When I slap my hand down on my desk, my hand comes to rest atop the desk. But if each atom in the desk is 99.99% nothingness, and each atom in my hand is 99.99% nothingness, shouldn’t my hand just go through it?
I suspect the reason has something to do with electrical repulsion forces on the atomic level. Just wanted to get the straight dope on this.
Electromagnetic repulsion of your hand’s and the desk’s atoms’ electrons.
Yep.
“… though the forces between the equal charges are wholly responsible for the solidity of matter, which is really mostly empty space.”
as sai, it’s the electric replusion if the electrons in your hand acting agains teh electrons in the desk. In fact, when you slam your hand down on the desk, no matter how hard you hit it, nothing in your hand actually makes physical contact with anything in the desk. It’s all EM. I found that very surprising when I first learned that.
So not only is everything 99% nothing, but nothing is touching anything?
:dubious: :eek:
::walks of to punch boss and claim I didn’t touch him ::
You can then test the solidity of the handcuffs that the police haul you away in. 
Electron repulsion works just as well for them, unfortunately.
So what’s actually going on? Are the electrons in my hand repulsed by the electrons in the desk? Are the protons in my hand repulsed by the protons in the desk? Or both? Or something else?
Electrons (which form a “shell” around the nucleus) are repulsing the EM fields of other atoms’ electrons. Proton charge is contained (negated) by the electrons, unless the electron has incomplete shells, which gives you covalent and ionic bonds (and hydrogen) bonds.
And things just get stranger from there.
Stranger
Since electrons have a negative charge and like charges repel each other the electrons on nearby atoms repel each other. So no matter how hard you hit something your atoms will never touch it’s atoms.
So this is how the tasers work!
In the end you are a summed smear of quantum probabilities interacting with other smears.
And the faster you go, the more lost you get. 
Stranger
It’s the electrons. Sure, the protons repel each other, too. In fact, the protons and electrons attract each other. But the strength of the repulsion (and attraction) between charges falls off as the square of the distance (if I remember correctly). Thus, the force between the electrons in your hand and the electrons in the desk is much larger than either the repulsive force between the respective protons, or the attractive force between protons and electrons.
The protons of a given atom are fairly far away from the electrons at the boundary of that atom. Since the electromagnetic force between any two objects falls off as the square of the distance between them, the repulsive force between the electrons of the two atoms is stronger than the attractive force between the protons of one atom and the electrons of the other. For instance:
P1------e1–e2------P2
Each electron is effectively a point in space, so P1 is distance 8 from e2, and P2 is distance 8 from e1. Let’s call the attractive force between each pair F. The distance between P1 and P2 is 16, so the repulsive force between them would be F * 8^2/16^2 = F/4. Finally, the distance between e1 and e2 of 2 would make the force between them F * 8^2/2^2 = 16 F. Summing the forces, we get an attractive force of 2 F, and a repulsive force of 16.25 F. Clearly, each atom will be repulsive to the other.
Of course, I have just shown that any two atoms will never get close to each other, thereby proving that no molecules would ever form between sets of atoms. Therefore, reality is an illusion, since it can’t possibly be true. That, or my physics teacher skimmed over some of the more complicated stuff.
Well, I guess it’s a philosophical point, but by your definition there’s no such thing as “touching” something else. I’d say that your hand “touches” the desk, but that touching ought to be defined on the basis of electrostatic repulsion. There’s not “solid” matter at that level anyway, or at least not enough of it volume-wise for it to count for anything. So even though touching works differently at the atomic level than at the macroscopic level, there’s nothing that could touch in the usual sense anyway.
Molecules form by “sharing” electrons to satisfy unfilled shells. They have an attraction, based upon needing to acheive charge balance, but aren’t strong enough to hold onto the electron exclusively, except for the noble gases like helium and argon, which are non-reactive under normal conditions.
Molecules are like political alliances; they come together by a shared interest, but enough conflict will rip them back into their components.
Stranger
According to many physicists, there’s at least a chance every time you try that your hand will simply pass through. The odds are uncounted trillions-to-one, but it could happen.
Ok, so I understand that the electrons in the desk and those in my hand are repelling each other.
But why don’t the electrons **within **the desk and **within **my hand repel each other? Or maybe they do, but there’s something stronger keeping them from just flying apart?
You are applying macroscopic rules regarding the electrostatic forces in places where quantum mechnics rules apply. For example, the nucleus is composed of all positive (or neutral) electrical charges and should, by rules derived from macroscopic effects, fly apart except for hydrogen. Obviously it doesn’t so on the atomic level there must be either some other rules or some other forces. There will soon be a quantum physicist along to go into the subject more fully.
The atoms themselves don’t fly apart from each other because, as was mentioned above, they interact through their outer electron shells to fill up incomplete shells by sharing electrons, or they stick together through electrostatic attraction by one atom grabbing an outer electron from another leaving a positive ion and a negative ion which stick togetner.
I can see this.
Now what happens if the probabilities line up such that my hand gets part way into the desk before the probabilities stop lining up and the electrons start getting in each other’s way again?