Time travel into the future is easy, just sit on your couch and enjoy the ride. There is a lot of science fiction movies and TV shows that have time travel but it is extremely rare for one to do time travel in a way that is logically consistent.
Einstein came up with a theory Special Relativity. Special Relativity makes the claim that if an observer is moving faster, then the passage of time is slower relative to an observer moving slower. For example, if you have a guy sitting on the couch, a guy jogging by outside on the sidewalk and another guy driving by in his car; the guy in the car will be aging the slowest, and the guy sitting on the couch will be aging the fastest. The clock in the house will be ticking faster than the clock in the car. The difference in velocity between the clock in the house and the car, is so small, that the time dilation would be too small to detect. This phenomenon has been confirmed with satellites, which have a much larger difference in velocity and satellite operators have to reset their clocks to compensate for time dilation. If a spacecraft is traveling at 99% of the speed of light to the nearest star and back, the passengers may only experience days while those on Earth would have experienced 8 years. Unfortunately, traveling at relativistic speeds isn’t easy. A group of passengers could hypothetically travel up the 99% of the speed of light, do a lap around the stellar neighborhood and come back, only having aged a year and Earth will be a hundred years older.
The more conventional time travel would require a wormhole. Wormholes are mathematically possible IF there is negative energy but none of the current theories predict that negative energy is even possible. Assuming negative energy is possible, there is way to manufacture it, manufacturing it is efficient enough to do it within the lifetime of our civilization, we master wormhole construction and it’s possible to connect wormholes to the past; there are only a few plausible ways time travel could work.
There is an underlying assumption of science, that things make sense, that there are rules and if you understand those rules, you can predict the answers. It’s like arithmetic. If you understand the rules, 1+1=2 and 2+5=7 then you can take two numbers you’ve never added together before, like 46546+4 and figure out the right answer, which is 46550. Science is basically the philosophical concept of Determinism, that everything is cause and effect. If you know the state of a system at a given point in time, have an understanding of how the system works, you can predict the next state, and the next, predicting the future 100% accurately for all eternity.
One scenario of time travel is there is only one universe, our reality. You can go back in time using a wormhole and make a change and then travel back to your time of origin. When you return to your time of origin everything would be exactly the same. Your present was already effected by you going back in time and making a change, you just fulfilled your destiny. If you went back in time and killed your grandfather, then you would never be born, and you would never go back in time and kill your grandfather, then you would be born. This would create an unstable universe and unstable universes would probably never come into existence in the first place. One theoretical perspective is that a universe is created, beginning to end, at the same time. It’s more like a song that has already been recorded and is on your cellphone. You’ve never heard the song before, you are playing it second by second, but the song has already been sung. Any time travel into the past, would have already happened and already effected your present. If it is possible to go back and kill your own grandfather, then for whatever reason, the universe would just never have a single being try, because any universe that did, would simply never have come into existence in the first place. It’s possible you went back in time and thought you killed your grandfather, but you were actually adopted and the newspapers were wrong about your grandfathers death.
Another version of time travel is based on the Many Worlds Interpretation. The Many Worlds Interpretation is an attempt to solve a problem in Quantum Mechanics where it appears to break Determinism. In Quantum Mechanics there are certain things like the position and momentum of an electron, the spin of particles that is undetermined until it is observed. Imagine you have a dice, you roll it on the table and it falls off behind the table. You can’t see what side it landed on but you know it landed on a side. The side it landed on would depend on the angle and velocity it hit the table, the hardness of the die and table, the material of the floor, etc. If you had enough information and understanding, you knew the state before it went out of sight, you should be able to calculate what side the die landed on. In Quantum Mechanics there are areas that seem to break this rule. If you roll the die, it rolls off the table, out of sight, it does not land on the floor. Instead it is hovering in mid air, just out of sight. When you get up to look, as soon as you see it, it falls to the floor and lands on a side but the side it lands on, was not determined by the preexisting state, it seems to be random. A determinist would say randomness is an ignorance of an underlying order. Einstein once said, “God does not play dice with the universe”. The assumption is that it is not random, that there is a greater system beyond our ability to observe, and if you knew the state of the whole system, it would be predictable. MWI is an attempt to restore order by saying there are alternate realities. Whenever one of these quantum waves of probability are collapsed to a specific reality, it doesn’t just land on one side of the die, it lands on every side. There are six alternate realities and in each one, the die landed on a different side. To extend this to a time travel scenario, imagine you create a wormhole, go back in time, make a change, then return to your point of origin. Everything would be different but you didn’t change your time of origin. The moment you arrived into the past, you caused the universe to branch off, creating an alternate reality, where a time traveler arrived. Your arrival already changed the past. Any change you make, would not change your time of origin, but the same date of the newly created alternate timeline. The energy of the universe may have been divided in two but since everything is relative, the universe would not notice. If everything in the universe got equally smaller, how would you notice? If the universe was created, beginning to end, all at once, then you already went back in time, already created a new alternate reality and you were just fulfilling your destiny. The universe would not branch into an alternate reality for just anything, just certain quantum events, which are happening an astronomical amount of times per second. We would have to assume the energy of the omniverse is infinitely divisible. If time travel was possible and there were infinite alternate Earths; ones with humans, ones with intelligent dinosaurs and even intelligent marsupials; and the future has already happened, you would think there would be millions of time travelers arriving every second from millions of different alternate futures and there would be no universe with enough sentient beings to enforce any kind of time travel laws.
There are no wormholes in Fractal Federation but time travel is an interesting topic to confuse myself with.