Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How close before touching ?

  1. Sep 17, 2010 #1

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    How close before "touching"?

    If I slowly lower a book to a table from a height of say 1 m, it is at some point, 0.5m away, then 0.4 m away, etc., until ultimately it rests on the table and I let it lie there. Now i understand (correctly?) that the book is not actually touching the table, there is some sort of photon exchange, but the molecules/electrons of the book never actually 'touch' the molecules/electrons of the table. So my question is, just exactly how close to the book molecules/particles get to the book molecules/particles of the table. Is the answer

    a.) The Planck Length
    b.) A silly fraction of a nanometer
    c.) 0
    d.) none of the above

    I expect the answer is (d), but I'd like an explanation. (I'm not talking about Zeno's paradox). Thanks!
     
  2. jcsd
  3. Sep 17, 2010 #2

    DaveC426913

    User Avatar
    Gold Member

    Re: How close before "touching"?

    It's not a photon exchange, it's an electrical repulsion between electrons in the book and electrons in the table.

    As for what the distance is, that is, of course not a simple answer. First, let's pretend the book and table are perfectly flat down at the molecular level. This is pretty much impossible, since neither book not table are single elements and crystaline in structure (think about the giant cellulose molecules that make up the paper fibres in the book and table).

    But let's say both are a perfectly flat layer of one homogenous element.

    Ultimately, the distance comes down to the atomic substance in the outermost layers of the molecules of the book/table (hydrogen? carbon?), more specifically, the replusive factor of the outermost electron shell in that particular element.

    So,

    d.ii) The sum total of the electrorepulsivity between the outer electron shells of the atoms in the surface of the book and the atoms in the surface of the table.



    All that being said, note that, whenever we talk about proximity of macroscopic objects, we are always talking about the electroreplusive interaction between their outer orbitals. The boundaries of a macroscopic object are defined by its electron shells.

    This means that it's kind of meaningless to talk about when they're "really" touching versus when they're "just" interacting electrically.

    Simply, any two obejcts are "really" touching when their electro-repulsivity is enough to offset the force pushing them together.

    So,

    c.) 0 - They're really touching, in any meaningful sense of the word.
     
    Last edited: Sep 17, 2010
  4. Sep 17, 2010 #3

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: How close before "touching"?

    Dave, thanks very much for your excellent response. One more question...when an object passes thru space moving from one point to another, does it traverse through a "Planck Length" of space while doing so, or is the motion actually in a series of 'quantum jumps' as it's moving, rather than a continuous smooth motion through all 'points' in that space of movement, if that makes sense.
     
  5. Sep 17, 2010 #4

    DaveC426913

    User Avatar
    Gold Member

    Re: How close before "touching"?

    There is no reason to think that space is quantized like that, but we don't know. Or at least, I don't. :wink:
     
  6. Sep 17, 2010 #5
    Re: How close before "touching"?

    http://en.wikipedia.org/wiki/Pauli_exclusion_principle#Stability_of_matter
    http://en.wikipedia.org/wiki/Exchange_interaction
     
    Last edited: Sep 17, 2010
  7. Sep 17, 2010 #6

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: How close before "touching"?

    Ok, Thank you!
     
  8. Sep 17, 2010 #7

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

  9. Sep 17, 2010 #8
    Re: How close before "touching"?

    I find wikipedia to be almost useless for learning physics. It always assumes you know the subject matter. The definitions are probably accurate but seem to be built for those who already know the subject matter.

    Unfortunately, knowing the material doesn't qualify one as a good writer or teacher.

    I love wikipedia but the physics explanations are lacking unless you are already a physicist, IMHO.
     
  10. Sep 17, 2010 #9

    DaveC426913

    User Avatar
    Gold Member

    Re: How close before "touching"?

    I found it a fascinating read, but I have yet to divine its relevance to the thread. I'm pretty sure there's a PF rule somewhere prohibiting the posting of a link without an explanatory comment.


    That's because Wiki isn't a learning tool. It's a reference tool, like an encyclopedia, which is why it's divided into discrete, manageable chunks.

    If you want to learn Physics, https://www.amazon.com/books".
     
    Last edited by a moderator: May 4, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook