what is energy?

Tom Mattson

Actually, it's an abstraction that is used for the purpose of theorizing. No one actually measures energy. Rather, experimentalists measure state variables that are used in mathematical functions which we call the different forms of energy.

Velocity is measured, from which kinetic energy is calculated.

Temperature is measured, from which thermal energy is calculated.

Position relative to a mass is measured, from which gravitational potential energy is calculated.

Frequency of a photon is measured, from which photonic energy is calculated.

Energy isn't measured, it's calculated.

That's a misuse of the term "energy". A photon is not to be identified with energy, a photon has energy, which as I noted above is calculated from its frequency.

Just as with photons, the same can be said of strings: They aren't to be identified with energy, they have an energy.


edit: fixed some tags

Tom Mattson

First thing to note is that energy is not a tangible substance. In fact, energy doesn't exist anywhere in the universe!

Energy has no reality apart from its (mathematical) functional dependence on state variables that do have reality (see my post to Les above).

As I just remarked above, photons are not to be identified with energy. Energy is a mathematical invention, but photons are real. Having said that, deexcitation of atoms is not the only way to produce them. They can be produced from any deexciting system (semiconductor, molecule, nucleus, nucleon, etc.) or by accelerating charges.

Math Is Hard

Thank you. I appreciate all the responses. This was the clarification I was looking for.

jdavel

Tom Mattson siad:

"Velocity is measured, from which kinetic energy is calculated.

Temperature is measured, from which thermal energy is calculated.

Position relative to a mass is measured, from which gravitational potential energy is calculated.

Frequency of a photon is measured, from which photonic energy is calculated."

I don't understand the distinction you're making. What's special about velocity? Why can't I say, "Distance and time are measured, from which velocity is caculated"?

In fact, can't I even work everything back to just length and say that's the only state variable, and everything else is just a definition or a mathematical abstraction based on length?

But why would I want to do that?

PRyckman

photons are both a particle and an energy wave. Depending on the circumstances

Tom Mattson

Actually, I do think that length is really the only things that is ever measured. But I was trying to explain that energy isn't really a physical "thing" the way it is typically portrayed.

Moonrat

I suggest, in layman's terms, that energy is simply motion. It is not the ability of motion, but rather the motion itself.

Matter/Motion comprise the physical universe, and they are so intertwined, one does need to turn to 'art' or 'artistic expression' to gain a full picture, I dont think scientific literature can cover it enough for the layman without the layman stumbling upon paradox.

Motion and energy seduces us to discover it's source, so it takes us into the quantum realm, which becomes even more paradoxical..

Motion is everywhere the center, while matter is the circumfrence nowhere can be found...(and that's poetic ;-0)

Moonrat

Math Is Hard

hmmm...I rather like thinking about it that way - and it makes sense. Motion's not tangible, but things definitely have (or can have) motion. Potential motion and kinetic motion seem to work by definition, too.

Thank you for the suggestion.

Moonrat

thanks for the question! kinda cool too when you think that all physical objects have motion affected by gravity, which 'motions' everything toward a center, or center source. (our moon centers around our planet, our planter centers around the sun, and our sun centers around the galatic core)

perhaps this motion can be quantified, perhaps, by 'omni-direction', uni-direction, and non-direction? (ternary quantifications are always pretty)

zoobyshoe

I can't think of an example of energy that doesn't involve relative motion, so I think this is a good way to look at it to unify all the kinds of energy we speak of into one thing.

Tom Mattson

The problem with Moonrat's identification is that the term "motion" is already attached to a very different concept, namely that of velocity. Energy is not the same as velocity. The two have different units, and are described by different dynamical laws.

PRyckman

If motion is energy then a PARticle of light travelling light speed would have infinite energy???? I think I just figured out why there is an uncertainty principle. Because if you know the exact position of that photon and know its exact speed(light) then it is disobeying the laws of physics!!!! Score one for d=E(t)

chroot

PRyckman:

A) The energy of a particle always given by [itex]E = \sqrt{p^2 c^2 + m_0^2 c^4}[/itex]. Photons have no rest-mass, so [itex]m_0[/itex] is 0. That collapses the equation down to just [itex]E = pc[/itex], where p is the momentum carried by the photon. Photons do not have infinite energy.

B) You cannot simultaneously know the position and momentum of a photon to a precision that would violate the uncertainty relation.

C) Do not post your personal theories in the general physics forum.

- Warren

PRyckman

ok sorry, got ahead of myself. But if you could measure that photon's momentum and place at the same time without interacting with it. Would it's M make it an impossibility?

chroot

You can't measure a photon's momentum and position to infinite precision at the same time.

- Warren

PRyckman

What are the observations of it's speed, I ask you ,You know it must exist somewhere in there in probability. So If it were possible, what would your calculations show, objevtively

chroot

Are you asking "how do you know light travels at c?"

- Warren