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The speed of light and time travel

  1. Apr 15, 2012 #1
    So, I'm certainly not a physicist but I think I've just about started to grasp the concept of relativity and spacetime - maybe. A little bit. Hopefully. Anyway, I've always felt the concept of time travel is ridiculous, or at least backwards time travel, and I was trying to prove this to myself. I'm starting to understand why time slows as you travel faster, particularly as you near lightspeed, but does this not effectively mean you cannot ever reach light speed because your relative time would stop - completely? Which I understand as being impossible because if time froze you would be stuck for all eternity and nothing would ever happen beyond that point. Now, this may be a long-winded question (sorry) but I'm hoping to get a reasonably simple answer that I can understand.
  2. jcsd
  3. Apr 15, 2012 #2
    It’s not impossible for things to move at the speed of light. They can and do – light, for instance. And in such cases, no time passes for them on their journey. But only things that have zero rest mass can do this. Everything else is confined to move at speeds less than light.

    If we wanted to accelerate an object with non-zero rest mass up to light speed we’d find that, no matter how much energy we used, we’d just get ever closer without ever reaching the speed of light. Even trying this with two objects and sending them off in opposite directions, and hoping their combined relative velocities would do the trick, wouldn’t work. This is usually offered as the reason why ordinary objects cannot be accelerated to the speed of light - that an infinite amount of energy would be required.

    As you say, as things move relative to us, the rate at which time passes for them, as measured by us, slows down. But that’s only as measured by us. To observers travelling with the ‘moving’ object, their time is running at the normal rate, and indeed, if they measured the rate at which time was passing for us, they’d conclude that our time was running slowly.

    If we allow ourselves a little imaginative licence (well, OK, a lot), suppose a photon of light could carry a wristwatch. The hands on its wristwatch wouldn’t move at all between the start and the end of the photon’s journey. While we would measure the journey to take a finite amount of time, the photon would experience no time at all passing. So photons arriving now on Earth from the Sun have been travelling for over 8 minutes, as measured by us, but to them the journey has been instantaneous. Persisting a little longer with the ridiculous notion that photons could experience anything at all, they’d also notice that the distance between the Sun and Earth had contracted right down to zero. So no wonder it seemed to them to take no time at all to make the crossing. And this, of course, applies to any photon journey.
  4. Apr 15, 2012 #3
    Right, thanks. So, if a photon did have a perspective it wouldn't be within the realms of time or space anyway? It sees no distance as we do? It does not have any concept of time to be moved back or forward through as we do, we just witness it in this way, through other dimensions, our perceived dimensions? Wow, I think that's a little too much to get my head around.
    Okay, well then is this one of the reasons (or another way of explaining) why other matter can't move at light speed, because light has no mass and therefore doesn't really exist in the same realms of physics? Meaning we can't really time travel because we'd have to stop existing in time or space rendering the whole thing fruitless because you're no longer in the universe which we perceive anyway... so there is no time to move backwards through????

    Am I just very muddled now? Perhaps I should give up?

    Basically, mass moving at or beyond the speed of light is impossible and the you can't suggest that if it could then it would time travel because it simply can't work in that way, not hypothetically or anything, it's just a physical nonsensical impossibility. Is that fair to say?
  5. Apr 15, 2012 #4
    The bigger question I think is is light really a thing.
  6. Apr 16, 2012 #5
    You're far from the first person to feel confused about this - sooner or later, anybody thinking about physics says to themselves 'that's just the way the universe is'. So it is with the speed of light. Our universe is one that has a speed limit, and that limit is the speed of light. Could other universes have different speed limits or no speed limits at all? Very possibly, if they exist.

    But light does exist within the realms of physics - very much so. Physics equations decsribe its behaviour very accurately. You might think that just because light has no rest mass, and 'experiences' no passage of time or distance, that it doesn't somehow exist - but this would be a misinterpretation. Light has many effects and properties that we can measure, so, although it is in many ways strange to our common sense, it is every bit as much a part of the physical world as the atoms we are made of.

    Travelling backwards in time by moving faster than light is not physically possible - you just can't get to light speed in the first place (unless your object is first converted into things of zero mass as can happen when ordinary matter completely annihilates with anti-matter, producing photons - so I suppose you can say that an ordinary object can travel at the speed of light - as long as you convert it into light!)

    (And since I've gone all philosophical, Passionflower, I'll suggest that an even bigger question is "What is a 'thing'?")
  7. Apr 16, 2012 #6
    Aha, okay, thank you Goodison Lad. I think I can pretty much put that at rest for now then. But a couple of bonus questions if you don't mind: How can it be that light does not experience any time, yet we can measure it's time to cross a certain distance from our perspective? And, is it then completely impossible (in our universe or known/perceived dimensions) for anything at all to travel faster than light speed, even forces, light etc?
  8. Apr 16, 2012 #7
    Here is a suggestion, something that is observable in the time & spatial dimensions simultaneously, regardless of relative motion.
  9. Apr 17, 2012 #8
    Special relativity is based on two assumptions, or postulates, which can be roughly stated as follows: a) the laws of physics as determined by observers in uniform relative motion are the same, and b) the speed of light is always found to be c when measured by any such observer.

    These postulates weren’t derived – they are, if you like, inspired hunches based experiment and theory. At first sight, the first postulate (sometimes known as the Principle of Relativity) perhaps looks more reasonable than the second.

    But if we take the postulates at face value, and follow their implications to their logical conclusion, we unavoidably end up with all the familiar effects predicted by special relativity such as the Lorentz contraction and time dilation. And the conclusion that no time passes for photons, even though everything else experiences aging, is the most extreme example of time dilation. Another inevitable conclusion, if both postulates are to be true at all times, is that nothing can be accelerated from below light speed up to speed – and therefore nothing can be accelerated to speeds greater than light. The speed of light really is the cosmic speed limit. The postulates lead us to conclude that time and space aren’t inflexible, but alter according to relative motion.

    So why do scientists believe the postulates are true? Because there is a massively impressive amount of experimental evidence that confirms relativity’s predictions (see the FAQ at the top of the threads in this section). Of course, it is always possible that some day an experiment will be done that contradicts relativity. If that were to happen (and there's no sign whatsoever at present), we'd simply have to follow Richard Feynman's (paraphrased) line: "it doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with nature, it's wrong"

    And while I'm in a quoting mood (again paraphrasing) the scientist JBS Haldane siad, “Not only is the universe stranger than we imagine, it is probably stranger than we can imagine”.

    (nitsuj: I reckon, then, that I can probably get away with calling a photon a thing?)
  10. Apr 17, 2012 #9
    Okay, but how? How do you measure the spatial & time coordinates of a wave simultaneously?
  11. Apr 17, 2012 #10
    Well, a photon is considered to be a particle, so if you detect it on screen you have both. For extended waves, you can measure the position of any part of the wave (say, a crest) at an instant. If I'm not careful, we'll be moving into the quantum mechanics threads!
  12. Apr 17, 2012 #11
    That's my slope\

    But I can't escape part of defining quanta (photon = quantum of em) "......one finds the fundamental notion that a physical property may be "quantized,"" from wiki "Quantum/Quanta" entry.

    That's definitely something.
    Last edited: Apr 17, 2012
  13. Apr 18, 2012 #12
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