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Mass and the Speed of Light

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"The notion of mass "increasing" with velocity is an outdated one. Under current definitions of "mass", mass doesn't increase at all with velocity; instead, it is an intrinsic property of the object in question, which is the same in all reference frames.

... If you're inside your rocket ship traveling at 99.99% of the speed of light relative to Earth, you wouldn't be able to tell you were moving at all without looking out the windows (or at your instruments, etc.) Specifically, you would not somehow have more trouble moving because of your increased inertia, and you would not see time slow down for yourself. It's only observers on Earth who would see your time slow down and your inertia increase."

The magnitudes of most of the famous effects of special relativity (time dilation, length contraction, increase in inertia) are determined by the "gamma factor",


This is where Allan Steinhardt's answer comes from. It's the factor by which inertia increases, and it's the factor by which we used to say your mass increased, before "mass" was redefined. (Again, this is all according to an observer who sees you moving at 99.99% of the speed of light relative to them; according to you, everything about you is "normal".)" Source


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"Of course eventually you would get vaporized. I'll explain why. At 99.99% of c you are not only moving at that whopping speed away from Earth, but also plowing through the interstellar medium (ISM). About 1% of the mass of the ISM is dust. The dust particles are about 10^-17 kg in mass and there is about one of those dust particles every cubic hectometer, or 1 particle every 1million cubic meter of ISM. Now let's assume that the front or our needle-like ship has an area of 100 square meter. That means every 10 km we travel we hit one such particle. The energy of the hit is mcc, where m is multiplied by the gamma factor of 70 (see other answers) and c squared is about 10^17. So every dust spec hits with an energy of 70 Joule. Compare that to a 0.1 kg apple hitting you at about 100 km/h. At almost the speed of light we are doing 300,000 km/s which is 30,000 dust specs hitting like apples per second, at 70 Joules per hit, is 2,100,000 Joule per second or 2.1 MW, that is Mega Watt! So we do not need headlights. Isn't this barrage of hits slowing us down? Let's check it out. The impulse delivered per dust spec is gamma times rest mass times light speed times number of hits per second, or 70 times 10^-17 times 3 times 10^8 times 30,000, or number of hits per second, or about 0,0063 kg m/s^2 or o.oo63 N for Newton, that is the like the force corresponding to the weight of less than a gram on Earth. That is very easy to compensate by a tiny rocket engine that is ignited for a short time during a day. So at these phantastic velocities we see the difference between Energy and Momentum. The higher the speed, the higher the energy and the momentum just can't keep up. So why would you eventually get vaporized?Because you would eventually hit a dust particle of a microgram, or 10^-9 kg, which would have a yield of about 10^10 Joule, which would basically kill the ship, because that is like a 10 ton truck hitting you at 3600 km/h. Sorry Trekkies, no Enterprise, ever. " - Ben H.

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