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Is light matter, or just a concept of vibrations in a medium?

  1. Jul 30, 2013 #1
    I've heard light compared to vibrations under the earth. The "waves" between the tectonic plates are simply a 'state', or 'action', i.e, the action of millions of atoms "bumping" into each other with their opposite charges (all the valence electrons repel other electrons). From my understanding, physical waves such as in plates are not composed of actual matter.
    So I heard light travels in a medium called "luminiferous ether", and obviously light is a "wave". So, I logically think that light has no actual matter, and is, as stated above with tectonic plates, simply the action of atoms bumping into each other. BUT I also know that light is made of photons, and I've heard that these particles have a very tiny MASS!! So is light a WAVE like in tectonic plates, or an actual PARTICLE with MATTER?!?!? I am extremely confused on the intuition behind light's nature, and if someone can clear this up, that would be GREAT!!!



    As a supplementary side note, why does light have different wavelengths in the first place? Also, I know the length of the wavelength has to do with color, but WHY does this have anything to do with color?!? It's like, you can tell me how my pet cow affects the weather conditions in Antarctica, but I never know why. It's the same for me with light, and I can't find an answer. Is it just that, we, as human beings, are not technologically advanced enough to know?? Thanks to those who answer.
    And also, do you think it would be possible to "cancel out" the reflection of light off of matter, making stuff invisible?
     
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  3. Jul 30, 2013 #2

    Simon Bridge

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    Neither.

    "Luminiferous Ether" is junk science these days.
    You'll find the topic so problematic that it is actually banned here.
    See: http://www.lhup.edu/~dsimanek/philosop/ether.htm ... for a potted summary.

    Light and matter are different forms of energy - energy can be described as having a characteristic frequency. Light and matter can be described as waves or particles depending on how you are looking at it at the time.

    The wavelength of light is related to the energy and momentum it carries.
    The light-detector in our eyes sort out the energy of the incoming light into groups.
    "color" is the name we give to the groups.
    Thus, colors are associated with wavelengths.


    You will find a lot of articles online about "wave-particle duality", as well as color vision, go look.
     
  4. Jul 30, 2013 #3

    davenn

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    Hi DGonzo
    welcome to PF :)

    sounds like you had better do some reading up on earthquakes and the seismic waves they produce to catch up with some real physics :)

    cheers
    Dave
     
  5. Jul 31, 2013 #4

    Drakkith

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    Light is an electromagnetic wave that interacts with matter and other light by transferring energy in little chunks that we call photons. This is different than what you would expect from something that is a pure wave, as a pure wave would transfer energy in a continuous way, not in small packets. Photons do not have mass.


    Consider an antenna. A voltage and current is applied to the antenna and is then oscillated from + to - at a certain rate per second. This rate is the frequency and the antenna will emit EM waves with that same frequency. This is how radio waves are created. (Radio waves are just EM waves with a frequency from about 1000 hz to 300 mhz) Light, also being an EM wave, obeys the exact same principles, but with a MUCH smaller wavelength and MUCH higher frequency than a radio wave.
     
  6. Jul 31, 2013 #5
    I feel like that statement needs some qualification.
    If you have a box with perfectly reflective mirrors on the inside and you fill it with light it will have a greater resistance to acceleration and it will gravitate more then the same box without light. Energy and mass are the same thing so a photon obviously has mass. What it does not have is rest mass because a photon cannot be at rest. By definition it moves at the speed of light.
     
  7. Jul 31, 2013 #6

    ZapperZ

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    Actually, what you wrote needs some qualification.

    What light has is MOMENTUM and energy, not mass. The term "mass" that we all use, and what the general public UNDERSTANDS, is NOT the "mass" that you are describing here. If it is, then someone should go correct the particle data book and phone CODATA that they've made a mistake.

    Mass and energy are NOT the same. Simply stating that they are the same is like saying a cow and a watermelon are the same since they are both edible!

    The "mass" that we state in all our books are the standard, covariant mass that do not require that we state at what speed. The mass of electrons, protons, muons, neutrinos, etc. that you find in the PDG book are not accompanied by the speed that these were measured, nor the qualification that these are "rest mass".

    Zz.
     
  8. Jul 31, 2013 #7
    Hmmm, this is kind of confusing...what my main problem is, I don't what light actually is (or how it physically looks like). For example, we all can visualize (up to a certain extent) an atom as a cluster of spheres in the middle, and tiny electron spheres far out "orbiting in circles" around the central part.
    Since many people are saying light does not have mass (yet has momentum), my assumption is that we cannot visualize, in our minds, light as having a certain structure, or what it would theoretically look like.

    And I think that it IS kinda absurd that we're trying to learn all this stuff with so little and obscure information. What we say today (not just in terms of light, but theoretical physics, quantum mechanics, and all the such) is pure absurdity to people 20-30 years ago, and so will physics in the next 20-30 years seem weird to us now, because science is (annoyingly) constantly changing, especially in the realms of physics, string theory, and generally, the very little known.
     
  9. Jul 31, 2013 #8
    Check out the wiki page for some ideas about visualizing light (apart from just looking at anything:smile:) Note that the wave diagrams they have are simple, coherent light waves. The light we normally see would have lots of these added on top of each other. The "medium" that is fluctuating is the electric and magnetic field. Your intuition might want a medium like air instead of a field, but luckily your intuition can change.

    The information is obscure to YOU and a lot of it is obscure to me, but there is a lot of information and a lot of very intelligent people that devote their lives to studying it. The study of light (beyond the phenomenological study) started about 150 years ago. The "final" understanding of light came with QED in the late 1940's. It does seem weird when you first see it, but so does a hippopotamus.

    Most modern physics would be understandable to physicists from 20-30 years ago, a lot of it even 40-50 years ago (most of the standard model would probably be a bit foreign to a lot of the physicists from 50 years ago). I understand what you're saying about how annoying it is that science changes. As soon as you learn something it seems that your "facts" are not completely correct. BUT don't forget that this is actually a good thing. When we learn new things, we change the theory. As the quote attributed to Keynes says
     
  10. Jul 31, 2013 #9

    Drakkith

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    I can tell you exactly what it is. Light is an electromagnetic wave that transfers momentum and energy from one object to another through the electromagnetic field. That's what it 'actually' is.

    And while you can never see it, you can visualize it any way you want that makes sense to you. I personally visualize it as an expanding wavefront in the shape of a sphere or other shape depending on the situation.

    You're right. We can't. Not because of limitations within our minds, but because light really doesn't have a structure. And it both theoretically and actually doesn't look like anything.

    Also I have no idea what not having mass but having momentum has to do with this.

    Are you kidding me? We have mountains of information about how subatomic physics works! Thousands upon thousands of experiments! Every microchip and transistor ever made is a testament to how much information we have on the subject!

    Just because YOU don't understand it doesn't mean that it's obscure and lacking in information.
     
  11. Jul 31, 2013 #10

    ZapperZ

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    The problem here is that you are trying to learn this from the middle, not from the beginning. There are no shortcuts! If you do not understand the basics, then of course things will appear ad hoc to you. You have no reference frame to place all these information in!

    Either you start from the beginning, i.e. go through the classical E&M stuff first, or you just have to accept things as they are told to you.

    Zz.
     
  12. Aug 2, 2013 #11
    I believe it is generally understood that when you state the "mass" of something you are giving the covariant mass and that therefore you cannot provide such a number for a photon because the photon has no rest frame. In this sense I agree with you. Covariant mass is simply not a property that a photon has, it cannot even be said to be zero.

    That being said mass is also generally understood as that which gravitates and resists acceleration. These are things that light certainly does do.
     
  13. Aug 2, 2013 #12

    ZapperZ

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    I disagree. "gravitates" needs explanation, because the bending of light due to gravity is NOT because it has mass, but because it follows the "geodesic" of spacetime. It is not the same. See the FAQ!

    "resists acceleration" is also puzzling, because photons are BORN at c. They "resist" no acceleration because they do NOT undergo such a thing!

    So yes, it is your response that requires more of a qualification than the one you were responding to.

    Zz.
     
  14. Aug 2, 2013 #13
    The previous example I gave is a good one. If you have a box with a perfectly reflective inside surface and you have a test particle and you chart the course of the test particle from a distant reference frame you will see that the gravity produced by the box alters its path. If the box contains some trapped light it will alter the course of the test particle more. Hence, light gravitates.

    If you try to accelerate the box you will find that it accelerates at a certain rate with a certain force. Trap some light in the box and you will find that it accelerates less given the same force. Hence, light resists acceleration.

    If you don't like the idea of a box with a perfectly reflective inside surface then feel free to substitute a black hole.
     
  15. Aug 2, 2013 #14

    ZapperZ

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    Find me such a box and the experiment that did this.

    Zz.
     
  16. Aug 2, 2013 #15

    Simon Bridge

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    MrSpeedyBob appears to be talking about the inertia of light.
    That topic has been discussed before.

    Classically, mass is inertia and the source of gravity.
    When someone asks a question involving "mass", it is not so important what is generally understood amongst phyicists but what is currently understood by the person asking the question... at least, when it comes to figuring out how to answer the question. Is DeGonzo asking in terms of a more classical or relativistic understanding?

    So to the comment of having heard that a photon may have a very tiny mass ... we can point out that photons are not usually well understood in terms of mass - explain about rest-mass etc. But, to be complete, we should point out that by the mass-energy relation, it is a fair observation. However, drawing a parallel with tectonic plates from this is not valid.

    As far as post #1 is concerned - nothing to do with light in a box. OP has shown such a Gordian tangle of confusions, probably from overexposure to junk-science, that picking at individual strands is unlikely to help: an Alexandrian solution is probably called for.

    Still - feedback from OP is called for at this point I think.
     
    Last edited: Aug 2, 2013
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