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Is time travel possible?

Is time travel possible? According to my friend, it is possible to go to the future but not the past.

In Physics, particles move faster than the speed of light,$c$. However, consider a train moving close to the speed of light along the Earth's equator, and a particle moves in that train in the opposite direction of the train's motion. Thus, the speed of the particle with respect to Earth now becomes the products of the two velocities. Since the particle cannot travel faster than light, what happens? Is it theoretically possible to time travel into the future using this concept?

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marked as duplicate by David Z Feb 4 '13 at 6:12

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Discover the cryochamber for yourself and your dreams will become reality. –  Anixx Feb 6 '12 at 11:37
Your wording is very convoluted and hard to understand. Please please reword your question to make it clear. –  Siyuan Ren Feb 6 '12 at 11:47
I don't think Q7823 is a duplicate, because that one explicitly asks about time travel to the past (closed timelike curves and the like). This question seems to be just about interpreting time dilation. –  David Z Feb 6 '12 at 20:50
@David Zaslavsky: yes, you're right. There is also some addition of velocities thrown in as in this question, although OP's question has an extra complication of dealing with an accelerated motion. –  Qmechanic Feb 6 '12 at 23:56

1 Answer 1

up vote 4 down vote accepted

It would have been easier if you had phrased your question better, with line breaks and clearer language. I'm answering my interpretation of your question here, let me know if you wanted something else (Use the comment button below).

Well, first thing let's explain this concept of "time travel to future". You're already doing it. Now. Just by surviving, you are moving forward in time and thus into the future. The main issue is, can we travel through time faster?

Now, time is relative. What you measure as a time interval between two events depends on the position and velocity of the events. If you (or the event-generating object) move with some velocity, you will disagree on time intervals with someone who is at rest with respect to the event generating object. This can be thought of as coming from your example of going-close-to-speed-of-light-in-a-fast-train example. Infact, you will disagree on many things, including lengths and simultaneity, bet that's uneccessary here.

Now, if you go at any speed with respect to the earth, assuming you start from rest and come back to rest, you will age less, i.e., while people outside notice that one year has passed, you might feel that it is only a month. This is probably what you mean by "travelling to the future".

This is not a hypothetical situation, and you do not need to go near the speed of light. It will happen even if you walk very slowly, but then the time difference will be negligible, so we don't notice this.

It does come into play when considering GPS satellites, though (actually their time difference comes from gravity and not their speeds, but the underlying concept is the same). They would not work with accuracy if this dilation of time/contraction of space was not taken into account.

Similarly, it is observed daily in particle accelerators. Particles have a fixed decay time, but particles accelerated close to $c$ live a bit longer.

Now, your question. "Does it make sense?". That's for you to decide. At first, it may feel absurd that time behaves this way, but what's the big deal with time anyways? Think for a moment, why shouldn't you be able to move faster through time? After some thought (and maybe studying of special relativity), it does not feel absurd anymore (to me, that is).

You may want to look at the twin paradox for more info: Short and simple version; more detailed with explanations

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