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Why isn't it possible to walk thru a wall?

  1. Aug 10, 2013 #1

    If the atom is mostly empty space, why isn't it possible to walk thru a wall?

    The science documentary I saw the other day told me that if you where in a church, the nucleus itself would be represented by a fly on the floor while the electrons would be encircling the church cealing.

    So much emptiness is said to recide in the atom.

    Being an amateur at this, I think that there really are not that many electrons in an ordinary atom which would make it impossible not to find a "window" where you could climb into the atom, so to speak.

    Best regards, Roger
  2. jcsd
  3. Aug 10, 2013 #2

    Simon Bridge

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    A net is mostly empty holes - how come they catch fish?
    Have you tried pushing a net through another net?

    Actually, an atom is not mostly empty space.... it's mostly electromagnetic fields.

    In atoms, the electrons are basically around the outside.

    Since "like charges repel", the electrons in your body repel the electrons in the wall - this repulsion increases in strength the closer you get to the wall and what we call "physical contact" is the proximity where the repulsion matches how hard we are pushing.

    The electrons don't have to strike each other to affect each other because they have charge: the electric field pervades all space. So where is the "window"?

    Atoms in a solid are (ballpark) 10-10m apart ... so to pass one solid through another basically involves passing electrons closer together than this ... you can figure out the work needed to get two electrons even that close if you like.

    We can get an electron past the atomic electrons by firing it from a gun ... the kinds of forces you can generate just by walking are nowhere near high enough to overcome the repulsion. However we could get you through a wall by firing you from a gun too...
    Last edited: Aug 10, 2013
  4. Aug 10, 2013 #3


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    (Classical description here - we don't need any quantum mechanics for this)

    When you force two atoms together, the resistance doesn't come from the particles bumping into one another. It comes from the negative-charged electrons repelling one another. The trick here is that because the electrons are on the outside, they end up much closer, hence stronger force between them, then any of the other parts of the atom.
    Last edited: Aug 10, 2013
  5. Aug 10, 2013 #4
    Sorry, I feel stupid for asking this.

    But at least I have learned to spell "through" :)

    I also remember from that same documentary that when you for instance clap your hands, the hands aren't really touching themself. Only the repelling force of the electrons are.

    I can appreciate the electromagnetic forces in these electrons but it still seems like that should not be enough due to being so few while there is so much empty space.

    But your net-analogy isn't that bad. Thanks!

    Finally, I posted it here under this sub-topic because I've heard of the fact that there actually is a probability for being able to walk through a wall. And this probability comes from quantum mechanics. Or so I've heard.

    But maybe I'm wrong.

    Please correct me if so.

    Best regards, Roger
  6. Aug 10, 2013 #5


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    That's quantum-mechanical tunneling, a completely different phenomenon.

    At a hand-waving level, tunneling is based on quantum mechanical uncertainty: There is a non-zero probability that a particle last known to be at one position might be found at another position the next time we look; and if every one of the 1027 particles in your body suddenly all happened to be found 10 centimeters to the left, all at once, then you could end up on the other side of a wall. That's not going to happen. It is enormously less likely than that your kitchen table might fly up into the sky because all the air molecules underneath it just happen to be moving upwards at the same time - and that's something that isn't going to happen even once in the entire lifetime of the universe.

    We do see tunneling with single particles, much lower energy barriers, and much shorter distances. Google for "alpha particle tunneling" for examples.
  7. Aug 10, 2013 #6


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    Don't go with "it still seems", try calculating the forces between two atoms as they're pushed towards one another.

    There's a long thread at https://www.physicsforums.com/showthread.php?t=703248.
  8. Aug 11, 2013 #7

    Simon Bridge

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    I'm with Nugatory on this one - do the math.
    Lots of things will "seem" that they "should be" a certain way ... that's just your every-day intuition misleading you. You cannot tell if the intuition is right or not without checking with the figures.

    That space is not "empty" - it's just that not a lot of the volume seems to be filled with matter. Recall - what it is filled with is "electromagnetism".

    But even if that were so - the electrons are all moving, and there are a lot of them (iirc in 1 cubic centimeter of pure copper there are about 10^27 or so of them) - so there is a very high chance that enough electrons get close enough to push you away. Your intuition does not know how to handle those sorts of numbers.
  9. Aug 11, 2013 #8


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    Actually they are touching. The repelling electrons do to one another when your hands meet IS touching. Consider the fact that particles are believed to be point-like, meaning that they have no physical size, so it is utterly impossible for them to actually 'touch' in the usual way of thinking. And if it is impossible, then it seems we need to change our definition of 'touch' to something that is possible, wouldn't you agree? (At least when talking about the atomic/subatomic world)

    There is no empty space in an atom. None. If you try to probe an atom with an electron you will find zero holes, zero empty spaces, no way for it to go through unless you give it enough energy to overcome the repulsion and just punch its way through.

    Also, the EM force is VERY strong. Plenty strong enough to keep you from walking through a sturdy wall.
  10. Aug 11, 2013 #9
    Thank you all for your answers.

    Earlier I refused to just accept the formula for Gravitational Time Dilation (GTD) and asked this nice forum where the formula or effect actually came from.

    I got several nice and pedagogic answers so now I know.

    I should be equally thorough here and not just accept my intuition as you have said.

    But I am a little lazy and proud with regard to what I actually think I know so I won't google it unless I know I am wrong.

    Here is what I believe I know:

    [tex]F=\frac{q_1*q_2}{r^2} N[/tex]

    which equals

    [tex]F=\frac{e^2}{r^2} N[/tex]

    when it comes to two electrons with the unity charge of -e.


    [tex]e=1,6*10^{-19} As[/tex]

    which shoud mean

    [tex]F=1,6^2 N=2,56 N[/tex]


    [tex]r=10^{-19} m[/tex]

    Thus at this "short" distance we already have a repelling force of around 2.5N.

    I don't know the radius of an electron (never heard of it actually) but if we consider it to be "point-like" then we could move even closer to the electron which will build up the force needed to get closer fast because of 1/r^2...

    This points to a strong electromagnetic force.

    I hope this is correct.

    Best regards, Roger
    Last edited: Aug 11, 2013
  11. Aug 11, 2013 #10
    Point taken! :)

    Last part taken, first part...was there something wrong in the documentary?

    I mean, the proportions should at least have been true, right?

    I agree that this was a stupid assumption. I only thought of the atom like the common man do, that is the simple model with a nucleus and electrons spinning around the nucleus in shells. But I've heard that this model oversimplifies the atom. Anyway, this model along with the documentary was the reason for my "window" idea.

    Reading your first comment above, did however strike me hard. Simply because I did consider one tiny atom only...

    I think I've never laughed so much! :D

    Best regards, Roger
  12. Aug 11, 2013 #11

    Simon Bridge

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    Yes, there was.
    The documentary was taking some experimental results and drawing misleading conclusions from them ... they did this for dramatic effect. It was not an education program but an entertainment program.

    Consider: if my bucket only has a few drops of water in it, does that mean it is mostly empty?
    Could there be something else in there as well as the water?

    At best the results show that the "matter" component of an atom does not occupy a lot of the volume.
    A better conclusion is that the "hard sphere" model of fundamental particles is wrong.
    This is the bit that the doco left out.

    For details, look up "Rutherford experiment".... and follow from there.
  13. Aug 12, 2013 #12
    Hi Simon!

    Jesus, I have much to learn!

    But I do recognize The Shrödinger Equation even though I then did not understand it.

    I will continue studying this and not disturb you with any more stupid questions.

    Thank you for your help!

    But I do want to make some corrections regarding my wrong calculations above.

    It struck me on the bus today that the units where wrong...

    The formula for two electrons yields a repelling force of:


    Let's say we want to know the distance for 10N (~1kg) then


    which calculates to

    [tex]r=4,8 fm[/tex]

    Best regards, Roger
  14. Aug 16, 2013 #13

    Claude Bile

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    Hi all, a little nitpick,

    The repelling force does not come from electrons, but rather from the chemical bonds that hold the "wall" together.

    This is why we can move through gases and liquids (where there are no strong intra-atomic forces), but not solids.

  15. Aug 16, 2013 #14


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    To re-word this, I'd say the repelling force DOES come from the electrons, but the attractive force between bound atoms/molecules are strong enough to keep them from being pulled apart. Liquids and gases are not bound together, or only very weakly, so the repulsion simply moves the atoms/molecules out of the way.

    That sound good?
  16. Aug 16, 2013 #15

    Simon Bridge

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    rogerk8's question is about moving the atomic nuclei past each other without breaking the chemical bonds. There's lots of space, after all. In practice you won't get the nuclei closer than order of angstroms from each other.

    @rogerk8: that's better - but that's how close two electrons would get if a single pair of electrons had to balance the entire 1kg of weight. What you've shown is that two electrons can easily fit inside an atom. You've also shown that we can fire electrons at atoms and use them to probe atomic structure.

    An actual 1kg weight would contain many more than a single electron though.
    The surface it is sitting on, the same.

    i.e. 12g of carbon 12 contains 1 mole of carbon atoms in a volume of about 6cm3[/sub]
    That gives about 2x1023 valence electrons to be involved - depending on geometry.

    Lets balance 1kg of carbon, cut thin so most of the valence electrons get a shot at helping out, on a carbon surface, and work out the average separation - back of envelope like:

    $$W=9.8\text{N} \\ k=\frac{1}{4\pi\epsilon_0} = 9 \times 10^{-12}\text{Nm}^2\text{C}^{-2}\\
    q=\frac{1000}{12}(2\times 10^{23})(1.6\times 10^{-19}) \approx 3\times 10^5\text{C}$$
    $$\Rightarrow r=0.3\text{m}$$ ... which is rather a lot more than what you observe.
    That's reasonable, since the geometry of the situation is that not all the valence electrons will be involved (they will be busy holding the solid carbon together in C-C covalent bonds).

    It should give you an idea of what sort of repulsions are involved here though.

    But this is where the analogy of trying to push a net through another net works quite well.
    Imagine you had one net in a loose ball sitting on another net - only the knots are luminous and the scene is shot in the dark. You would wonder why the ball-net does not fall through the other one. The reason it's a puzzle is because you are not seeing the whole picture. And that is how the documentary you saw misleads people.

    The simple pictures we are using to explain it here do not give the whole story either - they are supposed to help you see how there can be more to it than just lumps of mass floating in an empty vastness.
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