Faster than light travel might be possible – but only over very small distances. And only because the light speed that we think of as absolute is actually already being slowed down. Here's why you’re not getting the light speed you’re entitled to.
We think of light speed as the upper speed limit on objects in the universe. What we often forget is that it’s perfectly acceptable to go below the speed limit. Whenever light hits a transparent material it slows down – whether that material is air or water or diamond. Generally speaking, the denser the material, the more light slows down. This slowing is a technicality; the photons don’t actually drop below what we consider light speed, they just interact with a lot of objects on the way. The massive amount of detours and interactions between one end of, say, a thick prism of glass, and the other end of it causes the light that travels through it to be measured as going at slightly less than what physics books list as the constant “c.” The only place that light is completely free to move at speed c is the vacuum.
Or is it? Recently we’ve talked about quantum foam. This arises from the masses of tiny particles that physicists think pop in and out of existence in the vacuum. Those tiny particles should also interact with light as it moves through the supposed vacuum. Granted, the interaction should be very weak, but it should still slow down light.
This might be an entirely theoretical point if there were no circumstances under which the density of the particles in the vacuum lessens, but there is one instance when they might. In the mid-1990s, scientists observed the Casimir Effect. All the particles in the vacuum can also be described as waves. Put two plates close together and there are fewer waves that fit between them than waves that fit to either side of them. They’re pushed into each other.
This space between the plates necessarily has few particles than the space around them — or than any part of the vacuum of outer space. One physicist, Klaus Scharnhorst, figured that the lower density of particles popping in and out of the space between the plates had to allow light to go faster than what we think of as light speed. Gabriel Barton, another physicist, came to the same conclusion independently. Obviously, testing this is a technical challenge.; even if we do confirm it, the tiny distance over which it works leaves faster than light travel still theoretical rather than practical. But it might be a way to go faster than the light speed we know.