# What is energy?

by Math Is Hard
Tags: energy
Emeritus
PF Gold
P: 5,532
 Quote by Math Is Hard I have trouble understanding and visualizing this concept because I want to think of something tangible. I am not sure if I should be thinking of something tangible, though.
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).

 Are photons energy, or are they a just a by-product of electrons jumping between energy levels?
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.
 Emeritus Sci Advisor PF Gold P: 4,922 Thank you. I appreciate all the responses. This was the clarification I was looking for.
 P: 618 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?
 P: 137 photons are both a particle and an energy wave. Depending on the circumstances
Emeritus
PF Gold
P: 5,532
 Quote by jdavel 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?
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.
 P: 169 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
Emeritus
PF Gold
P: 4,922
 Quote by Moonrat I suggest, in layman's terms, that energy is simply motion. It is not the ability of motion, but rather the motion itself. Moonrat
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.
P: 169
 Quote by 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.
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)
P: 5,630
 Quote by Moonrat I suggest, in layman's terms, that energy is simply motion. It is not the ability of motion, but rather the motion itself.
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.
 Emeritus Sci Advisor PF Gold P: 5,532 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.
 P: 137 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)
 Emeritus Sci Advisor PF Gold P: 10,427 PRyckman: A) The energy of a particle always given by $E = \sqrt{p^2 c^2 + m_0^2 c^4}$. Photons have no rest-mass, so $m_0$ is 0. That collapses the equation down to just $E = pc$, 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
 P: 137 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?
 Emeritus Sci Advisor PF Gold P: 10,427 You can't measure a photon's momentum and position to infinite precision at the same time. - Warren
 P: 137 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
 Emeritus Sci Advisor PF Gold P: 10,427 Are you asking "how do you know light travels at c?" - Warren
 P: 137 No I am asking, If you knew Where a photon was at a given time, and knew it's momentum to be c would that violate the laws of physics, other than the uncertainty principle
P: 5,630
 Quote by 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.
Yes, I see the problem. It isn't exactly accurate to say energy is motion. However, I don't see that the term motion is attached to velocity in a completely exclusive way. We don't say motion is velocity. I think it would be possible to arrive at a way to accurately describe all the forms of energy such that the relative motion in each is viewed as the unifying concept.

This would be a big help to those people who get hung up on grasping the concept. It explains alot about the interconvertability of the different forms, and hence about the conservation of energy. The motion of one thing gets transferred to another and to another: the kinetic motion of a coil relative to a conductor causes the motion of electrons that cause the motion of their electric fields which results in the motion of photons. Is it not safe to say that all things which are in motion have energy, and that all things that have energy are in motion? (Potential energy = potential motion, of course.)

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