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WorldWeird: Chinese Physicists Prove Time Travel Impossible

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    • CommentAuthor256
    • CommentTimeJul 29th 2011
     (10075.1)
    @Oddcult

    I think you need to move away from the idea that photons are being "propelled" by their source, be it the Sun or a crappy torch. Moving at c is just a property that photons have, as soon as they're released. The amount of energy the photons carry doesn't have any relevance to their speed.

    To the second point... Quantum stuff isn't my strong point, so I think I'm going to stay out of that one. Could probably do more harm than good. I will say, though, that quantum theory is really not a place to apply common sense reasoning, and I think trying to do so is where we run into a lot of trouble.

    @Purple Wyrm: If you wanted the full rundown -

    e = mc2

    joule = kilogram x meter-per-second2

    kg x m2 / s2 = kg x (m2 / s2)

    ... which all works out nicely.
    •  
      CommentAuthorArtenshiur
    • CommentTimeJul 29th 2011
     (10075.2)
    @Oddcult wrt entanglement: I think you are misinterpreting how entanglement works. It's true that if I measure this entangled particle and it "goes left", the other particle will "go right". But I can't make it go left. It randomly decides whether to go left or right. I am not transmitting information of my choosing. There is no way to put information across; it's more reasonable to say that the same random information appears simultaneously to both of us, which is neat, but is not a telephone.
    • CommentAuthorZJVavrek
    • CommentTimeJul 29th 2011
     (10075.3)
    Artenshiur, that makes the difficulties with entanglement presented already in the thread make much, much more sense to me. A heartfelt thank you.

    256, thank you for the Lorentz graph. I'd always thought of it as "Well, it does take more force to change velocity from 10m/s to 20m/s than it does from 20m/s to 30m/s, just not much more", but hadn't seen the actual graph.

    Ariana, thank you for sharks.
  4.  (10075.4)
    @256 - Excellent! Thank you :)
    • CommentAuthorOddcult
    • CommentTimeAug 2nd 2011
     (10075.5)
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      CommentAuthorjohnjones
    • CommentTimeAug 2nd 2011
     (10075.6)
    @ 256

    2) That c, the constant with that value of the fastest speed possible, turns up all over the place in physics. Sometimes in things that have nothing to do with light or photons. c is really a property of the universe in general, rather than just light in particular.


    What about the idea of tachyons or other "ftl" particles? I understand that nothing can accelerate to c, but are there potential particles who, by their very nature, start out going at c or beyond, that they don't start from a lower speed and go to ftl but begin already going ftl?
    • CommentAuthorOddcult
    • CommentTimeAug 2nd 2011
     (10075.7)
    As I understand it, 'tachyon' is just a name for any hypothetical faster than light particle that may exist. Not that they're something which has been observed and described which does exist.
    •  
      CommentAuthortaphead
    • CommentTimeAug 2nd 2011
     (10075.8)
    Just read Ariana's SCIENCE. I think she should have a weekly science thread.

    (For reals!)
    • CommentAuthor256
    • CommentTimeAug 2nd 2011
     (10075.9)
    You know, I really don't know where the "tachyon" thing comes from. It's certainly not something I've ever heard mentioned by physicists. As per Oddcult, no FTL particle has ever been observed and, as far as I know, there's no particular effect that's been observed which would require an FTL particle to explain it.

    I suppose the idea is that there's some sort of symmetry to having a can't-go-slower-than-light particle to match our can't-go-faster-than-light particles. But it doesn't make much sense beyond that.

    [And, for that matter, if normal particles can move at speeds from 0 up to c, then tachyons should be able to move at speeds from c, up to.. or down to ... ???]

    It seems sort of possible that the whole idea formed out of a cloud of pot smoke sometime in the '70s.

    Without wanting to muddy the waters, there are some effects that "travel" FTL. The classic example is shining a spotlight at a distant wall - by turning the spotlight quickly, you could make the projected spot move along the wall faster than the speed of light. The trick here is that no matter or energy is actually moving FTL. There are other examples, some complex and pretty exotic, but there's never been anything yet that demonstrates a violation of The Law.
    •  
      CommentAuthorAriana
    • CommentTimeAug 2nd 2011
     (10075.10)
    Just read Ariana's SCIENCE. I think she should have a weekly science thread.
    Alternatively: Bob, the Fastest Shark in the World, weekly. Week one in which Bob is threatened by his shadow, because it's always one step ahead of him.
    •  
      CommentAuthorAlan Tyson
    • CommentTimeAug 2nd 2011 edited
     (10075.11)
    Bob the Shark needs his own series of Children's Educational Television shows. Like Barney + Bill Nye + Jaws.

    Also, is it just me, or has it become apparent that, between Sharkpony, Space Shark, Bob the Shark, and Shark Bukkake, that Sharks have become the unofficial Whitechapel mascot?

    Also consider that Si Spurrier, not-so-secretly in love with the mysterious, predatory sea, will soon be taking over as Head Batman.

    Coincidence? I think not.

    Anyway, SCIENCE QUESTION: So, if c is the speed of light in a vacuum, presumably then there are other speeds of light in different environments - what, in non-vacuums, slows light down (besides brick walls, I mean)?

    EDIT: For a second, I thought I'd figured out my own question, thinking the answer was "gravity." But, gravity only affects things with mass, and photons ain't got it... unless, for the purposes of gravity, they do...

    Help.
  12.  (10075.12)
    what, in non-vacuums, slows light down

    Anything that light can pass through. Moving between substances of different density causes a change in speed, which is what causes refraction.
    Gravity does bend light, but yeah, I don't properly get it. Spacetime curvature?

    Edit: Internet says yes, you can express gravity as a curvature of spacetime with the light traveling in a straight line through it.
    Edit2: I'm learning so much today. Apparently light changing speed is due to absorption and re-emission, rather than the photons moving at a different speed.
  13.  (10075.13)
    You know, I really don't know where the "tachyon" thing comes from.


    It began as a thought experiment and is used just as a mathematical formalism to balance equations. Since relativity theory renders their existence impossible they are in the end done away with via tachyon condensation AKA you replace these parts of the equation with math that works in real life.

    Reading the article posted by the op I can just say: Agh, sensationalist pop-science journalism at its best.

    Of course photons don't travel faster than light, Einstein proved that with doing the math a century back. Later, other predictions of his theory were experimentally verified, verifying the rest by extension.

    The reason the work of the Chinese scientists is noteworthy and thus appears in such a prestigious journal like the Physical Review Letters is because the measurement of individual photons is fiendishly difficult from a technical point of view.

    They researched the nature of so called "optical precursors", i.e. photons that travel like the wind that moves ahead of a speeding train "before" other photons. This is called anomalous dispersion and causes the photons, which are both particle and wave, to exhibit a phase velocity (the rate at which the phase of the wave propagates in space) that exceeds the speed of light in a given medium. This doesn't mean there's any true FTL information or energy transfer happening, though.

    Turns out, photons and optical precursor photons don't actually move faster than light, which was pretty much what everyone expected to happen.

    Time travel via something that travels at superluminal speeds is impossible anyway, so no surprise here at all.
  14.  (10075.14)
    So tachyons are a thought experiment, like an imaginary number, only there to balance the theory and ultimately replaced with real life stuff that can actually exist.

    So how about photons? It being a particle and a wave sounds like uber-fudging on the scale of inventing tachyons.
    Isn't it a third thing that hasn't been invented yet and is just called 'a particle and a wave' for now?

    (Disclaimer: I have two B's in Science GCSE and no fucking idea what I'm talking about)
    • CommentAuthorOddcult
    • CommentTimeAug 3rd 2011
     (10075.15)
    Okay, well, if a shark swims really, really fucking fast, like, light speed, then it looks like one looooooongshark is long shark, yeah? But it's really still only just one shark. So, it's both a shark and a longshark. Little things are particles, but long things, even if they just look liek they're long because they're moving at supersharkswimspeed, can behave like waves.
  16.  (10075.16)
    oh dear god
    • CommentAuthorOddcult
    • CommentTimeAug 3rd 2011
     (10075.17)
    To give a slightly more serious answer, it's more to do with the nature of observing light. A photon is a single thing that's moving, but because it's moving so fast then the observable effect it has seems to be continuous to the extent that it can interfere with other things in the manner of a wave. This doesn't mean that it *is* a wave, just that it shows some properties of one when it comes into contact with other sources of light.

    I remember the slits and torch experiment...
  18.  (10075.18)
    You can make it stop any time it gets too much, Warren. We DO have a safe-word, right?

    Ohshit. It's "shark" isn't it?
    •  
      CommentAuthorsneak046
    • CommentTimeAug 4th 2011 edited
     (10075.19)
    So if there was anything with a higher energy/greater velocity than a SHARK, how could we detect it - if we are looking for it with SHARKS then surely they wouldn't be going fast enough to catch up it this FASTER-THAN-SHARK-TICLE?
  20.  (10075.20)
    Things I have learned in this thread:

    -There needs to be a foundation which explains what E=mc^2 means to EVERY CHILD. It has everything to do with converting matter to energy and very little to do with the speed of light.

    -Ariana should be the President of this foundation.

    -Connie Willis is still who I go to for sensible constructions of time travel.

    -BAD Science press. NO. No.


    And finally, I'm so sorry for doing this, but Artenshiur, doesn't that mean you could, say, force entanglement between two molecule-scale telegraphs, and then extrapolate information about telegraph one from telegraph two? "Pressing" the telegraph would place constraints on what its constituent particles could do (e.g., bound its wavefunction) and that would be "communicated" via entanglement to telegraph two, a receiving telegraph. Detectors could then read effects on the constraint and extrapolate the local conditions of telegraph one, communicating data via a nonclassical method.

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