Is time travel possible? Technically no, since time is inseparable from space. The question should really be whether or not one can travel forwards or backwards through a particular path in space-time and return via that same path. As I’m not a physicist, take everything I say with a grain of salt. Still, I can’t help myself from speculating on how time travel might work.
Let us conduct a thought experiment where a time traveler builds a stationary time capsule capable of voyaging backwards and forward in time. As he began to move backwards in time, he would discover that the capsule also simultaneously travels through space because its really going through space-time. Even though we appear to be stationary on the Earth, the universe is constantly in motion. The Earth rotates as it revolves around the sun, as the sun revolves around the galaxy core, as the galaxy moves through an expanding universe. If a time traveler attempts to move through time, he might see his time capsule appear to move as all other celestial objects retrace their paths through space-time. The question is whether the capsule will move through space in the same way that it moves when time is moving forward at its “normal” rate.
In the first case, we consider motion backwards in time at the same rate that one normally moves forward. This requires some kind of relativistic measure of speed. For instance, going backwards at a “normal” rate would equal minus one second per second (-1/1). Going backwards at ten times normal rate would be minus 10 seconds per second (-10/1). If the capsule traveled at a normal rate of -1 sec/sec, then we can presume that all other forces will be the same but opposite. Any movement would have the same acceleration in the opposite direction.
If the traveler starts to move at an accelerated rate through space-time, would he experience changes in the forces of gravitational acceleration (or effects of the shape of space-time) to account for the compressed nature of time? I don’t believe so, because time would move ten times as fast, but the gravitational force (acceleration) should also be ten times as strong due to the compression of time. If the capsule was located on the surface of the Earth, the increase in gravitational acceleration should enable it to stay in the same position. The normal force of acceleration is 9.8 meters per second. At -1 sec/sec, it should be -9.8 m/s and at -10 sec/sec, it should be -9.8*10 m/sec. So, acceleration through space should take place at the same rate as acceleration through time. Any relative motion taking place when the time travel starts should continue at an accelerated rate.
But what would the time traveler perceive? According to the theory of relativity, as one approaches the speed of light, time slows down for the traveler, but his perception of time does not change. Time also slows down in the presence of large gravitational forces (masses), as described in the movie Interstellar when the astronauts landed on a planet located near a black hole. But how would the traveler perceive movement backwards in space-time? Would it like be rewinding a video feed? I doubt that reverse acceleration could be infinitely fast, so time would first have to decelerate to zero before starting to go backwards. If the traveler’s time appears to stay the same while the outside world starts to slow down, that is the relativistic equivalent of having the Earth accelerate away from him at the speed of light until time appears to stop for them.
But that is not an option. We just want to move through time but stay in the same spacial position. Making time go slower on Earth would also be the equivalent of increasing the mass of the Earth or reducing the mass of the traveler such that time moves more quickly in the capsule relative to the Earth. Increasing the mass of the Earth and everything on it does not sound like a viable option. Not only would people feel heavier and be unable to move, they would also see celestial objects speed up because time would be moving slower on Earth. Somehow, the traveler would need to shield himself from the relativistic time-slowing effects of mass in order to speed up the time in his capsule until time on Earth appears to stop. In effect, the ship would have to neutralize gravity, which is associated with mass. Maybe antimatter can neutralize matter and gravity by creating anti-gravity and speeding up the passage of time. In this case, the traveler needs to be able to generate enough antimatter within the ship to counteract gravity. Scientists suspect that antimatter will attract both matter and antimatter, but this has not been experimentally confirmed.
NOTE: Neutralizing gravity seems to be what UFOs are able to do. Not only have they been sighted hovering with no known form of propulsion, they have also been seen accelerating at an extremely high rate. The only way to physically survive such high rates of acceleration is to change the passage of time on the ship so that it doesn’t feel like the acceleration is really that fast. It sounds to me like UFOs must be speeding up the passage of time within their ship.
But, back to time travel. Even if one could speed up time within the ship until time appears to stop outside, how would one break through the barrier and actually start to move backwards in time? Hmmmm. Maybe if the presence of mass (matter) slows time, the presence of antimatter does something even weirder rather than just neutralizing matter and speeding up time. I can’t quite figure out how that would work other than to speculate that the traveler would have to pass the point of zero gravity/mass until it experiences negative gravity due to the presence of an excess of antimatter. Maybe negative gravity results in a relatively infinitely fast passage of time such that where the outside world actually moves backwards.
If the traveler starts to move backwards at ten times normal speed, would he also feel the increased force of gravitational acceleration (a -10G force)? If so, this would probably cause him to pass out or rip him or his capsule apart. Or would the passage of time and the forces of acceleration seem to stay the same for his own frame of reference? What does negative gravity due to antimatter feel like? Instead of attracting objects together, does it force them apart, much like the universe seems to be expanding at an ever accelerating rate?
It might be too dangerous to travel through time while stationary on the surface, since other objects might move through the same space and destroy the capsule or be destroyed by the required increase in the mass of the capsule. Storms, fires, moving objects or people, or other threats to the capsule might emerge, making it too risky. For this reason, it might be safer to travel backwards in time from a position in space or at least above atmospheric disturbances.
However, if the capsule were located in space and was in motion, it might not stay in the same position. If the capsule was in space on the leading edge of the Earth (i.e. in front of its absolute direction of motion through the universe) and moving slightly away, his capsule might increase its acceleration away into space as the Earth retraces its path backwards through the universe and the capsule lacks an equal force to accelerate it backwards with the planet. The speed of this relative motion would correspond to the speed of the change in time. If the capsule were moving slightly towards the Earth when reversing time, the capsule might increase its acceleration towards the surface and crash.
If the traveler moved relatively quickly through time, the Earth would change its relative location even faster and its speed of motion would differ from the speed of the time traveler after being accelerated by gravity. The traveler would move relative to the planet unless he was in perfect geosynchronous orbit. If he reversed the direction of time back to the forward direction, he would find himself accelerating back in the opposite direction.
If the time traveler was located on the trailing edge of the Earth (i.e. in back of its absolute direction of motion through the universe) and moving slightly towards the Earth, his capsule would accelerate down as the planet retraces its path backwards through the universe. If the traveler attempts to move too quickly through time, the capsule may crash into the Earth at a high velocity, resulting in its destruction.
Let’s assume that the traveler has a stationary capsule, but no spaceship. In this case, he would have to anchor the capsule to the ground and limit the speed of his motion backwards in time in order to attempt to avoid interacting with objects that might damage or destroy the capsule. As he travels back through time, he will eventually encounter other objects or sentient beings as they attempt to move through the same physical space. This will, of necessity, change the course of time as these objects or individuals are forced to alter their course as they hit or go around the object. For instance, when the capsule moves to a point in space-time where it becomes visible to another human, the course of that human’s future will change or diverge into a new path through space-time. As the capsule continues to move backwards in space-time, the time at which the other human first sees it will also change, so he will not be able to react to its appearance. Only by moving forwards again in space-time will the traveler see the effects of his travel on the future course of time. If he tries to move forward again in time, the human observer may attempt to interact with the object, including the possibility he may attack and destroy it.
The next question to ask is whether there would now be two possible paths forward through time. Could one find the path originally followed backwards or only the new, still unknown, path created by the effects of the backwards motion?
It should now be obvious that cosmic geometry would be an important factor in travel through space-time. It would also therefore be necessary to be able to perform calculations to project the path of motion through space-time before attempting such travel. In doing so, a spaceship would be advantageous as it would enable a traveler to remove himself from the proximity of large gravitational bodies and to avoid crashing into such objects during the trip.
Assuming the time traveler obtains a spaceship, he will most likely want to move into the open space in front of the planet as it moves through the expanding universe. Then, as he begins the process of reversing space-time, he will be able to start navigating his ship without immediate fear of collision. The better his ability to navigate a course and make real-time corrections, the faster he will be able to travel backwards through time. Navigation at high rates of time change would require high speed processing, so such a spaceship would probably require an automated navigation system programmed to avoid obstacles and follow objects of interest as they move relative to the ship.
Sounds pretty complicated to me, but then why wouldn’t it be? The very act of changing the direction and speed of time would have to require some powerful kind of device anyway, so it seems obvious that this would require a large space ship capable of manipulating matter, antimatter, and their resultant gravitational or anti-gravitational forces (or warping of space-time). Einstein says it isn’t possible to exceed the speed of light and that you can’t go back in time. He is probably right, but I’m not sure he had sufficient knowledge of antimatter or the possibility of anti-gravity. If it is possible to travel backwards in time, it still would probably require some mind-bending technology and a pretty advanced space ship. Is it worth the effort? Probably not.