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  1. 24 Jul '13 18:21 / 1 edit
    This is speculative, but:

    http://physicsworld.com/cws/article/news/2013/jul/24/what-is-the-lifetime-of-a-photon

    -not sure how you could test this theory.
  2. Standard member DeepThought
    Losing the Thread
    24 Jul '13 19:20
    Originally posted by humy
    This is speculative, but:

    http://physicsworld.com/cws/article/news/2013/jul/24/what-is-the-lifetime-of-a-photon

    -not sure how you could test this theory.
    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 Standard Model Photon is massless, so checking bounds on its mass and possible decay time is a way of testing the Standard Model, which is the real importance of the work.

    Incidentally, the article has recombination happen during inflation which is about 380,000 years too soon.
  3. 24 Jul '13 19:26
    Originally posted by humy
    This is speculative, but:

    http://physicsworld.com/cws/article/news/2013/jul/24/what-is-the-lifetime-of-a-photon

    -not sure how you could test this theory.
    You could, theoretically, observe the decay of photons, but since photons interact with lots of things that is quite difficult.

    Also, if the photon's energy is high enough, it can annihilate with another photon to form an electron-positron pair. (this has been observed)
  4. Standard member sonhouse
    Fast and Curious
    24 Jul '13 19:27 / 1 edit
    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.
    Photons are by definition massless. They can impart inertia but have no mass.
    How can they be going at c if they have mass, even if it is like they say 1E-54 kg. It seems to me it would still take an infinite amount of energy to get even that small a mass to c.
  5. 24 Jul '13 19:28
    Originally posted by sonhouse
    Photons are by definition massless. They can impart inertia but have no mass.
    They are most certainly not massless "by definition". They are assumed to be massless in the Standard Model.
  6. Standard member sonhouse
    Fast and Curious
    24 Jul '13 19:32 / 1 edit
    Originally posted by KazetNagorra
    They are most certainly not massless "by definition". They are assumed to be massless in the Standard Model.
    Yes, I thought of that too late, read the second half of my post, we were typing at the same time.

    If photon's have some non-zero mass, maybe the true speed of light is faster than what we know of as c.
  7. 24 Jul '13 19:51 / 1 edit
    Originally posted by sonhouse
    If photon's have some non-zero mass, maybe the true speed of light is faster than what we know of as c.
    And we couldn't call it 'the speed of light'.
    I thought the equation E=mc^2 could be used to find the 'true' c. Or is there a units issue that makes it impossible to work out from a nuclear reaction?

    I also thought that neutrinos had been measured to go very close to the speed of light and that they are believed to have a small mass. When checking this I found:
    http://en.wikipedia.org/wiki/Neutrino#Speed

    Also see:
    https://en.wikipedia.org/wiki/Speed_of_light#Propagation_of_light

    Apparently a very small mass would not change the speed of light significantly enough to measure by current experiments.
  8. 24 Jul '13 19:55
    Originally posted by sonhouse
    Yes, I thought of that too late, read the second half of my post, we were typing at the same time.

    If photon's have some non-zero mass, maybe the true speed of light is faster than what we know of as c.
    Well, "c" is just the speed at which massless particles travel.
  9. Standard member DeepThought
    Losing the Thread
    24 Jul '13 20:52
    Originally posted by twhitehead
    And we couldn't call it 'the speed of light'.
    I thought the equation E=mc^2 could be used to find the 'true' c. Or is there a units issue that makes it impossible to work out from a nuclear reaction?

    I also thought that neutrinos had been measured to go very close to the speed of light and that they are believed to have a small mass. When checking this I found:
    http://en.wikipedia.org/wiki/Neutrino#Speed
    No, c is defined to be 299,792,458 m/s. It is a constant of nature, the energy and mass scales are related by it, this is why particle physicists quote masses in MeV (which is an energy). Yes, neutrinos are very fast. If the mass is much smaller than the total energy you can treat it as going at the speed of light to a good approximation.

    This is difficult to test. One could try sending signals between satellites to test this, if one sent visible light and long wave. If there is a mass the long wave should arrive after the visible. The wavelength of visible light is of the order of a micron (~600nm) and long wave is of the order of a kilometer (radio 4 is 1515m). These correspond to frequencies of around 200,000 GHz and 20 Kilohertz respectively. Extremely Low Frequency (ELF) gets down to 3Hz. These correspond to energies of the order of 1eV for visible light, 0.1 nano eV for long wave and 10 femto eV for ELF.

    E = m / sqrt(1 - v^2) (We have units where the speed of light is 1)
    so
    v = sqrt (1 - (m/E)^2)
    expanding the square root:
    v = 1 - (m/E)^2 / 2

    The difference from the speed of light is then (1/2)(m/E)^2

    The mass bound is 1 atto-eV, so for visible light that's of the order of 1 part in 10^36. For long wave I get one part in 10^18, and for ELF I get 1 part in 10^8. I think it is difficult to measure a delay that small (over the distance to the moon the ELF will turn up 10 nanoseconds late) but not out of the question.
  10. Standard member lemon lime
    blah blah blah
    26 Jul '13 05:11
    At c there is no passage of time, so doesn't this mean the photon itself exists in a state of zero time? If this is true, then it seems a photon would remain unchanged no matter how long it has existed.
  11. 26 Jul '13 05:31
    Originally posted by lemon lime
    At c there is no passage of time, so doesn't this mean the photon itself exists in a state of zero time? If this is true, then it seems a photon would remain unchanged no matter how long it has existed.
    The point is that if a photon has mass it will not travel at c.
  12. Standard member lemon lime
    blah blah blah
    26 Jul '13 06:28 / 2 edits
    Originally posted by KazetNagorra
    The point is that if a photon has mass it will not travel at c.
    My question has to do with whether or not light can have a half-life if photons are locked into a state of zero time. Einstein said at the speed of light time stops, so that's my basis for wondering if light itself is frozen in time. If anything is frozen in time, then it will not change or experience entropy or be affected by any other force other than something that will stop it during its journey through space.

    A photon has no intrinsic mass, meaning it doesn't have enough energy to become even the tiniest particle of mass. However, I did read of a theory that said photons could be quickly vacillating between an energy state and something like an electron, but it would happen so rapidly it can't be measured or tested so they can't be sure if this is true or not.
  13. 26 Jul '13 07:38 / 3 edits
    Originally posted by lemon lime
    At c there is no passage of time, so doesn't this mean the photon itself exists in a state of zero time? If this is true, then it seems a photon would remain unchanged no matter how long it has existed.
    At c there is no passage of time,

    If c is defined by the actual speed of photons, then the assumption that there is no passage of time for a photon is assuming a photon has no mass.
    BUT, IF the photon has a mass (and that's a big IF ) then there WILL be a passage of time for a photon!
    -I think that answers all your questions here in one go.
  14. Standard member DeepThought
    Losing the Thread
    26 Jul '13 11:08
    Originally posted by lemon lime
    My question has to do with whether or not light can have a half-life if photons are locked into a state of zero time. Einstein said at the speed of light time stops, so that's my basis for wondering if light itself is frozen in time. If anything is frozen in time, then it will not change or experience entropy or be affected by any other force other than s ...[text shortened]... ppen so rapidly it can't be measured or tested so they can't be sure if this is true or not.
    If the photon has a mass it has a half-life if there is something lighter for it to decay into. If it does not have a mass then it does not have a half-life, as there is nothing lighter for it to decay into. Electrons and protons are stable because they have quantum numbers which prevent decay - there's nothing for them to decay into. Neutrons are more massive than protons, so they are unstable against beta decay into protons, there's enough extra mass to make an electron anti-neutrino pair and charge is conserved. I'll talk a bit more about how the photon can interact with anything if "time is frozen" below.

    In the Standard Model of particle physics the photon is massless. If photons have a mass then the U(1) symmetry that QED is based on would be broken. The way the electro-weak model has it's symmetry broken you'd expect a residual unbroken U(1) symmetry and the electro-weak model is basically confirmed with the discovery of a candidate Higgs at LHC. Also, if it did have a mass there is also a fine tuning problem as to why it is so small; if it is around (say) 1eV, visible light would travel fairly slowly ~3/4 c, and probably render life difficult to impossible as the force would be short ranged on the order of the size of organic molecules.

    However QED is inconsistent anyway due to something called the Landau pole - which predicts that the coupling strength becomes infinite at a finite, but huge, energy scale. This would be a problem for the theory, but we expect new physics and some kind of unification with the other forces on this side of the Planck scale. Since we don't know the correct way to unify all the forces we can't rule out the photon having a mass.

    So the question of whether the photon has a mass or not is experimental. There is an upper bound on it's mass of 10^-18 eV which is tiny.

    Regarding your question about "what the photon sees". The "on board flight time" is measured in something called proper time. If there is a mass and it is 10^-18 eV, then photons would take a small but finite amount of proper time. In that case there isn't a problem. So what about if the photon is massless? We have to be more careful about the concept of proper time. More formally it is the elapsed time measured in the reference frame of a co-moving observer, i.e. one who keeps up with the photon. If the photon is massless then it travels at the speed of light and there cannot be a co-moving observer. The frame of reference doesn't exist. This means that the question you are asking is mal-formed. But, with that in mind, as far as the photon is concerned it is emitted and absorbed at the same instant.

    As Kazet mentioned earlier it is possible for them to produce positron electron pairs. If a gamma ray has energy >~1MeV then it can split into a positron electron pair which can either annihilate and give the gamma back, or if one of them emits (or absorbs) a photon they can continue to exist as real particles and travel their separate ways. This is a mechanism for supernovae (see pair-instability supernova on Wikipedia) and has been observed in cloud chamber pictures.
  15. Standard member lemon lime
    blah blah blah
    26 Jul '13 16:32 / 1 edit
    Originally posted by DeepThought
    If the photon has a mass it has a half-life if there is something lighter for it to decay into. If it does not have a mass then it does not have a half-life, as there is nothing lighter for it to decay into. Electrons and protons are stable because they have quantum numbers which prevent decay - there's nothing for them to decay into. Neutrons are mor ability supernova on Wikipedia) and has been observed in cloud chamber pictures.
    If the photon is massless then it travels at the speed of light and there cannot be a co-moving observer. The frame of reference doesn't exist. This means that the question you are asking is mal-formed. But, with that in mind, as far as the photon is concerned it is emitted and absorbed at the same instant.

    When you say emitted and absorbed at the same instant, do you mean from the perspective of the photon (the photon is the observer) no time has elapsed? If so, I'd like to jump into my wayback machine to the year 1967 and show this to my smarty pants MIT friend and say "See? I was right!"
    LOL