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Thread: Could photons have mass?

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    Could photons have mass?

    I remember when neutrinos were supposed to be massless. Now they have mass. How do we know that photons are massless?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by Tom Mazanec View Post
    I remember when neutrinos were supposed to be massless. Now they have mass. How do we know that photons are massless?
    Photons are massless and travel at light speed. They have energy dependent on frequency and energy has a mass equivalent. So it becomes relevant in interactions such as impacts. Photons carry the EM force field in our standard model. So in a way we name the photon as what we measure or experience as light or the EM spectrum, so we call photon speed, light speed and define that as massless.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    We don't 'define' the photon as being massless in the same way that we define lightspeed to be the speed a photon propagates in a vacuum. Massless means having zero rest mass, and we don't define zero with respect to the photon.

    There are a number of compelling theoretical and experimental arguments for photons being massless.
    - Massive photons break the gauge invariance of QED which leads to it being non-renormalisable. Since we do renormalise it to get a number of high precision predictions that match observations this is a good sign that the photon is massless.
    - The electric and magnetic field equations would change which would lead to detectable phenomena not observed.
    - The conservation of electric charge is closely linked to the phase and gauge invariance of the underlying photon fields. Thus a massive photon leads to charge conservation breaking down, which we see no evidence for.
    - The photon's speed would be change with its frequency, something we don't see.

    The possibility of a massive photon is one that has been checked experimentally to a high degree of precision (although the experiments only put an upper bound on the photon mass) because it would be an area where we might see new physics. So far the mass of the photon is indistinguishable from zero in any experiment that has been done. I just looked it up and the Particle Data Group give an upper limit of about 10e-27 eV.

    I think it is fair to say that we don't 'know' that the photon mass is zero, but we do have a number of good arguments for it being zero and no experimental evidence for a detectable mass.

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    Quote Originally Posted by Shaula View Post
    We don't 'define' the photon as being massless in the same way that we define lightspeed to be the speed a photon propagates in a vacuum. Massless means having zero rest mass, and we don't define zero with respect to the photon.

    There are a number of compelling theoretical and experimental arguments for photons being massless.
    - Massive photons break the gauge invariance of QED which leads to it being non-renormalisable. Since we do renormalise it to get a number of high precision predictions that match observations this is a good sign that the photon is massless.
    - The electric and magnetic field equations would change which would lead to detectable phenomena not observed.
    - The conservation of electric charge is closely linked to the phase and gauge invariance of the underlying photon fields. Thus a massive photon leads to charge conservation breaking down, which we see no evidence for.
    - The photon's speed would be change with its frequency, something we don't see.

    The possibility of a massive photon is one that has been checked experimentally to a high degree of precision (although the experiments only put an upper bound on the photon mass) because it would be an area where we might see new physics. So far the mass of the photon is indistinguishable from zero in any experiment that has been done. I just looked it up and the Particle Data Group give an upper limit of about 10e-27 eV.

    I think it is fair to say that we don't 'know' that the photon mass is zero, but we do have a number of good arguments for it being zero and no experimental evidence for a detectable mass.
    Thanks for the excellent addition. When I said “define” it was in relation to what we “know” which is a model, the standard model, which makes really good predictions using what we call mass, speed, light speed and so on to all the complications. These are all models we have refined to be self consistent in making predictions and extending explanation of observations. I wanted to point out that the how do we know question is like the why question.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    Quote Originally Posted by Tom Mazanec View Post
    I remember when neutrinos were supposed to be massless. Now they have mass. How do we know that photons are massless?
    Neutrinos are also hard to detect and analyze. Their interaction with matter is weak. Photons are common and highly interactive with matter, so we can and do examine them and their properties in detail. We know photons very well. If they had mass their actions and results would be different than what we observe.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by Noclevername View Post
    Neutrinos are also hard to detect and analyze. Their interaction with matter is weak. Photons are common and highly interactive with matter, so we can and do examine them and their properties in detail. We know photons very well. If they had mass their actions and results would be different than what we observe.
    Good point - it is also worth noting that neutrinos are not gauge bosons. There are far fewer theoretical constraints on their mass since they are not force carriers. That is why the constraints on their mass were never as restrictive as the ones for photons. There simply wasn't a strong argument that they should be massive or massless.

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    Quote Originally Posted by Noclevername View Post
    Neutrinos are also hard to detect and analyze. Their interaction with matter is weak. Photons are common and highly interactive with matter, so we can and do examine them and their properties in detail. We know photons very well. If they had mass their actions and results would be different than what we observe.
    For one thing, we can very easily observe photons that were emitted many, many light-years away and determine their initial energies quite precisely, and correlate photons of a wide range of initial energies closely in time of emission. If they had mass, their speed and travel time would vary, and we can detect very, very tiny variations.

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    I remember when neutrinos were supposed to be massless.
    Even this part isn't true. They were never "supposed to be massless". They were supposed to be a sibling of electrons, and electrons have mass, so the most straightforward presumption was always that they'd have mass. That mass just happened to be so tiny that we couldn't detect it at first, which allowed some people to stretch their imaginations that maybe the obvious answer (that of course they would have mass) might not be right this time.

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    Quote Originally Posted by Delvo View Post
    Even this part isn't true. They were never "supposed to be massless". They were supposed to be a sibling of electrons, and electrons have mass, so the most straightforward presumption was always that they'd have mass.
    I don’t know what the position was with nuclear physicists, but very definitely when I was young, I had read as a standard statement that neutrinos were massless. I actually remember having a debate about that with fellow science nerds. We were all going by science texts or articles that were easy to find pre-internet and not going through a research librarian at a university. I think neutrino mass started to be debated when some started to take the solar neutrino problem seriously, and that was part of a possible explanation (but there were multiple hypotheses surrounding the solar neutrino problem). This was long before the experiments that validated neutrino oscillation. That process took decades.

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    Quote Originally Posted by Delvo View Post
    Even this part isn't true. They were never "supposed to be massless". They were supposed to be a sibling of electrons, and electrons have mass, so the most straightforward presumption was always that they'd have mass. That mass just happened to be so tiny that we couldn't detect it at first, which allowed some people to stretch their imaginations that maybe the obvious answer (that of course they would have mass) might not be right this time.
    That's not correct. In the Standard Model, neutral leptons are expected to be massless. They clearly aren't, but why they have mass isn't understood.

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