# Does a photon have only kinetic energy or can it have potential energy as well? [closed]

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.

• 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.
– user196418
Commented Dec 28, 2019 at 15:20
• @ggcg I assume "pointy" means "pointlike", based on the context. Commented Dec 28, 2019 at 15:33
• @ggcg Someone with a high reputation on this site did. I can't remember the name anymore, so... Commented Dec 28, 2019 at 16:10
• @probably_someone, that's fine but make the connection between that and the statements that follow.
– user196418
Commented Dec 28, 2019 at 16:17
• look at 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.. Commented Dec 28, 2019 at 17:17

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

• And right now I know that what I read was that the person in question said that there does NOT exist something like PURE energy. So I don't think he was right about that. Photons ARE pure energy. Or not? Commented Dec 28, 2019 at 18:10
• @descheleschilder yes they are the quanta. Commented Dec 28, 2019 at 18:34
• @descheleschilder physics.stackexchange.com/questions/15122/what-is-pure-energy Commented Dec 28, 2019 at 18:36
• What do you think about the second answer and the third (by Ron, off which I think that it's the best one). Look here:physics.stackexchange.com/questions/257628/… Commented Dec 29, 2019 at 2:20
• @descheleschilder yes those are good ones. Commented Dec 29, 2019 at 2:37