Can I transfer information faster than the speed of light by using a really long rod?
July 12, 2004 11:33 AM   Subscribe

[PhysicsFilter] This has been troubling me for a while and the answers I've gotten haven't satisfied me. Nothing travels faster than the speed of light for all intents and purposes, information included. I think I have a way around that, but I've been told it won't work. [More inside]

My idea: build a rod that stretches a long, long distance, perhaps to another solar system. I suspect that materials science would say this is impossible also, but bear with me. If you were to whack the near end of the rod at the same time as a light flash was emitted, would the other end of the rod move before the light was detected? Or would something cause sufficient delay to make them appear at the same time at the remote end. All I've ever heard was that it wouldn't work. My question now is, "why not?"
posted by tommasz to Science & Nature (47 answers total)
Think about it from an atomic perspective. Objects don't move as a single entity; they only appear to do so from our macroscopic perspective.

What would happen is you would exert a force on the atoms at the near end of the rod. They in turn would exert a force on atoms further down the line due to repulsive electromagnetic forces and such. What you'd really have would be a compression/decompression wave travelling down the rod. Imagine a really, really stiff spring. When you push on one end of a spring, the other end doesn't move instantly.

So since this wave is really just the motion of atoms, it would travel slower than the flash of light, and hence, the light would arrive before the other end of the rod would move.
posted by Khalad at 11:44 AM on July 12, 2004

Best answer: IANAP, but it seems clear to me why not--because a rod is not actually a perfectly rigid object. No matter how hard it "seems"--even if it were made of diamond--any material actually compresses in waves that ripple through it, like if you smacked one end of a rod made of gelatin.

Mechanically, for your example, I think a rod made of diamond or adamantium is actually more like a rod made of gelatin than it isn't. The differences are really only in how big the compression waves are, and how quickly they travel. You just can't see the compression waves in a stiffer material, because they're smaller and move more quickly, but they're still there.

So in a rod that long, even made out of some super-hard material, the compression waves would actually have to propagate down the rod from one end to the other, and that rate of propagation is actually closer to the speed of sound, probably, than the speed of light. You'd have to postulate an impossibly hard material for your example to work, and in that case, why not just stipulate leprechauns?
posted by LairBob at 11:45 AM on July 12, 2004

The impact will be transmitted as a (compression) wave, so it won't go faster than light (probably lots slower), sadly. Imagine at the moment of impact, only the atoms at the end of the rod "know" they've been hit -- they have to transmit that information to the next atoms in a very short (but still non-zero) amount of time.
posted by j.edwards at 11:45 AM on July 12, 2004

the rod is made of atoms. atoms are basically fuzzy balls of electrons. the electrons (and protons) fasten the atoms together using electrostatic forces - they act on each other using electromagnetic radiation.
light is electromagnetic radiation.
so the movement of atoms within the rod is basically done by the same "stuff" that is light and with similar constraints - in practice, when you hit one of the rod the movement will move down the rod as a presuure wave with a speed less than that of light.
sorry that's a bit confused - jet lag.
posted by andrew cooke at 11:45 AM on July 12, 2004

The rod would compress/deform as whack it. The speed at which this deformation runs along the rod is always lower than the speed of light.

on preview: it seems that ask.mefi answers faster than light!
posted by swordfishtrombones at 11:46 AM on July 12, 2004

Indeed. Speed of sound in diamond is only about 12000 m/s, not even close to fast enough.
posted by sfenders at 11:47 AM on July 12, 2004

No; the motion of the far end of the rod is not instantaneous, and not even faster than the speed of light. The "pulse," (essentially a compression wave) if you will, of you hitting one end of the rod moves through the rod at the speed of sound in whatever material the rod is made of. The earliest that the far end of the rod could move, after you hit the near end, is the length of the rod divided by the speed of sound in the rod.

The speed of sound through, say, steel, is a lot faster than the speed of sound through air, but still a lot slower than the speed of light.
posted by DevilsAdvocate at 11:48 AM on July 12, 2004

It shouldn't be necessary to make a really long rod to test this, either, except as a way of magnifying the difference in propagation speed.
posted by coelecanth at 11:50 AM on July 12, 2004

Also, consider that the compression wave would most likely be dissipated as heat due to friction long before it could reach the other end of the rod, if said rod is really as long as you imagine. So it's actually unlikely that you could even exert enough force on the near end of the rod to move the far end.
posted by Khalad at 11:54 AM on July 12, 2004

if this stuff interests you, see the EPR Paradox for more superluminal fun.
posted by badstone at 11:55 AM on July 12, 2004

Response by poster: I've got it now. Since there's no way to make a rod like that that isn't made of more than one atom, the wave propagation through the rod will always be slower than light. Thanks for the lesson.
posted by tommasz at 12:03 PM on July 12, 2004

While on the subject - is there a decent theoretical answer to the Stephen Wright joke/question: "If you are in a spaceship that is traveling at the speed of light, and you turn on the headlights, does anything happen?" I can't recall ever seeing a good answer to that one.
posted by kokogiak at 12:05 PM on July 12, 2004

well, you'd be in business if you had a wormhole to poke your rod through, tommasz. and no, i didn't mean that to sound as bad as it does. but yeah, wormholes are still a theoretical open door, not yet proved, but also not yet disallowed by anything. (damnit, didn't mean to make that open door pun...)

kokogiak - well, the light that bounces back at you would be blueshifted waaaaaay out of the visible.
posted by badstone at 12:09 PM on July 12, 2004

kokogiak- At the speed of light, matter is infinite and time is zero. So, there would be no lightswitch, nor a span of time in which to turn it on ;)

LiarBob- I like your props to Marvel chemistry, but I don't think adamantium actually exists.
posted by mkultra at 12:11 PM on July 12, 2004

posted by LairBob at 12:14 PM on July 12, 2004

Response by poster: Don't wormholes effectively shorten the distance and therefore "cheat"? I realize no one's ever detected one and making a stable wormhole is even less likely, but they would make a lot of sci-fi acheiveable, wouldn't they?
posted by tommasz at 12:15 PM on July 12, 2004

Or how about this:

Given a perfectly rigid rod, and enough energy to swing said rod, couldn't we make the rod long enough that the end would be moving faster than the speed of light. Say the rod is a light year long. A fairly slow rate at the center of rotation would translate into a great speed at the end of the rod.
posted by y6y6y6 at 12:16 PM on July 12, 2004

kokogiak, IANAP, but I have read that weelchair guy's book.

You are only traveling at (really close to) the speed of light relative to something else. If you're just out there in space, not accelerating, you can't tell how "fast" you're going. So let's say you're whipping past some planet at the speed of light.

So, you're sitting there in your car, and you generate a photon (using a PHOTON GUN(!)) in front of you, traveling in the same direction as you are. You will percieve the photon traveling away from you at the speed of light. Everything will look normal to you.

For some guy sitting on a planet watching the two of you (you and your photon), the two of you will travel away from him at the speed of light. You and your photon will be all mushed up, compressed if you will, as far as he's concerned. But unless you make a cosmic u-turn and go talk to him, neither of you will notice anything especially weird going on.

It's all relative.
posted by Capn at 12:18 PM on July 12, 2004

As the rod spins faster, it will take an asymptotically larger amount of energy to speed the rod up a smaller and smaller amount.
posted by Khalad at 12:21 PM on July 12, 2004

Also, consider bracing the rod. If you attach the rod securely to the earth via a motor, the motor will spin the Earth more than the rod, because of the rod's inertia.

Now if we had a perfectly rigid weightless rod, it could work, but if it was weightless, applying any force to it would accelerate it to the speed of light, so we're done!
posted by Capn at 12:29 PM on July 12, 2004

"If you are in a spaceship that is traveling at the speed of light, and you turn on the headlights, does anything happen?"

If I'm watching you approach at lightspeed -- if my reference frame is "stationary" -- and you turn on your headlights, I'll see the lights as x-rays or gamma rays or other hard radiation and promptly die.

I understand it as the light from the headlights can't go any faster, because C is C, so it has to become more energetic another way, by shifting down the spectrum.

Similarly, if you're flying away at 0.999C and I try to zap you with an x-ray laser (because I'm very cross with you), you might perceive it as a dim red glow, or as a dot of heat, or as a faint radio crackle.
posted by ROU_Xenophobe at 12:30 PM on July 12, 2004

y6y6y6, usually that hypothetical is described as a gigantic (superluminal) pair of scissors. If you make the blades long enough, won't they have to be going at light speed eventually? The link says no. "No matter what material you use for the scissors, [special relativity] sets a theoretical upper limit to the rigidity of the material. "
posted by smackfu at 12:30 PM on July 12, 2004

y6y6y6--Nice try, but I'm guessing that even if such a contraption were physically allowable, you'd still be done in by the end of the rod's relative mass and time. As the rod sped up, its mass would increase until the energy required to accelerate it further would become infinite.
posted by vraxoin at 12:31 PM on July 12, 2004

kokogiak, in terms of the math, your question is a logical impossibility. What does blue smell like? Same kind of question. You can get as close as you like to c, but never reach it.

Here's a consider for you: there are galaxys which are receeding at very close to light speed, so we're going at almost c from their reference point. Now go down into your basement and turn on a flashlight.
posted by bonehead at 12:39 PM on July 12, 2004

y6y6y6: I actually considered posting that as a followup question, but guessed based on this discussion that, if you were to move along the rod away from the center, it would become more and more massive, and that you'd run into the problem of having infinite mass at light speed. Anyone care to comment?

(on preview vraxoin seems to concur.)
posted by alphanerd at 12:39 PM on July 12, 2004

Read up on "spooky action at a distance" and "entangled photons", which uses some quantum physics weirdness to theoretically enable transmission of information faster than light.
posted by falconred at 12:41 PM on July 12, 2004

This recent announcement on entanglement.
posted by biffa at 12:48 PM on July 12, 2004

"If you are in a spaceship that is traveling at the speed of light, and you turn on the headlights, does anything happen?"

All of a sudden, that's not as funny as it used to be : (
posted by grateful at 12:57 PM on July 12, 2004

Also, I believe IBM is using entangled photons to create faster computers.
posted by xammerboy at 12:57 PM on July 12, 2004

"you'd still be done in by the end of the rod's relative mass and time"

Fair enough. Good point.

But what would I see at the center or rotation? As I applied energy closer and closer to infinite, would the straight rod appear to bend? Would the end appear to be motionless as time goes to zero?
posted by y6y6y6 at 12:59 PM on July 12, 2004

not just IBM by far, there's a pretty huge amount of effort goign into quantum computing. actually, encryption via entanglement is almost practical already. but as far as instantaneous information via entanglement goes, see the EPR Paradox link above.
posted by badstone at 1:03 PM on July 12, 2004

y6, I think if you had some kind of perfect visual imaging system, then you'd be right--at any given moment, the rod would seem to curve back away from the anchor point, since the photons from successively more distant positions are coming from successively more remote points in time, as well. (At moment T, you'd be seeing the near portion of the rod in what would seem to be "real time", but you'd be seeing portions of the rod much farther out as they were a fraction of a second ago, making it look like it was trailing behind.)

Effectively, though, your eyes could never see far enough to actually perceive this effect. You'd have to have some kind of hypothetical camera with almost infinite depth of field (and tremendous sensitivity) to actually somehow capture that image.
posted by LairBob at 1:18 PM on July 12, 2004

y6y6y6: You would look out a straight rod at a blue-shifted starfield. To an observer at rest with you centre of mass and not spinning (or spinning in an opposite hand), your rod would curve with your speed of rotation (or the sum of yours and theirs, whatever).

Your question has a long history and has tripped up even really bright physicists.
posted by bonehead at 1:28 PM on July 12, 2004

sweet link, bonehead. there's an interesting aside about rigid rods (longitudinal acceln) and the bell spaceship paradox that's relevant to the original question, too.
posted by andrew cooke at 1:44 PM on July 12, 2004

There's forms of quantum synchronicity, where electrons at unlimited distances match each other's state. This is being studied and experimentally used for quantum computing and data transfer. So yes, information seems to be able to transfer faster (instantaneously, in fact) than light can travel.
posted by abcde at 2:17 PM on July 12, 2004

as far as i knew, no-one has transmitted info faster than the speed of light. has that now been done? i don't know the details, but thought there was some kind of statistical argument (not necessarily accepted by everyone) against using correlated states to transmit information in practice (i'm not arguing against action at a distance, only against practical information transmission).
posted by andrew cooke at 2:26 PM on July 12, 2004

abcde: So yes, information seems to be able to transfer faster (instantaneously, in fact) than light can travel.

Do you have a source for this? :) Action-at-a-distance is not the same as information traveling faster than light. The term "information" in this context is pretty well defined.
posted by vacapinta at 3:29 PM on July 12, 2004

this will be vague and anecdotal, since i don't have time to research links right now, but y6y6y6's idea is similar in some respects to something that has been acheived recently - the effect of making electrons appear to travel faster than light. the analogy is this: imagine a lighthouse, emitting a steady beam of light. imagine it's in a huge, um, velodrome or water filled warehouse - something like off the truman show. as the lighthouse rotates, the spot of light, where the walls of the room are hit by the beam, moves. rotate the lighthouse fast enough, and the spot [note, the SPOT, not the light] moves faster than the speed of light. this is not violating any laws of physics, since it's only the spot that moves, and since the spot is made up of a constant stream of different photons, everything's fine. it's pretty easy to calculate room sizes and rotation speeds in order for this to happen on the macrocosmic scale.
on the quantum scale, an electron appears on the other side of a gas-filled area seemingly before it's even entered it.
posted by nylon at 3:34 PM on July 12, 2004

I'll be less harsh because I think I know what abcde meant. Correlation does appear to happen instantaneously but this isnt a usable information channel (that we know of) Its coordination not communication.

The best analogy is think of a lighthouse halfway between you and me. We agree that when the lighthouse keeper puts a red filter on the lighthouse you and I will both, say, jump up and down. So, we end up jumping up and down simultaneously but no information has traveled from me to you. I cant use this method to send you a message. Nothing has traveled faster than light.

nylon above is, i believe, making a similar point.
posted by vacapinta at 3:46 PM on July 12, 2004

as far as i knew, no-one has transmitted info faster than the speed of light. has that now been done?

ISTR that some physicists claim to have transmitted music through some manner of dense medium at speeds > C. I don't remember how it was supposed to work, but I don't think it involved quantum stuff.

I am neither a physicist nor someone who plays one on tv, and know nothing about whether or not the work was any good.
posted by ROU_Xenophobe at 3:48 PM on July 12, 2004

here's a link about signals being transmitted faster than the speed of light (four times faster, in fact).
posted by nylon at 3:51 PM on July 12, 2004

nylon: Yup, but apparently not information, alas:

"While the peak moves faster than light speed, the total energy of the pulse does not. This means Einstein's relativity is preserved, so do not expect super-fast starships or time machines anytime soon.

Signals also get weaker and more distorted the faster they go, so in theory no useful information can get transmitted at faster-than-light speeds, though Robertson hopes his students and others can now rigorously and cheaply test those ideas.

That was two years ago.
posted by adrianhon at 3:57 PM on July 12, 2004

not information, alas

yes, i almost made the schoolboy mistake of putting the word 'information' in my post, but thought i'd better be more careful. shame that nothing's come from that, eh?

here's another cool thing, which is sort-of related: laser teleportation.

ROU: are you confusing light with sound? :)
posted by nylon at 4:08 PM on July 12, 2004

The superluminal scissors thing isn't about the tip of the blade being superluminal, it's that the intersection of the two blades can move along the blades at superluminal speeds. This is an example of "something" that can "move" faster than c. Of course, it's not really a "thing", and that's why it can. All sorts of events are like this.

And it should be mentioned that by c here we mean "in a vacuum". Light travels more slowly otherwise and it is possible for something to travel faster than light, for example, through water. If it does, though, Cherenkov radiation is produced, which is cool and is why highly radioactive waste stored in water glows blue.

A spaceship couldn't travel at the speed of light. But the whole point of relativity is that any observer at any relative speed will measure the speed of light to be the same amount. That is, turning on the headlights of a spaceship "near the speed of light" will result, to a person traveling in the spaceship, in light emanating from the headlights just exactly as it would have were the ship moving more slowly. But the bigger conceptual problem here is in the misleading framing of the question: "near the speed of light" relative to what? c is not an absolute in this conceptual sense; you can't measure against it in the sense implied by the phrasing of the question. When you measure c, you get the same value, regardless of how fast you're moving. When we say "near the speed of light" we mean that something is traveling relative to something else near the speed of light. Two things cannot travel relative to each other at the speed of light.

On Preview: I wish the EPR "paradox" had never entered the public consciousness. Sigh.
posted by Ethereal Bligh at 4:49 PM on July 12, 2004

When your material rigidity gets to the point where sound waves crack C (generally in some kind of collapsar) you're not really in Kansas anymore and any kind of signal is unavailable to me because you're on the other side of an event horizon.
So it is possible, but physics currently doesn't have anything to say about it. That's one of the things that a theory of quantum gravity is expected to deal with.
posted by snarfodox at 7:58 PM on July 12, 2004

nylon - what you describe is used to explain apparent superluminal motion in quasar emission (very distant, very bright astronomical objects).
posted by andrew cooke at 6:38 AM on July 13, 2004

ROU_X - i remember that too, from a few years back, but iirc it was marginal and i don't know if it's been confirmed (i believe there was also an objection that the shift was so slight that the "premature" signal could be predicted by extrapolating from the known data, implying no extra info was available).
posted by andrew cooke at 6:43 AM on July 13, 2004

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