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  1. #1
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    Quantum Mechanics

    Quantum Mechanics, while taking a little while to learn, is by far the most extraordinary theory in science. It completely flies in the face of everything you thought you knew about reality, calling into question some of our most basic assumptions about the universe, assumptions like cause and effect, determinacy, locality, and the existence of an objective reality regardless of whether or not anyone is observing it.

    So, since the science forum has been so quiet I figured I would explain it to anyone that is willing to listen. This is prety much what I do all day at uni (and some other topics as well), and probably what I will be getting involved in for the rest of my life. Feel free to ask me any questions along the way, I really love telling people about quantum mechanics, it's especially nice on those nights when you're sitting out under the stars, having a few beers and pondering the meaning of life, but I guess an internet forum is the next best thing.

    I should warn you there is a bit of learning involved, but no maths or anything like that. I'll try to keep it as interesting as possible, so don't be scared by any long posts, I'm going to try to condense down a year or so worth of physics into something fit for an internet forum, and while I'm only bringing up the really interesting ideas, for you to really understand just how amazing they are you need a little bit of an understanding of the concepts that underpin them. So like I said, feel free to ask me to clarify anything that you don't understand.

  2. #2
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    Re: Quantum Mechanics

    It all begins in pre-20th century physics, with debates over what exactly light is. In our world things can be split into two classifications – particles (like a tennis ball, a car, a plane, a mote of dust, or a crocodile) and waves (like the waves on an ocean or sound).

    Particles are pretty straightforward, but waves perhaps deserve a second to think about. A wave is simply a transfer of energy without a transfer of particles. When a sound wave travels through the air molecules vibrate back and forth in order to propogate the wave, but the particle itself ends up back in the same place it started. The image below illustrates this:

    Note that if you watch an individual particle it never goes anywhere, even though there is clearly a wave travelling from left to right. Two waves can travel through the same space at the same time, they form a superposition of waves, which is the same idea just a more complex pattern. This is why two people can talk to one another at the same time and the sound waves pass right through one another.

    Waves also demonstrate a property called interference, and this is crucial to an understanding of some of the experiments that I'll mention later. If you have two waves coming from opposite directions and they hit one another then one of two things can happen – they can cancel each other out or they can reinforce each other (called destructive and constructive interference respectively). Consider the image above – the wave can be thought of as dense regions and sparse regions travelling from left to right. If another wave is coming from right to left (you'll need to use your imagination) and impacting this one, then at some points a dense region from the left will hit a dense region from the right and you will get a super dense region, at other points a dense region will hit a sparse region and they will cancel out.



    See in this diagram one ball is in a position where the two waves always cancel out, the other is in a position where the two waves always reinforce one another.


    Ok, moving on, pre-20th century people weren't sure whether light was a wave or a particle. Most people thought that it was a wave because it demonstrated many of the properties of a wave like interference, and some other things like diffraction (the bending of waves around a barrier, which is why you can hear someone even if there is something directly between you). However, there were a few niggling things that couldn't be explained: blackbody radiation and the photoelectric effect. I won't go into detail on these unless you want me to, but Planck and Einstein found that these phenomena led to certain conclusions:

    1-The blackbody effect required that things that emit light do so in discrete steps, that is to say that you can't just emit light with whatever frequency you like, but it has to be at certain intervals. It would be like me saying you are only allowed to produce sound at 20Hz, 40Hz, 60Hz etc.. but not at 23Hz or 41.713Hz.

    2-Light is a particle. This comes from the photoelectric effect and was such an incredible discovery that it earned Einstein the Nobel prize. It is important but the explanation isn't really necessary if you trust me when I say that this proved that light has to come in discrete packets, just like a particle.

  3. #3
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    Re: Quantum Mechanics

    So now we have two competing, seemingly incoompatible ideas – light is a wave, and light is a particle. A great deal of work was done on this, and I'll just summarise some of the key things that were discovered in the early part of last century.

    1-Light definitely comes in discrete packets, this was proved by Einstein, and we call these discrete packets “photons”.

    2-Photons have momentum, which is another way of saying inertia. So when a light beam hits you it actually pushes on you, albeit very weakly.

    3-The amount of energy each individual photon has is given by the formula “Energy=H*frequency”, where the letter H is just a number that gets the units right.

    Despite this, the particle theory of light still could not explain why light demonstrates things like interference and diffraction, which are exclusively properties of waves.

    Now the astute reader may have realised something a little strange in what I posted above. Namely that the energy of a photon is given by “Energy=H*Frequency”. What's strange about that? Well, I've just said that the energy of the particle depends on the frequency of the wave.... This conundrum won't be resolved for a little while, but now its time to delve into the very heart of Quantum Mechanics with the most famous experiment in all of physics, Young's Double Slit Experiment.

    It's a very simple experiment, you take a light, shine it on a wall that has two very small slits in it and then project the image of those slits onto a screen. The results you get should tell you whether or not light is a particle or a wave, because a wave will diffract as it goes through the slits and in some areas you will get constructive interference and in others you will get destructive interference. The end result, if it is a wave, should be a band of light and dark patches, like is illustrated below (The bright patches are where you have constructive interference, the dark patches are where there is destructive interference).


    If light is a partcle then the result should be similar to shooting a machine gun at the two slits, and the pattern you get will be very different:


    So Young did this experiment, and these were the results:

    (Different pictures are due to different slit spacings)

    A pretty clear indication that light is a wave no? Well then what about all that stuff Einstein did that proved light is a particle... well... Some people said that light is a particle, but when there are lots of them all travelling at once they can behave like a wave, so Young decided to repeat the experiment, but he would use a light source so weak that he could be certain only 1 photon would be in the experiment at a time (This took months for him, but it can be done better nowadays, I've even done this myself). The results are as follows: (Image a is after only a few photons have been fired, b after more etc...)



    Now it is clear from photo A that only individual photons are being fired through the apparatus, but given enough time they eventually build up into the same pattern that Young expected to see for a wave. Somehow, even if there is only one photon travelling through the experiment it knows that there are two slits, and it knows what the interference pattern should look like. This diagram illustrates what is happening:



    So, I will finish up for now by asking a question... Of the individual photons you see in photo A, pick one and tell me, did that photon pass through the slit on the right or the slit on the left?

  4. #4
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    Re: Quantum Mechanics

    it passed through both?

  5. #5
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    Re: Quantum Mechanics

    Quote Originally Posted by Qi123
    it passed through both?
    Very good! Here's a smiley :).

    (In another sense though, it didn't pass through either and it was a nonsensical question for me to ask, but that is all interpretation)

    It turned out that our way of viewing the world was wrong, the world doesn't consist of particles and waves, it consists of things that experience "wave-particle duality" (we don't have a word for it, though some have been suggested. Things don't behave like waves, and they don't behave like particles, they just behave like they behave). Now the magnitude of these effects decreases as things get larger, which is why you don't notice it in everyday life. If you calculate the wavelength of a human being it is much smaller than the width of a single atom.

    So, physicists had found that something that was once considered to be a wave had both particle and wave characteristics, the next step was to ask if something that was a particle experienced the same effects.

    The experiment was repeated with electrons, firing single electrons at a time at the double slit, and the exact same pattern emerged. Then it was tried with neutrons, protons, even entire atoms. The most recent thing to have been tested is an atom called a "Bucky Ball" which consists of 60 carbon atoms. It is, relatively speaking, a very large atom. Now what will be really interesting, and what will probably give the philosophers a headache, will be when we are able to send something living through this experiment, like a virus.

    This whole situation started to get people a bit worried. What exactly was going on? They could predict what would happen based on a few simple ideas relating to waves, but other than that there wasn't really any idea of what was going on with these atoms. Does an electron really exist? How can it possibly pass through two slits at the same time? What happens if you watch the slits to see which one it passes through?

    This last question was one they could answer. Why not just set up a device that measures whether or not an electron passes through a slit, it should be very easy to do, and so they did it. Here's where things get (more) interesting, this is the result they got when they didn't check which slit it went through:

    And this is the result they got when they did check which slit it went through:

    Apparantely the electron not only knows whether or not someone is watching it, but if someone is watching it then it changes it's behaviour! This really was (and still is) extraordinary! It doesn't matter how careful you are to make sure that your measurement doesn't interfere with the electron as it goes through, even in principle (and we'll get to why this is true) you can not ever know which slit the electron went through and still obtain the diffraction pattern.

    This is a point that bears repeating, because it is a break from a very long held assumption. There is absolutely no way that you can observe the electron during it's voyage without making it revert back to it's particle nature. The act of observing changes reality.

    (What precisely can be considered an observation is still a debate that is taking place, is subject to various interpretations and is also the focus of many experiments.)

  6. #6
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    Re: Quantum Mechanics

    Apparantely the electron not only knows whether or not someone is watching it, but if someone is watching it then it changes it's behaviour!
    Seems as if we dont have the language to really describe this.
    As how can an electron KNOW?
    It could react to observation perhaps?

  7. #7
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    Re: Quantum Mechanics

    We all get a little self-conscious when someone's watching us.

    (sigh) Centuries of the scientific method leading us away from attributing human characteristics to non-human life and matter, and here comes QM, practically demanding that we anthropomorphise every smeggin' wavicle.

  8. #8
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    Re: Quantum Mechanics

    Quote Originally Posted by pancho
    Seems as if we dont have the language to really describe this.
    As how can an electron KNOW?
    It could react to observation perhaps?
    Yep, it's bad wording on my part, an electron can't 'know' anything.

    What is actually happening (and I'll get into all of this in a bit) is that when an electron isn't being watched it doesn't have a definite position. We are used to thinking that an object like a billiard ball has a position, but on a small scale there is a 'fuzziness' about the world. Things aren't in one position at a particular time, but they are sort of spread out all over the place. This is part of the reason the electron can go through two different slits at once.

  9. #9
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    Re: Quantum Mechanics

    Quote Originally Posted by dr poormouth
    We all get a little self-conscious when someone's watching us.

    (sigh) Centuries of the scientific method leading us away from attributing human characteristics to non-human life and matter, and here comes QM, practically demanding that we anthropomorphise every smeggin' wavicle.
    QM can be interpreted in an anthropocentric way, but there are other interpretations too. In fact, the most recent research is pointing towards it not being anthropocentric at all, but that an observation is merely an interaction between a microscopic system and a macroscopic system (but no one is quite sure where you draw the line between the two).

    Anyway, for the purpose of trying to understand the theory, it is probably best if you just think of an observation as the sort of thing we do in the laboratory. We use a ruler to measure position, or we use some other tool to measure velocity etc... Until you get into some really deep stuff it doesn't make much difference what is considered an observation and what isn't.

  10. #10
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    Re: Quantum Mechanics

    Busy for awhile today but I'll get to your "assignment" as soon as I can. Never liked school so much. Good on ya Kaz.
    The terminally stupid and certifiably insane.

  11. #11
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    Re: Quantum Mechanics

    Quote Originally Posted by pancho
    Seems as if we dont have the language to really describe this.
    As how can an electron KNOW?
    It could react to observation perhaps?
    who cares about electrons!?what i want to know is!?how can YOU know ANYTHING!?(me too!?)but/and of course 1 cannot ignore the IDEA of RELATIVITY!?hehe!!......just askn..
    i do not endorse/recommend any advertising on scam.com associated with my name /posts or otherwise. thank you

  12. #12
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    Re: Quantum Mechanics

    Quote Originally Posted by bogie
    Busy for awhile today but I'll get to your "assignment" as soon as I can. Never liked school so much. Good on ya Kaz.
    define "good"!?and do you think his status as super senior member implies super senior or super member and if so what's super about senior and what's super about member!?and no....dont go there!?hehe!!.....just askn...
    i do not endorse/recommend any advertising on scam.com associated with my name /posts or otherwise. thank you

  13. #13
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    Re: Quantum Mechanics

    Great thread! I think I'll sticky it even, if the lessons continue :)

  14. #14
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    Re: Quantum Mechanics

    Quote Originally Posted by lexx
    define "good"!?and do you think his status as super senior member implies super senior or super member and if so what's super about senior and what's super about member!?and no....dont go there!?hehe!!.....just askn...
    Kazza is a young pup. It's my member that is senior. :cool:
    The terminally stupid and certifiably insane.

  15. #15
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    Re: Quantum Mechanics

    Ok, I finally got time to go through it and I'm ashamed to say that for the things that I don't quite understand, I can't even formulate an intelligent question to ask. But I'll keep reading it over and over till I do.
    The terminally stupid and certifiably insane.

  16. #16
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    Re: Quantum Mechanics

    please continue with the omitted black body photoelectric stuff!?hehe!!....just askn...
    i do not endorse/recommend any advertising on scam.com associated with my name /posts or otherwise. thank you

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