# Fermilab

## Inquiring Minds

All About Light Main Page  |  Classical  |  Relativistic  |  Quantum

 Relativistic - c; the Ultimate Speed c and Einstein  |  c Top Speed  |  Speed & Speed  |  Faster than c? Mass Gain  |  E=m*c^2  |  Age Slower  |  Time Dilation  |  Twin Paradox Dopper Shift  |  Ticketed  |  Simultaneity  |  Competition  |  Fast Strudel Paradox? No!  |  Confusions  |
In this page, we begin our study of the consequences of Einstein's theory of special relativity. I will tryto be as rigorous as possible, but mostly I will only state the consequence and then relate it to everyday life,rather than derive it from basic principles.

I will start with the fact that the speed of light in a vacuum is the ultimate speed in our universe.

We do not know why this is so, and we do not know if any objects exist traveling faster than the speed of light in a vacuum. We only know that we have not seen any such objects yet. This gives us enough confidence to believe Einstein. (Hooh! Pretty weak, isn't it?)

Naturally, if anyone were to observe an object or signal traveling faster than the speed of light in a vacuum, we would have to throw away the whole beautiful theory of special relativity.

This ultimate speed thing is a bit more subtle than what I presented above, so I will be a bit more precise here to avoid any confusion. In our universe, we have two major kinds of particles. Massive and massless. The massive particles, --for example electrons, cars, apples ... -- have a non-zero rest mass.

The massless particles, --for example photons-- have zero rest mass. Einstein's theory of relativity dictates that in a vacuum, the MASSLESS particles move WITH the speed of light, and the massive particles ALWAYS move slower than the speed of light! No matter what you do, you cannot accelerate a massive object to the speed of light. You can approach the speed of light, but you will never reach it. Why so? Because theory dictates, and because you would need an infinite amount of energy to do that!!

On the next page, we will discuss why 1+1 is never two in the theory of special relativity.

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