The h Bar - 2021-03-25

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12:20 AM

Is it ok to ask a question again, if it has been marked as a duplicate of several questions with self contradictory answers? I have asked if energy is conserved in GR, and the question was a duplicate, however the answers provided on the questions it was marked as a duplicate by contradict each other, sometimes even on the same question. Some say that energy is conserved in GR, and others say that it is not conserved in GR.

@user400188 What makes you think asking the question again will yield less contradictory answers?

there are semantic problems with that question in GR - you have to be careful about what "energy" and "conserved" really mean in a relativistic context - and the contradictory answers are likely due to people interpreting the meaning of these terms differently

@ACuriousMind I can set requirements in the question for reliable references, or ask for a proof of either fact, and add a bounty to encourage users to do so. Furthermore, I can define precisely what I mean by energy, and its conservation, to remove ambiguity.

@user400188 Every question asks implicitly for reliable reference or a proof of the answers it gets, does it not? And you can also just offer a bounty on the existing question (there's a pre-made reason for that, canonical answer required, if you feel existing answers are lacking in references) without asking it again.

If you exhibit a really specific technical definition of energy and conservation, and then show us where you have problems showing it is or it is not conserved , that would probably qualify as a distinct question from the generic "Is energy conserved in GR?" questions and at least I would likely not close it as a duplicate

@ACuriousMind Thanks for the tip. This is what I had in mind, it might take me a week or so to write it however.

@ACuriousMind I would do this instead, however I wont be able to specify exactly what I mean by conservation and energy if I don't write it myself. Also, all of the duplicate questions are at best on topic to my original. They ask if the total energy in the universe is zero (interesting, but not what I had in mind), if energy is conserved in general (not just in GR, which is what I wanted to have answered), and if it is conserved in the big bang (related, but not the original question).

2 hours later…

2:35 AM

@user85795 Yeah, it's been a weird year....

they can't get much weirder @nitsua60

have you seen this?

2

5 hours later…

7:31 AM

Hi! Can someone help me with this question?

If I know all of the energy states of a quantum system, and now I define psi_g(t) as an integral over energies with integrand as f(E) psi(E), what information can I visualise from such an integral?

Mauro Giliberti

8:19 AM

can anyone explain what's going on here? physics.stackexchange.com/questions/623492/…

Specifically, why is everyone talking about quantum chickens?

8:39 AM

@MauroGiliberti look at the edit history of the question

bit of a drastic edit, no?

he could have at least left in the part about the chickens, now all the answers look weird out of context

I really don't like it but I've rolled back the edit.

thanks

top answer's currently at +9/-4 lol

what answer are people looking for

9:31 AM

And we now have a Nonsense tag :-)

Is 4D even on-topic in the site?

@MarkGiraffe spacetime is four dimensional, if that's what you mean.

10:01 AM

@MarkGiraffe Depends exactly what you're asking, that's much too broad of a topic to give a yes/no answer to.

1 hour later…

11:01 AM

@ACuriousMind there's an argument at the start of ch. 4 here that frames brst as arising, since the action "has lost its local gauge invariance by the introduction of a gauge-fixing term: it would be nonetheless desirable to maintain the infinitesimal gauge invariance of the theory" and basically says if you attach anti-commuting parameters it blocks you from exponentiating a local symmetry to a group symmetry. Any thoughts on that view of it?

@Charlie I think one way to appreciate this is that if you just do things directly in the Schrodinger picture, you're still using operators, and with higher order corrections one gets infinities as they come and it's hard to see the big picture when dealing with them, but the higher order corrections naturally fall out of doing an analysis of propagators from the Heisenberg picture for full fields (i.e. non just interaction picture fields) so e.g. P&S just start from Heisenberg from the get go

11:40 AM

@bolbteppa I think the "it would be nonetheless desirable to maintain the infinitesimal gauge invariance" is a rather weird thing to say - why would that be "desirable"? A gauge symmetry is a redundancy, and gauge fixing has gotten rid of the redundancy, so if the symmetry were gone for good, we shouldn't worry about it

It's not that we somehow desparately want to have some symmetry, it's that we need to figure out what the "real" observables of the gauge-fixed theory with ghosts are (since certainly the ghosts are unphysical and we need a proper way to decide whether any given quantity in this theory is observable or not)

and it turns out that "gauge-invariant observable" more or less corresponds to things in the cohomology of the BRST symmetry operator

I think the idea "BRST is infinitesimal gauge symmetry blocked from being finite" might have some sort of merit, but the "desirable" part definitely misunderstands the nature of the BRST formalism

12:01 PM

Yeah I agree

What happens to the harmonic function solutions $\partial^2 \lambda = 0$ even after you gauge fix with $\partial_{\mu} A^{\mu} = 0$, is that a 'remnant' of the gauge symmetry still lying around that's maybe related to brst

it sure is residual gauge symmetry but I don't see how it could be related to BRST

The ambiguity of "gauge-fixing" conditions is called Gribov ambiguity and discussed in the chapter right before the one you cited

BRST doesn't really care about Gribov ambiguities, it is a general procedure for gauge theories regardless of whether or not they show Gribov problems

In fact, BRST is precisely the method you want to circumvent this problem: In contrast to naive "gauge-fixing" attempts, the proper BRST formalism constructs a gauge-invariant BRST operator that is everywhere defined

12:57 PM

This and this paper seem to say the gauge-fixed Lagrangian is still invariant under $A_{\mu} \to A_{\mu} + i \partial_{\mu} \chi$ for residual $\partial^2 \chi= 0$ transformations, so e.g. in the Functional picture for Maxwell one imposes $\Psi_{phys}[A^{\mu} + i \partial^{\mu} \lambda,t] - \Psi_{phys}[A^{\mu},t] = 0$ on the fields.

Replacing $\chi(x,t)$ above with $\chi(x,t) = \lambda \eta(x,t)$ where $\lambda$ is an anti-commuting parameter and $\eta$ an anti-commuting field, this condition on $\Psi_{phys}$ (explained there) reduces to $\Omega \Psi_{phys} = 0$ for $\Omega$ the brst charge.

Replacing $\chi$ by $\lambda \eta$ implies $\delta A_{\mu} = \lambda i \partial_{\mu} \eta$ and says if you used commuting instead of anti-commuting parameters, $\Omega$ would not commute with $H$ so the 'residual gauge symmetry' would not be realized.

In other words, in a sense it looks like the reason you'd even expect a symmetry to exist after gauge fixing is due to these residual gauge transformations, and by some magic/genius if you let this residual symmetry act in a way that involves the new FP variables which themselves arose from gauge fixing, so in a sense it's already not crazy, of course you should get a symmetry...

1 hour later…

2:18 PM

0

Q: What are radio waves made of in the photon picture?

A few days ago, I still thought I knew what radio waves are made of. I thought they were made of photons of the same frequency range. But not anymore after I read an answer and exchanging a bit with its author HolgerFiedler: https://physics.stackexchange.com/a/623132/230132 He basically says that...

photons antennas radio-frequency

::sigh::

In other news, though

Google Scholar will now publicly shame authors who fail to comply with Open Access mandates from their funders

Mithrandir24601

3:08 PM

> What are radio waves made of in the photon picture?

Uhhh... Photons? Lots of photons?

3:49 PM

-3

Q: Do we necessarily need real number for quantum?

In the quantum mechanics, one asked if the complex number was necessary? A typical answer was that it was not, or that it's simple product of real numbers. However, consider rational number to real number, with additional uncountable irrational numbers. One then think, just because rational numbe...

quantum-mechanics complex-numbers

duplicate?

4:19 PM

@EmilioPisanty any of the questions linked here?

How is the existi question

How is the past questiin

damn mobile phone, sorry about that

How is the question given as one offering the answers to my question answers my question about photon frequencies?

-1

Q: What are radio waves made of in the photon picture?

A few days ago, I still thought I knew what radio waves are made of. I thought they were made of photons of the same frequency range. But not anymore after I read an answer and exchanging a bit with its author HolgerFiedler: https://physics.stackexchange.com/a/623132/230132 He basically says that...

photons antennas radio-frequency

Have you even bothered reading either one?

I appreciate thoughts from community members who can cast non-binding close votes as to whether the chicken question is a duplicate of this question about finding the surface area of a rock.

@Winston The duplicate relates to your title question. Your more specific question about photon frequencies is already answered e.g. by Andrew Steane's answer to your previous question. Holger's answer is simply dead wrong, as can be easily seen by considering that one of the hallmark experiment of QM, the photoelectric effect, is based on the photons in an EM wave having a frequency roughly corresponding to the frequency of the wave.

The alleged good answers give zero information about frequency

Ok thanks ACuriousMind, this is actually very helpful

I thought Holger was right, and you are clearly saying he is not

Should I delete my question?

don't be too quick to trust answers that haven't gotten any upvotes

@Winston there's rarely a need to delete a question manually - closed questions with no answers are deleted automatically after some time anyway, it's entirely up to you if you want to delete it before that or not

4:30 PM

You are right. But usually wrong answers tend to attract a reaction, downvote or comment and there was no sign. Anyway, thank you.

Yes I will delete because I have what I was looking for, thanks to you. Bye.

@rob chicken soup, rock soup, no difference, right?

> I'm working on building an army of rocks and I'm trying to find out how much metal or kevlar I'll need to armor them. You will be spared when my rocks take over the world if you give me a working answer.

4:46 PM

I like the question about chickens. Shows the site can still have humor.

5:21 PM

Hi again. Looking for a moderator opinion here.

Having recently been lectured by rob about my bad behavior and my lack of social skills (or motivation to use them in accordance to the code of conduct),

I am seeking an official opinon about how I am to react to what I believe is a joke

Here: physics.stackexchange.com/a/623745/230132

I am refering to JEB last comment.

He just added a second part "The is physics stack exchange, not QVC."

No idea what QVC is. But rfl's answer was helpful and wavelength shifters do exist.

@Winston I think JEB is just saying that their derivation of why reflection cannot shift wavelengths is not affected by the existence of products that can emulate the desired effects via other means (in this case absorption/emission via fluorescence). QVC is a somewhat famous shopping channel.

I don't find his tone "kind and welcoming" to paraphrase rob, though.

but in any case, you can always ask someone directly what they mean if you don't understand the comment - jokes don't always land well, especially over the internet

Ok thanks

in this case, I read JEB's first sentence as clear hyperbole, but I can see why you might not

1 hour later…

6:51 PM

hm, upvote 30 secs after posting - I don't think that voter even read my answer :P

7:25 PM

0

Q: Community polls on Physics SE?

I recently came across this (admittedly, pretty old) question on Mathematics Meta: Do we want community polls? In summary1, it is proposed to have a meta post to find out information about the site's users. Such polls have been conducted on sites such as TeX, Mathematica and Academia. Would such ...

7:40 PM

huh. this thing made HNQ?

that system never ceases to surprise me

here I was thinking that I had just provided the best answer on the newest up-and-coming HNQ

2

Q: Is it possible to create a mirror that red shifts light?

Mirrors are able to reflect light but are not perfect and after a number of reflections, light loses intensity. However I wonder, during the reflection by a different type of mirror, could the light photons lose some energy and thus be red shifted instead of just losing intensity? I am not talkin...

and it turns out I'm already on the previous one ¯\_(ツ)_/¯

@Winston My two cents -- I find JEB's response there inappropriate, though not necessarily enough to rank as a code-of-conduct problem.

I would say just ignore it and move on.

If two gauge theories have isomorphic BRST operator cohomologies what information does this provide about any relationships that may exist between them?

7:57 PM

@Charlie what do you mean by "isomorphic BRST operator cohomologies"?

if you're thinking that BRST cohomology is some sort of rough invariant of theories like ordinary cohomology is a rough invariant of spaces, it's not like that at all

Actually I was pretty "without thinkingly" assuming there was some analogy with the fact that you can get useful information out of de Rham cohomology isomorphisms, which is the only other kind of cohomology I know aobut

ah. Yeah I was :p

8:15 PM

2

Q: Folks, we need to talk about the surface-area-of-a-chicken question

This question How do I find the approximate surface area of a chicken? is undergoing a severe close/reopen yo-yo cycle. It was closed once by a moderator, then reopened by community members, then re-closed by community members, then re-opened again, and it currently has one active close vote. A...

discussion scope specific-question vote-to-close

@Charlie maybe to give a bit of intuition here: ordinary cohomology of topological spaces is something that measures the failure of closed forms to be exact - this is a very rough property, there's lots of ways in which a space can be meaningfully different from another without changing this property, i.e. cohomology "forgets" a lot of meaningful properties.

BRST cohomology gives you the set of gauge-invariant states/observables - but the set of meaningful states/observables is precisely the essence of what a physical theory is, so this is not something that forgets information about the theory

9:06 PM

0

Q: Should we encourage "fun questions"?

There are a number of questions on this site that start with a fun, sometimes ironic context. Here are some more recent examples How do I find the approximate surface area of a chicken? (meta discussion on this question here) I'm working on building a chicken army and I'm trying to find out how...

9:21 PM

Oh yeah that make sense

10:05 PM

I don't know if I'm mistaking, but the description says to just ask. @ACuriousMind I want to bring your attention again to the question I posted about a "Berry phase operator" and in particular to the comment I added. If no such operator exists, as your comment seem to imply, then it kind of makes me question the QM postulate about observables being represented by Hermitian operators

Here's the link: physics.stackexchange.com/questions/623549/…

If the definite answer to my question is negative, then I can close it and perhaps make another one specifically on this other thing

@KarimChahine I saw your comment, I'll get around to writing an answer later today or tomorrow. The short answer is that you're mistaken about what "observable" means - it does not mean "anything you could possibly infer from an experiment"

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