# What is energy?

1. Aug 18, 2013

### Epicurious

We have many different ways of describing and quantifying 'energy' eg kinetic, potential etc. We also know that mass and energy are equivalent. We intuitively know what it does and we talk about it across many fora but my question is 'What is Energy?'. What is this characteristic of the universe whose effects we freely discuss. Is it possible to define Energy in a non-tautological manner ie without reference to matter or forms and manifestations of energy?

2. Aug 18, 2013

### Atwun

I find your question very interesting especially because as it now stands, the post right below yours, although the thread was started earlier in the day, is the very question you are asking. In my thread that is named "the destruction of energy" I end up with asking the same question you have: What is energy? As far as explaining it in a non-tautological manner...hmm...not sure exactly what tautological means, I will look it up but I will say that there have been many people who have simply stated that everything is energy. This to me answers nothing about the question of what energy is. Once I look up the word tautological, I am hoping to discuss this matter further with you. Good luck in all ways.

Last edited: Aug 18, 2013
3. Aug 18, 2013

### Atwun

according to an online resource
tautological:
1.
a. Needless repetition of the same sense in different words; redundancy.
b. An instance of such repetition.
2. Logic An empty or vacuous statement composed of simpler statements in a fashion that makes it logically true whether the simpler statements are factually true or false; for example, the statement Either it will rain tomorrow or it will not rain tomorrow.

So you are searching for a possible definition of energy that does not include as you put it, form, matter or the words manifestations of energy?

4. Aug 18, 2013

### Staff: Mentor

Energy is a model used in physics to describe observations.

5. Aug 18, 2013

### Epicurious

Thank you mfb, but that seems like an almost Zen-like answer! The extrapolation is that matter is then also a model so all we are left with in the Universe is Space-Time and Observations, which I guess could be a neat marriage of Relativity and Quantum mechanics :-)
It does lead though to a possible avenue. We know of the energy/matter equivalence and we know that matter and energy affect the structure of space-time, so is there a way of describing energy (in a non-philosophical but mathematical) in terms of space-time?

6. Aug 18, 2013

### ZapperZ

Staff Emeritus
https://www.physicsforums.com/blog.php?b=3203 [Broken]

Zz.

Last edited by a moderator: May 6, 2017
7. Aug 18, 2013

### DrewD

Just be careful going through those threads and note that many of the responses are incorrect (which is an important thing to remember at all times on the internet). You will probably be able to pick out the right and wrong and ask specific questions about examples that you can't discern.

Last edited by a moderator: May 6, 2017
8. Aug 18, 2013

### Staff: Mentor

9. Aug 18, 2013

### Epicurious

Thank you DaleSpam. Your references have underlined the fundamental problem. They all give mechanical definitions of work/energy (ie the observed effects of energy and how to calculate the amount) or define energy tautologically by saying that energy is different types of energy (a rhododendron analogy, perhaps).
If energy is considered as an operator acting on a vector in Hilbert space, do the eigen values arise de novo?

10. Aug 18, 2013

### Staff: Mentor

What is wrong with a mechanical definition of energy?

Btw, I disagree that the sites I gave you gave a tautological definition.

11. Aug 18, 2013

### Epicurious

It's the difference between describing what something does and defining something. If I had asked 'what is light' then answers that would tell me how to measure its intensity, describing how it can illuminate things or be used as a laser would not be as defining as say, Maxwell's equations.
Ultimately, it may be that I just have to accept that energy is a state of space-time and to understand it further is beyond my lay capabilities (or possibly not just mine, if GĂ¶del was completely correct :-)

12. Aug 18, 2013

### Staff: Mentor

That is not a difference as far as I can tell. If it walks like a duck, quacks like a duck, etc. then it's a duck. What is wrong with defining something through what it does, particularly if what it does is the important feature that you are trying to define?

You are rejecting perfectly valid definitions for completely invalid reasons.

Maxwell's equations also are a description of what light does.

Last edited: Aug 18, 2013
13. Aug 18, 2013

### Epicurious

Exactly my point! Defining a duck by saying it is a thing that looks and quacks like a duck and can be used to do duck things does not actually define a duck!

14. Aug 18, 2013

### Staff: Mentor

Yes, it does.

You are heading down the exact same road in all of those previous threads cited by ZapperZ. It won't last long this time. Do you have a SPECIFIC question about the definition of energy? If you do, please ask it. If you just have more of the standard vague unfounded complaints and no real intention to learn science then I would be glad to lock the thread for you.

15. Aug 18, 2013

### DrewD

One wouldn't say say "that is walking like a duck" as the definition of a duck, but if you remove the word duck an instead put a description of the walking style it would be a good start. Of course biologists would choose better descriptions that would require more understanding of biology, but fundamentally a duck is described by a list of duck-like attributes.

Similarly, energy is defined by giving a description of what it does in the world. The more physics you learn, the more detail you can get into.

Have you studied QM? The energy is the eigenvalue of the Hamiltonian. The eigenvalues arise from the hamiltonian operating on some state. It is usually better to discuss subjects within a single framework (ie QM or Newtonian Dynamics, etc.) or have the specific subject be how two well understood situation in one framework relate to another. Maybe stick with classical here?

16. Aug 18, 2013

### Epicurious

Dear DaleSpam. Please lock this thread now.

Last edited by a moderator: Aug 18, 2013
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