They do repel each other; this is what prevents the matter of which your hand is made from collapsing into a tiny speck of mixed protons and electrons. The atoms in your hand are repelled by the atoms in the desk, but putting your hand on the desk merely results in the two resting alongside each other; they don’t fly apart.
I suspect though, if we were honest about it all, it isn’t that your hand ‘never touches’ the desk, it’s just that the definition of ‘touch’ isn’t what we thought it was.
Ouch
There is a possibility that your hand would happen to pass right through the desk, there’s a possibility that only, say, the top half inch of matter is in the right configuration, or the top half inch of matter in this particular two-inch square etc… - it isn’t a case of timing alone and it will depend on the precise position and presentation of your hand.
Of course it’s much more likely that a random, insignificant scattering of atoms in the desk and your hand are the right configuration for close flyby, whereas the rest aren’t.
I remember coming across this concept in high school physics, so I asked the teacher, “So, what makes the noise, when you smack your hand on the desk, if it’s never really ‘touching’?”
He replied, “Loud electrons”.
Well, the repulsion between the electrons in your finger, or arm, or butt, or whatever, and the desk results in distortion of the flesh on your finger, etc. That is sensed by the nerves therein and that’s what we feel when we touch the desk.
Although the finger atoms and the desk atoms never are in actual contact, on the macro level they are so near to it that it doesn’t matter. As someone else pointed out in a post, there really isn’t much matter there to be in contact, just a bunch of electric and magnetic fields originating from nearly point sources.
Quite; in fact it would probably be quite unpleasant if the various bits of the atoms did actually touch each other.
So that’s what goes on in Parliamant and singles bars! 
Well, I am not a Quantum Physicist, but I know the answer to this one.
After it was found that the atom consisted of negatively-charged electrons around a nucleus of postively charged protons and chargeless neutrons, someone pointed out that the like-charged protons ought to be repelling each other electromagnetically, which would mean that atomic nuclei would never come to be in the first place.
Which led to the hypothesis (later proven to be true) that there must exist some fundamental force keeping the protons together that is even stronger at the nuclear scale than the electromagnetic force.
It was named, rather prosaically, the strong force.
Um, no.
This is a common misconception among people who have a good grasp of classical mechanics (Hi, Physics 107 students!) and a superficial understanding of so-called “modern” physics, i.e. quantum mechanics (QM).
In QM, matter, like electromagnetic forces, is described as both a wave and a particle, leading people to visualize it alternately as one or the other. It’s a conceptual model that is valuable in terms of different mathematical treatments, but doesn’t really convey the true nature of matter as described on the quantum scale. (Skip the next two paragraphs if you don’t want an overly technical explaination of what is going on.)
Particles (electrons, protons, and neutrons) in three-space are described by phi(x,y,z), where phi is, for lack of a better description, a value that describes the amplitude of a probability function for a given energy level, n (which is an integer, at least within an infinite potential well). You therefore can generate a tensor with components of phi that describes the likelyhood of the particle being “at” any given value of x, y, z, and n. The idea that you can actually snapshot the particle and measure its literal position is a false notion, however. You can estimate its position within a radius about a locus x,y,z but only by increasing your uncertainty of the “momentum” of the particle, per the indeterminacy (or uncertainty) principle. There is no actual particle, in the sense of something you can metaphorically hold in your hand; it is better to think of it as a cloud, which is thicker in the middle and wispy on the outside without really having a defined location or boundary.
Each electron, having such a distribution, has limitation on how much their “clouds” can interact. Specifically, electrons are not permitted to be in the same quantum state, per the Pauli Exclusion Principle. That is to say, in relation to any given nucleus, no two electrons can share the same shell, angular momentum (subshell), and spin. They just will not do it, no matter how much you shake them around or how long you wait for them to be at opposite ends of their “orbits”. If you try really, really, really hard, you’ll force the atoms into a degernate state, where electrons have too high a momentum to remain within an orbital, and slide around between nuclei without stopping or attaching themselves to any body, like a cocktail waitress at a dental equipment salesman’s convention. This is called a degenerate state of matter, the post-fusion white dwarf state the Sun will fall into when it has burned up nearly all of the hydrogen and helium. If you push even harder yet (we’re talking supernova-class effort here), then you’ll force the atoms into a state in which electrons will join with protons to make more neutrons (and a bunch of neutrinos, I think…I don’t remember equation balance off-hand) and will compress to a “solid” ball of neutrons or a neutron star. The neutron star is more compact (denser) because the probability distribution of neutrons is much tighter by virtue of being much more massive. Beyond that, you lose any information about the individual particles and collapse into a singularity from which information (matter, light) cannot escape.
The short version is, no, the electrons can’t all just jump to one side and make way for other electrons to just “slip through”, not even given an infinite period. The interaction between the waveforms of two electrons (exclusion principle) in proximity affect each other such that this can’t happen, period, any more than you can miss the ground if gravity just doesn’t notice that you are unsupported. This isn’t to say that you can’t compress the crystilline structure of solid materials (in which there is typically a lot of “unfilled” space) or, given sufficent pressure and time, suffuse one solid material into another, but this requires the atoms to be physically redistributed, not just temporarily “in sync” with each other’s quantum states.
I hope that wasn’t too confusing. Feynman does a better job of explaining this stuff in depth. Try Feynman’s Lectures on Physics, Vol. 3 for a more extensive explaination.
Stranger
Or, as Raymond Hall of Fermilab put in once in a lecture I saw: “Stuff is made of particles. Therefore, particles can not be made of stuff.”
The particle soup, or the dental equipment salesman’s convention?
Okay, I have to nitpick…myself. :smack:
n is an integer in a potential well with finite dimensional boundaries (think of a box with the nucleus in the center.) The electron “vibrates” back and forth like a standing wave in a flute, or a guitar string. Like a string, it can only vibrate at certain frequencies, its fundamental frequency and integer fractions thereof, as determined by the length of the string. If the walls are infinitly high, n has an infinite number of integer values. Outside the box, n can have an infinite range of non-integer values, which is why free electrons often release photons (light, heat) when they fall in with an atom.
I hope that makes it all as clear as a Halliburton accounting statement.
Stranger
Yes.
Stranger
Hey, what do you know, his slides from that lecture
are still on his website, six years later. Not the same without his excellent presentation style, but detailed enough to read w/o it, IMO.
It’s just not true that 99% of matter is empty space. It’s all space, and none of it is empty. The space in atoms (and between them, for that matter) is full of fields, most notably, in this case, electromagnetic fields. It’s true that most of the mass of an atom is contained in a very small volume, but that’s completely irrelevant, since it’s the fields (which are everywhere) which cause all of the various interactions.
Bzzzt!
But, when you rub your finger across that desk, you will leave some skin cells from your finger on the desk and some desk fragments will be scraped onto your finger.
So that answer ain’t quite right. Or at least, it’s incomplete. Though I agree with the basic premise of fields.
4Evan
That’s just an effect of the fields in the desk interacting with the fields in your finger. Molecular and metallic bonds are “stressed” by the effect of nearby fields, which result in molecules or atoms gaining or losing electrons. When you slam your fist into the desk in frustration of not understanding this whole quantum mechanics bull$#!&, you aren’t pushing the atoms out of the way, you’re pushing against the fields those atoms exert on each other. The splinter you now have painfully wedged into between your knuckles, and the skin and blood that you’ve left on the desktop are kept there by the (very weak) electrostatic attraction between them.
One of the Master’s Minnions (that would be SDSAB Karen) speaks on this. (Go to the last paragraph for the relevent commentary.)
Stranger
If you jump off a building it will take the gravitational force several seconds to accellerate you to the ground. It will take the electrostatic force only a fraction of a second to make you stop. Thus we see that the electrostatic force is much stronger than the gravitational force.
“The more complicated stuff” here is chemistry, not physics (though the line between the two isn’t always clear).
I had a fan-freakin’-tastic biochem teacher who taught bonds through the analogy of relationships:
**Hydrogen bonds ** are like a couple who hooks up at a club. We might dance and share a drink, and maybe even “hook up” for the night, but dawn comes, and it’s sayonara, dude! Opposites attract, but there’s really nothing keeping us together.
**Covalent bonds ** are when we move in together and start sharing stuff. Two phone accounts become one. One internet access serves two. The more of these things we share, the stronger our bond is, if for no other reason than it’s a pain in the ass to move out and separate everything again. It’s more “coveni(val)ent” to stay together.
**Ionic bonds **, of course, are when one person in the relationship starts giving up little pieces of him or herself to support the other one. While dysfunctional, these bonds are as strong as or stronger than covalent bonds, but the more pieces of yourself you give away, the less like your original self you look and act. And, in the end, the selflessness of the relationship is “i®onic”, because the original partners are so changed.
Props out to Prof. Nichols. He rocks.
No simple answer is ever “quite right.” Actually, the skin cells you leave are dead cells since I think virtually the entire exterior layer of our skin is dead and shedding all of the time. The bond between cells is already essentially broken and the pieces are ready to fall off.
However, leaving cells behind or not, it is the distortion of the flesh by the electric field repulsion consequent nerve impulses that constitutes the sense of touch. And the nerves in the finger tips are exceedingly sensitive. I’ve been lightly brushing my finger tips across the desk and I’m not sure that “distortion of the flesh” is a good description because I can feel the contact even though I don’t think there is any distortion. If I had any fingerprint ridges left I would say that they were being distorted but I have very few ridges left.
AH! Far out! Thanks!