The h Bar

Nike Dattani

01:26

@ACuriousMind Thanks for letting me know. I didn't realize things were so strict here. Why do you say that the mainstream position is that quarks don't have size? QCD treats them as point-particles, but you've got 400+ beyond-standard-model questions and 2000+ string-theory questions.

123

03:51

Hi All...

Sidarth

04:09

hi

user2236

04:40

yo

Avantgarde

yo

Physics Meta

07:02

0

Q: When I post a self-answer, is it expected that in the question I should pretend I don't know the answer?

Stackexchange explictly encourages writing a self-answer, when appropriate. Derek Muller posted a video about a wind-powered vehicle that can go downwind faster than that downwind itself. (The design of the vehicle dates back multiple years, but building one that is large enough to move a human ...

discussion

John Rennie

08:45

@NikeDattani the "size" of an elementary particle is a somewhat unclear quantity. If we adopt the position that a particle is thing created when we apply a creation operator to a Fock state then it is basically an infinite plane wave so its size is infinite.

When we say the particle is "point-like" we mean it can be localised to an arbitrarily small region of space by applying enough energy. So "point-like" really means it has no minimum size.

Physics freak

09:04

hi, i had a question related to directional derivatives

in the formal definition, the directional vector, need not be normalized,

however while calculating the slope we should normalize the vector in the direction of which we calculate the directional derivative

my question is what would happen if we don't normalize the directional vector?

surely we would get different magnitude of the directional derivative but what does this difference in magnitude imply?

Charlie

If you're calculating the directional derivative of a function $f(x,y,z)$ as $(\nabla f(x,y,z))\cdot\hat n$ where $\hat n$ is some unit vector in the direction you're taking the derivative, then multiplying $\hat n$ by some number will just give you the directional derivative times that number.

This is how the dot product works with scalar multiplication if you like, $(\alpha\vec v)\cdot(\beta \vec w)=\alpha\beta(\vec v\cdot \vec w)$.

Physics freak

@Charlie no, in the formal definition of directional derivative the directional vector needn't be unitary

Charlie

And if it's not such that $|\hat n|=1$ then you'll just end up multiplying the whole thing by that constant.

It's not "the formal definition", it's just a generalisation of what I've written

Physics freak

@Charlie but my question is lets say we want to compute directional derivative along a vector u and vector 2u(both have same direction), we would get 2 different directional derivatives(the latter being scaled by a factor of 2), what does it mean physically. the former would give slope of the function but what would the scaled directional derivative mean in physical sense

Charlie

09:21

I'm not sure what physical interpretation you're looking for, you're just rescaling the answer by a factor of two. This is an instance where I wouldn't go chasing a "physical interpretation". Although if you're thinking of the directional derivative as "how the function changes in the direction specified by this vector", if the vector is longer you're taking a larger step in that direction, if you really want a (sort of awkward/misplaced) physical picture

Physics freak

@Charlie alright, I'll think more about it. Thank you

ACuriousMind

10:09

@NikeDattani 1. Being point-like essentially means that they have no resolvable substructure, see this question and its answers. This is the mainstream position because up to the resolution of our current experiment no one has found anything indicating such a substructure. For quarks in particular the issue is even murkier because they are confined at ordinary energies so you can't shoot them at each other in isolation.

2. beyond-the-standard-model is often for questions about specific extensions to the Standard Model (such as supersymmetric extensions, axions for dark matter, etc.), not for generic non-mainstream questions.

string-theory is a mainstream research topic (but not a theory accepted as fact by the mainstream), but it also does not claim anything about elementary particles having "size" - QFT just stops working properly at the Planck scale and we have to do string theory instead, that doesn't mean quarks have size, so I don't know why you mention it.

123

10:22

Hello World.

Charlie

18:23

Is $\hat a^\dagger(-\vec p)=\hat a(\vec p)$? It's not right, it would make a thing I'm looking at a bit clearer but I sense it might not be. $\hat a$ are ofc the c/a operators in qft

ACuriousMind

@Charlie no, why would the creation operator for a particle with $-\vec p$ annihilate a particle of $\vec p$?

creating a particle that moves in one direction does not destroy a particle that moves in the other

Charlie

oh yeah

lol

Ryan Unger

whats up

I have a physics question

Ryan Unger

18:40

@ACuriousMind does anyone here do fluid mechanics

ACuriousMind

I think tpg does, but he's not currently pingable

Nike Dattani

20:18

@JohnRennie Thanks for the clarification. So the question then just changes from "Which theories (if any) predict a finite size for quarks?" to "Which theories (if any) do not treat quarks as point-like?"

Nike Dattani

20:35

@ACuriousMind I'm getting confused. I wanted to ask if there's any theories that do not say quarks are point-like. For example the Rishon model has got 645 citations and 7 in 2021 alone: it might not be "mainstream", but since you're saying that "point-like" means "no substructure", it seems to be a valid answer to my question.

ACuriousMind

@NikeDattani My point is that the mainstream position is that quarks are point-like. Your question essentially amounts to "please list non-mainstream theories with this specific prediction for me", which is off-topic both as a list question and as a non-mainstream question.

Nike Dattani

"We deal with mainstream physics here. Questions about the general correctness of unpublished personal theories are off topic, although specific questions evaluating new theories in the context of established science are usually allowed" -- here I'm not asking about any unpublished personal theories and am only asking about theories published in the mainstream journals (Physics Letters B has an impact factor of 4.2 in 2018).

ACuriousMind

The answerable and non-list way to frame your question would be something like "Why do we believe the quark is pointlike?", but you would have to explain how this is really different from the questions about other pointlike elementary particles we already have

Nike Dattani

"In the case of modern physics, if a theory has not been published in a reputable journal, it is not considered mainstream." --- but I'm not asking for theories published in non-reputable journals or unpublished theories.

Yea, there's already the question about "why do we we believe the electron is point-like" by John Duffield.

I think your restrictions about "non-mainstream" questions is a bit too restrictive. The Rishon model was published in a journal with a fairly high impact factor, and has 645 citations. You're calling it a "non-mainstream" theory, which is fine, but I think people need to be able to ask about that paper and similar ones. On the other hand, the "mainstream position" is that particles are not strings, but we allowed 2223 string theory questions (I'm not saying we shouldn't).

ACuriousMind

@NikeDattani you can ask a specific question about the Rishon model

Nike Dattani

20:47

@ACuriousMind By the way, this whole topic came up just because I was trying to help this user.

ACuriousMind

but that's not the kind of question you're asking about - you're asking for an open-ended, general list question of theories with a specific non-mainstream reprediction. Again, this falls afoul of both our non-mainstream policy in which question have to be specific rather than general, and our general dislike of open-ended list questions

Nike Dattani

Yes, the "list-like" nature of the question, was precisely why I asked here first about whether or not it would be okay.

I'm confused about why it's non-mainstream nature is a problem, but I can't even pretend to understand how hard it is to moderate a site with ~100 questions/day, so I can accept that you don't want questions like that there.

The question asked in chat by the other user was "I'd like to know the probability of the matter shifting due to quantum improbability, for example if I had matter consisting of 5 quarks, what are the chances it would shift the length of one quark to the right?" and I replied by saying that quarks don't have a "length" in QCD. But knowing that beyond-SM theories are a big area of research, I was curious to see which ones don't treat quarks as point-like.

I did a thorough dupe search first and even answered and even answered a couple questions while I was at it.

If list-like questions aren't allowed, and questions about theories that predict non-mainstream results are not allowed, then I'm happy I asked in chat before posting on the site :)

bolbteppa

21:06

Special relativity forces one to treat particles as point particles, if it's not a point particle then it's either some kind of rigid body, but special relativity says rigid bodies do not exist, or a quantity made up of smaller degrees of freedom and there's no evidence these things are not fundamental, so one has to model them as point particles. I doubt an 'alternative' SR-questioning theory can even credibly address this point.

Strings question this assumption but in a way that works with special relativity i.e. instead of points you model them as mini relativistic surfaces

Nike Dattani

Would you say a molecule is a point-particle?

bolbteppa

A molecule is made up of atoms which are composite objects made up of (point-particle) quarks electrons etc

Nike Dattani

I suppose in "Special relativity forces one to treat particles as point particles" you meant "forces one to treat elementary particles as point particles"

bolbteppa

Right

There are problems with this perspective of treating elementary particles as point particles, one simply accepts physics breaks down when things get small enough, e.g. classically for electrons somewhere one eventually ends up dividing by zero due to the point particle model, so assume a 'classical electron radius' makes sense, so talking about modelling particles with a finite radius is also something that's done (for obvious reasons)

Renormalization

Renormalization is a collection of techniques in quantum field theory, the statistical mechanics of fields, and the theory of self-similar geometric structures, that are used to treat infinities arising in calculated quantities by altering values of these quantities to compensate for effects of their self-interactions. But even if no infinities arose in loop diagrams in quantum field theory, it could be shown that it would be necessary to renormalize the mass and fields appearing in the original Lagrangian.For example, an electron theory may begin by postulating an electron with an initial mass...

Physics Meta

21:28

0

Q: What if I ask a "what if" question?

What if I ask a "what if" question? What are the criteria for the question not to be closed? The question should be about physics. That's clear. The question shouldn't be completely random too. For example: -What if a massive ball hits my sleeping room wall? Or: -What if a massive ball (radius 10...

discussion questions close-reasons

Nike Dattani

@bolbteppa Are you saying that electrons are treated as finite-sized particles for the purpose of renormalization?

ACuriousMind

he's saying that the process by which one obtains the classical electron radius is superficially very similar to the typical folklore view of renormalization - the "correct" classical electron radius is the one that removes the infinite self-energy and yields the correct mass-energy (or charge), just like the quantum renormalization parameters "remove infinities" and fix certain values to their correct observable values

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