Is Time Travel Possible? (part 1 of 2)

I will take a couple of weeks break from quantum mechanics to talk about the possibility of time travel. As it happens, I was asked a few questions for an article in the popular science magazine Science Illustrated in Denmark. The instructions were to keep the answers short, but I could add additional info to be used as seen fit by the editor. Also the explanation level should avoid being technical. This generated an interesting exchange which I will show in this and next post. Enjoy.

1:

Is it – in theory – possible to travel back in time? Does nature allow such time travel?

2:

If so, will it ever become possible to construct a time machine capable of transporting human beings back in time?

3:

And if this might be the case, how is paradoxes like the grandfather paradox prevented?

1, Is it – in theory – possible to travel back in time? Does nature allow such time travel? The answer to the possibility of traveling back in time is not yet known, but time travel is highly unlikely. Einstein’s general relativity theory – a very successful physical theory at large scale - allows many solutions which exhibit time travel but general relativity is at odds with quantum mechanics –a very successful theory at small scale - and so far there is no known physical theory which consistently combines them. There are several proposals being considered, like for example string theory, but only when such a theory will be validated by experiments we could have a definite answer to the possibility or impossibility of time travel.

Background info:

It is important to understand how time travel solutions occur in general relativity and what it means. Einstein’s general relativity equations are local laws and they do not forbid global behavior like traveling back in time. Since space and time are not rigid, they can be twisted and stretched by the presence of mass and if you continue doing it in certain ways, you can eventually manage to turn time back on itself. This is not unlike how one can turn a car all the way around in an empty parking lot. Then all sorts of paradoxes can occur and to understand them physicists studied for example how billiard ball games can be played in the presence of a time machine. To avoid paradoxes, the billiard ball may collide with its younger self in a self-consistent manner, but here is the catch: consider replacing the billiard ball with an egg. When it collides with its younger self it will go “splat” and create a paradox. The only way to prevent the paradox is for the egg not to break, and this means that: global consistency conditions required to avoid paradoxes imply non-physical local behavior and this does not agree with our current knowledge of nature.

2:

If so, will it ever become possible to construct a time machine capable of transporting human beings back in time?

The answer is a double no. First, assuming that time travel is actually permitted by nature, you can only go back in time to the very moment a time loop was created. In other words, nobody will be able to go back in time to witness the extinction of the dinosaurs, or the invention of the light bulb. Second, the energy required to bend space-time on itself is of galactic magnitude and you need to harvest the energy of an entire galaxy to bend space-time on itself. A more practical approach is by creation of a wormhole, but this requires negative energy and while negative energy is a real possibility, when you generate negative energy you have to pay it back with interest. To create a macroscopic wormhole large enough for a human to pass through you need yet again an immense source of energy.

Background info: The reason enormous amounts of energy are required is that gravity is the weakest force in our universe. Mass is equivalent with energy (E=mc²) and you are required to have a large amounts of mass (corresponding to even larger amounts of energy) to bend space and time. Titanic was hard to turn by a small rudder. Turning space-time all the way to itself is extremely hard with weak gravity. And how do we know gravity is a very weak force? After all it does not look that way when we fall for example. Consider a magnet on a refrigerator. The small magnetic attraction between the magnet and the sheet of metal can easily overcome the gravitational pull of the entire planet.

3:

And if this might be the case, how is paradoxes like the grandfather paradox prevented?

There are only two paradoxes generated by time travel: the grandfather paradox and creation of information from nothing. For the grandfather paradox there are two solutions: “the banana peel type solution” and the splitting the world into multiverses in quantum mechanics. Here is how they work.

In the grandfather paradox, you go back in time and you try to kill your grandfather thus preventing your own birth. But suppose at the key moment of the murder when you want to shoot your grandfather you step on a providential banana peel, slip and miss. And no matter what you try, there is always something which goes wrong and nature always conspires against you. Another solution is the multiverse idea in quantum mechanics. The moment you shoot your grandfather, the universe splits in two identical copies, one in which you fire the gun, and one in which you don’t fire the gun (call them universe A and universe B). The bullet from universe A jumps into universe B and kills the grandfather in universe B. Since that was not your grandfather in universe A there is no contradiction in universe A. In universe B, a bullet out of nowhere kills the grandfather which prevents your birth in universe B. But because you did not fire the gun in the first place there, universe B is also free of paradoxes.

For the creation of information out of nothing, the paradox goes as follows: As a young person you meet an old person who hands you the blueprints of how to construct a time machine. You work your entire life building it and as an old person you take the blueprints with you, hop in the time machine, and go back in time handing the blueprints to your younger self. So far there is no contradiction, but who wrote the blueprints in the first place? This paradox has no known solution.

Background info: As farfetched as it sounds, the splitting universe solution is actually correct and is based on real science. Splitting the universe in quantum mechanics is one of the several interpretations of the theory and since other interpretations are possible it can be taken as a narrative which can help visualize complex mathematical computations. The banana peel arguments may seem to contradict free will but here is a simple counterargument (I think originally given by Novikov http://en.wikipedia.org/wiki/Novikov_self-consistency_principle ): it is my free will to walk on the ceiling but the laws of physics prevent it.