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What is light made of?

  1. Nov 25, 2006 #1
    Hey is light made of electrons or protons? I know that all matter in existence is made up of a combination of those two. I figure it is one of each that way that the total charge is 0 and it's all neutral and stuff (and due to the symmetry of the universe and all that) but i'm not sure. I tried to look it up but it wasn't really clear it said online that it was a photon but I know that's just a sci-fi kind of term (that they use in Star Trek and stuff). So can anyone clear this up?
  2. jcsd
  3. Nov 25, 2006 #2
    Last edited: Nov 25, 2006
  4. Nov 25, 2006 #3
    Hello! Thank you for the link, it is very informative but it does not answer the problem regarding the proton and electron but instead gives a vague answer in terms of this imaginary "photon". Please clarify! Thanks! :)
  5. Nov 25, 2006 #4


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    that's not quite true, either.
    probably not.

    if i were to say light (visible light is just particular frequencies of the electromagnetic spectrum) was "made from something", i guess i would say it is made from varying (and propagating) electromagnetic fields.
  6. Nov 25, 2006 #5
    lol. I am pretty sure you're confused :) The photon is for real! A proton is a particle in an atom which does have mass (And could never travel at the speed of light) , a photon what light is made of (and is massless, thus it can travel at the speed of light).

    edit: By the way my answer was directed to OP. rbj snuck a post in before me :).

    btw rbj's post is more accurate then mine.
    Last edited: Nov 25, 2006
  7. Nov 25, 2006 #6
    Sorry but this explanation doesn't seem quite right since protons and electrons are the basic building blocks of matter and can't be made of anything else (proton has a charge +1 and electrons -1, thus you can't have anything else since what would it be? The only other thing is neutral which is 1 p and 1 e and that is my theory of what light is). The notion of light being waves seems pretty silly (after all, when you turn the lights off they dont slowly ripple away like a stone dropped in a pond) so I don't know where that came from. Light is definitely a particle that is a proton/electron mixture but I don't know what that exact mixture is.
  8. Nov 25, 2006 #7
    1) Protons and electrons are not the building blocks of all matter.
    2) The notion of light being a wave is not silly. Your idea that waves slowly ripple off after you stop them or something, and comparing that to light, is silly.
    3) Light is massless. Protons and electrons have mass.
    Last edited: Nov 26, 2006
  9. Nov 25, 2006 #8

    m = mass
    c=speed of light (299 792 458 m / s )
    [tex]m_{o}[/tex]=rest mass

    Since I am sure you would agree that protons and electrons both have mass (if you don't, you are OBVIOUSLY trolling), it becomes apparent that the mass of any object comprised of protons/electrons becomes infinitely great as the object approaches the speed of light. According to that equation which you can read more about here http://en.wikipedia.org/wiki/Relativistic_mass light can't possibly be made of protons or electrons. Plus you are leaving out many many many many subatomic particles that make up the universe. Anyways, hopefully that site helps you, but my true suggestion is for you to pick up a physics book and read if you're interested.
  10. Nov 25, 2006 #9


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    Staff: Mentor

    Electrons are indeed one of the fundamental particles (building blocks, if you like), but protons are not. Protons are made up of quarks and gluons. So are neutrons, which you also find in an atomic nucleus.

    Light waves travel at a speed of [itex]3 \times 10^8[/itex] meters per second, so they do "ripple away" but so very very rapidly that it looks like they disappear instantly.

    Light has wavelike properties because it exhibits diffraction and interference, which are impossible with purely particle-like objects.

    Light also has some particle-like properties, but it's definitely not a proton/electron mixture. For a summary of the current state of generally accepted knowledge/theory about particles in general, try

  11. Nov 26, 2006 #10
    jtBell gave the correct answers and said them very concisely and nicely.

    If you then ask what a photon is, no one can answer that question because its considered indivisible just like a quark or electron or graviton is. These 4 particles are called fundamental particles and its thought that everything in the Universe is made up of them.

    P.S.What's trolling mean?
  12. Nov 26, 2006 #11


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    I am keeping a pretty tight fist on this thread. If you have had a post vanish, very likely it addressed the sincerity of the OP rather then making a effort to answer his question.

    At this point there are several good explanations, it is now up to the OP.

    Seminole: You need to read the remaining posts very carefully. They contain good information.
  13. Nov 26, 2006 #12
    Since when did photons have mass???

    They are just blocks of pre-packaged energy, and energy does not have mass.
  14. Nov 26, 2006 #13


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    Mad, if your response is to the section that you quoted, I think that you misread it. It says protons and electrons, not photons and electrons, and the relativistic mass gain is real.
    Integral, I have no issue with the sincerity of the original question, but the refusal to accept correct answers is a bit irritating.
    Seminole, I don't know where you ever got the idea that photons are fictional. They are the quanta of electromagnetic radiation, which is indeed composed of an electric field and a magnetic field oscillating perpendicular to each other. Light displays the characteristics of both a wave and a particle. Which one it seems to be at any particular time depends upon how it is observed. (Compare the dual-slit experiment with the photoelectric effect.)
  15. Nov 26, 2006 #14


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    oh, i guess since energy was discovered to be equivalent to mass.

    there is a serious proposal to redefine the kilogram in such a way as to define Planck's constant to a fixed value (rather than measure it, similarly to how the meter was redefined so that the speed of light is defined to be 299792458 m/s). one such wording is:

    The kilogram is the mass of a body at rest whose equivalent energy is equal to the sum of energies of a collection of photons whose frequencies sum to exactly (2997924582/66260693 x 1043) Hz.

    so if we could put that collection of these blocks of pre-packaged energy in a perfectly mirrored box of neglegible weight on one platter of an honest scale and put the kilogram prototype on the other platter, which way does the scale tip?

    photons have mass. the mass of a photon is

    [tex] m = \frac{E}{c^2} = \frac{h \nu}{c^2} [/tex]

    but because

    [tex] m_0 = m \sqrt{1 - \frac{v^2}{c^2}} [/tex]

    the rest mass is zero because the speed of photons are normally believed to be the same as the wavespeed of light (which is [itex]c[/itex]).

    photons have mass (inertial mass, what you get when you divide momentum by velocity), but no rest mass or "invariant mass".
    Last edited: Nov 26, 2006
  16. Nov 28, 2006 #15
    Electrons and Protons are matter, and have mass.
    Electrons are leptons, believed to be indivisible, and are not made of quarks.
    Protons are made of two Up quarks which each have a charge of +2/3 (+4/3) and also one Down quark which has a charge of -1/3, which adds up to +1.
    Neutrons are made of one Up quark and two Down quarks, which adds to 0. Neutrons are quite happy in the nucleus of an atom, but if you pull one out they tend to die in about fifteen minutes. One of the Down quarks explodes and throws away an electron (changing its charge from -1/3 to +2/3 since an electron has -1) and an electron antineutrino (which is a very tiny uncharged mass of antimatter) making the Down quark into an Up quark, which makes the whole particle a Proton now.
    Quarks are matter, and have mass.
    Leptons like the electron, muon, and tau are matter, and have mass.
    Their neutrinos are matter, much much smaller, and have mass.
    Quarks also come in Top, Bottom, Strange, and Charm , but we aren't really sure what they "do" yet, aside from affecting certain masses and half-lives.
    Quarks come in three distinct flavors: red, blue, and green. If you add quarks and get white (by having red, blue, green), they form a particle and their mass changes to reflect their new energy state of being bound together. Similarly if you have antiquarks (quarks of antimatter) in the mix, you can add quarks and get black (say you have a green, and an antimatter green) those also form a particle.
    Most of these particles are extremely short-lived.

    The electromagnetic force is responsible for making protons and electrons attract, and making protons repel each other, and making electrons repel each other, and making magnets fly and making your hair stick up when you get zapped by lightning. When an electron falls from a high energy orbit to a low energy orbit, it releases some electromagnetic energy which travels to another particle and adds that energy to it in the form of heat or by moving another electron to a higher energy state.
    There's a limit to how small of a packet of electromagnetic energy can be released, and we call this the photon. It has no -rest- mass, but it does have a finite -apparent- mass due to travelling at the speed of light. It is not made of matter. It does not have rest mass. It does not take up space, so you can overlap them as much as you like. It vibrates like a wave, however, so there is a limit to how small a hole it can go through.

    Light is made of pure electromagnetism.
  17. Nov 28, 2006 #16


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    That was a very informative and down to earth explanation, it cleared things up to me all right!
  18. Nov 29, 2006 #17

    Well said, very nice post.
  19. Nov 29, 2006 #18
    Existence of graviton does not carry same weight of experimental evidence as other three (photon, electron, quark), not sure we can say that the case is closed and these 4 particles are what "everything in universe" is made of--I suspect many future changes in this working hypothesis.
  20. Nov 29, 2006 #19


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    Echo that.
  21. Nov 29, 2006 #20
    I agree.

    In fact I don't think there is any experiemental evidence for the graviton at all. But still it is taught to be a fundamental particle, perhaps based solely on theoretical constructs and inferences, I'm really not sure why.
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