What is a particle?

Is the answer obvious?  It is perhaps not as obvious as you might think.  I was in a discussion with a colleague of mine yesterday about the “reality” of fields.  I’m not convinced that they’re anything more than a mathematical convenience championed by people wishing to sweep the “action at a distance” problem under a rug.  I can’t think of an experiment that can only be interpreted in terms of fields (most interference experiments can be interpreted statistically – let’s ignore the recent PBR paper for the moment).  He countered that the same could be said for particles.  I suppose I could concede that they could exist, but that we might not be able to prove it since it would be impossible to make a measurement of anything without involving a “particle” interpretation somewhere in the chain of interpretations leading from the measurement process to our conscious brains.  In any case, he countered my assertion in the most logical way by asking me: what is a particle?

As a foundationalist, I probably am guilty of over thinking this, but the only thing I could come up with that could differentiate a particle from a field was that a particle is a local concept whereas a field is not.  Fields have spatial extension (of course this requires the existence of space and quite possibly time as well, but that’s an argument for another day).  Particles – fundamental particles, anyway – are mere points.  Note that in my above assertion about the requirement of a particle somewhere in the interpretation chain, I make no distinction between the classical and quantum cases because, like many other quantum foundations and information folks, I take the view that the classical world is a special case of the quantum world.  As such, “spooky action at a distance” can be explained by virtual particles (for interactions) or mere correlations (for odder things like entanglement).

At any rate, I am still at a loss in terms of trying to find a better differentiation between a particle and a field that is not, in some way, self-referential (in other words, saying that a particle is the quantization of a field is, in my mind, self-referential to some extent).  However, stay tuned for a commentary on the nature of mass that might add a little twist to this whole thing.


11 Responses to “What is a particle?”

  1. The fact that wave functions allow us to predict results of the double-slit experiment simply tells us that we should abandon the naive idea that particles are small balls moving along classical trajectories. At the quantum level, the best approximations to classical particles are still Gaussian packets with imperfect localization and with phase relations. Since I prefer the MWI of QM, I view particles as packets smeared out across not only classical space, but across the multiverse as a whole, and understanding fully them entails taking all of the possible trajectories into account — but you knew I was going down this road, no? Perhaps I’m just kicking the can down the road 😉

    By the way, glad to see you on Google +

    • David Batchelor Says:

      Gordon Kane says fundamental particles are point-like, indivisible objects with no apparent substructure, and they make up composite particles like the proton (built of quarks). Even though a wave packet has some descriptive power regarding a particle’s probable motion, particles clearly don’t disperse as they pass through a cloud chamber the way wave packets do. If the wave packet is a field, then it is not identical with the particle that it is associated with.

  2. I say overall, that many questions are not simple or obvious. The point-like nature of the particle is its collision cross-section and per field concentration etc, it is in another context than the spread of the wave function. That’s what complementarity is about, inequivalent traits in the abstract but “it depends.” People just can’t believe or accept that because of the pull of the mathematical mode – which has to take its pick which way to go.

    This impulse leads to views like MWI. I can’t accept MWI for various reasons, but not just some intuitive revulsion. But MWI leads to the mass-energy of the original particle being duplicated entire at multiple locations! Post-modernist arm-waving about decoherence etc, does not actually ameliorate the problem. Note that the Schroedinger equation describes a given amount of mass-energy, refers to momentum (and there is a “charge” as applicable) not just some abstraction of “probability of being there” which could be massaged into being “more” than before. Hasn’t that worried you, David?

    As for the particle really being like a wave – think about electrons in atoms. You don’t really think they literally go around in simple circular or elliptical orbits and are just surrounded by some “other field”? It makes more sense to think, the electron orbital clouds really are as close to being</i< "the electrons in flight" as it gets. And the charge of an electron is spread out thereby as well, not focused serially along a specific orbit. That also means, you can't have more than e of charge popping up as a result of one electron getting split into many worlds, whatever that means. Sorry to ramble, I just couldn't make it pithy.

  3. (In any case, does anyone think we can address these issues with de Broglie – Bohm mechanics? I find the idea clunky, but who knows.)

  4. Is the basis of existence binary?…

    This is an interesting and long-standing open question.  John Wheeler certainly thought the answer to that was ‘yes.’  He referred to this as ‘it from bit’ in a 1990 paper (available in Complexity, Entropy, and the Physics of Information edited by …

  5. I don’t know what a rest mass is, as some particle. I know I easily can differ between bosons and ‘matter’ in my normal life though. I can’t wade through (solid) matter but I can wade through bosons. Against that we have Einstein suggesting that you could see bosons and matter as the same thing, as I understands it that is. And then use the stress energy tensor to describe it all together, as a undivided ‘whole’ SpaceTime.

    But I still don’t know what a ‘uniform motion’ would be in a such a perspective, or any ‘motion’, accelerating or not. Are all uniform motions equivalent for example? If they are, when measured relative a observer to they differ? And how would you describe the difference, relative what?

    • quantummoxie Says:

      Good point. Heck, I don’t even know what mass is, per se. I know what we think it is, but at the quantum level things are bizarre in the least (particularly given that the vector bosons have mass). As for uniform motion, that’s another good point.

  6. Eh, ‘do’, not, ‘to they differ’ 🙂

  7. keep writing 🙂
    And tell me what you think.

    I’m interested, and I’m sure there are more than me 🙂

  8. Been thinking of ’emergences’, wondering if that idea would be applicable to particles, and ‘joined particles’ (molecules) into matter, too?

    What I recently saw as a definition of a emergence is that although you can factor in what you know about its constituents, a emergence will bring with it new behaviors, that’s more than the mass of its parts, as I understands it meaning that you will meet behaviors unable to be back-traced to the way their constituents act.

    From that one might ask what matter can produce, in form of ’emergences’, how about thoughts? (well, as a example?) Emergences are interesting concepts, reminding me of the idea of ‘synergy’

    Anyway, have fun 🙂

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