The h Bar

12:08 AM

I think I just noticed a funny bug:

On this question

45

My family members dislike the idea of having many devices communicating wirelessly in our house, arguing that the signals have negative effects on our physical health. I would like to tell them the EM signals are in fact weaker than the light from our lights but I cannot really confirm this. Cou...

I hit the rep cap today, but then a user was removed and brought my earned rep below 200. But because I hit the cap before the user was removed, I think I'm cut off from earning more rep today.

skullpatrol

:-(

sad bug

DanielSank

::shrugs::

skullpatrol

yeah, water off a duck's back

Avnish Kabaj

1:33 AM

Ello

skullpetrol

1:49 AM

Hola

vzn

2:50 AM

@RyanUnger maybe your nickname should be the joker... am thinking of dark knight :P

spking of physics this is/ was very fun today... they had oculus rift quest hardware only $1/min... ps does anyone wanna hear my QM theory based on pinball? :P pinballshowdown.com/index.html

vzn

3:33 AM

@Mazura theres some study of forces of stress on solids/ rigid bodies associated with mechanical engineering...

skullpetrol

5:59 AM

@RyanUnger we are only a select few of the many that he has and will continue to scar.

Novalium Company

7:38 AM

Ok maybe not never, but very rarely only on special occasions.

skullpatrol

and always remember: in moderation

> the quality of doing something within reasonable limits

Novalium Company

11:12 AM

@skullpatrol yep, meaning, don't get too drunk

I know my limits (since I tested them already) so I know how much is moderate :)

Emilio Pisanty

@DanielSank curious. The question is, does it still count as progress towards the rep-cap badges

PM 2Ring

1:21 PM

@JMac FWIW, Negative Quantum Space-Time guy has started a thread on the xkcd Science forum: forums.xkcd.com/viewtopic.php?f=18&t=126636

Semiclassical

1:50 PM

@RyanUnger one of the more amusing things I’ve seen on the arxiv was the suggestion of a Bohmian/pilot wave interpretation of string theory

Which I sorta want to be right, if only for how perverse that would be for the kind of people who like string theory

Slereah

I don't think they've even managed a correct version of relativistic Bohmian mechanics

PM 2Ring

Hey @Semiclassical How did your talk go?

JMac

@PM2Ring Wow, thanks. Watching that unfold somewhere that actually allows conversations is basically exactly what I expected to see. I'm surprised they haven't threatened to hold everyone on that forum accountable yet. They are dismissing him much more directly than we did lol

PM 2Ring

2:08 PM

@JMac I guess the OP is happy that the thread hasn't been locked yet. Yes, several posters have been dismissive, but some are trying to extract some meaningful content from the OP's vague ramblings.

vzn

← continuing long theme of semirandom googling + "ideas mix™" just found 2 deep papers linking strings + solitons... does anyone care? :| o_O

JMac

@PM2Ring Yup, I guess suggesting that he was not using the site appropriately was taken as more dismissive to him than straight up telling him many reasons why it is nonsense. Kinda weird, but I guess they feel like being disregarded is worse than being told you make no sense

PM 2Ring

It's nice that his insults have been ignored. But if the rude behaviour gets out of hand, the mod (gmalivuk) will step in. FWIW, swearing & rude words are perfectly acceptable on xkcd, but being nasty isn't tolerated.

vzn

3

Q: Moduli spaces in string theory vs. soliton theory

In both string theory and soliton theory, moduli spaces are frequently used. As far as I known, for soliton theory, moduli spaces are something like collective coordinates for solitons, and for string theory, moduli spaces is the spaces of all metrices divided by all conformal rescalings and di...

quantum-field-theory string-theory mathematical-physics solitons

These are very different moduli spaces, but they both follow the same idea. Have you looked at, e.g., moduli spaces on the nLab? — ACuriousMind ♦ Sep 9 '14 at 12:50

PM 2Ring

@JMac I get the impression that he really doesn't get it when people tell him that his "theory" doesn't actually explain anything. He knows what his words mean to him, and he thinks that the meaning should be obvious to anyone else. In contrast, a bunch of mathematical equations doesn't explain anything that "normal" people can understand. :rolls eyes:

Semiclassical

2:19 PM

@PM2Ring pretty good, I think?

JMac

I don't know what it is really that leads people to think that is how "theories" work. I'm curious if they've ever taken a science course. Odds are probably like high-school physics at best

vzn

lol maybe xkcd is the perfect forum for pittsburghjoe. anyway he should get a blog. would be amused to at least read his "about page". if you guys are really into it try inviting him into this chat :| :P

PM 2Ring

@Semiclassical Oh, good. :)

JMac

I just love the circularity of it. "How would you show that this is true?" "Well you're the science people, you know what it would mean if it is true, and I know it is true, so prove it for me"

Semiclassical

It was a tricky audience: for the quantum info people in the audience it was probably familiar and a bit labored. But for the foundations people it seemed like the right pitch

vzn

2:24 PM

@JMac putting aside the crank denigration & trying to extract something worthwhile, you might find it interesting how much HUP-like principles enter into classical physics, as in some of the responses eg citing fourier analysis. havent seen a really deep analysis/ survey anywhere, have seen scattered bits + pieces.

bolbteppa

@Semiclassical link to the bohm string thing? Vaguely remember it

PM 2Ring

@JMac I think it's because they believe the world runs on narrativium.

"Our minds make stories, and stories make our minds. Each culture's Make-a-Human kit is built from stories, and maintained by stories. A story can be a rule for living according to one's culture, a useful survival trick, a clue to the grandeur of the universe, or a mental hypothesis about what might happen if we pursue a particular course. Stories map out the phase space of existence."

bolbteppa

@vzn there is no HUP principle in classical physics, Fourier analysis simply lets you go from position to momentum space and from this you could expect some HUP thing to come out of it in QM because the foundations encode it unlike classical physics

vzn

@bolbteppa the math seems to be nearly/ exactly the same, would dig up refs but probably nobody would read them and/ or be convinced. there seems to be a "math pattern aversion" almost as part of physics training...

bolbteppa

Depends on what you're talking about, you can use Fourier analysis to maybe solve some F = ma equation, or in analyzing waves etc... but so what?

It sounds like you're saying because two words in the alphabet use some of the same letters they must mean the same thing :p

Just because you can use the same math tools in different areas, doesn't mean anything

vzn

2:29 PM

@bolbteppa its more than "tools" its nearly identical formulas. are you familiar with fourier analysis?

Semiclassical

@bolbteppa it’s a Nikolic paper I think? Lemme look

bolbteppa

@vzn yes, some brief examples would be good

Here's one thing but I remember a different article

Semiclassical

Maybe arxiv.org/abs/hep-th/0702060

bolbteppa

That's it

Semiclassical

I have no idea how seriously to take Nikolic tbh

bolbteppa

2:35 PM

Debates on PF a lot

Semiclassical

But it’s a delightfully perverse proposition

Plus, the suggestion that one sidestep the problems of Bohmian field theory by looking at strings instead...I dunno, kinda appealing to me

Tho again I’m not at all prepared to defend it

vzn

@bolbteppa am willing to dig into this but it would take extended analysis (with someone with the nec attn span)... have never fully grasped it myself, have wanted to survey it, as stated am always looking for good refs myself. again googling, here is at least one author that describes something along the lines. arxiv.org/ftp/quant-ph/papers/0306/0306173.pdf

Semiclassical

@bolbteppa yeah, I noticed that. Kinda weird

vzn

> The uncertainty relation of the Heisenberg in a traditional statement actually is the product not quantum, but classic wave theory. In the classic theory from link of frequency (energy) and temporary patterns by means of Fourier transform follows, that for the complete characteristic of the physical object it is necessary to use not numbers, and distributions. At a jumboizing of particles, i.e. at transition to macroscopic objects the appropriate distributions tend to δ-functions.
> For optical impulses the transition to a macroscopic limit is impossible. The optical impulse should be cha…

bolbteppa

I don't know what to make of this BM string stuff either haha

Semiclassical

2:43 PM

When I try to describe how I feel about Bohmian mech in general, a Simpsons joke comes to mind

Pritt Balagopal

Hello!

bolbteppa

@vzn the references in that article are the books I keep referencing, which is good

vzn

@bolbteppa there is some mythology associated with HUP relating to bohrian mystique, it got tied up in the ideological framework of "complementarity" nearly playing a central role. it needs a very careful look/ historical "disentanglement".

Semiclassical

The bit from 0:20-0:25 here: youtu.be/NwsUsshY4Tc

Pritt Balagopal

Does anyone happen to know how Reynold's number should be derived for non-newtonian fluids? I suspect we should replace $\mu$ with $\frac{\tau}{\left(\frac{dv}{dz}\right)}$, but I am unsure. Can someone help me out?

bolbteppa

2:47 PM

@vzn that article is pretty much unreadable, idk what they mean in the conclusion

@Semiclassical haha

Semiclassical

lol yeah

Pritt Balagopal

By $\tau$, I mean the shear-stress

vzn

@bolbteppa think he didnt fully make the case mathematically. the point is many credentialed authors have made similar assertions about HUP and dont think theyre all wrong. think its just not fully understood yet (because its hard to understand). aka blind men + elephant o_O

bolbteppa

Fourier transform

The Fourier transform (FT) decomposes (analyzes) a function of time (a signal) into its constituent frequencies. This is similar to the way a musical chord can be expressed in terms of the volumes and frequencies (or pitches) of its constituent notes. The term Fourier transform refers to both the frequency domain representation and the mathematical operation that associates the frequency domain representation to a function of time. The Fourier transform of a function of time is itself a complex-valued function of frequency, whose magnitude (modulus) represents the amount of that frequency present...

"Generally speaking, the more concentrated f (x) is, the more spread out its Fourier transform f̂ (ξ) must be. In particular, the scaling property of the Fourier transform may be seen as saying: if we squeeze a function in x, its Fourier transform stretches out in ξ. It is not possible to arbitrarily concentrate both a function and its Fourier transform."

"In quantum mechanics, the momentum and position wave functions are Fourier transform pairs, to within a factor of Planck's constant."

Things are different in classical mechanics

Semiclassical

My main feeling with BM: there are perfectly good reasons to not think it’s really workable as a picture of the world; one does not need to invoke bad reasons!

vzn

2:53 PM

@bolbteppa suspect a HUP like relation appears somewhere in classical mechanics with waves, not sure what plancks constant corresponds to.

Semiclassical

(Tho the way Bohm presented his ideas didn’t really help matters)

knzhou

Well, of course uncertainty relations are all over classical mechanics, because they arise whenever a Fourier transform comes up.

That doesn't mean the Heisenberg uncertainty relation is "classical", because it comes from assuming position and momentum are Fourier conjugates. Which is very much not a classical thing.

Semiclassical

BM, as far as I know, is compatible with neither the principle of inertia (save in the sense of a classical limit) nor relativity of simultaneity

knzhou

You might as well say that the Schrodinger equation is the same as Newton's second law because both have time derivatives in them.

vzn

@bolbteppa nice find. exactly! the wikipedia Fourier transform article actually implies that theres a HUP like relation associated with any wave mechanics scenario. thats my point. its presented rather abstractly.

Semiclassical

2:57 PM

My first point means that BM is not Newtonian, but the latter means it’s going to be awfully hard to square with special relativity

So while I think calling BM “Newtonian” (or “classical”) is a silly insult, the charge that it’s inherently in tension with relativity...yeeeep

bolbteppa

@vzn roughly the wiki is saying that the HUP generalizes to the claim that "It is not possible to arbitrarily concentrate both a function and its Fourier transform", so anywhere you use Fourier theory the HUP 'could' arise, but who cares, QM is saying we should replace the position and momentum (independent variables in phase space) with Fourier-conjugated variables which have this HUP property built-in, the question is WHY do we replace those variables, you'll never "derive" this from math

Semiclassical

Luckily, the latter doesn’t stop me from using BM as a visualization tool for non-relativistic problems :)

vzn

@bolbteppa QM has airbrushed out the deep link to CM in the HUP to the point that even physics experts are barely aware of it. something is missing™

knzhou

On the contrary, I'm pretty sure it's textbook material. This was all in my Quantum Mechanics I class.

bolbteppa

@vzn you're missing how radical the claim is... We're talking about replacing the set of independent labels required to fully specify the motion of a particle with a different set labels that inherently have this HUP inequality between them, thus preventing us from specifying both labels simultaneously, and the only reason we do this is because experiments show we can't specify both variables simultaneously ever, otherwise some form of classical mechanics would work, Newton/Einstein+

vzn

3:02 PM

@knzhou ok, can anyone cite a classical analog of HUP...?

knzhou

One easy example from acoustics is that it's harder to hear pitches of really short notes.

Which has been experimentally confirmed plenty of times.

vzn

@bolbteppa QM is less radical than is widely realized, the HUP CM connection is a major case for that. as stated early on, am not expecting anyone around here to realize/ agree with that. :|

knzhou

But that doesn't mean that music is quantum mechanics. They just have a few things in common.

vzn

@knzhou right, has anyone written out a HUP-like relation for sound waves anywhere? think its gotta be somewhere but cant pinpoint it.

knzhou

In the context of acoustics and signal processing it's called the Gabor limit.

It looks just the same, it's Delta x Delta k > 1 where k is the wavenumber.

vzn

3:06 PM

ah! thx! :)

PM 2Ring

"if we squeeze a function in x, its Fourier transform stretches out in ξ" And vice versa. That's a very core feature of the Fourier transform, so anyone using Fourier transforms in any branch of physics is fully aware of it.

vzn

from wikipedia HUP

> In the context of signal processing, and in particular time–frequency analysis, uncertainty principles are referred to as the Gabor limit, after Dennis Gabor, or sometimes the Heisenberg–Gabor limit.

bolbteppa

@vzn uncertainty principle when tuning musical instruments

vzn

now, how many QM textbooks cite Gabor? did heisenberg even cite him? o_O :P

knzhou

Gabor did his thing after Heisenberg, so I would expect not.

QM textbooks don't mention Gabor because they don't have to talk about everything that also uses the same mathematical tools they use.

vzn

3:12 PM

lol hmmm, really? nevertheless classical wave mechanics was discovered long before QM... maybe it wasnt fully understood in historical order...

Slereah

I have a qm textbook from a signal processing standpoint

This is not a particularly obscure idea

vzn

ok, googling, heres a comprehensive ref on gabor that indicates he developed the principle in the 1940s for signal processing/ information theory by analogy with QM its.caltech.edu/~matilde/GaborLocalization.pdf however, its clear this theory is not dependent on QM, its a basic mathematical property of CM wave mechanics.

Slereah

Yes

bolbteppa

It's a basic property of mathematics

Slereah

As is well known???

I mean it is pretty basic qm that momentum space is the fourier transform of position space

vzn

3:18 PM

> Heisenberg's Uncertainty Principle follows from a classical result, which is at least as old as Fourier.

Slereah

That QM has analogies with wave mechanics is not at all a new fact

It is literally the oldest thing about QM

vzn

@Slereah its not a new fact but feel its a misunderstood fact. isnt it interesting that the Gabor connection to CM took ~2 decades after HUP to uncover? does that give no modern physicist any pause?

ACuriousMind

One needs to be careful with terminology here: It is not "the HUP", it is the HUP for position and momentum that can be seen from the classical Fourier uncertainty once one knows that quantumly position and momentum should be differentiation and multiplication on an $L^2$ space. The uncertainly principle for generic observables is much more general and does not reduce to Fourier uncertainly since by far not all observables are Fourier conjugate.

PM 2Ring

@vzn The "if we squeeze a function in x, its Fourier transform stretches out in ξ" thing is basic Fourier. That position & momentum are related by the Fourier transform is a new idea that QM brought to the table.

ACuriousMind

@vzn We can easily invert that reasoning: Does it not give you any pause that everyone is telling you that this is well-known and unsurprising yet you insist it is somehow "misunderstood"?

3

bolbteppa

3:23 PM

@vzn for a single particle in CM, one uses points $(x,p)$ in phase space to label where it is at each instant of time. QM says we can't do this, we have to replace $x$ and $p$ by operators which are Fourier conjugates and can't be used to simultaneously produce labels we can interpret as position and momenta, just because some multiparticle classical systems exhibit overall wave behavior, the individual particles themselves are still specified by $(x,p)$ labels, something QM eschews

vzn

← again, stated early on, did not expect anyone around here to agree with me even as have not really written any incorrect statements, quite to the contrary...

ACuriousMind

In order to claim that "physicists" misunderstand something, one should first demonstrate that one in fact knows how physicists understand something.

8

vzn

its a fact that physicists understanding of physics has changed over the years, and its not a very credible argument that it will never change in the future... but it seems no individual physicist will admit that maybe his own understanding of physics is incomplete in some (key) way...

ACuriousMind

It's fortunate that no one has made such a non-credible argument, then.

bolbteppa

@vzn "Heisenberg’s uncertainty principle is a consequence of the classical Cauchy–Schwartz inequality and is one of the cornerstones of quantum theory... Dennis Gabor (1946), inventor of the hologram, was the first to realize that the uncertainty principle applies to signals. Thanks to wave-particle duality, signals turn out to be exactly analogous to quantum systems. " link

vzn

3:28 PM

@bolbteppa lol... Thanks to wave-particle duality, signals turn out to be exactly analogous to classical systems.

or... more radically (who was just talking about radical?) QM systems turn out to be exactly analogous to classical signals/ classical wave mechanics...

Slereah

sigh

bolbteppa

lol

@vzn where are the signals in the Tenev stuff?

Semiclassical

These conversations inevitably remind me of the old “I’m not Rapaport” routine

Slereah

it just reminds me of every crank conversation

3:47 PM

← somehow reminds me of Waiting for Godot...

user2723984

if you see uncertainty relations from an information theory point of view, there certainly are both classical and quantum uncertainty relations, the latter being a more general case

and not only between Fourier conjugate quantities either

vzn

@bolbteppa you seem to routinely ½ mock me or more, making it hard to tell individual cases, but will respond to your question seriously. the implication from all this is that HUP is the Gabor uncertainty relation for waves that propagate thru the spacetime fluid-fabric much like in a classical way. (know this is a radical assertion, but the points lead to it...) QM is a special case of CM.

bolbteppa

@vzn see I knew you found a link ;) That last sentence is completely ridiculous

vzn

@bolbteppa QM emerging from CM is asserted by many highly credentialed physicists, its a basic theme of "emergent QM" of which theres been many conferences now. agreed its a minority pov... currently

bolbteppa

4:07 PM

@vzn no serious people assert this, it's almost like you knew where this was going to end up from the beginning...

Slereah

QM as fluid mechanics has been tried before I believe

Like 90 years ago

vzn

@bolbteppa judge for yourself from lists of attendees, looks like 4 conferences, every 2 yrs dating to 2011 latest 2017 emqm17.org ... seems many have many more credentials than pseudonymous internet chatters with sketchy backgrounds...

Slereah

I don't doubt them

I doubt you

vzn

@Slereah people tried to create flight machines pre 20th century also.

Slereah

Oh man are we going with platitudes now

Here's mine then

From good old James Randy

"They laughed at Galileo, they also laughed at Bozo the Clown"

bolbteppa

4:15 PM

haha

user2723984

@vzn it seems to me the conference in the link doesn't talk about QM emerging from classical mechanics, but emerging from some deeper theory

tpg2114

@knzhou I clicked the wrong button on your latest flag and it marked it declined, but it was helpful and has been addressed. Sorry about that!

user2723984

"The focus is the search for a “deeperlevel” theory for quantum mechanics that interconnects three fields of knowledge: emergence, the quantum, and information. Contributions will be featured that present current advances in realist approaches to quantum mechanics, including new experiments, work in quantum foundations, and the physics of the quantum observer and the conscious experimenter agent."

bolbteppa

They pretty much always mean classical mechanics when they say 'deeper theory'

vzn

@user2723984 the papers/ povs/ proponents are diverse. nobody has all the answers yet, at least some are asking the right questions™

user2723984

4:17 PM

@bolbteppa do they really? well that boils down to what people mean when they say classical mechanics

Slereah

I don't dismiss the Bohmian people

user2723984

if they mean local deterministic theory, then yes I agree

Slereah

I'm entirely fine with pet theories

I just think you are particularly dismissive yourself of consensus in general on no particularly good ground

2

danielunderwood

@Slereah you mentioned a QM book from a signal processing standpoint...any chance you have the title? Or is it a worthwhile read?

Slereah

without any good understanding of them

@danielunderwood Caractère quantique matière et rayonnement

maybe not ideal for you

but then again, teaching QM as just wave mechanics is no revolutionary method

Especially in older textbooks

danielunderwood

4:19 PM

ah yes it sounds like it may be a bit of a difficult read for me

My QM courses kind of started with wave mechanics, but it always seemed pretty hand wavey

Slereah

Modern textbooks try to do it more from the abstract Hilbert space standpoint I suppose

vzn

@Slereah theres some very strong consensus against new ideas, it cannot be dismissed lol

danielunderwood

I think that's the way Dirac begins as well? Though I got distracted and swapped to something different while reading that

Slereah

shrug

I think modern day physics is mostly fine with new weird ideas

The whole relativity/QM shenanigan has given most physicist the impression that physics may have to get weird

some theories tend to be thrown out due to being shown wrong again and again

They may be patched up or not, I don't know

but I don't think you particularly care

you are at heart a contrarian

vzn

new theories always have a key element contrary to existing paradigms, its unavoidable... ofc do care about science/ physics... science/ physics means different things to different people, even scientists/ physicists...

bolbteppa

4:27 PM

Literally the very first thing one has to confront with their new theory is QFT i.e. relativity and QM, address all the issues QM+SR bring up, which are crazy things

A simple thing is - does your new theory say anything about Compton scattering, if it doesn't your theory is a total waste of time basically

I'm simply amazed people haven't bluffed BM to account for QFT yet

Slereah

oh they tried

But Bohm isn't very nice to adapt to that

for fairly obvious reasons

bolbteppa

Yeah, but from what I've seen they seem to keep trying by starting from the non-relativistic schrodinger equation (ahaha)

Slereah

well what else can they do

user2723984

the theory is perfectly relativistic in the non relativistic limit

Slereah

The pilot wave violates causality

Bad starting point

There's a paper on arxiv that attempts it if you want

bolbteppa

4:30 PM

This should be the end of BM if the starting point is so tied to the Galilean group, idk if they even know it is

I feel like it is eminently bluffable to repeat what they do (or want to do) with the Poincare group

Slereah

"QFT as pilot-wave theory of particle creation and destruction"

"Pilot-wave theory and quantum fields"

"The De Broglie-Bohm theory of motion and quantum field theory"

there's a few papers

though from what I remember they're kind of vague sketches

"Bohmian-type QFT and Malament no-go theorem"

o no not Malament

bolbteppa

4:47 PM

I'm not sure what to make of BM papers such as these, like on the second page it seems like fundamental misunderstandings are made, e.g. wanting wave function probability distributions in QFT and claiming a generalization to spacetime is an 'advance' when e.g. photon position-space wave functions demonstrably do not even exist in QFT

Slereah

Well you know

Most physicists misunderstand qft

It is usually poorly explained

bolbteppa

5:17 PM

Another thing is the claim about making multi-particle QM consistent with creation and annihilation of particles, like the formalism is already consistent, in terms of the formalism you're kind of only talking about whether your theory has conservation or non-conservation of particle number, seems like the issue is

"How to make a theory that describes deterministic continuous tra-jectories compatible with the idea that a trajectory may have a singular point at which the trajectory begins or ends?"

Physics Meta

1

Q: Referring people with SR-questions to test-bodies in the Schwarzschild solution?

I guess mostly because of how relativity is taught in schools, with a strong focus on the sometimes "unfinished" theory of special relativity and then a little bit on general relativity there are in my opinion a lot of questions on this site that could be answered by treating the effects (mostly ...

discussion answers best-practices

bolbteppa

i.e. a conceptual thing, but if BM has issues with something so fundamental the whole thing is clearly nonsense, so again I really don't understand why they can't bluff BM better since the formalism they use should allow it

Slereah

5:30 PM

Trying to mix the poorly understood QFT and the poorly understood quantum interpretation isn't a very good recipe

I would only trust an insane person like Malament to do such a thing

ACuriousMind

@Slereah I think the problem, in the end, is that QFT is rather young, so we usually teach it "historically". If we'd teach Newtonian mechanics relying on the original Newton's texts, it would also look very different. There haven't been enough generations of physicists learning QFT to complete the cultural evolution of the pedagogy into a form that is best suited to learn it from scratch

heather

hello

i'm having a bit of a misunderstanding with how renormalization works for qubit measurement. so when you measure a single qubit out of, say, a two qubit state, i know you create a projection matrix based upon the measurement results to remove the inconsistent states.

in my case, in my simulator i create a two qubit system in a zero state, apply a hadamard gate, and then measure the first qubit, and say the first qubit collapses to a 1 state

then the projection matrix is $\begin{bmatrix}0&0&0&0 \\ 0&1&0&0 \\ 0&0&0&0 \\ 0&0&0&1 \end{bmatrix}$

is this correct so far?

ACuriousMind

Depends on what the basis for that matrix is :P

heather

[and then i would multiply the projection matrix by the q state - in this case $\begin{bmatrix}1/\sqrt{2} \\ 0 \\ 1/\sqrt{2} \\ 0\end{bmatrix}$

ACuriousMind

(I'm not into q. information, so I don't know if there's a canonical choice)

heather

5:45 PM

@ACuriousMind i'm using 00, 01, 10, 11 which as i understand is the standard basis

ACuriousMind

then the projection matrix is correct

heather

okay, so that happens, and then you normalize the state, right? so $1/\sqrt{\text{norm}}$ times the qubit state (which has been multiplied by the projection matrix)?

i'm just getting a divide by zero error in my code, so i know i'm doing something wrong somewhere in the process.

i think i must be incorrectly calculating the norm

no, that looks right too...

is the norm calculated based upon the original qubit state, or based upon the qubit state multiplied by the projection matrix?

but if it's calculated upon the original state, it would always just be one, so that doesn't make sense.

the problem is when i multiply the projection matrix by the qubit state, i just get $\begin{bmatrix}0\\0\\0\\0\end{bmatrix}$

and then i get a norm of 0...

Mithrandir24601

@bolbteppa I'd argue that you could create such a photon state, with some work, but doing anything with it would be rather hard :P

(Or at least, some reasonably reasonable approximation, in at least some situations)

Oh wait, bother this - we're back to 'it depends on what you mean by a photon' again...

@heather the latter

heather

6:03 PM

@Mithrandir24601 okay...that makes sense...argh, can't figure out what i'm doing wrong =P

Mithrandir24601

@heather ah, then, either something is wrong, or there's zero probability of getting the state you're trying to get

heather

@Mithrandir24601 there's not zero probability of the first qubit becoming one, i don't think

Mithrandir24601

@heather apply H to which qubits?

heather

@Mithrandir24601 first qubit

Mithrandir24601

@heather I'm trying to figure out what the projection operator is :P (I'm on mobile)

heather

6:07 PM

@Mithrandir24601 diagonal matrix with the 1s only on the second row and fourth row - everything else is zero.

Mithrandir24601

@heather I believe you're trying to project the 2nd qubit into state 1

heather

crap you're right

well

that explains a lot of things =P

Mithrandir24601

@heather possibly :P

heather

@Mithrandir24601 i was wondering why my measurement results were so screwy, lol

there we go

it was an indexing error, so it was looking at the acceptable basis states if the second qubit was one, not the first

Mithrandir24601

Hmm, so that's what an indexing error looks like in quprogramming

heather

6:14 PM

er...i don't know if it is, i'm writing a simulator, not using qasm

you can look at it here if you want

it's actually reasonably close to done with the ideal part...just one more bug i have to nail down, then it'll be usable...and then i have improvements i want to make beyond that.

e.g., custom parser for user input in one spot so there's no use of eval().

Sir Cumference

Hey folks

Mithrandir24601

Oh yeah, @heather did I ever mention that they're happy with you applying to the Bristol summer school, if you want - they're more concerned about 'not started uni' over exact details about what exams you have/haven't done. I was told to mention that it's just ran by busy students, so it's not going to be anywhere near perfect or anything, considering you're so far away

heather

@Mithrandir24601 oh, no you didn't...it's probably too late to apply now though

and yeah, i'd have to travel all the way there so =/

unfortunate though, because it's sounds super cool

Emilio Pisanty

@heather don't let that stop you

if it's past deadline then don't take it hard if they say no

but there's rarely harm in trying

or at least getting in touch

heather

...actually it's not even past deadline, wow

Emilio Pisanty

6:30 PM

@heather so go for it

Bristol is awesome

heather

@EmilioPisanty i'd like to; i'll talk to my parents about it. remember i live in the u.s. so there would be somewhat significant travel expenses

Emilio Pisanty

@heather yes, I know

have you been to the UK? To Europe? Outside the US?

Mithrandir24601

@heather deadline is the 31st May. I won't lie - having someone from so far away would just add pressure on them, so if you did apply, they might say no as they don't want more pressure, but they might say yes, as getting someone from outside Europe kind of makes it a 'bigger' event

heather

@EmilioPisanty yes, yes, and yes

i've been very lucky in that respect

Emilio Pisanty

@knzhou what's with $\epsilon_{abcd}\epsilon^{efgh}=-\delta^{efgh}_{abcd}$ here? It looks dead wrong to me. Why should the efgh slots care about what's happening on the abcd slots?

@heather ah, so you don't have the novelty aspect when arguing for why you should go =P

knzhou

6:35 PM

@EmilioPisanty The generalized Kronecker says "the top 4 entries have to be the same set as the bottom 4", with an extra sign for permutations between them.

The two epsilons mean that abcd and efgh both have to be permutations of (0, 1, 2, 3).

Mithrandir24601

But yeah, Bristol's great - we have a big lab (and some smaller ones) and feedback has been really good since it started. It's a long journey through. :P

knzhou

It's not really that abcd and efgh are related, but that they just happen to be constrained in the same way.

heather

@Mithrandir24601 yeah, i'll talk to my parents about it for sure =)

skullpetrol

\o @SirCumference

Emilio Pisanty

@knzhou hmmmmm.

OK, so it's not that the formula is wrong, it's just that the object on the right makes no sense

knzhou

6:38 PM

It's not nonsensical, it's just kind of unusual! You can see it defined this way in, e.g. Schutz's Geometrical Methods.

Also it wouldn't work if the number of indices on each epsilon wasn't equal to the dimension, so it's kind of fragile.

Emilio Pisanty

@knzhou mostly, I don't see the justification for using the symbol $\delta$ on an antisymmetric object

bolbteppa

The idea is something like - if $\varepsilon_{abcd} = \delta_{abcd}^{1234}$ then $\varepsilon_{abcd} \varepsilon^{efgh} = \delta_{abcd}^{1234} \delta^{efgh}_{1234} = \delta^{efgh}_{abcd}$

knzhou

It is pretty weird! But you can define it however you want, I guess.

bolbteppa

Should be a minus due to the Lorentz metric, hmm

Emilio Pisanty

@knzhou you can define your notation however you want, but if you define misleading notation then your notation is misleading

knzhou

6:40 PM

With regards to this question I feel like we should modify the non-mainstream reason to "non-mainstream, or is quantum gravity"...

bolbteppa

Kronecker delta

In mathematics, the Kronecker delta (named after Leopold Kronecker) is a function of two variables, usually just non-negative integers. The function is 1 if the variables are equal, and 0 otherwise: δ i j = { 0 if i ≠ j ,...

Last equation of this section

knzhou

I don't think we can make a credible case that the OP's quantum gravity thing is wrong. But nobody can make a credible case that it's right either.

Most of our quantum gravity questions end up turning out that way...

Emilio Pisanty

@knzhou I can make a credible case that OP's post makes no sense at all

knzhou

I hear far vaguer things from real quantum gravity researchers all the time, though.

Basically we would need a criterion that is powerful enough to say that is wrong, but not say that, e.g. this is wrong.

> We reject here the concept of universe as machine not least
because it "has to postulate explicitly or implicitly, a supermachine, a scheme,
a device, a miracle, which will turn out universes in infinite variety and infinite
number. [...] This circumstance reminds us anew that no account of existence can ever hope to rate as fundamental which does not translate all of continuum physics into the language of bits.

heather

@Mithrandir24601 another question for you - is the standard basis for 3 qubits 000, 001, 010, 011, 100, 101, 110, 111?

ACuriousMind

6:49 PM

@knzhou I wish I could disagree with you there...

Mithrandir24601

@heather yep

heather

@Mithrandir24601 sweet, thank you

Mithrandir24601

I've wondered before what writing the basis in Gray code would make easier/harder, especially for photonics

Sir Cumference

@skullpetrol How's life

skullpatrol

Still alive @SirCumference how about you pal?

Emilio Pisanty

6:55 PM

@heather more generally, the standard basis follows the standard order of its binary digits (i.e. 011 goes before 100 because 3 goes before 4)

heather

@EmilioPisanty ah, okay.

bolbteppa

@Slereah bad EU ref exit results in France so far :\

Sir Cumference

@skullpatrol Same, just relaxing for once

Summer break is very welcome

Semiclassical

7:26 PM

Hello from London

@bolbteppa oof

Emilio Pisanty

@Semiclassical for how long will you be there?

Novalium Company

8:20 PM

Why is Feynman so admired? I mean, I haven't heard anything about this guy.

ACuriousMind

@NovaliumCompany He was one of the fathers of quantum electrodynamics, which is to date our best theory for the quantum behaviour of charged particles, and of the path integral approach. He was also a bit of a showman :P

dmckee

@NovaliumCompany He was (a) a very good physicist, (b) a good communicator, and (c) a character who was not in the least adverse to a little self-promotion.

It's the last two that separate him from most other physicists of his caliber.

Many really great physicists are not particularly interested in popularization and communication with the lay masses. And most of those who are communicators are not—in and of themselves—really interesting people: they just present some interesting stuff.

ACuriousMind

I think there's an interview with Gell-Mann where he complains about Feynman getting all of the fame

Then again, Gell-Mann was known to be something of a curmudgeon :P

PM 2Ring

8:36 PM

@NovaliumCompany This short article will give you a taste of Feynman's style. Although it was written about Brazil over half a century ago, the topic is still important in many regions today. v.cx/2010/04/feynman-brazil-education

dmckee

@ACuriousMind He was certainly in a grumpy mood the time I met him.

Emilio Pisanty

@dmckee are you using em dashes in chat?

dmckee

And my advisor said that was par for the course.

@EmilioPisanty They are easy to type on a mac. Option-shift-hyphen.

Emilio Pisanty

@dmckee huh

go figure

dmckee

I get hugely frustrated about the difficulty of getting them on other platforms.

Emilio Pisanty

8:38 PM

@dmckee you know, they're not that important

PM 2Ring

But you should read his book QED: The Strange Theory of Light and Matter and the autobiographical Surely You're Joking, Mr. Feynman!

dmckee

@EmilioPisanty I should know. I tell myself that. I just find that I still care.

Mithrandir24601

@dmckee and d) an extremely unpleasant person

Semiclassical

@EmilioPisanty my return flight is noon Friday

dmckee

@Mithrandir24601 I've heard that from time to time. I sounds to me like you were either under his spell or baffled by how other people fell for it.

Mithrandir24601

8:41 PM

@dmckee I heard about it, was it second-hand?

Sounded horrible

Emilio Pisanty

@Semiclassical I'm there from Tuesday through Saturday, with some time to kill, particularly Tuesday. If you're up for it, get in touch and we can get coffee or some such.

Mithrandir24601

(possibly third-hand, I've forgotten)

PM 2Ring

en dash is easy enough on *nix systems, using the Compose key. fsymbols.com/keyboard/linux/compose

Mithrandir24601

I'll hopefully be back in Bristol at some point this week

But then, neither of you will probably be near Bristol

dmckee

@Mithrandir24601 That was meant to be a generic you, so I meant "People were either under his spell or ..."

Mithrandir24601

8:45 PM

@dmckee Ahh, I see, yeah

But yeah, getting people to believe in you doesn't excuse atrocious behaviour that would at the absolute least deserve to get you fired today

(that was also a generic you :P )

Emilio Pisanty

@Mithrandir24601 I'll be back in the UK in August, get the structured-light group at Bristol to invite me to give a seminar and I'll be happy to oblige ;-)

ACuriousMind

@EmilioPisanty I...think you responded to the wrong message :P

Emilio Pisanty

@ACuriousMind uh

no I didn't

=P

Mithrandir24601

@EmilioPisanty Excellent! How does Thursday lunchtime sound?

(I'll need to ask, but we have a weekly seminar then and there's free pizza)

Emilio Pisanty

@Mithrandir24601 that trip is still a formless blob right now =P

Mithrandir24601

8:52 PM

(also won't specifically be the structured-light group, but either 'photonics' or generic QETlabs)

@EmilioPisanty I'll ask whoever the right person to ask is when I get back to Bristol then :)

Semiclassical

@EmilioPisanty hmm, that’d be interesting

I need to reserve one day for going out to west England—there’s something I want to see out there—but I’m pretty flexible beyond that

Emilio Pisanty

@Mithrandir24601 I need to be in the Liverpool area, and I'd like to visit Birmingham, but it's all essentially still in the air at this point. Any opportunity to talk about arXiv:1808.05193 is welcome; at Bristol the highest interest will be in the Light and Matter Theory group, primarily Knots in Physics and Wave Geometry & Optical Field Theory. But I would definitely also like to visit the quantum photonics side :-).

Mithrandir24601

@EmilioPisanty Ahh, that group - I know one of the postdocs, so if she's still around then, I'll ask her

Semiclassical

Let me know how that modifies the schedule. Like I said, my schedule is flexible for the moment

Emilio Pisanty

@Mithrandir24601 maybe I'm reading the group layout wrong, though -- it's entirely possible that Mark Dennis is no longer around at all and neither is Michael Berry, and the topics I want to tap into have partly or mostly dissipated. Or maybe they're still going strong. I should have a better sense in mid-June. In either case, it'd still be awesome to visit, and there's a good window in August.

@Semiclassical If it works for you, Tuesday is best for me, I reckon. Email me and we can work it out from there =).

Mithrandir24601

9:08 PM

@EmilioPisanty Mark Dennis is now in Birmingham (as of a few months ago) but Michael Berry still pops in now and again

Semiclassical

I’ll see what I can do, yeah

Emilio Pisanty

@Mithrandir24601 btw what's wrong with the personnel links on the Bristol website? every link to a non-PI person's name on the sidebar at bristol.ac.uk/physics/research/theory/research/geometrical is broken

Mithrandir24601

@EmilioPisanty Huh. I don't actually know. Strange

Emilio Pisanty

¯\ _(ツ)_/¯

ACuriousMind

@EmilioPisanty It's telling you to not care about non-PIs :P

Mithrandir24601

9:12 PM

@ACuriousMind :'(

ACuriousMind

:'(

Emilio Pisanty

@Mithrandir24601 maybe everyone there has actually graduated, and it's the sidebar that's wrong?

Mithrandir24601

@EmilioPisanty Entirely possible - the list of PhD theses hasn't been updated in years

Emilio Pisanty

@Mithrandir24601 presumably they'd be in the central repository, though, right?

I searched there but couldn't find anything

ACuriousMind

@EmilioPisanty So Mithrandir is The Last Grad Student?

Emilio Pisanty

9:21 PM

@ACuriousMind maybe they haven't graduated any PhD students since 2011

you never know

ACuriousMind

Jeez, I graduated high school in 2011 :P

Time flies

Mithrandir24601

@EmilioPisanty They're... Somewhere... I've never really looked ::cough:: The name Danica rings a bell though, so if it's outdated, I don't think it's that outdated

Emilio Pisanty

@Mithrandir24601 the Dennis group page marks her as already graduated

markrdennis.com/group

Mithrandir24601

@EmilioPisanty I know Teunja - she finished within the past year (I hope she doesn't mind me saying this) and isn't listed on any of those pages that I can find

Emilio Pisanty

@Mithrandir24601 is she still at Bristol?

Mithrandir24601

9:26 PM

@EmilioPisanty Yep

She's actually the one I sent a message to mentioning that you might be interested in giving a talk :P

Emilio Pisanty

@Mithrandir24601 so her thesis is indeed listed research-information.bristol.ac.uk/en/theses/…

Danica Sugic's isn't

Mithrandir24601

@EmilioPisanty I feel that I should actually read that, or the abstract and conclusion anyway :P

cows

10:11 PM

Taking a moment to remember the name school and affiliation of every physics professor whose ever thought less of me.

Now back to enjoying the breeze and cloudy day

images.app.goo.gl/jvyvihiTT7PiXMm49

2

heather

10:37 PM

For those interested, I've written a quantum computer simulator that is now reasonably ready for use. If anyone's willing to help test it, I'd very much appreciate it! Feel free to ping me with any questions =)

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