# Origin of photons

1. Aug 11, 2007

### DanielS

This sounds like a simple question to me, but the answer eludes.

When you light a match, how are photons created? Or even any light source.

I cant seem to wrap my brain around this one.

2. Aug 11, 2007

### malawi_glenn

Many processes; friction that makes heat -> Chemichal reactions begin -> Energy is released, as heat and photons.

Thas is probably the easiest derivation of that issue I can give. Or are you asking from a quantum physics point of view?

3. Aug 11, 2007

### DanielS

Yeah, in a quantum physics view.

I'm wondering how the photon comes into existence.

Also, do photons eventually cease to exist?

4. Aug 11, 2007

### malawi_glenn

Okay

What sources have you considered?

5. Aug 11, 2007

### DanielS

I cont find anything. I can find loads of information about photons, but nothing about how they are created.

6. Aug 11, 2007

### malawi_glenn

I know that the answer must lie in Quantum Field theory, and that is nothing I am familar with at the moment, only conceptual knowledge. I think there should be some information about this if you search on wikipedia for photons.

7. Aug 11, 2007

### quetzalcoatl9

you are initiating a chemical reaction. the free energy for this reaction is exothermic overall, after you can surmount an energy barrier that is significantly higher than kT at room temperature. the energy difference between the two states (unlit match vs. lit match) is given off in two forms: kinetic (consider the momenta of the nuclear centers) and emission (electronic transitions from an excited state back down to the ground state, giving off the light that you see as E=hv).

the overall reaction is a complicated combination of those two contributions. in fact, predicting the exact spectra from that reaction a priori would be a difficult research project in itself (since the two processes are actually coupled) and would involve molecular simulation techniques such as ab inito molecular dynamics (carr-parinello, born-oppenheimer MD, tight-binding methods, etc). in short, an excellent and practical question to which most ppl will probably blow smoke up you-know-where when asked.

8. Aug 15, 2007

### premagg

May be,I think that the chemical energy of the reaction is converted to heat.Then this heat generates photons.But I also wonder how at once the photons are into existence.May be that the molecules of compound radiate and that radiation brings the photon into existence.Since the radiation are waves and have energy asociated with it,we can treat the radiations as light which is comprised of photons.

9. Aug 18, 2007

### quetzalcoatl9

it's not "maybe" it is what i have already described

10. Aug 19, 2007

### reilly

If you look carefully at Quantum Field Theory, you'll see that emission and absorption of photons is assumed. So it's fair to say that we know photons are emitted, but we really don't know why that is so . However, that acceleration causes radiation in classical E&M is well known, and can be thought of as an adjustment in the fields,to insure that there is no instantaneous propagation of signals. Perhaps this idea might be generalizable to QM.
Regards,
Reilly Atkinson

11. Aug 20, 2007

### Son Goku

My understanding of Quantum Field Theory is that you assume that the Dirac Field and Electromagnetic field interact, which upon quantisation leads to electrons emitting and absorbing photons. However the fact that the Dirac Field interacts with the Electromagnetic Field can be derived from the local $$U(1)$$ gauge invariance of the Dirac Field.

12. Aug 20, 2007

### Idjot

This is a curious thread. How can there be so much detailed expert discussion without even an utterance of the process' name? The physical details are easy to find when you know the name of the process. Incandescence.

13. Aug 21, 2007

### Son Goku

I think searching for incandescence will only get the OP information about details such as what spectra of light is emitted from certain substances and other similar details. i.e., you would information about light emission as a property of bulk systems, rather than information about the mechanism of photon emission. For information about the creation of photons you could look at introductory QED.

14. Aug 21, 2007

### arunma

To put it simply, the photons are created by electron orbital transitions. When chemical bonds are broken, various processes (described by other posters) occur which ultimately result in the transition of electrons to lower energy orbitals, and the excess energy is radiated away as photons.