# Are photons energy? What is energy, anyway?

Discussion in 'Physics & Math' started by origin, Aug 19, 2019.

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1. ### originHeading towards oblivionValued Senior Member

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No light is not energy. Photons carry energy. A rock can carry PE or KE but a rock is not energy.

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To be pedantic, it is more correct to say that light follows geodesics in curved spacetime, but most know what you mean.
Light itself is part of the electromagnetic spectrum, which exhibits the whole known range of electromagnetic radiation, from radio and microwaves to gamma waves, including the visible part of that radiation we call light, the quanta of the EMS being the photon.
Taking that further and noticing some more pedant, the EMR is the flow of energy at "c "

Last edited: Aug 19, 2019

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And supplementary, a photon can take the guise of any part of the EMS/EMR, and the energy associated with any guise of that photon, depends entirely on the frequency and/or wavelength.

7. ### arfa branecall me arfValued Senior Member

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Yes, it is.
What does that mean? How do they "carry" energy, and in what?

8. ### originHeading towards oblivionValued Senior Member

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Look it up. I don't want to hijack the thread.

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Hope that helps.

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https://en.wikipedia.org/wiki/Photon
extract:
The photon as a gauge boson
Main article: Gauge theory
The electromagnetic field can be understood as a gauge field, i.e., as a field that results from requiring that a gauge symmetry holds independently at every position in spacetime.[102]For the electromagnetic field, this gauge symmetry is the Abelian U(1) symmetry of complex numbers of absolute value 1, which reflects the ability to vary the phase of a complex field without affecting observables or real valued functions made from it, such as the energy or the Lagrangian.

The quanta of an Abelian gauge field must be massless, uncharged bosons, as long as the symmetry is not broken; hence, the photon is predicted to be massless, and to have zero electric charge and integer spin. The particular form of the electromagnetic interaction specifies that the photon must have spin ±1; thus, its helicity must be {\displaystyle \pm \hbar }

. These two spin components correspond to the classical concepts of right-handed and left-handed circularly polarized light. However, the transient virtual photons of quantum electrodynamics may also adopt unphysical polarization states.[102]

In the prevailing Standard Model of physics, the photon is one of four gauge bosons in the electroweak interaction; the other three are denoted W+, W− and Z0 and are responsible for the weak interaction. Unlike the photon, these gauge bosons have mass, owing to a mechanismthat breaks their SU(2) gauge symmetry. The unification of the photon with W and Z gauge bosons in the electroweak interaction was accomplished by Sheldon Glashow, Abdus Salamand Steven Weinberg, for which they were awarded the 1979 Nobel Prize in physics.[103][104][105] Physicists continue to hypothesize grand unified theories that connect these four gauge bosons with the eight gluon gauge bosons of quantum chromodynamics; however, key predictions of these theories, such as proton decay, have not been observed experimentally

11. ### exchemistValued Senior Member

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With any wave, energy is present in the travelling displacement from equilibrium. In the case of the photon, the displacement is in the electric and magnetic field.

So Origin is quite right. Photons carry electromagnetic energy, but what they are is a travelling displacement in these fields.

(I am less concerned than Origin about hijacking this thread, since is it is nonsense thread created by a sockpuppet of Theorist, for the purpose of yet more attention-seeking - and I'd far rather be discussing science.

)

12. ### arfa branecall me arfValued Senior Member

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A candle flame gives off light, and it gives off heat, what we sometimes call radiant heat.

Is heat a form of energy?

13. ### originHeading towards oblivionValued Senior Member

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Heat is the transfer of translational kenetic energy from hot to cold.

In the case of radiant heat the energy is transferred by photons.

Photons are not defined as energy by mainstream science.

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14. ### arfa branecall me arfValued Senior Member

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The energy of a photon is given by $E = h\nu$ where $\nu$ is the frequency and h is Planck's constant.

So does the frequency "carry" the photon's energy or what?

15. ### exchemistValued Senior Member

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The kinetic energy of a cricket ball is given by 1/2 mv².

So does the mass carry the energy, or the velocity, or what?

16. ### exchemistValued Senior Member

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Yes, heat is a form of energy.

As is electromagnetic energy, for instance in the magnetic field created by a current flowing in an electromagnet.......or in a photon.

Energy is one property of a photon, alongside its other properties, including momentum, spin and frequency. Just as heat content is one property of any material object.

Energy is also a property of a sound wave, in that case from compression and rarefaction of the air, away from the equilibrium value of the air pressure. Nobody would say that sound "is" energy, though. That would be a fairly useless description of it.

17. ### originHeading towards oblivionValued Senior Member

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The amount of energy of a photons is proportional to its frequency and wavelength.

18. ### arfa branecall me arfValued Senior Member

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My comments are really only about how "useless" or otherwise it is to say energy is "carried by" photons, or by anything (like a cricket ball being bowled). We can equally say a photon "has" energy; we can make "the energy of" a photon meaningful (as in the photoelectric effect).

But that's all just because we need familiar linguistic terminology. Saying photons carry energy, from place to place, is meaningful if that's what you want energy to mean. You can just as meaningfully say that photons, or cricket balls, carry 'information' from place to place.

Does that mean we (inadvertently) are saying energy and information are equivalent?

19. ### Michael 345New year. PRESENT is 72 years oldlValued Senior Member

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Would you settle for

"A" has energy (the ability to perform work) and

when A" transfers energy from itself ("A" losses energy by working)

to "B" ("B" gains energy)

20. ### exchemistValued Senior Member

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Self-evidently, no, it doesn't, any more than energy and spin are equivalent.

You can also say a photon "carries" momentum, or spin, too, if you like as it is equivalent to saying that it "has" (i.e. possesses) those properties. Nobody has any real difficulty with such terminology, as everyone understands that these are properties, that is to say, attributes, of the photon.

But what you were actually maintaining earlier, which was that a photon "is" energy, is a wrong idea. That was what Origin was pointing out. It is a popular misconception that energy is "stuff" of some sort, when it is a property of a physical system.

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21. ### arfa branecall me arfValued Senior Member

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What "self-evidently" do you mean?

When you want to know a photon's energy, the photon has to be annihilated, right?

22. ### James RJust this guy, you know?Staff Member

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You mean, if you want to measure the energy? Yes. But that doesn't mean the photon is energy. It's just that in the process of annihilating a photon you get energy transfer to some other system.

23. ### exchemistValued Senior Member

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And to measure its spin, what has to happen?

Or its momentum?