# One photon at a time?

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1. Jun 30, 2015

### Ponderer

This school page goes over the basic math of calculating the photon energy and wavelength emitted by an electron in a hydrogen atom. It comes to one photon near 490nm. Is one photon always emitted by atoms? Why can't it be two or more photons at lower energy? Has a fraction of photons ever been detected such as 1.2 photons? Thanks

http://www.schoolphysics.co.uk/age1...ons/text/Energy_levels_in_hydrogen/index.html

2. Jun 30, 2015

### BiGyElLoWhAt

Just out of curiosity, how would you interpret 1.2 photons? What would .2 photons mean?

3. Jun 30, 2015

### Ponderer

It would mean h*f*0.2

4. Jun 30, 2015

### Ponderer

SPDC is the only exception?

5. Jun 30, 2015

### Physicist97

Well the point of the photon being quantized is that you can only have an integer number of photons (1, 2, 3... etc). You can't have 1.3 or 1.7 photons. The formula h*f tells you the energy of the photon, so h*f*0.2 could at best be interpreted as a photon with an energy that is 0.2 of some other measured photon, but it is still a whole photon.

6. Jun 30, 2015

### BiGyElLoWhAt

I would interpret that as a photon with one fifth the frequency. h*(0.2*f)

We don't really have a good idea of what a photon is, so to actually give physical meaning to a fraction of a photon (outside of an average) would be really hard to do.

7. Jun 30, 2015

### BiGyElLoWhAt

Not the only, but it is one example, of which there are many reliable sources to read.

8. Jun 30, 2015

### Ponderer

What about thermal energy from say a hot glowing piece of iron? IOW, would an electron in the hot iron that emits two or more simultaneous photons be a rare event?

9. Jun 30, 2015

### BiGyElLoWhAt

I would venture to say so. What's different about this case, is that the energy is stored in kinetic energy of the atom, versus with SPDC, it's stored in potential energy of the electron jumping to an outer shell. When the electron jumps out a layer (from say s to d [I think that's the right order, my chem sucks]) it isn't stable, so it collapses back to the previous layer, emitting energy. If the crystal is oriented properly (i.e. non linear) it can emit 2 photons which are entangled. My college uses BBO for this.

As far as why kinetic energy doesn't seem to release 2 photons when it's emitted, I'm not sure, honestly.

10. Jun 30, 2015

### bhobba

They never occur in our theories.

Thanks
Bill

Last edited: Jun 30, 2015
11. Jun 30, 2015

### bhobba

And exactly in what way is QED lacking in giving us a 'good idea'?

QED is the most accurately verified physical theory of all time.

Thanks
Bill

12. Jun 30, 2015

### bhobba

I an not sure 'jump' is the best language to use here - transition is probably better. What's going on really requires QED:
https://en.wikipedia.org/wiki/Spontaneous_emission

Thanks
Bill

13. Jun 30, 2015

### Ponderer

But if one formulates equations to predict how a ball bounces, does that explain what's inside the ball or what any possible fields around the ball are made of?

14. Jun 30, 2015

### Ponderer

"The most energetic photons from electron transitions in helium have energies of around 39 electron volts.The photon energy scales as Z2, so analogous photons observed in the helium-like atoms witha nuclear charge of Z=22, should have energies that are (22/2)2=121 times higher.The most energetic photons from the helium-like atoms, studied with high precision bent-crystal spectroscopy, have energies around 4,750 electron volts, which is in the soft x-ray region.The energy vs. Z of the most energetic photons from these studies of helium-like atoms were compared with the predictions of quantum electrodynamics, a part of the Standard Model that, up to now, has had an essentially unblemished record in predicting the results of experimental measurements.It was found that the data are systematically larger in energy than the 3-body QED predictions by about 0.1 to 0.6 electron-volts, depending on the value of Z.Further, the deviations in the heavier high-Z helium-like atoms appear to grow as Z3.The reported discrepancy with QED has a statistical significance of about 5 standard deviations. Thus, QED, a central and highly trusted component of the Standard Model, seems to be failing in a very fundamental and consistent way."

http://www.npl.washington.edu/AV/altvw167.html

http://www.nist.gov/pml/div684/ebit-112712.cfm

15. Jul 1, 2015

### BiGyElLoWhAt

@ bhobba how does QED define a photon? Just out of curiosity. Specificly, please.

16. Jul 1, 2015

### bhobba

I think you need to cognate on what explain means.

Our theories depend on assumptions that explain the predictions of the theory.

In your example classical mechanics explains how the ball bounces and doesn't require the other stuff you mentioned.

QED explains the behaviour of photons.

Thanks
Bill

17. Jul 1, 2015

### bhobba

18. Jul 1, 2015

### bhobba

Don't know that one.

But if true it would be earth shattering news comparable to the paradigm shift that occurred early last century. Since that hasn't happened its likely, at the most controversial, and probably wrong.

I suggest you do a separate post about it where people up on that sort of thing can comment.

Thanks
Bill

19. Jul 1, 2015

### Derek Potter

Ah yes, but "our" theories don't believe in virtual particles - do you? :)

20. Jul 1, 2015

### bhobba

And its relevance to actual photons or the question asked is?

Virtual particles are part of the theory. They are in the Dyson series and pictorially represented in Feynman diagrams: