Science fiction novels and movies have put forward the concept of time travel for a long time. However, it seems as if it is not quite possible. Firstly, there are two kinds of time travel: traveling back in time and traveling into the future.
Talking of traveling back in time, it is utterly impossible. Here a concept of causality comes in. In simpler words, causality refers to every “action” having an “effect”. Each of these effects brought about by a specific action draw a line into the future, and every event in the future would unfold on that line.
Suppose someone needs to move back in time and fix their mistakes. Once they have fixed their mistakes with a separate set of actions, those actions would draw a separate line. Following that line of events would lead to a different future where the person had never made a mistake in the first place. If the person had never made a mistake, they would not have then felt the need to move back into the past. Hence, causality strikes going back in time off the list.
Moving on to traveling into the future, this is slightly possible. Everything, whether living or non-living, is traveling into the future at any given moment. Everyone travels a second into the future as the clock ticks. However, as velocity is relative, when everything is moving into the future every passing second, that means no one is traveling into the future.
This means that if someone tries to move into the future a little faster, they time-travel into the future. This is proved by the concept of time dilation. For someone on a flight, their time would pass by more quickly, than for the one waiting at the airport. However, this time-travel is very minute and is next to being none.
Speed is the only factor behind time travel that puts forward a query that if humans build a machine that could reach extreme speeds, they can build themselves a time machine to take them into the future. The speed needed to actually travel into the future is the speed of light and the fastest machine developed by humans up till now is capable of reaching speed up to 0.064 times the speed of light. Therefore, humans are not as advanced yet to build a time machine that could take them into the future.
Traveling physically into the past is impossible, however, looking into the past is quite simple. It takes light nearly eight minutes to reach the Earth from the Sun. Hence, Sun as seen from an eye is from eight minutes back in time. Similar is the case when Canis Major dwarf galaxy is observed through a telescope. It is 25,000 light-years away from Earth, which means that 25,000 years old version of Canis Major dwarf galaxy is seen from the telescope at any given time.