# Photons don't decay, right?

1. Jan 4, 2008

### moe darklight

Ok, so my dad says that he remembers when he was young, the first Discover Magazine he ever bought was one that had the question "do photons decay?" on the cover. He also says that the answer was yes... which sounds weird to me... I mean, if photons are pure energy, and energy can't be destroyed; and they are also an elementary particle, which means that they can't "break down" into any smaller pieces, my logic just tells me that it would make no sense for them to decay...

so do they decay? ... my guess is that my dad misread the article; what do you think is most likely what the article was talking about?

(the article was from the spanish version of discover, so the english word "decay" was probably not the one being used)

2. Jan 4, 2008

### George Jones

Staff Emeritus
Are the Spanish words for "photon" and "proton" similar? If so, it's quite plausible that the magazine your father bought had an article about the possibility of proton decay.

3. Jan 6, 2008

### blechman

yes, PROTON decay was (and still is) a *very* big question that started getting a lot of press in the mid-1970's (through the present). So that makes much more sense.

BTW: as far as we know, the answer is *NO*, not yes...

photons cannot decay due to conservation of energy and momentum, but your suggested reasoning is somewhat flawed. First of all, everything is "pure energy" thanks to Einstein's famous E=mc^2. Second of all: many elementary particles can decay; in fact, MOST of them! The only ones that don't are the electron, photon, up quark and (possibly) neutrino. The real reason that the photon cannot decay is because it is massless. A counter-example is the Z boson, which is (VERY roughly speaking!!) a "fat photon", which can decay (and does so **VERY** fast - about 10^{-24} seconds!). However, it is just as "elementary" as the photon.

Massless particles can never decay. You can prove this mathematically by just writing down the energies and momenta of the decay products and showing that there are no values of these things that conserve everything. More intuitively: massless particles move at the speed of light. Particles that move at the speed of light do not experience "time" due to Einstein's special theory of relativity (infinite time dilation - they stop aging). Therefore they cannot decay since that would require a clock (the particle has to know how long it has to go before it decays).

BTW, this argument is how they relate neutrino oscillation to neutrino mass: if the neutrinos were truly massless, then they could not oscillate by this argument. Thus, even though we still cannot directly measure the neutrino mass since it's so small, we can still infer that it's not zero since we see the oscillations.

4. Jan 6, 2008

### DaveC426913

IIRC, protons do decay, they just have a half-life of like, 400 trillion years.

5. Jan 6, 2008

### mgb_phys

Proton decay is an interesting test of several GUT candidates. It has never been observed and the half-life must be greater than 10^36 years

6. Jan 6, 2008

### moe darklight

cool, that clears up a lot guys thanks

7. Jan 6, 2008

### Parlyne

Given that neutrinos are the lightest fermions, the lightest neutrino definitely cannot decay, since that would either violate conservation of 4-momentum or conservation of angular momentum.

8. Jan 7, 2008

### blechman

In the standard model, proton lifetime is INFINITY! Only in extensions like GUT's does the proton possibly decay.

9. Jan 12, 2008

### Bowles

The argumentation above could be understood that photons can not turn in something else. But of course there is pair production.

10. Jan 12, 2008

### Riogho

Photons can 'decay' into a positron and an electron.

11. Jan 12, 2008

### Staff: Mentor

Pair production requires the photon interact with a nuclear field.

Then there are photo-neutron emission and photo-dissociation of deuterons. Then there is the Compton and photo-electric effect, and ionization.

Bottom line is that photon energy is transformed, but AFAIK, they cannot spontaneously decay in and of themselves.

12. Jan 14, 2008

### DaTario

Considering that a photon can be produced in a state of superposition of different frequencies, it seems to have some sense to speak of quantum decay of a photon, as you put this photon to propagate in a medium where the absortion probability depends on the frequency, for instance.
In atmospheres studies, the probability of scattering of photons, by dipoles, at 90 degrees being proportional to the fourth power of frequency may be an example of enviroment where a photon may be said to decay.

Sorry in advance for it seems not be correct.

Best wishes

DaTario

13. Jan 15, 2008

### xlines

I agree with replies of others that it is probably article on proton decay, but ... IIRC , intensive light in very strong magnetic field will produce axions(?!) or some other exotic particle thus, perhaps, photons might "decay" beyond Standard model. OTOH I could be wrong about this one=)

14. Jan 15, 2008

### Parlyne

These are not decay processes, as they involve the photons interacting with some other object. For a process to be a decay, it must occur spontaneously in the absence of any interaction.

15. Jan 15, 2008

### Ricardo19

wrong answer blechman absolutely electrons and protons can decay since they are beta particles and they undergo beta decay read it yourself.
http://en.wikipedia.org/wiki/Beta_decay

16. Jan 15, 2008

### blechman

I will not waste my time with a wikipedia article that is obviously been either mis-read or is flat wrong! Protons cannot decay due to baryon conservation, and electrons cannot decay due to lepton conservation. You must go beyond the SM to allow for such decays!

17. Jan 15, 2008

### peter0302

While I have lots of sympathy for beginners and people who genuinely don't know but want to learn, the arrogance of that statement, combined with its utter incorrectness, forces me to chime in. Even someone with a high school education should be able to read the article you just cited and understand that beta decay means the decay of a NEUTRON into a Beta Particle (electron or positron), proton, and neutrino. None of those things decays further in Beta Decay.

Again, it is the NEUTRON that decays into a beta particle and other things; Beta Particles do not decay spontaneously.

As has been said many times in this thread and others, if the proton decays, its half life is longer than the age of the universe. Meaning it will probably never be observed. The electron CANNOT decay in the standard model.

18. Jan 15, 2008

### Staff: Mentor

Protons in certain nuclei can decay in a fashion similar to neutron decay:

$$p \rightarrow n + e^{+} + \nu$$

This is the "beta+ decay mode" of those nuclei. However, it is possible only when the total mass of the final nucleus, positron and neutrino is less than the mass of the initial nucleus, so as to provide energy for the decay.

A proton that is not bound into a nucleus cannot decay this way because the mass of a neutron is greater than the mass of a proton. There is no other way for a free proton to decay in the Standard Model. Various GUTs propose mechanisms for proton decay, but so far none of them have been observed experimentally.

19. Jan 15, 2008

### blechman

That's interesting. I'm not familiar with such a decay mode for a nucleus, but as I've said many times, i'm not a nuclear physicist. Can you give an example or two of such a process and an estimate of the widths (how "rare" they are)? Is this a "spontaneous" decay?

20. Jan 15, 2008

### Staff: Mentor

Beta+ decay is actually about as common as beta- decay. The short Wikipedia article on positron emission gives some specific examples. This chart from BNL plots all the nuclides with number of neutrons (N) along the horizontal axis and number of protons (Z) along the vertical axis. Black squares mark stable nuclides. The bluish-gray squares above them are nuclides which decay mainly via beta+. The pink squares below are nuclides which decay mainly via beta-.