Time paradoxes aside, there are two other reasons that argue against time travel.

First of all, we must remember the whole universe is in motion. If we want to go back to yesterday, our "time machine" has to move back in time and space. If someone claimed he was at home when he suddenly travelled 200 years back in time, he either had an illusion or made the story up. Two hundred years ago, Earth was in a different place in space, so how can you travel back 200 years without moving in space? Don't get me wrong. I am not knocking the genius of science fiction writers. H.G. Wells' "The Time Machine" is a great work of fiction, but a fiction nonetheless. I have read many other time travelling science fiction stories since reading H.G.Wells, but none address this problem of spatial displacement or the other problem I am going to present.

My own argument for the impossibility of time travel is that physical states of the past no longer exist and those of the future are not here yet. To be able to move back and forth in time requires everything that has happened, is happening, and will happen everywhere in the whole universe at every single moment in time–past, present, and future–to be stored as a "reality" somewhere—like the save game file of a computer game that contains every single byte of information of the game at the point it was saved, except you would need an almost infinitely large file and almost infinitely many of them–so it can be re-entered and interacted with, and not just light signals for viewing. That, to me, is not possible. But then, there is so much about time and space that we don't know yet.

Yes, we are all looking at the past when we look at something. The clock that Einstein looked at every morning on his way to work showed the time as it was millionth of a second ago, as it took time for light reflected from the clock to reach him. When we look at a star 100 light years away, we are looking back one hundred years. But the information of that world, and indeed, the whole universe in which that clock tower or that distant star was in, was not stored somewhere as a "tangible reality" that one can enter and interact with.

Without getting too technical, let me state categorically that I am fully aware of the negative delta-t problem in Einstein's special theory of relativity when an object, such as the theoretically postulated tachyon, travels faster than the speed of light, the quantum time reversibility equation, and the Einstein-Rosen bridge. But until we have a resolution to this "sheets of time", like individual frames on a reel of film, time travel remains in the realm of science fiction.

DS/JWC/Yeti Sep 27th, 2010