I thought this video was a really good question dissolving by Richard Feynman. But it's in 240p! Nobody likes watching 240p videos. So I transcribed it. (Edit: That was in jest. The real reasons are because I thought I could get more exposure this way, and because a lot of people appreciate transcripts. Also, Paul Graham speculates that the written word is universally superior than the spoken word for the purpose of ideas.) I was going to post it as a rationality quote, but the transcript was sufficiently long that I think it warrants a discussion post instead.
Here you go:
Interviewer: If you get hold of two magnets, and you push them, you can feel this pushing between them. Turn them around the other way, and they slam together. Now, what is it, the feeling between those two magnets?
Feynman: What do you mean, "What's the feeling between the two magnets?"
Interviewer: There's something there, isn't there? The sensation is that there's something there when you push these two magnets together.
Feynman: Listen to my question. What is the meaning when you say that there's a feeling? Of course you feel it. Now what do you want to know?
Interviewer: What I want to know is what's going on between these two bits of metal?
Feynman: They repel each other.
Interviewer: What does that mean, or why are they doing that, or how are they doing that? I think that's a perfectly reasonable question.
Feynman: Of course, it's an excellent question. But the problem, you see, when you ask why something happens, how does a person answer why something happens? For example, Aunt Minnie is in the hospital. Why? Because she went out, slipped on the ice, and broke her hip. That satisfies people. It satisfies, but it wouldn't satisfy someone who came from another planet and who knew nothing about why when you break your hip do you go to the hospital. How do you get to the hospital when the hip is broken? Well, because her husband, seeing that her hip was broken, called the hospital up and sent somebody to get her. All that is understood by people. And when you explain a why, you have to be in some framework that you allow something to be true. Otherwise, you're perpetually asking why. Why did the husband call up the hospital? Because the husband is interested in his wife's welfare. Not always, some husbands aren't interested in their wives' welfare when they're drunk, and they're angry.
And you begin to get a very interesting understanding of the world and all its complications. If you try to follow anything up, you go deeper and deeper in various directions. For example, if you go, "Why did she slip on the ice?" Well, ice is slippery. Everybody knows that, no problem. But you ask why is ice slippery? That's kinda curious. Ice is extremely slippery. It's very interesting. You say, how does it work? You could either say, "I'm satisfied that you've answered me. Ice is slippery; that explains it," or you could go on and say, "Why is ice slippery?" and then you're involved with something, because there aren't many things as slippery as ice. It's very hard to get greasy stuff, but that's sort of wet and slimy. But a solid that's so slippery? Because it is, in the case of ice, when you stand on it (they say) momentarily the pressure melts the ice a little bit so you get a sort of instantaneous water surface on which you're slipping. Why on ice and not on other things? Because water expands when it freezes, so the pressure tries to undo the expansion and melts it. It's capable of melting, but other substances get cracked when they're freezing, and when you push them they're satisfied to be solid.
Why does water expand when it freezes and other substances don't? I'm not answering your question, but I'm telling you how difficult the why question is. You have to know what it is that you're permitted to understand and allow to be understood and known, and what it is you're not. You'll notice, in this example, that the more I ask why, the deeper a thing is, the more interesting it gets. We could even go further and say, "Why did she fall down when she slipped?" It has to do with gravity, involves all the planets and everything else. Nevermind! It goes on and on. And when you're asked, for example, why two magnets repel, there are many different levels. It depends on whether you're a student of physics, or an ordinary person who doesn't know anything. If you're somebody who doesn't know anything at all about it, all I can say is the magnetic force makes them repel, and that you're feeling that force.
You say, "That's very strange, because I don't feel kind of force like that in other circumstances." When you turn them the other way, they attract. There's a very analogous force, electrical force, which is the same kind of a question, that's also very weird. But you're not at all disturbed by the fact that when you put your hand on a chair, it pushes you back. But we found out by looking at it that that's the same force, as a matter of fact (an electrical force, not magnetic exactly, in that case). But it's the same electric repulsions that are involved in keeping your finger away from the chair because it's electrical forces in minor and microscopic details. There's other forces involved, connected to electrical forces. It turns out that the magnetic and electrical force with which I wish to explain this repulsion in the first place is what ultimately is the deeper thing that we have to start with to explain many other things that everybody would just accept. You know you can't put your hand through the chair; that's taken for granted. But that you can't put your hand through the chair, when looked at more closely, why, involves the same repulsive forces that appear in magnets. The situation you then have to explain is why, in magnets, it goes over a bigger distance than ordinarily. There it has to do with the fact that in iron all the electrons are spinning in the same direction, they all get lined up, and they magnify the effect of the force 'til it's large enough, at a distance, that you can feel it. But it's a force which is present all the time and very common and is a basic force of almost - I mean, I could go a little further back if I went more technical - but on an early level I've just got to tell you that's going to be one of the things you'll just have to take as an element of the world: the existence of magnetic repulsion, or electrical attraction, magnetic attraction.
I can't explain that attraction in terms of anything else that's familiar to you. For example, if we said the magnets attract like if rubber bands, I would be cheating you. Because they're not connected by rubber bands. I'd soon be in trouble. And secondly, if you were curious enough, you'd ask me why rubber bands tend to pull back together again, and I would end up explaining that in terms of electrical forces, which are the very things that I'm trying to use the rubber bands to explain. So I have cheated very badly, you see. So I am not going to be able to give you an answer to why magnets attract each other except to tell you that they do. And to tell you that that's one of the elements in the world - there are electrical forces, magnetic forces, gravitational forces, and others, and those are some of the parts. If you were a student, I could go further. I could tell you that the magnetic forces are related to the electrical forces very intimately, that the relationship between the gravity forces and electrical forces remains unknown, and so on. But I really can't do a good job, any job, of explaining magnetic force in terms of something else you're more familiar with, because I don't understand it in terms of anything else that you're more familiar with.
Thank you for the transcript. I can read things a lot faster than most people say them, so I almost never listen to podcasts or watch, say, TED talks, because I get really frustrated with how slow they are.
In this case, Feynman is worth listening to slowly. There is something about the way he explains this that the transcript does not do justice to.
There are various tools for speeding up audio while leaving it listenable. The tool to use depends on your platform, but you can find quite a few options by Googling.
I use VLC to listen to audio and watch videos at about 2x speed. VLC has keyboard shortcuts to change the speed and pitch gets adjusted automatically. Keypad + or - to change speed in increments of 0.5x and "[" or "]" keys for increments of 0.1x
You can't skim audio either. But, yes, that might help. (I just like text, I guess.)
I greatly appreciate transcripts. Thanks a lot. And this particular one is really interesting.
But I don't think it's an example of dissolving the question. He mainly seems to be (1) explaining why a topic is too complicated to talk about with precision to a layperson, and (2) giving names to the concepts that are the most complicated.
Question-dissolving is what to do when you're confused, not just when you don't know something. Confusion is what tells us there's something wrong with the question; it's a matter of the map, not the territory.
I think he's implicitly answering a more important meta-question: when people ask "why", they're usually wanting a narrative explanation, and so scientific explanations are often found unsatisfying.
As I understand it, Feynman's tentative explanation for why ice is slippery (which he himself qualified with "they say") has since fallen out of favor. This isn't to quibble with Feynman -- if anything, the point he alludes to here and emphasizes in a lot of other works is that science is a continuing process, always updating itself, and individual scientists are not prophets or oracles.
I don't think his explanation for why a chair pushes back on your hand is quite right, either. I've mostly been told that material solidness comes from the Pauli exclusion principle, not electrostatic repulsion.
I don't know quantum mechanics, so I don't have a good perspective on the problem, but the electrostatic explanation has always seemed lacking to me. The electric charge in a neutral atom is fairly well-approximated by a symmetric sphere of negative charge with a bunch of positive charge at the center, so two atoms shouldn't experience much electrostatic repulsion until their electron clouds overlap. At which point [I've heard] the PEP should dominate the electrostatic force.
Can any physicists or physics students comment?
Both the Pauli exclusion principle and electrostatic repulsion contribute. There is a brief discussion of this on Wikipedia, which cites the work of Freeman Dyson.
I guess Feynman includes the Pauli principle as electric force. Remember, he got a Nobel prize for this stuff.
It would not surprise me if he just didn't want to start talking about quantum exchange interactions in response to an interview question about how magnets work. Electrostatic repulsion does count for some of the effect of solidity, so his answer wasn't wrong so much as incomplete. That was the point of his entire discussion: there are many different levels on which we can answer a "why" question.
The Pauli principle isn't a force at all. It's a symmetry, or at least a special case of one. Also, it has nothing to do what causes magnets to repel. If you violated the symmetry, magnets would still work.
I'm not sure the distinction between a force and a symmetry is a useful one. Any use of "force" in modeling physics can be equivalently expressed via conservation of linear momentum, which itself is equivalent to the fact that the laws of physics are symmetric in translation (i.e. translation-invariant, i.e. have the same form when the origin of the coordinate system everything is expressed in is moved around).
Literally, for any force, you can say, "that's just the playing out of a necessary symmetry in the laws of physics".
I don't see how. Either you're misunderstanding something, or you have a higher background in quantum mechanics than I do (I've had one in-depth class, and I've read the quantum physics sequence), and it works out like this for reasons I do not currently understand. Which is it?
In any case, force is clearly defined in the simplified version of quantum physics I've learned. It's the gradient of potential energy, which must be specified in the Schroedinger equation. The Pauli principle is not a force. It may be that force is always due to symmetry, in which case calling the Pauli principle a symmetry doesn't separate them at all, but the Pauli principle is still not a force.
Indeed, conservation laws correspond to symmetries: http://en.wikipedia.org/wiki/Noether%27s_Theorem
That has a lot of explanatory power for why I linked Noether's Theorem the first time around.
I must have missed that. Sorry.
I must have missed that. Sorry.
Conservation laws are not forces. There are hypothetical patterns of force that would not conserve these things, but the way things normally move is not the only one. For example, if there were no forces, all the conservation laws will still work.
Also, from what I understand, that's more a symmetry in the laws themselves, where the Pauli principle is a symmetry in the waveform being operated on.
The Pauli principle is not a force in the sense that gravity is not force. Yes, you can distinguish between a "force" and the phenomenon responsible for the force (gravity vs gravitational force). What is the difference between these two statements?
1) That's not a force, it's the playing out of the fundamental symmetries in quantum physics, normally phrased here as the Pauli exclusion principle.
2) There's no force on that falling object in a vacuum, it's just following the geodesic dictated by the symmetries in General Relativity.
PEP is not a force, in the sense that it's not 'dynamical:' it can't actually affect the Hamiltonian/Lagrangian of the world. And it's not a symmetry either, it's a consequence of the behaviour of 'fields' under rotations: see spin-statistics theorem. (Explanation of the field business: modern physics postulates that at every point in space and time there are a certain number of degrees of freedom, and we call them fields and 'quantising' gives us particles - and particles are just spatially localised excitations when you don't look closely at them.)
The rest of the forces, however, do come from symmetries called local gauge symmetries; roughly, since the wavefn is a complex no, change the phase by some amount which depends on the point and then requiring that physics be invariant under this. (Even gravity, though only in classical field theory as of now: it can be found by a Lorentz transformation by a different amount at every point.)
This explanation is horrible, so sorry; but on the bright side, the math is simple enough that you may actually understand wikipedia on these things.
Gravity can be interpreted as a force. To my knowledge, the Pauli principle cannot.
I'm not a physicist, but when I looked into this, I found this well-written article:
The Stability of Matter: From Atoms to Stars
It goes into lots of detail of what's happening with a single hydrogen atom, then a large atom, then bulk matter. It doesn't require quantum physics knowledge from a reader, but it does require mathematical maturity, and isn't easy reading.
The short of it is that you're right, the Pauli exclusion principle is more important than electrostatic repulsion.
Thanks! I love docs like these, that take the a broad approach.
I note Fyenman above is quoted as saying, "There's other forces involved, connected to electrical forces" in the context. One can quibble about how "connected" the Pauli exclusion principle is to electrical forces, but he is explicitly acknowledging there's more going on than just the immediate results of the electromagnetic force, in the midst of an extemporaneous explanation as to what's going on with magnets.
It's mostly this stuff. Dispersion forces involve both pauli exclusion and the electric force, working in sweet harmony. Which is the one that actually pushes the heavy nuclei around and stops your hand? The electric force.
I have never been taught this in particular, but it seems unlikely that the Pauli exclusion principle could do it. It's a symmetry, not a force.
From what I understand, if you sent two fermions at each other, assuming they don't otherwise repel, they'd just pass through each other. The Pauli principle would merely guarantee that they do so at an anti-node. You'd never find them at the same spot. You also wouldn't find them at any other anti-nodes that appear along their trajectories, or more accurately, their joint trajectory in configuration space, or still more accurately, their joint waveform in configuration space. In any case, their momentum and energy would be completely unaffected by this.
The Pauli principle might be why electrons end up in a pattern in which they repel each other so well, but I don't see what else it can do.
If I'm wrong, please correct me, and send me somewhere where I can read more about how it works.
Pauli exclusion holds neutron stars and atomic nuclei apart. ie. much denser than atomic contact.
Even with the clouds overlapping, I think it's mostly electromagnetic. They are too sparse for exclusion to be significant.
To get any deeper, we would need someone who understands the source and mechanism of exclusion.
The Pauli exclusion principle applies to all fermions, including both electrons and nucleons. The PEP for nucleons is what keeps neutron stars from collapsing (normally). But the PEP for electrons keeps electron clouds from overlapping (much).
On Hacker News: http://news.ycombinator.com/item?id=3440423
Typo: "undue" for "undo".
Another benefit of transcription: I can pretend the interviewer is Joseph Utsler.
Then what's even the point of studying physics if we can't get answers to the 'why' questions and just have to accept the fact that this is what happens in nature? And if we don't ask the why questions, how would physics even advance?
Some of Feynman's response makes more sense if you watch this video.
You know what curiosity looks like? Richard Feynman.
Yeah, this is one of my favorites.
Typo: missing "knew" (inserted in brackets).
Reminds me of this. See 2m45s.
There is possibly a better-quality video by this link (which I can't see from Russia; could someone who can see it indicate if the video there is better?):
They play for me in the UK. The quality is a little better, but they're the same size.
I couldn't see it in the United States, so I figured it was only open to British people or something.