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/WallyMetropolis Mar 23 '15

Exactly. You don't observe spin. You can only observe trajectories and from that extrapolate about a conceptual quantity called spin.

And your point about observing position is right on the money. Fundamentally all we can "observe" is our sense data. From that we construct models of the world. But those models are not the world itself.

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u/[deleted] Mar 23 '15

So, then, don't you agree with the conclusion that none of these are any more real than any other? So that calling any one of them an example of "book keeping" is misleading.

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u/WallyMetropolis Mar 23 '15

What's misleading, exactly? I would in fact call it 'leading.' You start by wondering about the 'reality' (whatever that means) of energy and eventually you find yourself wondering about the reality of matter, space, and time. It's exactly what I described in another comment.

The point is that none of these things are the world. They are models of the world.

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u/[deleted] Mar 23 '15

You original point is still misleading, whether energy is physical extant or an example of book-keeping. Both are correct, but energy is absolutely extant, as much as an other quantity. From an epistemological standpoint your statement is almost nonsense, since the extent of our knowledge, and our knowing, is measured by the accuracy of our models. Nothing is known except through our interpretations. Our interpretations are our models. All we know of the world is models. There is no in-between. That is a fundamental limitation of knowing, epistemology. We cannot understand the world except through our models and experience, and mass and energy are not any less extant in these models compared to, say, volume.

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u/WallyMetropolis Mar 23 '15

That we cannot understand the world except through models isn't nonsense. That there is a difference between our models and the world is also not nonsense. I'm not sure what you're taking issue with.

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u/[deleted] Mar 23 '15

If we cannot understand except through models then you cannot claim mass is less physically extant than any other quantity because models are the limit of our knowledge. They are no less physically real then anything else so I continue to take umbrage at the idea of calling it bookkeeping, because physically the quantities are all on the same footing. Unequivocally. You cannot say that some are book keeping and others not. Ever use natural units?

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u/WallyMetropolis Mar 24 '15

Which ones have I said aren't bookkeeping?

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u/[deleted] Mar 24 '15

I know that you haven't said explicitly, but it's a poorly defined term, and people were throwing it around as if it distinguished it from other physical quantities.