This is the most interesting debate I’ve ever seen in GQ! 
Scott, to answer your latter questions, at least, the reason people care about what Feynman thought is for a couple of reasons… One is that he’s one of those god figures in physics, like Einstein and Dirac and so forth, so his opinion naturally carries a lot of weight, and I think more importantly, he’s one of the few physicists who wrote about complicated theories in a way to make them accessible to the layperson. Many physicists, and I’m as guilty of this as anyone, have a tendency not to look far enough behind the mathematics that the simple physical picture emerges, in part because by the time you get to QM, of course, the physical picture is anything but simple to begin with.
Still working on trying to give a useful and concise definition of exactly what is meant by the terms “particle” and “wave.”
Oh, and Lib, I agree completely!
I don’t think you can do it. In the every day macro world these terms have a very definite and intuitive meaning, but in the micro world they do not. The reality of the situation is that we don’t know (in non mathematical / non macro terms) what a sub atomic particle is, and we probably never will.
We have math that accurately predicts an experimental outcome. But precisely interpreting the math in everyday language is impossible. We don’t have an intuitive knowledge of the quantum world and therefore we don’t have the language or concepts necessary to accomplish the task. The best we can do is string together a bunch of imprecise analogies that sort of get the point across.
Well, of course I wouldn’t try to give a definition of particle and wave with reference to the quantum world, because as has been said by most of us, these aren’t really terrifically accurate concepts in that realm. I’m just having a hard time finding even the appopriate words to give a simple, correct, and concise definition in the macro world… It’s one of those “fer Chrissake’s, I know one when I see one and so should everyone else” sort of things.
I’ll note in passing, by the way, that I’m not sure I would even be willing to define particle, now that I think about it, since at least in my mind, a particle is inherently part of the microscopic world and hence would be quantum mechanical from the beginning.
Well, we got sidelined into this by both me and scotth relying on Feynman as gospel. I’m therefore going to do the irrational move of a long post about how Feynman’s book QED is both deeply interesting and possibly misleading.
Having thought about it a bit more more, I now think that Feynman was doing something rather more subtle and complicated in the book than he let’s on in the passages scotth has quoted. This is going to be a bit roundabout, so bear with me.
By way of background, sum-over-histories is one example of a quantisation procedure. This is a set of rules that tells you how to take a classical system and derive its quantum equivalent. The system might involve just particles, in which case you’re doing quantum mechanics, or it might involve fields, in which case you’re into QFT. Any half-decent quantisation procedure should apply to either. Very roughly, sum-over-histories quantises a particle as follows: for all classical paths it assigns an amplitude (in practice, a precise complex number) and you sum over all these to find the amplitude you’re interested in. That then gives you the probability for the process.
The philosophical problems start when you ask what any of this means. How do we relate the mathematical formalism to real things? Now as a general observation, when people argue about interpretations of QM, they tend to ignore quantisation procedures - I’ll outline several reasons why below. However, let’s try to take sum-over-histories as the basis for various interpretations. One possibility is “throw everything away.” Amplitudes are real, but everything else is just ficticious scaffolding there to calculate them. This is actually a pretty common default way of thinking in practice. It’s “calculate the amplitudes and don’t worry what they mean.” Or you might take the next step and decompose the summed amplitude into the amplitudes for each path. This is a very wavy picture, with amplitudes rippling all over the place, interfering with each other. All the quantum amplitudes actually exist, but not the classical particles associated with them. Finally, you can take the classical particles as really there and “attach” an amplitude to each in some way.
And so back to QED, because what Feynman does there is use this last picture as his way of explaining things. The question then is did he actually believe that such a picture was what is really going on? My guess is no. Granted, that’s largely based on my impression from the rest of his writings; for example, the passage from Character I quoted. It’s certainly possible that he changed his mind prior to delivering the lectures in New Zealand in 1983(?), but that seems unlikely. I usually think of him visuallising things in terms of the “wavy” version above. This might be me imposing myself on his texts - personally, I find the QED version a bravura bit of explanation, but too ugly for it to be the way he usually thought.
So what’s with:
I think it’s possibly a slight cheat. Note that in the first two interpretations above, we’ve introduced ficticious items, but not in the third. Now it’s an incredibly common tactic in theoretical physics to introduce concepts that are fictional, to work with in intermediate steps and then throw away. People can devote careers to such concepts. This thus becomes a terribly easy, even habitual, turn of mind amongst the professionals. But I’m not sure it’s natural for the general public. In trying to understand popularisations, they tend to latch onto everything as being of equal weight. Feynman’s trick in QED is therefore to work with the ultra-literal interpretation. This allows him to explain the formalism non-mathematically without cheating. Now he knows that his audience is likely to have some preconceptions about electrons and photons being waves, so he has to issue the blunt warning that the particles he’s using are classical. As indeed they are in all the interpretations above ; it’s just whether the classical particles involved are real or ficticious. (I’d also read a dash of showmanship into the passage. And the truly brilliant technical touch is that he’s stepped into the quantum theory to realise that light is similar to electrons, then stepped back out into a quasi-classical world where light is made up of classical photon particles and quantised that as his starting point.)
To pick up on Scott Dickerson’s point, even if Feynman did take the classical underpinnings of sum-over-histories literally, it’s far from clear that anybody else has. He’s the only original figure I can think of who’s even tried basing an interpretation of QM on a quantisation procedure. I suspect that the main reason for this is the slightly banal one that we can talk about all the flashy, philosophically interesting bits of QM/QFT using just the results of the theory. For instance, this is the normal starting point for discussing wave-particle duality: under some circumstances, we see behaviour that suggests waves, in others particles. And then there’s the fact that any quantisation procedure starts from a classical system. In interpreting QM, the usual urge - unless you’re from Copenhagen - is to avoid tying yourself to a classical realm. Finally, there’s the technical point that no particular procedure ever applies to all cases. This was Schwinger’s bugbear about sum-over-histories, that there were too many circumstances where it couldn’t be used at all. Well, that and he hadn’t invented it.
Can we stop talking about particles and waves as if they actually exist at the quantum level, or are analagous there to what we see in the “real” world. Even Feynman has said it (in 1995, for heaven’s sake)…
Observation from an amateur: I believe the Master was always very careful in his choice of words (as when first attacking the Maxwell wave theories). Light, he said, comes “in pointlike quanta of energy e”. Nothing there says or implies a particle specifically. People may have tried to comprehend him by interpreting that he was saying light is a particle, but I think he grasped wave/particle duality well before anyone else did.
Question from an amateur: where does String Theory come in all this?
This seems exceedingly unlikely as he was dead for 7 years at the time. (1995)
Sorry, the thing was published in 1995 -
Libertarian: Hear, hear! I’m sure learning a whole lot!
Bonzer:
“…he knows that his audience is likely to have some preconceptions about electrons and photons being waves…” If your reference is still to “the general public,” I can only say something between “if only!” and “you wish!” My defeasible impression is that the itty-bitty-solar-system model of the inner atom still rules among the unaccelerated masses; and, though LIGHT is supposed by them to come in waves, PHOTONS are presumed to be particles, and never the twain shall meet!
(“Gee, I can never remember…was Jesus a little baby or that guy on the cross?”)
Question: Was English Dr. Feynman’s native language?
Questionlike comment: wasn’t Feynman the same guy who propounded the view that opposite charge polarities can be understood as a consequence of reversed motion in time? (My bad bald summary, of course.)
Just to be usefully clear, my question “…what is to be meant by the terms ‘particle’ and ‘wave’…” is a request for something like a general operational-type definition, not something along the fruitless lines of “So, what is a particle really made of, guys?”
I suppose I’m asking:
>Are we really on the same page with these terms that we are arguing over? (Evidently not, since I think of “particle” as something like a grain of sand or salt.)
>If not, why not?
>And isn’t it kind of an unfortunate choice to controvert with some passion, prior to establishing a commonly shared definition of the relevant terms?