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A question and theory about light

  1. Apr 5, 2007 #1
    Hey guys! I'm pretty new to physics, so please forgive me if this is a dumb question.:confused: Anyway, I was thinking yesterday, "What the heck is light made of?" So I looked it up on the internet and found out light is made of particals and waves.(or something like that.) So I just kinda sat down and watched the discovery channel and saw a show that was talking about the displacement of water when something falls into it. Then I thought, "Does light displace water?" Since its particals and all. Can you guys tell me the answer to this question?
     
  2. jcsd
  3. Apr 8, 2007 #2
    actually light is nothing but electromagnetic waves and they travels in the form of small bundles called "Photons"
     
  4. Apr 8, 2007 #3

    Mentz114

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    Hi Mato. Although it's possible to think of light as partcles, they are not made of matter and have no mass or weight in the usual sense. So they wouldn't displace water. Light can travel through water, as you can see and gets deflected in the process.
     
  5. Apr 8, 2007 #4
    This isn’t exactly true, light can displace water. Optical tweezers for example can move a single molecule of water. Because light has momentum it can move water, given the right constraints. Albeit seeing "light" (EM waves) move water is not going to be observable on our macroscopic scale.
     
  6. Apr 8, 2007 #5

    Hurkyl

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    The third thing you say here is right.

    Before quantum mechanics, we had classical mechanics. In classical mechanics, we talked about things we called "particles" and "waves". Light was confusing because it behaved a little like a classical particle, and a little like a classical wave.

    Now, we have quantum mechanics, which says that classical particles and classical waves simply don't exist. Instead, everything is this new quantum mechanical "stuff". And even if we continue to use the word "particle", we really mean a "quantum mechanical particle" which is not a "classical particle".
     
  7. Apr 8, 2007 #6

    Mentz114

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    Wizardsblade, please check the definition of 'displace' as used in the OP. It does not mean 'move'.

    But you're certainly right about the optical tweezers.
     
  8. Apr 8, 2007 #7
    Mentz114,

    Your right I did not read it correctly.

    Could you displace water with an inverted optical tweezers, i.e. what if intensity goes as R (radial distance)? Would that push/pull the water out of place?
     
  9. Apr 8, 2007 #8
    Hurkyl, what exactly does this mean? I have been trying to study quantum mechanics for a few years now and honestly it sounds more like a religion than something based on facts. Not saying there are no facts, just that when you attempt to locate them you are bombarded with a "You just have to believe" sort of argument. So is it a particle? Or is it a wave? quantum mechanics talks of a bunch of strange things and then never really seems to answer anything. Just raises more questions.

    Please forgive my statement as I'm not a scientist so a lot escapes me I'm sure... :-)

    glenn
     
  10. Apr 8, 2007 #9
    Glenn,
    This will sound odd but light is both a wave and a particle. If you do a particle expiroment on light it will act like a particle, but if you do a wave expiroment on light you the light acts like a wave. In other words a wave and a particle are the same thing. As far as understanding how/why this is true QM does not go there, it just states the math. There are some theories on what it all means but nothing that is proven.
     
  11. Apr 9, 2007 #10
  12. Apr 9, 2007 #11

    Hurkyl

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    It means that quantum mechanics is not a kind of classical mechanics -- so it is wrong to insist on understanding quantum mechanics in terms of classical ideas.

    What do you mean? There are tons of facts; QFT is an extremely well-tested theory.

    It suspect you mean that QFT does not explain things in terms of things that are "intuitively obvious". That is not a failing of QFT -- it simply means you have not yet developed the necessary intuition, which should be one of the aims of your study.


    Neither. Light is not a classical object. It is wrong describe light in terms of these classical ideas.

    (Of course, for many purposes, these classical ideas are often sufficient descriptions, even if they aren't exactly correct. And since the classical ideas are simpler, it would be right to use the classical ideas for such purposes)
     
  13. Apr 9, 2007 #12
  14. Apr 9, 2007 #13
    But see, this is where I think things are off. Specifically what experiment determined that light was a wave? The only experiment that I've seen that supposedly proves this as fact is the way light behaves when it is split and how it interferes with itself causing an interference pattern on the wall if you will.

    However, a little deeper thought may produce another explanation for what you see during this transaction so I'm not totally convienced the earlier statements of light being a wave were accurate.

    Would make a very interesting conversation though... :-)

    glenn
     
  15. Apr 9, 2007 #14
    But doesn't this make quantum mechanics some sort of black magic? I don't deny that it predicts certain things based on probability, but does that in and of itself justify people taking it to the extreme in human comprehension?

    For example: Based on the results of the two slit experiment and the "wave" functions that quantum mechanics uses to predict things, people have gone off on multi-world universes where nothing exists until someone looks upon it and it becomes measured. Whereupon it snaps into 1000 different universes with all the possible solutions to the measurement along with 1000 different measurers that were conducting the test.

    So instead of looking at the fact that quantum mechanics is made up of wave functions that make predictions (not absolute predictions, but best guess solutions), it seems we instead try to give it a magic quality with religious intent. Something to strive to understand because you can't possibly understand it without great study and reflection... Just seems we've gone too far here...

    I'm not sure what a classical measurement is anymore. Taking the claims of quantum mechanics, one would seriously have to backup and look at all aspects of past science and question if we've done anything right at all.

    glenn
     
  16. Apr 9, 2007 #15

    Hurkyl

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    No. At least -- I don't see why one would think so.


    Undergraduate linear algebra is hardly the "extreme in human comprehension," and is all you need to understand most of quantum "weirdness". Well, I suppose you also need to admit you may need to learn something new, which does appear to be extraordinarily difficult for some people. But that's only a limitation for those people, not for humanity as a whole.

    I think it's extremely arrogant for a person to insist that everything can be explained in terms of things that he already knows. (And even more so for him to insist that if he doesn't understand it, then nobody can) However, I do have empathy for those who simply find it difficult to learn new things.


    Who is this "we" of which you speak? As far as I can tell, you are the only one here who is making any claims that quantum mechanics is "magic" or "religious".
     
  17. Apr 9, 2007 #16
    I will be taking linear algebra next spring so am happy to hear that it will help me to grasp the concepts of quantum mechanics. Not sure why you feel the need to take questions to personal insults, but then thats just were some people live. If I didn't enjoy learning new things I wouldn't even be here asking questions to begin with.

    I'll keep studying and try not to ask too many questions so as to not affend anyone.

    For those of you that offered suggested reading materials, thank you very much. I am looking through them and will continue to study.

    Thanks,

    glenn
     
  18. Apr 9, 2007 #17

    Mentz114

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    There's nothing weird about quantum mechanics. I don't know where people get this idea. The popular press has made a fortune pushing this notion so maybe that's it.

    I've studied several books on quantum mechanics and quantum field theory and not one of them has a label on it saying 'warning - this stuff is weird'.

    Nor do they say things like - 'please toss a coin and choose formula A if it's heads or formula B if it's tails'. Now that would be magic.

    In fact far from being weird, QM is completely deterministic.
     
  19. Apr 9, 2007 #18

    Hurkyl

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    I'm not nearly as offended as I sound. :smile: I love questions, and so do most of the people here -- don't feel that you should ask fewer questions.

    It's the other little things that are troublesome -- for example, many people will insist that if we cannot explain {fill in the blank} to them in a few paragraphs over the internet, then we must not understand it, or it must be obviously wrong, et cetera, et cetera.

    When you asked loaded questions like "does that ... justify people taking it to the extreme in human comprehension?" it sounded like you were beginning to go down that road. Thus the response. Maybe talking about arrogance is too much, but I think it really puts things in perspective.


    As a caveat, I don't know where you can actually learn about quantum weirdness, from just linear algebra. I was sort of lucky: there was a brief "quantum computing for mathematicians" in one of my classes, where we didn't do any physics at all... we just got an overview of how to program a quantum computer. But because that involves mathematically manipulating quantum "weirdness" (and this required only basic linear algebra), including a prototypical "measurement" (a CNOT-gate), it resulted in my biggest epiphany regarding quantum mechanics. Alas, the result is that I can puzzle my way through the weirdness of Bell tests, counterfactual computation, et cetera... but I can't actually do any physics. :frown:
     
  20. Apr 9, 2007 #19
    Thats fine. no hard feelings here either. I understand what you're saying and apologize if I misled you. I actually have a very open mind and love learning new things. I look forward to the day where this stuff makes sense.

    Also, keep in mind that I've only read those off the shelf books about how mysterious it is and how there are lots of people using it but none that can explain why it works (hence the wierd comments). They have done a good job of making me feel its a strange science indeed but perhaps not in the correct sense.

    I'll keep asking questions as long as you guys don't kick me off. Just don't think I'm being hard headed, I am just the type person that has to ask questions until it sinks into my head.

    Thanks,

    glenn
     
  21. Apr 9, 2007 #20

    jtbell

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    You must not yet have encountered Bell's Theorem and the experiments that test its predictions. :biggrin:

    If that is the case, you might want to read the following article which describes a thought experiment that encapsulates the "weirdness" demonstrated by these experiments:

    http://qt.tn.tudelft.nl/~lieven/qip/extra/mermin_moon.pdf
     
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