Ever since Einstein developed his Special Theory of Relativity, the speed of light has been seen as the ultimate limit of how fast we can hope to travel. This theory has been with us for over a century now, and it seems to hold wherever it is tested. So I'm not trying to overthrow special relativity, but I feel that it might not be the whole story. Below I list a number of things which suggest that the speed of light might not be such an absolute limit.

### General Relativity

Special relativity is all very well, but to describe space-time fully you need the more advanced theory of general relativity, and that suggests that there may be ways to exceed the universal speed limit. For instance, it has been shown that Wormholes are a valid solution to the equations of GR, and if you have a wormhole then you can jump from one place to another in an instant. Also the 'Warp Drive' of Star Trek might be more than just fiction - examples have been devised within the mathematics of GR. It's true that the realisation of either wormholes or warp travel seem to require some sort of exotic matter, and we don't know whether such matter might be possible. But there's a lot we don't know about quantum gravity, and when we know more we might find that it allows this sort of matter.

### Bell's inequalities

You're probably familiar with Bell's inequalities, which are seemingly violated in certain quantum mechanical experiments. This implies that on the one hand it is impossible to transmit information faster than light, but on the other, if you want to model what is going on then your model must include faster than light information transmission, just hidden from any possible detection. The standard response to this is 'Yes, well the universe is a pretty weird place'. Well that's all very well while you're reading the works of Douglas Adams, but shouldn't we be trying to interpret the universe in the simplest possible way? And that would suggest that there may well be a new physics which underlies that which we presently know, in which there is an absolute frame of reference, and in which faster than light travel is indeed possible.

### Cosmological Event Horizon

You may have heard of galaxies travelling away from us faster than light in another context. Anything beyond the Hubble sphere can be said to have superluminal recession speed. However, these are still visible, with finite redshift, and my feeling is that this is due to the choice of coordinates used, rather than being an example of true FTL motion. |

Recent results in cosmology show that not only is the universe expanding, but that this expansion is accelerating. This means that there will be what is known as a Cosmological Event Horizon. As our observations approach this horizon, so the objects we see will be more and more redshifted, until the redshift is infinite at the horizon itself. But there is matter beyond the horizon, which might be thought of as having greater than infinite redshift, and so to be travelling away from us faster than light. However, some of this matter was once within our Cosmological Event Horizon, and so the Dark Energy which is driving the acceleration must have somehow pushed it beyond the light barrier. If we could find out more about this Dark Energy, then maybe we could use it for FTL travel of our own.

### The enigmatic Neutrino

It used to be thought that, like light, neutrinos had no rest mass, and so always travelled at the speed of light. However, recent experiments indicate that they do in fact have a small rest mass. Hence, although they move very fast, their speed will always be below that of light. Thus there is the possibility of overtaking a neutrino. Now neutrinos have spin, and the peculiar thing is that the spin is always in the same direction - every neutrino is left-handed. But if we went fast enough to overtake a neutrino then it would appear to be right-handed. But it's a fundamental tenet of relativity that nothing in physics depends on how fast you are travelling. The neutrino suggests that there really is a distinction between low and high speed travel, thus undermining relativity and its prohibition of faster than light travel.

### Doubly Special Relativity

The units of measurement we use in everyday life such as the mile or the kilogram are fairly arbirary. Using what we know about physics, it is possible to devise much more fundamental units of measurement. These are known as the Planck units. The Planck length is 1.6 × 10

^{-35}metres, and can be thought of as the smallest possible length. This, however, causes a conflict with special relativity, since if an object's length was equal to the Planck length in its rest frame, then relativity tells us that it would have a shorter length in other frames of reference, which is impossible. This might not seem so important, since we don't yet have a theory of quantum gravity, and one would expect such a theory, when devised, to have some way of resolving this problem. What Giovanni Amelino-Camelia did was to turn the problem around and examine what could be deduced starting from the assumption that this problem must disappear. Thus*Doubly Special Relativity*was born. I won't go into the details of this theory here, but I just want to note that it's another example of a problem with special relativity as it was originally devised.### Summary

The above examples suggest to me that all is not well with special relativity. It may be fine for physics at the scales we have so far examined, but it looks to me as if it might be a theory which emerges from a more basic underlying physics - one in which there is an absolute frame of reference, and the speed of light is not the ultimate limit. It's true that none of the above are cast iron arguments, so maybe the light barrier will win out in the end. But I wouldn't give up on you're dreams of interstellar travel quite yet.

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