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Plethra of questions.

  1. Oct 26, 2007 #1
    I am interested in knowing some things.

    When an electron moves down an energy level, it releases a photon. How does it do that? What is the process it goes through that a photon is actually released.

    Waves are electromagnetic, Why is it that they travel at 90 degrees angles to each other, like on a copper wire with electricity flowing through it. Why perpendicular? Also, why does it fluctuate on the same axis, if it was perpendicular to the wave, why doesn't the magnetic part of the wave stay horizontally parallel to the electric part of the wave, in other words, why does the magnetic part flux around the center. Why does it fluctuate at all. It's positive, 0, negative 0, positive, 0, negative 0. Is that because of the electron going back up energy levels? It can't be that. Is it the electron spin? What is the wave at 0, does it exist?

    EM waves have mass, its very little, but they do, it's pretty commonly well known. I've seen 2 red light waves crossed paths at an angle that was mathematically determined to produce this result. At where they crossed, they performed destructive interference, No light! What happens at that point? Is the mass canceled out too? How can the light waves keep going after they canceled each other out? Why won't they die?

    What is electricity made of? Do we actually know? We know it's characteristics, I don't want to hear that. Caused my movement of electrons, right? OK, but what IS it? What causes the electron movement to produce the observed effects. What causes the electron to keep it's movement, if you stuck a magnet on it, why wouldn't the electron just stop moving, and be attracted to the magnet? How do electrons transfer their energy from one to another?

    How does the electricity know when there is a complete circuit?

    Also How does sonolumenesence work?
    Last edited: Oct 26, 2007
  2. jcsd
  3. Oct 26, 2007 #2


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    I'll try my hand at a few of these...

    No, they don't! Where did you get that? Every experiment done confirms that photons traveling in free space are massless. The interference follows from the wave-nature of the light. If the light is out of phase by just the right amount, it will cancel. Exactly the same effect occurs with water waves. Next time you're in the tub, try it out. It's a cute little experiment.

    sorry, I don't understand what you mean by "characteristics" - what we perceive as "electricity" is nothing more or less than the movement of electrons through a wire. That's not the CAUSE, that is what it IS. It's movement is "kept" thanks to the existence of a electrical force. And they transfer energy and momentum to each other through collisions and exchanges of the "force carrying particles" called photons.

    As to questions like "where do photons and electrons come from?" You have to ask a religious expert. Why is QED a U(1) gauge theory and not an SO(32) gauge theory or any other of the infinite number of gauge theories? Again, I don't think there's any way to answer that question. There are arguments from string theory, but even those just postpone the question (what made the vacuum what it is, etc).
  4. Oct 27, 2007 #3
    maybe I can help with the others.

    - the mechanism by which a photon is emitted by an electron going down in energy levels is quantum mechanical. You can think about it, however, on a more heuristic level, and reason that it must. Clearly the electron looses energy when it transitions, and this energy has to go somewhere; this implies something is emitted. Since the electron isn't loosing mass, the emitted particle must be massless. Of the massless particles, photon makes most sense: since this is an electrically charged particle moving some sense (at least the mean position of the wavefunction moves) it makes sense that an electromagnetic wave would be produced.

    - The magnetic field moves perpendicular to (and 90 degrees out of phase with) the electric field because the electric and magnetic fields generate each other as the wave propagates. Take some point on the wave and look at the E field; clearly it is either increasing or decreasing, but its curl is non-zero. From maxwell's laws we know that a changing E field generates a changing B field in a perpendicular direction. Now we have a changing B-field, which in turn generates a changing E field in a perpendicular direction. These must be perpendicular to the direction of travel, because if they weren't, then the photon would randomly change direction, but the photon has momentum, which means that it requires a force acting on it to change direction and this cannot happen in vacuum.

    The more rigorous solution is that a traveling plane wave with E,B and v orthogonal is the solution to the Maxwell equations in a vacuum.

    - As Blechman said, electricity is made of electrons. Electrons flow because, when metals bond they give up the few electrons in their valence shell into this sea of free electrons (in QM we would say that these are in delocalized states). This sea of electrons isn't really bound to anything, and can simply move in the presence of an electric field. When you apply a voltage across a wire, suddenly the electrons are sitting in an electric field, and thus want to move. Current is rigorously defined as number of electrons passing through a unit area per unit time. What causes them to move is the applied E field. Also, electrons are not attracted to magnets.

    Electrons don't "know" when they complete a circuit, per se. you can only apply a voltage across a circuit if the circuit is complete, and since the electrons are delocalized, they don't care where they are in the circuit, as long as they are moving with the field lines. For instance, when you turn the key in your ignition, it completes a circuit. It actually takes days for an electron from the key to do a complete cycle through the circuit, but the electric field which can come on once the circuit is complete appears almost instantaneously (it propagates at the speed of light) so the length of time it takes an electron to move is irrelevant.
  5. Oct 27, 2007 #4


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    Indeed what blechman says is true, photons appear to be massless to measuring precision. Nevertheless electromagnetic waves carry energy and as such they have a gravitational mass depending on their energy density. This is probably what caybo meant in his question.

    So there are generally two kinds of mass: quantum mechanical mass (representing a relation between frequency and wavelength) and classical mass/energy density (as a source for gravitation). The first is zero for EM waves, the second is not.
  6. Oct 27, 2007 #5


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    Physics isn't supposed to tell us what things are but rather how they work. There is often substantially different but equivalent descriptions of physical processes, so even from this POV the question of what things really are can't be answered unequivocally.
  7. Oct 27, 2007 #6
    Thanks for your answers

    The energy has to go somewhere, but I'm curious what is the actual process of how it's released?

    If the magnetic field is "over there" to the left and the electronic field is "up there" over it, then how come the magnetic field doesn't pull the electronic field over that way, or the thing the electronic field is attached to. Is there no attraction? Pick a point on the axis, from which it fluctuates is it that point on the axis that is charged, producing the electronic and magnetic fields? What quality does that point have that says it has to produce those fields? please to not say, it just does, because it has a charge, what about the charge makes it produce the field?

    Yeah, I got that. But what are electric and magnetic fields made of, and what is the process through which they are produced? Also I wonder, how are electric fields able to be propagated that quickly, at the speed of light, if it takes some times for an electron to move? It takes some time for one to bounce into another and start the chain reaction.

    Precisely the thing to which i was referring. Blechman- asking where electrons come from, and where photons come from, are 2 totally different things. I did not ask where electrons come from, I merely asked how photons were released. Photons are propagated all the time, I don't think a religious leader can satisfactorily answer my question.

    So when the wave is at 0, and it has no field, what does it have. It still has energy doesn't it? So you might be able to cancel out a point of a light wave with interference, but the waves are still moving. They still have energy right?

    I've got some more questions-

    When I increase my kinetic energy, in some direction, relative to point A, am I getting heavier, relative to point A? So when I jump, as I am moving, I am slightly heavier to the earth?
    Last edited: Oct 27, 2007
  8. Oct 29, 2007 #7


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    I have a couple of comments on the above:

    In regards to OOO and the many masses of physics: You are correct that mass and energy are related thanks to Einstein. Back in Einstein's day (and before), people talked about masses changing with energy, with regards to "rest mass", "gravitational mass", etc. However, that's not the way it's thought of now (or should I say, it's not the way I was taught to think of it). Rather, you should say that "gravity couples to *energy*" Mass is one form of energy, but it's not the only form ("mass" and "energy" are NOT synonyms). This is why a photon couples to gravity, even though it's massless. The word "mass" is reserved for "inertial mass", the invariant quantity that describes its motion in *any* reference frame. If caybo meant that the photon has gravitational energy, then I stand corrected. But when he talked about "little mass", it sounded like he meant inertial mass, since there are photons in the universe that are accelerated to near- (or even possibly trans-) Planck energies, and I wouldn't call those "little"!

    In your statement of "How vs Why": Personally, I think "How?" is the question of engineers, while "Why?" is reserved for us egghead-physicists! ;-)

    Caybo: In response to "what is MAKING the EM field?" The exact answer to that question is that what you perceive as the EM field is actually a coherent state of a large number of photons, the quanta of the field. They all add up constructively to give you a macroscopic field. This is the "physical object" that is "making" the field. This also answers your question about the fast-propogating EM field: it's the PHOTONS that move at the speed of light, carrying the information of the field, not the electron (or more general charged object).

    As to "how do photons get physically released?" Well, I don't think there is an agreed-upon answer. If you were a string theorist, you might say that strings break apart into a "photon string" and an "electron string", but even that just postpones the question. I don't think anyone can give you an accepted answer to that question. All I can say is that "it does!" I'm not trying to be condescending, but rather I'm trying to tell you that we ASSUME that it does according to the laws of QFT, and we find experimental agreement to 15 decimal places! (I'm refering to g-2 of the electron - one of the best measured and predicted quantities of physics!). This is why I made the reference to "religion" - I just mean that nobody has any real idea. But on the other hand (going to OOO's statement), why do we care?! What I mean by that is that we have an explanation that does very well. It's not complete in the strict sense, but it is quite powerful as it stands, and it gets us very far. Maybe that's not very satisfying to you, but I think that is the general attitude of high-energy physicists in regards to questions like that.

    As to your comment about waves and interference: the thing to remember about light (or EM waves) is that it has two different descriptions - a wave or a particle. The wave-nature of light is in a certain sense "nonlocal" - this is a consequence of wave-mechanics, not particularly QM or EM. Again, think of a water wave. Think of the "double-slit" experiment if you know what that is. These experiments show that there is some non-local thing going on in the sense that the wave at x "knows" what the wave at "y" is doing. This is nothing mystical, it's just a consequence of wave mechanics. Thinking of it like that, I don't think there's any contradiction. Make your life simpler: don't worry about EM, just think about water waves (ironically a much more difficult subject in general, but not in this respect). Think about how they form, propogate, and interfere. I think that will help hone your intuition on how light works.

    Finally, as to your last question, you are forgetting that gravity is "conservative." This means that an increase in your kinetic energy is precisely cancelled by a decrease in your potential energy. Since gravity couples to TOTAL energy, not just mass (see my comments above), no, you do not get "heavier". This is a consequence of the "equivalence principle" which is just to say that there is ONLY ONE MASS - the inertial mass.

    It is true, however, that as you go further and further away from the center of the earth (weigh yourself at v=0 going up), you do get LIGHTER. This is just a consequence of the fact that the gravitational field of the earth weakens as you go further and further away from the center. However, even though you weigh less, you still have the same MASS.

    Let me just end by pointing out that there are some great experimentalist who are testing things exactly like this. However, so far everything is looking just like Einstein says it should. Check out Eric Adelberger from the U.Washington Physics dept and his research, for example.
  9. Oct 30, 2007 #8


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    Yes, you're right. All forms of energy get summed up in the end to express energy-momentum conservation. I just wanted to point out a possible source of caybo's misunderstanding.

    That's the way physicists (I'm one of them too) usually think of themselves. But I think some engineers do a lot of smart stuff (and a lot of trivial things as well, just like physicists), and they couldn't do that if they didn't ask "why ?" at a certain level.

    So how do we physicists answer the question "why?" ? Sometimes we just notice some internal inconsistencies of our existing theories (Maxwell->displacement current, Einstein->Lorentz invariance...). On other occasions we postulate new theories with minimal consistency impact on existing ones.

    In both cases, I'd say answering the question "why?" amounts to answering the question "how?" more carefully and detailed.

    By the way, I guess it was Einstein, who once said: if you want to understand someone you have to look at what he does instead of listening to what he says.
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