E equals mc 2

[jerryyu]

E equals mc 2 proved that mass can be converted to energy. But how?

[swansont]

E equals mc 2 proved that mass can be converted to energy. But how?

 

"How is this true?" or "how do you do it?"

 

If it's the latter, change the center-of-mass energy of a system. A hot cup of tea has more mass than a cold one. An excited state has more mass than the ground state. A free collection of particles has more mass than a bound state of the same particles. (or change the net binding of a system, which is how fission and fusion work). Annihilate a particle with its antiparticle.

[between3and26characterslon]

"How is this true?" or "how do you do it?"

 

If it's the latter, change the center-of-mass energy of a system. A hot cup of tea has more mass than a cold one. An excited state has more mass than the ground state. A free collection of particles has more mass than a bound state of the same particles. (or change the net binding of a system, which is how fission and fusion work). Annihilate a particle with its antiparticle.

 

 

Does this mean as the universe cools down it looses mass?

[swansont]

Does this mean as the universe cools down it looses mass?

I'm not sure how to answer the question.

 

In the coffee cup example, it will lose mass as it cools because energy is being lost from it — you have a system, and a boundary. With the universe, there is no boundary. The energy is still there. However, the description of the universe uses General Relativity, and the cooling of the universe is due to its expansion, so you have to be very careful how you account for energy, because energy conservation is only applicable within a reference frame. The situations aren't analogous to each other.

[between3and26characterslon]

I'm not sure how to answer the question.

 

In the coffee cup example, it will lose mass as it cools because energy is being lost from it — you have a system, and a boundary. With the universe, there is no boundary. The energy is still there. However, the description of the universe uses General Relativity, and the cooling of the universe is due to its expansion, so you have to be very careful how you account for energy, because energy conservation is only applicable within a reference frame. The situations aren't analogous to each other.

 

Of course, should have thought it through before I asked.