Happy Back to the Future Day!
In honor of today, I thought I'd make a post about time travel.
So, is time travel possible?
That's a hard question to answer, so let's look at what one of our brightest minds have to say on the subject.
There a two possibilities that Stephen Hawking puts forth in this lecture. To understand both of these theories we need some background info into Einstein's General Theory of Relativity. The basic concept is that time exits as a fourth dimension. This makes sense if you, for example, wanted to describe an event. You'd have to give the location, for which you use the first three dimensions, and also the time, which would count as the fourth dimension. The connection between time and space may not seem inherent, but Einstein also figured that the time of an event was dependent on the speed of the observer. In doing so, he cemented the connection between time and space. His finding also meant that if two observers moving at the same speed would agree on the time of the event, but the greater the relative speed between them, the more they disagreed. So, if one could increase the relative speed faster than the speed of light, we may influence the time of the event enough to travel back in time. The problem is that Einstein found that the rocket power needed to reach the speed of light became exponentially higher the closer one go to the speed of light. So we would need infinite rocket power.
So the first possibility deals with Comic Strings, a cosmic phenomenon that sci-fi writers don't seem to have caught onto yet. Cosmic Strings, Hawking tell us, can be seen as rubber bands. These "rubber bands" would be under about a hundred billion billion billion tons of tension, and be able to accelerate at a speed close to the speed of light. If you had two of the strings, perhaps the relative speed may be enough to allow for time traveling.
The second possibility is the sci-fi fan-favorite wormhole. The idea of wormhole is that we bend space-time so far that it gets saddled shaped:
If we could do this, we could travel faster than light. Hawking argues that if the ends of the wormhole were moving at relative speeds, we could travel back in time. How do we go about this? We would need negative mass and negative energy density. Quantum Theory allows for the existence of negative energy so long as a surplus of energy exists elsewhere. Therefore, in the future, with the right technology, we may be able to build a wormhole and travel back in time.
This, of course, posses a paradox. If time travel is possible in the future, why hasn't anyone visited from the future yet?