
#1
Feb110, 10:19 AM

P: 112

Why is a photon considered to be its own antiparticle? What distinguishes a photon from an antiphoton? When photon and antiphoton collide do they annihilate?




#2
Feb110, 10:26 AM

P: 26

I'm not very good at particle physics, but I think only charged particles can have an antiparticle. The neutron has an antiparticle, but it is made of different charged quarks.




#3
Feb110, 10:27 AM

Sci Advisor
P: 4,491

Actually, it would be more correct to say that antiphoton does not exist.




#4
Feb110, 10:32 AM

P: 112

Photon and its antiparticle 



#5
Feb110, 10:55 AM

Sci Advisor
P: 1,395

Anyhow, what would be emitted in the annihilation of a putative photonantiphoton pair? I mean, energy and momentum have to be conserved, right? EDIT: so, to further qualify my own question, I know electronpositron pair production is possible if the photon energy is high enough ... I am trying to understand the general situation that would be valid at all photon energies. 



#6
Feb110, 01:30 PM

Sci Advisor
HW Helper
P: 1,930

Particles like the photon and the pi zero that are their own antiparticles are eigenstates of the charge conjugation multiplicative operator C. The photon has eigenvalue 1, and the pi zero has eignevalue +1.




#7
Feb110, 01:41 PM

Sci Advisor
P: 1,395

I guess this amounts to what the OP was asking in the first place ... 



#8
Feb110, 03:33 PM

Sci Advisor
HW Helper
P: 1,930

Two photons can ( and do) annihilate into other particles in just the same way that an electron and a positron can.




#9
Feb110, 07:22 PM

Sci Advisor
P: 1,395

Again, I am just following my intuition here, so please forgive me if these are naive questions ... 



#10
Feb110, 07:49 PM

P: 640

Torquil 



#11
Feb210, 03:41 AM

P: 112





#12
Feb210, 04:36 AM

Sci Advisor
P: 3,363

They can have different energies, but note that this depends on the reference system.
The two photons can only anihilate if they do not travel exactly in the same direction. But then you can find a reference system where one is red shifted and the other one blue shifted so that their energies coincide and they impact head on. This is analogous to the scattering of massive particles in the laboratory frame vs the rest frame of the center of mass. 



#13
Feb210, 04:37 AM

Sci Advisor
P: 3,363

They can have different energies, but note that this depends on the reference system.
The two photons can only anihilate if they do not travel exactly in the same direction. But then you can find a reference system where one is red shifted and the other one blue shifted so that their energies coincide and they impact head on. This is analogous to the scattering of massive particles in the laboratory frame vs the rest frame of the center of mass. 



#14
Feb210, 06:31 AM

Sci Advisor
P: 1,395

I can't see how any other particles besides more photons could be emitted in this case, and in that case, how can you tell the difference between an annihilation event and a scattering event? Of course, I am not familiar with the full "menagerie" of subatomic particles ... are there other candidates for emission in such an annihilation? 



#15
Feb210, 06:56 AM

Mentor
P: 11,221





#16
Feb210, 02:41 PM

P: 1,540





#17
Feb210, 02:49 PM

Sci Advisor
P: 3,363

Two photons may anihilate e.g. into an electron and a positron. This requires at least two photons having 511 keV, the rest energy of the two electrons formed. I think the only anihilation process possible for visible light would be the production of a neutrino and an antineutrino which are nearly massless, which is a phantastically improbable.




#18
Feb210, 03:18 PM

P: 1,540




Register to reply 
Related Discussions  
The neutrino its own antiparticle?  High Energy, Nuclear, Particle Physics  13  
Antiparticle annihilation  Quantum Physics  6  
do antiparticles really go back in time?  Quantum Physics  12  
The destiny of an antiparticle  Quantum Physics  39  
Particle and antiparticle  Quantum Physics  35 