# What is energy?

1. ### mrhoneycinnamo

8
Is matter and energy the same thing?Is energy something that "is there" or is it just a description of matter, like the term speed?

2. ### Jimmy

742
Last edited: Aug 1, 2014
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3. ### Delta²

730
Energy and matter are the same thing, it is just that our brain and our senses conceive energy and matter in a different way. In simple words we could say that matter is a highly compressed form of energy.

Einstein was the first that gave the famous formula $E=mc^2$ that shows the equivalence of mass and energy and the factor $c^2$ shows why the energy from a piece of matter with mass m is so big (if we could completely transform this matter m into some form of pure energy like electromagnetic energy or heat energy, example of such a transform happens in nuclear plants where part of the mass of the nuclear fuel (uranium) is transformed mostly to heat energy).

Last edited: Aug 1, 2014
4. ### Jimmy

742
What definition of matter would one have to use in order for this to be true? I thought matter was a generic term for something that has mass and volume, and energy is just one of many properties that matter can have: charge, momentum, density, shape, etc.

I'm really not trying to be difficult or snarky. I've seen posts by members—Sci Advisors as well as others—stating the same thing and I simply don't understand it. Am I being too nit-picky? :)

### Staff: Mentor

Energy and matter are not the same thing. Energy is a conserved quantity related to work that is associated with matter and fields. The amount of energy an object or system of objects has can change depending on reference frames. In many ways it is like momentum, which is also a conserved quantity of matter and fields.

Note that energy is most definitely not a substance. You will not find globs of energy floating around like you might see in fictional stories, cartoons, anime, movies, etc. It is simply a mathematical quantity that we track when doing physics because it is very, very convenient and gives us an easy way of making predictions that we wouldn't have otherwise.

For a more in depth description of energy, see here: http://en.wikipedia.org/wiki/Energy

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6. ### Delta²

730
Yes the classical view of matter and energy is as you say. But this view changes in the Quantum Field Theory (QFT) where particles and fields are unified. A QFT treats particles as excited states of an underlying physical field. Particles gain mass by interaction with the Higgs Field. Its all about fields and interaction between fields , i.e its all about the energy states of those fields.

Last edited: Aug 2, 2014

### Staff: Mentor

I don't agree, Delta. There's a reason that we have separate uses for "matter" and "energy". Even in QFT you could not simply replace the term "matter" with "energy".

8. ### Delta²

730
But particles have mass because of the interaction with the higgs field right? I ve to admit i dont know much about the higgs field and the higss mechanism by which particles get their mass, but i *think* the property of mass relates to the energy the particles have inside this higgs field.

### Staff: Mentor

The higgs interaction gives rise to invariant mass, which can be seen as the energy content of a fundamental particle, but there are other types of energy and mass that don't involve the higgs, such as kinetic energy.

10. ### Delta²

730
What do you mean there are other types of mass, i thought all particles's mass are due to the higgs field. Yes there are many forms of energy like kinetic energy or potential energy or electromagnetic field energy, all we say here is that mass relates to the energy of the higgs field.

### Staff: Mentor

I'm far from an expert on the higgs, so I don't really want to get into a discussion on it, as I don't feel I can provide accurate answers.

Still, saying that matter is energy seems to be a massive oversimplification of the big picture of things. Its true that energy can be released or absorbed by matter, and it's true that you can create new matter using energy, but I can't see how matter and energy are the same thing. Particles are "things" which have properties. In addition to mass (energy), they also have charge and spin. Does this mean that matter is the same thing as spin and charge as well?

Does that make sense?

12. ### Jimmy

742
Matter is such an ambiguous term and energy is a well-defined quantitative property. What definition of matter are you using?

I'm pretty sure that composite particles do not get all of their mass from the Higgs field. Most of the mass comes from the binding energy. I'll have to check up on that though...

BTW, thanks for the replies, Delta².

13. ### Delta²

730
It makes some sense, but i will wait for someone good with the higgs field and higgs mechanism to explain us if and how the mass of a particle relates to the energy state of the higgs field.

14. ### Delta²

730
Well in my mind matter consists of particles that DO HAVE mass. I dont consider massless particles like photons part of matter. But of course if we go by QFT, matter consists of particles which are excited states of some field, and photons are excited states of the EM field, so photons are part of matter as well. In any case we have to do with excited states of one or more fields and what i am thinking is if mass corresponds to some sort of excited state of the higgs field.

Thank you Jimmy too.

15. ### Jimmy

742
Thanks again.

I apologise to the OP for turning his thread into a niggly semantic discussion. :P On the other hand, there are a ton of these "What is energy" threads so I'm content with the discussion so far.

I've spent the last few months trying to demystify energy in my mind and just when I think I've a good handle on it, something else pops up and leaves me scratching my head.

I was happy simply considering energy as book-keeping and not something tangible like matter: matter has energy, energy is a property and not something that exists by itself, and so on. But QFT is well above my pay-grade so I'll let the experts sort it out.

I did find a page about energy and matter which relates to this discussion:
Matter and Energy: A False Dichotomy

I'm not saying this is the final word on the subject and I'm sure other physicists have different opinions, but I did find it interesting.

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### Staff: Mentor

Drakkith is right. Matter and energy are NOT the same thing. Energy is a property of matter. Saying that they are the same thing is like saying that car and color are the same thing just because color happens to be a property that cars have.

19. ### Delta²

730
well yes it seems energy is a property of matter and property of fields as well (cause fields have energy too dont they?)

BUT what confuses me is that matters consists of particles (right?) and wikipedia says that QFT treats particles as excited states of an underlying field. So matter consists of some things that are excited states of some underlying fields. Excited states means elevation in the energy level at some level above the ground level...

So it seems to me that all what matter is elevation in the energy level of some fields. Do i miss something?

(Sorry if i sound ignorant i dont mean to, i know very little about QFT and i vent dwelve deep into Quantum Mechanics).

### Staff: Mentor

First, particles are excitations of the Fock space, whose eigenvalues are occupancy, not energy. The associated single-particle Hilbert spaces may or may not have quantized eigenvalues in an energy basis, but the Fock space itself certainly should not be assumed to have them.

Second, even if your comments were correct, energy still is not the only proper of matter.

Third, I think it is a bad idea to bring QFT into discussions outside of the QM forums since it generates replies like my first point here, which is probably not appropriate for the level of question being asked, but necessary in order to respond to your comments. Please, ask QM questions in the QM section and don't bring in complicated explanations when a simple one will do.

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