 12:05 PM on April 21, 2011 [13 favorites]

Dark matter is just shorthand for whatever the reason is that galaxies seem to have more mass in them than we can see.

I would rephrase that as, dark matter is shorthand for the reason why we predicted the universe to be expanding at a different rate than what we can see. That may because there is hidden mass, or because our models of movement are incorrect.
posted by molecicco at 12:05 PM on April 21, 2011 [3 favorites]

we should just accept that as true prima facie, since most of us lack the expertise to even begin to evaluate his claim.
posted by [citation needed] at 2:57 PM on April 21

Well, I think the peer review process needs to work itself out before this theory is established as the scientific truth, but the latter part is certainly correct (and something I just wrote about!).
posted by Pastabagel at 12:05 PM on April 21, 2011 [2 favorites]

He may be right, but his author lists make me doubt it. I seriously doubt that not one, but two of the biggest challenges in modern astrophysics are going to be solved by a single guy toiling in obscurity. Any significant effort to try to understand / prove / disprove the existence of both dark matter and dark energy is going to require the collaboration of many different scientists from lots of different sub-disciplines.

It's worth pointing out that, even as a professional astrophysicist, I don't feel like I have the expertise to evaluate his claims. He could be right. However, his patterns suggest to me that it's very unlikely.

On a side note, my professional opinion is that dark energy may end up to be BS, but dark matter's going to be pretty hard to shake.
posted by Betelgeuse at 12:06 PM on April 21, 2011 [5 favorites]

It was luminiferous aether all along!
posted by Blazecock Pileon at 12:06 PM on April 21, 2011 [16 favorites]

Yeah, dark matter wasn't invented to plug a whole in an equation, it's the variable in the equation that we know fits in but aren't sure why.
posted by shakespeherian at 12:07 PM on April 21, 2011

Upon seeing Christoffel symbols again I felt like Carol Anne towards the end of Poltergeist when the closet door flips open and murmured "No more."

I hated tensors with a passion, necessary as they are for G&R, but damned if my college simply had no-one capable of teaching them well. Working through Ohanian made me feel like a chimp trying to assemble a submachine gun: best case scenario was that I couldn't get anything done.

Still, if 't Hooft is working on something in the same area, and it has no adjustable parameters, it's worth looking at. And Carl Bender has co-authored with him — I know Dr. Bender, so this guy probably isn't a complete kook.

*keeps reading*
posted by adipocere at 12:08 PM on April 21, 2011 [3 favorites]

When I was in college, my engineering major roommate and a couple of his friends were having trouble on a particular homework problem. I got a laugh out of them when I remarked they should simply "add more values" to make it balance.

So um, yeah, "dark matter".
posted by Xoebe at 12:08 PM on April 21, 2011

recognize it's true beauty

/weeps
posted by docgonzo at 12:12 PM on April 21, 2011 [2 favorites]

Dark matter is a question, not an answer.

Yeah, this. Dark matter doesn't plug a hole, it describes the hole.

However, our observations of dark matter's effects describe that hole pretty darn well. If we don't see evidence for dark matter at the LHC in the next ~5 years, we need to start thinking harder, but we're pretty sure we have a good grasp on some basic properties of dark matter

Dark matter is just shorthand for whatever the reason is that galaxies seem to have more mass in them than we can see.

I would rephrase that as, dark matter is shorthand for the reason why we predicted the universe to be expanding at a different rate than what we can see. That may because there is hidden mass, or because our models of movement are incorrect.

The first one is more right, actually. Dark matter originally was proposed because galaxies were rotating too fast. There's now very convincing evidence from both rotation curves and gravitational lensing that most matter in the universe must be made up of something that we don't observe.

I think molecicco may be confusing dark energy with dark matter; dark energy is the thing that seems to be affecting the universe the most on a global scale. It's worth pointing out at this point that dark matter and dark energy are probably totally independent phenomena. "Dark" is just our code word for ignorance (as in "in the dark").
posted by Betelgeuse at 12:12 PM on April 21, 2011 [2 favorites]

He forgot to divide by the Bacon Constant.
posted by It's Raining Florence Henderson at 12:12 PM on April 21, 2011 [1 favorite]

I seriously doubt that not one, but two of the biggest challenges in modern astrophysics are going to be solved by a single guy toiling in obscurity.

Yeah. To pull that off you'd have to be a regular Einstein.
posted by scrowdid at 12:14 PM on April 21, 2011 [34 favorites]

Or a certified genius.
posted by It's Raining Florence Henderson at 12:15 PM on April 21, 2011 [4 favorites]

Don't we need to wait for other scientists to respond to this before declaring victory?
posted by Measure at 12:15 PM on April 21, 2011 [2 favorites]

Some of us are other scientists. And by "us" I mean "not me."
posted by It's Raining Florence Henderson at 12:18 PM on April 21, 2011 [2 favorites]

I'm reading it, and he doesn't seem like a crank, but I tensor calculus makes my head spin, so I'm not really following it that well.
posted by empath at 12:18 PM on April 21, 2011

Nobody knows what dark matter is, so how can it be bullshit?

Well, bullshit is demonstrably dark matter.
posted by dirtdirt at 12:19 PM on April 21, 2011 [2 favorites]

I seriously doubt that not one, but two of the biggest challenges in modern astrophysics are going to be solved by a single guy toiling in obscurity.

You know, like an obscure patent examiner in Bern.
posted by Chocolate Pickle at 12:23 PM on April 21, 2011 [4 favorites]

Tell me about this again next week after the piranhas of science have finishing their work.
posted by TwelveTwo at 12:24 PM on April 21, 2011 [1 favorite]

If it doesn't lead to cheap ftl travel, the theory is obviously incomplete.
posted by happyroach at 12:24 PM on April 21, 2011 [6 favorites]

It's Raining Florence Henderson: He forgot to divide by the Bacon Constant.

I thought it was the Bacon Radius.
When you continue to compress bacon more and more then you will eventually reach a point density where none of its glorious bacony goodness particles can escape its own gravitational pull. The bacon becomes virtually undetectable by any means other than through indirect measurement of suspiciously low levels of baconosity in a given volume of space.

Also, today is bacon thursday at work. Yes, they give us free bacon for lunch every thursday.
God bless'em.
*wipes tear from eye*
posted by Hairy Lobster at 12:27 PM on April 21, 2011 [3 favorites]

If this keeps Wil Wheaton employed, well, warp speed ahead, sir!
posted by Blazecock Pileon at 12:28 PM on April 21, 2011

This is interesting, but way above my head.
posted by klangklangston at 12:30 PM on April 21, 2011

Again, as a layman, it seems to me that the dark matter hypothesis is in place because it fits the currently accepted model. Discrepancies between predicted and observed phenomena can be explained, in theory, by other factors–maybe our idea of gravity is incorrect or incomplete, for instance. I think "dark matter" is convenient because it doesn't require reworking some cherished theories that worked to describe the observed universe beautifully well for some time now.

Now, my humorous postscript to the contrary notwithstanding, if we do get more evidence for dark matter, other than the circumstantial evidence that we have detected so far, I will become a dark matter believer. I respect the power of the evidence for dark matter - it does elegantly describe and predict observed phenomena, and it fits into the accepted model of the universe really well. I, however, am really excited to think that maybe our model of the universe is substantially incomplete! I would love to be alive during the days when we humans really figure this all out. It's exciting to contemplate. And that's really why I'm a dark matter hater. I love surprises!
posted by Mister_A at 12:35 PM on April 21, 2011 [1 favorite]

This is interesting, but way above my head.

It kinda describes the curve of my sloping brow as it passes over, though.
posted by Devils Rancher at 12:40 PM on April 21, 2011 [4 favorites]

Um, is the answer 42?
posted by Mental Wimp at 12:40 PM on April 21, 2011 [3 favorites]

I'm sort of sick of the implication that Einstein was isolated (and sometimes even uneducated) and that he dreamed up relativity in a vacuum. He was an avid student of mathematics and physics, obtaining a teaching degree from ETH in Zurich. While working as a patent officer, he formed a scientific club and was working towards his PhD, which he eventually obtained during his "miracle year."

So no, he was not working inside a university system, but he was avidly reading scientific theory and research at the time, was a regular contributor to a physics journal, and perhaps most importantly as a patent examiner was directly dealing with mechanical analogs to his theoretical proposals.

Also, he was only 26 during his miracle year, unlike Mannheim. Physics is demonstrably a science for the young.
posted by muddgirl at 12:42 PM on April 21, 2011 [12 favorites]

I'm sort of sick of the implication that Einstein was isolated...

Shorter version: Einstein was awesome!
posted by Mental Wimp at 12:44 PM on April 21, 2011 [2 favorites]

but I tensor calculus makes my head spin,

That's a feature, not a bug...
posted by Mental Wimp at 12:46 PM on April 21, 2011 [2 favorites]

Can we get a little context on this? The Wikipedia page on conformal gravity makes it look like Mannheim's been publishing papers on the topic for a while. In fact (as far as I can tell) he's been making the argument that dark matter and dark energy aren't necessary for a while, too. What's new here that wasn't in the earlier work? What do the cosmology and astrophysics communities think of it? What problems did people have with it? Does it look like this work remedies those? What testable predictions does this work make?
posted by Serf at 12:46 PM on April 21, 2011

Oh yes, Einstein was awesome. Totally a genius, no doubt. But if we're looking for the next Einstein perhaps we should be looking at the subset of struggling grad students.
posted by muddgirl at 12:48 PM on April 21, 2011

I respect the power of the evidence for dark matter - it does elegantly describe and predict observed phenomena, and it fits into the accepted model of the universe really well.

It does describe what we see well, but it doesn't help predict anything, it's more of a sign of the failure of generally accepted physical theories to predict the behavior of galaxies. It's a fudge factor. As is dark energy.
posted by empath at 12:49 PM on April 21, 2011 [1 favorite]

Dark matter clearly exists. It's obvious to any numbskull that can reproduce the following simple experiment:

1: Rearrange your living room furniture evenly across the room.
2: Tape over all the windows with black sheets.
3: Leave the room and turn off the lights
4: Blindfold yourself and attempt to run into and across the living room and back using a random path.

If you either fall flat on your face or bruise your shins, then clearly there is dark matter in the room.

QED*

*Quite Enough Done
posted by Brockles at 12:50 PM on April 21, 2011 [3 favorites]

The Bacon Radius is derived from mc² . Except that in my equation, c is not a constant, it's the transformational limits of the system. As derived from the Lorentz Transformation, it still limits velocity v as v approaches c, but since this is true for any value of c, by refusing to beg the question of c's value, the equation can be used to express the transformational energies of any discrete system without pretense as to whether the energies involved represent an absolute universal limit. The limit applies only within the internal constraints of the defined system.

Or maybe, B=mc² isn't an equation at all. Maybe it's better described as the integral of m(ƒ(c))ndc from 0 to c with respect to c, where the solution (B) is the potential available Bacon Radius of a system in which units of energy (m) can be delivered at a maximum rate of c. Hence, Special Relativity is true for c=300,000 km/hr (electromagnetic systems), but is not universally constrained by it. With c not defined as a universal constant, but instead, merely the common limit to all of the reference points within a given transformation set, the integral can be used to calculate energies in systems of lesser energy than light speed – flow rates, for example – or in systems of higher energies than light speed, should such systems be discovered at some point in the future (the speed at which new memes become intolerable suggests itself).

Hence, my version of B=mc² is superior by three distinct measures:

1) The cosmic speed limit is not constrained by the speed of light, but rather by the limits of the system of bacon delivery.

2) B=mc² can be used to describe the maximum potential "goodness" value of a food item, where m is the weight by volume and where c is the maximum throughput as determined by such factors as maximum chewing and swallowing velocity before excess pressure causes system failure, AKA dark matter leakage.

3) Bacon is good.
posted by It's Raining Florence Henderson at 12:51 PM on April 21, 2011 [8 favorites]

Space is flat in the absence of matter

My flat has no space, but it doesn't matter.
posted by Decani at 12:51 PM on April 21, 2011 [6 favorites]

Could some philosopher type explain to me the epistemological basis of theory creation? What is the difference between"something invented to satisfy a mathematical formula" as opposed to " something for which there was compelling evidence." Isn't the formula the evidence in structured form? In other words, why should the universe be expressible in understandable form, as opposed to mere accurate equations? I can appreciate why one might want it to be understandable, but isn't this just an aesthetic consideration?
posted by Obscure Reference at 12:52 PM on April 21, 2011 [1 favorite]

I'm okay with this, just so long as we're still all agreed that everything is held together by tiny strings.
posted by Sys Rq at 12:53 PM on April 21, 2011 [2 favorites]

It's like receiving a post card from Mars.
posted by klausman at 12:55 PM on April 21, 2011

Dark matter isn't really a question, it is an assertion that our theory/model/math is exceeds our observation/reality, and this dark matter/energy is simply a method of putting one's thumb on the scales and tilt everything back so that our neat model worked out.

Which is why I also found it deeply suspect. So much so that when we had a black goldfish, we titled it: MACHO!

Which stood for Massive Astro-physical Compact Halo Object, or to put it laymans terms, something really heavy, and really far away, but really small and really really hard to see, and look, fits our theory! Unfortunately it turned out there isn't so many MACHO's out there so it didn't solve the central issue they was hypothesized/created to service. But the fish was real enough, and I always loved the opportunity to tell folks that I had, in fact, found all that missing dark stuff. It was in my living room, eating shelled peas. He's really really heavy I would tell them. Like the sun, I would insist, and it balances our model of the universe, I would claim. This sort of thinking is likely why I am not an astro-physicist.

Of course, when the fish fully matured he turned almost entirely gold.
posted by zenon at 12:58 PM on April 21, 2011 [9 favorites]

I seriously doubt that not one, but two of the biggest challenges in modern astrophysics are going to be solved by a single guy toiling in obscurity.

You know, like an obscure patent examiner in Bern.


Just came in to type that.
posted by Ironmouth at 12:58 PM on April 21, 2011

(I thought [citation needed] was being sarcastic?)
posted by kmz at 12:59 PM on April 21, 2011 [3 favorites]

In other words, why should the universe be expressible in understandable form, as opposed to mere accurate equations?

There's no reason that it has to be understandable. There's no reason there has to be a universe at all.

Though to say that anything about advanced physics is understandable is a stretch.

The deeper you go, the weirder it gets. When you're learning classical mechanics and newtonian physics, it's really easy to see the numbers you're manipulating as stand-ins and symbols for real objects, but as you get into quantum mechanics it gets harder and harder to think of the numbers you're manipulating as referencing or standing in for some real thing. Once you get down to that level it seems that reality is math, and nothing else. The numbers don't stand in for something else, the numbers are just what reality is.
posted by empath at 12:59 PM on April 21, 2011 [3 favorites]

I'm sort of sick of the implication that Einstein was isolated (and sometimes even uneducated) and that he dreamed up relativity in a vacuum.

I've found this claim to be put forward by people toiling on the internet, in time cubey fashion, and those who like to think that their lack of understanding of advanced science makes them more likely to make the breakthrough that will astound us all. It's annoying, and strangely, much more annoying than it ought to be. It may, in fact, tap into "dark annoyance," the energy that fuels those things that should just slide off you, but which you find yourself unreasonably yearning to fight over.
posted by OmieWise at 1:03 PM on April 21, 2011 [18 favorites]

I think the very very short version is that where Einstein's theories were based on the invariance of length -- a stick measures the same wherever and whenever you measure it -- Mannheim's theory requires that angles are preserved. This results in relativistic equations which are similar to Einsteins in the neighborhood of earth (just as Einstein's equations reduce to Newton's at non-relativistic speeds) but which diverge radically the further out you get in space or time.

Mannheim's theory says that at sufficiently large distances gravity may become repulsive. Experimentally this could be shown if distant objects are red shifted more than would be predicted by current theories.

That said there seemed to be a LOT of handwaving in the paper, assuming he got the math all right.
posted by unSane at 1:03 PM on April 21, 2011 [4 favorites]

I've found this claim to be put forward by people toiling on the internet, in time cubey fashion, and those who like to think that their lack of understanding of advanced science makes them more likely to make the breakthrough that will astound us all.

"They laughed at Galileo. They laughed at Newton. But they also laughed at Bozo the Clown."
posted by kmz at 1:06 PM on April 21, 2011 [2 favorites]

unSane's description seems accurate, as far as I can tell.

I will readily admit that this is over my head, but conformal gravity seems to be a system where the Ricci tensor Einstein used to describe curvature is replaced by a more complicated Weyl tensor. If I'm understanding correctly (which I may not be), the result is apparently that, as the distance between two objects grows larger, gravity does not necessarily just grow weaker, as Einstein predicted -- it can change and even become repulsive.

If true, it's a neat little solution ... but for those complaining that dark matter and dark energy are nothing but mathematical tricks to explain what's been observed, without additional evidence to support their existence, this kind of has exactly the same problem. Gravity changing at long distances would solve the same irregularities currently commonly explained by dark matter and dark energy ... but I'm not sure there's any evidence for it other than those same irregularities.

Poking around, conformal gravity also may have some hurdles to get over. I've seen cautions that it goes against the evidence of the cosmic microwave background, and it seems to be unable to account for the creation of large amounts of deuterium during the Big Bang. Doesn't mean it's right or that it's wrong, per se, but I personally don't think this can be considered a nail in the coffin of dark matter just yet.
posted by kyrademon at 1:07 PM on April 21, 2011 [6 favorites]

3) Bacon is good.

Sing it on high, brother! Amen!
posted by KingEdRa at 1:08 PM on April 21, 2011

Nobody knows what dark matter is

Wormhole-linked Dyson spheres built by the Type III civilization whose assimilation probes are currently speeding toward our system.

Duh.
posted by adamdschneider at 1:10 PM on April 21, 2011 [5 favorites]

made me feel like a chimp trying to assemble a submachine gun: best case scenario was that I couldn't get anything done.

I love this remark, because it so perfectly describes my life.
posted by aramaic at 1:20 PM on April 21, 2011 [7 favorites]

I seriously doubt that not one, but two of the biggest challenges in modern astrophysics are going to be solved by a single guy toiling in obscurity.

Yes, toiling in the obscurity of University of Connecticut, after serving on the faculties of the University of Oregon, Lawrence Berkeley Laboratory, Stanford Linear Accelerator Center, the Institute for Advanced Studies at Princeton, and in Belgium.

Also the obscurity of having nearly 100 professional papers published in the past 40+ years.

Yes, he's very obscure.
posted by hippybear at 1:22 PM on April 21, 2011 [2 favorites]

"It seems like it was invented to fit a hole in a theory, and then convoluted experiments have been done (and are being done) in order to back up this hole-plugging."

Hardly.

If you look at a galaxy spinning around in space, the rate of spin for the outer stars wildly defies the physics of motion. But if you assume there is a large amount of matter we can't see in any wavelength, the rate of spin is just fine. That's dark matter. Just that.

There is no "convoluted experiments" or dubious theory. It's looking at a thing spinning, and some high school math.
posted by y6y6y6 at 1:23 PM on April 21, 2011 [1 favorite]

Dark matter is how Shepard is going to destroy the Reapers, duh.
posted by kmz at 1:24 PM on April 21, 2011

> On a side note, my professional opinion is that dark energy may end up to be BS, but dark matter's going to be pretty hard to shake.

So dark energy isn't just dark matter multiplied by the speed of light squared?
posted by Brak at 1:25 PM on April 21, 2011 [2 favorites]

I tweeted Neil deGrasse Tyson about this. Letcha know if he replies.
posted by BeerFilter at 1:28 PM on April 21, 2011 [2 favorites]

Make sure he knows about my Bacon formula, too. If you read it carefully, it will change your life.
posted by It's Raining Florence Henderson at 1:33 PM on April 21, 2011

So just having visited Bern and the Einstein house there, I thought I would weigh in. While it is true that Einstein was trained and was working on his dissertation, he also worked largely outside of the academic establishment. He was the only person in his graduating class at the Polytechnic that did not get a job when he graduated - thus he had to find a position at the patent office.

Also he initially withdrew his first dissertation in 1902 before finally having it accepted in 1905. So I wouldn't exactly characterize him as the golden haired boy of the science world.

So I don't think it would be that big of a stretch to say he was toiling in obscurity. It is surprising that even after 1905, it took over 4 years for him to land a junior professor's position (see same link above).
posted by spaceviking at 1:46 PM on April 21, 2011 [2 favorites]

So dark energy isn't just dark matter multiplied by the speed of light squared?

'Dark matter' is also energy, but it's not 'dark energy'.
posted by empath at 1:50 PM on April 21, 2011

It is surprising that even after 1905, it took over 4 years for him to land a junior professor's position (see same link above)...So I wouldn't exactly characterize him as the golden haired boy of the science world.

That was going to be my second point - it's not like his theories immediately catapulted him into fame and fortune (or even an academic job).

Grand Philosophy of (Crackpot) "Scientists":
(1) Great science is always done by complete outsiders,
(2) Great science is and should be immediately embraced by all who read it.
posted by muddgirl at 1:53 PM on April 21, 2011 [1 favorite]

(3) Great science will corrupt the scientist into a sociopathic megalomaniacal super-villain with huge veins on his or her head.
posted by Mister_A at 1:55 PM on April 21, 2011 [1 favorite]

On Dark Matter:

Rotation curves were originally the evidence for Dark Matter. Vera Rubin was the first (and still the most prominant) scientist to put this "problem" into words. However, most convincing to me is gravitational lensing data. Lensing has been one of the great triumphs of General Relativity (famously accurately predicting the change in position of background stars due to the Sun during the eclipse) and results in the last few years from the Bullet Cluster have made dark matter hard to avoid.

My point is that it's more than just rotation curves. Some have suggested that an alternate gravity theory like MOND might equally well explain the rotation curves without the somewhat peculiar addition of an exotic form of matter. However, the lensing results (particularly combined with the rotation curves results) are very compelling and suggest that a picture with cold dark matter is likely.
posted by Betelgeuse at 1:55 PM on April 21, 2011 [8 favorites]

So I don't think it would be that big of a stretch to say he was toiling in obscurity.

My masters-degree-holding spouse had been jobless for over a year before he went back to grad school - does that mean the papers he published during and immediately after his joblessness were produced in obscurity?

In other words, every scientist is Obscure until they become a Genius.
posted by muddgirl at 1:56 PM on April 21, 2011 [1 favorite]

This is a cool paper, although it goes over my head as well (my undergrad GR course from 10 years ago didn't stick with me). Mannheim appears to be a seriously good writer, though, because while I can't follow the math, the text is pretty clear and bits and pieces of the picture make some sense.

That being said, someone's going to take issue with this theory. The reason I think that is some of the text on page 23:


In this sense Newtonian gravity is local in character, since to explain a gravitational effect in some local region one only needs to consider the material in that region. Thus in Newtonian gravity, if one wishes to explain the behavior of galactic rotation curves through the use of dark matter, one must locate the dark matter where the problem is and not elsewhere, i.e. within the galaxies themselves....

...In conformal gravity one cannot ignore the rest of the universe, with a test particle in orbit in a galaxy being able to sample both the local field due to the matter in the galaxy and the global field due to the rest of the matter in the Universe. Unlike Newtonian gravity then, conformal gravity is an intrinsically global theory.


It certainly sounds like he's saying that his version of gravity must take the entire universe into account in all cases. So, when you drop a ball on earth, it is somehow important to take into account where andromeda is at the time. Which seems unlikely.

Of course, I don't follow the details well enough to understand under what conditions this affect is significant (maybe it's really small when you're dropping a ball). But the quote above is very odd, and I would guess that I have either misunderstood what he's saying entirely or that the theory has some serious problems.
posted by bessel functions seem unnecessarily complicated at 1:59 PM on April 21, 2011

Came here to say what Betegeuse said about the Bullet Cluster. I just gave my astronomy class an exam about dark matter and dark energy. It'd be nice for a theory to explain all this simply. "Everything should be as simple as possible... but not simpler!" There's a lot of theoretical wiggle room when it comes to modifications of GR. I agree it'll take some scrutiny of these claims to shift the consensus on dark matter (which is firmer than the consensus on dark energy).
posted by Schmucko at 2:00 PM on April 21, 2011

You just have to reverse the polarity of the neutron flow.
posted by joannemullen at 2:00 PM on April 21, 2011 [2 favorites]

2D space certainly explains a lot of odd plot lines in star trek...

Captain: we have to get through that fleet of (Romulan/Borg/Klingon/Dominion) ships!
Officer: isn't space 3D? can't we just go the long way around?
Captain: did you even read the script?
posted by Glibpaxman at 2:01 PM on April 21, 2011 [8 favorites]

Yes, toiling in the obscurity of University of Connecticut, after serving on the faculties of the University of Oregon, Lawrence Berkeley Laboratory, Stanford Linear Accelerator Center, the Institute for Advanced Studies at Princeton, and in Belgium.

Also the obscurity of having nearly 100 professional papers published in the past 40+ years.

Yes, he's very obscure.

OK. You're right. "Obscurity" is not the right word.

My main point was just that his small number of collaborators on this work (his student was his only collaborator on his most recent papers) suggests that his work hasn't been accepted by the larger community of folks like him. He's published enough that, if he were really onto something, there would at least be some other people who would be working with him.
posted by Betelgeuse at 2:02 PM on April 21, 2011

After many years of lurking, I thought I'd finally bite the bullet and join. I'm a particle physicist working on dark matter (among other things), and I wanted to weigh in on whole dark matter thing (though not dark energy: they are very different and I have no good ideas about it).

I thought it might be useful to lay out exactly why physicists believe so strongly in dark matter, and how it’s more than us “putting our thumbs on the scales.” The the thing to keep in mind is that there are several different measurements that lead us to dark matter. The one people are talking about here (rotation curves) is only one.

The rotation curve thing is the observation that the spiral galaxies are spinning too fast to hold their stars in. One way to picture the problem it is to imagine spinning a weight on a string over your head. If you spin the weight very fast, the tension on the rope, required to hold the weight on it’s circular course, will snap, and the weight will go flying off in a straight line

Were galaxies composed only of luminous matter, these fast moving stars would have long ago escaped their observed orbits. Clearly, there was something else at work here. Physicists, demonstrating their infinite ability to come up with a catchy name, called this invisible stuff “Dark Matter.” This is, to some degree a fudge factor, and it could be anything that gravitationally attracts. The MACHOs mentioned by zenon above were the best candidate at the time, since we know protons and neutrons exist, and we don’t like making up new particles without a good reason.

Even before this, Fritz Zwicky had demonstrated that the Coma cluster, a group of galaxies, had individual component galaxies moving way too fast to be held together over long time scales. Either the Coma cluster only existed briefly (exactly when we happen to be looking at it), or it’s much heavier than the stars and dust predict. Again, MACHOs are a fine answer. We looked though (with gravitational lensing), and we don’t find enough objects that could be MACHOs in our own Galaxy.

The real evidence that dark matter is really new and different is the early Universe and the Bullet cluster. The latter is a collision between two clusters of galaxies. We can see the dust that each cluster contained: it stopped in the middle of the collision (forming the eponymous bullet-shape), and using lensing measurements, we can see where the mass of the clusters is, and it doesn’t line up. The “dark” matter of the clusters passed right through each other.

This is a real problem for theories that fix the dark matter problem by modifying gravity. In any exampleI know about (MOND is the main on, but this paper by Mannheim I think has the same problem, and I notice he avoided talking about the Bullet cluster at all), has mass trace the normal matter (called baryons). Here, you have to have mass displaced from all the baryons, which is really hard to do without a new particle. MOND models that do fix this do it by... introducing a new particle that you could have just called dark matter.

Finally, the early Universe (via our picture of it from the Cosmic Microwave Background: CMB), tells us when matter started clumping. Baryons can’t clump early enough to form the galaxies we see today, or the clumping we see in the CMB. Dark matter, which doesn’t talk to photons, can. Note that this wasn’t a motivation for dark matter, but dark matter fixes this problem anyway.

Also, we get a different measurement of the baryon density in the early Universe from the number of heavy elements formed. If all the mass we see in galaxies were baryons, we’d get the totally wrong answer. With Dark matter, we get the correct one. However, I’m not sure what happens with this if you start changing gravity a la MOND. I think you have problems though.

So while just saying Einstein was wrong sounds like a better answer, there’s a host of experimental evidence that needs to be answered. To me, dark matter seems like the simplest solutions. Hopefully, we’ll have some solid evidence of dark matter in the lab very soon.
posted by physicsmatt at 2:02 PM on April 21, 2011 [192 favorites]

but I tensor calculus makes my head spin

Well, spinors make me tense, so maybe we should collaborate?

But if you assume there is a large amount of matter we can't see in any wavelength, the rate of spin is just fine. That's dark matter. Just that.

And when I was in school mumble years ago, the proposed explanation weren't very exotic: things like a bunch of cold planets in the galactic halos (MACHOs) or some unknown weakly interacting massive particle (WIMPs). I guess further observation have ruled out the simple explanations, though.

as a layman, it seems to me that the dark matter hypothesis is in place because it fits the currently accepted model. Discrepancies between predicted and observed phenomena can be explained, in theory, by other factors–maybe our idea of gravity is incorrect or incomplete, for instance. I think "dark matter" is convenient because it doesn't require reworking some cherished theories that worked to describe the observed universe beautifully well for some time now

Physicists are not unaware of the history of their field, and Kuhn notwithstanding, I think most physicists are willing to consider total rewrites of parts of physics if it makes everything simpler (and who wouldn't want to be the next Kepler or Einstein?). Thing is, our current theories explain such a huge number of things so extremely well that it takes a ton of work even to see whether a new theory comes close to working as well as the standard theories.

As I understand it (IANAP), the main alternative to dark-matter ("there is invisible stuff") theories is dynamics theories ("the visible stuff doesn't act like we think it does"). This page on MOND also has a link to a tree of the various categories of explanations people have come up with. From Kyradaemon's post, I guess that Mannheim's theory lives in the upper-right, with "Weyl gravity".
posted by hattifattener at 2:02 PM on April 21, 2011 [2 favorites]

"Jim, I'm just a doctor! Go ask Spock to explain the difference between a PT symmetric and a Hermitian theory!"
posted by rdone at 2:05 PM on April 21, 2011 [1 favorite]

(ah, and right before I post, physicsmatt and betelgeuse jump in with the infos I was half-remembering. Listen to them, it's been ages since I tried to understand this stuff.)
posted by hattifattener at 2:05 PM on April 21, 2011

physicsmatt: Welcome to MetaFilter! I hope you enjoy participating here!

I wonder -- have you looked at the paper linked in this FPP? Can you make any sense out of it? (I can't, but then I was a music and psych major in college.) Maybe you can provide us with some better insights about the matter (dark or not) at hand here with this paper.
posted by hippybear at 2:08 PM on April 21, 2011

So while just saying Einstein was wrong sounds like a better answer, there’s a host of experimental evidence that needs to be answered.

He's not saying einstein is wrong is he? Only that there are other ways of writing equations of motion that are equally correct?
posted by empath at 2:23 PM on April 21, 2011

So it seems to me (not a physicist) that the rotational problem arises because of a loop in the calculations.

1) We say that the outer stars of a galaxy are moving too fast given the mass of the galaxy
2) We calculate the mass of the galaxy's central black hole by the speed of rotation of nearby stars given our assumptions about mass and gravity

If black holes were more massive that we expected couldn't that solve #1 given that we're calculating it wrong? (Due to gravity being slightly different than expected?
posted by CheeseDigestsAll at 2:24 PM on April 21, 2011

If it doesn't lead to cheap ftl travel, the theory is obviously incomplete.

ftl travel alone wouldn't be enough for you? it needs to be cheap, too?
posted by finite at 2:27 PM on April 21, 2011 [3 favorites]

So it seems to me (not a physicist) that the rotational problem arises because of a loop in the calculations.

1) We say that the outer stars of a galaxy are moving too fast given the mass of the galaxy
2) We calculate the mass of the galaxy's central black hole by the speed of rotation of nearby stars given our assumptions about mass and gravity

If black holes were more massive that we expected couldn't that solve #1 given that we're calculating it wrong? (Due to gravity being slightly different than expected?

It's worth pointing out that the central black hole has basically zero influence on the vast, vast majority of the galaxy. One of the key issues in wrapping your head around issues like this (and lot of issues in Astronomy) is the problem of scale. Picture in your head for a second the size of the solar system. OK. I can guarantee you that the actual size of the solar system is several orders of magnitude greater than what you're picturing. Plus, the distance to the next star is WAY bigger than that distance (again by many orders of magnitude). Finally, the size of our Galaxy is WAY bigger than that distance (again, by orders of magnitude). For this reason, it's hard to imagine that something like a supermassive black hole would have, essentially, no influence on most of the galaxy.

The rotation velocity disagreement can be understood by the comparison of two masses in two experiments/observations.

Experiment 1:

1. Measure the rotation velocity of the galaxy.
2. Use that rotation velocity to infer a mass using our theory of gravity.

Experiment 2:

1. Count up all the stars, gas, and dust in the galaxy.
2. Make some reasonable assumption about the mass of each of these things (using our physical understanding of these objects to constrain our assumptions).
3. Add up the masses.

You then compare the masses that you got from experiment 1 to the mass you got from experiment 2 and see if they are the same. The thing that Vera Rubin (and others) have found is that the dynamical mass (i.e. from experiment 1) is way larger than the mass that you get from counting everything up (i.e. experiment 2).

So, the argument about a new theory of gravity vs. Dark Matter is an argument about whether we went wrong in step 2 of Experiment 1 (i.e. our idea of gravity is incomplete in some way) or went wrong in step 1 of Experiment 2 (i.e. you missed some mass). Most astronomers think we're missing something in Experiment 2; a small minority thinks we're missing something in Experiment 1. Moreover, there is now pretty good evidence from other experiments (see all the posts on the Bullet Cluster) that we are missing some kind of matter that, when added on to all of the other things in Experiment 2, makes the two experiments agree.
posted by Betelgeuse at 2:41 PM on April 21, 2011 [12 favorites]

Maybe we're not missing mass. Maybe we're missing a force acting on the mass. Call it, oh... I don't know... the Bacon Force.
posted by It's Raining Florence Henderson at 2:49 PM on April 21, 2011 [1 favorite]

ftl travel alone wouldn't be enough for you? it needs to be cheap, too?

I don't think I want to live in a world where only the rich have access to violating causality.

Then again, I imagine I wouldn't get much say in the matter.
posted by Zed at 2:50 PM on April 21, 2011 [1 favorite]

hippybear, thanks! There was a post on the Fermilab anomaly last week that I wanted to join to talk about, but I was so busy at the time (something to do with an anomaly in data...). This time around, I thought I should contribute.

I glanced at the paper before posting, just to see what it said about dark matter (and to check whether it tried to fit the Bullet Cluster, as that's the Achilles Heel of these kind of models in my opinion). Unfortunately, I haven't been paying attention to any modified gravity models, and so evaluating this right now it a bit of a slog. I've spent the last N minutes reading this, and while there are a bunch of things I'd complain about if I were a referee (mostly in the regime where I work and the author doesn't), most of those are about the need for a model without dark matter or dark energy. If you really want to build such a model, you're free to. That said, I can't give a quick coherent explanation of his theory; if I get some more time this evening, I'll give it another shot. Sorry.
posted by physicsmatt at 2:56 PM on April 21, 2011 [1 favorite]

Apology not accepted! How DARE you have a life of your own when people are asking you complicated physics questions on the internet! Get to work!

I mean, right on. I understand how it may take some time to digest. If you do have things to share once you get the chance to look more closely, I'd love to hear what you think.
posted by hippybear at 3:06 PM on April 21, 2011 [1 favorite]

(I thought [citation needed] was being sarcastic?)

[citation needed]
posted by mek at 3:08 PM on April 21, 2011

Welcome, physicsmatt!

[On a somewhat tangential note, I have the following sockpuppet name I thought I'd just toss out there for anyone who might be interested:

Orion's BLT

You're welcome.]
posted by It's Raining Florence Henderson at 3:08 PM on April 21, 2011 [2 favorites]

Maybe we're not missing mass. Maybe we're missing a force acting on the mass. Call it, oh... I don't know... the Bacon Force.

Kevin Bacon? Irresistable force meets unmoved film critics.
posted by Blazecock Pileon at 3:09 PM on April 21, 2011

Orion's BLT

That's my favorite restaurant on Betelgeuse.
posted by Mental Wimp at 3:14 PM on April 21, 2011

Wherever someone has fried eggs and toast, they will be there. Whenever a salad has only croutons and dressing, they will be there. Whenever a dog smells breakfast cooking and can only look forlorn, they will be there.

They are the Bacon Force!
posted by Mental Wimp at 3:15 PM on April 21, 2011

(I thought [citation needed] was being sarcastic?)

This would be the Dark HAMBURGER Hypothesis.
posted by rewil at 3:34 PM on April 21, 2011

I always knew dark matter was bullshit! Seriously. Even though I don't understand it at all

So you're proud that you rank gut instinct and folk wisdom over actual knowledge?

The Dark Ages? Your fault.
posted by obiwanwasabi at 3:38 PM on April 21, 2011 [3 favorites]

Dark matter is a question, not an answer.

To every fanboy out there, dark matter is THE TRUTH.
posted by telstar at 4:11 PM on April 21, 2011

Well, spinors make me tense, so maybe we should collaborate?

Tenser said the Tensor
Tenser said the Tensor
Tension, apprehension, and dissension have begun!
posted by Crabby Appleton at 4:23 PM on April 21, 2011 [5 favorites]

It's Raining Florence Henderson, if we're throwing out nerdy sockpuppet names, I see your Orion's BLT and raise you Neutrino Kill Zone or Proud Parent of a Boltzmann Brain.

hippybear, the problem is that this is just close enough to what I do that I can understand most of it, but its far enough away that, to do it justice, I'd have to take a solid evening to read the paper and some of the other citations. I've never met the author, but I don't want to go running around the internet screaming his theory is wrong if I don't take the time to do due diligence.

One thing I can talk a bit about is this conformal gravity idea. Or actually, just what a conformal theory is. Conformal here basically means "scale free," where by scale I mean a characteristic energy. Conformal theories tend to be handy as one can actually do a lot of the nasty calculations that plague certain classes of quantum theories. Mapping those intractable theories into a conformal version is big business these days. In this example, the author claims he can avoid a cosmological constant (which is special energy scale) in conformal theories (which makes sense: these theories don't have special scales).

One problem I don't know quite how he gets around is, in real life, there ARE special energy scales. For example, the higgs boson defines one such scale, and once you have that scale through some other mechanism, you'll get a cosmological constant. He talks about this a bit, but I found his solution a bit handwavy (however, I didn't read the papers cited, so maybe its plausible). If I had to guess, I'd imagine that the solution to dark energy will be something along the lines of a conformal theory. I don't think it will have nothing to do with dark matter, mostly because as I said previously, dark matter doesn't seem to track regular matter at all in the universe.
posted by physicsmatt at 4:30 PM on April 21, 2011 [1 favorite]

... and this thread went exactly as I had hoped. Seriously.

I will add that I've spent months reading through his Comprehensive Solution paper, and it makes some intuitive sense.

It is true that there is a background gravitational potential due to all of the matter-energy in the universe, and it also establishes a natural length scale (in a scale invariant theory... symmetry breaking creates length scales) for galaxies. Newton is recoverable. Special relativity is recoverable, and pertinent aspects of General Relativity are recoverable. GR fails at the galactic level and cluster level. This theory does not (thus far).

Will the bullet cluster be addressed? (I believe is the same type of question as whether the rotational curves of galaxies would be addressed. One down. One to go.) It has not been explicitly studied as yet... probably due to the lack of people working on this theory, which doesn't make it any less correct... if it is. Actually, I should say that hypothesizing dark matter as a solution is just as ad hoc as it was for the rotation curves.

He is not obscure, and the biggest impediment to his theory's development has been the conjecture that 4th order theories are plagued with negative norms. He has recently found the solution to this problem. All eigenvalues are shown to be real and positive.

Why now will it take years to work itself through the academic gauntlet? The short answer is in fact due to its inherent complexity (pure 4th order theory). Occham's Razor would be misapplied here since the divergences he accounts for are in fact quartic and necessitate a fourth order theory... I believe, but I'm not the expert's expert.
posted by quanta and qualia at 4:32 PM on April 21, 2011 [1 favorite]

I am, however, obscure.
posted by quanta and qualia at 4:37 PM on April 21, 2011 [1 favorite]

I saw this and thought, oh, I should tell my brother about this — then I realized he posted it.

Hey Tim.
posted by defenestration at 4:39 PM on April 21, 2011 [11 favorites]

So dark energy isn't just dark matter multiplied by the speed of light squared?

Well, no, of course - dark energy is dark matter multiplied by the speed of dark squared.
posted by ivancho at 4:41 PM on April 21, 2011 [10 favorites]

If black holes were more massive that we expected couldn't that solve #1 given that we're calculating it wrong? (Due to gravity being slightly different than expected?

Well that's what all these non-dark-matter theories are about, coming up with new ways to do the calculations so that things work out.

I'm surprised by all the people who say "Dark Matter sounds like bullshit" or whatever. Dark Matter is just matter you can't see. Planets and things like that are all 'dark matter' The earth would be made of 'dark matter' for an observer far enough away that they couldn't see the reflected sunlight.

The problem is that 'ordinary' ('baryonic') dark matter isn't enough to satisfy the equations, and so this non-baryonic stuff has been proposed.
posted by delmoi at 4:44 PM on April 21, 2011 [1 favorite]

dark energy is dark matter multiplied by the speed of dark squared.

There is no such thing as light. Light is merely the absence of darktons.
posted by hippybear at 4:47 PM on April 21, 2011 [6 favorites]

Hmm, I was reading the wikipedia article on Dark Energy and came across an article on Dark Fluid which is apparently a proposed synthesis of dark energy and dark matter. And also Dark Flow which isn't related to either, as far as I can tell.
posted by delmoi at 4:49 PM on April 21, 2011

So what you're saying is, the universe is basically held together by Guinness. I can buy that. And often do.
posted by It's Raining Florence Henderson at 4:52 PM on April 21, 2011 [2 favorites]

I bailed as soon as I saw the equations.

Fucking math.
posted by bwg at 6:03 PM on April 21, 2011 [1 favorite]

There is no such thing as light. Light is merely the absence of darktons.

Reminds me of this old gem. I was just a kid and I think I saw it in a k12 group, but there always seemed to be more to that theory than is publicly accepted.

DARK IS ALSO HEAVIER THAN LIGHT: If you go swimming, just below the
surface of a lake you will see a lot of light. If you swim deeper
and deeper, you notice it slowly gets darker and darker. When you
reach a depth of approximately 50 feet, you are in total darkness.
This is because the heavier dark sinks to the bottom of the lake and
the lighter dark floats to the top.

posted by polyhedron at 6:11 PM on April 21, 2011 [3 favorites]

quanta and qualia, the problem is that the scale set by galaxies is 1/(10^5 ly) ~ 10^-28 eV. The conformal breaking scale I'm worried about is the higgs vev of ~200 GeV.

Additionally, f(r) theories of gravity (which it seems this bears a pretty strong resemblance to) are known to fit the rotation curves of galaxies better than cold dark matter. However, this assumes there's one set of rotation curves to be fit to, as the observational and simulation people will tell you, every galaxy is unique (we're all individuals!), so getting a good universal fit doesn't tell me that much. Plus, the simulations for cold dark matter were ignoring baryon effects which are known to be important in the very region where the CDM simulations seem to be most problematic. It's a work in progress.

Now that I think about it, I'm wondering what this paper did (if anything) about the very small dwarf galaxies we know are out there. Their mass to light ratios are huge, and I'd be interested to see how well his model fits those particular ones. The missing satellite problem is a bone of contention in cold dark matter, my naive guess is this model would do much worse, but the paper doesn't break down his fit to the rotation curves well enough for me to be sure.

The Bullet Cluster is a huge problem for all MOND theories, because it's a clear example of mass not tracing the baryon distribution. It isn't something you can trivially tweak away, and I have never seen any modified gravity theory that can explain that observation without introducing a new massive, stable, noninteracting particle. In which case, congratulations! You now have a theory more complicated than dark matter... that contains dark matter.

Finally, there's the early Universe results for BBN and the CMB, which dark matter and dark energy explain amazingly well. Maybe this model can do that too, but it's a very non-trivial problem, and the safe money is on no.

Not to be too much of a wet blanket, but there is a very good reason that modifying gravity isn't as popular as dark matter. This theory is almost certainly mathematically correct, but he has to make some assumptions early on about how the Universe works that aren't axiomatically true. I also don't see a direct way to test this theory. If the experimentalists don't see dark matter this year, is it this theory? another theory of MOND? or is dark matter just axions? Testability is a general problem with all explanations involving dark energy, and that's a big reason I don't work on it.

Now that I've run my mouth off about something I'm not an expert in though, I'll say that clearly something crazy and new needs to come along to explain dark energy (if not, in my opinion dark matter). Maybe this guy has found something just crazy enough to work. In which case, he'll get all the money, power and women (or men) which were the big reasons we all went into physics in the first place.
posted by physicsmatt at 6:17 PM on April 21, 2011 [9 favorites]

delmoi, I once spent a serious amount of time on a research project just so I could use the phrase "dark light" in talks. It's no penguin diagram, but sometimes you have to make your own entertainment.
posted by physicsmatt at 6:21 PM on April 21, 2011 [3 favorites]

Physics for the 21st Century
Unit 10: Dark Matter (PDF)

which led me to this.
posted by clavdivs at 6:53 PM on April 21, 2011

Fantastic.
Next targets: Big Bang, Redshift, and Time. Things have been getting far too cluttered and ad-hockey. The price of elegance is *never* too high.

As Bronowski said, When people believe that they have absolute knowledge, with no test in reality, this is how they behave.
posted by Twang at 7:00 PM on April 21, 2011

Inelastic Dark Matter sounds sexy. nice post and I am thumbing through Flatland for some clue as to matter-less space.
posted by clavdivs at 7:04 PM on April 21, 2011

Oh weird, this and Mister_A's comment reminded me:

The other day I was reading this and asking my husband if he knew anything about it; he works for NASA and has to deal with gravity when making some calculations, but he's an aerospace engineer and not a physicist so he knew of some of the discrepancies but not enough particle physics to really judge it. Nearly simultaneously, though, we both sort of wrinkled our noses and said a variation of "Dark matter seems like bullshit" in a wondering sort of way -- and not "dark matter" in the broad sense of "whatever is wrong/missing with the theory of gravity," but "dark matter" in the sense of "what is missing from the theory of gravity is a particle that is supplying this unseen mass or making things behave differently -- more or less attractive -- in this context." Which I'm not sure is actually how physicists use the term or even what they actually look for, but as laypeople that's how we've always heard dark matter described, so that's how it's framed in our heads. In that sense, it made us wonder if dark matter is less a quirky thing that will eventually fit into our current theories of gravity, or a band-aid to keep the current theory of gravity intact when some fundamental assumption of it is flawed.

To be clear, we don't know shit about shit so I'm not saying any of this means anything; we probably misunderstand something. The question we both had, though, was wouldn't it make more sense to think that we misunderstand how gravity arises than imagine there's all this invisible mass or a missing particle that explains the discrepancies? Not that it's not possible and it's certainly worth looking for that too, and of course we shouldn't throw out a theory that's still predictive in so many circumstances, but it seems like it's just as reasonable to ask "are we wrong about something basic?" as it is to ask "are we right but we have to uncover what accounts for these situations?" For example, gravity as an emergent property of thermodynamics is the kind of explanation that seems more likely, not in specifics (since we don't know shit about shit), but in that the very underpinnings are held to be wrong and there is no need for extra mass or particles to explain anything if you abandon the idea that gravity is a fundamental force, or that it's based on space-time curvature, or anything else that seems reasonable or even predictive much of the time but may not necessarily work out after the cards are all turned over.

So anyway, this is just to say that this is interesting to me because, right or wrong, any theory that doesn't need dark matter in the sense of extra invisible mass or another particle appeals to me... but I can't even get the gist of what this paper is proposing, haha. I am shocked that not knowing shit about shit will only get me so far. :(

I read physicmatt's comment above, and I'm not sure if it's answering quite the same question that my husband and I have...? I wouldn't necessarily be sophisticated enough to tell, but it seems to be more about alternate theories of gravity that only "eliminate" dark matter in the sense that they say "dark matter is this particle, or has to do with this particle" -- rather than saying "everything makes perfect sense and there is no invisible mass or new particles if you derive gravity from this different set of assumptions." Which, err, it occurs to me that "if you derive gravity from this different set of assumptions" may be too broad because there's a sense in which the hidden mass or particle stuff would fit that, but I don't have another way to phrase it except maybe to say in the same sense of gravity being emergent in some way instead of fundamental. Am I reading the comment correctly, i.e. the issue addressed is the idea that most alternate theories of gravity are basically giving "dark matter" specific names and have their own discrepancies? Or does it also apply to what seems weird to me, or is it sort of the same thing in some way I don't see, or something like that?
posted by Nattie at 7:39 PM on April 21, 2011

physicsmatt, I am glad you signed up for metafilter. When I first applied to grad school, I looked into MACHO and WIMP experimental groups. I ended up in an ultracold quantum optics theory group.

First, LBCs are what you were asking about: Fits of many galactic rotation curves.

Second, MOND is an ad hoc theory. This is a pure 4th order theory. MOND is a competing theory that Mannheim's Conformal Gravity Theory must both account for its successes and avoid its failures. The linear and quadratic potentials arise from the math. I am headed back to the text for their connexion (sp?) to the theory.
posted by quanta and qualia at 7:56 PM on April 21, 2011

In brief, they become background terms in a fourth derivative, I believe. This coincidentally also closes the cosmological constant and zero-point energies as well. (I may have to rescind this gut explanation.)
posted by quanta and qualia at 7:59 PM on April 21, 2011

I'm sorry for that dead link above: Actual link to Galactic Rotation Curves: LBCs
posted by quanta and qualia at 8:01 PM on April 21, 2011

In our work we have required symmetry breaking to be dynamical, with the scalar Higgs field that is commonly used in symmetry breaking in particle theory having to only be a c-number order parameter in an effective Ginzburg-Landau theory. Being a c-number, such a Higgs field would not be detectable as a particle in an accelerator experiment, and in addition, there would be no quadratically divergent self-energy hierarchy problem. Mannheim, Comprehensive Solution...

This is what he says about the Higgs field, and subsequently about a prediction at the LHC.(?)
posted by quanta and qualia at 8:12 PM on April 21, 2011

The sky is green! But my observations don't fit my model, so I'll make up some bullshit to fit.

blue + yellow matter = green
posted by gjc at 8:41 PM on April 21, 2011

That galactic rotation curve fitting paper is a massive piece of work and has some really interesting observations in it towards the end, such as a seemingly universal term which drops out of the empirical data and is not accounted for by the galaxy-by-galaxy dark matter theories, plus some indications as to how experimentally you could decide between conformal gravity, MOND etc.

It seems to have the advantage over MOND that it proceeds from a fundamental idea (the confrormal bit) rather than being a modification of Newtonian dynamics which fits the data better.
posted by unSane at 8:47 PM on April 21, 2011 [1 favorite]

Redshift Controversy.

I just found this online. All hail the internets! I believe unSane was hinting at this?

The cosmological constant can thus be huge and yet its effect on cosmic evolution would still be small ... with the coherent state in which it is to be evaluated redshifting as the universe expands.
Mannheim, Comprehensive Solution ...

posted by quanta and qualia at 8:48 PM on April 21, 2011 [1 favorite]

Nattie, the thing is that, in order to correct general relativity to fix the problems that dark matter is trying to fix, you end up either introducing other problems or not solving all of the things you started off with. It is HARD to change something as basic as gravity without getting even the simplest observations incorrect.

Dark matter doesn't count as changing gravity because it doesn't change how space-time bends in response to mass. The equations that govern that are still the same, but there's just an addition thing wandering around that has mass. So, we can ask: if there's this extra particle around with properties like "doesn't interact with light or the strong nuclear force", what would it act like? And the results are extremely similar to observations. For example, the simulations of dark matter clustering in the Universe do a pretty good job of matching the structures we see. For some awesomeness, check out the Millennium simulation. It's the Universe, if you could see the dark matter. And had some great seats.

One thing that I think people outside the field don't appreciate is that, to a theorist, adding a new particle is not a unprecedented step. Changing Einstein's equations is, because they are simple, elegant and experimentally tested to extremely high precision over a wide range of distances. However, the history of physics in the last 100 years has been adding particles to fix problems. For example, the neutrino was introduced to avoid having to postulate energy and momentum violation. So a particle for dark matter is easy for me to imagine, especially since so many theories that were introduced for other reasons, such as supersymmetry, contain ready-made candidates for dark matter. If anything, the problem is that there are too many candidates, and we desperately need some new data to help us move forward.

The entropy and gravity thing you linked to is very interesting. I remember when it came out a few years ago. No idea if that particular formulation is correct, but it is clear to me that gravity and entropy have to have a very strange relationship. There's some fascinating stuff with Hawking radiation and Bekenstein bounds, and if I were smarter or more daring, I'd work on it. I'll shill for Sean Carroll's book "From Eternity to Here" and say if you're interested in those kind of questions, read it.
posted by physicsmatt at 8:55 PM on April 21, 2011 [7 favorites]

Thank you so much for the response, that cleared up a lot for me. :-) I'll get ahold of that book asap, too!
posted by Nattie at 9:14 PM on April 21, 2011

I hope this physicsmatt character sticks around. He seems to have a gift for simplifying big concepts for little brains like me.
posted by hippybear at 9:17 PM on April 21, 2011

For example, the neutrino was introduced to avoid having to postulate energy and momentum violation.

And mind you it took another decade to even figure out how to detect the buggers, and more than another decade after that to actually find one. So these sorts of things can be open questions for quite some time, even when the theorists are on the right track.
posted by Zalzidrax at 9:23 PM on April 21, 2011

Aw, thanks hippybear. I've been lurking for about 5-6 years, so it was time to pay the $5. Plus, talking about physics is fun (though I've already noticed at least one error I made about the f(R) stuff above, that's less fun).

Zalzidrax, exactly. I think the astrophysical observations just far outstripped the dark matter detectors for a while there. However, I'm actually pretty optimistic that this summer will bring some interesting new results. It better, because if it doesn't I'm going to have to wait several more years before anything big enough comes online. All in all though, with the dark matter detectors getting to the sweet spot, the Tevatron reporting wacky shit, and the LHC not exploding, this is an awesome time to be a physicist.
posted by physicsmatt at 9:31 PM on April 21, 2011 [1 favorite]

Redshift Controversy.

For more on Halton Arp and his research on non-cosmological sources of Red Shifts (i.e. caused by other than an expanding universe), I recommend Universe: Cosmology Quest Pt 1.

Pt 2 examines an alternate explanation to gravity-only physics that doesn't require epicycles like dark matter: Plasma Cosmology. The two are essentially companions of each other, though viewable independently

Note: I'd recommend downloading both those videos if you want to each them as they seem to be coming down at the end of the month.
posted by Pirate-Bartender-Zombie-Monkey at 10:05 PM on April 21, 2011

ACK... Universe...
posted by Pirate-Bartender-Zombie-Monkey at 10:08 PM on April 21, 2011

Fucking tensors... How do they work?
posted by cman at 1:19 AM on April 22, 2011 [5 favorites]

Redshift Controversy.

I didn't want to wade into this as I think Halton Arp is seeing things that aren't there, but Mannheim's theory has a very straightforward explanation for redshift, since there's a repulsive component to his gravitational field which becomes significant at cosmological distances.

One paper of Mannheim's (Implications of Cosmic Repulsion for Gravitational Theory) deals with this pretty much directly, showing how a negative spatial curvature predicted by the theory could account for the apparent cosmic repulsion that shows up in recent Hubble Plot results.

The sort of seat-of-the-pants objection that gravity should be local, and balks at the idea that when we calculate a gravitational field there may be a significant contribution from extremely distant objects, which increases the further away they are, doesn't bother me a bit. We just think gravity is entirely local because that's the way it seems around here.
posted by unSane at 4:48 AM on April 22, 2011 [1 favorite]

Fucking tensors... How do they work?

Invariantly, of course.
posted by benito.strauss at 5:55 AM on April 22, 2011

If this is the thread where we get to post our questions about dark matter and have real physicist address them, I want my turn.

The only scenario where I come close to understanding the need for dark matter is the original issue with galactic rotation curves. "There isn't enough matter to explain the rotation speeds we observe", right? So (unless we go the path of re-formulating gravity) we imagine that that there must be more matter out there.

What I don't get is why we assume it must be a totally new type of matter, made up of new types of particles. Why isn't it possible that there's a bunch of "ordinary" matter out there, but we currently don't see it? Is there some explanation that says "if there was boring old matter out there, we would see X, and we don't see X."

In short, why can't dark matter be a bunch of neutrons?

Set me straight, physicists!
posted by benito.strauss at 6:05 AM on April 22, 2011

Not a physicist, but as I understand it the Big Bang nucleosynthesis model doesn't provide for the production of that amount of baryonic dark matter. I think it doesn't square with the cosmological background radiation either.
posted by unSane at 6:25 AM on April 22, 2011

Dark Energy... without it, the cosmos would be inexplicable
posted by doiheartwentyone at 6:27 AM on April 22, 2011

So, it's like blowing up a balloon?
posted by Trochanter at 6:30 AM on April 22, 2011 [1 favorite]

Hey, physicsmatt, while i have an Actual Physicist here, I'm trying to learn more about physics from Susskind's lectures, but while he does explain the math as best he can, I really need help getting up to speed. I pretty much get Calculus, but I need help with linear algebra, tensors, matrices -- aside from wikipedia, can you recommend any books that better explain math for physics?
posted by empath at 6:35 AM on April 22, 2011

unSane, I'm guessing that both the Big Bang nucleosynthesis model and CBR relate to universal abundance of baryonic matter. Wouldn't a galaxy be a local clumping of matter in any model? I don't see how global limits would affect how dense local clumpings could get.

/being a little bit contrary.
posted by benito.strauss at 6:41 AM on April 22, 2011

benito.strauss, well first off, neutrons decay with a half-life of 11 minutes, so what you'd have to imagine is neutral atoms. Which is what astrophysicists first guessed dark matter to be. It can't be dust (i.e. very small grains spread evenly) because dust glows, and we can trace the location of dust by its glow. These objects would have to be some large object, like a planetoid or maybe things as big as Jupiter, but small enough to avoid becoming a star. The term is Massive Compact Halo Object (MACHO), because they have to sit in the halo of the galaxies (in order to get the right rotation curve).

We can't look for these things in any wavelength of light, because by assumption they're very cold. But we can look for the gravitational lensing they would induce. So astronomers did was look at millions of stars, waiting for a MACHO to wander in between us and the star. If it did so, it would very slightly bend the light from the star, and the star would seem to flicker very, very slightly. By NOT seeing this happen, we can place a limit on the density of MACHOs, and that limit turns out to be less than the number necessary to explain galaxy rotation curves.

Now, we have two other measurements that really put the nail in the coffin. The first is Big Bang Nucleosynthesis (BBN). The early Universe as it cooled consisted of a soup of ionized protons, neutrons, electrons, photons and other elementary particles (like dark matter!). If that's all that happened, the Universe would have started with only hydrogen (once it cooled enough for electrons and protons to combine into neutral atoms). As I said, neutrons decay after 11 minutes, so before those 11 minutes are up, every heavier element (helium, lithium and some beryllium) must have been created (everything heavier was created in stars much later). We can go and look at clouds of gas that we have reason to believe have never been "processed" by a star, and find out a good guess for the primordial abundances of these elements. You can then predict the amount you'd get in a Universe with and without dark matter. The equations are not very illuminating, but basically what's going on is you are balancing having matter density (which tells you some information about how the Universe is expanding back then) versus more baryons (which are the fuel for the heavier elements). We get the right answer when we have more matter than baryons, so we need something that acts like matter ("acts like matter" has a specific definition in this context), but isn't protons or neutrons.

Finally, there's the CMB (Cosmic Microwave Background). Do a google image search on this and you'll find a picture of the sky at 2.75 Kelvin (the temperature of the cosmos these days). The CMB started as the "surface of last scattering," the light emitted when the Universe was a few hundred thousand years old, just as it cooled enough for ions to trap electrons, and thus stop interacting with the photons (as much). When that happened, the Universe became transparent, and those last emitted photons just went off forever, cooling as the Universe expanded. There are VERY small variations in the CMB, which come from very small over- or under-densities at that surface of last scattering. Each overdensity grew up to become the largest structures around today - galaxy clusters, so we now have a picture of how structure started forming way back when.

If matter were only baryons, then the photons floating around before the CMB decoupled would have prevented structures from forming. This is because baryons talk to photons, so if they started clumping up, they'd heat up, and the resulting increase in photon temperature would drive the nascent clump back apart. Dark matter, whatever it is, doesn't talk to photons, so it can start clumping earlier, and give us the CMB we see today. Incidentally, this tells us that the dark matter must have been "cold," which means non-relativistic, since it's very hard to trap something moving at 90% of the speed of light inside a small gravitational potential.

Finally, as a fun aside, dark matter was once postulated to be neutrinos, we we know exist, and are "dark." The problem is actually that you can't cram enough neutrinos inside a galaxy. The Pauli exclusion limit tells you that you can't stick more fermions into a volume then you have states to place them in, since they won't share a quantum state with another fermion (elitist bastards that they are). Unless neutrinos were much heavier than we now know them to be, there simply isn't enough room in a galaxy to get enough mass out of neutrinos (plus, they're not cold dark matter, but we didn't know that at the time, as the CMB measurements weren't taken). That should give you an idea of how much dark matter we're talking about here.
posted by physicsmatt at 6:49 AM on April 22, 2011 [8 favorites]

empath, what are your goals? It might give me a better idea of what to recommend.
posted by physicsmatt at 6:52 AM on April 22, 2011

General Relativity. That was the course that made my head hurt to the point where I had to stop watching them.
posted by empath at 7:04 AM on April 22, 2011

(specifically tensors and curvature, and Cristoffel symbols)
posted by empath at 7:05 AM on April 22, 2011

(specifically tensors and curvature, and Cristoffel symbols)

you might try reading some introductory differential geometry, like say do Carmo thick green book.

(you can find it free on the internet....)
posted by ennui.bz at 7:09 AM on April 22, 2011

empath, I've got a degree in math, and "tensors and curvature, and Cristoffel symbols" hurt my brain too. I found the mere mechanics of manipulating them so painful that I could never develop any intuition about them.
posted by benito.strauss at 7:09 AM on April 22, 2011

empath, I don't know your background, but if you want to learn real, unabridged, no-bullshit GR, I'd recommend Sean Carroll's "Spacetime and Geometry." I used that and Wald in grad school; usually I'd read Carroll's chapter on something, then read Wald. I think he goes through enough of the math to give you some idea of what's going on. Not going to lie though, GR is fucking hard. I didn't learn much of the specialized mathematics before going in to GR, and all my books that are relevant are much, much worse to learn from, but Appel's "Mathematics for Physics and Physicists" is supposed to be good (I have it sitting on my shelf, but haven't used it much). Is this for fun? for a class? for your warp drive project? (if so, can I get in on that?) There's a real gap between the serious texts on GR and the popular science stuff, which I'm imagining you've already read and want more than. You could check out t'Hooft's How to Become a Good Theoretical Physicist page, I see he recommends Sean as well as providing his own notes.

(ok, this is the 3rd time I've shilled for Sean in two days on metafilter, so I'll state that I am neither Sean Carroll or Scott Adams. I have worked with Sean in the past, I think he's GR book is the best written grad text I used, and I really respect his abilities as a popularizer of science. Plus, he works on a lot of sexy Big Questions that the non-science community likes to talk about, so he's going to come up. Also, he's a fun guy, and that's a rare trait in science.)
posted by physicsmatt at 7:24 AM on April 22, 2011 [3 favorites]

I'm trying to learn it because I get frustrated by reading pop-science articles that attempt to summarize what papers are saying when it's pretty clear that the reporter doesn't have any idea, so I'd like to be able to read the original papers directly and have a clue what they're talking about.
posted by empath at 7:26 AM on April 22, 2011

physicssmatt, thanks for that fantastic explanation. I'd be happy to re-reimburse your $5 just for that explanation alone.

First off, neutrons outside of a nucleus have a half-life of 11 minutes?!?!?! Holy crap. I never knew that. I guess if they didn't decay, we could have big old piles of neutrons laying around and interfering with traffic.

And dust glows. I feel a bit of a fool here, and I'm sure it does. But does all dust glow? For sure? I guess I'm driven by all my experience debugging computer programs. If the universe were a computer program, I can imagine someone reporting "Well, it turns out that really difficult bug was because everyone assumed dust always glows. Turns out, under very rare circumstances, some dust doesn't glow. Yeah, I'm surprised too."

I really appreciate your explanation of Big Bang Nucleosynthesis. I know you said the equations aren't very illuminating (pun?), but I kinda dig equations. If you've got a convenient reference I'd appreciate it. [DiffEqs don't scare me. The sometimes defeat me, but they don't scare me.]

The idea of the CMB as a picture of the Universe at the moment it "de-ionized" is great too. I'd always heard the CMB poetically called "an echo of the Big Bang", but it's really a snapshot taken a hundred thousand years later. The universe's sixth grade graduation picture.

And I've had enough quantum mechanics to get a little laugh and a little thrill out of seeing the Pauli Exclusion Principle applied to a volume the size of the Milky Way.

Many thanks for your reply.
posted by benito.strauss at 7:32 AM on April 22, 2011

And dust glows. I feel a bit of a fool here, and I'm sure it does. But does all dust glow?

Everything emits black body radiation.

First off, neutrons outside of a nucleus have a half-life of 11 minutes?!?!?! Holy crap. I never knew that. I guess if they didn't decay, we could have big old piles of neutrons laying around and interfering with traffic.

The fact that it even lasts for 11 minutes rather than decaying instantly is a big part of the reason why they are looking for the Higgs Boson. The reason it doesn't decay faster is that the bosons that carry the weak force (W+ and W-) have mass (unlike photons and gluons). Bosons aren't supposed to have mass, so another field had to be hypothesized that interacts with W bosons and stops them from acting at large distances.

If you see phrases like gauge symmetry and spontaneous symmetry breaking in papers, it's usually talking about this. Someone who understands that stuff can explain it better than me, though.
posted by empath at 7:40 AM on April 22, 2011 [1 favorite]

One thing that surprised me about learning about the standard model (and I'll freely admit that I might be misunderstanding) is that all free particles just randomly change into other particles (or just randomly be absorbed into a spot in space and where another particle will be spontaneously re-emitted).

Up quarks can change into down quarks or electrons, quarks can change color, electrons can change into photons, etc, and it all happens pretty much instantly and constantly.

Motion of particles can even be thought of a bunch of particles which are not moving at all, but which are just absorbed at one point in space time and re-emitted at another point in space time that's very close.
posted by empath at 7:48 AM on April 22, 2011 [1 favorite]

God, the mismatch between what you say to a reporter and what they actually WRITE is truly amazing. Some of the dedicated science reporters do pretty well, but in general, it's abysmal. If you're a brave soul, you might try Sean's lecture notes (which have the advantage of being free online). Keep in mind that, even if you understand all this completely, the average cutting edge science paper will be still hard to read. GR is old hat, to some degree, and the stuff going on now requires it (at least in cosmology), but often won't even reference it in a direct way. You'll notice that I basically had to step back from directly engaging quanta and qualia on the paper that this thread is ostensibly about (derail complete!) just because, though I've had all the coursework I'd need, I'm not actively engaged in that area of research, and simply can't spare the time to read all the papers that I'd need to answer the questions I have about it honestly. So don't get discouraged if you can't understand what the hell most of the science papers are about even after a lot of hard work.

benito.strauss, the canonical reference for the BBN abundance is Kolb and Turner's book. I don't have an arxiv paper handy for it (since it was all figured out in the 80s), but I imagine if you googled those two names (or checked inspire) you might figure it out. They assume some things from GR early on, just to get the expansion of the Universe, but if you take that as a given, it just becomes a set of coupled differential equations.

As for neutrons, the thing to remember is that, if the particle isn't the lightest object with a preserved quantum number, it is going to decay (eventually. the 11 minutes is, as empath said, a function of how weak the weak force is at our energy scales). Electrons are stable because they're the lightest thing with charge, protons because they're the lightest thing with strong interactions (or baryon number), neutrinos because they have lepton number, and photons because they're massless. Neutrons are heavier than a proton plus an electron, so they are not going to stick around. This then begs the question: what's keeping dark matter stable (or stable enough to live 14 billion years)?

empath has it on the dust. If you're in the galaxy, you get hit with radiation from stars, and then you heat up, and re-emit the radiation as infrared (I think thats the right wavelength, I'm not an astronomer). You'd have to imagine dust WAY away from the galaxies to avoid heating, but then it couldn't be the stuff fixing the rotation curves.
posted by physicsmatt at 7:50 AM on April 22, 2011 [2 favorites]

Another big welcome to physicsmatt! And thanks for the details comments from everybody.

Just to be clear - I named my fish MACHOS as a joke (and I can't even tell you how great it is to have an even bigger audience for it, because I can assure you only a very small subset of the population even considered anything but odd*) and claiming I was suspect of dark matter was part of the set up for the joke. I understand that the dark stuff have strong arguments supporting them - and it has been really great hearing some of the contemporary issues around it. Science has generally had a long track record of being able to postulate somethings existence long before we were able to actually prove or even perceive them.

*I mean, who doesn't love an astro-physics joke involving fish? It was that or call him Mönch (Munk) Fritsch.
posted by zenon at 7:51 AM on April 22, 2011

Motion of particles can even be thought of a bunch of particles which are not moving at all, but which are just absorbed at one point in space time and re-emitted at another point in space time that's very close.

Are you telling me the entire universe is just a movie being projected at 2 x 10⁴³ frames per second (1/t_P)?
posted by benito.strauss at 7:54 AM on April 22, 2011

Thanks for the reference, physicsmatt. Kolb and Turner it is. Wish me luck.

And I finally get it about the dust. Whatever the mass out there is, it's not subject to the law of black body radiation. Yup, it's gotta be weird.
posted by benito.strauss at 8:03 AM on April 22, 2011

Keep in mind that, even if you understand all this completely, the average cutting edge science paper will be still hard to read.

Yeah, not expecting to be able to judge them on their merits, but I at least know what most of the words mean and have a decent idea of the problems they're trying to solve.
posted by empath at 8:12 AM on April 22, 2011

Well, if you're going to run the camera with Planck time resolution, now you have to deal with the fact that at those scales, causality is propably just a suggestion at best. So now you have particles going backwards in time, popping in an out of existence (more than usual), and generally acting like a bad LSD trip. Though in fairness, that's when quantum gravity takes over, so we really don't know what's going on. LSDions are as good a guess as most.
posted by physicsmatt at 8:37 AM on April 22, 2011 [2 favorites]

you have to deal with the fact that at those scales, causality is probably just a suggestion at best

So like Buñuel? Or more early Lynch?
posted by benito.strauss at 8:41 AM on April 22, 2011

Maybe Joyce a la Finnegan's wake.
posted by physicsmatt at 8:44 AM on April 22, 2011

now you have particles going backwards in time

For example, positrons can be seen as electrons moving backwards in time. And in fact, it has been hypothesized that all electrons and positrons are the same particle with an incredibly complicated path through space and time.
posted by empath at 8:45 AM on April 22, 2011 [1 favorite]

Hey, empath, I think I'm coming in at about the same level as you. If you should find one of the recommended books useful, I'd appreciate knowing.
posted by benito.strauss at 8:47 AM on April 22, 2011

Physicsmatt:

Thank you so much! This thread has been great.
posted by thsmchnekllsfascists at 9:03 AM on April 22, 2011

The problem with the single positron as a backwards electron idea is that there isn't enough antimatter to make it work.

The whole idea is an aspect to charge-parity-time symmetry, which states that the laws of physics are invariant under a charge (matter-antimatter switch), parity (mirroring, right becomes left) and time inversion. So you can't tell the difference between our world and an antimatter world run backwards in a mirror. Fun, isn't it?
posted by physicsmatt at 9:29 AM on April 22, 2011

What does that mean, "run backwards"? It seems like there are lots of one-way, irreversible processes in the universe (e.g. pissing in a swimming pool).
posted by ryanrs at 9:56 AM on April 22, 2011

Yeah, that's entropy. That's where things get REALLY strange. As I said, every fundamental law of physics obeys CPT (charge-parity-time) symmetry, except that, for some reason, entropy always increases. Nothing prevents all the milk in my coffee from jumping straight out of the mug and into a glass next to it, but we all know it will never happen (likewise with pissing in swimming pools). How this comes to be out of time-invariant laws of nature is a big mystery.

Yesterday there was just an askme about this, coincidentally enough. One argument I've heard and can't see any fault with is that it's because the Universe started in a low-entropy state, which then requires one to answer why it started that way. The short version is that, if you start in a low entropy state: coffee and milk separate, for example, then at every step forward in time there are far more states to evolve into that had coffee and milk mixed, and far fewer that result in them separated (this is what entropy counts, actually), so just statistically you're more likely to end up with the non-reversible world we see around us.
posted by physicsmatt at 10:09 AM on April 22, 2011 [1 favorite]

What does that mean, "run backwards"? It seems like there are lots of one-way, irreversible processes in the universe (e.g. pissing in a swimming pool).

If you look at a single particle, it's completely reversible.

On a macroscopic scale with lots of particles, you run into something called entropy.

Any given system can exist in a large number of states. Say, you have a vibrating plate with a 100 coins on it. The plate causes them all to flip randomly. If you add up the number of heads and tails at any given random point in time, you'd expect to have roughly the same number of heads and tails. That's because out of all the possibilities of combinations of coin states, there's vastly more combinations that end up with a roughly equal number of heads and tails than there are with, say, all of them heads.

Now imagine you're filming this system of randomly distributed and equal numbers of heads and tails, and you later watch the tape forwards and backwards. You would have a very difficult time telling which way the tape is going, because all of the states of the system are more or less indistinguishable. This represents a high entropy state.

Now, imagine you prepare the system such that all of the coins are heads. This is an incredibly improbable state -- there's only one out of millions of possible states where they're all heads. The chances of this happening by chance are astronomical.

Let the system run for a while, and purely by chance, you'll soon find that you'll end up witih roughly equal numbers of heads and tails.

Now run that tape forwards and backwards, and it's obvious which direction the tape is playing, because the backwards tape will be showing entropy decreasing, which is something that never happens in reality, purely as a result of probability.

But if you zoomed in on a single coin, you'd once again be unable to tell the difference between forward and reverse, because any result is equally probably in a system of a single coin.

So pissing in a pool is an example of a low entropy state (two liquids that aren't mixed) becoming a high entropy state -- the dispersion of one liquid into another.

However, if you look at the microscopic level of individual molecules in the pool, it wouldn't be at all clear which way time is flowing.
posted by empath at 10:13 AM on April 22, 2011 [3 favorites]

(although gravity is also not reversible, from what I understand, which is why a lot of people think that gravity is related to entropy somehow).
posted by empath at 10:18 AM on April 22, 2011

What about that neutron mentioned upthread?
posted by ryanrs at 10:20 AM on April 22, 2011

The paths of particles in gravity wells are reversible, but the aggregate action of gravity is to move things to higher entropy states (which is lucky for us, as we sort of need the Sun around). There's also the really interesting fact that entropy of black holes is proportional to their surface area, which is of course defined by how gravity works. This turns out to be true for any smooth surface: the entropy of the state inside is always lower than the entropy of a black hole with equal surface area. Gravity is up to something, I just wish I knew what.

ryanrs, when a neutron decays, it turns into a proton, an electron and a neutrino. If I threw all three decay products towards each other with EXACTLY the right momenta and energy, I'd (sometimes) get a neutron out. That's what we mean by the laws of nature being reversible. Now imagine a bunch of neutrons sitting around in a box. They're all identical, and their entropy is low (I can specify it as "X neutrons, sitting in a box"). Now they start decaying. So now I have a large number of protons, electrons and neutrinos running around. That system is much more complicated than the neutron-only one (there are more things to keep track of: number of each particle, and their momentum, for example), and so the entropy is higher. If I kept this system and periodically rechecked it, eventually I might open it and find one or two new neutrons that got created out of the bath of decay particles, but it's very, very unlikely for me to open it and find all the neutrons recreated (like, never happen in the lifetime of the Universe unlikely). Does that make sense?
posted by physicsmatt at 10:32 AM on April 22, 2011 [2 favorites]

re entropy and reversibility

The best image I have of this is my kids playing in the basement. Imagine you start with a tidy basement with all the toys in a neat pile in the center. Now the kids come in and start randomly picking up toys and putting them down.

(This is the big bang).

It is just as easy for the kids to remove a toy from the neat pile as to put it back neatly. So the process of removing a particular toy is perfectly reversible. If all I told you was that a toy was at A at time T1 and at B at time T2 you would have no way of knowing whether T1>T2 or not.

Nevertheless, the initially tidy basement always ends up looking like a bomb has hit it, with the toys more or less evenly distributed through the room. This is maximum entropy, or heat death, and usually the point at which one of the kids comes up the stairs crying.

The reason being that if the probability distribution of where the kid will put the toy is uniform acrosss the room, the probability of all the toys ending up back in the center becomes vanishingly small.

Looking at the room over a cosmological time period (a couple of hours, say) you would have no difficulty in discerning whether T1>T2 or not.

So entropy is a statistical, rather than a physical law.
posted by unSane at 10:39 AM on April 22, 2011 [1 favorite]

If I kept this system and periodically rechecked it, eventually I might open it and find one or two new neutrons that got created out of the bath of decay particles, but it's very, very unlikely for me to open it and find all the neutrons recreated (like, never happen in the lifetime of the Universe unlikely).

And one (almost definitely wrong) theory about why the initial state of the universe appears to have low entropy is that given an infinite amount of space and time, eventually a locally low entropy state WILL be created, even if it takes 10^100000000000000 years to happen, which would spontaneously create an arrow of time from that point.

The problem with that theory is that it's almost infinitely more likely that a single human brain with the illusion of consciousness and memory, etc would spontaneously coalesce out of a high entropy gas of a particles than that an entire universe would spontaneously form a low entropy state like we see at the big bang.
posted by empath at 10:43 AM on April 22, 2011

I have speculated in the past that maybe some of these things don't make sense because we are coming at them from a direction where they distorted by the perspective that we are coming from.

For instance as a Gedankenexperiment, what if matter is generated by gravity? It seems like at lot of the thinking about gravity and matter starts with matter and works towards explaining how gravity comes from matter. It might seem kind of backwards to start with gravity, and from a practical standpoint it might not make much of a difference, because we don't know how to generate gravity without having matter. But, if you could make some kind of "gravity field", would matter start to appear in it once a certain threshold was crossed? Does nature abhor a gravity vacuum?

I guess part of the reason that I raise this particular line of thought is the idea that dark matter might be some kind of "sub-critical" matter, or an area where there is gravity, but not quite enough to make regular matter appear.

I guess that when I think about this stuff, I definitely run up against the limits of my math. When I think about it, it's all in pictures and I don't have a good way to describe it mathematically, but I was always way better at geometry than any other type of math that I took.
posted by jefeweiss at 10:48 AM on April 22, 2011

unSane and empath, both are great examples.

empath, as you've noted, one issue with the spontaneous formation of a low entropy patch idea is that we're in a low entropy patch of the Universe that has MUCH less entropy than the minimum necessary for there to be interesting processes going on inside (i.e., us). It is not only terribly unlikely that there's a fluctuation to a low entropy state, it's fantastically unlikely that this fluctuation is as big as the one that we see containing the visible Universe. This kind of line of reasoning starts making me nervous though, since as you've noted, by the end you'll have convinced yourself that you're just a Boltzmann brain and the rest of us are figments of your imagination (it's far more likely than the alternative). In that case, you're imagining this conversation, and in the next instant you'll be vaporized by the thermal background of a nearly empty Universe.

...or not.

unSane, funnily enough, the 2nd law of thermodynamics is actually the only law of nature I'm not ok with a theory violating. There's a brilliant quote (which I can't seem to find right now) that goes something like "if your theory violates conservation of energy, so much the worse for energy. If your theory violates the 2nd law, I can give you no help, and may god have mercy on your soul." Statistics is a harsh mistress.

jefeweiss, you're sort of describing matter and a quantization of gravity. However, the mathematics don't really hold up. Gravity creates spin-2 quanta (gravitons), not the spin-1/2 matter fields, the spin-0 higgs field, or the spin-1 vector field that mediates the other forces. To re-rail slightly, some of what the original paper this thread is supposed to be about talks about is a different way of quantizing gravity. However, I think its safe to say that if all these fields can be unified in some way, that only occurs at extremely high energies, and at the more pedestrian energies I think about (you know, like the temperature less than a second after the Big Bang), any unity of the gravity and matter fields has long ago been lost. It's not that its not an interesting idea, its just that every tool I have at my disposal fails to work at the energies you'd need to consider to get it to work, so its easier for me to treat them as different sorts of things, because in the Universe we live in, they effectively are.

Hope I don't come across as too negative on this sort of thinking. I genuinely love to find that people are as interested in these kind of things as I am; and believe me, I wish more of these innovative ideas worked when you get down to brass tacks. I am just trying to get across how complicated and subtle the Universe turns out to be, and why we're forced to consider the more down-to-Earth alternatives. Not that I really classify as boring the fact that most of the Universe is made out of mysterious stuff I can't interact with but is none-the-less necessary for my existence.
posted by physicsmatt at 11:03 AM on April 22, 2011 [2 favorites]

(oops, that should read "spin-1 vector fields" There is more than one force of nature, after all.)
posted by physicsmatt at 11:18 AM on April 22, 2011

For instance as a Gedankenexperiment, what if matter is generated by gravity?

Think of space time as a gigantic grid where every point in space and time has a value associated with it. Just a number. It can represent anything, it doesn't matter.

Okay, now lets say that nature wants to balance these numbers out. So if you have a high number in one place, and a low number right next to it, then nature tries to balance them out by making the higher numbers lower, and the lower numbers higher, but it can over shoot, so it corrects again, and this oscillation continues over and over again.

So let's say you have a one dimensional line, with numbers like so:

4 2 0-2-4-2 0 2 4

the next instant you might have:

2 4 2 0 -2 -4 -2 0

and the next instant you have

0 2 4 2 0 -2 -4 -2

Which kind of looks like a wave travelling from left to right, if you graph it out.

That's energy -- think of it like a photon travelling through space.

You can compact that wave or stretch it out as much as you like. The more compact (the higher frequency) the wave is, the more energy the photon carries.

if you keep stretching out the wave enough, you get less and less variation from one place to another, and less energy -- .000001, .000000001, 0, .00000001 ,etc.. until when you reach a wave of infinite wavelength, the 'field' of numbers becomes just 0 everywhere and the photon disappears and there's no energy.

The only difference between matter and energy, is that when you stretch out the wavelength of something with mass as far as it can go (which means that it's not moving), it doesn't equal 0. There's energy left over, and that energy is its 'rest mass'. The reason this happens is mysterious as far as I know.

(I might have completely botched this)
posted by empath at 11:44 AM on April 22, 2011

Regarding physicsmatt's comments on the 2nd law

If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations — then so much the worse for Maxwell's equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation. — Sir Arthur Stanley Eddington

Snagged from Wikipedia
posted by It's Never Lurgi at 12:27 PM on April 22, 2011 [1 favorite]

I knew it had to be someone British and aristocratic, and thus capable of much more pithy put-downs than I am.

empath, the thing your explanation is missing is the quantization; you're doing more of a classical picture (which is fine until you get down to that last quanta...). Also, "matter" to me just means spin-1/2 fields, so a neutrino would be matter, even if they didn't have mass (turns out they do, but we didn't know that till recently). Energy seems to be a property of fields, not a thing in and of itself - but I'm running into a terminology issue, since the physics definitions of certain commonplace words (matter, energy, etc) are a bit slippery, and it's easy to misinterpret. For example, the previous sentence might make one confused about E = mc^2, what does that mean, if energy isn't its own thing? The way I interpret that equation is to say something like: energy is a property of fields, which can take many forms. Mass is one form of energy, and here's the conversion factor. If I'm clever, I can find ways to liberate that energy from an amount of mass, but it will always take the form of some property of other fields (as photons, motion of a particle, etc).

The way I think about mass is to look at what mass means in quantum field theory. Mass terms (in the Lagrangian, for those of you playing along at home), are terms that tie the field value to itself (M^2 \phi \phi^* for a field \phi). What's going on is that you have a background field, say the electron field. An "electron" is a local excitation of the field, which because of quantization can't really relax back into the background and disappear. An electron has mass because the field talks to itself in this particular self-coupled way; and so there is a minimum energy that you need in order to pull an excitation out of the background. A photon, conversely, doesn't have this particular kind of self interactions (it's Lagrangian only contains interactions with electric charges and "kinetic term" which tells you how much energy it has due to movement), so you can pull an photon out of the background with arbitrary low energy.

The fact that some fields talk to the forces of nature also can create mass terms. A proton doesn't have mass because the individual quarks are massive (they are, but that mass is totally negligible), but because the quarks feel the strong interaction. So creating a proton REQUIRES that I also spend about 1 GeV of energy to create a ball of gluons and quark/antiquark pairs around the 3 quarks. Since strong interactions don't yield themselves to a calculable field theory at these scales, I cheat and just call this a mass term for the proton, which it is, once I've chosen to work in this realization of the theory.

I feel like I'm kind of dominating this thread, which isn't how I want to kick off my time at Metafilter. I also don't want this become the "physicsmatt slaps down other people's ideas about Nature" Happy Hour, which isn't fun for anybody, so I'm going to step out for a bit. I should probably go do some actual work as well. I'll drop back in here at some point, or I'll see you all in the next physics-y ffp.
posted by physicsmatt at 1:02 PM on April 22, 2011 [1 favorite]

Entropy is an anthropocentric idea.

The universe doesn't care if all 100 coins are heads up; you could just as easily colour half the coins blue and half red and say red coins down and blue coins up is the "special" state. Or you could say that all coins with years ending in an even number are up is the special state. The point I'm getting at that defining a "special" unique state is something that intelligences do -- not the universe, which just doesn't care.

Low entropy is "stuff organized in a way I think is significant." High entropy is "stuff kind of scattered around."

The definition of the significance, e.g. "all coins up", is a cognitive product and the universe doesn't care what you think. And that's why entropy doesn't participate in CPT parity -- because entropy is a human construct.
posted by seanmpuckett at 1:13 PM on April 22, 2011

Don't feel shy about posting, physicsmatt, your contributions have been great.

One of the things that's always attracted me about Feynman's approach is that the whole field side of things basically disappears (which was one of his goals from the get-go). You're just left with this cosmos in which Schroedinger's cat and wave function collapse and all those other weird observer-centric ideas just sort of step back into the shadows.
posted by unSane at 1:13 PM on April 22, 2011

The universe doesn't care if all 100 coins are heads up; you could just as easily colour half the coins blue and half red and say red coins down and blue coins up is the "special" state.

You can define the states however you want, but entropy is going to always increase no matter how you define them. Any particular definition of states is arbitrary, but the end result is going to be the same.
posted by empath at 1:30 PM on April 22, 2011

Entropy is an anthropocentric idea

Your understanding of entropy is off. It has a precise meaning in thermodynamics which can't be conjured away halfway along the timeline.

In my example above, you can choose any arrangement of toys on the floor as your 'ordered' state, but once you let my kids in for an hour or two, it will be disordered by your own metric.
posted by unSane at 2:10 PM on April 22, 2011 [1 favorite]

(unless of course you reject the idea that there's any meaning to 'ordered' and 'disordered' which solves the problem in the same sense that death cures everything)
posted by unSane at 2:12 PM on April 22, 2011

> choose any arrangement of toys

I choose the arrangement of toys that your kids leave them in after an hour or two. Et voila, entropy decreases.
posted by seanmpuckett at 2:20 PM on April 22, 2011

Buzz Lightyear decides to run away and the gang has to break out and persuade him to return. In the process, valuable lessons are learned, and Disney stock goes up. Adventures are had and entropy increases.
posted by It's Raining Florence Henderson at 2:26 PM on April 22, 2011

What you are describing is not entropy.
posted by unSane at 2:27 PM on April 22, 2011

Entoypy?
posted by It's Raining Florence Henderson at 2:28 PM on April 22, 2011

Another way of thinking about entropy is as information.

Suppose I have encoded a secret message in the arrangement of toys on the floor. Before the kids come in, it can be perfectly decoded using a cipher key that I give you. However the longer the kids are playing in the room, the less of the message can be decoded until eventually, once the toys are randomly scattered, you cannot retrieve any of the message at all.

You can encode your own message in the new arrangement if you wish, but if you want to know what my message was you are SOL. That is entropy.
posted by unSane at 2:31 PM on April 22, 2011

I believe the term you're looking for is sophistry.
posted by empath at 2:33 PM on April 22, 2011 [2 favorites]

Or SOLphistry.
posted by It's Raining Florence Henderson at 2:34 PM on April 22, 2011 [1 favorite]

As an aside, the Bacon Force is likely evidence of "pork matter", not dark matter.
posted by SPrintF at 4:27 PM on April 22, 2011

4 8 15 16 23 42
posted by bwg at 4:42 PM on April 22, 2011 [1 favorite]

Pork matter in, dark matter out.
posted by It's Raining Florence Henderson at 4:44 PM on April 22, 2011

I had not realized how much my brain needed exploding when I clicked on that sidebar entry.

"physicsmatt slaps down other people"... but it feels sooo gooo.... ;-) no but really you seen incredibly patient and polite. Another thanks, I had not really grasped the problem of dark matter before, really interesting, and some actually makes sense.

So when can we get to time travel and FTL! Sigh-lol.
posted by sammyo at 6:44 PM on April 22, 2011

Late to the thread - I'm an astrophysicist and work in a department that specialises in the dark matter and dark energy problem, and alternative theories of gravity. I'm at the observational end of it however - I deal with what falls out of the back of telescopes and help massage it into data suitable for the people looking at all the theories like the one in the post. Relieved that physicsmatt has done such an excellent job of things here!

On unSane's comment "One paper of Mannheim's (Implications of Cosmic Repulsion for Gravitational Theory) deals with this pretty much directly, showing how a negative spatial curvature predicted by the theory could account for the apparent cosmic repulsion that shows up in recent Hubble Plot results." - I skimmed that and it's not clear given its date of publication how well you can make this work given current constraints on flatness. I didn't see it specify just how much negative curvature this needs, but the universe definitely looks pretty non-curvy at the moment.

I can ask around for what the modified gravity community's feelings on Mannheim's stuff is currently (but everyone's on holiday for a few days). My only comment is that the first linked article only really fits to rotation curves which we know already have a peculiarly tight fit to a simple MOND like model, and there's a lot more I'd like to see it fit to first before it takes over a standard cosmological model.

Essentially there's a lot of people working on lots of different models to try to deal with the mildly distasteful amount of dark whatever we put in right now, and I'm not sure there's a clear candidate yet to really take on LCDM. Personally I'm prepared to put a fair amount of money on dark matter being found (as in directly detected) in the next decade or two, but I won't exactly be falling off my chair if I lose that bet either.
posted by edd at 7:43 AM on April 23, 2011 [1 favorite]

MetaFilter is spawning dark matter physicists right and left these days!

I think they should all be drafted into the cabal and put to work helping speed the inevitable MetaFilter world domination.
posted by hippybear at 9:23 PM on April 23, 2011

Pork matter in, dark matter out.

Who is "Nibbler," Alex?
posted by Sys Rq at 9:51 PM on April 23, 2011

Hi edd,

Glad to have you here. Here are more rotation curves.

Welcome to the investigation!
posted by quanta and qualia at 8:40 AM on April 24, 2011

So it seems to me (not a physicist)

*hand to cheek, eyes wide, jaw dropped*

Seriously, why do you even bother? "It seems to me (not an [expert in relevant field]) that the solution to [problem challenging our brightest minds for decades] is [some stupid shit you think is awesome deep but somehow said brightest minds have, like, totally overlooked it!]
posted by obiwanwasabi at 12:38 AM on April 26, 2011 [3 favorites]

As I said, rotation curves aren't exactly a dark matter killer. Sean Carroll discusses some recent rotation curve results that fit MOND very well. A colleague also puts it pretty clearly in a Physical Review Letters Viewpoint. I think we're at the point where it's going to take more than additional observations of rotation curves to settle the issue - some new line of evidence is going to be needed to properly sell people on a modified gravity theory. I also seriously doubt they'll let you decide between the various modified gravities that are being proposed by the community.
posted by edd at 3:41 AM on April 26, 2011

Entropy is an anthropocentric idea.

The universe doesn't care if all 100 coins are heads up;

That's completely ridiculous. First of all, entropy is a mathematical thing, it's not even a property of 'the universe'. It would exist in any conceivable universe. To say that the universe "doesn't case" makes little sense because the universe doesn't "care" about anything at all.
posted by delmoi at 10:34 PM on April 26, 2011

delmoi: "

Entropy is an anthropocentric idea.

The universe doesn't care if all 100 coins are heads up;

That's completely ridiculous. First of all, entropy is a mathematical thing, it's not even a property of 'the universe'. It would exist in any conceivable universe. To say that the universe "doesn't case" makes little sense because the universe doesn't "care" about anything at all"

There is a school of thought that would hold otherwise, depending on your definition of universe. The anthropic principle puts forth the philosophical position that the physical laws of our universe HAVE to be compatible with the intelligent life forms that observe it. That doesn't necessarily make any statement about what other possible cosmos would be like.

I'd be willing to bet that you could come up with a universe that follows internally consistent physical laws that allow for time to go backward and forward. Just because there can be a "universe" where time is reversible, but that doesn't mean that it's anything that we can experience.
posted by jefeweiss at 10:03 AM on May 4, 2011

I'd be willing to bet that you could come up with a universe that follows internally consistent physical laws that allow for time to go backward and forward.

It's easy to come up with such a universe, it's one with maximum entropy.
posted by empath at 10:11 AM on May 4, 2011

Hah, so it is. That was a case that did not occur to me.
posted by jefeweiss at 10:37 AM on May 4, 2011

I'd be willing to bet that you could come up with a universe that follows internally consistent physical laws that allow for time to go backward and forward.

A newtonian universe consisting of bodies in stable non-intersecting orbits round each other also completely satisfies this.
posted by unSane at 10:38 AM on May 4, 2011