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Reason why the chair can support me

  1. Jan 9, 2006 #1
    A physicist said that the reason why the chair is able to hold my weight is because of Heisenberg's Uncertainty principle. His reasoning was that when the atoms of my body are in contact with that of the chair, the electrons in the my body push against the electrons in the chair. Thereby forcing the electrons to a smaller region of space. Due to HU princple the more accurately we can determine the position of particles, the more wide ranging their momentum. Therefore when I sit on a chair, the electrons in the chair due to their high momentum push against my electrons hence providing pressure to hold me up. That is the reason why the chair is able to support me.

    I am not fully comfortable with this explanation but that is due only to my basic understanding of QM

    My inital reaction would be simply that when I sit on a chair, the electrons in my body will repel that in the chair due to the electromagnetic force. It is this repelsion that provides the reason why the chair is able to support me. The protons in my body and that of the chair are on average too far away to exert any attractive force.

    It seems that my idea and that of the physicist is completely different. Can someone offer some insight here?
     
    Last edited: Jan 9, 2006
  2. jcsd
  3. Jan 9, 2006 #2
    When you say a "physicist," what exactly do you mean? The reason the chair does support you is due to the fact that the "vacuum" field of electromagnetism generates enough virtual photons, especially in the presence of the electrons of you and the chair, so as to transfer enough momentum so as to keep you from falling into it.
     
  4. Jan 9, 2006 #3
    A professional physicist and a good one because he was interviewed (that is where I got the infomation from) in a software about physics that contained two physics nobel prizewinners. The CD was called 'The mystery of the universe' or something like that.
     
  5. Jan 9, 2006 #4

    LURCH

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    I do not think it is true that your idea is "completely different" from that of the physicist you have quoted. It is indeed the repulsion of like charges between the electrons in your butt and those in your chair that hold you up. But that is only the direct cause. The next obvious question is, "then why don't my electronics simply push the chair's electrons out of their way, and keep going?". The fellow on the CD was answering that question.
     
  6. Jan 9, 2006 #5

    Galileo

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    There's another purely quantum mechanical effect which looks very different from the above two, and this comes from Pauli's exclusion principle. I believe it's called 'degeneracy pressure' and is unrelated to electric repulsion and thermal motion.
     
  7. Jan 9, 2006 #6

    Gokul43201

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    Either you are misquoting the "physicist" or he's not really one. Does he have a name ?
     
  8. Jan 9, 2006 #7
    This is all backwards. You don't say "what reason does physics give that the chair can support me?" Physics doesn't make the rules, the rules are already there. You've already proven many times that chairs usually support you.

    The real question is, "What predictive system can I suppose that describes why the chair supports me." The answer will be unlike anything listed here, since describing macro-scale obviously classical phenomena with quantum mechanics is a dismal waste of time.

    What you might be really asking is how you can get from quantum mechanical systems to classical ones. The answer is, as I pointed out in another thread, that you really can't, or at least not in a complete way. You can make some general statements, but they are just assertions, and can't be proven.
     
  9. Jan 11, 2006 #8
    The physicist's name is Jean Dalibard. The software's name is called 'Secrets of the Universe'.

    So you don't like the HU principle as an explantion for the reason why people don't fall through a chair?
     
  10. Jan 11, 2006 #9
    What I said which was "what reason does physics give that the chair can support me?" and what you say "What predictive system can I suppose that describes why the chair supports me." are to me, the same thing. Physics does not give laws to nature but merely tries to find them which I have assumed all along. But because of their close connection I might sound as if physics actually determines the law of nature. So you have stated my thoughts in a clearer way.

    The quantum and classical do not mix but what I was assuming was coming up with what microscopic things tend to behave on average. And it is this averge state that dictates or contributes to the macroscopic level. I read this from Erwin's Schrodinger's "What is life?"
     
  11. Jan 12, 2006 #10
    But they aren't the same at all! The reason they seem so to you is revealed at the end of the above quote - you feel that physics finds laws of nature. I said nothing about laws of nature, but instead wrote of predictive systems, which are entirely man-made. How closely those describe "laws of nature" (whatever that is! I would argue there is only one law, and it cannot be fully described mathematically) is another question entirely.

    You also speak of microscopic things on average. My argument is that you cannot deduce macroscopic phenomena by averaging microscopic phenomena. Note that this is primarily a philosophical discussion, as there is no definitive proof either way.

    As a counter example to your physicist's argument, I remember a chair I used to use as my computer chair. Your physicist's argument (because it is qualitative, and not even a good qualitative argument) would suggest that that chair would support me just as this one does. Of course, it didn't; it broke in two and I ended up on the floor. The strength of materials is just one of many things that you should (by such reductionist ideals) be able to produce using QM, but cannot. The reason the chair holds your weight is not adequately described by the HUP.

    I like the above example because it illustrates the philosophical lackings of such reductionist arguments while at the same time making their absurd uselessness obvious.
     
  12. Jan 12, 2006 #11
    I never said that physcists will find what nature or reality 'really' is. I wanted to say that physcists try their best to find it by theorising which from experiments looks like a pretty good way of going about it but they are all approximations to reality. Your predictive systems are what I would call theories. So actually we share the same view.


    So you are saying that there are emergent properties that arise from microscopic systems when looked at macroscopically and these properties cannot be described by the microscopic laws. Maybe if you model the atoms in the chair classically such as how classical thermodynamics models the physical systems than you might get a result that matches your experiment. But that is cheating a bit because you are soley using macroscopic laws.
     
  13. Jan 12, 2006 #12
    That doesn't really seem to be a counter proof. The example used by the physicist is probably the standard "Assuming a chair where strucural weakness can be ignored". Even so the chair broke into two, you didn't pass through the chair as if it were immaterial.
     
  14. Jan 12, 2006 #13
    I was waiting for that! No, see, that's the whole point - how do you think you can possibly show you cannot pass through the chair without making a case for its structural integrity? Every argument listed in this thread (HUP, electron repulsion, etc) so far could be applied to air, and yet it doesn't support anyone most of the time. You must try and describe the physical properties of the material - and you can't do that using strict reductionism.
     
  15. Jan 12, 2006 #14
    Exactly! It's worth mentioning that studies of very low-level systems (atomic structure, etc) are essential to properly understanding some higher level systems. They just aren't enough. You end up having to take measurements at whatever larger scale you are operating at.
     
  16. Jan 12, 2006 #15
    I guess my main question is how, if at all, does the triangle between HU principle, electromagnetic repulsion and with Paul's exclusion principle connect together. What relationship exists between the three physical theories. I gather that Paul's exclusion principle includes the charge so the electromagnetic repulsion and Paul's princple are more similar than the HU principle, which seems to stand on its own.

    The three all explain why broadly speaking, matter are structured and organised in some way.
     
    Last edited: Jan 13, 2006
  17. Jan 13, 2006 #16
    What?

    Can you hit us with that again, worded differently?
     
  18. Jan 13, 2006 #17

    Sorry about that. I have fixed post 15 and have made it make sense, hopefully.
     
  19. Jan 13, 2006 #18

    Galileo

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    No. That pressure has nothing to do with electron-electron repulsion. It's a purely quantum mechanical effect that arises from the antisymmetrization of the wavefunctions.
     
  20. Jan 13, 2006 #19
    Heh, true. It is macroscopic properties of the chair that prevent you from going through it.
     
  21. Jan 13, 2006 #20

    Gokul43201

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    Exactly ! That's why (among other reasons), I said the original argument was essentially gibberish.
     
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