One way of looking at this is in terms of the energy involved
One of the quirks of relativity is that it shows mass and energy to be interchangeable, with the result that, the faster something moves, the more mass it appears to have. When you're accelerating something, the force you have to apply to achieve a given acceleration depends on the mass of the object.

So, as something gets faster, you have to apply progressively more force to it to keep on increasing its velocity; i.e., you have to put progressively more energy into the system. But, of course, as you do that, it becomes heavier, and so on in this continuing circle.

If you plot this on a graph, then you see that it's not a simple, straight-line relationship. As you get closer to the speed of light, the amount of mass/energy in the system appears to tend to infinity. The upshot being that you'd have to put in an infinite amount of energy (or your spaceship/Honda Accord would become infinitely heavy) to reach the speed of light.

Because of this mass/energy interchange, therefore, the only things that we know to be able to travel at the speed of light are photons (i.e., particles* of light) which have no mass.

*_{Well, wave-particle dualities, but that's a different can of worms...}
(LongJohnBaldry, Thu 22 Oct 2009, 13:09, 3 replies, latest was 16 years ago)

I knew it was worth waiting for you to reply
rather than saying the same thing but really badly.
( Vipros. clever got me this far, then tricky got me in, Thu 22 Oct 2009, 13:11, Reply)

You're too kind
It's good to know I'm useful for something...
(LongJohnBaldry, Thu 22 Oct 2009, 13:24, Reply)

Ok then Mr. Smarty pants, explain this
If God doesn't exist, who pops up the next Kleenex?
(The Oscillating Gibbon is eating curried squid, Thu 22 Oct 2009, 13:14, Reply)

I knew you'd come back with something like that
Always the deep ones with you, isn't it?

Oh, and I think the answer is "Ganesh" - isn't he "the maintainer"?
(LongJohnBaldry, Thu 22 Oct 2009, 13:16, Reply)

surely the correct answer
is ym
(bamboozled Can hear you getting fatter, Thu 22 Oct 2009, 13:34, Reply)

According to Einstein's theories of relativity
this is indeed the case.

Before Einstein came along, such laws of kinetics were based on Newtonian dynamics, which were derived from everyday observations. It was impossible to prove them wrong and no-one thought there should be any reason for them to be wrong. But Einstein showed that Newtonian dynamics were only a low energy approximation to the 'correct' theory. At high energies, like particles travelling at close to the speed of light, it all goes out the window.

Now, we've tested relativity pretty thoroughly, and it's always been right so far. But that's not to say that we won't be able to come up with other theories in future, which may or may not be testable, which allow for faster-than-light (FTL) travel.

In fact, some people already have come up with this sort of thing. Google 'tacheon'.
( K2k6 has a proper job these days, Thu 22 Oct 2009, 13:31, Reply)

Ah, yes, I've heard about these tachyon types
Not to discount it, I'm sure it's a perfectly valid theory rather than just a quirk of the maths (after all, that's how we first discovered antimatter) but someone is going to need to come up with a way to test that theory...
(LongJohnBaldry, Thu 22 Oct 2009, 13:33, Reply)

It's even out of the league
of the LHC boys at CERN. I think they'd need to harness the power of a whole star and build an interplanetary accelerator to do that!

And you're right - it's tachyon, not tacheon. My mistake.
( K2k6 has a proper job these days, Thu 22 Oct 2009, 13:35, Reply)

The whole tachyon thing
Is totally allowed in GR, it means that they must always be travelling faster then c just as all us slow-coaches must always travel slower than c.
(Shippy likes linebreaks and doesn't care what you think, Thu 22 Oct 2009, 14:17, Reply)