Most recent answer: 10/22/2007
- Matthew Ervin (age 16)
The short answer to "are there anti-photons" is "yes", but the disappointment here is that anti-photons and photons are the same particles. Some particles are their own antiparticles, notably the force carriers like photons, the Z boson, and gluons, which mediate the electromagnetic force, the weak nuclear force, and the strong force, respectively. Particles that are their own antiparticles must be electrically neutral, because an aniparticle has the opposite electrical charge as its partner particle. Other things must also be zero, like the number of quarks. A neutron cannot be its own antiparticle because it is made up of quarks and an antineutron is made up of antiquarks. A pi_0 is made up of a quark and an antiquark and is in fact its own antiparticle also.
You can find lots out about particles at , part of the Particle Data Groups .
You also ask, in a follow-up question:
I have just thought of some stuff to add to my other question. When the antiphoton and photon collide, would they fuse? And if so, would they form a particle that has mass, or one that is massless. And what kind of particle is it?
The answer is yes, photons may collide and produce other particles. One familiar reaction is the low-energy annihilation of an electron and an anti-electron (known as a positron)-- the result is usually a pair of photons (sometimes you get more than two). You need at least two, in order to conserve both energy and momentum. This reaction also works in reverse -- a pair of photons may collide to make an electron-positron pair. This happens all the time in particle physics experiments.
In high-energy electron-positron collisons, often what collides are not the electrons or the positrons, but the photons in the "entourage" around the beam electrons and positrons. These photons come together with enough energy to produce a pair of particles like an electron and a positron, a muon and an antimuon, or some quarks, depending on how much energy is available. The quarks may have lots of energy to pull on the strong force holding them together so that they may produce jets of subatomic particles.
These collisions also happen in proton-antiproton collisions at high energy. The protons and antiprotons also have a cloud of photons around them which may interact with the photons in the opposite beam to produce pairs of particles which may be observed in a detector. The energies of the photons has to be really high, however.
For low-energy photons (like visible light, radio waves, x-rays and just about anything outside of a high-energy physics laboratory), photons for the most part just go right past each other. This is because the equations of electricity and magnetism are "linear" -- the local field strentgths of two electromagnetic waves colliding is just the sum of the two, with no interaction. It works just like waves on a pond -- they will pass through each other without interacting. There is a very tiny effect, called "light-on-light scattering", where, with a very low probability, a photon will "bounce off" of another one. This proceeds by exchanging a virtual electron around in a loop. The resulting photons are massless just like the incoming ones. So if you ask if photons when they collide make massive or massless particles, the answer is: they can make either, but the mass of the whole system (that is, the total energy in a frame in which the total momentum is zero) is the same before and after the collision.
Back to high-energy collisions: There are ideas floating around the high-energy physics community to build a "photon collider" out of an electron-positron collider. This can be done by focusing laser light head on on a beam of very high-energy electrons. The photons which bounce backwards from the electrons will have very high energies, taking a large fraction of the electrons energy. The same can be done pointing in the opposite direction, and the beams of high-energy photons can be brought into collision. This is proposed to study the production of, for example, Higgs bosons, which can be made in this way via loops of W bosons and top quarks.
You can do a search on Google for "photon collider" -- there is quite a lot of information available. You can ask us more questions about anything that sounds weird.
(published on 10/22/2007)
Follow-Up #1: Are neutrinos their own anti-particles?
- An (age 17)
This is a very interesting, and fundamental, question. In order for a particle to be its own anti-particle it must be invariant under the three simultaneous operations, Charge Conjugation, Parity Inversion, and Time Inversion. The CPT theorem (that physics as a whole is invariant under this operation) is one of the tenets of modern field theory. No experiment has proven it wrong. Even though some particles may violate P or C the combination of CPT always turns out to be invariant.
As you pointed out in a previous question any charged particle cannot be its own anti-particle since under the operation of charge conjugation the sign of the electric charge is reversed. Neutral bosons (integer spin particles) can be their own anti-particles, for example the photon and presumably the graviton, but not necessarily if there is an additional associated quantum number. The neutral K meson is an example of the latter since it has a quantum number, called strangeness, that is not charge conjugate invariant.
Back to neutrinos. If the neutrino has mass, which we now believe it does, then it has the capability of having a magnetic moment. If so, then it cannot be its own anti-particle because, like electric charge, the magnetic moment changes sign under the operation of charge conjugation. If the mass of the neutrino were zero then the anti-neutrino could in principle be its own anti-particle. There is a complete theory of this type of particle, called a Majorana neutrino (See:) that describes this possibility. So far, no experimental evidence for these particles have been found, the absence of neutrino-less double beta decay being the best test.
Even if the ordinary Dirac-type neutrinos have mass, but no magnetic moment, then they can be their own anti-neutrino. Lowest order calculations using minimal extensions to the Standard Model give a moment value of the order of 10-20 times that of the electron's Bohr magneton. Several experiments have set upper limits of around 10-11 or so. We have to improve our experimental limits by quite a bit before we will know the answer.
(published on 02/05/2011)
Follow-Up #2: does a photon stay the same?
- Joe Esposito (age 28)
Pittsburgh, PA, USA
The sea of virtual photons in the quantum vacuum doesn't affect the real photon propagation. Perhaps photons are affected by the sea of virtual particles of some deeper type, sort of as quarks are affected by the Higgs field. But in that case the thing we call a photon would already be an object that included the interactions.
You ask if the initial photon could annihilate and be replaced with a different one. I believe that that process would have no symptoms whatsoever, even in principle. Therefore I think that the question has no meaning. We all often stumble into questions like that when we take try to picture quantum processes in classical terms. Classically, no matter how similar two particles are they are in some subtle way not identical. Yet quantum particles of the same type truly are identical, so it doesn't mean anything to say that one is "new".
I'm not positive what change a pilot-wave (Bohm) interpretation would make for the discussion.
(published on 01/02/2014)
Follow-Up #3: how does a pilot wave work?
- Joe Esposito (age 28)
Aha, I see what you're wondering about- if some mechanism can be given for the pilot wave force on the particle coordinate in the Bohm picture. maybe some sort of little collisions with local particle fluctations, etc. It's a nice thought, but I think that if anything comes out of an attempt like that it will be even weirder than the view in which the wave function is the sole ingredient. The reason is that all such processes can violate the Bell Inequalities. That means that there is no local picture at all (other than universal conspiracies) that can reproduce the observations. So there's little motivation to pursue yet another local picture.
The emitted photo in a Hawking picture is a real photon, and, like Pinocchio, can do all the things a real photon can do.
(published on 01/06/2014)
Follow-Up #4: antiparticles and the two-slit experiment
- Travis (age 39)
Phoenix, AZ, USA
The basic two-slit behavior works the same for particles that are their own antiparticles (e.g. photons) and ones that aren't (e.g. electrons, buckyballs). The particle-like aspect of quantum waves is that they have a "number operator" that gives them something that has discrete integer counts. That also holds for each type of particle. It has a little different behavior for bosons (e.g. photons, 4He) than for fermions (e.g. electrons, 3He), but that is a different distinction than that between ones that are their own antiparticles and ones that aren't.
(published on 05/20/2014)
Follow-Up #5: What a photon knows, and where it goes.
- David (age 38)
It is true that photon amplitudes take all possible paths in an interaction. For example, if you make an interferometer, half of the photon's amplitude goes down each path, and the two amplitudes interfere at the beamsplitter. Most physicists don't try to make statements about the photon's past (beyond saying that it has amplitudes for every possibility), and instead just discuss the outcomes of detector measurements.
That said, you can make some statements, which I believe are true:
1. The photon does NOT have a huge nonlocal database telling it everything about all the other particles in the universe. However, it can have properties which are (nonlocally) correlated to ("entangled with") the other particles that it has interacted with, or that are connected with the chain of events leading to its creation.
2. Attempts to determine which path a particle took can certainly be successful. However, even if such a measurement doesn't destroy the photon, it always collapses the photon amplitudes from "all possible paths" to the measured path, and so limits any further wavelike behavior. (For example, if you put a nondemolition measuring device in one arm of an interferometer, then you won't get the usual interference at the output.) The physical (i.e. classical-sounding) explanation of this effect is that any position measurement transfers random momentum to the particle, which washes out any fringes. (A less classical but more robust description can be made in terms of information: correlations and density matrices.)
3. Since the photon doesn't know about all the other particles, this question doesn't make sense. However, you can ask something similar: if a photon is entangled with a partner particle, and something affects the partner particle, then when does the photon find out? Really, it never "finds out." The relationship between distant entangled particles is not causal, it is just a correlation. The correlation can change without the photon "having any knowledge" about its partner. (See https://van.physics.illinois.edu/qa/listing.php?id=24896.)
4, 5, and 6: The interference pattern you see in any such dynamic experiment will depend only on what configuration the photon saw locally at each point in time. For example, if you emit a photon towards two slits, but yank them out a picosecond before the photon reaches the slits, then you won't get any interference pattern. There isn't anything surprising here, as far as I can think of: the photon really does travel from the source to the detection screen, and the pattern doesn't change if you change something before the photon gets there or after the photon has left. The photon interacts with objects at the same spacetime coordinate, that's all.
Hope that made sense. Let me know if you want further details or explanations, of which there are many.
(published on 06/09/2014)
Follow-Up #6: getting photons
- Alex (age 11)
Ordinary light is made of photons so you already have plenty of them!
Ordinary materials are made of electrons, protons, and neutrons, so you have plenty of them too. of course theyre usually stuck together. When you rub a balloon on a sweater and get it electrically charged up, you're actually rubbing some electrons off the balloon onto the sweater, or maybe the other way around. (I forget.) The protons and neutrons mostly stay put.
(published on 01/17/2018)
Follow-Up #7: photon mass
- R.sugun (age 17)
It's nice that the Youtube explanation was correct as far as it went.
We've explained about this before:
(published on 01/31/2018)