We've all seen science fiction on TV where travel between the stars is considered to be routine. So when is it going to happen? Unfortunately, the fact is that interstellar travel is a lot harder than it appears on TV. We've used rockets to send men to the moon, and probes to the planets so why not the stars? Well for rockets like the ones we have at present - forget it. The stars are so much further away that it's never going to work. What if we could build more powerful rockets? Well the British Interplanetary Society did put forward project Daedalus, which was a fusion powered mission to the Barnard's star, one of the nearest stars. This is just about within the bounds of conceivable technology (it does require getting Helium-3 from Jupiter though), but it still takes about 50 years. Hardly the stuff Star Trek is it? Even using antimatter, the best possible energy source, doesn't do much better.
Warp drive and Wormholes
So what possibilities are there for faster travel? One sci-fi concept for crossing the galaxy is the wormhole in space-time. You go in to one mouth and come out the other. All very well if you happen to find one, but if not then you have to make one - is that possible? Well it's allowed by general relativity, but to be able to manipulate space in this way would appear to require some sort of 'exotic' matter, and there's no evidence that its possible to create this. The other problem with wormholes is that they allow travel backwards in time. This is may be commonplace in science fiction but I wouldn't pin my hopes on something which leads to such paradoxes.
What about the warp drive? Is this anything more than fiction? Interestingly the answer is.. maybe. Miguel Alcubierre has shown that if we had general relativistic compression of space in front of a spaceship and expansion behind it then normal travel would correspond to faster than light travel with respect to the external universe. Unfortunately this suffers from the same problems as wormhole travel in that it needs exotic matter and allows time-travel paradoxes.
Beam me up Scotty
I know that in science fiction transporter beams are generally used for relatively short distances, but there's no reason why they couldn't be used for interstellar travel. Such a device works by measuring the details of all of the atoms in your body, and transmitting this information to a distant location where they can be reassembled.
Now there are two problems with this. Firstly to transport a person there is a huge amount of information to be collected and transmitted. The second is that it is impossible to measure fully the quantum state of an atom - due to the uncertainty principle, measurement changes the state of whatever is being measured. In Star Trek they have a Heisenberg Compensator to deal with this problem. How does it work? "Very well, thank you." is the reply, that is to say they don't have an answer as to how such a device might work in practice. So you might think that there is a fundamental problem with creating such a device. Interestingly this isn't the case. The problem does appear to have a solution, using what is known as Quantum teleportation. This is currently an active field of research, and we are likely to hear about quantum states being teleported over ever increasing distances. Intriguingly this cannot be used to duplicate an object, which to my mind sounds like reasonable behaviour if you think in terms of teleporting a person.
That's all very well if you just want to teleport a sub-atomic particle, but for a reasonable sized object there's still the first problem of capturing and transmitting all that information, which doesn't look likely to be practical any time soon.
There is another possibility for a transporter beam type of device. We are beginning to understand how the mass of objects fits in with the physics of the universe. The idea is that there is an underlying Higgs field, and it is interaction with this which gives objects mass. One would also hope that a 'Theory of Everything' such as String theory would give further insights into this area. And if we understand the mass of an object then maybe we can alter it - this might be thought of as a rotation of the object in one of the extra dimensions we hear about so often. If we could set the rest mass of an object to zero then relativity tells us that has to travel at the speed of light. This seems to me to be an ideal candidate for interstellar travel.
Such a transporter is likely to be restricted to travel at the speed of light, so it still takes you four years to reach the nearest star. Hardly suitable for a weekend break you may think. But that depends on how you view time. In fact as far as you were concerned you would get there immediately. It is only when you returned to earth that you would find that eight years had passed there. All of your acquaintances would be eight years older - unless of course they had been doing some interstellar travel too. But if you can build a device that transports you across the galaxy then you could certainly build a similar device that didn't transport you anywhere - keeping you suspended animation if you like. So its simply a case of all of your acquaintances arranging things so that none of them age too much more than you. This may seem a strange way of living, but I can't help believing that one day we will adopt it. So maybe you should start getting used to the idea - it may arrive sooner than you think!