I explained to my friend the theory of relativity and how it can basically be used to time travel, he didn't know anything about it earlier, but he came up with this idea. I'd like to hear your thoughts on it. We take a "train" or tube something of that sorts and we speed it up in a huge underground track to a very high speed, the fastest we can get it. Then we have the tube around it, with it moving, also put to that speed, so relative to the earth it's moving twice as fast as we possible can move it now, we continue this process untill it's fast enough to slow time in a big enough way for us to time travel.
Relativity does say that if you move an object or piece of information from one place to another faster than light it is effectively time travel, because that object could be observed from some frames of reference to have arrived at its destination before it left.
That said special relativity also specifically says that it is impossible to accelerate any object beyond light speed as it would require infinite energy. General relativity makes room for some possible loop holes that get around this by distorting space to travel FTL (think wormhole) rather than actually accelerating the object in a traditional way.
That said while we can't totally rule out all proposed methods of FTL they probably don't work. When you consider the 3 concepts of relativity, FTL travel, and causality you see that only 2 of the 3 can be true, otherwise they contradict each other, so these are your options:
1. Relativity is true, Causality is true, FTL is false - This is the most well supported option and the opinion of most scientists. Relativity has tons of experimental evidence backing it up and common sense tells us the universe is causal.
2. Relativity is false, Causality is true, FTL is true - Although its unlikely we may one day discovery that relativity is wrong somehow. If that's the case and there exists something other than the universal constant C to maintain causality then maybe FTL travel could work just fine.
3. Relativity is true, Causality is false, FTL is true - This would be really weird but maybe the universe isn't as causal as we think, maybe arriving somewhere before you left is just fine with nature.
Ok could you explain better what you mean by time travel then? Because that would slow down time (for observers on the fast moving ship or train or whatever relative to observers on Earth) yes traveling at such high speeds but I wouldn't call that time travel.
Hawking explains that if you're moving at very fast speeds, very very fast, not the speed of light. Time for you would move the same speed but everyone would be going much "faster" same way on how living on another planet is basically like travelling through time
Ok so you're talking about a scenario like the Twin Paradox.
One twin goes on an interstellar journey on board a ship that travels nearly light speed, the other remains on earth, when the first twin returns he has hardly aged but his earth bound brother has grown very old.
Seems weird at first and I suppose on its surface it seems like one of them has time traveled but not really. First of all you should note that both twins would still feel time passing normally for them. Any clocks on Earth would still appear to run fine as would any clocks on board the ship. Only when you tried to compare the clocks would the difference become apparent.
Also while you could argue that the twin who took the trip has "extended" his life span there is really very little practical benefit to extending your life in this way.
As for time "speeding up" or "slowing down" it all depends on how you want to look at it. The guy on the ship would say time was going faster on Earth and the guy on Earth would say time was going slower on the ship. They're both right and nothing really all that crazy has happened, nobodies time lines got messed up, no one jumped forward or backwards in time, they just experienced time differently.
Sending people moving in a train at .7 C would not "speed up" the rest of us, we would just observe them to have "slowed down" through time and they would observe us to have "sped up" through time.
Its all relative I think you are still looking at it as if there is some absolute timeline and traveling fast somehow speeds up time in an absolute sense, it doesn't.