We all know that stuff can't go faster than the speed of light - it's length becomes negative and all kinds of weird stuff happens.
However, this is in relation to what? If two objects, each moving at 0.51 times the speed of light in relation to some point pass each other, do they disappear?
in relation to anything else that can make such measurements.
As the speed of light is universal, nothing can see any other massive field moving at the speed of light (which is reserved for massless fields)
your 0.51 number suggests that you expect that naive addition of velocities holds when velocities approach the speed of light. This is wrong. Here is an article explaining the relativistic velocity addition expression:
This is essentially the same as lurscher's answer, but from a different perspective.
Special Relativity is often thought of as some kind of mystical force that acts on objects and stops them moving faster than light. This misconception is the reason for questions like this one. Special Relativity is actually just a prescription for telling us what events in another inertial frame look like in our inertial frame.
So if your object moving at 0.51$c$ fires a bullet at 0.51$c$ SR tells us that as measured in our inertial frame the velocity of the bullet is given by the relativistic addition law and cannot exceed the speed of light. SR doesn't mean objects can't move faster than light, and indeed galaxies farther away than about 50 billion light years are moving faster than light relative to us. Indeed, if the universe is infinite (and the FLRW metric applies to all of it) infinitely distant galaxies would be moving infinitely fast relative to us. However SR tells us we will never measure the velocity of a distant galaxy to be faster than light.
The speed of light seems to be the undisputed speed limit of the universe, in relation to the fact that to travel faster than a massless photon is able to travel would not only be physically impossible because no object containing mass would be able to stand the blinding speed, even in the vacuum of space without atmosphere to create drag.
Also, there is the time aspect. If you are on a spaceship going 99% the speed of light, and your twin brother is on another spaceship going 9% the speed of light, traveling to the same location, by the time your twin were to arrive, he would have experienced 90% more time than you have, even though you have both travelled the same distance.
Also, traversing the universe at the speed of light, for a human, would require technologies that we could only dream of.
First, you would need to somehow find a way to shield the inhabitants of the ship from the deadly effect of inertia and g-forces inside of the ship. For a human, 186,000 miles/second would kill in a fraction of a millisecond, the force would flatten all of your internal organs, your brain, your eyes, everything, against the back of your body. Bones would literally liquify, the heart wouldn't be able to beat against the immense force of inertia.
