I'd love to have a real physicist explain this, but:
When we think of what a particle IS, we often think as though it were dirt, or a billiard ball, or something. As though there were some other substance of which it's made. At least I do.
But the definition is as low as you can go. It's hard to wrap your head around that. Unless you're trained to do so, I guess.
It's not quite right to call basis vectors particles, even in a free field theory. The particles correspond better to the creation / ladder operators that take you from one Hamiltonian eigenstate to another.
In a perturbative interacting case, people still think of particles as these same ladder operators, but they don't connect eigenstates so simply (the interactions generically mix all the states with the same quantum numbers).
In a strongly interacting case the story is even more subtle, because composite operators may be closer to the ladder operators between the asymptotic states, even though they're built of other... particles? Language isn't great in this instance.
On basis of that, I'd be more likely to say "QM needs to describe everything as a wave, and sometimes certain kinds of localized 'wave-packets' move around coherently, and that's what we'd call 'particles'". That also seems to gel with less coherent states where it feels like there's not really a particle to be found.
So, I'm curious why you'd prefer to relate the eigenstates themselves as particles. Again in the oscillator case, the eigenstates themselves seem less coherent and seem to behave less classically than I'd hope.
My best guess is that the property those states have that is not as well replicated by the "particle as a coherent wave packet phenomenon" is that they have well-defined energy quanta. But that's just a bit of a stab in the dark here. It perhaps makes more sense from the perspective of "particles are the things that we're able to measure in detectors" POV, though.
Imagine a ball that’s rotating,
Except it’s not a ball, and
It’s not rotating.
(popular particle physics meme)
From what I understand of QFT, the Universe is made of fields of different types, and a “fundamental particle” is just an excitation (wave) in the corresponding field.
For example, a photon is a wave in the universal electromagnetic field, A charm quark is a wave in the universal charm quark field, etc.
I’m not a trained physicist, so I might be wildly wrong.
It’s just the statement that the object spinning is attached to its surroundings in a smooth and continuous fashion. Less rigid object, more a patch of space-time fabric spinning.
There’s a video here: https://youtu.be/LLw3BaliDUQ?feature=shared
The problem lies that it is hard to imagine something that does have zero dimensions. You can get the example of ant walking into 2D and it is unaware of third dimension to explain we are have something similar for space-time 4D (although not the same picture exactly as time is different from spatial dimensions). But we don't have an idea how to approximate a mental picture of what a zero dimension could be. So you have something that does not occupy a volume in space (Talking strictly about elementary particles here) in the classical sense.
This does not mean they are abstract concept. According to QFT -Quantum field theory- you would think (by training) of particles are excitations or quanta of their respective fields. Fields are there always (vacuum is just filled with fields) and particle appears when they are excited (more complex processes occurs). So you would think of each particle as a manifestation of a quantum field that permeates the universe. What is interesting (and probably confusing to most people) is that these fields are not zero-dimensional, instead, they exist everywhere in space and time. But the quanta (particles themselves) are considered point-like with no spatial extension.
In practice physicists will think about particles properties (i.e charge, mass, interactions, spin) ..etc instead of what this particle actually is from that point of view. This is often for practical reasons. You are a working physicist and you learned from your training that you shut up and calculate (or implement if you are doing experimental particle physics as you spend most of your time coding) by this stage.
Topological defect. The unpictured thing the contour lines are swirling around. It's influence is only felt by seeing the effects on higher dimensional space, but you can never see the ghost itself.
Or a magnet
Do you really think so? It’s not hard to picture the real number line, with the point zero (or any other single point) distinguished. Sure — if you draw it in the standard schematic way you have to give it some area, but it still seems quite intuitive that it’s ‘zero-dimensional’. Especially if you play around with converging sequences and open sets and stuff; you quickly develop intuition for what it means to be a point rather than something higher dimensional.
How can something come from nothing?
You can't explain it in terms of anything else, which was sorta my original point. Maybe he could have been more touchy-feely in his answer, but that wasn't his nature.
https://physics.stackexchange.com/questions/46573/what-are-t...
What are strings made of?
One answer is that it is only meaningful to answer this question if the answer has physical consequences. Popularly speaking, string theory is supposed to be the innermost Russian doll of modern physics, and there are no more dolls inside that we can explain it in terms. However, we may be able to find equivalent formulations.
His attitude really bugs me, and it’s on full display in this clip - everything about this to me says “I am so smart and you have asked me a question so dumb that I don’t know how to dumb down my answer for you, now I will show how smart I am in the course of explaining how dumb your question is in excruciating detail, so next time you’ll know not to waste my smart time with dumb questions.”
Instead of playing games and being evasive and ‘owning’ the interviewer, he could have just worked with the guy to clarify the question and to talk through the guy’s understanding - he didn’t try to do that, though, and you can tell how delighted he is for this chance to show the guy up. His autobiography is written the same way, it’s a series of anecdotes about times where he was smart and other people were dumb. Really rubs me the wrong way.
You must have missed the part where he says that it's an excellent question.
> he was smart and other people were dumb
But he was smart. A lot smarter than you and me and the vast majority of people. He was also pretty humble about it, once famously saying that if he couldn't explain something to a freshman it's because he himself didn't understand it.
Those behaviors are something like:
- it has momentum - it's state is uniquely defined by a position in space and a velocity
What's not a particle? A wave (well, until 1900 or so ...).
Sort of like asking "what is a number?"
A number is a thing that obeys certain rules. (You can add them; there's an `identity `, for every number there's a number which if you add together gives zero, etc).
That allows things like `(3 + 5i)` to be a number, for example.
Except when they don’t
They're analogies. The concepts need names, but I think they do more harm than good because people then start with a mental model of a membrane or a surface - something they have experience seeing waves in. And then after 1 or 2 steps where the analogy helps, it breaks down, and people start being confused.
Of course the alternative isn't any better. If they had named it a "Wazoo function" and a "Quantum Flarg" everyone would've just kept asking "OK but what IS a Wazoo? What IS a Flarg" and not been satisfied with a "Yeah, it's a fundamental own thing".
Feynman, of course, has a pretty definitive response on the difficulty of this problem: https://www.youtube.com/watch?v=Dp4dpeJVDxs
The trouble is that an electron is an electron and it is nothing like anything you have ever seen in your macroscopic classical world. It shares some aspects with billiard balls and some with water waves but it is not like either. And it does not switch between being a billiard ball and a water wave, it always is the same thing, it always is an electron.
It just happens that in certain situations the billiard ball properties are more apparent and in others the water wave properties and in yet other situations neither of the two analogies will help. I think that is what trips people really up, they want to visualize their electron as one thing they know, as something they have an intuition for, but no such thing exists.
And electrons being electrons also means that they are not excitations in quantum fields. Those fields are mathematical models that describe the behaviour of electrons, they are not the electrons. Certainly not in the very direct sense of nature is just mathematics because I can differentiate, integrate, and square fields at will but I can not do this to electrons. And even the less direct interpretation, there are real entities in the universe that behave exactly like our mathematical fields, does not seem likely, what would the gauge symmetries mean?
You’re going against the dominant interpretation of QFT here, aren’t you?
Quantum fields have gauge symmetries which means that they are a redundant description, i.e. any given physical situation is represented by an entire equivalence class of field configurations which makes me highly suspicious of there being real quantum fields. Quantum fields are a nice mathematical tool but I do not think we have any good reasons to think they are real, but I am not a physicist and I am certainly in dangerous half-knowledge territory here.
I have been wondering for years whether this might actually be a non-issue, could the universe secretly have fixed a gauge and just ran with it? Or would this somehow be inconsistent?
I worked in particle physics for years and never once saw an electron. :-)
I must be under-thinking this, but that’s what’s worked pretty convincingly for me.
Sometimes when particles meet or even spontaneously they can split or merge altering other parts of their nature (unrelated momentum, energy and angular momentum). This happens for example when neutron decays into proton and electron.
Sometimes they get stuck together because of electromagnetic force and they resonate in interesting harmonies and travel together. That's atoms. Interestingly when they are resonating in those harmonies they become quite fussy about amounts of energy they prefer to exchange and they do it only in a very specific quanta.
And there's a class of particles called quarks that travel together all the time as they are always tightly bound with each other and can never get free despite possessing incredible amounts of energy they continuously exchange. That's nucelus.
We really don't like this image because fuzziness is actually two dimensional in every point of our already 4 dimensional space-time and described by complex numbers so we prefer to focus on those brief moments when particles interact since if we have a lot of particles that are bound together to form measurement apparatus they are so sharp that the interaction they participate in squash other particles nearly to a point and we can declare that the measure particle collapsed to have some momentum, or location, or spin described by a single vector instead of a cloud. It neatly turns out that the square of complex number fuzziness describes the probability that a fuzzy particle will interact with a sharp one (one of those bound together in measurement apparatus) with a specific outcome.
(I'm aware we don't have an understanding of how quantum physics interacts with singularities, but the whole billiard ball metaphor certainly is incoherent with it)
I only got the bachelors' version of physics, though I did take some grad classes, so here is what I will tell you:
The human mind learns from experience and it thinks of things in terms of the past experiences it has had. We are big assemblages which exist in a narrow range of temperatures (think in terms of Kelvin). Our experience is classical, in the Newtonian sense: we move at not a particularly notable fraction of c, we are too warm to note the strangenesses which happen below, say, twenty or four or a thousandth of a Kelvin (superfluids and BECs are out), we are too cold to have a great internal experience of plasma, leaving us to be creatures of solid and liquid, with a sort of inferred understanding of gas. We are too large to feel the quantum realm, in the sense that the uncertainty principle is not obvious to us from what we have felt.
So, we must make do with abstractions, with fictions, with approximations. Conscious that we are the epitome of the six blind men trying to understand the elephant through touch alone, we try to break our understanding, to search for flaws in our inferences. Yet this does not grant us true experience when we run across, say, the electron. We try to think of it like a billiard ball, but we can say that a billiard ball is this wide, yet we are fairly sure at this time that the electron has no radius, no diameter, that it might as well be a geometric point. Every time we try to measure, we can only establish a smaller and smaller upper bound for the confounded thing's radius. That's not like our lives at all!
The reality of this electron is that if we get it going fast enough, it stops getting much faster no matter how hard we smack it. That's not like our reality. If we try to pin down where it is, the more we do it, the harder it is to figure out how fast and in what direction it moves. And as we work to ascertain the velocity (and therefore momentum), we lose sense of this bit of weirdness' position.
You eventually have to develop an understanding based not on experience at all.
Perhaps this was unique to me, but the first time I understood integration in calculus, I had a brief moment of dizziness as I apprehended this new thing. You know how you are working a math problem and you have a good idea of what the answer is already, a sense of what the magnitude and direction might be? I had ground my way through vector and tensor calculus, and had been working a problem in gravitation and relativity class when I sensed what the resulting tensor would look like, the shape of it, in the sense that I would know if my figures were way off. I nearly fell off the chair, my head spun so.
If you care to, you can do this for a particle.
Fun fields discussion on what particles are…
Also, the soul discussion was pointing more at the history of language and concepts versus a crude equation of the two :)
There's also a lot of overlap with the article itself however. From the sibling comments too. At risk of breaking the rules, it's very much worth the read if you haven't read it!
If that gets around your head, you’ll throw the physicality and real world away and you’ll come to see everything as information interaction.
Reading the article, I understood so little of it. And I guess it’s because so much of the language is just words chosen through some sort of consensus to represent an abstract idea itself composed of such words-idea-representations which I’ve never encountered before.
What we don't understand is the fundamental structure of that matter or of our Universe. I personally feel that the people ostensibly "in charge" of this effort are a little chagrined at their decades of inability to produce not only a cohesive result but even a reasonable intermediate explanation that they intentionally couch these problems in the most arcane and impenetrable language available to them.
In any case, you shouldn't feel discontent for humanity, as we've simply discovered all the easy problems, cleverly worked out all the average problems, and now all we're left with is the intractably hard ones. It's very likely that a different type of effort we haven't engaged in yet will be necessary to make progress.
I mean heck, even something as mundane as concrete is still the subject of active research as to the chemical reactions and complexities involved.
I guess it's more a spectrum of understanding than a yes/no situation.
For myself, I find it exciting to keep discovering how little we understand, despite our abilities. We seem to be barely a step removed from alchemy, from some points of view.
Going the other way, you most likely cannot explain why your body or your brain works, yet, here we are, using and understanding them just fine.
Which leads to what I was trying to get at. Perhaps our tentative understandings and our means of receiving them are what gets in the way of deeper comprehension.
If I recall correctly, the we can't really solve the equations for anything more complex than a helium atom (or is it hydrogen?). That's not to say there isn't useful work we can do, numerical approximations, etc. But things do get astoundingly complex very quickly, even with the "mastered" bits.
Me: Looks up Poincare group
Also me: Oooookay, that makes absolute perfect zero sense to me
From what I gather, a set of possible transformations is a group in group theory
Physical space is a type of group, i.e. a Poincare group, and is described by the set of all transformations on objects, or something (i.e. motion or lack thereof)
An irreducible poincare group is a tinest example of physical space, i.e. a 'particle'
So although it has no physical space, yet the irreducible Poincare group is intrinsically (but not practically) capable of those same types of transformations within itself as in the larger Poincare group within itself
E.g. a larger object (many particles) can undergo shears and strains, i.e. internal motion. In theory an infinitesimal particle can, it just doesn't have the space to undergo those
I'm inferring from this that a subset of physical space is also a Poincare group?
Where the irreducible representations come in is in quantum mechanics, where physical systems are described by Hilbert spaces. These are spaces in the abstract mathematical sense of a vector space. They're used in quantum mechanics as a way to mathematically describe the fact that there are multiple possible outcomes of an experiment or interaction. The way this is done is by describing a quantum state as a unit vector in Hilbert space, each possible outcome corresponds to a dimension (a basis vector) in this space, and the probability of each outcome corresponds to the projection of the unit vector onto one of these dimensions. You can picture this as an arrow of unit length with its tail at the origin, and evolution of the state makes the arrow rotate about the origin, consequently changing its projection onto the basis vectors.
A representation of a group on a vector space is a way of describing the action of that group on the vectors in the space. Any vector space can be decomposed into subspaces which are invariant under the group action, meaning that if a vector starts off in that subspace, then the action of the group will not move it out of that subspace - these subspaces are the irreducible representations of the group.
So the irreducible representations of the Poincare group correspond to the components or properties of a physical system that are invariant under the basic symmetries of spacetime, i.e. that are independent of one's perspective, and in that sense they're considered basic or fundamental.
It is interesting to note that (correct me if I'm wrong) perhaps our mathematical constructs are based on a classical intuition and perception of the universe?
And here we are trying to fit that classical intuition into the quantum realm
Perhaps that can be related to why shit gets complicated with particle physics?
That said, model-theoretic semantics go back to, at least, William of Ockham, who developed a formalized subset of Latin (which kind of reminds me of the Google C++ style guide in a way). We're clearly not utterly incompetent in this space, but it helps to stand on the shoulders of the giants to see clearly.
I am not sure if the modern mix of group theory and differential geometry which forms the foundation of theoretical physics today can qualify as such, even though the word "geometry" does bring the expectation that we should be able to "see" the things we are talking about.
This makes me remember the time I looked at a random book at my university library, and it happened to be a physics book from 1905.
Which was both fascinating and unintentionally hilarious due to it proudly asserting that we knew most of physics now because we knew about atoms, and assuring the plum pudding model as how atoms worked.
n.b. Plum Pudding was the old-school idea that atoms were a positively-charged blob with negative electrons embedded. It was refuted when you measure the radiation scattering patterns off gold foil and discover that, actually, there's an extremely dense nucleus.
The key problem humans must grapple with an solve, if we are to make it long-term, is how to harness greater and greater power without destroying ourselves. Nuclear destruction has been a very real option for 80 years. Biological weapons could do the job, too. And of course, climate change looms. For many years I agreed with the thinkers that wish for a "backup plan" for humanity; however, I don't think Mars will do it simply because its environment is even harsher than a post-nuclear holocaust Earth. The important reality that THIS is our home, and we must protect her even if it's hard. Yearning for advanced physics to solve all our problems has the unfortunate side-effect of undermining motivation to solve the problems we have now with the tech we have now. It underlies an unfortunate "disposable planet" attitude that we'd be better off without.
Some discussion then: https://news.ycombinator.com/item?id=25091742
Quarks get most of their mass from QCD with very minor contribution from Higgs Boson. And nobody has any idea where the mass of neutrinos comes from.
It also has no influence on photons and gluons.
Higgs seems to be very peculiar and not very universal mechanism. I wonder if one of the potential future approaches won't do away with Highs Boson (together with virtual particles) as artifacts of specific math approach and interpretation without any physical manifestation.
Higgs was detected, sure, but it was detected through an interpretation of the data through the best available mathematical model which some postulate might contain some purely mathematical constructions along the way to the ultimate real world result.
Thanks for sharing, TIL and it’s fascinating.
What Is a Particle? - https://news.ycombinator.com/item?id=25085286 - Nov 2020 (37 comments)
(1) With Itself:
Consider Young's double slit experiment: So, have plane with two slits and some distance away a parallel plane with detectors. (A) Several times, shoot a photon at the slit. Observe that the detection locations form parallel lines, i.e., fringes. (B) Cover one slit, repeat, and observe that the detection locations from a smooth hill without fringes.
So, from (A) we conclude that the something about the photon went through both slits and interacted with itself to form the fringes, the ones we didn't see from (B).
Q. Between the two planes, where was the energy?
(2) Mass and Charge
Set aside (1) with its photons and two planes.
Now one at a time shoot electrons, i.e., with not just energy but also mass and charge. And shoot the electrons at a beam splitter, i.e., a plane, partially transparent to the electrons, and at 45 degrees to the path of the electrons.
Some electrons pass through the plane with no change in direction and some get deflected 90 degrees.
On the paths after the plane, have some very sensitive detectors for mass and charge. These detectors are distant enough that what they do cannot affect the electron, i.e., the electron does not know about the detectors.
Q. What do the detectors read? For each of the two paths, whole mass and charge, half, or something else?
Even though it sounds as though your arguments are gotchas that prove quantum mechanics to be nonsense, it turns out the world really is that way.
My idea for resolving this is that electron is never a point-like particle. It's always a cloud, just larger or smaller. When it's detected it gets reshaped to be narrower. Mass, energy, momentum and such are a quantities ascribed to the whole cloud and exchanged only on the moment of interaction.
Think about diffraction. Photon or electron that passes through a small hole had it's moment messed up proportionally. It becomes large again.
Interesting question is where's the gravity in all of this. There are various ideas how to match quantum uncertainty to shape of space-time.
"These detectors are distant enough that what they do cannot affect the electron, i.e., the electron does not know about the detectors."
We detect gravitational waves without "affecting".
The electron mass and charge send out signals. Have the detectors sufficiently far away that they can't affect the particle yet. Get the detection and then know where the particle was and its mass and charge then. Have the particle reflected by some mirrors and then know the current path of the particle and its mass and charge, all without affecting the particle.
What about interaction-free measurements, such as in the Elitzur–Vaidman bomb-tester thought experiment [1], which was later shown experimentally to be correct?
[1] https://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_t...
General relativity treats energy and momentum jointly so I guess basically energy-momentum is a quantity preserved in space-time translations and spin is a quantity preserved in space-time rotations. (in flat space-time, I think?)
I guess that's why those Poincare symmetries are rarely mentioned when talking about particles. They seem to come more from sheer geometry of space-time than anything else. Particle physicists are mainly interested in all other symmetries (because they were harder to figure out). It also must be bad feeling that while you are trying pull gravity into your framework, more than half of the symmetries that the objects you spent your career observing obey, come from general relativity not from your framework.
1.1 INTRODUCTION: THERMODYNAMICS AND STATISTICAL MECHANICS OF THE PERFECT GAS
Ludwig Boltzmann, who spent much of his life studying statistical mechanics, died in 1906, by his own hand. Paul Ehrenfest, carrying on the work, died similarly in 1933. Now it is our turn to study statistical mechanics.That appears to be sufficient to allow non-fatal studying of statistical mechanics. Goodstein wrote that book 49 years ago and didn't die until a few months ago at the age of 85.
[1] https://www.amazon.com/States-Matter-Dover-Books-Physics/dp/...
We have to use them to talk about things and work with them, but they all “leak.”
When I consider your heavens,
the work of your fingers,
the moon and the stars,
which you have set in place,
what is mankind that you are mindful of them,
human beings that you care for them?
You have made them a little lower than the angels
and crowned them with glory and honor.
You made them rulers over the works of your hands;
you put everything under their feet:
all flocks and herds,
and the animals of the wild,
the birds in the sky,
and the fish in the sea,
all that swim the paths of the seas.
You missed the wisdom in your own statement.
A concerning statement for a four year old article. Has anything in it been superseded?
“Space”, however, is a derivative attribute emerging as the “distance” between particles in the graph; there is no “space”, only metrics.
Reality does not exist between measurements/interactions; the outcome is calculated on demand.
Anyway, what exactly is a field besides a mathematical object? What is it made of?
I would be personally interested in proton decay as it could be indirect indication for magnetic monopoles [2].
[1] https://en.wikipedia.org/wiki/Proton_decay?useskin=vector#Pr...
[2] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.52...
Why are magnetic monopoles interesting to you? I've seen some articles on them but I can't still wrap my head around how they'd work.
Hobson (2012): There are no particles, there are only fields
If so, disable True Tone and disable auto brightness on all screens. That might help.
We should have been told that science is about the prediction and description of things. This is very different to what things actually are. If only scientists didn't believe from the very beginning that they were studying what reality of things are, they wouldn't spend so much time unlearning this later in life.
And, don't get me wrong, that doesn't mean particles don't exist. They do. But, a particle is whatever we say it is.
So no, countless people around the world aren't wasting their lives researching particles when the answer is simply, it's whatever we say it is.
What gives you the faith that a "particle" is not a human construct? What on earth does "fundamental" even mean except being the bottom turtle we can see on the particular mountain of turtles at which we're looking.
"Countless"(?) people around the world are not researching particles. They're doing particle physics as they understand "particles". That's how it should be and particles are whatever they say they are. In that field, no measure means no reality. *Unless*, of course, you have faith in some platonic reality.
Positive materialists will disagree.
If you are saying even objective things are whatever we say they are, then this is a useless discussion. Obviously every word is defined with other words and all words are human creations. So yea in that sense, literally everything we know of is whatever we say it is. That's a pointless statement to make in response to this article or really ever.
The big philosophical problem with much of this is that people assume that the smallest things are the most fundamental. So people think that stuff, whatever that stuff is, is fundamentally made up of much smaller stuff, and that stuff is fundamentally made of yet smaller stuff, and so the smaller you get, the more fundamental you get. And so (they think) if you want to work out what is really going on at any layer of reality, you need to figure out what the smallest possible things are.
Yet this is ultimately a philosophical posit -- it's not empirically-informed. There's no good reason for thinking it.
To be clear, none of this is about physicists doing physics. It's about the philosophy that many people bring into, and therefore take away from, these kinds of discussions.
[1] "God in the beginning formed matter in solid, massy, hard, impenetrable movable particles." Isaac Newton, Optics, 1704, Book III, page: 375