Talk:Faster-than-light

Contents

1 Odd questions 2 Article was rewriten since these comments 2.1 Loose concepts 2.2 Mass increase 3 Pushing a rod; FTL reaction? 4 Relative speed 5 Light travels at different speeds? 6 NPOV/Clarity 7 Explaining an edit 8 Speed and distance or time 9 "Impossible" vs. "Probably impossible"

Odd questions

Please move these to a seperate category if they ever evolve into a discussion

• If you absorb light from all sides, is there no limit to the "speed" you can go? After all, there is no light to clock you. - User:hackwrenchRobert Claypool
  • Short answer is that it's useful to imagine using light to measure distances and times due to its properties; the speed limit is a property of spacetime itself, not something artificially applied by light. See special relativity and minkowski space for more details, and ask questions on their talk pages. --Christopher Thomas 02:34, 30 May 2005 (UTC)

• What about quantum entanglement?
  • Short answer is that it lets you transmit gibberish faster than light, but information at the speed of light or slower. See phase velocity, and group velocity for more information. Apparent FTL photon transmission observed to date has been triggering of a reconstructed photon pulse on receiving only the leading edge of the original photon's wavefunction. This turns out to not let you do anything useful. Talk:quantum entanglement is probably the best place to ask about this. --Christopher Thomas 02:34, 30 May 2005 (UTC)

• What about Cherenkov effect? - Superborsuk 12:21, 22 Aug 2004 (UTC)
  • Short answer is, what about it? The speed at which the actual photons are propagating remains C. The phase velocity changes in a medium with a refractive index, which makes anything (like a fast charged particle) causing an EM disturbance that moves faster than the phase velocity to do strange things (in this case, a cone of light is emitted from the point of disturbance). Still, no actual FTL here. --Christopher Thomas 02:34, 30 May 2005 (UTC)

• If a particle had a rest mass that was a negative imaginary number would it's mass at a speed faster than light be a positive real number?
  • See tachyon and tachyon condensation (and their talk pages) for a more detailed discussion of this. --Christopher Thomas 02:34, 30 May 2005 (UTC)

Article was rewriten since these comments

Loose concepts

"these concepts are outright impossible, as well as being well beyond our current technology"

Isn't this sentence kinda contradictory? It can't be both impossible and beyond our current technology, beyond our current technology implies that it is possible. Fuelbottle | Talk 22:52, 29 Aug 2004 (UTC)

Or does it? It means we can't do that yet, it doesn't specify wheter we'll ever be able to do that.

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I removed This process cannot transmit information, as nothing is getting closer to anything. since we definitely have transmission of information from high redshift galaxies :). - Boud 15:17, 17 Jun 2004 (UTC)

Would you care to provide a source for this? - 70.18.182.39 03:41, 17 Apr 2005 (UTC)

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Note: I changed the phrase "the limit may not be quite as absolute in general relativity" back to "the limit IS not quite as absolute ...". It is inarguably the case that general relativity allows bizarre metrics that let an object move faster than light, or back in time, from the point of view of a distant observer. Physicists don't like this, so they look for a more comprehensive theory that would prohibit it. Such a theory would supersede general relativity; it wouldn't BE general relativity. (Does anyone know what current hypotheses of quantum gravity say about wormholes and the Alcubierre metric?) - Zack 00:04, 12 Jan 2004 (UTC)

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Removed the following parapgraph. Anyone capable of understanding it can rewrite and replace it: In the context of quantum field theory, in the framework of local quantum physics, this is the requirement that if O is a bounded open subset of spacetime, then the observable algebra of the causal completion of O is the same as the observable algebra over O. There are QFT models where this axiom does NOT hold, and so why this axiom holds is an open question. - Ec5618 10:18, Apr 18, 2005 (UTC)

Mass increase

I had posted an entreaty, asking for further consensus, but got no responses, so I lost patience (sorry) and forged ahead on my own with a major MAJOR revision. Why? because I found the existing approach horribly misleading. I DO however compliment the previous author(s) for their diligent efforts at circumspect wording, but even so, to base the speed limitation (of a rocket propelled craft) on relativistic mass increase is dubious at best. The reader would've been grossly misled and would have concluded that a speeding rocket is ever harder to thrust, which is NOT the case! And that thing about time dilation preying upon exhaust gas speed was likewise awkward and obtuse. So I've replaced all of that.ETP 23:08, 26 Jan 2005 (UTC)

I want to reemphasize that the attribution of relativistic mass increase is quite okay, but it's a lousy way of explaining the speed limitation of a rocket propelled craft. It is a perfectly fine and dandy way to explain the speed limitation of a charged particle in a particle accelerator, because those particles are being pushed along by electromagnetic field-producing coils that are positioned in the (relatively) stationary lab frame. From that viewpoint, the mass increase to the speeding particle is entirely pertinent as an impediment. But the thrust applied to a rocket propelled spacecraft originates in the craft's own rest frame. I hope the readers here can appreciate that crucial difference! If you try to apply the relativistic mass increase explanation to such spacecraft, naive readers will indubitably be misled and will likely end up loosing sight of the fact that motion through space is after all only relative. ETP 00:03, 31 Jan 2005 (UTC)

Pushing a rod; FTL reaction?

Something that's been bugging me: Imagine a light-year long platform (For the purposes of this experiment we're assuming it wouldn't collapse under its own gravitational field) with two stands on the end, with a super brittle metal rod resting on both stands. If you pushed the rod at one end of the light year long platform, no part of the rod would ever move faster than a few inches per second, but a person at the other end could immediately see that it had been pushed forward a few inches.... Right?

Not quite... No information can be transferred faster than light. Thus, it takes at least a year for the deformation to reach the other end. Otherwise you could send "instant morse" to another person far-far away by just pushing or pulling the rod. From here we can conclude that there is no such thing as infinite stiffness or zero elasticity — one end of the rod is already compressed while the other end is still waiting for the information, lying in perfect peace. The length of the rod is always decreased, when one pushes an end of it and increased, when an end is pulled. Any more interesting questions puzzling you? — Pt _(T) 23:06, 25 Jan 2005 (UTC)

Relative speed

Could someone explain how this works

It is also possible for two objects with mass to travel faster than light relative to one another, from the viewpoint of a stationary observer. For example, particles on opposite sides of a cyclotron.

No. The space-time distortion caused by near-lightspeed travel means that no two objects travelling slower than light can ever perceive each other as moving faster than light, no matter how high their relative velocities with regard to a stationary observer. Lee M 23:45, 11 Jan 2004 (UTC)

Both are correct.

If you put a "stationary" observer in the middle of a circular particle accelerator, they can observe a particle on one side of the ring moving at .999c in one direction, and a particle on the other side moving at .999c in the opposite direction. They will then conclude that the particles are moving at 1.998c relative to each other.

i've tried to correct this. (1) It doesn't matter where the observer is standing. She knows how to measure and calculate distances and times, even if she is not at the centre. (2) i don't understand how she gets 1.998c - she can add up the two numbers, but that just means she does not know how to add up two velocities, it doesn't have any physical meaning. Boud 15:30, 17 Jun 2004 (UTC)

Agreed. There is a difference between Newtonian physics and relativistic physics. --Seraphis 04:54, 16 Apr 2005 (UTC)

However, an observer traveling with either particle would observe the other particle moving at .9997c (figure made up - I don't remember how to calculate this) relative to themselves.

I'll try to clarify the article.- Zack 23:52, 11 Jan 2004 (UTC)

Light travels at different speeds?

how can this article incorporate the following:

To quote Richard Feynman "...there is also an amplitude for light to go faster (or slower) than the conventional speed of light. You found out in the last lecture that light doesn't go only in straight lines; now, you find out that it doesn't go only at the speed of light! It may surprise you that there is an amplitude for a photon to go at speeds faster or slower than the conventional speed, c." -- Chapter 3, page 89 of Richard Feynman's book "QED".

NPOV/Clarity

This page is rediculous. While I do not grasp the subject matter (can anyone say they do?) I did notice what I believe to be some obvious problems.

I added a {cleanup-rewrite} tag to suggest this page needs some cleaning up. When I first saw the page, I thought of adding a {tag}. I found more and more that fitted:

• {neutrality} - This article includes just one theory, one interpretation.
• {Limitedgeographicscope} - Please include metric measurements. 12 inches per nanosecond?
• {OriginalResearch} - I guessed this page wasn't put together by people remembering bits from highschool science.
• {disputed} - Without references and source material (even wiki links) no-one can validate this article's claims.
• {technical} - Obviously. Please add some plain descriptions. "Or in other words, how can thrust originating from a rocket's very own rest frame labor under any relativistic effect? It makes no sense at all."
• {cleanup-tone} - The last quote should show that the language leaves something to be desired, for an encyclopedic article.

Well, someone's rewritten the article, and it looks good to my (admittedly unexperienced) eyes, so I'm tentatively removing the {cleanup-rewrite} tag for now, as well as making a few writeovers myself. 70.18.182.39 04:24, 17 Apr 2005 (UTC)

Explaining an edit

I took out this html comment:

"Could someone please re-write this to something Stephen King could understand? What information does this paragraph contain?

The universe on large scales appears homogeneous and isotropic. On second thought, this is rather surprising, since it also holds for those parts that are too far apart to influence each other (for example, seen in opposite direction). Technically, this is expressed by saying they are not within each other's horizon which is limited by the speed of light. Homogeneity and isotropy is explained in the theory of cosmic inflation. The idea is, that objects (such as particles with mass) cannot travel faster than light, but space itself can. In the inflation theory it is assumed that space itself dramatically expands (with velocities much larger than the speed of light) in the first few pico seconds after the Big Bang."

I removed it because the information summarized in that paragraph is the same as that presented lower down:

"The expansion of the universe causes distant galaxies to recede from us faster than the speed of light, if comoving distance and cosmological time are used to calculate the speeds of these galaxies. However, in general relativity, velocity is a local notion, so velocity calculated using comoving coordinates does not have any simple relation to velocity calculated locally."

70.18.182.39 04:36, 17 Apr 2005 (UTC)

Cleaned up wording, sectioning, and removed incomprehensible section on .. eh, something. - Ec5618 10:18, Apr 18, 2005 (UTC)

Speed and distance or time

I've been wondering; if adding up velocities isn't as simple as v1+v2=Vtotal, what does that say about the distance covered. Time dilation can't be dependant on the subjective frame of reference, can it. The article doesn't cover this.

So, what is the relation, in relativity, between speed, distance and time?

Ec5618 12:21, Apr 20, 2005 (UTC)

Time dilation is a relative phenomenon, although this seems paradoxical. If you and I have high relative velocities, I see your clock as running slow - and you see my clock as running slow. This means that we can synchronize our watches when we pass each other, but after that neither of us can agree which event happens first: an hour passes on my watch, and an hour passes on your watch. That's because for events that are not at the same location, observers may not agree on which one happens first. --Andrew 12:57, Apr 20, 2005 (UTC)

So, you're saying time is relative. What does that mean about distance? When I accelerate to half LS, and so so again (relative to a probe or something I drop at half LS), will I be moving at full light speed? Will I, and my frame of reference, reach alpha centaury in 4.4 years, or will it take longer? Will people on Earth think I've travelled something like 40 years when I land? Is that what the time dilation means? -- Ec5618 13:27, Apr 20, 2005 (UTC)

This is quite complicated; I think it's better explained at special relativity than I can pull off here. --Andrew 13:54, Apr 20, 2005 (UTC)

"Impossible" vs. "Probably impossible"

An anonymous user revised the opening paragraph to state that FTL was outright impossible under our current understanding of physics. I've replaced this with a more reasonable caveat (I'll agree that the original version that the anon user replaced was badly worded). FTL isn't outright forbidden by the laws of physics as currently known. It just requires fairly exotic conditions, and would have very peculiar side effects. Anything that generates a closed timelike curve in GR will allow time travel, which looks like FTL from other reference frames (the two are equivalent in SR and GR). Further, the laws of special relativity (and, if I understand correctly, general relativity) are compatible with the existence of particles that travel FTL, for which the speed of light is a _lower_ bound to velocity (see tachyon). While it is strongly suspected that some as-yet-undiscovered mechanism will outlaw all closed timelike curves, and that the idea of FTL particles like tachyons is only valid in certain cases (see tachyon condensation), nobody's actually managed to _prove_ it in an all-encompassing manner yet, so it remains an open question. --Christopher Thomas 02:19, 30 May 2005 (UTC)