That is a remarkable question! According to Einstein's Theory of Relativity, it turns out that if an object could travel faster than the speed of light, extremely weird effects could be observed:
An observer traveling with the object would be able to observe an effect before its cause! That means, for example, that he or she could detect a particle before it was created, or observe a house being destroyed before it was built. We all accept that a cause has to precede an effect (of course, it might not be true. This is a philosophical question - but we believe that it's true), thus such effects cannot be observed, and hence nothing can travel faster than the speed of light.
Furthermore, there are no experiments (so far) where particles are observed to travel faster than the speed of light. In one famous experiment done by W. Bertozzi (aptly named the "Ultimate Speed" experiment), the speeds of electrons were measured as they were given more and more energy. As their energies increased, their speeds also increased, but not indefinitely. Their speeds were getting nearer and nearer to the speed of light, but could never be equal to or over it. (Where could the energy go, then? It turns out that the mass of the electrons increased as their energies increased!)
Here is a neat illustration of Bertozzi's experiment: http://tap.iop.org/atoms/accelerators/518/file_47173.pdf
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Hope this helps!
(published on 02/27/2011)
I'm assuming you're asking about how we know it's possible to observe an effect before a cause if one could travel faster than the speed of light. The truth is that this has never been observed; since there hasn't been a case where any object could travel faster than the speed of light (hence you could say there is no direct evidence).
The argument that proposes this weird "fact" that effect can precede cause is a purely mathematical one - one that stems from Einstein's theory of relativity. To get a glimpse of what happens, imagine a toaster sitting on a spaceship. Then imagine two events happening: putting a piece of bread in the toaster (event 1) and taking a toasted bread out (event 2) after 30 seconds. A person standing still on the spaceship measures the distance between these two events as zero (the toaster was always at the same position to him), and measures the time difference as 30 seconds. If the spaceship was moving relative to you at some speed, the distance between these two events wouldn't be zero to you (because the toaster had to move backward relative to you as you were moving - although it's not that simple). More interestingly, you would measure the time difference between events 1 and 2 as more than 30 seconds. These effects are entirely real, but it's only obvious if you travel at a speed comparable to that of light. The speeds we encounter in our daily lives are so slow compared to light speed that these effects are too minute to be observed. Back to the discussion, the set of equations that tells us how these time differences, distances, and speeds are related to each other are called the Lorentz Transformations.
Based on these equations, say we were to change the scenario by having the spaceship (with the toaster) travel faster than light relative to you (though not physically possible). By using the equations to figure out what any other observer sees, you would find that some observers, moving at some speed relative to you, would measure a negative time difference! That means that to those observers, event 1 happened after event 2, which means that the toasted bread was taken out before the bread was even put in! However, since the laws of physics must be the same no matter where you are or how fast you travel, this scenario is impossible - hence scientists conclude that faster-than-light travel is impossible.
If faster-than-light travel were really possible, there would be many possibilities which only appear in science fiction - like traveling backwards in time, sending messages to the past, and so on. But just imagine the inconsistencies that would arise. If one could travel back in time to prevent one's parents from ever meeting each other, what would happen then?
We do not have to worry, though. Faster-than-light travel will probably always remain as a purely theoretical concept.
Thanks for the question!
(published on 04/15/11)