Can Photons die?

[Silvestru]

I know that atoms break down into smaller particles through radioactive decay but do photons ever decay?

 

Considering they have no mass leads me to think "no" but I do not have strong physics knowledge so was hoping someone could shed some photons on this question.

[swansont]

No, they don't decay, which is a spontaneous reaction. They can interact, though.

 

You have to conserve charge, spin, energy and momentum. A photon can't spontaneously become a massive particle, for that would violate conservation of energy and/or momentum, at the very least.

[DrP]

I don't think so - we see photons that were emitted a very long time ago from galaxies far far away.

 

They can be absorbed, giving an electron higher energies for example - but I don't think they decay.

 

 

(sorry swansont - simultaneous postings)

Edited May 11, 2017 by DrP

[Silvestru]

They can be absorbed, giving an electron higher energies for example - but I don't think they decay.

 

I don't think that's the proper interaction between the electron and photon. By the same logic the electron can also emit photons.

[DrP]

It can emit photons - when it drops down in energy levels.

 

It Goes up energy levels when it absorbs a photon of the exactly right amount of energy to make the jump.

It depends on the energy different between the orbitals it jumps between as to whether it emits a photon or an x-ray for example.

[Janus]

 

I don't think that's the proper interaction between the electron and photon. By the same logic the electron can also emit photons.

Yes, and this is exactly what happens. The property of fluorescence relies on it. High energy photons are absorbed by the atom jumping electron upwards by more than one energy level. The electrons fall back to their original energy state one energy level at a time, emitting lower energy photons in the process. Fluorescent colors look bright because they absorb ultraviolet and re-emit in the visible range. Fluorescent lights work in the same way. "glow in the dark" material work by atoms absorbing photons and storing the energy in electron's higher energy states, which slowly fall back down over time, emitting light.

The energy level increase of the electron doesn't even have to originally come from the absorption of a photon. With chemiluminescence, the electrons are jumped up by chemical reaction and then release the energy as photons (a common example is the "glow stick".)

[swansont]

I don't think that's the proper interaction between the electron and photon. By the same logic the electron can also emit photons.

It can emit photons - when it drops down in energy levels.

 

It Goes up energy levels when it absorbs a photon of the exactly right amount of energy to make the jump.

 

It depends on the energy different between the orbitals it jumps between as to whether it emits a photon or an x-ray for example.

 

To be more precise, that photon interacts with the atom, and the energy level of the system changes. We say it's the electron, because that exists primarily outside the nucleus and absorption yields a distinct change in its wave function

 

Free electrons can only scatter photons. There are no excited states involved.

[DrP]

out of interest.... I do not suppose it is the SAME photon that gets emitted... Swans, do you know? Is it more like if you add 10 ml to a litre of water and then take 10 ml out later, you won't have exactly the same molecules of water extracted. What happens with the energy in the system that is the atom when it absorbs a photon... does it re-emit the same photon with the 'same' energy, or is the energy just taken from the pool?... I would expect that we do not know the answer to this fully. What do you think swan?

[swansont]

out of interest.... I do not suppose it is the SAME photon that gets emitted... Swans, do you know? Is it more like if you add 10 ml to a litre of water and then take 10 ml out later, you won't have exactly the same molecules of water extracted. What happens with the energy in the system that is the atom when it absorbs a photon... does it re-emit the same photon with the 'same' energy, or is the energy just taken from the pool?... I would expect that we do not know the answer to this fully. What do you think swan?

If it was in the same state how would we be able to tell? Photons are bosons and their numbers are not conserved, so it's perfectly reasonable to consider photons being created and destroyed, and there are processes where photon number goes up and where it goes down. But if the emitted photon we identical to the absorbed one, there's no way to tell if it's the same or if it's different. In e.g. spin statistics, the fact that particles are identical is crucial to the result. You would get a different answer if the particles were somehow distinguishable.

 

However, in a simple excitation process, there are many instances where we could tell that the same photon is not emitted, because the atom does not return to the same exact state, so the energy and/or the polarization are different than the one which was absorbed. So AFAIK it's generally accepted that it's not the same photon being emitted. You don't store photons in atomic excitations (despite the fact that some clumsy pop-sci articles imply you can). I suppose you could argue that you have the same photon but you've changed it somehow, but enter a realm where you are are arguing a distinction without a difference.

[DrP]

K-thanks. That's kind of what I thought. The photon is probably totally destroyed when absorbed imo and a new one is formed if the electron drops back down again, even if it goes to the exact same energy level. I doubt it would ever be the same photon. As you said - it's impossible to tell. I guess we could say it has died then after absorption, but it obviously isn't really the right terminology as it has never really lived as such.

[Bender]

If you make a wave in water, and it bounces off a wall, it is still the same wave?

 

I think that is a pretty meaningless question, just like it is meaningless to say two photons are "the same". Is a photon deflected by an electron "the same"? After all, due to the Compton-effect, the scattered photon has a different wavelength.

 

I know that atoms break down into smaller particles through radioactive decay but do photons ever decay?

 

Considering they have no mass leads me to think "no" but I do not have strong physics knowledge so was hoping someone could shed some photons on this question.

High energy photons can "decay" in particle - anti-particle pairs, but only near a nucleus. The nucleus is required for conservation of momentum.

[swansont]

High energy photons can "decay" in particle - anti-particle pairs, but only near a nucleus. The nucleus is required for conservation of momentum.

 

 

Which makes it an induced interaction rather than a spontaneous decay.

[Bender]

True.