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Very basic quantum mechanics questions

  1. Jan 18, 2012 #1
    I hope this is the right place to post my questions. I have very basic questions that I just don't know the answer to and they are based off of my trying to understand quantum mechanics. Also, my questions will likely build and have follow up questions. It might come off as more or less a discussion even...

    For instance, when it is said that photons act as both waves and particles, what exactly does that mean? Because I know that a photon is in fact a particle (correct?). Is this to say that the particle more or less 'wiggles' as it travels? I was under the impression that if something acts in the sense of a wave, that it was more like a sound wave; where no particles were really moving much, but instead acting like jolting one end of a stretched slinky and down the line the other end will feel the jolt.

    Everything I read does not really give a simple explanation to the very basics I've always wondered.
  2. jcsd
  3. Jan 18, 2012 #2
    You can't really say a photon is a particle, nor can you say it is a wave. It is something that has the properties of both, a quantum particle , sometimes it acts as a particle and other times it will act as a wave. They don't really "wiggle". For instance young's double slit experiment; firing one photon at a time still left an interference pattern. And, there is the photoelectric effect where photons act as particles.

    They are described by a mathematical construct called a wave function, which can be used to find the probability of where a photon is at a certain time. Particle like behaviour is most evident from measurement of the photon. If you measure its position its wave function will "collapse", you'll know its position accurately, but you won't know its momentum, as shown by the uncertainty principle. If you don't measure the position, the photon is "smeared out", it can be anywhere, more like a wave.

    There are different interpretations of quantum mechanics that try to explain this, like the Copenhagen interpretation, or Many Worlds to name a few, however there is no conclusive evidence for one over the other.
    Last edited: Jan 18, 2012
  4. Jan 19, 2012 #3
    hey diggs!

    This is definitely a tricky one, and ultimately there doesn't seem to be an easy or definitive answer. At least not in the way that we want. The key point is that photons and electrons and indeed all quanta can be conclusively proven to act as waves AND as particles... well, almost. This leads to a logical contradiction, but seems to be true.

    Consider the double slit experiment mentioned in the previous post. If you shine light at a barrier which has two thin slits cut in it, the light will pass through the slits and display as an interference pattern on whatever surface it ends up hitting (look up the double slit experiment). This pattern is characteristic of waves. You would actually get the exact same sort of pattern if you used sound or water or anything else that travels in waves. This has been considered definitive proof that light is a wave since the beginning of the 19th century, but since then we've tacked on a couple observations that contradict it. The best modern example of light acting as a particle is when observing low levels of light using a photo multiplier (basically an extremely sensitive light amplifier). What we see using photo multipliers is that light doesn't come in pulses of varying intensity. If light were a wave, then even when there is very little light, you would get a constant reading at very low intensity. And as you raise the amount of light, the constant reading would raise in intensity. However, we find that we actually observe individual 'blips' all of the exact same intensity, and that the amount of light was actually determined by the total number of these full intensity blips. The blips are what we call photons, or the smallest indivisible packet of light.

    Ok so light is definitely a particle? Well, observing the photon as a particle doesn't disprove the observation in the double slit experiment. Light IS observed as a wave, and also as a particle. And to be sure it is often described in both forms. It seems that the truth is that it isn't a wave, exactly. And it isn't a particle, exactly. Those are just good ways of rationalizing the strange behavior we observe.
  5. Jan 19, 2012 #4
    Welcome to the forum!

    You are correct! The photon is, by definition, a particle. When we detect a photon it is always localized as a particle. For example, on a detection screen, we see a dot where the photon hit. But, all quantum particles, including photons, have an associated wavefunction that allows us to calculate probabilities. If we repeat an experiment with photons many times, we get a statistical distribution of all the dots on the screen that looks like an interference pattern. It is that statistical distribution that we identify with wave properties.

    As far as we know, the photon does not ‘wiggle’ and there is no vibrating continuum for the associated waves, like in sound waves or with slinkies. I hope this helps.

    Best wishes
  6. Jan 19, 2012 #5
    I think this is a little too crass. For instance this kind of obscures why we can say things like the frequency of light, but also light is made out of photons.
  7. Jan 19, 2012 #6

    Ken G

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    My take on this is that wave/particle duality is not a logical contradiction if it is interpreted as a unification. Waves and particles were always the same thing! That's because a wave of small enough wavelength can (and does) do everything a trajectory does. So even sound waves are also a particle phenomenon, but the particles aren't the air molecules, they are the quantized excitations of the collective modes of the air molecules. Those quantized excitations are called "phonons". So for centuries when we made music, we thought we were "sending out waves" of sound, but in fact we were sending out particles of sound-- but like all particles, they obey wave mechanics. There never was any difference between particles and waves, so there's no contradiction, but we didn't realize that. Still, the unification of particles and waves requires that there be a principle of complementarity-- we can't simultaneously know the momentum and position of a particle.
  8. Jan 19, 2012 #7
    How about the other way around is a particle then also allways a wave? I mean we can measure both the position and momentum of a train!
  9. Jan 19, 2012 #8
    The difficulty here is that light itself is not either a particle or a wave. It depends on the experiment that we do. Some experiments with light exhibit wave properties. In the beginning this was always the case. We were all told that Young’s experiment proved conclusively that light was a wave, because “Waves exhibit interference. Particles do not.” But later, Planck and Einstein showed that other experiments with light exhibit particle-like properties.

    The point is this - light “behaves” like waves or particles depending on the experiment performed.

    I did not intend to oversimplify the difficulties we have with the wave-particle duality issue. If it were easy to understand, we wouldn’t be here discussing it.

    Best wishes
  10. Jan 19, 2012 #9
    I don't know whether the following comment will clarify or only make things harder to understand, but here goes.

    When talking about quantum mechanical things it allways pays to keep in mind that photons/electrons/wavefunctions do not exist in the way a for instance a couch does. In the fact that you cannot really ever 'find' a wavefunction in the world. You can describe certain systems using wavefunctions. However every measurement will just give you a value. A whole lot of experiments on identically prepared systems gives you a distribution. The wavefunction can predict this distribution of vlues for you.

    When we say a photon behaves like a wave or behaves like a particle we mean that the mathematical framework of systems we already had from for instance sound or bullets can be used to describe this photon system as well. However as it turns out we cannot allways use the same mathematical framework. Or at least the same one doesn't allways work. Sometimes we can describe the systems using what we know about waves, other times we do it using what we already know about particles.

    Of course there are also some entirely new concepts that needed to be used.
  11. Jan 19, 2012 #10
    Ken, I'm gonna have to disagree. Wave-particle duality is not a unification. They are not always the same thing. Indeed they are never the same thing simultaneously. As it was discussed earlier we know that on the fundamental level photons are not waves, they are particles. But they behave as waves in terms of their probability distribution and how they interact with the probability wave functions of other particles. This is fundamentally different than phonons and other macro wave phenomena because in those cases there is nothing physical that is moving from point a to point b. But in the case of electrons there IS a physical something moving from a to b, and it is a discrete packet of energy and mass.

    A clean way to conceptualize the difference is just to think about containment. If you drop a pebble in water, the ripples radiate outward uniformly and over time reduce in amplitude in proportion to the radius swept out until the wave gradually dissipates into nothingness. The waves aren't contained into perfect straight beams with definite direction. But particles are. Assume a electron-positron collision resulting in the creation of gamma radiation. So we get an evenly distributed "wave" of light emanating in all directions? No, we get two distinct individual photons that have their own set energy and direction. They travel in straight lines in their own set direction, which would be imposible for a "wave" which by definition must radiate in all directions. The way in which it acts as a wave is that the way in which the photons, for lack of a better term, choose which direction to go is determined by a wave function, which we wouldn't expect to see if the photons were like little tennis balls or something like that.

    I stand by the concept that these things are neither particles NOR waves exactly. But using our limited instruments and experience of the physical world those are good analogues to describe certain aspect of their behavior.
  12. Jan 19, 2012 #11

    Ken G

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    Not to arbitrary accuracy, no. But when the action is very large, the simultaneous precision we can achieve is certainly adequate.
  13. Jan 19, 2012 #12

    Ken G

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    Sure they are. You just have to change your understanding of both, because it was a wrong understanding of each that got us to think they were ever anything different. To me, that is just exactly what quantum mechanics is trying to tell us about dynamics.

    And so are all waves.
    And so do all particles!

    Oh no? Why not?
    That isn't true for photons. So you're now saying that photons are not particles?
    That is certainly one way to frame it. I think it misses the point that particles and waves are exactly the same thing.
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