The many problems of time travel

Time travel is a popular concept in works of fantasy, with many different flavors. In the Back to the Future trilogy, Doc Brown built a time machine out of a DeLorean just because he could. In Harry Potter and the Prisoner of Azkaban, Hermione Granger uses a time-turner so she could study every subject in Hogwarts, and even Superman turns back time to undo the death of Lois Lane.

Seems like Superman really is quite super. But scientifically speaking, how much sense does time travel actually make?

Let's start from the most obvious type of time travel: forward in time. Clearly we are all automatically traveling towards future time by default, since we age and seasons shift and whatever. But the interesting thing is that according to the theory of relativity, it is possible to slow down the passage of time for yourself, while everything else around you continues to age normally.

This effect is called time dilation, and it is an important effect in the universes dynamics. Anything that is within a gravitational field, or moving, will experience the passage of time slowing down, which is equivalent with time accelerating for everything around you. So theoretically speaking, it's totally possible to build a contraption that flies really fast, jump in, and fly around the globe for a while and then jump out of it to find yourself (back) in the future! Though you would have to fly very close to the speed of light to achieve any noticeable time travel, but still, totally possible.

Sounds a lot like what Superman did, right? Well, not quite. The big difference is the direction of time, which brings us to the problematic part: traveling backward in time. To do this, Superman needs to fly faster than the speed of light, and you just know that's gonna cause a whole lot of problems.

First of all, to exceed the speed of light, Superman would require an infinite amount of energy. This is caused, again, by the theory of relativity. It states that a massive body, such as Superman (pun intended), will approach infinite kinetic energy as it approaches the speed of light. Then, logically speaking, he would never even reach light speed. But what if he is so super that he actually has an infinite supply of energy?

Then we come to problem number two: before Superman has the chance of reaching the speed of light, his mass would exceed the limit needed to collapse into a black hole, and he would... well, collapse into a black hole. Given the fact that he was flying so close to Earth, that would be rather worrying. Thanks Superman!

But what if he has some kind of mechanism in his body that prevents the formation of a black hole? Let's say that he is really super and can reduce his mass on will, or that he has internal pressure that prevents gravitational collapse, or something. Surely then Superman can move faster than light, turn back time and make poor Lois undead?

Now we find problem number three: If you do exceed the speed of light, you will propagate backwards in time. Back to the point in time when you didn't yet break the absolute speed limit. Oh dang it.

So everything we know is against any attempt to go backwards in time with the Superman method. But there are conceptual difficulties as well. Probably everyone is familiar with the famous grandfather paradox. The paradox states that if you go to the past and do something that causes your grandfather (or any other direct relative) to die, then you will not exist in the time where you came from, which in turn means that you would have never time traveled, and ergo, your relative would have not died because of you.

This by itself is one of the biggest problems in traveling backwards in time, and there are oh so many attempts to reconcile the paradox. But I don't think this is even the biggest problem.

Let's say that a time machine that goes back and forth in time is possible and humans are smart enough to build them. Let's also say that this technology becomes accessible to the masses, as technology always does at some point. In that case, we would have a nearly infinite influx of people from different times flooding our time and every other time as well, because people are curious creatures.

Since we don't see such things happening we can conclude one of two things: 1. Traveling backwards in time is, and will always be impossible, or 2. One can at most look to the past, but not physically travel there.

So there you go, time travel as it is presented in science fiction is almost definitely not possible. But I hope that this didn't ruin time travel as a fantasy genre for you. Although they usually don't make any physical sense, I occasionally enjoy a good mind bending film or book on time travel and it's unlimited possibilities.