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All About Photons

  1. Jul 25, 2012 #1
    Photons seem to play an enormous role in not only our universe, but also our daily lives. from my understanding our eyes are just photon detectors... It also acts as the mediator for the EM force, is one of the few particles that can reach the cosmic speed limit, distributes heat or energy throughout the universe, and has countless technological uses too. so what id like is an explanation of how a single particle can do so much. what sets it apart from the electron or quarks for instance, both of which are building blocks of atoms.

    please feel free to correct any mistakes ive made and dont hold back on technical info, i wwant to learn this
     
  2. jcsd
  3. Jul 25, 2012 #2

    mathman

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  4. Jul 25, 2012 #3

    sophiecentaur

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    And is is very far from 'just' being a particle. The word 'particle' is only one description that can describe the behaviour of electromagnetic phenomena under some conditions.
     
  5. Jul 25, 2012 #4
    ok so a photon(s) is a family of particles/waves in a spectrum from radio to gamma. we see a small part of this. but when we see things, the only thing our eyes are detecting are photon waves in the visible range, right? also if photons, as there are so many of them, are mediators of the EM force, how do they do this? looking at the standard model its obvious that the bosons which are the force carriers have neutral charges, and the fermions which make up matter generally have a charge either positive or negative. if you make a metal coil and pass a magnet through it you can produce electricity. at the atomic level where is this energy comming from. the way i see it is atoms are being bombarded by photons increasing their energy levels and releasing electrons.. these electrons are the "electricity" we use. one part im not quite getting though is how the energy would be transfered from the magnet to the coil.
     
  6. Jul 25, 2012 #5

    davenn

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    bombarded by what photons -- where from?
    Photons "bombarding the coil" from any ambient light "hitting" the coil wont produce any significant emf in the coil. may not even be measurable
    Now if the coil was part of a radio receiver tuned circuit, then the photons of RF energy hitting and being absorbed by the coil will generate a tiny voltage (emf)
    but this is much much smaller than when you move a magnetic fild through the coil. The voltage in the coil is being generated by the moving magnetic field
    have a look at Faraday's Law in this wiki link.....
    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

    cheers
    Dave
     
    Last edited: Jul 25, 2012
  7. Jul 25, 2012 #6

    Drakkith

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    The basic idea is that the photon is the "quantum" of light and other EM radiation. This basically means that when an EM wave interacts with something it does so in little packets that we label as photons.

    Yes.

    This is a complicated question. The simple version is to imagine two ice skaters on ice. One tosses a ball to the other one. The toss causes the tosser to move away from the other skater, and when the ball is caught the other skater moves away as well. So we have a very very simple analogy of a repulsive force.

    This is covered just fine in classical electromagnetic theory and doesn't need to invoke "virtual photons" at all. (Virtual particles are the mediators of forces, such as virtual photons in the case of the EM force) Attempting to think of it in terms of virtual photons will only confuse you, as the way you imagine it is incorrect and it would take a lot of time and effort to learn the math required to understand the theory, along with the theory itself.
     
  8. Jul 26, 2012 #7
    "This is a complicated question. The simple version is to imagine two ice skaters on ice. One tosses a ball to the other one. The toss causes the tosser to move away from the other skater, and when the ball is caught the other skater moves away as well. So we have a very very simple analogy of a repulsive force."

    ok so newtons 3rd law is the main culprit there the two skaters being atoms, the ball being the photon. the person trowing the ball is is exerting a force tossing the ball, with an equal force pushing in the opposite direction as the pass pushes them back. repulsion makes sense on the atomic level, but what about attraction. if little energy packets are being exchanged between "particles" and repulses them, then how do they "attract" other particles. i know opposite charges attract and to my understanding that's why quarks attract and form protons n neutrons, an up quark having a 2/3 charge and and a down having -1/3. these two quarks bond in 3's either to make a positively charged proton or a neutrally charged neutron. 2/3+2/3+(-1/3)=1 or a proton, and -1/3+(-1/3)+2/3=0 or a neutron. where does this charge come from?
     
  9. Jul 26, 2012 #8
    To follow up on the analogy above, this would be the skaters throwing two balls in opposite directions (left skater throws ball left, right skater throws ball right), skaters attract eachother (but it's just an analogy, of course).
    The charges "come from" the particles themselves. A better way of expressing it is "charge is a basic property of particles" (recent thread about this here). If you then ask "where does this charge come from?" the answer would be "nobody knows" (today).
     
    Last edited: Jul 26, 2012
  10. Jul 26, 2012 #9

    Drakkith

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    I only know the analogy I gave earlier, sorry.

    Quarks are bound into hadrons because of Color charge, not because of Electric charge. As for where it comes from, it's simply a property of the particles, like mass and spin are.
     
  11. Jul 26, 2012 #10
    There is another analogy concerning attraction (I saw it on some lecture on the net, don't remember which one/where). It went like this: left skater throws ball left, ball goes upwards left, turns around, then moves to the right of the right skater, turns downwards right and hits the right skater from the right; skaters attract eachother. But it's quite confusing, that's some magic ball :biggrin:. I really don't know any good analogy concerning attraction. But it's described with virtual particles and momentum here (Baez).
     
  12. Jul 26, 2012 #11
    Another analogy for attraction is that as two skaters pass they exchange a ball but both hold on to it briefly. None of these analogies hold up to much scrutiny though do they :-)
     
  13. Jul 26, 2012 #12

    Drakkith

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    That's why they are merely analogies!
     
  14. Jul 27, 2012 #13
    Does anyone have a graphical representation of a single photon traveling through space?
    I know how Electric and Magnetic fields oscillates around the path of a photon (light).
    My question is as a photon moves from one point to another point what happens to
    the E and M waves at the previous point? How fast do they disappear or die off?

    Does each photon in a light beam travels as a short (spatially) E and M wave packet ( a bundle of E & M waves but terminated at both ends) ?
    I'm not sure if I have asked the question clearly.
     
  15. Jul 27, 2012 #14
    I disagree. These are all the same kind of particle, just with different momenta. They are all inter-convertible by means of Doppler shift; therefore, gamma rays and radio waves are the same thing from a different point of view. A particle doesn't suddenly become a different particle just because you move toward it!
     
  16. Jul 27, 2012 #15

    sophiecentaur

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    Absolutely not. The photon is 'everywhere' until it is detected and the uncertainty has been resolved. It was produced and it is detected. In between, you can't nail it down at all. It is very tempting to draw a sort of little squiggle that starts small, gets fatter and then tapers to a point again and to say "that's what a photon looks like". This is because of the popular phrase 'wave-packet'. It's easy to extend this to say a light beam is a whole shower of these little squiggly things. But it is a totally flawed model and there is no justification for using it. The 'extent' of a photon is a question that cannot be asked (validly). If you try to tie it down to the wavelength, in some way, then the photons of a lf radio signal would need to be many km long: no more 'little bullets' here, as in some people's descriptions of the photon 'particle'.
     
  17. Jul 27, 2012 #16

    Drakkith

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    I prefer to think of EM radiation as an expanding wavefront. Photons are simply the interaction of the wave with matter. Not sure if it's very accurate, but it greatly simplifies things for me.
     
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