# Does energy really exist?

I'm not sure if I understand what energy really is but it seems to me that energy should be just the force that moved some object and not actualy an existing thing in the sense that it is physical in form. Is there a difference between the energy involved in moving an object and the energy like electricty and EM?

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Dale
Mentor
energy should be ... not actualy an existing thing in the sense that it is physical in form.
I don't know what you mean by this. Can you explain?

DavidSnider
Gold Member
Energy is a measurement, like height. Height doesn't "exist", yet it describes something that does.

I haven't gone that much past the classical theory of physics, so I'm not sure if the definition of energy becomes less or more vague, but in general you can postulate that all things need 'something' to change state - (change position, temperature, velocity, etc...). Work cannot be done without changing state. This 'something' is referred to as energy. A force is something that changes the state of a system (it accelerates an object), so it is a method of transferring energy.

This postulation is somewhat similar to how people postulated an ether.

What was described above were changing states; so we were referring to the transfer of energy (work, heat). It can also be said that for a transfer of energy to occur, each system should have an amount of energy that varies based on the state of the system. Therefore, it could be said that even a stationary system may have some energy.

I didn't really discuss its existence...but it might be helpful to know that its a concept we can measure and it is useful.

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Dale
Mentor
Energy is a measurement, like height. Height doesn't "exist", yet it describes something that does.
I don't know about this. Height is measurable and has some testable and predictable physical consequences, so it is hard for me to say that it doesn't "exist". Your usage of "exist" would imply that things like mass and charge also don't "exist". I am not saying that it is an illogical position, but it is certainly not the only possible definition and it may lead to consequences that are not really desirable. I just don't think it is so simple.

Yes, there is a difference in the sense that energy can be in different forms. It does really exist. In fact, it can be converted back and forth between mass and energy.

However, don't expect that to be intuitive. The human brain has, afterall, only understood basic physics intuitively for the majority of the human timeline.

Not sure if energy is real.

At first note a post by me
https://www.physicsforums.com/showpost.php?p=2250182&postcount=26
where I show the for usual interactions the conservation of energy as defined by
$$E_\text{kin}=\int v\mathrm{d} p$$
is an artificial consequence of the conservation of momentum.

And then Einstein came along and said the $E_\text{kin}=mc^2$, i.e. mass as defined by $m=p/v$ is equal to this artifical(?) kinetic energy.

In my opinion this equation states that normal particles can create or absorb photons.... somehow.

Actually from my definition of kinetic energy
$$E_\text{kin}=\int v\mathrm{d}p$$
and the brilliant
$$E=\frac{p}{v}c^2$$
the equation for mass in special relativity follows.

ZapperZ
Staff Emeritus
And then Einstein came along and said the $E_\text{kin}=mc^2$, i.e. mass as defined by $m=p/v$ is equal to this artifical(?) kinetic energy.

In my opinion this equation states that normal particles can create or absorb photons.... somehow.
Can you show valid experimental evidence to back this up? For example, I have this free electron. Can you show me evidence that this electron can "create and absorb photons" based on that Einstein equation?

You've also made a bunch of your own "definitions". In case you've forgotten about the PF Rules, please review it now, especially the part on speculative post.

Zz.

Can you show valid experimental evidence to back this up? For example, I have this free electron. Can you show me evidence that this electron can "create and absorb photons" based on that Einstein equation?

You've also made a bunch of your own "definitions". In case you've forgotten about the PF Rules, please review it now, especially the part on speculative post.
There is of course no experimental evidence since in the end I have nothing more than the normal equations and no new physical insight. Only the derivation starts from another point. The results are valid for both classical and relativistic mechanics depending on how you define p(v). I find it more insightful than just defining conservation of energy which is somewhat comparable to creationists ideas.

The statement that E=mc2 connects photon energy and kinetic energy of objects, is a very vague one as I indicated - that's why I didn't go much into detail. It comes from thinking that "my kinetic energy" is due to the force and hence photon action and "my mass" is due to the kinetic energy of an object. Since energy and thus the mass obey the conservation laws, they could probably convert into each other.

What do you mean by own definitions? First, at least in undergrad physics, there is no alternative definition apart from saying "let there be mass" and second these proposed definitions don't conflict with neither classical nor the special relativity picture. I did mention the word "my" and "my opinion" at all places.

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I'm not sure if I understand what energy really is but it seems to me that energy should be just the force that moved some object and not actualy an existing thing in the sense that it is physical in form. Is there a difference between the energy involved in moving an object and the energy like electricty and EM?
Energy exists like money. Particles, systems exchange with it - give away and obtain. Exchange property allows measurements.

Born2bwire
Gold Member
There is of course no experimental evidence since in the end I have nothing more than the normal equations and no new physical insight. Only the derivation starts from another point. The results are valid for both classical and relativistic mechanics depending on how you define p(v). I find it more insightful than just defining conservation of energy which is somewhat comparable to creationists ideas.

The statement that E=mc2 connects photon energy and kinetic energy of objects, is a very vague one as I indicated - that's why I didn't go much into detail. It comes from thinking that "my kinetic energy" is due to the force and hence photon action and "my mass" is due to the kinetic energy of an object. Since energy and thus the mass obey the conservation laws, they could probably convert into each other.

What do you mean by own definitions? First, at least in undergrad physics, there is no alternative definition apart from saying "let there be mass" and second these proposed definitions don't conflict with neither classical nor the special relativity picture. I did mention the word "my" and "my opinion" at all places.
m_0c^2 is not a kinetic energy, it is the rest energy of the object, it will never be converted to kinetic energy, heat or anything of the kind without annhilation of the object.

I find it more insightful than just defining conservation of energy which is somewhat comparable to creationists ideas.
This is just nonsense, the conservation of energy is a basic tenets of physics, part of the fundamentals of thermodynamics.

...the conservation of energy is a basic tenets of physics, part of the fundamentals of thermodynamics.
Conservation of energy is an experimental fact. Of course, it exists in the theory too, as should be.

m_0c^2 is not a kinetic energy, it is the rest energy of the object, it will never be converted to kinetic energy, heat or anything of the kind without annhilation of the object.
Please look up E=mc2 for yourself. In that equation m is the relativistic mass and not m_0. At no point I mention the rest mass.

This is just nonsense, the conservation of energy is a basic tenets of physics, part of the fundamentals of thermodynamics.
It's kind of naive to believe everything they tell you. I mean you don't have to disbelieve your teachers at school, when they tell you about energy conservation, but it's a wise choice to try to find reasons for laws whenever possible.

They have told you that entropy increases, right? And then one day you might find a system where this doesn't seem to apply.
A person who knows where the increase of entropy derives from could distinguish between systems where the 3rd law of thermodynamics applies and where it doesn't. That's basically the difference between engineering and physics.

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I believe that energy needs to be classified into two separate categories:
1) Intrinsic energy contained in a system per degree of freedom, such as the pressure or temperature of a gas.
2) Free energy, which is useful to do work only until the entropy of a particular closed system is maximized, meaning that the energy density content of everything has reached a final equilibrium for all degrees of freedom, and no more work can be extracted.

So intrinsic energy will always exist, until the Universe expands to infinity and until the CMB temperature reaches absolute zero. Free energy will exist until the universe reaches a steady state equilibrium.
Bob S

ZapperZ
Staff Emeritus
Please look up E=mc2 for yourself. In that equation m is the relativistic mass and not m_0. At no point I mention the rest mass.
I think it is you who need to look up what that means. For example, read E. Hecht, Am. J. Phys. v.77, p.799 (2009). I will quote just the first paragraph:

Einstein's first paper on relativity appeared when the concept of speed-dependent electromagnetic mass had already become a topic of considerable interest. He accept this idea but changed his mind after being confronted by a far more compelling insight. We will show that after reading Planck's 1906 article in which the concept of relativistic momentum was introduced, Einstein came to realize that it was the relativistic equations for energy and momentum that were primary. From that perspective, it became clear that the inertial mass m was invariant, and he never again spoke of mass as being speed dependent.
Further verification of that came from Lev Okun in L.B. Okun Am. J. Phys. v.77, p.430 (2009). It is very clear that that "m" in the Einstein equation is meant to be just the rest (invariant) mass.

Zz.

Further verification of that came from Lev Okun in L.B. Okun Am. J. Phys. v.77, p.430 (2009). It is very clear that that "m" in the Einstein equation is meant to be just the rest (invariant) mass.
Well, today any course on special relativity teaches to use the relativistic mass for E=mc^2.
Just look up Wikipedia or in fact any resource
http://en.wikipedia.org/wiki/Einstein's_Mass-Energy_Relation

Maybe there was another history or other meaning at first. I'm still sure that m is the relativistic mass. Once I get that paper I can think about it again. Meanwhile you can explain why it is written everywhere that m is the relativistic mass.
(of course for particles at rest m=m_0)

If you use E_0=m_0c^2 then this E_0 has no meaning as it is not the term needed for energy conservation laws.

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ZapperZ
Staff Emeritus
Well, today any course on special relativity teaches to use the relativistic mass for E=mc^2.
Just look up Wikipedia or in fact any resource
http://en.wikipedia.org/wiki/Einstein's_Mass-Energy_Relation

Maybe there was another history or other meaning at first. I'm still sure that m is the relativistic mass. Once I get that paper I can think about it again. Meanwhile you can explain why it is written everywhere that m is the relativistic mass.
(of course for particles at rest m=m_0)

If you use E_0=m_0c^2 then this E_0 has no meaning as it is not the term needed for energy conservation laws.
Besides the fact that you're using Wikipedia as a source (which should scare you, but it doesn't, obviously), where exactly in that article does it say that "m" is the relativistic mass in that Einstein equation?

If you're referring to this passage:

The relativistic mass is always equal to the total energy divided by c2.
then you obviously missed a day in your class on SR, because TOTAL ENERGY is the key phrase here. Read our FAQ in the General Physics forum if you have never seen that in your SR class.

While you're idling around, try to wrap your head around this one:

If I have 1.022 MeV photon, and it undergoes a pair production of e+ and e- (which is what is allowed per the E=mc^2 equation), then what kinetic energy do these two particles have? I've just applied that Einstein equation, so show me where this "relativistic mass" is.

Zz.

Besides the fact that you're using Wikipedia as a source (which should scare you, but it doesn't, obviously), where exactly in that article does it say that "m" is the relativistic mass in that Einstein equation?
Cmon, stop being pedantic. Absolutely all equations you find an books read
$$E=mc^2=\gamma m_0 c^2$$
Throughout the text they write E and m as I suggested. They write E=mc^2 and m=\gamma m_0.

There is no equation with
$$E_0=m_0 c^2$$
where E_0 is given any meaning. Just give me any SpecRel book and I scan and post it here.
Any now don't say "oh you mean *total energy*?!". Of course "total energy" since E_0 has no physical significance.

If I have 1.022 MeV photon, and it undergoes a pair production of e+ and e- (which is what is allowed per the E=mc^2 equation), then what kinetic energy do these two particles have? I've just applied that Einstein equation, so show me where this "relativistic mass" is.
You cannot have "single photon -> e+ and e- (with identical mass)" (unless there is another body like a nucleus)
Energy and momentum conservation wouldn't be valid. That's basics from the SpecRel class!
So I'm not sure what you calculated.

Cmon, stop being pedantic. Absolutely all equations you find an books read
$$E=mc^2=\gamma m_0 c^2$$
Throughout the text they write E and m as I suggested. They write E=mc^2 and m=\gamma m_0.
Wrong, See Beiser, A. Concepts of Modern Physics Pg 27

There is no equation with
$$E_0=m_0 c^2$$
where E_0 is given any meaning. Just give me any SpecRel book and I scan and post it here.
Any now don't say "oh you mean *total energy*?!". Of course "total energy" since E_0 has no physical significance.
Again incorrect, see the same pg on Beiser for a clear statement $$E_0=mc^2$$ as the rest energy. I am unclear if you are trying to suggest that there no energy in a system at rest...or that is somehow isn't a real physical quantity? It certainly is a physical quantity, the whole field of chemistry doesn't work without it.

ZapperZ
Staff Emeritus
You cannot have "single photon -> e+ and e- (with identical mass)" (unless there is another body like a nucleus)
Energy and momentum conservation wouldn't be valid. That's basics from the SpecRel class!
So I'm not sure what you calculated.
But the nucleus or a heavy body does NOT contribute to the energy of the system. If it does, then I don't need a minimum energy for the photon since it can suck in energy from other sources to produce the pair production. The nucleus is simply there for momentum conservation! So where is the relativistic mass in that system?

Again, read BOTH of the references I gave. You'll see that the concept of "relativistic mass" is very faulty, and there are an increasing number of textbooks that are moving away from using that concept.

Zz.

Wrong, See Beiser, A. Concepts of Modern Physics Pg 27
By the end of this week I hope to get this book and the articles from Zapper to clear up things.

Again incorrect, see the same pg on Beiser for a clear statement $$E_0=mc^2$$ as the rest energy. I am unclear if you are trying to suggest that there no energy in a system at rest...or that is somehow isn't a real physical quantity? It certainly is a physical quantity, the whole field of chemistry doesn't work without it.
Of course that is the rest energy and I wrote that. But this rest energy is not conserved in general, unless the particle is really at rest - when of course this rest energy coincides with the relativistic energy.

My statement is that if you want to have something to obey energy conservation in scattering processes, then you have to take $E=\gamma m_0c^2$
Only for particles at rest you can incidently take $E(v=0)=E_0=m_0c^2$
Or in chemistry $\gamma\sim 1$ so it is imprecise to take E0, but doesn't make much of a difference.

Question:
Two 1MeV photons collide head on, annihilate and create an electron-positron pair. What is the energy of the electron? It's 1MeV indeed. So why is your definition of energy E=m_0c^2 not giving this? Because you have an "energy" that isn't conserved. And in fact what do you ever use it for (apart from for particles at rest)?

Born2bwire
Gold Member
Please look up E=mc2 for yourself. In that equation m is the relativistic mass and not m_0. At no point I mention the rest mass.
You are still missing the point. mc^2, m_0c^2 are not kinetic energies. When will mc^2 be zero for a particle at rest?

It's kind of naive to believe everything they tell you. I mean you don't have to disbelieve your teachers at school, when they tell you about energy conservation, but it's a wise choice to try to find reasons for laws whenever possible.

They have told you that entropy increases, right? And then one day you might find a system where this doesn't seem to apply.
A person who knows where the increase of entropy derives from could distinguish between systems where the 3rd law of thermodynamics applies and where it doesn't. That's basically the difference between engineering and physics.
So please, please, explain to me how conservation of energy is in any way comparable to creationism?

You are still missing the point. mc^2, m_0c^2 are not kinetic energies. When will mc^2 be zero for a particle at rest?
I presumed that people who answer to this have at least some knowledge of SpecRel so I only loosely refered to m as kinetic energy. The kinetic energy as you know it is $E_\text{kin}=mc^2-m_0c^2$

So please, please, explain to me how conservation of energy is in any way comparable to creationism?
I make a short note, but if you do not accept it then argueing about it is as useless as argueing with a creationist.
Some people just like to define everything as laws of nature like energy conservation and 2nd law of thermodynamics or Ohms law. Equally creationists avoid any explanation but define that every question can be answered by "Because god wanted so" (which isn't strictly wrong but just useless).
Now someone who is interested in understanding the structure underneath will know better when to apply a concept and when the preconditions are not given. Whereas the former people will be shocked when Ohms law R=V/I doesn't hold. Just as you will be shocked when you find that the 2nd law of thermodynamics (might hold for all gases, but) doesn't hold for particular systems. I saw a paper once - maybe I can dig it out again.

ZapperZ
Staff Emeritus