Three theories of time travel

Time travel has always been a part of many science fiction shows and movies. The idea of time travel became popular when Albert Einstein proposed his famous Theory of Relativity. According to this theory, time is a dimension and is affected by both "gravity" and "speed". When you're closer to the surface of Earth, the gravity that you experience is stronger and hence time slows down for you. Similarly, when you travel at higher speeds and try to approach the speed of light, time slows down. This is the very reason why astronauts at the International Space Station travel to the future. At the space station, astronauts experience weaker gravitational force which fastens the time for them relative to people on Earth. However, the speed at which the International Space Station orbits around Earth is too high (7660 m/s). The higher speed slows down the time for the astronauts while the weaker gravity fastens the time for them. However, the factor by which time has slowed down or dilated due to the speed is much greater than the factor by which the time has fastened due to weaker gravity. Hence, the overall effect is that the time is slowed down for the astronauts. Thus, their clocks tick slower as compared to our clocks. (In other words, when our clocks show that one second has passed, their clocks show 0.9999 seconds (say) have passed.)

Astronauts experience time dilation at the International Space Station
      There are three theories in which time travel in the past can happen. Time travel can either happen in a fixed timelinedynamic timeline or through the Multiverse.

1. Fixed Timeline:

According to this theory, you can travel back in time in a fixed or given timeline. That is, when you travel to the past, the future will remain unchanged. Your actions in the past have already become a part of the history.

For example, consider that you travel to the past and kill Adolf Hitler as a baby so that the second World War is prevented. However, you replace him with another baby so that the family does not notice and then you travel back to the future, and the replaced baby himself grows up to become Adolf Hitler.

2. Dynamic Timeline:

Grandfather paradox is one of the reason why past time travel is not possible

Here, the actions in the past have an effect on the present. Suppose that you travel back in time and kill your Grandfather. As a result, you prevent your own birth. That is, now, you cannot travel back in time. In turn, your grandfather is never killed and you are born again, you go back in time and kill your grandfather anyway. This loop will keep on continuing infinitely and will result into a paradoxical situation.

3. Multiverse:

If Multiverse exists, you could time travel to the past!
    If Multiverse exists, there would be a possibility of infinite alternate timelines. When you travel back to past, a totally new timeline would be created different from your original timeline. Here, you could do anything, kill anyone and only the new timeline would be affected. There would be no effect on the original timeline.

There would be no paradoxes. If you kill your grandfather, you will not exist in this new timeline. A new Universe will be created where you would not exist. However, you cannot return back to your original timeline.

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