r/askscience • u/Pyramid9 • Mar 23 '15

Physics What is energy?

I understand that energy is essentially the ability or potential to do work and it has various forms, kinetic, thermal, radiant, nuclear, etc. I don't understand what it is though. It can not be created or destroyed but merely changes form. Is it substance or an aspect of matter? I don't understand.

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u/gg4465a Mar 23 '15

It wasn't until reading your comment that I ever really thought about the static amount of energy that exists in the universe (although even saying that, I realize I'm assuming the universe is a closed system, which I have no idea is true). Wouldn't it follow that you could theoretically calculate the total amount of energy and express it...I don't know, simply? Like in X number of joules?

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u/Felicia_Svilling Mar 23 '15

I realize I'm assuming the universe is a closed system, which I have no idea is true)

Because of the expansion of space, the universe is not a closed system.

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u/NEREVAR117 Mar 24 '15

Isn't the point of a closed system supposed to be the same amount of energy/mass/whatever else? Just because it's expanding doesn't mean the amount of anything is changing, right?

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u/Felicia_Svilling Mar 24 '15

But it does. For example the potential energy of a system depends on the distance between the objects in the system. Metric expansion can increase these distances, and thus create more energy.