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Homework Help: How come we can't walk through walls?

  1. Dec 5, 2006 #1
    Hey all......
    im still at school and doing the IB diploma, and for Theory of Knowledge subject i have to write an essay, the topic being
    "Our sense say that a table is solid, yet science tells us it is mainly empty space, so how come we cant put our hand through it?"
    I have been looking and it seems it has something to do with electron repulsion and the Heisenburg Uncertainty Principle can someone help me please! Why cant we put our hands through tables or walk through walls?
  2. jcsd
  3. Dec 5, 2006 #2
  4. Dec 5, 2006 #3
    Because tunneling is a QM phenomenon and the physics that describes humands colliding onto a wall is not valid on atmic scale : ie classical physics.

  5. Dec 5, 2006 #4


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    "...science tells us it is mainly empty space..." ! Does it, really? I think not, and I wish high school textbooks would stop propagating this ill-formed idea.
  6. Dec 5, 2006 #5
    Why is this idea ill-formed? Because the probability distributions of quantum particles fill all space? Or is there some classical reason why the idea is flawed?
  7. Dec 5, 2006 #6
    Think of it like this. When your ceiling fan is off, you can easily put your hand through it and touch the ceiling. Now when it's on full blast, the blades start looking almost like 1 object, and if you put your hand in there, it's going to stop you.

    Now think about atoms and electrons and such orbiting at light speeds. It may give the appereance of being 100% solid, but I doubt that's even possible. Anyways, I hardly know what I'm talking about, just referring to an example my chemistry teacher gave me way back in highschool, maybe it'll help.
  8. Dec 5, 2006 #7

    Claude Bile

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    Agreed 100%.

    To the OP, you should consider challenging this statement in your essay, provided of course that you are able to use arguments grounded in physics.

  9. Dec 5, 2006 #8
    so what grounds do i challenge it on? The fact that "science" in general does not state that atoms are mostly free space so therefore the hypothesis is wrong?
    thanks for the fan analagy that helped alot :)
    check this url for some deductive reasoning about the subject:
    i mean, theoretically, it must be possible to walk through a wall?
  10. Dec 5, 2006 #9
    Theoretically yes, the probability is just really low...
  11. Dec 5, 2006 #10
    Electrons 'repel' other things with the EM force. This makes it so your particles and a table's particles repel each other to a point where you cannot go through the table itself.
  12. Dec 5, 2006 #11
    ok so the outer most electrons repel each other with the EM force..... thats all good. So it means that we never actually touch something? its only the force between the outer most electrons? how can we then tell something is rough or smooth, and why do different things "feel" different?
    wasnt there an experiment before done about quantum tunnelling and proved it worked?
  13. Dec 5, 2006 #12
    Then how do you explain Rutherfords experiment where he was able to shoot helium through a sheet of gold. QM definitely tells us that what was previously thought as solid i.e. a smooth rigid continuum is not entirely true. In fact, QM can show us that our previous notion of the phases of matter, primarily say Solid, liquid and gas, although macroscopically having very different properties, on the microscopic scale they look almost identical (there are many ways to characterise this difference, correlation lengths, order parameters etc), however this is a very different notion than what was previously believed about matter.
  14. Dec 5, 2006 #13
    things are all the same on a microscopic level? How come a helium atom can go through when say an atom on my hand cant?
  15. Dec 5, 2006 #14
    The gold foil was pretty thin, but the point of the experiment was that if the 'size' of nuclei were what they were supposed, the scattering angle would have been much larger. Instead of you hand lets talk about making the gold thicker and thicker. Forget all that crap about tunnelling through walls and the like, its one of those popular physics notions which frustrates the hell out me and others I know. If a solid was a continuum then if I was to take an infinitesimal slice, nothing would be able to pass through it without breaking it. However if the slice is made out say a single sheet of atoms (think of filling a plane with equilateral triangles, at each vertex is placed an atom), then there will be 'space' between the atoms (regions where there is most likely little or nothing to impede the motion of a projectile) and an object will pass through. Let us say that statistically if we randomly shoot projectiles at this sheet, the probabilty that it will pass through is

    Pr(passing through 1 sheet)=\frac{1}{q}

    Now if I build up a 3d solid by adding layers to this sheet, the probability of passing through the solid will be a function of the number of layers

    Pr(passing through n sheets)=\frac{1}{q^{n}}

    Now say a sheet of gold is effectively 5.0 Angstrom thick (mean plane seperation in fcc structured gold), and the transition probability is nearly one, let's say [tex]\frac{99}{100}[/tex] for some particular momentum projectile (it will be different for a whole lot of parameters). Now take a slab of gold to be 1cm thick, that's approximately [tex] 10^{9}[/tex] layers of gold, such that the transition probability will be

    Pr(passing through 1cm gold)=\frac{99}{100}^{10^{9}}

    which is an incredibly small number, although it is still theoretically possible.
    Last edited: Dec 5, 2006
  16. Dec 5, 2006 #15
    "Forget all that crap about tunnelling through walls and the like, its one of those popular physics notions which frustrates the hell out me and others I know."

    thanks for that, it makes sense there. These popular physics notions? They would help to know so i can use them in the essay about generalisations (see the original post)
    Can u explain the one i just used (about the tunneling?) what really happens there?
  17. Dec 5, 2006 #16


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    You might appeal to the Pauli Exclusion Principle. The reason that your hand stops when you slam it on the table-top is that the electrons in the molecules making up your hand refuse to be forced into the same quantum state as the same-spin electrons in the molecules that make up the table. Of course, the PEP could be entirely wrong, and then it would be your job to explain why you can't walk through walls.... ;)
  18. Dec 5, 2006 #17
    is PEP proved? Or is it one of those things we use to explain something that we dont quite understand properly?
  19. Dec 5, 2006 #18
    The Pauli exclusion principle is phenomenoligical. However the mathematics behind such mechanisms is well understood in that if we say

    'If a type of particle has the propertu that only one particle is able to any particular state'

    then we can go quite a long way. However no one really knows why some particles (half integer spin, fermions) can only singularly occupy states and others (integer spin, bosons)may all occupy the same state. This is one of the open problems in physics
  20. Dec 5, 2006 #19
    Because the "atoms" in your hand are molecules and even at the smallest size are 4-5 times bigger than a helium atom. Not to mention pseudo-surface tension in the phospholipid bilayer and the spacing between the individual units as described by the fluid mosaic model dictate what can/cannot pass through it.
  21. Dec 6, 2006 #20
    indeed, it is annoying but how else are quantum physicists gonna get funding support from the public? :biggrin:
  22. Dec 6, 2006 #21
    ok thats all good and fine thanks guys :) let me clarify something thou? electrons "pop" in and out of existance within their sub orbital forming this "cloud" of varying shapes (dumbbell etc) thus appearing to be everywhere at once like the fan analagy on the previous page?
  23. Dec 6, 2006 #22


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    Don't forget that the atoms and molecules in the table, or your car, our yourself form very tight bonds, to the point where it takes a sledgehammer, or an ax or a saw or a drill to break a table top, or simply put, to put something through it. On the other hand, putting your hand through liquid water or air is not exactly a challenge.

    So, lo and behold, the real issue is what distinguishes solids from liquids and gasses. There's plenty of folks who have written on this subject, Google is certainly one place to check. some chemistry books will touch on this issue, as will books on solid state physics. (The probability of your hand tunneling through anything solid is so small that it is less likely than a snow blizzard in tropical weather.)

    Reilly Atkinson
  24. Dec 6, 2006 #23
    when you put your hand into liquid water or through air, your not putting your hand "through" them like i mentioned with a table or walking through the walls are you?
    another question... what would happen if in rutherfords experiment, the helium atom that was fired at the sheet of gold entered into it, but became caught within, ie stopped before it could make it the whole way through. Would this have any effects on the gold or the helium?
  25. Dec 6, 2006 #24

    Claude Bile

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    Quantum physicists AFAIK have reached a consensus that the wavefunction that represents a bound electron actually represents the spatial distribution of the electron, and not the electron whizzing around behind the scenes (i.e. the electron exists everywhere within the wavefunction at any one time).

    Since the electron wavefunction occupies most of the space in the atom, you could say that an atom is filled with electrons, not empty space. Empty space has no mass and charge etc, while the space occupied by an electronic wavefunction does.

  26. Dec 6, 2006 #25
    ok thats good, i needed counter arguments and arguments from the perspective of another culture... can anyone help me on that all i can find is this story about a chinese monk getting "stuck" while he was reportedly attempting to walk through a wall, and that sounds pretty absurd to me.

    still to follow my last post on this topic... putting your hand through water and air is not the same as putting it through a table is it?
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