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General questions about fields and light

  1. Dec 21, 2005 #1
    First, I should warn you as to my "level" when it comes to physics. I'm quite decent at electricity and magnetism and I have taken up to differential equations (though I really only remember up to Calculus 3 -- Multivariable calculus). I've only read non-mathematical stuff about light's particle/wave duality and the two-slit experiment.

    First question:
    It seems to me that "matter" as we understand it, at least the way it is explained, seems kinda lacking. I was wondering why I can't walk through walls, and I think it's because the fields of the electrons in me and the wall's fields are somehow pushing against each other. Now, I'm not sure if it's the electric fields that are not allowing me to walk through walls, but it's certainly not this notion of a single "object" pushing the other. What exactly is pushing back? And if it's a field, then how come it's pushing so hard only over a short distance (as opposed to working the way two electrons push against each other via 1/r^2).

    Second question:
    I absolutely hate how light is described in most text books. Does anybody actually understand what it is? I know light can be slowed down through various methods, what would it be if it was stopped completely? It's shown in textbooks as an electric field and a magnetic field that increase and decrease in magnitude perpendicularly. Sorry, but I can't really imagine what that means. Can somebody shed some "light" on this?

    Third question:
    What is a "spin"? How do elementary particles have a "spin"? Why is it 1/2 or 3/2 and why doesn't that have a unit associated with it? I know a ball can spin angularly at 30 radians / second, but that doesn't seem to be what they're talking about.

    Fourth question:
    What happens when light is shined at an object? I know that some objects will reflect light, while others will "absorb" it, and still other times it'll just pass right through. How exactly is it "reflected" and where does it go when it's "absorbed" and why will it sometimes just pass through?

    Fifth question:
    If you move an electron to the right really quickly, will its field instantly move with it, or will there be a "lag" at some speed as the changes propagate outwards...

    Thanks for any help! And sorry if this was asked before... If it was I'd appreciate a link or something...
    Last edited: Dec 21, 2005
  2. jcsd
  3. Dec 22, 2005 #2
    Welcome itistoday,
    You have many questions ! Congrats, that means you are eager to learn.
    I'll try to give you some elements of answer.
    First question:
    If you can not walk through walls, it is indeed because of electronic repulsion. It is really a complicated problem if you think about it deeply. At the interface between the two surfaces, the electronic clouds tend to repell each other. However, this is indeed not a [tex]\sim\frac{1}{r^2}[/tex] interaction. The description I give here is over simplified, and I could not go much further into it (maybe some more competent member will do it). Since the overall electric charge is zero, the potential energy decreases much faster. Maybe you are familiar with the potential : it is called Lennard-Jones and has the form [tex]
    \sim\frac{\alpha}{r^{12}}-\frac{\beta}{r^6}[/tex]. It is very repulsive at short distances.
    Second question:
    Photons always travel at c (constant). Now of course, in a medium photons are constantly absorbed and re-emited, and this is what causes light to travel slower in air or water for instance. Now I am much sorry if you do not like Maxwell's theory of light. At the point when he wrote it (without the use of curl and div operators) it was really ugly, but even then, it was true. Nowadays with hindsight, it is also very beautifully formulated. Light is a wave in the electromagnetic field.
    Third question:
    Quite tough question !
    First, spins come in integer values of [tex]\frac{\hbar}{2}[/tex] so they do have unit of energy times time (Js). Second, the reason for values to be integer is technical : it is due to a "group" being "compact" and "non-simply connected". Many symmetry groups indeed have those property, and this why we dubbed the microscopic theory "quantum" in the first place : many things come in integer values at small scales ! Third, the spin is a quantum analog to the angular momenum. Alas, one cannot push the analogy too far without running into difficulties.
    Fourth question:
    Waow ! You need to read the entire Born&Wolf :wink:
    This is a very long question. One can do the following calculation : suppose an incoming plane wave in an homogeneous medium is incident on an interface between this medium and another homogeneous one. The difference between the two media is only the speed of light. Then you will find that some part of the wave is reflected and some part is transmitted. Light can be absorbed by exciting the atoms of the second medium, those atoms in turn releasing the energy in (for instance) thermal waves. Those excited atoms can also release the energy in the form of light, some part being reflected and some part being transmitted in the medium.
    Fifth question:
    If I understand correctly your question, you want to know what happens if you accelerate a charge really hard during a short period of time. In that case, light will be emitted. This can be interpreted as "a lag in the changes of the proper field of the charge".
    Hope that was some help :smile:
    Last edited: Dec 22, 2005
  4. Dec 22, 2005 #3
    Thanks a bunch humanino for taking the time to answer my questions!
    Unfortunately I am not familiar with Lennard-Jones potential, but I will look that up when I get the chance.
    Well, what I meant was that I cannot picture a wave in an electric magnetic field. I can imagine a wave in a pond or an ocean, or a sound wave, but I cannot imagine, or even understand, why and how two perpendicular fields (E and B) act as waves and how that makes light. I also thought that there's no such thing as a magnetic monopole, yet in drawings of light you see a B-field that does not wrap around like it does in a magnet. What's the deal with that also?
    When you say the "spin is a quantum analog to the angular momentum", how is it? I something actually spinning? Really, I think I'm just asking how the "spin" is detected and what it tells us about the particle. Do scientists use really powerful microscopes and see an electron spinning like a top??? (I doubt it...)
    Yeah, I too think the answer to this question will require a lot of reading...
    What I mean is that an electron has an electric field associated with it. Now if you move this electron to the right really fast, will its field lines move with it instantaneously or not? Better illustrated, if there was another electron suddenly placed a distance away from it, would it detect the changes in force on it the instant the other electron is moved or will it take a while for it to "realize" that the electron was moved?

    Thanks again for your help.
  5. Dec 23, 2005 #4


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    Nothing can go faster than the speed of light.
  6. Dec 23, 2005 #5
    I searched google for some articles on this (except searching for the speed of gravity, since that seems more popular), and with the exception of a crazy guy named Tom Van Flandern, most articles that I read said that gravity has a finite speed, and is probably equal to that of light.
  7. Dec 25, 2005 #6


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    Gold Member

    Thats right, according to GR, gravity moves at the same speed as light.
  8. Dec 26, 2005 #7
    Yes. As daniel_i_l pointed out, nothing can move faster than c, including the electric field. We normally treat these in electrodynamics with advanced and retarded potentials.
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