Do photons have a half-life?

A thought just struck me about this. Suppose we have a fairly high energy "Standard Model" (specifically I'm assuming it's massless) photon trundling along. It can split into an electron positron pair, the electron emits a W- particle and turns into a neutrino. The W- emits a photon and is then absorbed by the positron which turns into an anti-neutrino. Since the electron type neutrino has a mass less than 0.1 eV (based on a cosmological bound on the sum of neutrino masses being 0.3eV see the Wikipedia page on neutrinos) an upper bound on the lowest energy photon that can do this has an energy of 0.2eV which is near-infrared. The Feynman diagram corresponding to this process is non-zero (the photon can't connect to the electron as switching the electron and positron around picks up a minus sign so the graph cancels), so unless it's suppressed by Ward identities or otherwise canceled it should contribute to photon decay. It won't happen at much of a rate as there are 4 powers of Planck's constant and it's mediated by the weak force (which reduces the rate a lot), but I don't see an immediate reason why this shouldn't be a process whereby a sufficiently high energy (>~ 0.2eV) massless photon could decay into a couple of neutrinos.

This process won't work if the photon is massive but lighter than the 2 neutrino masses we need at the threshold, as then one would be able to find a frame of reference where the photon is stationary - in that frame it is prevented from decaying by conservation of energy - the same must apply in all other frames, so if the photon has a mass this decay channel is ruled out.

Originally posted by DeepThought
I don't think anyone seriously expects the photon to be massive, but there isn't a particular reason why not. The work uses the cosmic microwave background to establish a lower bound on the possible decay rate (the higher the half-life the lower the mass). The mass bounds come from laboratory experiments on electromagnetic fields and is good. The Stan ...[text shortened]... , the article has recombination happen during inflation which is about 380,000 years too soon.

There is reason to expect the photon to have mass. We know that it reacts to a gravitational field predictably. It may have very low mass but I believe it has mass. If somehow one could stop one in its tracks its mass would be very very tiny. Could low mass explain a long half life? The probability of it reacting to virtual particles within its sphere of influence is less than larger particles and much less than say the nuclei of atoms. Are particles nothing more than the center of a cloud of virtual particles of like nature with the probability of it being in the center? When particles react with one another electrically, gravitationally, or by nuclear force, isn't this because the universe is trying to even out the virtual particles in a system where we have persistent particles?

Originally posted by joe beyser
There is reason to expect the photon to have mass. We know that it reacts to a gravitational field predictably. It may have very low mass but I believe it has mass. If somehow one could stop one in its tracks its mass would be very very tiny. Could low mass explain a long half life? The probability of it reacting to virtual particles within its sphere of rse is trying to even out the virtual particles in a system where we have persistent particles?

There is reason to expect the photon to have mass. We know that it reacts to a gravitational field predictably

Are you talking about gravity bending light here by "reacts to a gravitational field predictably"? If so, this is not a reason to expect the photon to have mass for gravity will bend light whether it has mass or not because gravity causes the curvature of space and it is that, not mass, that causes light to bend in response to gravity.

So far, I haven't heard any compelling argument why photons should have mass although I also haven't heard any compelling argument why photons shouldn't so, personally, until if or when that changes, I have no opinion on this.

Originally posted by humy

There is reason to expect the photon to have mass. We know that it reacts to a gravitational field predictably

Are you talking about gravity bending light here by "reacts to a gravitational field predictably"? If so, this is not a reason to expect the photon to have mass for gravity will bend light whether it has mass or not because gravi ...[text shortened]... shouldn't so, personally, until if or when that changes, I have no opinion on this.

You could be right here about the mass of photons but the curvature of space may well be another phenomena that appears to be bent space. Does gravity cause things with mass to attract to one another or could it be bent space or is there another possibility where the universe is pushing them together? Is the universe pushing magnets together with opposite poles and pushing them apart when the same poles are near? Same with electrical charges? How about nuclear force. I have read how we can get gravity theoretically by space time theory but what about the rest? Light that may have travelled long distances I would think would converge together if photons had mass. But then again the mass could be so low that the effect would be nil.

Originally posted by joe beyser
You could be right here about the mass of photons but the curvature of space may well be another phenomena that appears to be bent space. Does gravity cause things with mass to attract to one another or could it be bent space or is there another possibility where the universe is pushing them together? Is the universe pushing magnets together with opposite ...[text shortened]... ether if photons had mass. But then again the mass could be so low that the effect would be nil.

You seem to have the universe as a whole as an active player, which is an unusual point of view. The forces in the standard model of particle physics are understood in terms of Gauge Field Theories. These theories are based around geometrical constructions called principle bundles. Gravity is also a geometrical theory but doesn't require the principle bundle structure the other theories have.

Originally posted by DeepThought
You seem to have the universe as a whole as an active player, which is an unusual point of view. The forces in the standard model of particle physics are understood in terms of Gauge Field Theories. These theories are based around geometrical constructions called principle bundles. Gravity is also a geometrical theory but doesn't require the principle bundle structure the other theories have.

The gauge field theories and models are very useful for us. I don't think that we can say that we know the nature of what makes things tic based on them alone. We can predict things mathematically from our observations, but how does it all work together? It is interesting that the guy who came up with the experiment to disprove quantum entanglement admits to still not being able to understand quantum mechanics. Of course his experiment went on to prove entanglement and this is yet another feature of our world that has to jive with a theory of everything. I think there is so much more to our world beyond our direct observation that it will be hard if not impossible to unravel. The effects in our dimension of reality is all we have to go on so understanding the universe is not intuitive. We can come up with mathematical models that work for us in a practical sense but do we really understand?

Originally posted by humy

There is reason to expect the photon to have mass. We know that it reacts to a gravitational field predictably

Are you talking about gravity bending light here by "reacts to a gravitational field predictably"? If so, this is not a reason to expect the photon to have mass for gravity will bend light whether it has mass or not because gravi ...[text shortened]... shouldn't so, personally, until if or when that changes, I have no opinion on this.

You beat me to that one🙂

The reaction to the curvature of space would alter ANYTHING, mass or no.

Just as an aside, bringing in the idea of matter V antimatter, I see these reports about particles that are its own anti particle at the same time.

Is this really possible? I mean if it is its own anti, how could it be stable? Why wouldn't it just explode immediately?

Would there be bits of itself as matter and other bits as antimatter held apart internally somehow?

This moggles the bind!

Originally posted by sonhouse
You beat me to that one🙂

The reaction to the curvature of space would alter ANYTHING, mass or no.

Just as an aside, bringing in the idea of matter V antimatter, I see these reports about particles that are its own anti particle at the same time.

Is this really possible? I mean if it is its own anti, how could it be stable? Why wouldn't it just exp ...[text shortened]... s matter and other bits as antimatter held apart internally somehow?

This moggles the bind!

An anti-particle is just a particle with the signs of all their intrinsic quantum numbers reversed. So because an electron has negative charge it's anti-particle has positive charge. A photon has no charge, and doesn't couple to the weak or strong force, so it's anti-particle is indistinguishable from itself.

A nice example is the neutral pion, which is a composite composed of a quark and it's anti-quark in a bound state. To get the anti-particle of a composite you have to replace each of the constituent particles with its anti-particle. In the case of the neutral pion that leaves you with the same particles, so the neutral pion is its own anti-particle.

Originally posted by DeepThought
An anti-particle is just a particle with the signs of all their intrinsic quantum numbers reversed. So because an electron has negative charge it's anti-particle has positive charge. A photon has no charge, and doesn't couple to the weak or strong force, so it's anti-particle is indistinguishable from itself.

A nice example is the neutral pion, whic ...[text shortened]... pion that leaves you with the same particles, so the neutral pion is its own anti-particle.

So I guess it is the gluons that keep them separated. I assume if they ever did get together there would be a violent but sub micron explosion.

Originally posted by sonhouse
So I guess it is the gluons that keep them separated. I assume if they ever did get together there would be a violent but sub micron explosion.

Neutral pions are highly unstable and decay (or "explode", if you insist) with the highest probability to 2 photons. The mean lifetime is less than 0.1 femtoseconds.

Originally posted by sonhouse
You beat me to that one🙂

The reaction to the curvature of space would alter ANYTHING, mass or no.

Just as an aside, bringing in the idea of matter V antimatter, I see these reports about particles that are its own anti particle at the same time.

Is this really possible? I mean if it is its own anti, how could it be stable? Why wouldn't it just exp ...[text shortened]... s matter and other bits as antimatter held apart internally somehow?

This moggles the bind!

If I remember correctly, one of the ways they proved the curvature of space was using the deflection of starlight through a gravitational field. By doing this the assumption was that photons have no mass but still bent and viola! curvature of space time was proven. Interestingly though the derivation of a photons momentum actually uses a substitution for mass. This doesn't prove a photon does have mass or not but getting back to the original question of half life, maybe because it has at least a mass equivalent, is has a half life equivalent. The idea of using photon sails makes me wonder if some of the mass equivalent of a photon is used up when it imparts momentum.

Originally posted by joe beyser
If I remember correctly, one of the ways they proved the curvature of space was using the deflection of starlight through a gravitational field. By doing this the assumption was that photons have no mass but still bent and viola! curvature of space time was proven. Interestingly though the derivation of a photons momentum actually uses a substitution for ...[text shortened]... makes me wonder if some of the mass equivalent of a photon is used up when it imparts momentum.

There's a bit of confusion here. It comes down to this equation E = mc^2. In the early days of relativity they used to talk about rest mass and equivalent mass. But the equivalent mass is just a scaled version of the energy, and doesn't tell you anything new. Also it is a frame dependent scalar quantity, which makes no sense. So physicists now almost never refer to equivalent mass, when they say mass they mean rest mass.

The full equation is E^2 = (pc)^2 + (mc^2)^2, m = rest mass, p = momentum. If you set p (momentum) equal to zero then you get E0 = m c^2, in the rest frame of the particle. In a frame of reference where the particle has speed v the energy is given by a Lorentz transform and comes out as:

E = E0/sqrt(1 - (v/c)^2) let y = 1/sqrt(1 - (v/c)^2) (called the gamma factor, y is the symbol most like gamma available to me).

So E = ymc^2. For equivalent mass one makes the identification M = y m, to give E = Mc^2. But this quantity is no longer used. The only mass modern physics refers to is the rest mass.

Since Planck, the energy of a photon of frequency f is given by E = hf. Louis de Broglie demonstrated that the momentum was given by p = h / wavelength. At no point in this has anyone referred to a photons mass. It is not necessary to define a momentum. The statement "a photon has zero mass" means that the photon has zero rest mass. If you define mass as energy over c^2 then it has an equivalent mass, but that's just the energy.

Originally posted by DeepThought
There's a bit of confusion here. It comes down to this equation E = mc^2. In the early days of relativity they used to talk about rest mass and equivalent mass. But the equivalent mass is just a scaled version of the energy, and doesn't tell you anything new. Also it is a frame dependent scalar quantity, which makes no sense. So physicists now almos ...[text shortened]... define mass as energy over c^2 then it has an equivalent mass, but that's just the energy.

Most excelente!!! So would you say a photon is a particle or just a perturbation of the universe? If it is a perturbation, could what we perceive as a photon is really just the most probable effect of the perturbation at a given time of observation? One other curiosity is that in the experiment to prove the bending of space time, the observation that photons change course within a gravitational field might not be proof.The energy/mass equivalent of a photon moving may react as if it has real mass. Kinda like a photon sail would maybe. I like to think that someday we may be able to manipulate the stuff of space to be able to achieve interstellar travel. It seems that we are very good at mathematically describing our observations but still don't know what makes the universe tic. If and when a theory of everything comes down I believe there will be a lot of scientists shocked in that the answer was right in front of them the whole time as is will agree with everything from particle tracks in accelerators to momentum to quantum time etc. It might take a little different viewpoint or model to jar us into understanding.

Originally posted by joe beyser
Most excelente!!! So would you say a photon is a particle or just a perturbation of the universe? If it is a perturbation, could what we perceive as a photon is really just the most probable effect of the perturbation at a given time of observation? One other curiosity is that in the experiment to prove the bending of space time, the observation that phot ...[text shortened]... tum time etc. It might take a little different viewpoint or model to jar us into understanding.

Not sure what you mean by "a particle or a perturbation."

There are lots of "theories of everything", but the problem with all of them if that gravity is hard to measure or the microscale, so it's hard to verify them.

Originally posted by KazetNagorra
Not sure what you mean by "a particle or a perturbation."

There are lots of "theories of everything", but the problem with all of them if that gravity is hard to measure or the microscale, so it's hard to verify them.

I was thinking along the lines of space or whatever it is comprised of takes up the conservation of energy in what appears to be a photon. In other words if an electron shifts to a lower energy level, does its surrounding environment of space get perturbed and put an energy/mass equivalent out into our world, or does a discrete object get formed and sent out by the electron? In a way, from a photons point of view (if it could have one) its time is nearly stopped if not stopped. After it gets emitted it is instantaneously absorbed by something possibly light years away. From its point of view distance is nothing. So in a way it would appear that one atom giving up energy causes instantaneous change light years away in another atom. From our perspective when we see instantaneous change at distance with entanglement, what does that mean? What sort of energy or information is able to overcome time in our frame of reference, and how does it do it?