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Implications of constant speed of light in all inertial frames

  1. Jan 22, 2014 #1

    Lately I have been wondering about the implications of the speed of light being constant for all observers, to me this seems to imply that there are different versions of reality for different observers. I will use a scenario to illustrate my reasoning. After you read this I would like people to tell me if my reasoning to come to this conclusion is valid or if it is erroneous and if so, how.


    Two space ships (ship A and ship B) which are both capable of instantly attaining light speed are about to race each other while a stationary observer watches. As the race commences the observer sees the two ships racing side by side, as they are both traveling at light speed, resulting in a tie. However from the perspective of an observer on board ship A, ship B shoots ahead at light speed, winning the race. Similarly, from the perspective of an observer on board ship B, ship a wins the race.

    So you have three different realities for three different observers, each of which involve different race results which are summarised as follows:

    From the stationary observers perspective, the race is a tie

    From ship A's perspective, ship B wins the race

    From ship B's perspective ship A wins the race

    Please let me know if this is accurate and if not how so.
  2. jcsd
  3. Jan 22, 2014 #2


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    Wow. Not only light speed, but instantly. Sorry but massive ships cannot attain light speed.

    Relativity makes no predictions about such a perspective. Even if you replace the massive ships with photons, you cannot construct a valid frame of reference of the photon.
  4. Jan 22, 2014 #3

    Thanks for the reply, I am talking theoretically, so please disregard technicalities like ships not being able to achieve light speed.
  5. Jan 22, 2014 #4


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    This is the theory.
  6. Jan 22, 2014 #5


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    I'm not sure what "different versions of reality" means. If it means that the notion of "now" is different for different observers, this conclusion is correct. I'm not sure that's what you actually had in mind, but at least something that could have been what you had in mind is true.

    But let's now look at the reasoning to led you to you're conclusion.

    Ooops - you're reasoning has already started from a false premise, that there are ships that move according to the theory of relativity that are capable of reaching light speed instantly. It is (or should be) fairly well known that material objects can't ever move at "c" according to relativity.

    Unfortunately, once you've started with a false premise, any conclusions you draw from cannot be relied on. So your logic doesn't hold up. Sorry.
  7. Jan 22, 2014 #6
    Thanks for the reply, I'm not literally suggesting that ships could actually travel at light speed, I'm using this unrealistic example for the sake of simplicity, I guess to make the scenario more realistic I could have had the two ships gradually speeding up in tandem to near light speeds, I'm not really asking for a critique of the scenario itself, rather whether the end result of different observers observing different realities is possible.
  8. Jan 22, 2014 #7

    D H

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    That is not a mere technicality. It is a central concept.

    As pervect just noted, you are starting with a false premise. I'd say it's even worse than that. You are essentially asking us to answer the question "what do the laws of physics say will happen if the laws of physics are 100% wrong?"
    Last edited: Jan 22, 2014
  9. Jan 22, 2014 #8


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    If they are in tandem the front one will win.

    No, all valid observers must agree who wins.
  10. Jan 22, 2014 #9
    What's your problem with "instantly"? There is no limit on acceleration.

    And about reaching light speed, yes, that's not possible, but it is possible to get arbitrarily close to it.

    To the OP, you are wrong in the part that each ship thinks the other will win. But you need to provide more formal specifications if you want details about why.

    For example, if both ships are located at the same spot, after they accelerate they will see the other one still with an apparent velocity 0 relative to them, and so they will reach the destination together (or rather, in this point of view, the destination will reach them)

    If they start an equal distance away from the destination on opposite sides of it, each will see the destination racing towards themselves at speed close to c, and their competition racing towards it at speed even closer to c, and it would even seem that the competition started the race in advance, but still having very hard time catching up to the destination, and in the end they still finish together.

    The math behind relativity is proven consistent, and all observations for the order of time-like separated events will always match.
  11. Jan 22, 2014 #10


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    As others have pointed out, you can't have a race between two spaceship traveling at the speed of light but you can certainly have a race between two photons or two bursts of light. They will travel at the speed of light and as long as they start together and follow the same path, they will arrive at the finish line together.

    But on the way, if you want to consider how each one observes the progress of the other one, you have learned that the speed of light is the same for all observers and so they would each conclude that the other one is going to win. And this leads to a logical contradiction: we can't have all three of your cases be true.

    So what do we conclude? Simply that photons or bursts of light cannot be observers. Technically, when we say that the speed of light is the same for all observers, we mean that in the rest frame of all observers, the speed of light is the same. Now I think it should be obvious to you that a photon or a burst of light cannot have a rest frame because we would be saying that the photon or burst of light is both at rest and traveling at the speed of light. That is a logical contradiction in and of itself.

    Your scenario is a good example of why photons or bursts of light cannot have their own reference frame and why they cannot be observers.
  12. Jan 22, 2014 #11


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    This is a theoretically possible scenario which can be analyzed with SR. If you do so, you will find that all three observers agree that it is a tie, although there do exist observers which disagree.

    Nobody ever is, but sometimes it is only possible to correctly answer a question by pointing out a fundamental problem with the assumptions in the question.

    Although some observers may disagree on whether it is a tie or not, that does not imply that different realities are possible. The same reality holds, but who wins is not a part of reality, it is merely an observer-dependent label placed on a given scenario.
  13. Jan 25, 2014 #12
    No DaleSpam, there can not be any disagreement regarding who wins no matter the observer. The two events have the same space and time coordinates in one reference frame, and they will transform equally no matter how wonky of a transformation you try to come up with. One can even say they are basicly a single event, not two.
  14. Jan 26, 2014 #13


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    The two events don't have the same space coordinates because the OP said the space ships were racing side by side. That means a fourth observer traveling along the finish line will not see the race as a tie.
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