Here is a physicis't angle on time travel:
1) Be aware that we are already travelling in time. Consider Einstein's definition of time as the 4th dimension. When we travel through space, we continuously change our position in some arbitrary x, y and z axes. But we are also changing our position along the time (t) axis. In fact, we advance through time at a constant speed of one second per second! Every second, we travel foreward in time at a distance of exactly one second. So countrary to x, y and z where we can vary our speed, travel through time is restricted to a single speed, one sec. per sec., and in a single direction: foreward.
2) When we travel through space, we do not magically teleport from one point to another, instead we gradually change our position at various speed, so that we pass through every point between our origin and our destination. This excludes quantum teleportation, which is another story for another day.
3) So for me, time travel means that we gain the ability to change our travel speed through time. It is not different from travel through space. If I want to time travel into the "future", I will have to increase my "speed" through time. If I want to time travel into the past, I will have to reverse my direction of travel.
4) Time travel through the future: So let's pretend that we can construct a mechanism that allows us to increase our travel speed through time. What happens? Say we advance at 10 seconds per second. In this example we would see everything around us evolve ten times faster than normal. The people around us would see us as well, but they see us behave in slow motion, by a factor of ten. Once we reach our "destination time", we resume our normal speed through time. Of course, with this process it is impossible to go back in time. We are stuck in the future. On the other hand, with this process there is no chance of meeting an older version of ourselves. We are the original. There is no paradox, and no violation of conservation of energy (more on this later)
5) I have more trouble justifying time travel in the past, since it involves reversal of time travel axis. So let's say we reverse our speed through time, so we go at -1sec. per second. Everytime the world moves foreward by one second, we move backward by one second. From our point of view, we see everything around us go backward, like a videotape that we rewind. But there are problems: I) Travelling backward in time does not mean we travel backward in space. Which means that the inertia that we possess initially (say our velocity due to Earth's rotation etc) is carried with us. But, when we see the world go backward, they also go backward in space, which means we see the Earth travel backward in its trajectory while we continue to travel foreward. Consequence: we are expelled from the orbit of Earth, violently. So on top of travelling backward in time, we have to travel backward in space relative to Earth. II) Now there will be a paradox, as we gradually travel backward we will exist along with the younger copy of ourselves. If not then it would mean we are changing the world by the mere act of travelling backward. III) Worse is the problem of conservation of energy. The total energy of the Universe is constant. This is a sacrosaint law of physics. When we travel backward, we are carrying our energy (E=mc^2) with us. We are introducing this new energy in the "past", and we are taking this energy away from the "present". So we actually violate conservation of energy twice. This is not acceptable.
- For those who want a teleportation type of time travel, as in I magically disappear from the present and appear in the past/future, please remember to account for energy conservation: you need to add energy in the time frame you left and remove energy to the time you visit, equal to your mass times the speed of light squared. Somehow none of the time travel movies seems to take this important little detail into consideration.
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