# walking through wallsDecember 5, 2009 6:56 PM   Subscribe

Why don't I fall through chairs?

Physics geeks needed. A friend and I, who both have dangerously little knowledge of physics, are trying to determine why solid objects can't pass through each other (given that atoms are mostlÝ 'empty space'). He says that the answer involves virtual photons and the Pauli Exclusion Principle. I say there must be a simpler, classical reason (Occam's razor and all). Which one of us is less wrong?
posted by bbuda to Science & Nature (15 answers total) 7 users marked this as a favorite

Electromagnetic force

"Electromagnetic force interactions are determined by electric charge. The reason you don't fall through your chair while reading this is that the negative charge of the atomic electron shells making up your body are repelled by the negative charge of the electron shells making up the chair beneath you. Photon waves diminish in strength according to the square of the distance of their source." -http://www.wisegeek.com/what-are-the-four-fundamental-forces-of-nature.htm
posted by CarolynG at 7:10 PM on December 5, 2009

Pretty much every force you feel is the electromagnetic force. Of the 4 forces, strong and weak nuclear are inside of atoms so you'll never feel them. Gravity you don't really feel directly, you feel the floor pushing back. Everything else is electromagnetism.
posted by DU at 7:11 PM on December 5, 2009 [4 favorites]

Richard Feynman discusses this very thing about 4 minutes into this video.
posted by Knappster at 7:21 PM on December 5, 2009

I'll give it a shot: You are less wrong.

You are not walking "through" the atoms/molecules in air; nor are you swimming through them in water - you are displacing or pushing them out of the way, because the strength of the bonds between the atoms/molecules of a of a gas are weaker than those of liquids, which in turn are weaker than those of a solid. The atoms in a solid are densely packed and therefore tightly bound to each other, either in a regular crystal lattice, or in an amorphous structure.

A solid placed into water displaces rather than passes though the molecules. Placing two solids of varying density next to each other does not permit one to pass "through" the atoms of the other until you break the bonds between the molecules. You break the bonds of a solid and you displace it as well - you do not pass though the empty spaces of the atoms.
posted by minimii at 7:30 PM on December 5, 2009 [4 favorites]

minimii has it. Your friend is wrong.
posted by phrontist at 8:58 PM on December 5, 2009

The paradox that arises here seems to be that atoms have a lot of "empty space"--so why can't two atoms just sort of pass through one another, as long as the particles of one pass through the empty space of the other?

As outlined above, the solution to that apparent paradox is that the empty space isn't nearly as empty as it seems. It's not filled with particles or matter, but it is filled with forces--very strong forces that make it very, very hard for a stray particle--or, say, an atom that is part of you--to impinge on all that supposedly empty space.

Very strong forces are at work inside the atom--and that is, in fact why there is so much "empty space" in the atom in the first place, because different particles are repelling and attracting each other until the whole system reaches an equilibrium.

As mentioned above, the main repelling force we encounter in everyday life is the electromagnetic force, and if (for instance) the electromagnetic repulsion between one electron and another suddenly became much less, then the electrons in an atom would pack down much, much tighter. It's exactly that electromagnetic repulsion that makes the atom's electron shell so much larger than the nucleus and gives the atom so much "empty space".

So when you start to sit down on the chair, the atoms on the surface of your body get close to the atoms on the surface of the chair and try to impinge into their space.

But the very same forces that keep the space within the atom empty also work to keep you and your atoms out of that space.

I know that's all sort of vague & handwavy, but it may be helpful in a conceptual way to think that the same basic forces that are creating such a large amount of "empty space" in atoms & molecules are also the same forces that keep other atoms and particles from easily impinging on that space.

Even though there is a lot of space, you can't go there . . .
posted by flug at 10:21 PM on December 5, 2009

Best answer: You're both right. Virtual photons are probably a distraction from understanding this, but the Pauli exclusion principle is kind of why atoms take up space. Electrons are pointlike particles but they still manage to occupy a volume, in the sense that pushing another electron into that volume takes energy.

You can kind of understand things with a classical Rutherford model atom, as long as you don't worry too much about the details.
posted by hattifattener at 11:01 PM on December 5, 2009

(Hmm, I had to poke around a bit, but I think you're more right. The exclusion principle gives atoms their bulk, but atoms repel each other by classical electrostatic means, unless you're living on a white dwarf.)
posted by hattifattener at 11:11 PM on December 5, 2009

Best answer: "answer involves virtual photons and the Pauli Exclusion Principle. I say there must be a simpler, classical reason"

I wouldn't say your friend is wrong at all, simply that he is thinking on a much deeper and more basic level.

The simple classical explanation why one atom doesn't pass through another is that at large distances, atoms are electrically neutral (the positive charge of the nucleus balances out the negative charge of the electron shell).

At atomic-scale distances, however, the fact that the electromagnetic force decreases by the square of distance means that when one atom approaches another, the negative charges of the two electron shells dominate the interaction, because at these distances they are far closer to each other than the positively charged nuclei.

Thus the two atoms repel each other--but only when they are close enough to be "touching" at an atomic scale.

And you can work out all the mechanics of this process just by knowing the charges, the positions and distances involved, and how the electromagnetic force works (Coulomb's Law).

* The electromagnetic force itself is best explained by a continual exchange of "virtual photons" between the two particles exchanging the force.

So yeah, when your rear end gets close to the chair, suddenly the electrons in your atoms & those of the chair's atoms are exchanging virtual photons like crazy. That's the underlying mechanism behind every electromagnetic interaction--like every time two atoms or electrons or protons in the universe interact.

* When two atoms come close to each other, why can't their two electron clouds just sort of blend together? The reason the electron clouds take up so much space in the first place, and the reason more electrons can't just willy-nilly fit into them is summed up via the Pauli Exclusion Principle. The Principle involves the wacky quantum properties of particles that become very important at the atomic and sub-atomic level, and it together with the electromagnetic repulsion between electrons and some other factors determine how an atom's electron shell functions and the different states it can assume.

So your roommate is right at least in the sense that the Pauli Exclusion Principle helps explain why atoms take up so much space in the first place and (by extension) why the electrons of second atom can't just horn in on that same space as the second atom 'passes through' the first atom.
posted by flug at 11:12 PM on December 5, 2009 [1 favorite]

By the way, if you want to get a deeper understanding about how the electromagnetic force works I can't recommend Richard Feynman's lectures on Quantum Electrodynamics highly enough. (Previously on MeFi.)

He explains all the basic interactions (photon-photon, electron-electron, electron-photon) in a way that's pretty understandable if you have any background in science/math at all.

Those interactions form the basis for most everyday phenomena--everything from light to radio waves to chemistry to why you don't fall through the floor with every step--so it's definitely worthwhile getting a basic grasp about how these things work.
posted by flug at 11:23 PM on December 5, 2009

Of course, Buckaroo Banzai did demonstrate that the Oscillation Over-thruster can overcome the above limitations.
posted by FuManchu at 12:49 AM on December 6, 2009

On an atomic and subatomic scale, things don't have well-defined edges any more.

To improve your intuition about what things without well-defined edges are like, grab a pair of bar magnets, grip them so both their north-seeking poles are buried in your fists and their south-seeking poles are sticking out, close your eyes, and feel out the shapes of the repelling magnetic fields.

If the magnets are half-decent, you will be very nearly unable to make them touch each other. Instead, you'll feel like you're holding a couple of squashy, springy, slippery things that push back harder the closer they get.

Open your eyes. Now, what's between those magnets is clearly mostly empty space, right? But according to your hands, that space has a shape.

Electron orbitals work pretty much the same way when you start pushing them close to one another (the forces are electrostatic, not magnetic, but roughly the same relationship between force and distance applies). Even though the electrons themselves are tiny, the space around them has a squashy, springy, slippery shape that's perfectly capable of pushing on something similar when it gets close enough.
posted by flabdablet at 1:41 AM on December 6, 2009 [3 favorites]

I like flug's explanation. Starting with Coulomb's law (the electric force between two charges diminishes as the square of the distance) and a model for atoms that have positively charged centers, a bunch of empty space, and then a negatively charged electron shell at the outside, you can argue that when two surfaces are far enough apart, the overall neutrality of the atoms that make up the surfaces means virtually no net force on the objects. However, for surfaces that are sufficiently close to one another, the proximity of the negatively charged outer surfaces becomes the dominant effect -- the repulsion of the electrons on the outer surface of the chair and the electrons on the outer surface of your body increase as you move closer to the chair, and when you are at the right distance such that this repulsion exactly balances your weight you stop moving downward.

Your friend's take on this is the next layer in the onion skin -- an attempt to explain how these electrons repel each other in the first place. (Coulomb's law accurately describes the important features of electric interactions, but is not an explanation of the cause of this interaction. In the case of gravitation, Isaac Newton recognized the utility of having a description even if you didn't know the cause, and sidestepped challenges to explain how gravitation came about.) A modern description of interactions between two particles that exert a force on each other involves an exchange of mediating particles. In the case of electromagnetic forces, these particles are photons. So if you take any pair of electrons from the two surfaces, the repulsion between them is the result of an exchange of photons.
posted by Killick at 8:55 AM on December 6, 2009

What's in a Touch
posted by Rhomboid at 11:05 AM on December 6, 2009

This is a question with several distinct, correct, and only partially overlapping answers, some of which are above. My short answer is that it's mostly the exclusion principle, and that virtual photons are a computational device that you don't need for this problem. I think that "atoms are mostly empty space" is a classical statement about atoms. Atoms, like everything else that comes in lumps, are quantum mechanical, and making classical statements about quantum-mechanical things is a dangerous game.

The assertion that an atom is mostly empty space comes from thinking in a Rutherford-like way about atoms and their constituents. Someone unaware of quantum mechanics (like everyone in the world in 1910) might imagine that nuclei and electrons are sort of like bricks that can be combined into atoms. How big are the bricks? An atom in a crystal or a nearly-ideal gas occupies a volume two or three angstroms across. In order for a fast alpha particle to scatter backwards from a single atom, the atom's mass and charge must be concentrated in a nucleus perhaps ten femtometers, or 0.0010 angstroms, across. And if an electron is "made of the same stuff" as a nucleus and has the same mass density, it would have to be very small, maybe one-tenth of a femtometer across. The combined volume of the nucleus and the electrons is much smaller than the volume of the atom. So the atom is obviously mostly empty.

But electrons and nuclei are not like bricks: they are waves, and the "size" of the wave depends on the momentum of the associated particle in whatever interaction you use to measure it. An outermost electron is bound to its atom by an energy of a few "electron-volts," or eV. If you know the electron mass this tells you the momentum of the electron relative to the nucleus, and from there you can find its wavelength: a couple of angstroms, the "size" of the atom. The protons and neutrons in the nucleus are held together by energies of a few million eV. In the same way you can find their wavelength: it is a few femtometers, the "size" of the nucleus. The atoms in, say, a crystal of aluminum are interacting with each other with thermal energies around room temperature, which is a few milli-eV, corresponding to wavelengths somewhere under an angstrom. The exclusion principle is a statement about wave-wave interactions between identical objects. If two atoms are a few electron wavelengths apart --- as they are in a typical solid --- then the electrons from one atom overlap those from the other and the exclusion principle matters. If you could get two atoms separated by less than a few atomic wavelengths, the atoms could interact coherently and the situation might be different: non-identical objects aren't restricted by the exclusion principle, nor are bound objects whose proton, neutron, and electron numbers are all even. But since room-temperature atomic wavelengths are smaller than outermost electron wavelengths, the electrons see each other first.

You can change this state of things by making your material extremely cold. Low-energy, low-momentum objects have bigger wavelengths than high-momentum objects. If the energy of a typical thermal interaction is small enough that the atom wavelength is larger than the few-angstrom wavelength of the least-bound electrons, then the atoms do begin to act like waves and can pass through each other. This is essentially the difference between an ordinary fluid and a "superfluid" like liquid helium: superfluid helium flows without resistance because the atom-atom interaction is wavelike rather than particlelike. Helium is a very light atom, so its atomic wavelength at a given temperature is longer than in other materials. It helps that helium has two protons, two neutrons, and two electrons, and is therefore obeys Bose statistics (or "is a boson") and therefore doesn't care about Pauli exclusion. But the odd isotope of helium, which singly does follow the exclusion principle, can still become a superfluid when the wavelength for a pair of atoms is longer than the typical distance between atom pairs. This whole situation is pretty complicated and took most of the twentieth century to figure out. I have heard people talking about "supersolids," where this same sort of coherent interaction happens in a system with a crystalline long-range order, but I don't really understand that and no one who does has written about it on Wikipedia.

In a very real way, the de Broglie wavelength of an object is its size, and an atom is not mostly empty but is completely full of cold (and therefore great big) electrons. People say that electrons are pointlike because, no matter what momentum they trade with another object, electrons show no evidence of fragmenting into any different sort of object. This stability is different from atoms, which of course can be made to eject electrons; different from nuclei, which can be made to eject protons or neutrons; and different from protons or neutrons, which can be made to eject things like pions or kaons. These particles all have a momentum scale associated with the objects they can be broken into, and it's tempting to think of that as their "intrinsic" size. I think it makes sense to call this a minimum size: if you look at length scales shorter than a femtometer, whatever you see won't be protons. But an atom with a three-angstrom electron radius is just as "full" of electrons as the nucleus with a three-femtometer radius is "full" of protons. You don't fall through your chair, or through the bedrock below it, because all the few-eV electron states are already full.

aside to hattifattener: the difference between bedrock and a white dwarf is that, in a white dwarf, the gravitational interaction between an electron and the rest of the star is, on average, stronger than the electrical interaction between that electron and the nuclei that happen to be nearby.

posted by fantabulous timewaster at 8:15 AM on December 7, 2009 [2 favorites]

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