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Is Light Matter?

  1. Sep 12, 2007 #1
    Is light a physical thing, in other words, is it matter?
  2. jcsd
  3. Sep 12, 2007 #2
    Light is another name for electromagnetic radiation, which is the transmission of energy. It is energy. Einstein's equation shows that energy and matter are the same concept, so in a way, it is.
  4. Sep 12, 2007 #3
    Light is a flow of a huge number of particles - photons. Photons have zero mass, but this doesn't make them any "less" matter than electrons or protons.

  5. Sep 12, 2007 #4

    Claude Bile

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    E = mc^2 is the equivalence of mass and energy, not matter and energy.

    Matter is traditionally regarded as stuff with rest mass, i.e. protons, neutrons, electrons and so forth, and is distinct from inertial mass.

  6. Sep 12, 2007 #5


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    In fact, it does.

    Light is not matter. Light is energy. This is not a point of debate.
  7. Sep 12, 2007 #6
    I agree, but I think there are plenty who will debate this (I hope). Not being smart, just trying to learn,
  8. Sep 12, 2007 #7


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    You agree, but you're looking for debate? Are you just looking to stir a pot or something?

    Really. There's no debate. It's not open to interpretation.
  9. Sep 12, 2007 #8
    But such a statement does not distinguish it from matter. Nonetheless, distinction from radiation is a practical meaning for modern use of the term "matter".

    On the other hand, language is fluid: "Space-time tells matter how to move".
    Last edited: Sep 12, 2007
  10. Sep 12, 2007 #9
    My purpose is not to stir. There are many people who argue that light is matter, and I would like to learn what their argument is and how credible it is. One of the hardest things with learning modern science is that there seems to be so many conflicting conclusions, it's hard to know which to believe.
  11. Sep 12, 2007 #10
    Swerdna, are you sure those people weren't merely asserting that light has (relativistic) mass?
  12. Sep 12, 2007 #11
    Matter to me is localized energy. In this instance light is matter. Thats my take anyway.
  13. Sep 12, 2007 #12
    If they are, they aren’t making that point clear. Does that light has “relativistic mass” mean it is matter?
  14. Sep 13, 2007 #13
    This argument has been put to me . . .

    If E = mc^2, and light = E, then light = mc^2. Therefore light is matter.

    Given, as Claude Bile stated (post #4), “E = mc^2 is the equivalence of mass and energy, not matter and energy”, I guess they are saying that because light has relativistic mass it is matter.
  15. Sep 13, 2007 #14
    Matter contains energy, but how does that make energy matter? A sponge can contain water, but that doesn't make the sponge water.
  16. Sep 13, 2007 #15
    While we are on the topic. Does this prove that light cannot have mass?:

  17. Sep 13, 2007 #16


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    Can you point out who "these people" are? I mean, if you're referring to crackpots who barely know physics, then we're wasting out time here, aren't we?

    There is a FAQ in here that addresses the misuse of that Einstein equation. You may want to start with that. After you've done that, then maybe you should then proceed with defining what you mean by "matter". Only after you do these two, then maybe we can use that criteria of what "matter" is to see if light has such a property. I think this is the only sensible way to approach such a thing, using valid definitions and a clear understanding of what we are talking about. The way this thread has proceeded appears to be more of a "make it up as I go along" type.

  18. Sep 13, 2007 #17


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    Algebraically, this is what you've said:
    A = Matter
    B = Energy
    c = localized (a modifier)

    Your argument is
    Since A = cB, therefore B=A. (Where did the c go?)

    Because matter is localized energy does not make light matter. Not the least reason of which is because light is not localized; it moves at c. And If it didn't, it wouldn't be light.
  19. Sep 13, 2007 #18
    Thats exactly what it makes it .... if true.
    Didn't say that light does not move at C, only that if it is localized, it can take on the characteristics of matter. A solor sail for instance one could consider as a situation where light is very briefly localized, by which light takes on a characteristic of mass by which a sail can be pushed.

    Light could be said to move in a straight line, but what if it follows an orbital pattern. I.E. localization.
  20. Sep 13, 2007 #19


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    No. It does not. This claim holds no water.

    One can "consider" all one wants. One can consider faeries, but that doesn't make them exist.

    You are making up definitions as you go. This has nothing to do with reality. Science is not a form of poetry.

    The OP's question has been answered. Further speculation merely degrades the quality of this thread for future readers.
    Last edited: Sep 13, 2007
  21. Sep 13, 2007 #20
    Mathematically speaking, c is a constant, however, it is possible for light to be slowed down in a Bose-Einstein condensate.

  22. Sep 13, 2007 #21
    Light affects matter, like sun rays evaporating water, therefore is physical. Einstein's famous equation, E=mc2, shows that (light) energy cannot exist without (matter) mass, and contrariwise. Furthermore, both light and matter can exist as particle or wave, according to de Broglie. The property of mass is mediated by a physical entity, the Higgs potential.
  23. Sep 13, 2007 #22
    Light has a wave-particle duality. In other words, the higher you go in frequency, the more light begins to look and act like a (massless) particle, with "probability amplitudes," or quanta. I think this is more or less the direction the OP was heading. Matter has definite distinction when examining light-matter interactions, so I would venture to say that no, light is not matter.

    Edit: Bah, you beat me to it Loren Booda, although you failed to mention that the Higgs boson is still a purely theoretical conjecture.
    Last edited: Sep 14, 2007
  24. Sep 14, 2007 #23
    I would have thought the contrary :

    [tex]E=mc^2[/tex] (einstein)
    [tex]E=h\nu[/tex] (Planck, where [tex]\nu[/tex] is the frequence of the wave)
    thus, m = [tex]\frac{h\nu}{c²}[/tex].

    so the higher you "go in the spectrum of wave length/frequency", the bigger the mass of the photon, and then the more it acts like a massy particle.
  25. Sep 14, 2007 #24
    Goddamn E=mc^2 !!! If only people would learn why that is they'd stop applying it to things outside of its applicability! The full formula:

    [tex]E^2 - p^2 c^2 = m^2 c^4[/tex]

    E=mc^2 only holds when p=0 -- for a stationary particle. Is a photon ever stationary? Furthermore, m=0 for a photon, so the energy and momentum is tightly linked: E = pc. Add to that, [tex]E = \hbar \nu[/tex] and [tex]p = \frac{\hbar}{\lambda}[/tex] and you've got everything. Thus higher energy gives higher momentum, which gives *lower* wavelength, thus a high energy photon is better localised in space, behaving more like a particle.
  26. Sep 14, 2007 #25
    I was right in principle, at least. Just take out "wavelength" save frequency and it all makes perfect sense.
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