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Asked by blatantlyninja to Marcus, Martin, Rob, Suzanne on 21 Mar 2012.
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Robert Thompson answered on 21 Mar 2012:
This is a tricky question, and I know there is also a little debate on the subject. As I understand it an anti-photon can not exist – some people say that the photon is its own antiparticle but this to me is quite difficult to comprehend.
As I understand, the photon has no charge and therefor there is no symmetry which requires the photon to have an anti particle of opposite charge but equal magnitude.
Hope this helps
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Suzanne McEndoo answered on 21 Mar 2012:
An antiparticle is a particle with the same mass and opposite electric charge. Since photons don’t have charge they are their own anti-particle.
Neutrons are neutral, but they are made of quarks, which do have anti particles, so an anti-neutron is made of anti-quarks.
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Martin Zaltz Austwick answered on 22 Mar 2012:
As I understand it, particles and antiparticle combine to make photons. So when that happens, the “anti” bits (like charge) cancel out. Neutrinos have no charge, but there are antineutrinos. There are other properties that neutrinos have which can have an anti- equivalent (e.g. “lepton number”) but then we’re getting into proper particle physics which is not really my field. So a particle needs mass, but not necessarily charge, to have an antiparticle. I think.
Comments
blatantlyninja commented on :
So could it be possible to have a ‘anti-photon’, but because they are identical to normal photons, we can’t tell the difference to label it as such? Or is it not possible because if a anti-photon existed, it would annihalate with a normal photon (then would that create particles like electrons, even maybe protons etc?), and we havn’t observed anything like that happening with photons, so we definitely dont have anti-photons?… i’ll stop rambling now =S
Are photons the only particles that dont make up matter/solid stuff, or do photons count as matter?
Suzanne commented on :
Hi ninja. Thanks for the comments.
For this, I think it’s more that the photon *is* it’s own anti-particle. If you tried to make an anti-photon, you’d just end up with a photon, rather than something that was distinct but we couldn’t tell the difference. Since particles and anti-particles annihilate to create photons, I guess that photons are the bottom rung of that ladder, they’ve nowhere to go.
blatantlyninja commented on :
Ohh, also, i read that all electrons are completely indistinguishable from each other, as are photons (of the same wavelength). is this true for all fundamental particles?
Suzanne commented on :
So, there are basically two types of particles, bosons and fermions (actually, this is true specifically for 3D space).
Bosons are particles that have integer spin, so 0,1,2,etc. Bosons are indistinguishable, so you can have multiple bosons in the same quantum state. Bose-Einstein condensate is when you cool bosons so much that a large number of them are in the lowest energy state so you get millions of atoms that all have the same quantum state and are indistinguishable. So if you do something to one, you do something to all.
Fermions, on the other hand, have half integer spin, so 1/2, 3/2, 5/2, etc, and they are distinguishable, so you can’t have more than one fermion in the same quantum state. If you cooled fermions down, they couldn’t all go to the ground state, but they’d occupy each of the lowest energy levels (like stacking shelves), and this is called a fermi sea.
Electrons, protons and neutrons are all fermions, but you can combine them (and thus add up their spins) to make bosons, so some atoms are bosons, some are fermions (even some isotopes of an atom are fermions, while others are bosons).
Photons are bosons, since they have spin 0. So, as you say, if two photons have the same wavelength, they’re indistinguishable.
Feel free to ask more! 🙂
blatantlyninja commented on :
So are you saying that electrons aren’t indistinguishable from each other? I read that they are, and a theory of feynmans was that only 1 single electron exists, and that it moves backwards and forwards in time, like ‘stitches in a tapestry..?!?
Suzanne commented on :
Yes, electrons aren’t indistinguishable. This is part of how atoms work, if electrons were indistinguishable then they could all sit in the lowest energy level together and chemistry wouldn’t work.
To be honest, I’m not familar with that theory of Feynmann, it’s a bit outside of my sphere. 🙁 Sounds interesting though.