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Asaf Pe'er

Physics for all

Is time travel possible?

Yes !! ... and no...

The thing is, that there is no physical law that prevents you from travelling into the future. The thing you can never do is travel back in time. Thus, if you want to discover now what will be 100 years from now, you can certainly do so (if you have enough money to invest in building the appropriate engine). But you better say goodbye to all your friends, because, unless they travel with you, you would not see them again, since you would not be able to return back in time to tell them how their future will look like. Unfortunately to many sci-fi movies...

Einstein's theory of relativity tells us that time is relative to the observer. That is, if I see an object moving with respect to me, If I look at a clock carried by this object (say, astronaut), I see it slows down. The faster (= closer to the speed of light) the object (astronaut) moves, the slower its clock will appear. Note though that the clock can never stop - unless the astronaut travels at exactly the speed of light, and it can never turn back. However, from the perspective of the astronaut, there is nothing wrong in her clock! It works as usual.

As a concrete example, let's say that you manage to build an engine that travels at 99% the speed of light. Note that there is no physical law that prevents you from building such an engine - it is merely a technical problem to design and build it. At such a high speed, a clock that you carry with you will appear to someone here on earth to slow down by a factor of approximately 7 (7 years of my time will pass until 1 year of your time will). Now, say, you build such as engine, and you use it to travel to a star which is 50 light years away. From the perspective of someone on earth, the time it takes you to get there is 50 / 0.99 ~ 50.5 years. Thus, if you travel there and back, the time on earth will progress by 101 years. However, your time will progress by 101/7 ~ 15 years. Thus, when you return, you will be 15 years older than your age when you began your journey ; but anyone else will be 101 years older, namely, most of your friends will be dead (or very old), unless technology will increase dramatically life expectance in the next 100 years. Thus, practically, if you take such a journey you jump into the future. But, as I stated, if you ever do so, there is no return.

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