The h Bar

tpg2114

03:18

@Slereah Any more specific hints than that? I know how to do perturbation analysis, but I'm not sure how it's going to show whether terms create higher wavenumbers/frequencies or their relative magnitudes in those new spectral bands

Secret

06:05

So some random thought of a quantum experiment:

Produce two beams of photons heading towards a screen connected to a computer

In the middle of the setup is some nonlinear crystal that function as a CNOT thus entangling the beams

and just before that, is some nonlinear crystal that has the effect of randomising whether beam 1, beam 2 or both can passes through classically

So you now basically have two product states, heading to the first crystal thus turning it into a mixed state of product states, and then whenever the outcome produced involve both beam 1 and 2, they are entangled before hitting the screen, otherwise, only beam 1 and beam 2 hits the screen

It's a simple setup suggested by last night dream, but I don't see how that will allow more complicated messages to be encoded

Let $\lvert 0\rangle, \lvert 1\rangle$ be beam 1 and beam 2 transmitted though the crystals respectively. Then the initial state is something like:

$\lvert 11\rangle = \begin{pmatrix} 1 & 0\\ 0 & 1\end{pmatrix}$

After passing through the 1st crystal, it transforms into the mixed state:

(sorry, should be $\begin{pmatrix} 1 & 0 \\ 0 & 0\end{pmatrix}$)

$\frac{1}{3}\left( \begin{pmatrix}1 & 0 \\ 0 & 0\end{pmatrix} + \begin{pmatrix}0 & 1 \\ 0 & 0\end{pmatrix}+ \begin{pmatrix}0 & 0 \\ 1 & 0\end{pmatrix}\right)$

ok nvm this is all wrong

The initial state, where both beams are not blocked, is:
$\lvert 11\rangle = \begin{pmatrix} 0 & 0\\ 0 & 1\end{pmatrix}$

After passing through the first crystal, it becomes the mixed state:

$\frac{1}{3}\left( \begin{pmatrix}0 & 0 \\ 0 & 1\end{pmatrix} + \begin{pmatrix}0 & 1 \\ 0 & 0\end{pmatrix}+ \begin{pmatrix}0 & 0 \\ 1 & 0\end{pmatrix}\right)$

And then it passes through the CNOT to become

$\frac{1}{3}\left( \begin{pmatrix}0 & 0 \\ 1 & 0\end{pmatrix} + \begin{pmatrix}0 & 1 \\ 0 & 0\end{pmatrix}+ \begin{pmatrix}0 & 0 \\ 0 & 1\end{pmatrix}\right)$

huh ok, so it actually does not do anything

ah... I need bell states for the CNOT to do something

So I might need to use other quantum gates instead

ok, I will try this gate instead:

$\begin{pmatrix} 0 & 0 \\ 0 & 1\end{pmatrix}\mapsto \frac{1}{\sqrt 2}\begin{pmatrix} 1 & 0 \\ 0 & 1\end{pmatrix}$ and leave unchanged otherwise

Then we have the final state be:

$\frac{1}{3}\left( \frac{1}{\sqrt 2}\begin{pmatrix}1 & 0 \\ 0 & 1\end{pmatrix} + \begin{pmatrix}0 & 1 \\ 0 & 0\end{pmatrix}+ \begin{pmatrix}0 & 0 \\ 1 & 0\end{pmatrix}\right)$

so 1/3 of the time, beam 1 hits the screen while beam 2 is blocked, 1/3 of the time, beam 2 hits the screen while beam 1 is blocked

and then 1/9 of the time both beams are blocked and 1/9 of the time both beams passes through

sorry, 1/6 of the time both beams blocked and 1/6 both beams passes

A quick check with $\rho^2 \neq \rho$ showed it is a mixed state

Secret

07:27

now by the Peres–Horodecki_criterion the partial transpose has one negative eigenvalue, thus this mixed state has some entanglement in it

And that is all can be said about it in general, due to the Quantum separability problem there isn't an easy way to work out how much the classical uncertainty or the quantum correlation contribute to the von Neumann entropy

Perhaps for this simple example here, one thing can be said is that signs of entanglement do not necessarily show up inside the correlations between one subsystem with another experimentally for a record of outcomes. It could be the very probability distributions between the outcomes of the overall system where the correlation is hiding

in this case, the way the probabilities between both beams blocked and both beams allowed are tied in a way you cannot decompose them

Slereah

09:48

"This result, which became known as O'Raifeartaigh's no-go theorem, showed that it was impossible to combine internal and relativistic symmetries other than in a trivial fashion, thus ending a widespread quest by the particle physics community to achieve this fusion. The O'Raifeartaigh theorem was later generalized to a result known as the Coleman–Mandula theorem."

Whaat

Never heard of O'Raifeartaigh in relation to Coleman-Mandula

journals.aps.org/prl/abstract/10.1103/PhysRevLett.14.575

the original paper, apparently

"Let $L$ denote the Lie algebra of the inhomogeneous Lorentz group $\mathcal{L}$n $G$ denote the Lie algebra of any Lie group $\mathcal{G}$ of finite order $r$, of which $L$ is a subgroup, and let $H$ denote Hilbert space on which any representation of $G$ operates. Let $P_\mu$ denote the infinitesimal generators of the spacetime translations in $L$.

If the mass operator $P^2$ and every finite power thereof, are self-adjoint on $H$, and if there exists a discrete point $m^2$ in the spectrum of $P^2$ on $H$, then the eigenspace $H_m$ belonging to the point $m^2$ is closed, and is invariant …

MadSpaceMemer

11:02

Hello, can you confirm the following? in Precession movement the object does not fall because the resulting Torque is constantly changing the direction of the momentum vector since Torque= F . r then ist the change of the momentum equal Torque / r

PM 2Ring

11:23

@Slereah I think you have a MathJax typo or 2 in the 2nd bullet point of physics.stackexchange.com/a/536557/123208

Slereah

too late to edit

PM 2Ring

12:04

We'll be able to follow questions soon, and to follow answers a little while later:

31

Q: Upcoming Feature: Follow Questions

We are planning to release a new feature on the network in the next few weeks that will give users the ability to follow and unfollow questions. This was mentioned by Teresa in the Q1 Roadmap. The plan is to roll out these changes in multiple releases. (Work is well underway, and we are aiming fo...

JMac

13:53

@PM2Ring physics.stackexchange.com/questions/536152/… I get the feeling that "pittsjoe" is pittsburghjoe. Looks like he might have been hitting us up with another round of questions.

PM 2Ring

14:11

@JMac It's hard to be certain from a single sentence, but it does sound like him. And if it is him, he's trying to get around a suspension. So I'm going to flag that post.

JMac

@PM2Ring Looks like that one is already suspended. I'm kinda curious if the "closely related" question was also asked by them on a less obvious alt.

PM 2Ring

14:44

@JMac Ah, right. I should've checked pittjoe's profile. :oops: That closely related one does look pretty similar to his style. And that one hasn't been suspended yet...

bolbteppa

@Slereah he's the guy whose notes said susy comes from iterating the transition from KG to Dirac with Dirac

MadSpaceMemer

Hey guys can someone check out my question above?

i will repost the message

Hello, can you confirm the following? in Precession movement the object does not fall because the resulting Torque is constantly changing the direction of the momentum vector since Torque= F . r then ist the change of the momentum equal Torque / r

also this might stupid, but for a gyroscope for that is NOT rotating, a force acting on its Point of mass will cause a torque right? but if it is NOT rotating, then the object falls just to the ground, my question is, why doesnt it also rotate to the direction of the torque? the formula for the torque just says Force multipleid distance. i know the other formula which says Angular acceleration multiplied Intertia, but in this case toeque would be unequal zero in the first case and zero in the2nd

Slereah

15:01

@bolbteppa I kinda suspect that we call it the Coleman-Mandula theorem is because he has a horribly Irish name

bolbteppa

I think the O' theorem assumes a finite parameter group or something

Slereah

Yeah but you know

the naming of a theorem is fairly flexible

Gauss didn't have much to do with the Chern-Gauss-Bonnet theorem

And yet

bolbteppa

15:19

O'Raifeartaigh may yet be found in some Gauss or Euler notebook

vzn

15:39

in theory salon, 1 min ago, by vzn

How to Make Sense of Quantum Physics / hossenfelder/ palmer, quanta

bolbteppa

@vzn superdeterminism??

"We beg to differ. Quantum mechanics is perfectly comprehensible. It’s just that physicists abandoned the only way to make sense of it half a century ago. Fast forward to today and progress in the foundations of physics has all but stalled. The big questions that were open then are still open today. ...

How can we overcome this crisis? We think it’s about time to revisit a long-forgotten solution, Superdeterminism, the idea that no two places in the universe are truly independent of each other."

John Rennie

't Hooft is a fan of that isn't he?

Abhas Kumar Sinha

@JohnRennie That GR Professor?

John Rennie

Gerard 't Hooft

Gerardus (Gerard) 't Hooft (Dutch: [ˈɣeːrɑrt ət ˈɦoːft]; born July 5, 1946) is a Dutch theoretical physicist and professor at Utrecht University, the Netherlands. He shared the 1999 Nobel Prize in Physics with his thesis advisor Martinus J. G. Veltman "for elucidating the quantum structure of electroweak interactions". His work concentrates on gauge theory, black holes, quantum gravity and fundamental aspects of quantum mechanics. His contributions to physics include a proof that gauge theories are renormalizable, dimensional regularization and the holographic principle. == Personal life == He...

Abhas Kumar Sinha

wow...

bolbteppa

15:48

Someone references him in the comments as (I think) saying something different

John Rennie

@AbhasKumarSinha he's a member of this site though he hasn't posted anything for years.

Abhas Kumar Sinha

I guess that he gota Nobel proze.

John Rennie

I admire 't Hooft for his approach to this. He is on record as saying that he thinks it's probably crackpot but that he's old and famous enough to waste his time on fringe physics on the off chance that it might work out.

Which I think is a jolly good show.

Semiclassical

one can respect his intellectual commitment while not agreeing that it's worth you personally working on it, of course

Abhas Kumar Sinha

@Semiclassical Good Evening sir.

Semiclassical

15:55

hi

Abhas Kumar Sinha

@Semiclassical What are you reading now?

Semiclassical

not much at the moment.

Abhas Kumar Sinha

@Semiclassical What are your plans for now?

Semiclassical

sorta interested in reading up on baby level twistor stuff. (by which I mostly mean how to do lorentz transformations in terms of su(2))

Abhas Kumar Sinha

Cool sir :)

John Rennie

15:57

@Semiclassical I think 't Hooft's point is that it would be career suicide for young researchers to work on this, so it has to be done by people like him who don't stand to lose anything. And good for him!

Semiclassical

yeah

for me the closest example would probably be my flirtations with pilot wave stuff

bolbteppa

What the hell is a twistor

Semiclassical

Twistor theory

Twistor theory was proposed by Roger Penrose in 1967 as a possible path to quantum gravity and has evolved into a branch of theoretical and mathematical physics. Penrose proposed that twistor space should be the basic arena for physics from which space-time itself should emerge. It leads to a powerful set of mathematical tools that have applications to differential and integral geometry, nonlinear differential equations and representation theory and in physics to relativity and quantum field theory, in particular to scattering amplitudes. == Overview == Mathematically, projective twistor space...

basically, something penrose came up with

bolbteppa

Something about null light rays and then going to matrices

PM 2Ring

Jan 23 at 15:37, by PM 2Ring

@Knight See HOW to BECOME a BAD THEORETICAL PHYSICIST by Gerard 't Hooft.

Abhas Kumar Sinha

16:00

@PM2Ring oh okay,,,

Semiclassical

@bolbteppa for the part I'm wanting to read up on, see the second half of this john baez page: math.ucr.edu/home/baez/physics/Relativity/SR/penrose.html

kb314

Hello. I am not sure that this is the correct forum to ask this question. I am still a little new and learning the ropes, so to speak. A question I posted was closed as off-topic, with the suspicion of it being a homework question. Is there a way to let the moderator now that this is not a homework question?

Abhas Kumar Sinha

@PM2Ring very good blog tho :)

Semiclassical

so 4-vectors are in bijection with 2-by-2 hermitian matrices, and (proper, orthochronous) Lorentz transformations are in bijection with PSL(2,C) elements

John Rennie

@kb314 hi :-) I agree your question in't homework and have voted to reopen, though it takes four more votes from other site members to reopen the question.

kb314

16:03

Thanks @JohnRennie

Semiclassical

which somehow goes over to the Moebius group?

"So, PSL2(C), the [restricted] Lorentz group, and the Moebius group are all isomorphic as abstract groups."

John Rennie

@kb314 We get so many homework questions that sometimes questions get closed by accident when they shouldn't be. I can't really blamce the moderators for this since they work so hard to close questions that really should be closed.

2

kb314

Is this an uncommon error on the part of the moderator or is it very common for legitimate questions to get down-voted or closed, @JohnRennie?

Abhas Kumar Sinha

I've got my room reenabled, if anyone wants to have talk about AI Ideas, discussions.

Artificial Intelligence

This is the year 2109 on Earth. The reign of Terminators and A...

John Rennie

@kb314 You could post a question in the Physics Meta asking for the question to be reopened. That's the usual way of approaching this.

kb314

16:05

Ok thanks for the clarification!

ACuriousMind

@Semiclassical A story familiar to anyone in conformal field theory ;P

John Rennie

@kb314 it's relatively rare, but it does happen.

kb314

I see @JohnRennie. That's good to know...

ACuriousMind

People don't usually call the correspondence between Lorentz and Möbius group "twistor" unless in the kind of QG context where Penrose coined the term

Yuvraj Singh...

@PM2Ring hi. How are you?

PM 2Ring

16:15

@YuvrajSingh... Hi.

kb314

I have posted a question on Physics Meta bringing attention to my question being closed due to being an off-topic homework-like question, although it clearly is not, @JohnRennie. Let me also ask for a re-open vote from @PM2Ring, who has seen this post earlier.

John Rennie

@kb314 you already have three reopen votes, presumably from people who saw it mentioned here, so with luck it will be reopened soon.

PM 2Ring

@kb314 Yes, I've already voted to reopen it, so it just needs 2 more reopen votes.

kb314

Thanks @JohnRennie, @PM2Ring! Another question I have is whether it is fine to use chat rooms to bring questions to the attention of people.

John Rennie

16:30

@kb314 yes I think that's fine. People will start getting annoyed if you keep banging on about it, but a polite post mentioning it is no problem.

kb314

Ok, got it! Thanks again @JohnRennie.

Physics Meta

1

Q: Question closed as an off-topic homework question although it is clearly not a homework question

A question I asked has been closed by a moderator due to the suspicion of being as off-topic homework question, although it is clearly not a homework question. It was suggested to me on chat that one can ask for re-open votes via Physics Meta, besides raising this issue.

support closed-questions reopen voting moderators

Aaron Stevens

@JohnRennie Is meta supposed to be used to just ask for certain questions to be opened? I feel like meta is more for questions like "Help me understand why this was closed".

Or "What can I do to improve this question?"

John Rennie

@AaronStevens well it is kind of a "Help me understand why this was closed" question.

Secret

@vzn @bolbteppa Even if that is the approach to be take, it still left behind an unsatisfying ontology: How on earth the universe get so linked up in the first place and what controls which parts are more stronger in relatedness and which part is not

Aaron Stevens

16:40

@JohnRennie It was suggested to me on chat that one can ask for re-open votes via Physics Meta It seems just like a call for reopen votes. Is that fine for meta posts?

John Rennie

@AaronStevens it seems like a fine distinction to me. The question could have been worded as "help me understand why this was closed" and would have been basically the same question.

ACuriousMind

@AaronStevens @JohnRennie No, "asking for reopen votes" is not a proper use of meta. "Why was this question closed?" is. (Even though of course both kinds of questions lead to attention and therefore potential reopen votes)

Aaron Stevens

@JohnRennie I guess I am not familiar with basing questions off of what someone thinks it could have been

Secret

I mean, Mach principle is basically the GR analogue of that for example. But philosophically, it does not really explain why relatedness governs physics any more than why maths governs physics

John Rennie

I'm sure the mods will delete or close the question if they think it is inappropriate, though the bottom line is that the question should not have been closed.

Aaron Stevens

16:43

@JohnRennie Yes, I agree with that. I was just asking specifically about the meta post :)

Secret

Having said that, I don't mind people trying out that more. If it is true we have no freewill, then we knew exactly what to do about it

John Rennie

@ACuriousMind there's the letter of the law and the spirit of the law. Do we have to rigorously adhere to the former?

ACuriousMind

@JohnRennie In this case yes, in order to not set precedent. We can close "Why was this question closed?" questions when the answer is obvious or explained in a duplicate. If we let "Please reopen this question" stand, it becomes hard to explain why not every single user whose question was closed should post about it on meta.

John Rennie

@ACuriousMind so vote to reopen the question then delete or close the meta post.

It has 4 reopen votes already so reopening it is a formality.

ACuriousMind

@JohnRennie Well, that DZ thought this should be closed as HW-like but many users seem to "obviously" disagree suggests this case isn't obvious, so there is a meta discussion to be had here. I've edited the meta post.

I understand that from a viewpoint focused on the post in question this seems like a lot of fuzz over nothing, but we have to keep the larger perspective in mind here.

John Rennie

16:55

@ACuriousMind :-)

Semiclassical

@ACuriousMind yeah, but it seems like a reasonable warm-up towards twistors proper

bolbteppa

@Semiclassical I think stereographic projection is how Mobius is seen as natural

Semiclassical

ah, of course

bolbteppa

Since rotations leave $x^2+y^2+z^2=r^2$ invariant on sending $(x,y,z)$ into a new $(x',y',z')$, and stereographic projection sends $(x,y,z)$ into some point on the complex plane, a rotation on the sphere translates into a mobius transformation of the plane representation

Semiclassical

right

bolbteppa