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Say I write following statement on a paper:

It's year 2014 at earth.

Then I give it to my friend. My friend then starts to leave me and earth to outside with velocity of light. Einstein says the time stops for my friend.

In my next year, the statement is false but it's true for my friend because there is no any time interval for her to change true to false.

Then the statement seems to be true and false in same time.

Do "change from true to false" need time to happen?

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    $\begingroup$ What has relativity to do with it? If Newton wrote "It's 1766." and you read the statement now, does that make the statement false or true? $\endgroup$
    – Raskolnikov
    Commented Jan 20, 2014 at 20:02
  • $\begingroup$ Also, read this and carefully restate your hypothetical: en.wikipedia.org/wiki/Relativity_of_simultaneity $\endgroup$
    – Zo the Relativist
    Commented Jan 20, 2014 at 20:05
  • $\begingroup$ Well first of all, the statement would be wrong since your friend has left the Earth. Secondly, I do not understand why you are asking this; there is no fundamental significance to any of this. $\endgroup$
    – turnip
    Commented Jan 20, 2014 at 20:06
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    $\begingroup$ Your friend can not leave at the speed of light, but it is true that no time passes on luminal paths. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Jan 20, 2014 at 20:38
  • $\begingroup$ @Raskolnikov, I do not know if time stops for a death man or not...it's my question...do an absolute statement need time to change from true to false? $\endgroup$
    – Yasser Zamani
    Commented Jan 21, 2014 at 10:50

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The problem here is that the statement "The current time is 'x'" CANNOT be globally true. It is true only with respect to a given reference frame, so what you've implied with the statements involved in your question (i.e. that it is either 2014 or not 2014 everywhere) is simply invalid in relativity. "At earth" doesn't cut it, since the earth is an extended object and velocity matters.

Also, the friend cannot travel at the speed of light.

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    $\begingroup$ OK, but I do not understand! Simply suppose I have no any respect to any frame but an absolute statement can globally be true or false, I think. The question "What year alive people on earth are calling?" can not have two or more answer...I'm some confused and sorry, I'm not expert in physics. $\endgroup$
    – Yasser Zamani
    Commented Jan 21, 2014 at 10:56
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I think you are asking "Are there statements whose truth or falsity does not depend on a frame of reference?", or rather, "is the same in every conceivable frame of reference?"

Without getting into metaphysics, clearly there are tautologies like "129 = 129". It's hard to think of a frame of reference where that's not true.

If you want to dabble a bit in information theory, where you ask "how likely is statement X?" there are a couple answers:

  • In Shannon information theory, information is based on probability, and probability is always in the eye of an observer who has a frame of reference, whether explicit or implicit.

  • In algorithmic information theory, information is based on program lengths, like how long of a program does it take to print such-and-so. That's interesting because it depends on what language the program is written in, which acts like a frame of reference. However, there is the concept of universality in computer languages. Any computer language, to be non-trivial, needs to be universal - a theory that has stood the test of time. Any universal language can essentially be translated into any other, so actually no frame of reference is fundamentally different from any other.

So - good question.

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  • $\begingroup$ Thank you so much! those two questions were what I tried to ask which you improved them in physics context. If you suppose me as observer, so (the statement is false and true for my friend in same time XOR for my friend, there is no need for time interval to change true to false). am I right? $\endgroup$
    – Yasser Zamani
    Commented Jan 24, 2014 at 16:45
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Un-ask the question. You seem to be applying the word truth as a binary proposition (IE. it's a rib-eye steak or it's a Chevrolet) and it is based on an assumption that any single truth must be true for all.

It is a question of frame of reference, truth having no relevant or scientific meaning here.

Every object is moving in relation to every other and that goes for every atom in the entire universe, relativity applying to each and every one. As JohnnyMo1 sais, nothing travels at the speed of light (not even light, but don't get me onto that!)

Tau (http://en.wikipedia.org/wiki/Tau) may approach zero with continuous propulsion, but even this for the length of the universe will not reach C, only approach it. Using E=mC^2 it would take approximately the average male (59Kg) more than Eta joules to apply the force needed to accelerate and decelerate his body to get within a fraction of a percentage of the speed of light. The closer to that speed the more energy is required to make significant progress.

Further, to do achieve anything like this within your lifetime, he would be subjected to such forces of acceleration/deceleration - same thing), that all you would get back would be a layered organic mass, unrecognizable as human.

BTW give my regards to your hypothetical and mushy friend. :)

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  • $\begingroup$ Thanks for your regards :) I'm not expert in physics and I got -2 vote :( anyway I love these thinks! $\endgroup$
    – Yasser Zamani
    Commented Jan 21, 2014 at 10:57

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