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flux capacitor or not..
July 25, 2011 3:17 PM   Subscribe

Sorry Marty.. but 2.21 gigawatts ain't gonna cut it. Scientists have proven nothing can travel faster than the speed of light, so no time travelling for you, bub. A new study published in the American Physical Society's Physical Review Letters journal concludes that even single photons have to obey the posted speed limits.

That said, some other guys did figure out how to slow light down to the speed of sound. So we've got that going for us, which is nice.
posted by analogue (145 comments total) 19 users marked this as a favorite

 
But how do we know that somebody hasn't come back in time and screwed with the test results?
posted by Flashman at 3:20 PM on July 25, 2011 [7 favorites]


Maybe it was a slow photon. I mean, they managed to catch it so it has to be slow, right?
posted by tommasz at 3:21 PM on July 25, 2011 [10 favorites]


Scientists have proven nothing can travel faster than the speed of light

That's not what they've proven. They've proven that photons can't travel faster than the speed of light. Not being comprised of photons, I'm still holding out hope.
posted by Civil_Disobedient at 3:22 PM on July 25, 2011 [12 favorites]


I'm sorry, I'm sorry, I'm sorry.

But, the BTTF reference is '1.21 gigawatts'.

Great Scott, etc.
posted by metaxa at 3:23 PM on July 25, 2011 [10 favorites]


1.21 GW +/- 1 GW
posted by griphus at 3:25 PM on July 25, 2011 [17 favorites]


Do you know what this means?

It means this damn thing doesn't work at all.
posted by The World Famous at 3:25 PM on July 25, 2011 [31 favorites]


"BTTF" - that looks awfully like a bad word.
posted by phaedon at 3:25 PM on July 25, 2011


My understanding is the "speed of light" is better understood as the speed of causality in our universe, light just being an example of something which isn't slowed down to much lower speeds in the way most everyday objects are. Do physicists really still consider something like a photon to exist except as a helpful concept?
posted by crayz at 3:27 PM on July 25, 2011 [2 favorites]


I'm holding out hope for artificial wormholes, C_D, but I'm sure someone will come up with a reason they won't work either.

I think this means that even if we aren't the only sentient race in the universe, we'll probably never get to meet any of the others. Which sucks.
posted by tommasz at 3:28 PM on July 25, 2011 [1 favorite]


I am sorry for stepping on BTTF. Can we pretend Mr. Fusion punched it up by 1 GW?
posted by analogue at 3:28 PM on July 25, 2011


That article is terribly written. I have no idea what they are trying to say. I'm fairly sure that their study wasn't about proving that photons can never go faster than the speed of light, but about testing whether individual photons were exceeding the speed of light under certain conditions where it might appear that they were.
posted by empath at 3:30 PM on July 25, 2011


Civil_Disobedient : That's not what they've proven. They've proven that photons can't travel faster than the speed of light.

It doesn't even prove that much...

It only demonstrates that the best known "trick" of making a pulse pulse of light appear to exceed c doesn't actually involve any particular photon moving faster than the speed of light.


Though that makes me perversely wonder, if a photon did exceed c, wouldn't that just increase c?

And yes, I already posted basically this same comment in the earlier thread. Same topic, so the same comment seems fair. :)
posted by pla at 3:30 PM on July 25, 2011 [6 favorites]


Could someone who knows what they are talking about explain how this relates to quantum entanglement's "spooky action at a distance" which (from what I have heard) is instantaneous?
posted by East Manitoba Regional Junior Kabaddi Champion '94 at 3:31 PM on July 25, 2011 [1 favorite]


That article is terribly written. I have no idea what they are trying to say. I'm fairly sure that their study wasn't about proving that photons can never go faster than the speed of light, but about testing whether individual photons were exceeding the speed of light under certain conditions where it might appear that they were.

Agreed. It is a good example, though, of the science news cycle; we appear to be around step 4 or 5.
posted by Johnny Assay at 3:32 PM on July 25, 2011 [1 favorite]


The size of the known universe is 40 billion light years.
The age of the universe is currently estimated to be 13 billion years old.

Obviously , if we assume the big bang theory, the Universe itself expanded at a rate several times the speed of light. Therefore movement at the speed of light is, or was at some point, possible.

Since I am in my fifties, I very much remember a time when I was taught in school that :
1. Manned travel to the moon is impossible
2. Planets other than our solar system are impossible or at least highly unlikely.

I also remember a time when a scientist could not conceivably be considered to publish , in a respected journal, an article regarding time reversal . But of course such articles about time parity violations are common in physics journals today.

So when Discovery magazine (of all places interprets something the APS published you must understand my doubts.
posted by Poet_Lariat at 3:34 PM on July 25, 2011 [18 favorites]


When asked to comment, representatives from the Hot Tub Manufacturing Association of America had two words: "No refunds."
posted by Blazecock Pileon at 3:34 PM on July 25, 2011 [7 favorites]


EXTRA!
'The stuff light is made of cannot get ahead of itself. Scientist: "yup, it's a fact"-Warp Field sought!'
posted by clavdivs at 3:34 PM on July 25, 2011


Somebody better get John Titor on this, stat.
posted by Chuffy at 3:35 PM on July 25, 2011 [2 favorites]


Here's a video of the speed of light travel, simulated in the game Outerra. To get a scale for how fast things are.
posted by hellojed at 3:36 PM on July 25, 2011 [4 favorites]


Time is what keeps everything from happening at once.
posted by Trurl at 3:37 PM on July 25, 2011 [3 favorites]


As Poet_Lariat pointed out, the speed of light is a relative thing. If the expansion of the Universe is accelerating, then the photons on the c train will have travelled faster than the speed of light.
posted by panaceanot at 3:37 PM on July 25, 2011


Space is what keeps everything from happening to me.
posted by wobh at 3:38 PM on July 25, 2011 [10 favorites]


How fast is gravity?
posted by doublehappy at 3:38 PM on July 25, 2011


Science does not know proof. Let the proof for the mathematicians.
posted by yoyo_nyc at 3:40 PM on July 25, 2011 [1 favorite]


Okay, after reading more, I think what they are talking about is the difference between phase velocity and group velocity.

A photon is can be thought of as a perturbation in the electromagnetic field, but localized in a specific place. Think of the light quantum or photon as being a tear-shaped packet, within which there's a wave that vibrates at a certain frequency, and the whole packet is transmitted through the electromagnetic field together.

The movement of vibration within the packet is the phase velocity, and the movement of the packet itself is the group velocity. The wikipedia article has some good animations of this.

Usually the group velocity is the speed of light, and is also the speed of the photon, and is also the speed that any information is carried. However, you can mess around with the medium that is carrying the light, and change the shape of the light wave, to make the group velocity faster than the speed of light, which makes it seem like the light is moving faster than the speed of light, but it's the signal is not being carried faster than the speed of light, and I believe what these scientists have confirmed is that the individual photons are not being transmitted faster than the speed of light.

Hopefully physicsmatt will be along to confirm this.
posted by empath at 3:40 PM on July 25, 2011


SHOCKING DISCOVERY ROCKS SCIENTIFIC COMMUNITY!
Speed of light does not exceed speed of light




This got funding?
posted by Sys Rq at 3:40 PM on July 25, 2011 [5 favorites]


Cosmos take me away!
posted by analogue at 3:40 PM on July 25, 2011


That fact that no photon can move faster than light is no surprise. This doesn't completely doom all possibility of time travel, however.

However, certain extremely complex varieties of singularities theoretically produce spatial folding that results in something like a wormhole (admittedly microscopic to the point that you MIGHT be able to send a signal through via X-ray laser). Of these, the only one that doesn't involve exotic matter (ie negative gravitational pull, etc.) are toroidal singularities rotating extremely fast, forming paired hyperports at the axial extremes.

Basically: imagine a gigantic black hole shaped like a doughnut standing on its side, spinning like a top. The top and bottom will have EXTREMELY small openings through which you might be able to beam yesterday's stock quotes into the past.

The reason doing so won't actually change anything, is the Novikov self-consistency principle. In essence: the probability of events that alter the timeline occurring is zero.
posted by Ryvar at 3:41 PM on July 25, 2011 [2 favorites]


Damn, should've previewed. Scratch one of those howevers. Either one.
posted by Ryvar at 3:42 PM on July 25, 2011 [1 favorite]


Help! I'm trapped in Poet_Lariat's parenthetical!)
posted by odinsdream at 3:42 PM on July 25, 2011 [8 favorites]


"This got funding?"

That's an easy take-away from any pop-science article. A better question is usually, "This journalist got funding?"
posted by panaceanot at 3:42 PM on July 25, 2011 [12 favorites]


East Manitoba Regional Junior Kabaddi Champion '94 : Could someone who knows what they are talking about explain how this relates to quantum entanglement's "spooky action at a distance" which (from what I have heard) is instantaneous?

IANAQP, but (I play one from my armchair)... Almost nothing. So far as I know, that remains an open problem. A pair of entangled photons with complementary but as-yet-undefined spins "transmit" that information across an arbitrarily large distance instantaneously on measuring one of them. Whether that involves some trick (such as EWG literally holding true), or actual communication between them, only the FSM knows for now.
posted by pla at 3:43 PM on July 25, 2011 [2 favorites]


Everything is deemed impossible until it happens. This means nothing, if it were possible would they (the royal they) tell us?
posted by thebestusernameever at 3:44 PM on July 25, 2011


Obviously , if we assume the big bang theory, the Universe itself expanded at a rate several times the speed of light. Therefore movement at the speed of light is, or was at some point, possible.

Well, no. Space itself has expanded. Photons moving through that space are still moving at the speed of light, though.
posted by empath at 3:44 PM on July 25, 2011 [1 favorite]


I REFUSE TO BELIEVE THIS IS TRUE UNTIL I READ IT IN THE DAILY MAIL.
posted by chavenet at 3:44 PM on July 25, 2011 [2 favorites]


Obviously , if we assume the big bang theory, the Universe itself expanded at a rate several times the speed of light. Therefore movement at the speed of light is, or was at some point, possible.

"[I]f we assume . . . . Therefore . . . ." When it starts looking like the "therefore" might not be true, it's time to start looking at the assumptions again.
posted by The World Famous at 3:44 PM on July 25, 2011


'entangled photons' transmitting stuff across large 'distances? Surely it makes more sense to ignore our perception of distance and say they're part of the same thing, it's just our perception of distance that thinks of them as discrete entities.
posted by panaceanot at 3:48 PM on July 25, 2011


Planck Time!
time
time

posted by clavdivs at 3:50 PM on July 25, 2011 [1 favorite]


"When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong."
--Arthur C.Clarke
posted by Guy_Inamonkeysuit at 3:50 PM on July 25, 2011 [8 favorites]


Right. Entangled photons aren't really two separate things. They're a single waveform until it's is measured. But that's a separate issue entirely from this study.
posted by empath at 3:50 PM on July 25, 2011


pla: Whether that involves some trick (such as EWG literally holding true), or actual communication between them, only the FSM knows for now.

Thanks for your answer! What is EWG? My Google-fu is lacking. (You don't need to help me with FSM)
posted by East Manitoba Regional Junior Kabaddi Champion '94 at 3:51 PM on July 25, 2011


Abstract from the paper TFA is discussing:
We report the direct observation of optical precursors of heralded single photons with step- and square-modulated wave packets passing through cold atoms. Using electromagnetically induced transparency and the slow-light effect, we separate the single-photon precursor, which always travels at the speed of light in vacuum, from its delayed main wave packet. In the two-level superluminal medium, our result suggests that the causality holds for a single photon.
doi: 10.1103/PhysRevLett.106.243602
posted by alby at 3:52 PM on July 25, 2011


"Saying science can't do something is like playing pool against a guy named after a state. You may be winning now, but don't leave your money on the table too long."
-- Etrigan's father, who was named after a county but knew his limits
posted by Etrigan at 3:53 PM on July 25, 2011 [14 favorites]


A pair of entangled photons with complementary but as-yet-undefined spins "transmit" that information across an arbitrarily large distance instantaneously on measuring one of them.

They've already proven that it's impossible to transmit information faster than the speed of light with entangled photons. You need an additional slower than light channel to transmit information about the measurement that was performed.
posted by empath at 3:55 PM on July 25, 2011


I REFUSE TO BELIEVE THIS IS TRUE UNTIL I READ IT IN THE DAILY MAIL.

PHOTONS CAUSE CANCER

(given sufficient frequency, insufficient melanin/suncream)
posted by kersplunk at 3:55 PM on July 25, 2011


Hang on, everybody. The 1.21 gigawatts didn't send the DeLorean past the speed of light, it was required to power the Flux Capacitor. How the Flux Capacitor operates is never really explained, but basically you only need to get the DeLorean to 88 mph, not the speed of light. Doc Brown's time travel machine is therefore completely plausible and the framing of this post is misleading.
posted by Hoopo at 3:56 PM on July 25, 2011 [29 favorites]


I'm all for empirical science but we keep doing every experiment at the bottom of a huge gravity well which is a bit of an ongoing bias in all these tests. Also, yes, I don't think we have a good measurement of the speed of a gravity wave yet.
posted by GuyZero at 3:57 PM on July 25, 2011


East Manitoba Regional Junior Kabaddi Champion '94 : Thanks for your answer! What is EWG? My Google-fu is lacking.

The Everett-Wheeler-Graham interpretation, aka the "many worlds" hypothesis.

Basically, it gets around the problem by saying that, at the moment the entangled pair came into existence, the universe split into two; In one universe, where photon-A has spin -1, photon-B has spin +1; In the other universe, photon-A has spin +1 and photon-B has spin -1. The act of measurement doesn't change the photons, it just forces the observer to get off the fence and pick a universe.


empath : They've already proven that it's impossible to transmit information faster than the speed of light with entangled photons. You need an additional slower than light channel to transmit information about the measurement that was performed.

Do you have a link for that? I somehow missed hearing about it... (not sarcastic).
posted by pla at 3:58 PM on July 25, 2011


I'm all for empirical science but we keep doing every experiment at the bottom of a huge gravity well which is a bit of an ongoing bias in all these tests.

The earth's gravity well is barely enough to affect the path of photons, afaik.
posted by empath at 3:58 PM on July 25, 2011


Empath: what if you use the timing offsets between performing measurements of one member of a paired particles as your communication channel?

Basically: we perform the double-slit experiment in parallel with entangled particles, I refrain from measuring or not in order to cause the interference pattern to show up (or not).

You simply sit somewhere, a light-month away, and write down whether or not there was an interference pattern for each particle.
posted by Ryvar at 4:01 PM on July 25, 2011


How the Flux Capacitor operates is never really explained, but basically you only need to get the DeLorean to 88 mph, not the speed of light.

Impossible.
posted by Sys Rq at 4:01 PM on July 25, 2011 [9 favorites]


There was a demon that lived in the air. They said whoever challenged him would die. Their controls would freeze up, their planes would buffet wildly, and they would disintegrate. The demon lived at Mach 1 on the meter, seven hundred and fifty miles an hour, where the air could no longer move out of the way. He lived behind a barrier through which they said no man could ever pass. They called it the sound barrier.
posted by kirkaracha at 4:03 PM on July 25, 2011 [1 favorite]


Do you have a link for that? I somehow missed hearing about it...

Quantum Teleportation.

. It is useful for quantum information processing, however it does not immediately transmit classical information, and therefore cannot be used for communication at superluminal (faster than light) speed.

No-communication theorem.
posted by empath at 4:03 PM on July 25, 2011


It's not impossible, you just have to consume enough coke so that everything feels like it's moving faster to begin with.
posted by Civil_Disobedient at 4:05 PM on July 25, 2011


Since I am in my fifties, I very much remember a time when I was taught in school that :
1. Manned travel to the moon is impossible
2. Planets other than our solar system are impossible or at least highly unlikely.


Poet_Lariat, I completely agree. We are nothing more than great apes, reveling in our advanced understanding of our own frame of reference.
posted by thebestusernameever at 4:07 PM on July 25, 2011 [1 favorite]


Since I am in my fifties, I very much remember a time when I was taught in school that :
1. Manned travel to the moon is impossible
2. Planets other than our solar system are impossible or at least highly unlikely.


There's a world of difference between what is practical and what is possible. I don't think anyone would have told you that it was theoretically impossible to go to the moon, or that there were planets around other stars, even 100 years ago, and if they did, they didn't know what they were talking about, even by the standards of the time. "What they teach people in school" is never a good measure of what scientists believe.

If you ask me now whether it's theoretically possible to travel to other stars, I would say yes, but I would say it's damn close to impossible, based on any theoretical technology we have in the pipeline.

I agree with Sean Carroll that the physics of every day life are darn near completely understood. And that what scientists don't know for sure now, they have a pretty good idea of what the answer will eventually look like. They haven't been surprised by anything big for quite a while -- since the 70s, at least.
posted by empath at 4:14 PM on July 25, 2011


doublehappy: How fast is gravity?

Theoretically, gravitational waves propagate at c.
posted by justsomebodythatyouusedtoknow at 4:16 PM on July 25, 2011


The earth's gravity well is barely enough to affect the path of photons, afaik.

We have no idea what the speed of light is outside of a gravity well. I mean, we have a pretty good idea it's not any different, sure, but we've never measured the speed of light at a significant distance from the sun and/or Earth. Just saying that it is systematic bias.
posted by GuyZero at 4:16 PM on July 25, 2011 [1 favorite]


We have no idea what the speed of light is outside of a gravity well. I mean, we have a pretty good idea it's not any different, sure, but we've never measured the speed of light at a significant distance from the sun and/or Earth. Just saying that it is systematic bias.

I don't see what your point is. Gravity bends space time, and light bends with it. We've measured light crossing all kinds of different gravity wells from various distances and sources nd from various detectors and we have very precise measurements for how gravity affects light.
posted by empath at 4:24 PM on July 25, 2011


But I can't drive 55.
posted by nola at 4:26 PM on July 25, 2011 [2 favorites]


some other guys did figure out how to slow light down to the speed of sound

Well, that's easy enough, but what I want to know is have they managed to figure out A'Tuin's sex yet?
posted by adamdschneider at 4:27 PM on July 25, 2011 [1 favorite]


empath : No-communication theorem

Interesting, but merely a thought-experiment, not "we measured them six light-seconds apart, timestamped the outcome, and they did/did not give the expected results within six seconds of each other". Though it does make a compelling point that you can't compare those results without a conventional side-channel.

The closest we have in that link, "B. Dopfer, a graduate student of Anton Zeilinger, has indicated via experiment that it is possible to cause or prohibit an ensemble of photons into making an interference pattern on a screen, by remotely manipulating their entangled twins", actually seems to support exactly what that link calls impossible - If one end actually controls what the other sees, rather than merely measuring a shared trait, that would appear to transmit classical, rather than state, information.

So as soon as we can put one side on Mars, we'll have our answer. :)
posted by pla at 4:27 PM on July 25, 2011 [1 favorite]


If one end actually controls what the other sees, rather than merely measuring a shared trait, that would appear to transmit classical, rather than state, information.

I think probably they're going to run into timing and noise issues, etc, and how you separate random chance from intentional signals, etc. It's definitely an interesting idea, though.
posted by empath at 4:29 PM on July 25, 2011


Because an interference pattern is a statistical phenomenon generated by a large number of measurements, not something that happens with an individual photon, so they'd have to perform a series of measurements on a large number of entangled photons to create or destroy an interference pattern on the other end, and I'm not sure you can reliably create enough entangled photons over two channels to create a detectable signal on the far end.
posted by empath at 4:32 PM on July 25, 2011 [1 favorite]


Wait, doesn't Hawking Radiation travel faster than the speed of light?
posted by justsomebodythatyouusedtoknow at 4:33 PM on July 25, 2011


Science is always right until it's wrong. I'm fine with the current science saying "nope, sorry--no time travel," and I'll believe it, because that's what they've proven/are proving. But so many things that have been scientifically proven--that is, fit the rules of the universe as we understand them--are disproved years and years later. I'll just keep on hoping for science to be wrong on this one.

(Also, transporters. Bring 'em.)
posted by tzikeh at 4:44 PM on July 25, 2011


The basic problem with thinking that the Speed of Light can be fudged around, is that it would break all kinds of everyday physics that depend on it, such as the fiber-optic networks these words were transmitted on.

Wait, doesn't Hawking Radiation travel faster than the speed of light?

No. In fact, AFAIK, the momentum of Hawking Radiation is 0, it's just that half of that is hidden behind the Event Horizon.
posted by KirkJobSluder at 4:44 PM on July 25, 2011


No. What gave you the impression that it does?
posted by flabdablet at 4:44 PM on July 25, 2011


Or, like poet_lariat says.
posted by tzikeh at 4:45 PM on July 25, 2011


The "moved 40 billion light years in 13 billion years" thing is confusing "moving" with "increased in distance". They are actually not the same, and the photon that left 13 billion years ago from a star 40 billion light years from us travelled 13 billion light years to get to us, not 40.

It's as if an ant were travelling on the surface of a balloon, and that balloon were expanding. You could mark the point that the ant left from, wait for a minute, and mark the point that the ant got to. Those two points could be ten miles apart, if you blew up the balloon really fast. The ant did not travel ten miles in that minute, though. Rather, the distance between the two points expanded by (almost) ten miles. Not the same thing.
posted by Flunkie at 4:56 PM on July 25, 2011 [5 favorites]


Do physicists really still consider something like a photon to exist except as a helpful concept?

Pfft, yeah, it's like, is this something I would have to be governed by classical mechanics to understand?
posted by No-sword at 5:00 PM on July 25, 2011


Does this mean I have to throw out all my Peter F. Hamilton? Because that shit is taking up a lot of shelf space.
posted by tumid dahlia at 5:01 PM on July 25, 2011


flabdablet : No. What gave you the impression that it does?

I would hazard a guess that one might think that because you have something apparently "escaping" an object with an escape velocity greater than or equal to the speed of light.

Of course, I personally consider the reality much spookier than "merely" exceeding the speed of light. "So we have this sea of imaginary particles that constantly pop in an out of existence, so often in fact that you can measure the pressure they exert on real stuff with the right setup; They always appear in self-annihilating pairs though, and quickly do so, so no harm, no foul... Except when one half of the pair accidentally gets sucked into a black hole. When this happens, that whole "never be created or destroyed" thing goes right out the window, and as a result, black holes leak like God after too many Olestra chips".
posted by pla at 5:05 PM on July 25, 2011 [2 favorites]


I don't give a rat's ass about time travel; I have a big enough challenge trying to live in the moment.
posted by bwg at 5:10 PM on July 25, 2011


No. In fact, AFAIK, the momentum of Hawking Radiation is 0, it's just that half of that is hidden behind the Event Horizon.

Photons always travel at the speed of light. Momentum is just a function of the frequency.
posted by empath at 5:21 PM on July 25, 2011


I would hazard a guess that one might think that because you have something apparently "escaping" an object with an escape velocity greater than or equal to the speed of light.

Of course, I personally consider the reality much spookier than "merely" exceeding the speed of light. "So we have this sea of imaginary particles that constantly pop in an out of existence, so often in fact that you can measure the pressure they exert on real stuff with the right setup; They always appear in self-annihilating pairs though, and quickly do so, so no harm, no foul... Except when one half of the pair accidentally gets sucked into a black hole. When this happens, that whole "never be created or destroyed" thing goes right out the window, and as a result, black holes leak like God after too many Olestra chips".


It's important to note that hawking radiation escapes from the boundary of the event horizon, that is the surface where the escape velocity is just below the speed of light.

Also, no energy or mass is created or destroyed by the hawking radiation. Positive energy escapes, and negative energy is absorbed into the black hole, which reduces the total mass/energy of the black hole. The effect is just as if the black hole were radiating a small amount of energy.

And by a small amount, i mean a very, very small amount -- the black hole is absorbing more energy from ambient photons around it left over from the big bang than it is emitting in hawking radiation. It will take a very, very, very, very, very long time before any black holes lose any mass on balance from hawking radiation, except perhaps for very small ones (smaller black holes emit more radiation than larger ones)
posted by empath at 5:27 PM on July 25, 2011 [1 favorite]


I've long thought physical time travel is wishful nonsense. Here's why:

If "you" (your body, or a collection of related data points representing your consciousness, whatever) travelled backwards or forwards in time, the planet/sun/galaxy wouldn't be there, because they are all moving at phenomenal speeds. Chances are you'd find yourself in empty interstellar space. If you're really unlucky, in the middle of a star. See, when people talk about time travel they are usually imagining temporal movement while maintaining physical integrity/position. But of course this makes no sense. Time and space are intimately linked.

So...let's assume that you can travel, say, back in time 100 years, but still maintain your spatial position and the specific alignment of atoms that makes up your body/brain/mind - that makes you, you - in order to do so, you will need to somehow "transport" those atoms into a past space already filled with other atoms - requiring you to rearrange those atoms in the past. That rearrangement would affect other atoms, in an infinite chain reaction. So at this point we're really talking about rearranging the entire universe at some point in the past so that the atoms representing 'me' are present in a way they were not 'before' the time travel event. Absurd. Unless...time travel really just involves reversing or 'accelerating' the arrow of time, much like pressing the fast forward or rewind buttons on a VCR (remember those? lost to time...). Of course if you simply hit rewind you would also rewind yourself, along with all memory, and you would never know it.

In short, time travel always seems to involve some form of a false Cartesian split - an implicit division of the physical world from a 'non-physical' concept such as Time, Information, Consciousness, etc. But as far as I'm concerned, any such split is illusory.
posted by jet_manifesto at 5:32 PM on July 25, 2011 [8 favorites]


I've long thought physical time travel is wishful nonsense. Here's why:

Because the only people who talk about it as if it's real are fictional characters.
posted by The World Famous at 5:35 PM on July 25, 2011


Re: 40 billion v 13 billion

If you start at the center and expand outwards, you've got 13 billion light years in each direction. (aka 26 billion.) That's only off by 50%, which isn't bad at that kind of scale.
posted by blue_beetle at 5:39 PM on July 25, 2011


Me? I'm holding out for receiving total consciousness on my deathbed.
posted by Ickster at 5:43 PM on July 25, 2011


Time is what keeps everything from happening at once.

That's what ya'll who are limited to 3 dimensions think.

I watching you be born Trurl AND looking at what you did with the sheep on that cold morning in '82. You naughty fleshblag.
posted by Brandon Blatcher at 5:47 PM on July 25, 2011


Because the only people who talk about it as if it's real are fictional characters.

And armchair physicists ;p
posted by jet_manifesto at 5:52 PM on July 25, 2011


Time travel is a profoundly misconceived idea. It's an unsuitable metaphor: travel is an intuitively grasped concept, based in our organismal sense of place, territory, and migration. But, despite the formal and mathematical intimacy between time and space, time is *not* space, and it doesn't share all if its (I don't have the education to use the right word here) geometric or proximal properties. An hour cannot be "traversed" like a mile can. It's just imagination and plot device.
posted by clockzero at 5:55 PM on July 25, 2011 [2 favorites]


If you start at the center and expand outwards, you've got 13 billion light years in each direction. (aka 26 billion.) That's only off by 50%, which isn't bad at that kind of scale.
There are at least three things wrong here:

(1) There is no "center". Or perhaps more accurately, every single point is the "center" from its own point of view.

(2) You seem to be saying "13 is a radius, 26 is the diameter, 26 is kind of like 40". But 40 is also a radius, not a diameter.

(3) It's not that there's a margin of error in measurement. It really is that space is expanding. In fact, its rate of expansion is accelerating.
posted by Flunkie at 5:56 PM on July 25, 2011


Does this mean I have to throw out all my Peter F. Hamilton? Because that shit is taking up a lot of shelf space.

No, as it's probably categorized as "Science Fiction/Fantasy", just cross out the first two words.
posted by sammyo at 5:57 PM on July 25, 2011


> I'm sorry, I'm sorry, I'm sorry.

But, the BTTF reference is '1.21 gigawatts'.


Actually, I'm pretty sure it's jiggawatts. cite
posted by cjorgensen at 6:39 PM on July 25, 2011


It's so nice to show up and see that empath is already ahead of me on the answering questions gig.

pla: Hawking radiation doesn't escape from inside the event horizon, but rather comes from the horizon itself. In fact it's not that the escape velocity is c at the event horizon, it's that the end required to escape is infinite. Otherwise, I could theoretically escape by moving slower than c but with an engine strapped to my back that continued to push me outwards slowly - just like I could escape from the Earth by climbing straight up a very tall tower (assuming one existed) and never exceed the escape velocity. This doesn't work for black holes, which is another crucial difference between General Relativity and classical gravity.

GuyZero: We actually have measured the speed of light over many different distance scales. Notably, from the Earth to Jupiter (this was known nearly in Galileo's day, as it makes the eclipse of the Galilean moons appear delayed by up to an hour or so depending on the relative position of the two planets), Earth to the Moon (via interference through the reflectors the astronauts put up there), or Earth to the heliosphere (communication with Voyager I, II and Pioneer). We also have circumstantial evidence from analysis of various cosmological phenomena (as c enters into the energy levels which can be deduced from early Universe observations), though I concede that there could be some sort of conspiracy of several things changing at once to make any observation seem consistent. Though now you're at the "Universe is intentionally fucking with us" theory of physics.

We also know that gravity propagates at c as well (I would have thought this could be done from observation of binary systems including a pulsar, but apparently, it was from an eclipse of a pulsar by Jupiter. Thanks Jupiter!). Also, the gravity field inside the solar system is big by terrestrial standards, but pretty much insignificant compared to the level of a black hole. We can't jump off the Earth by ourselves, but the gravity well here is actually pretty piddling.

Flunkie already answered Poet_Lariat: The "radius" of the Universe and the age of the Universe are separate numbers, meaning we can "see" things that are further away due to the huge propagation times involved. Nothing here is moving super-luminally. To address blue_beetle, the issue is that the Universe was expanding differently early on than it is today, so the "physical distance" is different from the "comoving distance" (the first is the actually mileage a photon must cover, which is 38 billion light years, the 2nd is the distance corrected for the changing expansion of the Universe, and is .... 13.7 billion light-years).

Also, I agree with empath that the violation of the speed of light is in a completely different category than the other "impossible things" mentioned in this list. No reputable scientist from 1950 (probably long before) onward would say that travel to the Moon is a violation of the laws of physics (though it certainly may have been "practically impossible"), but most of us would say that travel faster than light is impossible. More precisely, we would say that communication of information (which includes superluminal transportation of matter) violates most everything we know about the way the Universe works, and does so in non-trivial ways. So either we are incorrect in these other observations, or FTL isn't allowed.

Specifically, here is the problem, which boils down to this phrase: causality, relativity, and superluminal travel. Pick two.

If you pick relativity, but also allow communication via faster than light mechanisms, then you must allow time-travel, since I can always find a frame of reference in which the super-luminal connection is backwards in time. So there goes causality. If I don't believe in relativity, then I can say that certain frames are forbidden, but there's nothing particularly special about the ones I'm disallowing, so that idea seems very suspicious. Maybe you don't care about causality, which is fine, and I'll talk below about why that might or might not be a great idea.

First, note that the entangled photons are not a way to do FTL communication. Here's my handwavy reason why (there are proofs of this, which I see people above have discussed). Imagine I have two entangled particles, for example, photons. What does this mean? Well, if I set it up right, it might mean that the spins are anti-correlated. So if photon A has spin +1, then B has spin -1. Now, part of that entangling means that, in this case, all I know is that the sum of the spins is 0, so, until I measure A, I don't know B. Also, CHANGING the spin of A after the fact will destroy the entanglement, so I can't force A to have +1 spin (for example). Entanglement means that I have forced two wavefunctions to share a property (e.g. spin) but to do so requires that the wavefunctions have no yet collapsed, which means I do not know the value of that property.

Now, imagine that I want to send a message via this system of two photons. I have one photon, you have the other (so we must have been within causal contact with each other in the past, so we're within each others light cone), and at a predetermined moment I measure A to have spin -1 and you measure B to have spin -1. Now, I instantaneously KNOW you have spin +1, and and you know I have spin -1. But what good is that? We can't have agreed before, for example, that -1 means no and +1 yes, since we can't force the measurement to come out one way or the other and still retain the instantaneous transfer of knowledge about the other state. So no information is transferred. Thus, I've learned something about an object that's far away, but to do so I had to have held that object before and arranged things specially; there's no way of entangling an object here and on Mars at the same moment, for example without something moving between here and Mars "the slow way." The end result is, to transfer information via this "faster-than-light" mechanism, something must have traveled slower-than-light beforehand.

The real use of this kind of set-up is cryptography, where you can give someone a box full of 1/2 of the entangled bits, then measure your half, use your results to make a single-use cypher, and transmit the encoded text knowing that your contact has the decoding pad. This is especially handy since no one else can measure the transmitted bits first; if they did, that would destroy the entanglement. So there are uses, but it isn't an ansible, and there's no convincing mechanism to make this into one.

So why do I like causality. First, I have no reason to dislike it; the Universe seems to be pretty causal in every experiment known. So there's that going for it. Also, the 2nd Law of Thermodynamics goes out the window if you can violate causality, and if there's one thing people reading my posts should know by now is that I love me some 2nd Law. However, none of this is to say that the other laws of physics forbid time travel: if you give me access to sufficient quantities of exotic matter and an expense account to make Nero blush, I'm pretty sure I could construct a closed-time-like curve (though maybe some deeper problem will arise, which would be very interesting). However, if I was successful, then in that Universe, I should expect that the 2nd law is going to be violated in some way, and that "causality" is going to have to redefined in some interesting ways (avoiding time paradoxes is going to be fun). Now, such exotic matter might be impossible, but that hasn't been conclusively proven. What is proven (and I mean that mathematically) in my mind is that, if faster-than-light communication is possible, then we live in a Universe without causality or relativity. Since I have strong evidence for the latter two, and none for the former, I currently believe that FTL is not possible.

So, to sum up, there are various kinds of impossible. There are the practically impossible things: like going to the Moon in 1930, sustainable fusion (for the next 20 years! it's always 20 years...), or going to Alpha Centuri in 2011 (or 2061, most like) via slower-than-light mechanisms. There are the things that appear to be physically impossible, but maybe we just need to experiment some more: like construction of closed-time-like curves (barring discovery of exotic matter, this is impossible, and even then, it still might be). Then there are the things that are mathematically impossible: like communication through entangled particles, or FTL in a Universe with both causality and relativity (and by this, I mean local travel faster than c in flat space; closed time-like-curves are a different kind of beast. The latter includes the ever-fun Alcubierre Drive).
posted by physicsmatt at 6:44 PM on July 25, 2011 [22 favorites]


I can always find a frame of reference in which the super-luminal connection is backwards in time. So there goes causality.

How does this follow?

I don't see how self-consistent causal loops can be ruled out a priori. Obviously I didn't kill my grandfather, but why should it be logically impossible for me to be my grandfather?
posted by flabdablet at 6:54 PM on July 25, 2011


I think it's funny that we wandered so far afield in this thread that matt posted a multiparagraph response and never touched the original topic of the thread :)
posted by empath at 6:55 PM on July 25, 2011


I can always find a frame of reference in which the super-luminal connection is backwards in time. So there goes causality.

Yea I got stuck at this line as well… is there a reference or book that explains this too?
posted by polymodus at 7:06 PM on July 25, 2011


flabdablet and polymodus: Not sure if I can come up with a convincing non-mathy reason for that. The mathematical answer is that two events connected by a path moving faster than c have a "space-like" separation (for normal non-FTL events, it's time-like). If two events are time-like separated, then I can find a frame (by a boost with v < c) in which the two events occur at the same place but different times. The space-like separation means that there's a frame (related by a v < c boost) in which the two events occur at the same time but different places: which implies that the communication between the events is backwards in time (according to that frame). I'm skipping over most of the details here, but I think I've gotten the most important bits. What you want is a text on special relativity, look at what the interval ds^2 is (ds^2 = dt^2 - dx^2 in my particle theory metric, -dt^2 + dx^2 in the confusing other one...), and then look at how boosts transform the dx and dt parts. There isn't actually any calculus necessary, but there is matrix multiplication.

As for the consistent causal loops, that's my guess about how a Universe with time-travel would work. You never kill your grandfather, because you try and fail, but you always failed, and that was the way the Universe "always was." However, that's not me as a physicist talking, that's me as a sci-fi fan. Though, if time-travel was "free" or very cheap at least, it's hard to imagine how a consistent time-line could be established. Though most "practical" time-machines (ones build with exotic matter, like a Tippler cylinder) only allow time travel back to the point where the machine was built, which probably solves most of the problems. Though there are gonna be some fucked-family reunions in that Universe.

empath: It happens. My mind wanders. And people keep bringing up more fun things to talk about. If the conversation is "photons travel at c," then I have very little to add.
posted by physicsmatt at 7:15 PM on July 25, 2011 [4 favorites]


Just so I'm sure I understand the jargon here, because this does confuse me.:

Not sure if I can come up with a convincing non-mathy reason for that. The mathematical answer is that two events connected by a path moving faster than c have a "space-like" separation (for normal non-FTL events, it's time-like).

When you say connected by a path, you mean they're connected causally, yeah? That two events which are causally related have a time-like separation. Events which are not causally related have a space-like separation.

If two events are time-like separated, then I can find a frame (by a boost with c > v) in which the two events occur at the same place but different times.


When you talk about boosting, you're talking about putting yourself as an observer of the two events into reference frame where you are moving in relation to them (or vice versa), such that the two events appear to be occuring at the same place but at different times, I think, yes?

The space-like separation means that there's a frame (related by a c > v boost) in which the two events occur at the same time but different places: which implies that the communication between the events is backwards in time (according to that frame). I'm skipping over most of the details here, but I think I've gotten the most important bits.

So what you are saying is that if they two events have a space like separation (that is, further than speed of light), then you can boost yourself into a reference frame where the two events look like they happened in two different locations at the same time (or perhaps where the later event happened before the earlier event). In which case causality is broken.

As a concrete example, say I'm on a space ship, and I use a match to light a candle. Event A is me lighting a match, and Event B is me lighting a candle with the match. If the space ship is moving faster than light, you can create a frame of reference observing the events such that you observe me lighting the candle before I've lit the match, which means that cause and effect are reversed.

Do I have that right?
posted by empath at 7:29 PM on July 25, 2011


I think this means that even if we aren't the only sentient race in the universe, we'll probably never get to meet any of the others. Which sucks.

I find this to not suck. I find it reassuring on many levels. I do think there must have been and will be other sentient life in the universe. And I'm really glad we probably will not encounter it, for reasons of both time and space.
posted by 3.2.3 at 7:35 PM on July 25, 2011


next up, is the fine structure constant constant?
posted by ennui.bz at 7:40 PM on July 25, 2011


empath: "I think it's funny that we wandered so far afield in this thread that matt posted a multiparagraph response and never touched the original topic of the thread :)"

The gap between the hard sciences and social sciences has officially been bridged!
posted by Apropos of Something at 7:45 PM on July 25, 2011


And here's the actual equations for this. I will work in two dimensions: time and x-axis (adding in the other 2 space dimensions doesn't change much, but sadly, despite a nice email from pharm, I still can't get latex to work in html).

We have a space-like separation between two events. Which means that the separation in time t and space x in some frame respect the relation t < x (I've set the speed of light c to 1. With c in there explicitly, this should be ct < x). This says that the time it takes to get between two events t is less than the time it would take for light to get between the two locations (x/c).

Now, a Lorentz boost from one frame to another is quantified by the velocity that we change frames by. We call this beta (0 < beta < 1, or 0 < beta < c if you want to keep units, but why would you do that?). So, when you perform a Lorentz boost, how does the separations t and x change? After eliminating go to:
t' = (t-x beta)/sqrt(1-beta^2)
x' = (x-t beta)/sqrt(1-beta^2).

Now, to make no time separation, we can boost to a frame where t' = 0, or beta = x/t. But since t < x, beta is less than 1. Which means I can boost (using a legitimate, slower than light boost) to a frame where the separation is all in space and none in time. If I boost to a frame beta' > t/x, then I indeed find that the "order of events" is reversed (like your candle example, empath). This is all just fine, and not a problem as long as I'm not presuming a causal connection between these two paths.

To answer the rest of the questions, technically I meant two infinitesimally separated events in my path (so that the path is "straight"). Events separated by macroscopic distances could have time-like separations, but still I could find a path between them that was locally space-like. This is called "using my warp drive to get to Mars, but taking 20 years to do it.")

Boosting here just means a Lorentz boost, which implicitly requires beta < 1 (so that gamma=1/sqrt(1-beta^2) is finite and positive). Since it's a finite Lorentz boost, then you can imagine an observer living in that frame.

ennui.bz, didn't we already cover that recently?
posted by physicsmatt at 7:50 PM on July 25, 2011


Scientists have proven nothing can travel faster than the speed of light, so no time travelling for you, bub.

So how do you explain the Chappa'ai, nerds?
posted by hal_c_on at 7:50 PM on July 25, 2011


This sucks, first no jet packs and now no time machines. If I had a time machine I'd go back and find a past with a better future.
posted by doctor_negative at 7:51 PM on July 25, 2011 [2 favorites]


Damn.

Damn, damn!
posted by XhaustedProphet at 7:58 PM on July 25, 2011


Well, if you want a time machine, get started on matter that violates the weak energy condition (I think you just need weak. Might need null. Where did I put those plans?).

If you will had to have started once building one in a few years back, it would have once be already finished by now-then-will. Elders these/those days, no respect for their youths.

If you're going to complain about no time machines, I'm going to start using achronal tenses.
posted by physicsmatt at 8:03 PM on July 25, 2011 [2 favorites]


physicsmatt: "[...] We also know that gravity propagates at c as well (I would have thought this could be done from observation of binary systems including a pulsar, but apparently, it was from an eclipse of a pulsar by Jupiter. Thanks Jupiter!). [...]"

I want to read more about this! please give me some search terms.
posted by bleary at 8:13 PM on July 25, 2011


For another data point, it's surprising how many random people that you find in a bar playing trivia DO know that the DeLorean had to get up to exactly 88 MPH to go back to the future.

I am working on including this fact in my unified field theory of trivia.
posted by Curious Artificer at 8:17 PM on July 25, 2011


Speed of Gravity

According to the Standard Model, gravity is carried by gravitons, which are massless bosons, and like all massless bosons, they travel at the speed of light.

If we were to find out that gravity moved at a different speed, it would have huge repercussions for the Standard Model.
posted by empath at 8:18 PM on July 25, 2011


I just googled "speed of gravity" and found a series of popular science articles about Kopeikin et al in 2003 (it was a quasar, not a pulsar, which I got wrong above). In fairness, their experiment had some large error bars, so there's still room for gravity to propagate a bit faster or slower than light. So it goes with science.
posted by physicsmatt at 8:20 PM on July 25, 2011


and empath beats me to it again. empath, stop cheating and share the exotic matter with the rest of us.
posted by physicsmatt at 8:22 PM on July 25, 2011 [1 favorite]


Heh, I have never actually taken a college physics or math course -- just watched the susskind lectures on itunes u, and I read some physics blogs regularly. Participating in these threads helps me clarify things for myself in a way that just reading about these things doesn't.
posted by empath at 8:26 PM on July 25, 2011


physicsmatt: "I just googled "speed of gravity" and found a series of popular science articles about Kopeikin et al in 2003 (it was a quasar, not a pulsar, which I got wrong above). In fairness, their experiment had some large error bars, so there's still room for gravity to propagate a bit faster or slower than light. So it goes with science"

I'm crushed. I thought you had that all on the tip of your tongue.
posted by bleary at 8:31 PM on July 25, 2011


empath, I've been enjoying interacting with you on these topics. Whatever you're doing, keep at it. It's always really encouraging for me to meet people outside of academia who are as interested in this stuff as I am.

The exotic matter crack was just too good to pass up; I was just writing my response to bleary when you posted. I formally retract any accusations of temporal drug doping.
posted by physicsmatt at 8:32 PM on July 25, 2011


(though if you know where to get some of the good stuff, let me know. You know, really grade-A, w=-1 TARDIS. Possible street names: Time Lord, Who?, When? and Delorean. There are a few people in Stockholm who would be interested)
posted by physicsmatt at 8:38 PM on July 25, 2011


Though, if time-travel was "free" or very cheap at least, it's hard to imagine how a consistent time-line could be established.

Absolutely not a problem; everybody from our future who will travel to our past has clearly already done so.
posted by flabdablet at 9:04 PM on July 25, 2011


Yay! I love when physics threads actually get into some nice meaty physics, rather than the usual japery. Thanks for that, all!
posted by stavrosthewonderchicken at 9:08 PM on July 25, 2011


"The Vulcan Science Directorate has determined that time travel is impossible." -- T'Pol
posted by bryon at 9:14 PM on July 25, 2011


Sorry about that, Stavros.
posted by bryon at 9:14 PM on July 25, 2011


My own personal favorite Handwaving Theory of Everything when under the influence of... ahem... exotic matter: that every perceivable and measurable thing will turn out to be completely constructed from self-consistent closed causal loops, the simplest possible forms of which are those entities we currently know as "particles".
posted by flabdablet at 9:22 PM on July 25, 2011


If we're at the bonghits phase of the discussion, here's mine: the probabilistic nature of quantum mechanics is because causal violations which result in extremely similar universes occur constantly - like a temporal vacuum energy, and the universe is constantly reverberating back and forth between a virtually infinite number of near-identical timelines. We perceive this as the probabilistic nature of quantum mechanics until observation causes the a Many-worlds-theorem-style bifurcation of those timelines, and the past is the fixed path of our bifurcations.
posted by Ryvar at 9:54 PM on July 25, 2011


I have a really hard time reading lay 'interpretations' of quantum mechanics now, because it just reads like babble to me. QM is just the math and the formulas, and they just work. You don't need to 'interpret it'. Interpretations are just our way of turning the math into a physical metaphor that our monkey brains can understand, but our brains just aren't wired to understand what is happening with QM in any kind of intuitive way. Any attempt to understand it and interpret it intuitively is almost certainly doomed to failure. Just learn the math, and forget the interpretations.
posted by empath at 10:13 PM on July 25, 2011


It's too bad this interesting discussion got attached to *such* a crappy FPP. This is the worst kind of science writing: Claiming that there is some 'new discovery' which 'changes everything' and bla bla bla. It's always been impossible travel faster then light under relativity and (as far as I know) regular quantum physics. Now in theory there could be something wrong with those theories, some gap in the knowledge. But the experiment with the superluminal group speed (or whatever) never violated any known laws of physics. It was just a trick. And apparently these guys have illustrated why it's just a trick, or something.

Beyond that it has nothing to do with time travel! As far as I know time travel may be possible under normal relativity without requiring traveling faster then the speed of light, provided you can create a 'closed, time-like curve' or something like that (IIRC), from what I remember. I was watching a talk that Neil DeGrass tyson gave where he talked about a fellow student's thesis on what happens when black holes collide, apparently you do have a situation where you can go backwards in time (without needing to go faster then the speed of light)

It sounds like the author of the article didn't understand WTF the paper was about and decide to claim that something we've known since the 1950s was just prove true -- and then extended that to time travel based on some inaccurate nonsense he heard somewhere.

then there were a bunch of comments about how people used to think you couldn't go faster then the speed of sound (not true, bullets and rockets traveled faster then the speed of sound) or they thought space travel was impossible (on par with thinking the earth was flat, or perhaps that an airplane can't take off on a treadmill)
And by a small amount, i mean a very, very small amount -- the black hole is absorbing more energy from ambient photons around it left over from the big bang than it is emitting in hawking radiation. It will take a very, very, very, very, very long time before any black holes lose any mass on balance from hawking radiation, except perhaps for very small ones (smaller black holes emit more radiation than larger ones
Yeah, small black holes actually evaporate quickly.
posted by delmoi at 10:21 PM on July 25, 2011


(sorry, that wasn't directed at anybody in particular, I've read lots and lots and lots of pop-sci explanations of quantum mechanics and just found that none of them clarified what was happening as much as just learning about state vectors and hermitian operators and observables and wave forms and so on did in just a few hours...)
posted by empath at 10:21 PM on July 25, 2011 [1 favorite]


wonk t'nod uoy gnihtemos wonk i
posted by obiwanwasabi at 10:30 PM on July 25, 2011


Wow, the physics education community has some work to do if this thread has turned into a discussion of entanglement, Jupiter, and Hawking radiation.

The "superluminal fibers" that are the subject of this experiment have always been kind of a swindle. What happens is that you put a light pulse with some rise time and fall time through an optical system with high gain. The "tails" before the peak of the input pulse and after the peak of the input pulse are both amplified. If the gain is high enough, the amplifying material will saturate and its output will start to fall off -- even if the brightest part of the input pulse hasn't yet entered the system. If your pulse-detecting electronics tell you the arrival time of the peak, you have a "paradox": the peak of the output pulse leaves the system before the peak of the input pulse enters. If you continuously measure the intensity of the light, you see something that makes sense: two similarly-shaped but mostly overlapping bell curves. (There's a typical plot in the PRL, figure 3d, showing a forty-nanosecond "peak advance" between two 400 nanosecond wide pulses).

Of course if your incident pulse doesn't have that least-common-denominator "bell curve" shape, then the output pulse doesn't anything like the same shape anymore and the "paradox" is a little easier to get past.

What's new here, as far as I can tell, is that Zhang et al. have figured out a way to tag the first photon in a pulse to interact with the superluminal medium, getting rid of the leading tail. With no "tail" in front of the peak of the pulse, the superluminal illusion is gone. No surprise there. (You can tell that there's no surprise because instead of concluding with "more research is needed," the authors conclude their paper with "this may bring closure to the debate.")

(And the answer to crayz's question is absolutely yes: I talk about photons all the time. I happen to be doing an experiment where photons come one at a time, so I am always thinking about how I'm counting them. But even in an RF field it's usually productive to think about electricity-atom interactions in terms of photons.)
posted by fantabulous timewaster at 10:36 PM on July 25, 2011 [1 favorite]


According to the Standard Model, gravity is carried by gravitons, which are massless bosons, and like all massless bosons, they travel at the speed of light.


I don't want to be a pedant but... gravitons might be what you would get if you had a consistent, effective quantization of gravity.... which we don't as far as I know.

In general, particles i.e. photons, bosons, mesons, etc. are a way of talking about the 'discrete' effects for "wave" solutions of a quantum mechanical field theory i.e. a wave of light (non-static solution) in Quantum Electro-Dynamics is made up of photons i.e. if you resolve it into it's frequency distribution and change the intensity a given frequency will jumpin intensity by discrete amounts (in the right conditions, etc. etc.)

My point is that you can't talk about fundamental particles without a corresponding field theory that has been made quantum mechanical (and has wave solutions.) But this runs straight into 'naive' ideas about matter being made of tiny billiard balls. But then physical problems like light-pulses at a mesocopic scale, which you really depend on the full 'wave' properties, look paradoxical if you just think about them being made by shooting billiard balls.

You can have "particles" even without a field when you quantize sound (matter vibrations) so that sound-waves are decomposed into "phonons." But again, you are looking at quantized wave solutions where the field is the actual material lattice instead of some luminiferous ether...
posted by ennui.bz at 3:47 AM on July 26, 2011 [1 favorite]


Any attempt to understand it and interpret it intuitively is almost certainly doomed to failure. Just learn the math, and forget the interpretations.

Interpreting it to mean something other than what it says has always struck me as an odd use of time as well. But many people do see to have trouble with taking it at face value, and I think this is because of some of the worldview features that doing so requires:

1. Every experiment is unique; there is no genuine repeatability in nature in the sense of identical conditions guaranteeing identical results; there are only consistent and specifiable tendencies.

2. Although we can recognize and within probabilistic limits predict particle interaction events, we have no justification for any assumption that particles between such events display anything like the kind of persistence we associate with everyday objects. We simply cannot know what happens between a particle's emission and the subsequent absorption of the "same" particle; all we can do is correlate absorptions with emissions.

3. Some of the natural features we can model mathematically as particles do not display the kind of locality that a naive mental picture of a particle as a tiny but otherwise ordinary object would suggest they ought.

It seems to me that all the "paradoxes" associated with QM just go away once we realize that we do know what we're talking about, and QM is how we need to talk about it, and that QM does not map well onto the objects and concepts underlying classical mechanics.

I am quite sure that with QM as with any other field of knowledge, what's intuitive depends on how much time you spend actually working with the models. It should be no more surprising that QM is counter-intuitive to people who don't do much QM than that software failure modes are counter-intuitive to people who don't do much debugging or that effective colour choice is counter-intuitive to people who don't do much visual design.
posted by flabdablet at 4:06 AM on July 26, 2011 [1 favorite]


The problem of time will go away once a modern Reimann makes it happen.

But the Flying Spaghetti Monster can fly as fast as s/he wants!
posted by Twang at 4:14 AM on July 26, 2011


Excuse me, Rorschach, I'm informing Laurie 90 seconds ago.
posted by Guy_Inamonkeysuit at 5:03 AM on July 26, 2011 [1 favorite]


Time is an illusion. Lunchtime, doubly so.
posted by Guy_Inamonkeysuit at 5:24 AM on July 26, 2011 [1 favorite]


No offense taken, empath. It was totally an unabashed layman's bonghit that I haven't the least interest in defending.
posted by Ryvar at 5:51 AM on July 26, 2011


I don't want to be a pedant but... gravitons might be what you would get if you had a consistent, effective quantization of gravity.... which we don't as far as I know.

Right, but if the graviton doesn't exist, then the Standard Model (and String Theory) is in trouble, no? That was all I was saying.
posted by empath at 6:37 AM on July 26, 2011


Also, I think the interpretation that forces are mediated by particle exchange is just as valid as the interpretation of forces being wave-linke perturbations in fields. They're two equivalent descriptions of the same phenomena. Although, I don't think there is a workable particle exchange model of gravity, yet.

It seems to me that all the "paradoxes" associated with QM just go away once we realize that we do know what we're talking about, and QM is how we need to talk about it, and that QM does not map well onto the objects and concepts underlying classical mechanics.

I've been thinking that it would be a good idea to make a puzzle game out of QM, which would basically consist of setting up experiments with photons and polarizers, beam splitters etc, which would walk people through the basics, all the way up to Bell's Inequality and Quantum Computing.
posted by empath at 6:44 AM on July 26, 2011


If you think you understand quantum mechanics, you don’t understand quantum mechanics. – Richard Feynman
posted by jet_manifesto at 7:28 AM on July 26, 2011 [1 favorite]


Right, but if the graviton doesn't exist, then the Standard Model (and String Theory) is in trouble, no? That was all I was saying.

IANAPP, IANYPP

String theory presupposes that the Standard Model is in trouble i.e. it replaces the standard model with... String Theory. And, string theory can be jiggered to include gravity...

The 'standard model' itself is a quantum (gauge) field theory that manages to stuff in all of the "forces" needed to explain nuclear physics (from the last century) plus electro-magnetism. Note that "force" in this context is an impulse propagating through a "field." Quantum mechanics then says that the energy over frequency in these impulses distributes in discrete chunks or packets... these packets are explained by saying "particles" are received or exchanged.

The graviton is not part of the Standard Model because Gravity is not part of the standard model, nor will it ever be. Even if string theory doesn't work out, I think the consensus is still that the standard model will need to be replaced in order to have a unified theory of "forces" which includes gravity.

(I Am Not A Particle Physicst, I Am Not Your Particle Physicist)
posted by ennui.bz at 8:30 AM on July 26, 2011


And, string theory can be jiggered to include gravity...

String theory automatically includes quantum gravity because of the spin 2 particles it generates (ie, gravitons). That's the whole reason that it's generated so much interest.

The graviton is not part of the Standard Model because Gravity is not part of the standard model, nor will it ever be.

Why do you say it never will be? Almost all physicists have been spending the last 30 years trying to include it, which is why they are so interested in String Theory and Super-symmetry. If the Standard Model has to change somewhat to incorporate gravity, then it will still be the Standard Model. AFAIK most theoretical physicists assume the graviton exists and that it will eventually be incorporated into the Standard Model.
posted by empath at 8:46 AM on July 26, 2011


String theory presupposes that the Standard Model is in trouble i.e. it replaces the standard model with... String Theory.

String theory replaces the Standard Model, btw, in the same way that quantum electrodynamics replaces chemistry. The Standard Model still applies at certain scales, whether or not it arises out of the underlying reality of string theory.
posted by empath at 8:49 AM on July 26, 2011


I guess they're saying time travel into the past seems impossible from this result. Not at all surprising, since the idea of travelling into the past is nonsense.

Time travel into the future is trivially easy. You're doing it right now at the rate of (about) 1 second per second. If you could accelerate to 99% of c, you'd be travelling into the future at about 7 seconds per second.

According to this.
posted by General Tonic at 9:11 AM on July 26, 2011


Not at all surprising, since the idea of travelling into the past is nonsense

How so? Are you talking impracticality, or logical impossibility?
posted by flabdablet at 9:59 AM on July 26, 2011


String theory replaces the Standard Model, btw, in the same way that quantum electrodynamics replaces chemistry. The Standard Model still applies at certain scales, whether or not it arises out of the underlying reality of string theory.

the "standard model" is really something very specific: a non-abelian gauge theory with the symmetry group U(1)×SU(2)×SU(3). You could say that the standard model 'replaces' the theory of electromagentism, because E&M corresponds to the subgroup U(1) < U(1)xSU(2)x(SU(3).

String theory is something entirely different.

The standard model is a quantized gauge field theory, which is really 19th century physics. But, my point is that you can't talk about "particles" in the standard model using some kind of naive realism ignoring that they are particular aspects of quantizing a "field." The physics of the standard model is not billiard balls in constant collision but "fields" constantly interacting.
posted by ennui.bz at 5:24 PM on July 26, 2011


The higgs mechanism is also part of the standard model, and that's a scalar boson, not a gauge boson, and the standard model isn't set in stone and continues to be modified as new physics is discovered. If supersymmetric particles, for example, are discovered at the LHC, they'll just be included in the Standard Model, they won't replace it. I assume the same would be true of the graviton.
posted by empath at 5:35 PM on July 26, 2011


If you think you understand quantum mechanics, you don’t understand quantum mechanics. – Richard Feynman
You know, Feynman said this in the mid-sixties, when the Schroedinger equation was almost forty years old. That was almost fifty years ago -- fifty years in which there has been a lot of progress. I don't think that Feynman meant nobody would ever understand quantum mechanics, and I hate the implication that the last half-century of physics has been some sort of priesthood of ignorance. Sure there are some profound unanswered questions, like why gravity seems to behave differently on galactic length scales, or why the universe seems to be full of matter and have only incidental antimatter. But as a way of thinking about phenomena and trying to guess how a measurement will turn out, there are an awful lot of people with a very well-developed quantum intuition.
posted by fantabulous timewaster at 7:53 PM on July 26, 2011


I'll take your word for it, fantabulous, truly. I'm sad to say that I can't claim to have any such intuition myself. Even after decades of extensive reading on the subject, QM still feels like the shadow on the cave wall to me.

Do you feel that you have that 'intuition'? If so, I'd love to know how you came to that understanding - was it sudden, a flash of insight or inspiration? Or a slow accumulation of concepts and theories that eventually settled in your mind and 'made sense'?
posted by jet_manifesto at 10:23 PM on July 26, 2011 [1 favorite]


jet_manifesto: Did you actually try to learn the math?
posted by delmoi at 2:10 PM on July 27, 2011


Even after decades of extensive reading on the subject, QM still feels like the shadow on the cave wall to me.

Watch Susskind's videos on Quantum Mechanics at iTunes U. He doesn't assume you know much except calculus, and you can pick that up at khan academy. He doesn't even bother trying to interpret what the numbers mean, he just explains the experiments and the math, of the course of many, many hours.
posted by empath at 2:21 PM on July 27, 2011 [1 favorite]


jet_manifesto: I do okay in the quantum-intuition department. I have colleagues who are better. Definitely a slow accumulation of information --- they don't call it gradual school for nothing.

As delmoi suggests, reading (and consuming lectures) will only get you so far. If you were taking a course, you'd also have to work problems. This is the key to figuring out how quantum systems work: you take some well-posed quantum systems, and you figure out how they work. A lot of interesting systems are actually accessible without even calculus, if you can handle complex numbers and two-by-two matrices.
posted by fantabulous timewaster at 7:36 PM on July 27, 2011


Do you have a good source for exercises? I've been wanting to actually try my hand at working out some of these.
posted by empath at 7:55 PM on July 27, 2011


Thanks for the suggestions, I'll definitely check out Susskind and take up the challenge to actually tackle some problems. It may take a while...but hey I've got the rest of my life to work at it :)
posted by jet_manifesto at 3:24 AM on July 28, 2011


Sorry, empath, but not really. You could read reviews and try to find an accessible textbook.
posted by fantabulous timewaster at 5:13 AM on July 28, 2011


How is it that gravitons are made compatible with the existence of black holes?

If photons are unable to escape black holes because of their energy, how would gravitons escape black holes?

Yet if gravitons do not escape black holes, how does the matter inside continue to exert the same gravitational effect it did when it was outside-- via some kind of graviton Hawking radiation?
posted by jamjam at 10:55 AM on August 6, 2011


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