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Do photons keep atoms stable?

  1. Aug 25, 2004 #1
    I read recently that photons are largely responsible for why matter doesn't collapse in on itself!? For example, the electrons (negatively charged) and protons (positively charged) inside each of the atoms that make up our bodies should ordinarily attract each other and thereby cause us to collapse in a heap(?), however, because photons interact between electrons and protons this interaction suffices to keep the two apart. Does that sound about right?

    Related to the last point above... are there still photons in the air when it is pitch black?
  2. jcsd
  3. Aug 25, 2004 #2
    I think the best answer to your first question is a simple no. Photons are not the reason electrons do not collide with protons. How good is your quantum mechanics? If you break down the probability distribution for the electron in its lowest state, the probability that it is found in the nucleus is not zero, but it is very very small.

    The answer to your second question, is absolutely yes. Remember that we only see a tiny tiny portion of all light with our eyes. A great deal of light is "dark" to us.
  4. Aug 25, 2004 #3
    Thanks. Maybe i should quote directly from the book i read it in to be sure of what the author was actually trying to say. I'll try and do that as soon as i can. In any case, i got the following answer to the same question elsewhere in another thread:

    "Yes this is right, according to quantom theory, there are things called virtual photons which are responsible for the elctromagnetic force. These virtual photons ar what keeps elevtrons and protons appart in an atom."

    So still confused as the two answers appear to differ!

    Just starting out... read a couple of books...

    But it doesn't rule out that the electron will never be found in the nucleus... what happens when that occurs?
  5. Aug 25, 2004 #4
    Virtual photons keeping an electron in orbit? I'm not sure how that would work. Maybe someone else can explain that. In any case, I don't see how it is necessary.

    I tried laying out the equations in LaTex but found my skills to be insufficient. Besides, this is a pretty common problem, I remember it distinctly from my QM class. Here is a website that has a reasonable walkthrough of it:


    Skip the background and go straight to the Shroedinger Equation. The derivation of the basic orbital levels is there, and note that an n=0 energy level is simply not allowed. The probability of finding the electron in a shell some radius from the center is given later, but also note the assumption on a non-small radius shortly after the coulumb force is inserted in the Sshroedinger Equation. This is for the lowest energy level, which is closest to the nucleus. There is no angular momentum, so the electron is not moving "around" that atom.

    If I remember right, it is that assumption on a reasonable radius that causes the probability to be absolutely zero in the center, but even without it the probability of the electron being detected there is very low; so much I don't even remember the consequence.

    Given the reliability of that equation, I don't see any reason to "beef" it up conceptually with virtual photons. Maybe there is a QED effect that helps explain it? Someone else will have to answer that, but I think the derivation in the link is plenty substantial.
  6. Aug 25, 2004 #5
    Perhaps you should also tell us which book you're reading? In any case, saying virtual photons keeps the electron bound is (almost) the same as saying the electron is bound by the classical EM field of the proton. Virtual photons are the means by which the EM interaction is transmitted, at least in the perturbation theory picture. Once the interaction is established, one solves the Schroedinger (or better yet, Dirac) equation for the system and discovers a discrete spectrum of bound states. Since there exists a "lowest energy" state, crashing into the nucleus is not energetically allowed.
    The only feature of the hydrogen atom that requires quantum electrodynamics is the Lamb shift. There's plenty info on that, google it up.
  7. Aug 25, 2004 #6
    That certainly makes sense. It doesen't agree with whomever told him that virtual photons keep the electron apart from the atom... but I don't think they were correct, either.
  8. Aug 25, 2004 #7
    In the last month i've read:

    - Light Years and Time Travel (Brian Clegg)... the chapter on QED sparked my interest in the topic.

    - Schrodinger's Kittens and the Search for Reality (John Gribbin)

    - The Elegant Universe (Brian Greene)

    Still, most of what's been said in this thread is still over my head it seems :frown:
  9. Aug 25, 2004 #8
    The good news is that, upon a cursory look, those books seem to convey a generally accurate picture of the current state of physics. I've only looked at their tables of contents so I cannot vouch for them, but they seem alright.
    The bad news is that IMO it's very hard - if not impossible - to explain modern physics in a way that everyone can understand, without sacrificing some rigor. It's simply far too difficult, much more so than the classical F=ma. One can explain what happens in the hydrogen atom qualitatively, but try to do any sort of calculation and you're stuck at best with the Schroedinger equation - a second order partial differential equation that requires mathematics at the year 3 level (here in Canada, anyway).
    Anyway, going back to your first post: elementary quantum mechanics is sufficient to explain the stability of the hydrogen atom, there is no need of virtual photons to conclude the atom is stable. QED is a theory that is one generation more advanced than ordinary QM and only contributes tiny corrections to the atom's description.
  10. Aug 26, 2004 #9


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    In addition, obviously we only see the light that hits our eyes - visible light can be (often is) shooting by all around us, but if it doesn't hit our eyes, we won't see it.
  11. Aug 27, 2004 #10
    The exchange of virtual photons is what QED is all about. Schrodingers equation can tell you how a charged particle acts in an external electromagnetic field, but does not tell you anything about the electromagnetic field produced by the particle. Classically the function satisfying a wave equation for an electromagnetic wave is the electromagnetic field. Thus it makes sense to think of the electromagnetic field as the wave function of a photon(technically there are some minor differences, however they satisfy the same wave equation). An electron can emit or absorb virtual photons(which don't quite satisfy the wave equation, but when they are too far from satisfying it they are not significant). When this happens the electron's momentum changes, however the change in the electromagnetic field compensates(due to the new virtual photon), so that momentum is conserved and the wave equation for an electron is still satisfied.
  12. Sep 2, 2004 #11
    nothing keeps electrons from falling onto protons. in fact if we consider the atom of hydrogen, electron does not even move around the protons (orbital momentum is zero for ground state, l=0). Only uncertainty principle prevents matter from collapse to a point.
    Virtual photons in fact are just a medium between interacting proton and electron. So their effect is attraction, not repulsion.

    Yep, even in very very pitch dark room, there will be photon of infrared spectrum and lower.
  13. Sep 2, 2004 #12


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    Put yourself in a cylindrical room whose wall is a mirror. Along the central axis of this cylinder is a linear, vertical light source that is ON. As long as you are not facing this light source, you will see nothing - you'll experience darkness even though the entire room is flooded with light (except for the small portion that lies in your shadow). Draw a simple ray diagram to see why this is true -neglect diffraction effects.
  14. Sep 2, 2004 #13


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    Regarding the first question: no, what keeps matter from collapsing is essentially the uncertainty principle. The electromagnetic force tries to collapse the electrons on the protons, and in classical electromagnetism, this would occur extremely rapidly. No atoms could exist very long if it wasn't for quantum mechanics. The key point is that if you try to squeeze an electron (or any particle) into a smaller and smaller box (let's say), its momentum becomes more and more spread and this means a stronger and stronger force againts the surface of the box. You would find it impossible to squeeze the box to zero volume, no matter how strong you are. The em force tries to squeeze the electrons into the protons but this quantum effect pushes back and the atoms reach an equilibrium state (this is a crude picture and I could get into solving the Schrodinger equation and even QED but I think that's the level of answer you were looking for).

    Notice that if you pile up a huge amount of matter together, then the force of gravity would become important and would "try" to squeeze the atoms together with a large force. Well, at first the ordinary electromagnetic repulsion between the outer electrons would be enough to overcome gravity and this is what is hapening with planets, asteroids, etc. But if you pil eup more and more matter (and there is no nuclear fusion to produce an outward pressure like in stars), then electric repulsion is no longer able to win against gravity. The atoms collapse until the quantum effects become so large that gravity can't compress the electrons into smaller space anymore. At that point it's only this quantum effect that keeps the object from collapsing under its own gravity and you have a white dwarf (if you want more info about this, search for "degeneracy pressure", that's the technical term).

    If you keep piling more matter on the white dwarf, at some points the electrons and protons have a large probability of interacting through the weak nuclear force and they are turned into neutrons (and other stuff). It turns out that the degeneracy pressure (the pressure due to this weird quantum effect) is smaller when you try to squeeze neutrons than when you try to squeeze electrons (it's related to the mass of the particles). So gravity wins and suqeeze the neutrons into smaller and smaller volume until the degeneracy pressure of the neutrons is finally strong enough to stop gravity. Then you have a neutron star. Whereas a white dwarf is typically the size of our planet (with a mass of the order of the mass of the Sun!), a neutron star of roughly the same mass has a diameter of the order of only 10 kilometers! That's because it's easier to squeeze neutrons than to squeeze electrons.

    If you keep adding mass, then even the degeneracy pressure of the neutrons can't resist gravity and finally you get a black hole. At that point we don't even know how to do a calculation that properly includes quantum mechanics and gravity.

    Anyway, the key point is that what keeps a neutron star or a white dwarf from collapsing under gravity is an extreme example of what keeps atoms from collapsing under the em force.

    related to the second question: let's say you turn on your radio in the pitch black room. Will you get some radio station? Yes, and the reason is that there are photons around corresponding to radio wave wavelength. Likewise, a picture of you taken with an infrared sensitice camera would show your body quite clearly and that is because your body emits infrared radiation, hence infrared photons. The point is that there are tons of photons around, they just don't correspond to visible wavelengths (assuming a perfectly pitch black room)

    Last edited: Sep 2, 2004
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