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SilasBarta comments on Open Thread: November 2009 - Less Wrong
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Semi-OT: It's discussions like these that remind me: Whenever physicists remark about how the laws of nature are wonderfully simple, they mean simple to physicists or compared to most computer programs. For most people, just looking at the list of elementary particles is enough to make their heads blow up.
Heck, it nearly does that for me!
Seriously? Dude, it's a list of names. It should no more make your head asplode than the table of the elements does, and nobody thinks that memorising those is a great feat of intellect. Are you sure you're not allowing modesty-signalling to overcome your actual ability?
Now, if you want to get into the math of the actual Lagrangians that describe the interactions, I'll admit that this is a teeny bit difficult. But come on, a list of particles?
"Antimony, arsenic, aluminum, selenium, and hydrogen and oxygen and nitrogen and rhenium..."
I followed the link Silas provided. Rather than seeing a list to be memorised my brain started throwing up all sorts of related facts. The pieces of physics I have acquired from various sources over the years reasserted themselves and I tried to piece together just how charm antiquarks fit into things. And try to remember just why it was that if I finally meet my intergalactic hominid pen pal and she tries to shake hands with her left hand I can be sure that shaking would be a cataclysmic-ally bad idea. I seem to recall being able to test symmetry with cobalt or something. But I think it's about time I listened to Feynman again.
Point is, being able to find the list of elementary particles more overwhelming than, say, a list of the world's countries requires a certain amount of knowledge and a desire for a complete intuitive grasp. That's not modesty-signalling in my book.
Everyone knows what a country is. Few people know what the term "elementary particle" means. (It's not a billiard ball.)
It's not a billiard balls from the movie they showed? Then surely 'elementary particles' must refer to those things on the Table of the Elements that was on the wall!
I have a metaphorical near-head-explosion for different reasons than the average person that I was referring to. For me, it's mainly a matter of the properties shown on the chart being more abstract and not knowing what observations they would map to (as wedrifid noted in his signaling analysis...).
Compared to the Periodic Table, elementary particle chart also has significantly less order. With the PT, I may not know each atomic mass number, but I know in which direction it increases, and I know the significance of its arrangement into rows and columns. The values in the EPC seem more random.
Granted, but there are also nowhere near as many of them. Besides, fermion mass increases to the right, same as in the PT; charge depends only on the row; and spin is 1/2 for all fermions and 1 for all bosons. This is not very complicated.
I would also suggest that the seeming randomness is a sign you're getting closer to the genuinely fundamental stuff: The order in the periodic table is due to (using loose language) repeated interactions of only a few underlying rules - basically just combinations of up and down quarks, with electrons, and electromagnetic interactions only.
Nu, mass and charge are hardly abstract for someone who has done basic physics; that leaves spin, which just maps to the observation that a beam of electrons in a magnetic field will split into two. (Although admittedly things then get a bit counter-intuitive if you put one of the split beams through a further magnetic field at a different angle, but that's more the usual QM confusion.)
Alright! Point taken! The chart is less daunting than I thought. You mind loosening your grip on my, um, neck? ;-)
An especially good point -- maximally compressed data looks like random noise, so at the fundamental level, there should be no regularity left that allows one entry to tell you something about another.
Oh, a bit off topic, but mind clarifying something for me? My QFT knowledge is very limited at the moment, and I'm certainly not (yet) up to the task of actually trying to really grasp the Standard Model, but...
Is it correct to say that in a sense the force carriers are, in a sense, illusory? That is, the gauge bosons are kind of an illusion in the same sense that the "force of gravity" is? From what little I managed to pick up, the idea is that instead one starts without them, but assigns certain special kinds of symmetries to the configuration space. These local (aka) gauge symmetries allow interference effects that basically amount to the forces of interaction. One can then "rephrase" those effects in a way that more looks like another quantum field interacting with, well, whatever it's interacting with?
ie, can the electromagnetic, strong, and weak forces (as forces) be made to go away and turn into symmetries in configuration space in the same sense that in GR, the force of gravity goes away and all that's left is geometry of spacetime?
Or have I rolled a critical fail with regards to attempting to comprehending the notion of gauge fields/bosons?
Thanks. Again, I know it's a slight tangent, but since the subject of the Standard Model came up anyways...
Ok, I'm not touching the ECE thing; as noted, I'm not a theorist. I just measure stuff. I've taken classes in formal QFT, but I don't use it day-to-day, so it's a weak point for me. However, it seems a bit odd to describe things that can be produced in collisions and (at least in principle) fired at your enemies to kill them by radiation poisoning as 'illusory'. If you bang two electrons together, measuring the cross-section as a function of the center-of-mass energy, you will observe a classic 1/s decline interrupted by equally classic resonance bumps. That is, at certain energies the electrons are much more likely to interact with each other; that's because those are the energies that are just right for producing other particles. Increase the CM energy through 80 GeV or so, and you'll find a Breit-Wigner shape like any other particle; that's the W, and if it weren't so short-lived you could make a beam of them to kill your enemies. (With asymmetric electron energies you can produce a relativistic-speed W and get arbitrarily long lifetimes in the lab frame, but that gets on for being difficult engineering. In fact, just colliding two electrons at these energies is difficult, they're too light; that's why CERN used an electron and a proton in LEP.)
Now, returning to the math, my memory of this is that particles appear as creation and annihilation operators when field theories with particular gauge symmetries are quantized. If you want to call the virtual particles that appear in Feynmann diagrams illusory, I won't necessarily argue with you; they are just a convenient way of expressing a huge path integral. But the math doesn't spring fully-formed from Feynmann's brow; the particular gauge symmetry that is quantised is chosen such that it describes particles or forces already known to exist. (Historically, forces, since the theory ran ahead of the experiments in the sixties - we saw beta decay long before we saw actual W bosons.) If the forces were different, the theorists would have chosen a different gauge symmetry and got out a different set of particles.
I'm not sure if I'm answering your question, here? My basic approach to QFT has always been "shut up and calculate", not because of QM confusion but because I find it very confusing when someone says that a particular mathematical operation is "causing" something. I prefer to think of the causality as flowing from the observations, so that the sequence is thus:
I wasn't bringing up the ECE thing.
I meant illusory in the same sense that "sure, the force of gravity can cause me to fall down and get ouchies... but by a bit of a coordinate change and so on, we can see that there really is no 'force', but instead that it's all just geometry and curvature and such. Gravity is real, but the 'force' of gravity is an illusion. There's a deeper physical principle that gives rise to the effect, and the regular 'force' more or less amounts to summing up all the curvature between here and there."
My understanding was that gauge bosons are similar "we observe this forces/fields/etc... but actually, we don't need to explicitly postulate those fields as existing. Instead, we can simply state that these other fields obey these symmetries, and that produces the same results. Obviously, to figure out which symmetries are the ones that actually are valid, we have to look at how the universe actually behaves"
ie, my understanding is that if you deleted from your mind the knowledge of the electromagnetic and nuclear forces and instead just knew about the quark and lepton fields and the symmetries they obeyed, then the forces of interaction would automatically "pop out". One would then see behaviors that looks like photons, gluons, etc, but the total behavior can be described without explicitly adding them to the theory, but simply taking all the symmetries of the other stuff into account when doing the calculations.
That's what I was asking about. Is this notion correct, or did I manage to critically fail to comprehend something?
And thanks for taking the time to explain this, btw. :) (I'm just trying to figure out if I've got a serious misconception here, and if so, to clear it up)
I guess you can think of it that way, but I don't quite see what it gains you. Ultimately the math is the only description that matters. Whether you think of gravity as being a force or a curvature is just words. When you say "there is no force, falling is caused by the curvature of space-time" you haven't explained either falling or forces, you've substituted different passwords, suitable for a more advanced classroom. The math doesn't explain anything either, but at least it describes accurately. At some point - and in physics you can reach that point surprisingly fast - you're going to have to press Ignore (being careful to avoid Worship, thanks), at least for the time being, and concentrate on description rather than explanation.
Well, my question could be viewed as about the math. ie: "does the math of the standard model have the property that if you removed any explicit mention of electromagnetism, strong force, or weak force and just kept the quark and lepton fields + the math of the symmetries for those, would that be sufficient for it to effectively already contain EM, strong, and weak forces?"
And as far as gravity being force or geometry, uh... there's plenty of math associated with that. I mean, how would one even begin to talk about the meaning of the Einstein field equation without interpreting it in terms of geometry?
Perhaps there is a deeper underlying principle that gives rise to it, but the Einstein field equation is an equation about how matter shapes the geometry of spacetime. There's no way really (that I know of) to reasonably interpret it as a force equation, although one can effectively solve it and eventually get behaviors that Newtonian gravity approximates (at low energies/etc...)
(EDIT: to clarify, I'm trying to figure out how to semivisualize this. ie, with gravity and curvature, I can sorta "see" and get the idea of everything's just moving in geodesics and the behavior of stuff is due to how matter affects the geometry. (though I still can only semi "grasp" what precisely G is. I get the idea of curvature (the R tensor), I get the idea of metric, but the I currently only have a semigrasp on what G actually means. (Although I think I now have a bit of a better notion than I used to). Anyways, loosely similar, am trying to understand if the fundamental forces arise similarly, rather than being "forces", they're more an effect of what sorts of symmetries there are, what bits of configuration space count as equivalent to other bits, etc...)
I guess I'm not enough of a theorist to answer your question: I do not know whether the symmetries alone are sufficient to produce the observed particles. My intuition says not, for the following reason: First, SU(3) symmetry is broken in the quarks; second, the Standard Model contains parameters which must be hand-tuned, including the electromagnetic/weak separation phase that gives you the massless photon and the very massive weak-force carriers. Theories which spring purely from symmetry ought not to behave like that! But this is hand-waving.
As an aside, I seem to recall that GR does not produce our universe from symmetries alone, either; there are many solutions to the equations, and you have to figure out from observation which one you're in.
If you like, I can quote our exchange and ask some local theorists if they'd like to comment?
What is the difference between saying gravity is a force and saying it's a curvature of spacetime?
What is your definition of "a force" that makes it inapplicable to gravity? Is electromagnetism a force, or is it a curvature in the universe's phase space?
I don't know much about physics, please enlighten me...
To say that gravity is a curvature of spacetime means that gravity "falls out of" the geometry of spacetime. To say that gravity is something else (e.g., a force) means that, even after you have a complete description of the geometry of spacetime, you can't yet explain the behavior of gravity.
Isn't it equally valid to say that the geometry of spacetime falls out of gravity? I.e., given a complete description of any one of them, you get the other for free.
What is a force by your definition? Something fundamental which can't be explained through something else? But it seems to me that "the curvature of spacetime" is the same thing as gravity, not a separate thing that is linked to gravity by causality or even by logical necessity. They're different descriptions of the same thing. So we can still call gravity a fundamental force, it's not being caused by something else that exists in its own right.
...but forces fall out of something - electromagnetic interactions, for example. As an engineer, I am inclined to call something a force if it goes on the "force" side of the equation in the domain I'm modeling, and not worry about whether to call it "real".
(Then again, as an engineer, I rarely need to exceed Newtonian mechanics.)