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Mass of light

  1. Sep 3, 2003 #1
    dear reader,
    what is the mass of light particels?
     
  2. jcsd
  3. Sep 3, 2003 #2
    deppends on their frequency m=hν/c^2
     
  4. Sep 3, 2003 #3

    jcsd

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    Photons have zero mass.
     
  5. Sep 3, 2003 #4

    jcsd

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    relativistic mass is not the same as mass and is hardly ever used.
     
  6. Sep 3, 2003 #5
    Relativistic mass, mass at rest, mass... It's a question on nomenclature.
     
  7. Sep 3, 2003 #6

    jcsd

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    Not really, because mass that is invariant under a Lorentz transformation is a much more useful defintion than one that isn't. The term 'mass' means invariant mass only.
     
  8. Sep 3, 2003 #7
    It's worthwhile to discuss about this. However I think that a definition of mass that include the term 'mass' is not very good.

    Of course, in my first post, I was talking about the 'relativistic mass'.
     
  9. Sep 3, 2003 #8

    selfAdjoint

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    And the "relativistic mass" is wrong because it would be infinite. The invariant mass of a photon is zero, and its energy and momentum are what depend on its frequency.
     
  10. Sep 3, 2003 #9
    Can you explain it please? :smile:
     
  11. Sep 3, 2003 #10

    jcsd

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    No: the relativistic mass is a function of the kinetic energy and therfore frequency of a photon and the correct formula for this was given by Dr. Luz. This is obviously not invariant under a Lorentz transformation as different observers will observe different frequencies and therfore each give a different value for it's energy and relativistic mass.
     
  12. Sep 3, 2003 #11

    marcus

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    Luz seems to be asking for a primitive definition of mass

    (he says a definition of one kind of mass based on another kind of mass would not be very good----so he probably wants a definition of mass in terms of the most basic kinds of measurement)

    the only one I know is this

    "the mass of a body is the inertia of the body at rest"

    inertia is a ratio of force to acceleration

    the potential circularity of this definition is a long-recognized minor problem in the foundations of physics

    it may not be a perfect definition but at least it has a clear operational meaning and removes some of the ambiguity

    I agree with several of the other posters here that relativistic mass does not seem to be a very useful concept---Einstein explicitly advised against using it (there is a letter to this effect)----and it is apt to lead to confusion. Both selfadjoint and
    jcsd are right although they contradict each other----defined one way the "relativistic mass" of some light would be infinite if the light had any mass to begin with and defined another way the "relativistic mass" is just a redundant jargon synonym for the energy of the light and is not infinite but simply E/c2.

    So one might as well follow Einstein's advice and not use the concept. Saves endless useless discussion about terminology.

    Following majority usage among working physicists, since mass is the inertia of a body measured at rest, since light cannot be at rest it has no mass.

    However boxes CONTAINING light can be at rest and part of their inertia can be due to the light which they contain. The sun is soaked full of light even to its very core and that light (which has zero mass) contributes mass, inertia, gravitational attractiveness, etc. to the sun.

    In other words the notion of mass prevailing in modern physics is not additive-----which is tough for some people to accept. So they have this irresistible urge to try to get people to change the way they talk so that mass can be more of an alias for energy and have the additivity that we associate with energy.

    I'm for the simplest least ambiguous use of words----getting mass aligned with what most physicists mean by it.

    I also appreciate when at least some types of quantity can have simple operational meanings, without a lot of theory mixed up in them.

    Force can be measured purely electrically by a device called the "watt balance", which is kind of interesting. Maybe it is more primitive than mass.
     
  13. Sep 3, 2003 #12
    Thank you for your efforts and this great explanation. Realy I did not wanted a primitive definition of mass, I only was remembering my old logic teacher when he told me that "when you are going to define the concept A you must not include the word A in the definition". Do you understand me?

    From now when I read mass I will think in the "invariant mass-mass at rest"

    Then answering the initial question, the mass of photons are 0
     
  14. Sep 4, 2003 #13
    If mass is "the inertia of the body at rest", there are several problems. Firstly, there is no absolute "at rest". Motion is relative. Secondly, a photon at rest has never been observed, so how can you say that it has 0 mass?

    How do we measure mass, and what purpose does the concept have, other than in inertia/momentum-qualities that a photon exhibits?
     
  15. Sep 5, 2003 #14
    Can it be said that a photon has mass when it is localized, or briefly localized? Perhaps all mass is localized photons.
     
  16. Sep 5, 2003 #15

    pmb

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    Wrong - It has zero *proper mass*
     
  17. Sep 5, 2003 #16

    pmb

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    Wrong. Relativistic mass is just another name for mass. When the term "mass" is used it means one of two things - "proper mass" or "relativistic mass" and the later is closed to being mass than the former since it retains all the properties associated with mass.

    And your comment about the usage is incorrect as well.

    Pmb
     
  18. Sep 5, 2003 #17

    pmb

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    As far as proper mass being more useful - that's a matter of usage - i.e. for what problem is being solved. And even then it's a matter of point of view since different people sovle the same problem in different ways.

    And "invariant mass" is not what people mean when they use mass. That's just plain wrong. In almost every case the author will simply say what he means by the word "mass" and then use it as such. If it's not exlained like that then one can tell by the context. And it
    also depends on the journal. Mass means relativistic mass in almost all cases in the American Journal of Physics while in Physical Review D it means proper mass in almost all cases. If you go online to a place like FermiLab then it might mean relativistic mass. One only has to look to find counter examples to your claim. Examples are all over the place. Invariant mass is important as quite useful but does not have a well defined meaning in general. For example: th term "invariant mass" (aka rest mass aka proper mass) has liitle meaingin for several particles moving in an EM field. And its quite easy for people to make mistakes if they think strictly in terms of proper mass. For example: If you asked someone if a moving particle weighs more than the same particle at rest then they're likely to say no. And that's the wrong answer. Weight increases as a body moves faster since the mass increases (no. Not proper mass. Passive gravitational mass = relativistic mass = inertial mass = just plain 'mass')

    Note: Weight W = mg is defined as the magnitude of the supporting force required to supoort a particle in a gravitational field where g is the local acceleration of gravity

    Pmb
     
  19. Sep 5, 2003 #18

    jcsd

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    I'm afraid your very wrong, 'mass' always means invariant mass, the idea of relativistic mass as 'mass' went out the window a very long time ago.

    There are reasons for this for example when talking about the Chanderskar limit, relativistic mass is a completely useless defintion.

    Yes some pop-sci books do occasionally muddy the water in this way, but when a scientists says mass he means invariant/rest/proper mass.
     
  20. Sep 5, 2003 #19

    pmb

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    Sorry to disappoint you but that's quite far from the truth. You've gotten the wrong idea somewhere along the line. There are many new relativity texts which clearly use the concept - some heavily. In fact one of the most prominent relativists, i.e. Wolfgang Rindler, use this concept in his new text, published in 2001. Many universities do as well. The Chanderskar limit, as I recall, is an inherent property and as such it would be incorrect to refer to is as anything other than rest mass. And just because you've never found relativistic mass to be useful - in NO way implies that everyone does - That's quite far from the truth.

    And when I say that it's used I don't mean in pop-sci books. I mean at the undergraduate and graduate level texts in both special and general relativity. And I also mean recent texts too. And I aslo mean in physics journals.

    Pmb
     
  21. Sep 5, 2003 #20

    jcsd

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    PMB, you must of learnt your physics about 20 years ago because 'mass' these days means exclusively invariant mass, you will simply not find a recently published paper that refers to relativistic mass as 'mass'.
     
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