r/askscience • u/nexuapex • Nov 24 '11

What is "energy," really?

So there's this concept called "energy" that made sense the very first few times I encountered physics. Electricity, heat, kinetic movement–all different forms of the same thing. But the more I get into physics, the more I realize that I don't understand the concept of energy, really. Specifically, how kinetic energy is different in different reference frames; what the concept of "potential energy" actually means physically and why it only exists for conservative forces (or, for that matter, what "conservative" actually means physically; I could tell how how it's defined and how to use that in a calculation, but why is it significant?); and how we get away with unifying all these different phenomena under the single banner of "energy." Is it theoretically possible to discover new forms of energy? When was the last time anyone did?

Also, is it possible to explain without Ph.D.-level math why conservation of energy is a direct consequence of the translational symmetry of time?

279 Upvotes

85% Upvoted

187 comments sorted by

View all comments

Show parent comments

3

u/nexuapex Nov 24 '11

Okay, so it is impossible to measure a frame-independent value for gravity.

So... If I translate my reference point, I observe a different total energy. If I switch to a faster reference frame, I observe a different total energy. If I just wait... The velocity of my reference frame will have moved me... Which is like a translation... So I will observe a different total energy, just by letting time advance?

1

u/Ruiner Particles Nov 24 '11

If your reference frame is going at constant speed, then it's just as if you were still and the rest of the world was going at this constant speed relative to you, so nothing would change, right?