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Qm 101

  1. Nov 16, 2004 #1
    Hello, I ran across a couple of articles about Quantum Mechanics and I can't believe that I only now discovered this awesome, down to the bottom, fundamental branch of science. I envy all of you who are masters at it. I'm trying to understand the theory, at least conceptually, but in my thought process, when reading a book, I run into questions that need to be answered before I can move on. If left unanswered, the picture gets blurry, I make wrong assumptions and lose it quickly. I have tons of questions, but I'd like to ask a few general ones that will implicitly answer some of my specific questions. I would greatly appreciate your insight into the matter, and if you don't have time or desire, at least reference links with answers would be of great value as well. In advance, thank you!

    1. This one might be very naive, but please explain, specifically, how the propagation of light is slowed down by the air. This is what I don't get: an electron of a specific atom will be "bumped up", or accept a photon only of some specific frequency (fact?). The photons of other frequencies will pass through the atom like there's nothing, empty space (fact?). Let's say we have a tube filled with the hydrogen gas and shine a beam of white light in it. Given the premises above, I would conclude that only certain frequencies of the white light will be absorbed by a hydrogen atom, at which point the atom becomes "excited", as the electron is moved to a higher orbital/energy state; and then released into a random direction when the electron comes back down to its original state. That's how the spectral lines work, isn't that? However, that would mean that most of the light would slice through the gas unaffected and therefore should not lose its speed. But that's not what I hear when they say "speed of light in the vacuum....." So, if it does lose its speed, exactly how? The other thing, let's say we shine only the frequency that does get absorbed by the hydrogen atoms. As I said earlier, the photons will be absorbed and then released into random directions. Does that mean I would completely lose the beam aspect of light, as it'll be evenly scatted all over? Then how fast will this absorb/release action take place in a chain of atoms in one direction? That is, how fast will the light reach the end of the tube in this manner?

    2. I'm having difficulties understanding this whole "don't know until you look" concept when dealing with Schrödinger’s view on atoms. It is said to have demolished the classical physics perspective that would view the cat in the gas chamber being dead or alive regardless of whether we verify it or not. The new perspective sites that both states, mutually exclusive states, are true at the same time, but not determined until the act of the verification. Obviously, I misunderstand something, as I strongly believe that the proposition "The cat is either dead or not dead" is true a priori, regardless of any sense datum, radioactive decay, or what have you. That is, I don't have to observe anything to be absolutely certain that the cat is in either one of the two states, not both. Yes, I acknowledge my archaic use of the principles of analycity, but they still remain applicable in the case. In other words, the view seems to be playing a dangerous game with logic, in my opinion, and you can't do that - if you throw the fundamental law of excluded middle out of the window, all bets are off, you might as well throw your argument too, since in the argument, you employ the very law itself. Anyway, it would belong in the philosophy forum to expand on this topic, but I want to understand what exactly is meant by this counterintuitive concept. Is it that:

    a. the act of looking determines the position of the electron in the "orbit"? In other words, you could say that to observe an electron you have to shine a photon of certain frequency on it. That action makes the electron absorb the photon, which puts it into a certain position, or something to that effect.... That is, our act of looking - thus interference, would be part of the equation of determining the position of the electron...... If that's the case, it's very understandable, but would be true for a lot of my daily activities as well, not just quantum mechanics. So, I doubt that's what's going on. I think that when you use the word "look", or "verify", you're talking about verifying in principle! Sort of like the scenario where I could squeeze myself to a subatomic size without influencing the surrounding particles and make all the appropriate observations....

    b. a more radical and somewhat "anti-scientific" version of a) - is it my consciousness that interferes and determines the position of the electron, making the act of looking so important?

    c. or is it a timing issue? another kind of relativity principle where the position of the electron is undetermined and meaningless when considered out of context. That context is the state of other particles at that specific moment of observation...

    d.. or is it more of an epistemological / pragmatic issue, belonging in the same category as "does the tree make sound when there's nobody there?".... Is the electron in any position if there is nobody to observe it? This one would open up all kinds of further questions / issues depending on what philosophy dominates your view of reality. But I'll wait because I'm having a feeling the real explanation of the uncertainty behind the electron has nothing to do with anything above. So, please, clue me in.

    Lastly, if you made it this far, what specifically is the electromagnetic field, or force. Physics, materialistic in nature, reduces everything to something material and tangible. I can understand the strong force being reduced to a particle (gluon) exchange between quarks. I can understand the weak force being expressed as particle decay. I can comprehend the gravitational force as a curvature in space, or the mysterious graviton particle exchange, whatever. But I still can't get the electromagnetic force. It remains to me simply some voodoo vibes in empty space. The photon is said to be the force particle for the electromagnetic force. Well, to me that means, just like with the gluons, the electron would be constantly exchanging photons with the nucleus, but that's no the case, is it? So, what is it, specifically, that holds the electron in its "orbit" around the nucleus. Or simply sticking with the electric field, when I have two electrically charged rods in vacuum, what exactly is that between the rods that starts ripping the electrons of one of them and transferring them into the other? It's not magic, is it?

    Again, thank you kindly for your help!

  2. jcsd
  3. Nov 17, 2004 #2
    My RAM in my brain does not support such a huge question, but I will try to answer. The velocity of light in a medium is becuse of a collective phenomenon : the medium is providing it with a " field " that is due to all the particles of the medium. So when you want to think in terms of photons and atoms you will have to consider all the particles of the system. The individual interaction doesn't always means absorbtion of photon, it may be also a simple scattering and many other phenomenon.

    The answer to the second part is (vaguely), If you assume that an electron has superposition of two states with equal probablity (much like the cat), then you happen to "observe" the electron you will find it in one of the two state. Suppose you have 1000000 exactly same system of the electron then half of the time you will find it to be in one of the states.

    >>>>> So if you are in a dark room with the cat in front of you in a enclosure..... can you "observe" and tell if it is dead or alive. No for it you will need photons to "observe" the cat. Say you try this experiment with 100000000 exactly same system of the cat the final result will be 50-50.

    >>> The point is that the probes that are used for observation causes a interaction that brings in the final result.
  4. Nov 17, 2004 #3
    You were quite close to the answer when you talk about photons "observing". There are a vast number of photons and other particles in nature interacting all the time.

    I've studied some of the more recent books about this and the solution to Schrodinger's cat has become apparent over the last thirty years or so. It's something called "decoherence".

    The important thing to know is that the interaction between the cat and the vast number of particles in the box, the walls of the box and the rest of the universe destroy the cat's alive/dead superposition state.

    Even in deep space, a huge number of photons from the Big Bang are flying around and "observing" and "measuring" even a tiny grain of dust by interacting with it all the time. The photons scatter off the dust grain and will destroy any superposition state in a billionth of a second.

    The same is true for Schrodinger's cat. The alive and dead states decohere incredibly fast because all the particles interacting with the cat destroy the superposition state and you just end up with an alive or dead cat. Photons, electrons, etc. are interacting with the cat all the time. The cat simply cannot stay in a superposition state.

    Although the cat's superposition state is made up of both alive and dead states, it's not easy to say what a superposition state like this actually means. There are other kinds of superpositon states in quantum theory that are made up of two states that have a quite different meaning to the superposition state they make up, so it's not necessarily all that obvious what a dead-alive superposition state means in terms of a cat or anything like a cat.

    Anyway, that's the modern solution to Schrodinger's cat is that particle interactions destroy the superposition of the dead and alive states. :smile:
    Last edited: Nov 17, 2004
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