[quote=@Crimmy] For a Newtonian particle, with p = mu, the momentum is directly proportional to the velocity. The relativistic expression for momentum agrees with the Newtonian value if u ≪ c, but p approaches ∞ as u -> c. [/quote] [quote=@Crimmy] According to Newtonian mechanics, the momentum of a particle is equal to its mass multiplied by its velocity. Momentum is also a conserved quantity, which is true in all reference frames related by Galilean velocity transformations. However, the Galilean transformations are inconsistent with relativity. Say you're moving at 0.9c with respect to Earth, and then you shoot out an object that moves at 0.95c with respect to you; you'd expect the velocity of the object to be 1.85c with respect to the Earth, which is impossible because c, the speed of light in a vacuum, is the speed limit in all reference frames. But because you can't just throw the conservation of momentum out the window, however, you have to account for why it still works at significant fractions of c. It's an alright approximation when velocity "u" is small, but when you get big it starts going wonky. Instead, you'll have to use the "true time" measured by the particle itself. You'll have to use the Lorentz transformations (except using the velocity of the particle rather than the reference frame) to get relativistic momentum. [/quote] Translation: As objects approach the speed of light, the mass, and therefor the velocity of the object decreases proportionally. Even if an object is thrown at 95% of the speed of light from an object traveling at 90% the speed of light, it cannot exceed the speed of light in a vacuum. Also, as an object approaches the speed of light, the object apears to move in a slower time, and will have a different "true time" then objects not traveling at or near the speed of light. Did I get that right? The gist of it anyway.