Twice the speed of light.

Discussion in 'Physics & Math' started by Beaconator, Jul 18, 2021.

  1. dart_ship Registered Member

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    Write4U what is the mass (M) of a photon?
     
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  3. Write4U Valued Senior Member

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    @ c , it's mass is the equivalent of its energy divided by c^2

    What does E = Mc^2 mean?
    https://www.pbs.org/wgbh/nova/einstein/lrk-hand-emc2expl.html
     
    Last edited: Aug 26, 2021
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  5. DaveC426913 Valued Senior Member

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    Sss's point is that there is no such thing as a photon at rest.

    The postulates of SR mean that all photons in a vacuum move at c, as observed in all frames of reference.

    "A photon at rest" is an oxymoron.
     
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  7. Ssssssss Registered Senior Member

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    No its mass is 0 always. Dividing energy by \(c^2\) gives you relativistic mass which is a different and pointless thing.
     
  8. Write4U Valued Senior Member

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    It is mass! You can talk around it all you want.
     
  9. Ssssssss Registered Senior Member

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    No it isn't because mass is \(\sqrt{|p_\mu p^\mu|}\) and relativistic mass (aka total energy divided by \(c^2\)) is \(|p_\mu u^\mu|\) so even if you aren't up on tensor notation you can see that mass depends only on the particle's four momentum \(p\) whereas total energy depends on both the particle's four momentum and the observer's four velocity \(u\) so they are obviously different. The PBS source you keep quoting is wrong and what it should say is something like that separate units for mass and energy was a mistake and that mass is definitely a contribution to the energy of a particle but not that mass is energy. Saying mass is energy is clearly wrong because photons have 0 mass but not 0 energy and if you just say mass is energy there's no consistent way to describe the photon's mass as 0 because you can never bring it to rest and discuss its energy when it's not moving because that's oxymoronic.
     
  10. Janus58 Valued Senior Member

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    No, it isn't. Not by the modern convention of "mass". When we say "mass" today we mean what used to be qualified as "Rest mass". The term "relativistic mass" has fallen out of use, and just considered "energy". It's just that energy in of itself can imbue momentum without any "mass" being involved.
     
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  11. Write4U Valued Senior Member

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    Well, if you cannot have a photon at rest, perhaps it is oxymoronic to say that it has zero rest-mass. It will never have effective zero mass and always effective energetic momentum. When it hits your skin it is causal to heat.
    https://en.wikipedia.org/wiki/Causality_(physics)

    Interestingly:
    Wikipedia

    IOW, energy can be converted into mass and mass can be converted into energy.

    Are you telling me that mass can be converted into energy, but energy cannot be converted into mass?
    E = Mc^2 is a one-way equation?

    Please note that I am playing devil's advocate here. I have no objection to the notion of separate properties. It is the logic in the distinctions that interests me.

    Bohmian Mechanics holds that photons are particles which acquire mass from momentum.
    That effectively does away with that pesky particle/wave duality and the strange phenomenon that the wave collapse creates a "massive" impact.
     
    Last edited: Aug 26, 2021
  12. DaveC426913 Valued Senior Member

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    Notable.
     
  13. Ssssssss Registered Senior Member

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    I never use the term rest mass for exactly that reason and I always say either mass or invariant mass.
    Define "effective mass" in mathematical terms.
    No because I'm not stupid and I have no idea how you got the idea that I might think that from what I wrote.
    No you are not because a devil's advocate advances a coherent argument against a position whereas you are simply stating things that are wrong like that photons don't have 0 mass.
    Reference please.
     
  14. Write4U Valued Senior Member

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    Then there is this;

    Rest mass of photon on the surface of matter

    Highlights


    Abstract

    https://www.sciencedirect.com/science/article/pii/S2211379719330943#

    I am not talking about photons flying around. I am talking about what occurs when a photon hits a physical surface and with what kind of non-zero value.

    We assign zero value to the massive properties of a photon, and then turn around and purpose a non-zero property to a wave function.
    If not, where does it get its physical mass from on impact (wave-function collapse)? Is this the moment when the photon becomes a physical particle?
    When it becomes a particle does it acquire non-zero force (mass)?

    Is this the dilemma?

    When the same thing has a dual character, where one state has zero force, and the other has non-zero force, who're you gonna call, the particle or the wave? I see a logical conflict, a differential equation, if you will.

    I just happen to like Bohm, who assigned an inherent potential that a zero value particle @ c
    acquires mass, whereas the potential of a collapsing wave function @ c, converts energy into mass, however small. We can see the resulting "impact" patterns on the photographic plate in the double slit experiment. The mathematics must equate, therefore pM = eM at the moment the particle strikes the surface. It is a causal event and has an effective result.
     
    Last edited: Aug 26, 2021
  15. Ssssssss Registered Senior Member

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    That paper seems to be misrepresenting what its references are saying and you only have to look at the title of its reference 24 to see that those nonzero values are upper bounds on any possible mass of a photon not a value. We do not simply assume that photons are massless but scientists do experiments do to check and those figures are the limit of what possible nonzero mass the photon could have and be consistent with experiment and they are not the mass which we still believe to be 0. And also they are talking about photons in matter where they are coupled to matter fields and don't travel at c so would be expected to have mass.
     
  16. Write4U Valued Senior Member

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    a value is not a value? You need to get philosophical here, it's all mathematical and operates in accordance with certain mathematical guiding equations. This is the self-ordering aspect of the Universe.
    Everything has a value and when values interact they do so in a mathematical manner 1 + 1 = 2 (even in the Abstract)

    An equation means the description of the "same" pattern from two different aspects, but listing the common applicable values to both.

    I like Roger Antonsen's Ted Talk video.
     
  17. Ssssssss Registered Senior Member

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    I will rephrase: those numbers are upper bounds on the possible range of mass values and not best estimates of the actual value which is 0 a fact which would have been obvious to you if you'd looked at references 24-26 in the paper you cited instead of trying to score points with cod philosophy.
     
  18. Ssssssss Registered Senior Member

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    Reference please.
     
  19. Write4U Valued Senior Member

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    Bohmian Mechanics.

    Pilot Wave model of the Universe.
    https://en.wikipedia.org/wiki/Pilot_wave_theory
     
  20. Ssssssss Registered Senior Member

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    I know that. I was looking for a reference about Bohm's work that said that it "assigned an inherent potential that a zero value particle @ c
    acquires mass, whereas the potential of a collapsing wave function @ c, converts energy into mass, however small" which is what you said and none of the excerpts you provided say and I was looking for it because Bohm's work was as far as I know non relativistic and therefore says nothing about photons and was only much later extended by others to cover QFT concepts like photons and I don't think that later work is likely to say anything of the sort anyway.
     
  21. dart_ship Registered Member

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    Ssssssss light must have mass for black holes can pull it in (and stop it from escaping) and light reflects off objects (even in space)?
     
  22. Janus58 Valued Senior Member

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    No, The idea that you need Mass to have, or be effected by, gravity is an aspect of Newtonian physics. But we have moved past that to Relativistic Physics where Gravity is related to the Energy-stress tensor, of which mass may be a component, but doesn't have to be. In other words, just like in my previous post, where energy alone has momentum, energy alone participates in gravity. Light has energy, so it is effected by gravity.
     
  23. Write4U Valued Senior Member

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    Yes and that gravitational effect is identified as energy having "virtual mass".
    Else "no mass, no mas"
     

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