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Causailty and the speed of light

  1. Jul 23, 2014 #1

    I would be really grateful if someone could help me with two questions I have, both have puzzled me on and off for years. I have read what I can regarding these points, but a lot of it I don't understand (the maths specifically). I would love it is someone who understands this more than me could take a few minutes to help me understand what I am missing/misinterpreting.

    The first question can best be described with a thought experiment....

    Imagine you have two planets 1 light year apart. Lets call them planet A and planet B. Exactly half way between them is a small star and on both planets there is a person observing that star.

    At the moment both observers see a huge solar flare erupt from the star they fire a huge flare up into low orbit and a spaceship takes off from Planet A traveling to planet B at exactly the speed of light.

    The observers on each planet then turn their telescopes towards each other and zoom in.

    Over the next year the observers on each planet will watch each other with time passing normally. To start with they see each others planet as it was a year before and after one year of observing they will each see the other turn their telescope towards each other and the flare go up into low orbit.

    That bit I get :-) What I don't get is what happens from the spaceships point of view and why people say that that if it travels faster than light it can 'travel back in time' effectively.

    Whilst it is traveling at the speed of light, the pilot looks out the front window. I believe he would see events on the planet he was heading too happening twice as fast as they were for the people on that planet. Effectively when he launched he was 1 year behind events on planet B, and by traveling at the speed of light he is effectively just catching up.

    He looks out the back and he see's events on the planet he has left as completely frozen solid. They are frozen at the moment just after he hit the speed of light departing from Planet A

    E.g. nothing is actually happening. Basically he started at planet A one year ahead of what people on planet B would see when looking at Planet A. and when he arrives at Planet B and stops then looks at planet A out the back window he see's events resume but as there were when he left 1 year before. This is in agreement with what people on Planet B see.

    If he'd travelled at twice the speed of light events would have been going past 4 times faster than normal in front of him, and actually going backwards when he looked at the back window at planet A.

    When he arrives at Planet B he looks back at Planet A and sees things as they were 6 months before he leaves, in complete agreement with what the people on Planet B would see.

    Even if he turned round and flew back to Planet A faster than the speed of light he still couldn't arrives back before he left because as he was flying towards planet A he'd see events on planet A going past faster than normal, meaning he'd still arrive after he left.

    Even if he travelled at 100, 1,000 or 1,000,000 time the speed of light he still couldn't travel in time.

    Its all just an optical effect created by the speed of light and time appearing to distort.

    Basically, even if you could beat light, you still couldn't beat time.

    So, that is how I visualise things. I just cant see how traveling faster than light could allow time travel? What am I missing????

    Thank you!!!!!!!!!

    PS. I'll post the second question once I've found an answer for this one, my head hurts now! :-)
  2. jcsd
  3. Jul 23, 2014 #2


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    Relativistic effects are not "optical effects", but what is left after you already accounted for the finite signal speed.
  4. Jul 23, 2014 #3
    Thank you for the response. I'm afraid I don't follow though?

    If I understand you correctly (which I probably don't, probably more down to me than your reply)doesn't accounting for the finite speed of light in this frame of reference account for everything? Leaving nothing un-accounted for?

    If this is how it works then what room is there for time travel?
  5. Jul 23, 2014 #4


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    The relativistic effects (time dilation, length contraction) are still there if the signal delay has been factored out. More generally: The Lorentz transformation tells you what happens in a certain reference frame, not what somebody sees visually.
  6. Jul 23, 2014 #5
    I shall do some googling and reading and be back once I have educated myself (or at least tried too!).

    Thank you.
  7. Jul 23, 2014 #6


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    If for any observer, two events are separated by more space than by time (for example, they happen at two points one light-year apart, but within six months of one another) then there will exist some observers for whom the two events happened in one order and others for whom they happened in the other order. If the two events are "I start my journey" and "I arrive at my destination" then we end up with absurdity that some observer finds that I arrived before I left. That's not quite time travel, but is effects happening before causes.

    If the two events are not separated by more space than time (say I'm traveling a distance of one light-year, but it takes me more than one year to make the journey) then all observers will agree about the ordering of the two events: the departure came first, and there is no absurdity.

    By far the most widely accepted interpretation of these facts is that effects cannot happen before cause, and therefore that causal influences cannot travel faster than light.

    Google for "light cone" for more information.
  8. Jul 23, 2014 #7
    Thank you :-)

    That is really helpful and puts it into a language I can understand. I do understand light cones, as per my example in the original post I just didn't use the words :-)

    So if someone refers to faster than light travel leading to time travel should I just take it that they really mean 'effects before causes', which isn't really happening. Its just appearing to happen if you don't take into account the time it took the light to travel.

    E.G. In my head I see it as "just because you don't know that something has happened yet doesn't mean it hasn't happened"
  9. Jul 23, 2014 #8


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    It isn't happening, because nothing travels faster than light.

    If you extrapolate the math of relativity to faster than light, then it predicts that 'effects before causes' would actually happen. Not that it would just appear to happen optically.
  10. Jul 23, 2014 #9


    That was the sound of my head exploding :confused:

    Ok, let me mull over that for a bit.


    So (please correct me if I am wrong!) what you are saying is that nothing actually happens until it is observed to happen?

    Is this just a prediction of theory or is there actual experimental evidence to show this is the case? (Cant think what kind of experiment you could create to test it without FTL travel though?).

    This just feels so completely wrong and counter intuitive ! !?! !! ??

    'Common Sense' would say that when light reaches you from an event 1 light year away that event MUST have happened 1 year before.

    So, one year before the light reaching you that event must have happened or the light wouldn't reach you 1 year later.

    As such it must have happened 1 year before even though you didn't observe it.

    Therefore I cant get my head around this -> "It isn't happening, because nothing travels faster than light."

    The event happened, the light took a year to reach you. Just because you had to wait a year to see it happen doesn't mean it didn't happen. In fact it had to happen 1 year before or you wouldn't have seen it a year later.

    I really appreciate this by the way, thank you :-)
    Last edited: Jul 23, 2014
  11. Jul 23, 2014 #10


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    You're right that it's not about time travel. That only comes up when people (usually non-scientists) are speaking informally and not trying to be precise.

    However, as A.T. says above, the math of special relativity unambiguously predicts that some observers would find some effects happening before their causes if the things could travel faster than light. There's no "appears" to it, not if we accept the basic principle that if light emitted from an event five light-years away hits my eyes right now, the event must have happened five years ago. (If you cannot construct a result-before-cause example for yourself, speak up and someone here will be able to point you to one. There are a bunch of examples in previous discussions, I just don't have to time to look them up right now).

    This in turn leads to so many hopeless logical contradictions that we conclude that faster than light travel is impossible. The argument is basically a proof by contradiction: Valid logic plus the assumption that causal effects can travel faster than light leads to absurd contradictory unphysical results, so the assumption has to be wrong.
  12. Jul 23, 2014 #11


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    That is absolutely correct. However, if you consider two events that are separated by more space than time (the technical term is "spacelike-separated") you will find that different observers travelling at different speeds relative to one another will come up with different orderings of the two events.

    Google for "Einstein train simultaneity" to see why.
  13. Jul 23, 2014 #12
    Thank you :-)

    I have done some googling and further reading. I can find quite a few references to effect before cause but they all seem to be based on quantum effects which don't seem to have anything to do with traveling at or past the speed of light. For example..


    In that experiment the effect was based on a pair of particles being entangled or not, or more specifically the prediction of their state being made before the observation of their state.

    I cannot find anything regarding relating to FTL travel, but I will keep looking.

    I have looked up "Einstien train simultaneity" and I remember reading about this before. There is a problem with this though as I see it..


    When we say that the lightning strokes A and B are simultaneous with respect to the embankment, we mean: the rays of light emitted at the places A and B, where the lightning occurs, meet each other at the mid-point M of the length A —> B of the embankment. But the events A and B also correspond to positions A and B on the train. Let M' be the mid-point of the distance A —> B on the travelling train. Just when the flashes 1 of lightning occur, this point M' naturally coincides with the point M, but it moves towards the right in the diagram with the velocity v of the train. If an observer sitting in the position M’ in the train did not possess this velocity, then he would remain permanently at M, and the light rays emitted by the flashes of lightning A and B would reach him simultaneously, i.e. they would meet just where he is situated. Now in reality (considered with reference to the railway embankment) he is hastening towards the beam of light coming from B, whilst he is riding on ahead of the beam of light coming from A. Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A. Observers who take the railway train as their reference-body must therefore come to the conclusion that the lightning flash B took place earlier than the lightning flash A. We thus arrive at the important result:

    That last sentence bugs me.. why? It is only true if you assume that the observer doesn't know they are moving. If they know they are moving they may just as easily arrive at the conclusion that they don't know if the strikes took place at the same time. Or if the know their precise velocity and distance from the strikes they would come to the conclusion the strikes happened at same time.

    Ok you could say that that information isn't available to them during the experiment, but they have access to the timing of their perceptions of the flashes of light. Shouldn't they have access to all the information regarding the circumstances? I'm pretty sure you could create all kinds of interesting thought experiments if you based them on someone/thing making an observation and drawing a conclusion with only partial information.

    See, this is what I am thinking..

    1) In the example in the OP the scenario fits with my every day experience. Sure I have no experience of traveling at anything close to the speed of light but it fits with what I know of every day life.

    2) Special Relativity predicts clearly that events before cause would be possible at faster than light speeds, however there has not been from what I can see a demonstration of effect before cause due to travel at of past the speed of light. Only those based on quantum experiments which had no relation to velocity.

    So, is there any actual validation of Special Relativity's prediction of effect before cause due to travel at or past the speed of light?

    What about particle accelerator experiments? Don't the particles in them get closer to the speed of light than pretty much anything else with mass that mankind has access too?

    I'm sorry if this questioning is annoying, but I am trying really hard to understand and you are being really helpful :approve:

    EDIT:- "Observers who take the railway train as their reference-body" the implications of this has just twigged something. But it still feels wrong. If they are affected by something outside of their frame of reference shouldn't their frame of reference include the thing they were affected by? It seems wrong to construct an experiment (thought or otherwise) where you remove part of the information from the person/thing you require to make a logical deduction about the result.
    Last edited: Jul 23, 2014
  14. Jul 23, 2014 #13


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    What do you mean by validation? SR predicts that it is impossible. You can refute it by demonstrating the opposite.
  15. Jul 23, 2014 #14
    I was going from Nugatory's comment.

    He says that you were right and that special relativity predicts that effect before cause is possible.

    I was wondering if there have been and validations by experiment of this (although not sure how??) The only effect before cause I can find relates to quantum effects which have nothing to do with velocity.

    EDIT :- Ok, I think I see what you are saying. Special relativity predicts that traveling at or faster than the speed of light is impossible. Based on your previous comment it also predicts that effect before cause would be a problem.

    Yet there is no experimental evidence to support special relativity's prediction about effect before cause due to FTL travel as it can't be tested anyway.

    Does that not mean though that we cannot use the argument about special relativities prediction regarding cause before effect as solid and concrete argument for precluding travel at or faster than the speed of light? It cannot be tested and a far simpler approach of time dilation in the direction of travel and time (whatever the opposite of dilation is) in the direction away from the direction of travel makes more sense?

    Ok sure it may be prevented by other effects, like disappearing into your own singularity due to energy/mass equivalence and the gains associated with traveling at almost the speed of light.

    But as we cant test it, doesn't that mean we actually don't know what would happen IF you could travel at or faster than the speed of light?
    Last edited: Jul 23, 2014
  16. Jul 23, 2014 #15


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    As others and the FAQ has said, we know that if you could travel at or faster than the speed of light, relativity would be proven wrong. It's not something you can "fix up", you'd need a new theory of physics.

    A possible loophole to this is wormhole travel.
  17. Jul 24, 2014 #16
    Thank you everyone!

    This all makes a lot more sense now :-)

    I really appreciate your time.
  18. Jul 27, 2014 #17
    An excellent old causality violation illustration involving ansibles

    Violating causality is equivalent to having an “effect before its cause”, which is a form of “sending information into the past”, or “receiving a message from yourself before you sent it”. So a good way to understand how the theory of Special Relativity applied to a postulated device that can send information faster than c (for simplicity, let’s say send instantaneously, with an infinite velocity, and in a nod to Orson Scot Card, call this device an “ansible”) shows a message being received before it’s sent is to describe it in a thought experiment.

    This excellent blog article, written in 2003 by “physicist, software engineer, entrepreneur, extropian and science fiction fan” Richard Baker, does just that. Its conclusion is illustrated in this diagram
    In it, Alice send a message to Bob using an ansible, Bob sends it to Carol, who is near him, using any means, Carol sends it to Dave using an ansible, and Dave, who is near Alice, who is near him, using any means.

    The only thing I’d change about Baker’s article is adding specific distances to the example. In drawing the diagrams, a specific value, 0.4 c (119 916 983.2 m/s), is given for the speed V of Carol and Dave relative to Alice and Bob, but not for the distance D between events between event P and Q (which is the same as between Alice and Bob as seen by them, or Dave and Carol as seen by them), so a specific value for the time (duration) T between event R and P (Alice getting the message and sending it) can’t be given.

    Throwing in a nice, plausible value for D of about 700 light-seconds (210 000 000 000 m, the distance between Earth and a body sharing its orbit around the Sun 1/4th of a revolution away, we get a T of 280 seconds. So, if Alice and Bob are using this set-up to, say, cheat on the Lottery, Alice would have to be prepared to buy a ticket within 4 min 40 sec, less the time it took her future self to note the number and compose and click “send”, and the time it took Bob, Carol, and Dave to repeat it, of getting a message from her future self with the winning number.

    Note that the choice of an infinite speed “ansible” is for simplicity only. Any device that sends a message faster than c will work. For finite-speed devices, the slope of lines P-Q and R-Q will be greater, and for the same V and D, T will be less.

    Also note that imagining Alice, Bob, Carol and Dave as people is just a nicety. Any or all of them could be automatic machines. No sort of quantum mystical special “observer” status is needed in this thought experiment.

    In summary, you need more than just a body or signal traveling faster than c to describe a simple causality-violating thought experiment. Simpler examples might be possible, but I think Baker’s is about as simple as can be made.
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