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I have read on this site (I can't remember who): There is only ONE kind of energy.

I also read, in this question, that there is indeed a difference. In classical thermodynamics that one can speak of an energy density. But in the more fundamental explanation with elementary particles (or whatever kind of elementary objects), the concept of energy density seemed rather complicated, but it isn't.

If we consider the particles as point-like, then obviously they would have an infinite energy density (either potential or kinetic). And so also a huge collection (ensemble) of them will have the same infinite energy density (again, either potential or kinetic).

Unless we consider the particles as not-point-like (how, I think, doesn't matter). In that case, they do have an energy density.

Now let's look at the photon. What kind of energy (if it's not point-like) the photon will carry? According to my, it can't be potential. Because of "the simple fact" that they are the cause of this potential.

So can it only be kinetic, or am I supposing something wrong? That's my question.

Deschele Schilder
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  • This question seems very opinion based in its description. I can't understand why you are making these assertions, like "there is only ONE kind of energy" who said this? And the idea the being pointy leads to infinite energy? In what context? Please edit this do be more objective, perhaps cite some books or references. –  Dec 28 '19 at 15:20
  • @ggcg I assume "pointy" means "pointlike", based on the context. – probably_someone Dec 28 '19 at 15:33
  • @ggcg Someone with a high reputation on this site did. I can't remember the name anymore, so... – Deschele Schilder Dec 28 '19 at 16:10
  • @probably_someone, that's fine but make the connection between that and the statements that follow. –  Dec 28 '19 at 16:17
  • @descheleschilder, what is the definition of ONE. High rep on this site may or may not correlate with professional rep. Does Prof Higgs post here? Maybe. –  Dec 28 '19 at 16:18
  • @descheleschilder In what context do you say that a point particle has an infinite energy density? Newtonian, Relativistic, as a QFT? –  Dec 28 '19 at 16:19
  • @ggcg I dó know that Prof van 't Hooft (he's my country fellow, has questioned or answered. I have seen it with my own eyes (only). The definition of ONE is 1. I asked this question in the context of QFT. – Deschele Schilder Dec 28 '19 at 16:30
  • Regardless, one's online creds do not necessarily correlate to true knowledge. How is ONE kind of energy defined in the context of your question? A matter particle acquires "potential" via its interactions with other fields, an interaction term. In the absence of all other particles photons are not self interacting like quarks. Hence I would tend to think of them as not having potential energy. However in classical field theory we sometimes associate different field energy densities with kinetic and potential. This may just be for convenience and not literal. –  Dec 28 '19 at 16:52
  • You're right about the credits, but I do remember what was said. You write: "A matter particle acquires "potential" via its interactions with other fields, an interaction term." We are talking not about matter particles here (though matter and energy are equivalents), but about massless interaction particles. – Deschele Schilder Dec 28 '19 at 17:07
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    look at http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/vec4.html where the invariant mass is defined for four vectors. When itis zero all the energ is due to the momentum of the particle, so you could call it kinetic, which means having motion.. – anna v Dec 28 '19 at 17:17

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Photons are pure kinetic energy.

Moreover, you could say the energy of a photon is purely kinetic energy. In relativity theory, massive particles have both kinetic energy and a potential energy which is proportional to their mass. Photons have no mass, hence their energy is purely, and wholly, kinetic.

[The concept of Energy][1] in special relativity includes the energy inherent in the rest mass of the system . $$\sqrt{P\cdot P}=\sqrt{E^2-(pc)^2}=m_0c^2$$ Here p is the momentum vector of the particle, and one can say the $(pc)$ is the kinetic energy term of the particle in special relativity. When mass equals zero, as with the photon, the total energy is kinetic energy.

Do photons have kinetic energy?

It would not be correct to talk about gravitational potential energy of a photon either. You can talk about gravitational redshift of a photon though.

Does the potential energy for a given photon increase or decrease in quanta?

http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/blahol.html

Árpád Szendrei
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