Actually, the mass of an object is independent of speed. Some textbooks talk about relativistic mass, but what they are really talking about is that the kinetic energy and momentum increase more quickly than 1/2 m v^2 for large velocities, and if you stick with the old classical equations, you put the extra stuff in the mass term. But I prefer to think of the mass as being due to the number of atom the object contains, and use the correct relativistic expression for kinetic energy (gamma - 1)mc^2 or momentum (p = gamma m v), where gamma = 1/sqrt(1 - (v/c)^2).

In summary, this doesn't mean that mass changes with speed, it means that objects with mass cannot achieve the speed c. There is in theory a maximum velocity (less than c) for any object that is a function of its mass.

Also, it should be noted that the value of c is when it is in a vacuum. the actual speed of light changes with its medium...recently several labs supposedly broke the speed of light, but all that really happened is they created a medium in which light could travel faster than c. (just as a medium like water makes light travel slower than c) Inside this medium, light speed is still the maximum velocity....when those same photons transitioned to another medium, such as air, they would slow down.

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