Why does mass increase if all movement is relative?

Question

It is said that nothing in the universe is totally at rest. Motion is relative. So why would mass increase as an object aproached the "speed" of light? Wouldn't the speed of light be relative? And if so, then, the mass increase would be based on the perception of an observer and not absolute, right?

Answer 1

You are correct, the increase in mass is from the observer's standpoint. An alien in a spacecraft passing by the earth at near the speed of light would observe YOU as being very massive and himself as "normal" mass.

WHY the mass increases is a question I cannot answer. Nor can I explain why its length decreases or time slows. I know that it does, and some of the ramifications, but I don't know WHY it happens.

Answer 2

In physics over the centruies, and even before Einstein, it was well known that the total energy in a substance was a function as E = M V squared. Einstein rationalized that M and V are always "paired" and that if an object would be accelerated to a very high V, the limit would be the speed of light or 186,000 miles per second.

So inserting the limit of speed of V at 186,000 miles/sec one would have E = M x 186000 sq Thus since it would take enormous energy to reach that limit of light's speed one can see that E would approach infinity, and therefore so would M, the mass to balance out the equivalence. .

Answer 3

A body or bodies in space are affected by other bodies or body(gravitation or other forces) a body moving at high speeds behaves according to E=MCsq. Even here on earth that can be demonstrated and has been confirmed. Since mass becomes infinite near the speed of light a body would be stretched so that all atoms molecues would be pulled apart. The Question is what or who at the other end will put it back together upon arrival at destination. Light is a slow poke in the universe,just trying to get to the nearest star system(Alpha Centauri) would take 10,000 years(earth time) asumming that instant light speed would be achieved at launch time and instant deceleration at arrival and then repeat for return journey,20,000 earth years would then be total time assuming no time taken for observing and such. Would we want to try it , it may turn out to be a oneway trip. Will anyone be around if they arrive back here?