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Gravity is Optional
July 12, 2010 3:20 PM   Subscribe

Physicist Erik Verlinde proposed in a recent paper that the force of gravity can be derived from the principles of thermodynamics. NY Times explains. [Physicist Lee] Smolin called it, “very interesting and also very incomplete.”
posted by jjray (55 comments total) 10 users marked this as a favorite

 
Ok, that is supposed to read "force of gravity". Sorry.
posted by jjray at 3:21 PM on July 12, 2010


Entropy caused that typo.
posted by memebake at 3:25 PM on July 12, 2010 [2 favorites]


You accidentally the important part. We've discussed Verlinde's research before.
posted by GuyZero at 3:25 PM on July 12, 2010


I can bend this spoon with the force of
posted by Trochanter at 3:27 PM on July 12, 2010 [1 favorite]


Titular irony FTW.
posted by Atom Eyes at 3:29 PM on July 12, 2010 [4 favorites]


I once accelerated to the speed of
posted by GuyZero at 3:30 PM on July 12, 2010 [1 favorite]


I assumed it was a Jedi Mind Trick.
posted by yeloson at 3:31 PM on July 12, 2010


I assumed that the "very incomplete" quote referred directly to the dropped word.
posted by splice at 3:35 PM on July 12, 2010 [1 favorite]


These string people have a great deal of difficulty communicating.
posted by warbaby at 3:35 PM on July 12, 2010


Wonderful to see Hawking in the vomit comet on NYT article page two. Look at his smile.
posted by Trochanter at 3:35 PM on July 12, 2010 [1 favorite]


[Fixed that you.]
posted by cortex at 3:37 PM on July 12, 2010 [5 favorites]


I hate this use of language where people refer to concepts in physics theories as "real" or "not real". Entropic forces are real. Osmosis is real. It just has a deeper explanation in terms of statistical mechanics. 99% of pop-sci coverage of physics is ruined by a screwed-up need to make the research sound far out or edgy somehow, rather than trying to make it understandable.
posted by Humanzee at 3:41 PM on July 12, 2010 [7 favorites]


Or not.
posted by Jpfed at 3:44 PM on July 12, 2010 [1 favorite]


Is Time Disappearing from the Universe? These guys have another theory of dark mater/time.
posted by lee at 3:44 PM on July 12, 2010 [1 favorite]


I read the NYTimes piece and still don't understand the theory this guy has put forward. Is he suggesting (using the Scrabble tiles analogy that the article used) that gravity is a result of the chaos of the universe? What am I not gleaning from this article? Someone help me here.
posted by Marisa Stole the Precious Thing at 3:49 PM on July 12, 2010


Inasmuch as the standard model provides for a mostly predictable, workable understanding of gravity — with some exceptions — gravity seems by definition to be a force of order. Not entropy. I'm not a physicist but I'm definitely in the crowd that "doesn't understand" this idea, and the NYT article didn't go very far to elucidate the matter.

Which is too bad, because I really want to believe!
posted by The Winsome Parker Lewis at 3:53 PM on July 12, 2010


The standard model doesn't include gravity. General relativity provides the current best description of gravity. In general relativity, gravity isn't a force. Rather, it is the dynamics that result from warping and curvature of spacetime (which in turn results from the presence of matter and energy). It sounds like most string theorists don't understand this theory, and many in fact suspect that no one does. Even the author says that one shouldn't read the equations. That suggests that this is at the hare-brained idea stage ---not nearly mature enough for the layman to worry about.

That "time slowing down" article is pure gobbledygook. Time is not an invariant quantity in general relativity, so no theories depend on its "flow" "remaining constant" or any such similar nonsense. It sounds like a weirder intellectual cousin to MOND theories, which are also crap.
posted by Humanzee at 4:01 PM on July 12, 2010 [1 favorite]


To be clear, string theorists understand general relativity just fine. Just not this gravity == entropic force theory.
posted by Humanzee at 4:02 PM on July 12, 2010


Okay. I don't understand it. But I've still got 3 chapters left to read in "String Theory for Dummies", so I'll get through that first then revisit this paper.
posted by Jimbob at 4:09 PM on July 12, 2010 [1 favorite]


Gravity seems weirdly divorced from the rest of physics. We united electricity and magnetism into electromagnetism. We combined electromagnetism with the weak force in the electroweak. We even shoehorned the strong nuclear force (which is really more like the colourforce) in there. Gravity just sits off there, mocking us. Thus the question continues to arise, "What the hell do we do with gravity? It's not in the Standard Model!" That's why you get string superstring M-theory and all kinds of hypotheses: everyone wants to get gravity into the mix, somehow.

Gravity is interesting. It's inextricably linked with spacetime, naturally. The math is complex, but also results from an inspired direction. And, for all of the ways we attempt to treat time as "just another dimension" it has some irksome properties, like the pesky negative sign in the metric, or the way we seem to relentlessly ratchet forward (up to ... what?).

Entropy is interesting. The second law of thermodynamics is not empirically derived, but rather arises out of math itself. The math is not particularly complicated, just a little statistical jaunt, although the direction is inspired. For reasonably populated systems with enough entities, entropy seems to relentlessly ratchet forward (up to some maximal level).

The commonality in both of these is that you have this curious time element. The natural human inclination is to draw a line between the two and wonder if they are connected. Be interesting to find out.
posted by adipocere at 4:12 PM on July 12, 2010 [10 favorites]


I thought I understood this quite well until I tried to post a simplified version of it, at which point I realized I didn't really understand it at all.
posted by unSane at 4:18 PM on July 12, 2010


I thought I understood this quite well until I tried to post a simplified version of it, at which point I realized I didn't really understand it at all.

This is exactly how I felt after going to see Total Recall while tripping.
posted by Marisa Stole the Precious Thing at 4:22 PM on July 12, 2010 [5 favorites]


Smolin called it, “very interesting and also very incomplete."

I never understand the charge of a theory being "incomplete". Of course it's incomplete! If I knew everything, I'd be God! I'd know how to use some mind trick to make you publish my paper! Isaac Freaking Newton's theory of gravity was "incomplete"!

The question is not "have I explained everything it is possible to explain". The question is "have I explained something previously inexplicable OR explained something previously explicable but more simply".
posted by DU at 4:32 PM on July 12, 2010


I read the NYTimes piece and still don't understand the theory this guy has put forward. Is he suggesting (using the Scrabble tiles analogy that the article used) that gravity is a result of the chaos of the universe? What am I not gleaning from this article? Someone help me here.

It's like an elastic band. Unlike a metal spring, elastic does not have a ridged shape that it wants to return to. Instead, elastic is made up of lots of long molecular chains that can take lots and lots of shapes.

But the thing is, there are a lot more 'short' shapes then there are long ones. So as the band randomly changes shapes, it's far more likely to end up in a short shape then a long one.

When you pull the band, you make more of the molecules longer. That creates an 'elastic' force that, in large scales make the elastic band act like a spring.

---

So the theory is that if two particles are close to each other, the universe has more 'states' in which those two particles can have that 'distance' then if they are far away from eachother. On the large scale, that means that the objects have a tenancy to get bunched together.

So, why would there be more 'close' states then 'far' states? I guess that has to do with the holographic principle and other leading-edge physics ideas.
posted by delmoi at 4:37 PM on July 12, 2010 [2 favorites]


This is all to say that we're nothing more than information stored in a hologram that is part of a quantum computer preserving all that could be recorded of our "real" Human selves which, of course, have met their demise.

That's where this is all heading, isn't it?
posted by basicchannel at 4:50 PM on July 12, 2010


Okay imagine something like this:

You have a board with a bunch of rings on it, and each of the rings has a little indentation for a marble. The inner ring has got, say, 4 spots, the next one has 5 spots and so on, with each ring having the floor(2π*r) or whatever.

Now imagine dropping a marble on that board. You would expect the marble to land on a ring that's farther away from the center. So this is kind of like the reverse of gravity. Being farther away means higher entropy, because there are more spots for the marble at greater distances. There are more distant microstates (each notch) in the macrostate (the distance).

Now imagine that somehow there were more spots in the center, and fewer farther away. Now each time you shake the board, the object is going to get closer to the center. So it's kind of like gravity. (Except you only have one board and one fixed center).

Now just imagine that "the universe" somehow has more 'microstates' when things are close to each other then when they are far. Why would that be the case? Well, I don't really have any idea but I think it's supposed to have something to do with the holographic principle.
posted by delmoi at 4:51 PM on July 12, 2010 [2 favorites]


When I was creating artificial gravity for a novel I was writing about twenty years ago, I first realized that gravity is indistinguishable from acceleration. If those in the spaceship continually accelerate or decelerate (linear or curved) they will experience the force of gravity. You just need to plan the energy expenditure of getting there in terms of acceleration.
Which brings me to my theory of gravity: it is just our acceleration through a curved and expanding universe. This theory is better than truth. It is poetic.
posted by dances_with_sneetches at 5:01 PM on July 12, 2010 [1 favorite]


Does this theory have a testable, falsifiable hypothesis? Or is it just math?
posted by Nelson at 5:11 PM on July 12, 2010


Does this theory have a testable, falsifiable hypothesis? Or is it just math?

Pfft, that never stopped string theory.

When I was creating artificial gravity for a novel I was writing about twenty years ago, I first realized that gravity is indistinguishable from acceleration.

Genius! You should publish that!
posted by Jimbob at 5:14 PM on July 12, 2010 [1 favorite]


DU, it's not a ridiculous complaint. Capital-T Theories generally carve out a part of the physical world to explain, and "incomplete" theories are ones that don't fully fill that space, not ones that fail to explain the entire universe. So at the time of its creation, Newton's theory of gravity was quite complete, since the entire set of available observations (planetary motion, and that's it) were explained well with it. But if you take something like MOND, it explains part of what we want a theory of gravity to explain (the motion of stars in a galaxy), but it doesn't yet explain—and throws out our current explanation for—other parts that we have existing theories for (e.g., gravitational lensing is not explained in MOND). You could probably accuse MOND of being outright wrong for not explaining lensing, but most physicists are savvy enough to know that theories develop over time, and it's possible that these holes will get fixed soon. So take Smolin's "incomplete" in that way: there are holes yet to be explained in this new framework, but it's not yet clear that they are fundamental flaws. Nobody is expecting it to explain everything, but it should at least explain (eventually) everything that is explained under current theories.

On the paper itself, it's very interesting, and not a crank. Believe me, crank papers in physics are really obvious and funny. This is merely wacky. The idea is fun, and if I had to bet I'd say it's probably just some interesting "coincidence" rather than a full-blown unseating of GR.

dances with sneetches, tell me the fact that particles move on straight lines1 through a spacetime that's curved by matter isn't damn poetic too. Also, it's true. Well, ignoring the current paper under discussion.

[1] geodesics.
posted by kiltedtaco at 5:17 PM on July 12, 2010


No offense, but these elastic band/marble-board-rings/scrabble game in space analogues just don't make no sense. Could someone just Cliff Note this for me like a Fox News crawler or something?
posted by Red Loop at 5:19 PM on July 12, 2010


Newton's theory is incomplete as a theory of gravity since all it does is give a value for the force. It leaves undefined how that force arises or what exactly a "mass" is (and why inertial and gravitational mass are the same). It's like calling Linnean taxonomy a "theory of biology".
posted by DU at 5:20 PM on July 12, 2010 [1 favorite]


Does this theory have a testable, falsifiable hypothesis? Or is it just math?

What the author of the paper says is that it isn't even just math. He says it is a hair-brained idea that is worth thinking about and working on.
posted by Chuckles at 5:31 PM on July 12, 2010


Meanwhile, Satan continues to pull everything down towards hell, while laughing at these cute speculations.
posted by qvantamon at 5:42 PM on July 12, 2010 [2 favorites]


DU, I'm telling you how physicists use the term. I'm not sure what you want to accomplish by telling them that they're really saying something else. Smolin's not some hack, and I assure you that he has a far better understanding of how physics works than you are accusing him of.
posted by kiltedtaco at 5:43 PM on July 12, 2010


Just so you guys know, gravity is not my fault.
posted by Higgs Boson at 5:44 PM on July 12, 2010 [10 favorites]


Higgs, you'd save a lot of people time and money if you showed up in Geneva right about now.
posted by Jimbob at 5:46 PM on July 12, 2010 [1 favorite]


Also Higgs, I need to borrow Robin's Ferrari to check into this lead I have on the kidnapping.
posted by GuyZero at 5:49 PM on July 12, 2010 [3 favorites]


Well, since I brought up MOND by calling it crap, allow me to say that a) I will sincerely confess to not being the most savvy of physicists these days, but b) I've never been partial to MOND because I think it and its variants throw far too much out. I always thought the regression to Newtonian physics was telling (although I'm given to understand some variants rectify that, I'm not familiar with them) but the kicker is that the "extra" dark mass can be seen with a variety of different contexts (galaxy curves, gravitational lensing, galaxy-galaxy interactions) and similar numbers are obtained. MOND is in the position of saying, "I can't explain that, but it's all a coincidence! By the way, you get a HUGE extra fundamental length scale and you may have to give up general relativity!" Do not like.
posted by Humanzee at 5:51 PM on July 12, 2010


MOND has gotten the smackdown from various observations as of late. They're into the second or third epicycle of hypothesis adjustment. The funny thing is, if you plot the trajectory with all of the epicycles together, it looks a lot like a death spiral.
posted by adipocere at 6:36 PM on July 12, 2010


The funny thing is, if you plot the trajectory with all of the epicycles together, it looks a lot like a death spiral.

So where's Copernicus?
posted by VikingSword at 7:48 PM on July 12, 2010


It's hard to troll physicists.

The enterprise of math and sciences has two methods for determining Truth. We have mathematics, in which we prove true statements based on fundamental axioms of logic. And we have science, in which we hypothesize truths and then test those theories with carefully designed experiments that try to falsify our hypothesis. If a theory passes enough experiments we begin to believe that in the absence of a demonstration of falsehood, that scientific theory may be true.

String theory is not experimental science, it exists entirely free of falsifiable hypotheses. String theory is not mathematics either, since it's not effectively grounded in a series of axioms. So what is string theory? It seems a form of literary criticism, maybe, or postmodern analysis. Maybe it's psychohermeneutics. I'm not quite sure.
posted by Nelson at 8:25 PM on July 12, 2010 [1 favorite]


Rule number three hundred eleventy one: read the arXiv daily. It's been around since 1991. Heck, the web was invented to disseminate physics results.
posted by oonh at 8:47 PM on July 12, 2010 [2 favorites]


So there's an arXiv for computer science (and many other fields). How come it doesn't seem to be as influential in any other field?
posted by breath at 9:02 PM on July 12, 2010


Maybe the reason gravity and entropy seem one-directional is because they're actually each other's opposites. Thinking about delmoi's example; there's just more space the farther you are from something. So the natural entropic drive would be to expand to fill the available space. If you reverse the flow of time (most of physics doesn't give a shit about the way time is going), gravity pushes things away from each other (kind of like entropy), and entropy causes heat to flow together (kind of like gravity). The only difference is that one acts on mass and the other on energy. Maybe that's part of what dissociates mass from energy; they're actually opposites.
posted by Eideteker at 9:24 PM on July 12, 2010


I have no background in this. That being said, even if this is wrong, it will be this type of theory, that suggests a completely different way to think about these things, that will provide the next breakthrough in theoretical physics.
posted by Ironmouth at 10:02 PM on July 12, 2010 [2 favorites]


d_w_s: Did you used to work with that German guy at the patent office? 'Cause he had some pretty similar harebrained ideas.
posted by hattifattener at 12:28 AM on July 13, 2010


I am not a string theorist (though am a physicist) and enjoy teasing my string theorist friends, but all you who say that string theory isn't science really haven't learned enough about it. There is much more to it than whole landscape picture that Susskind did so much to popularize, in which you can get any physics you want if you find the right parameters.

First, there are good reasons why people got excited about it. By making some assumptions about the nature of fundamental particles (in this case, that they are strings that obey certain equations), you suddenly get, without any tweaking, several results that theorists suspect to be true. You get general relativity as a low energy limit, so that's a good start. You get supersymmetry, which would make a lot of sense with various collider results (though isn't bound to be trure). You get a good candidate for a dark matter particle, and we are probably more sure that dark matter exists than that a standard model Higgs boson exists. You get an answer for why gravity is so incredibly much weaker than the other forces. There are other, more esoteric results that are also interesting, such as a correct accounting for the entropy of a black hole based on microstates (the statistical mechanical notion of entropy) that matches the one from thermodynamics.

Second, it definitely makes predictions, it's just that they are unfortunately at extremely high energies that are well out of reach for us. To some extent, this is going to be a problem with quantum gravity theories in general, exactly because gravity is so much weaker than the other forces. This is one reason that a lot of people are looking these days is string cosmology, trying to see if they can eke out consequences of the theory.

Third, though most people who do string theory don't treat it with full mathematical rigor (they are, after all, physicists), don't underestimate the power of physical insight. It's much easier to prove a result if you have some notion that the result exists. String theory gives considerable intuition on dualities, in which the same object can be described in two very different ways. There's a reason that Ed Witten got a Fields Medal, and it wasn't because the committee was drunk. Also, some of the dualities discovered through string theory have already been useful in other fields. It turns out that in some situations, one can use dualities from string theory to take a strongly interacting system that is very hard to solve and turn it into a weakly interacting system that can tractably approximated. Strongly interacting systems of this sort show up in condensed matter and elsewhere, and these dualities have already proven useful in some situations.

Is string theory exactly the right answer? I would be shocked if it were. When we get more data on what is going on, will we be glad that we spent the time figuring out how this strange math works? Probably. (Except the small army of grad students who were trained to do jobs that don't exist after grad school, but that's a different rant)
posted by Schismatic at 1:04 AM on July 13, 2010 [8 favorites]


Man! I was just wondering how the fuck this worked.
posted by kittens for breakfast at 6:50 AM on July 13, 2010


also here :P

cheers!
posted by kliuless at 7:28 AM on July 13, 2010


Newton's theory is incomplete as a theory of gravity since all it does is give a value for the force.

that's because newton never described a theory of gravity, merely stated its laws. they are two different things. a law describes how things work, a theory describes why. you would be an idiot to try to deny that gravity exists, as it obviously does, but as to why it works, well that's still, apparently, (ahem) up in the air.
posted by sexyrobot at 8:52 AM on July 13, 2010


Maybe the reason gravity and entropy seem one-directional is because they're actually each other's opposites. Thinking about delmoi's example; there's just more space the farther you are from something. So the natural entropic drive would be to expand to fill the available space. If you reverse the flow of time (most of physics doesn't give a shit about the way time is going), gravity pushes things away from each other (kind of like entropy), and entropy causes heat to flow together (kind of like gravity). The only difference is that one acts on mass and the other on energy. Maybe that's part of what dissociates mass from energy; they're actually opposites.

Gravity warps space time, so there ISN'T more available space as you move away from gravity wells. The event horizon of a black hole seems to have maximum entropy, in fact.

Also, the fact that only gravity and entropy seem to be time-asymmetrical would imply that the low entropy state of the early universe and the inflationary force that caused the big bang would be related, wouldn't it?

Also, the fact that entropy decreases when you run the current universe backwards is a result of the fact that we had an incredibly, incredibly, incredibly improbably low-entropy initial state at the big bang.

If we didn't, there wouldn't be an arrow of time, and reversing the chronological flow would have no noticeable impact on entropy, and perhaps gravity wouldn't exist at all.
posted by empath at 9:22 AM on July 13, 2010


If you reverse the flow of time, gravity still causes masses to attract each other. General relativity is invariant under time reversal. One way to visualize this: imagine a planet orbiting on a highly elliptical orbit. It's faster when near the sun, slower when far, and changes speed smoothly in between. Now run that picture backwards in time: it's the same.
posted by Humanzee at 4:42 PM on July 13, 2010


Someone with expertise please correct me if this is all nonsense...but it seems like, in a holographic universe, there would be a higher proportion of close microstates than distant ones bc of the way surface area scales w volume. Small volumes have larger surface areas, per unit of volume, than large volumes. If the universe is holographic, then it is the surface area of a space that is relevant for describing it, not the volume occupied by the space. And so smaller volumes are less special, and thus have higher entropy per unit volume when compared to large volumes.

Is this what Verlinde is saying?
posted by Zerowensboring at 8:35 AM on July 14, 2010


If you reverse the flow of time, gravity still causes masses to attract each other. General relativity is invariant under time reversal. One way to visualize this: imagine a planet orbiting on a highly elliptical orbit. It's faster when near the sun, slower when far, and changes speed smoothly in between. Now run that picture backwards in time: it's the same.

Right, but I'm picturing a universe without the big bang -- A maximum entropy state -- which would be a gigantic black hole. Which I suppose would be affected by gravity technically, but when everything is already a singularity.
posted by empath at 5:52 AM on July 15, 2010


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