# Insights What is Energy? - Comments

1. May 27, 2015

### Staff: Mentor

2. May 27, 2015

### nasu

Great, it should be used as a reference. :)
It applies to other "hot" topics, not just energy.

3. May 27, 2015

### Greg Bernhardt

Nice first entry @DaleSpam!

4. May 27, 2015

### Staff: Mentor

Whoops, unfinished post inadvertently posted. Replaced by a later post.

Last edited: May 27, 2015
5. May 27, 2015

### Unified28

Energy does have it's definition. However there is a reason why we can define energy the way we do which is unknown to physicists today.

6. May 27, 2015

### baudrunner

7. May 27, 2015

### Staff: Mentor

I wasn't trying to list forms of energy. I was trying to list definitions of energy. For instance, using the mechanics definition you can define KE, elastic potential energy, and gravitational potential energy all as different forms of energy using the same definition.

8. May 27, 2015

### Staff: Mentor

Good post Dale, congratulations.

I do think that the sentence, "There are other definitions of energy which are used in thermodynamics, Lagrangian mechanics, and quantum mechanics. " could have been broader to specifically mention, chemical, nuclear, EM, rest-mass-equivalent and other forms of energy that are not thermodynamic, nor mechanics. Those are all domains where work = force * distance is hard to apply, therefore making "the capacity to do work" definition problematical.
Also worthy of mention is that we can freely convert between all these forms and that conservation applies to the collective sum of all the forms.

You're correct of course when you said, "Energy is not a thing with independent existence." But there is something special about energy that IMO goes beyond other properties like mass or momentum. That is the intertwined concepts of energy and time. I am thinking of the Heisenberg expression for the rate of change for any observable B. (sorry, I don't know how to do Latex in PF4.)

dB(t)/dt = (i/hbar) [H,B(t)]

where H is the Hamiltonian and B is any observable. This has always struck me as very profound. With zero H (zero energy), nothing can change ever. Without a nonzero d/dt of something, there can be no event of any kind. If time is defined as "the way to order events from past to present to future", then no events implies no time. I read into that simple equation that the existence of energy is a prerequisite to the existence of time.

Forgive me for going off the deep end. I know your focus was on more basic concepts. Perhaps if the title was "What Is Energy in Mechanics?", then I wouldn't have gone so far astray.

9. May 27, 2015

### Greg Bernhardt

Good distinction!

10. May 27, 2015

### Staff: Mentor

We can define energy the way we do because we can define any word anyway we want. By definition.

I think that you may mean that we don't know why the laws of nature are such that energy is conserved. But that is quite a bit different from not knowing why we can define words.

11. May 27, 2015

### Staff: Mentor

I agree, and in my experience on this forum such questions are answered clearly and directly. And then the discussion goes downhill from there.

12. May 27, 2015

### HallsofIvy

I've always thought of "Energy" as a book keeping device! We start with the definition of "kinetic energy" along with "momentum". Then we find situations in which Kinetic energy is not constant so we define potential energy to take up the slack. But when friction comes into play the sum of those is not constant. So we add "heat energy", etc.

13. May 28, 2015

### Staff: Mentor

BTW I agree with Dale.

But just for the sake of fleshing this out more why exactly cant energy be the conserved Noether charge related to time symmetry?

If that's the case we know exactly what energy is and why its conserved.

Thanks
Bill

14. May 28, 2015

### Jeff Rosenbury

Is there a tutorial somewhere?

15. May 28, 2015

### Jeff Rosenbury

My problem with the "Ability to do work" definition is that it abrogates the "energy is neither created nor destroyed" rule. The two are incompatible because one has to stretch the definition of "work" way too far to argue that the energy of a neutrino can do useful work. Yet there is clearly something very, very close to that definition going on. I certainly don't have a better one.

It is interesting that any discussion of energy seems to rely on things outside the energy. An electron is a thing. A photon is a thing. Energy is an ability. Energy does not exist by itself, but only in relationship to something else.

16. May 28, 2015

### nasu

Who said anything about useful work in the definition of energy?
There is no stretch in the definition of work. The neutrino can do work on the particle with which will interact in a neutrino detector.
And actually this is quite useful for the people working at the neutrino facilities. But utility is irrelevant.

17. May 28, 2015

### Khashishi

Like a lot of other terms in physics, the definition of energy depends on context.

18. May 29, 2015

### Islam Hassan

If we ignore the different classical classifications of energy (heat energy, sound energy, etc) and take a fundamental view, can we say that all energy at the fundamental level is ultimately one of these four categories:
• Kinetic;
• Static, ie deriving from an object/particle's position in a physical force field;
• Energy incarnated in mass; and
• Dark energy, which we know little about.

IH

19. May 29, 2015

### Unified28

Obviously kinetic energy is the most fundamental in all cases. At least Wes Tausend was right about that.

20. May 29, 2015

### Staff: Mentor

Some posts have been removed. Please don't post personal theories.