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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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1 Answers1

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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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