To understand this, let’s start with a short story. Once Stephen Hawking organized a party, with flowing champagne and trays covered food. But no one arrived until he sends an invitation after the party. Actually, this party was meant only for time travelers.
According to him if anyone in the future would have seen his invitation then must have shown up at the party. Therefore time travel is not possible. Like we know Stephen hawking had organized a party similarly people in the future will know too, so if time travel is possible, they will travel back in time to attend Stephen Hawking’s party. But no one did.
But wait there is something more called “Grandfather Paradox“. In this, a person who is a time traveler goes back in to meet his grandfather and killed him before the birth of his father or mother. This means if the time traveler’s father or mother doesn’t exist then that time traveler too won’t exist. And this goes on.
Time is relative according to Einstein’s theory of relativity which means if you and I synchronize our watches and I went into space and then come back, there will be a slight difference in time shown by your watch and my watch, this is called time dilation or in simple I did time travel at a very very very minute scale.
One more example, when astronauts living in space station came back to earth after 6 months, their age was 0.005 seconds less than those of us on the earth. Although it’s really small there is a difference. One more example, suppose you are traveling at the speed of light to the destination 10 light-years away, how much time will you take to reach? 10 years?
No, you will take 4.4 years. How? When you will travel towards your destination, your destination too will travel towards you this will reduce the time, although if someone sees from the earth, it will appear that only you are traveling towards your destination, so with respect to earth, you will take 10 years. Confused? Let’s get back to simple.
In simple it’s possible to travel into the future if somehow we achieve a speed of light, but traveling back into the past is more complicated. If there exists a wormhole there is a possibility of going back to the past. So concluding all if and else, time travel is/not/hopefully/ possible.