What happens to mass when it approaches the speed of light?

[nukeman]

Hey guys,

If something with mass comes close to the speed of light, the laws will adjust and slow down time in order to prevent the mass from passing the speed of light. Is that correct?

So, would the object start to gain mass as it gets close to the speed if light?

Where does this mass come from? And how is it created from nothing?

 

[Pengwuino]

It actually doesn't gain any mass. The term "relativistic mass" is a misleading term. The rules of special relativity simply act in such a way that the mass in question appears to be gaining mass. For example, say you have a rocket that you know gives a constant acceleration (and let's be unrealistic and say the rocket doesn't lose any mass). If we allow the rocket hooked onto a mass to go, eventually you'll see the acceleration decrease due to relativistic effects. Now, if you're a firm believer in F = ma, then if the force is constant and the acceleration is apparently decreasing, the mass must be increasing. Of course, there is no actual new mass appearing out of nowhere.

That's why people don't like the idea of relativistic mass. It makes a lot more sense to simply talk about relativistic momenta and relativistic energy. They're more concrete and subject to less misunderstanding due to what we know in the non-relativistic cases.

 

[nukeman]

Great, thanks!

Just 1 thing, as you said "eventually you'll see the acceleration decrease due to relativistic effects."

Can you please tell me some examples of this on the "rocket" you are talking about. ?

Thanks, you helped me clear this up :)

It actually doesn't gain any mass. The term "relativistic mass" is a misleading term. The rules of special relativity simply act in such a way that the mass in question appears to be gaining mass. For example, say you have a rocket that you know gives a constant acceleration (and let's be unrealistic and say the rocket doesn't lose any mass). If we allow the rocket hooked onto a mass to go, eventually you'll see the acceleration decrease due to relativistic effects. Now, if you're a firm believer in F = ma, then if the force is constant and the acceleration is apparently decreasing, the mass must be increasing. Of course, there is no actual new mass appearing out of nowhere.

That's why people don't like the idea of relativistic mass. It makes a lot more sense to simply talk about relativistic momenta and relativistic energy. They're more concrete and subject to less misunderstanding due to what we know in the non-relativistic cases.

 

[Chronos]

Relativistic mass is merely a way of expressing how [under general relativity] spacetime curves around mass as it approaches the speed of light. It has no macroscopic effect on the universe.

 

[Radrook]

If indeed it were truly gaining mass as we usually imagine that mass is gained, via addition of matter, then the question would be where is that matter coming from?

 

[Ken G]

Relativistic mass is merely a way of expressing how [under general relativity] spacetime curves around mass as it approaches the speed of light. It has no macroscopic effect on the universe.

No, relativistic mass appears also in special relativity. It is simply out of vogue, in favor of the invariant mass (rest mass). In special relativity, relativistic mass is a device for understanding why a fixed force, experienced in the frame of the object being accelerated, does not produce a constant acceleration, from the frame of an inertial outside observer. So it is basically just a way to make Newton's laws still work in special relativity. Modern pedagogy rejects the desire to make Newton's laws work, and simply relies on the more relativistically powerful concept of invariants (things that are the same in all reference frames, as I'm sure you know).

 

[Ken G]

If indeed it were truly gaining mass as we usually imagine that mass is gained, via addition of matter, then the question would be where is that matter coming from?

That's pretty much why the concept isn't used much any more, because the rest mass stays invariant, since the rest mass is the thing that would need to add matter in order to increase. If you like to use relativistic mass as a device, then you simply let go of the idea that you have to add matter in order to increase it, because relativistic mass is frame-dependent so can change simply by changing reference frame.

 

[Forestman]

Einstein showed that energy and mass were really two sides of the same coin, when an object approaches the speed of light; energy is added to it in order to speed the object up. And since energy is equivalent to mass the energy adds inertia to the object, thus making it harder for the object to speed up any faster. So the object doesn't gain any new atoms, but it does gain inertia.

Inertia is a resistance to a change in motion.

Somebody might have already said this, I did not read the other post.

 

[Chronos]

I hate well deserved semantical corrections, Ken.

 

[Ken G]

I'm happy to get corrected too, it's how we all learn.

 

[skeptic2]

In the case of a free falling object perhaps falling towards a black hole which in some reference frames appears to be accelerating, in its own reference frame it is at rest. In this case the acceleration will not be diminished as the object gains velocity.

 

[Ken G]

Yes, there are two flavors of acceleration, proper acceleration, which can be measured on an accelerometer, and coordinate acceleration, which just depends on some arbitrarily chosen coordinate system and never appears on any accelerometer. Confusion between those types creates a lot of problems. The idea that "acceleration diminishes when the object approaches c" really means "an object with constant proper acceleration will appear to have diminishing coordinate acceleration as it approaches c in any globally inertial coordinates." Not quite the same statement! It's not even easy to specify when there is gravity around, for that tends to ruin the whole concept of globally inertial coordinates-- you then avoid coordinate-dependent language altogether, and stick to the invariants, which are the measurables.