

Accelerating massive objects, Part 2 Physics states that Particles with mass such as protons and electrons can never truly travel at the speed of light in vacuum but they can get very close. Why not?
Dan, This follows from Einstein's Special Theory of Relativity which predicts a number of physical consequences for objects moving at large velocities, consequences which are outside our normal everyday intuition gained from observing objects moving at low velocities. One effect is that particles with mass acquire a "relativistic mass" equal to their mass at zero velocity (called the rest mass) divided by the square root of ( 1 minus (particle velocity/speed of light)squared ). So effectively a particle gets more and more mass and is therefore harder and harder to speed up further. So hard that you can't ever reach the speed of light. If you look at the equation, you see that if the particle velocity were to equal the speed of light, then you would compute a "relativistic mass" of the rest mass divided by zero. Something divided by zero is infinitely large. Another effect is that particles with exactly zero mass (for example a photon), MUST ALWAYS travel at exactly the speed of light! Pretty strange, and I suggest you consult your local library for books on Special Relativity for more interesting examples. Now, all this is a "theory". Is it correct? I can only say that in countless tests done since Einstein put forth his theory in 1905, it has held up to every challenge. But then, so did Newton's theory for 219 years until Einstein found the flaws with high velocity objects.
Hope this answers helps, 
last modified 3/31/1999 physicsquestions@fnal.gov 