Is time travel possible? Time travel into the future is possible but time travel into the past is most likely not.
It is difficult to say if something is possible or not because we can only judge the likelihood of it being possible with our current knowledge and technology. There is no way of knowing what humans in the future will learn and what will be possible. If you asked people a thousand years ago whether some of the things we can do today would be possible or not, they would not have thought so. However, with that being said, time travel into the past is not very likely to be possible.
Time travel into the future is possible. In fact, we are all time travelling into the future as we speak. By the time you finish reading this article, you will be three minutes further into the future than you are now. But that is obviously not what we mean when we talk about time travel. We want to know if it is possible to jump into the future. It is possible to travel to the future because time is relative. Time is relative because it depends on the speed and the gravity of the person observing it. What does that mean?
Time is relative means that it varies depending on where you are standing and how fast you are going. And by how fast, we mean percentages of light speed fast. The faster you go, the slower time moves. This keeps increasing until you get to light speed where there is no time at all. That is difficult to understand but for a light photon, there is no time at all. When light leaves our sun it takes it 8 minutes and 20 seconds, but that is from our frame of reference. For the light photon it is instantaneous because there is no time at light speed. The same is true for light that takes 4 years to reach us from Proxima Centauri is 4.246 light years but, again, that is from our frame of reference. For those light photons it is instantaneous again.
What that means is that if you want to time travel into the future you have to go much faster. We cannot travel at light speed because that is impossible for anything with mass, but one day we might be able to travel at 50% of light speed. If you could travel at half the speed of light, time would pass roughly 15% faster than people you left on Earth. That means, if you flew for one year, when you came back you would be one year and 54 days in the future. You would have time traveled by two months. The faster you go, the greater this difference becomes. If you travelled at 90% the speed of light, your time difference with the people you left behind would be about 44%. If you travelled for one year again, you would come back 1 year and 160 days into the future. If you could get to 99% the speed of light, 7 years would pass for every one year on Earth. So, this is one way that you can travel into the future.
A second way you can travel into the future is by going close to a very heavy object, such as a black hole. Very heavy objects have an enormous gravity and they curve the fabric of space. Space is often described as a sheet with heavy balls on it. Larger balls stretch the sheet more. When light passes a heavy object it has to travel further because of the stretch in space, so it takes longer to pass the heavy object. That means time slows down as you go past large objects. If you want to travel into the future, all you have to do is get close to the edge of a black hole where time is slowed down. Depending on the size of the black hole, one year near the black hole could be hundreds of years on Earth. We probably can’t get to 99% the speed of light, but getting close to a black hole might be possible. These are two ways we can travel into the future.
So, it is theoretically easy to go forward in time. Why is it impossible to go backwards? The main reason is because of the second law of thermodynamics, which is entropy. All things in the universe move towards a state of entropy. Organized things head towards the random and energy dissipates. A good example of this is when you pour milk into a cup of coffee. To begin with, you have two organized systems. You have your milk organized as milk and you have your coffee organized as coffee. When you pour milk into the coffee, the two organized systems mix and become disorganized. This is the way it is all across the universe and it is the reason why travelling into the past is impossible. To go back into the past, the state of entropy would have to reverse. Random things would have to become ordered. The milk would have to separate itself out of the coffee. And that is not possible. Once entropy has happened, the only way to undo it is by reordering things and that would take enormous amounts of energy. Plus, even if you could separate the milk atoms from the coffee atoms, you would be doing it in our present, not in the past. As far as we know, and with modern technology and knowledge, time travel to the past is not possible. And this is what I learned today.
Sources
https://www.amnh.org/exhibitions/einstein/time/a-matter-of-time