I was forgetting that the mass increase is another area where the Theory of Relativity is confirmed over and over again, especially in particle accelerators, where the particles approach the speed of light. If the designer of a synchroton (particle accelerator) doesn't account for the mass increase, the particles will simply get out of sync, and the whole apparatus won't work!

One reason that is often cited, why a mass can't reach the speed of light, is the increase in mass.

Another is the addition of velocities; for instance, if a spaceship travels at 2/3 of the speed of light (in relation to Earth), and fires off something, say, a bullet, at 2/3 the speed of light with respect to the rocket (in the forward direction), the speed of the bullet with respect to Earth would be 4/3 c, according to classical physics. In the Theory of Relativity, the addition formula is more complicated; the end-result is that the combined speed is less than c.

From my physics textbook (by Douglas C. Giancoli): "However, as was pointed out in the late 1960s, Einstein's equations do not rule out the possibility that objects exist whose speed is always greater than c. If such particles exist (the name "tachyon" - meaning "fast" -was proposed) the rest mass would have to be imaginary... In spite of extensive searches for tachyons, none has been found. It seems that the speed of light is the ultimate speed in the universe."

>Hi Hilmar,
>
>Here is something else that might help. I have heard several people state that it would be impossible for an object to travel at the speed of light, or beyond the speed of light; because, as the object accelerates towards the speed of light, the mass of the object tends toward infinite. As I understand relativistic mass, as the speed/velocity of matter accelerates, the matter gains mass. This definition of mass is rarely used by physicist today.
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>Here is a paper that explain the difference between relativistic mass and Newton more traditional mechanical definition of mass which has been converted into a more help geometric definition of mass. I hope I got that right, because it very confusing to me, other than it show how the mathematic of physics can change as our understanding increases or as the theory is applied to practical application.
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>http://www.weburbia.demon.co.uk/physics/mass.html
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>Regards,
>
>LelandJ