The h Bar

7:37 AM

Maybe it's time to protect that Schrödinger's Cat question, to prevent answers like this one: physics.stackexchange.com/a/500889/123208

8:08 AM

morgen

Loong

huomenta

Dammit

There was a post about if you could model physics with fairies and it was deleted

I was going to answer it

(The answer is "yes", by the way)

@Slereah Not deleted, bur closed: physics.stackexchange.com/q/500807/123208

It's a perfectly valid question

although it is more of a philosophy of science question

@Slereah Agreed, and they tend to get closed as Primarily Opinion Based.

8:14 AM

I'd say it's valid

If the model is appropriate, it fits the three basics requirements of a scientific theory

It may not fit some of the most peripheral requirements, though

If someone asked if QM satisfied requirements as a scientific theory I bet it wouldn't have been closed!

@Slereah Yes, it's valid, and if we don't have a good canonical Q&A on how the modeling process works, then we ought to have one. But the problem is that questions like that tend to evoke a lot of crap answers & fruitless circular discussions.

Hm

If you have a fleet of observers in GR

Along trajectories which are initially at the same point, for synchronization purpose

And the spacetime is globally hyperbolic

Is there always a Cauchy hypersurface which crosses each such observer at the same proper time?

That is, if $\gamma_i(0) = 0$, is there a foliation by $\Sigma$ such that $\forall p_i \in \Sigma \cap \gamma_i$, $t(p_i) = t(p_j)$

@Slereah From continuity considerations, that sounds reasonable. I assume none of these trajectories cross event horizons.

Q: The problem of measurement in general relativity

8:36 AM

Well hopefully not

0

I am looking for a good selection of articles and books on the topic of measurements in general relativity. The only one I'm really aware of is Reichenbach's "The Philosophy of Space and Time", which is fairly a fairly old one by now (1928) and does not really go in great details for all issues. ...

plz halp

@Slereah I fixed some typos

Everyone only cares for the measurement problem in QM

Very sad

It's so hard to measure anything in GR

8:52 AM

@PM2Ring hi will you help me in a math problem

@Slereah Science historian & philosopher John D. Norton has some nice material on clock synchronization in SR. He might also discuss measurement in GR, but I couldn't see anything in a quick look at the contents page of his site.

have quadratic polynomial p(x), with real coefficient ,such that for all real x such that 2(1+p(x))=p(x-1)+p(x+1) if p(0) =8 and p(2)=32 rhen sum of all coefficent of p(x) (it is a request please do not use math jax because i am using phone)
@JohnRennie i assume that p(x) be ax^2+bx+c=0 and i have vakue of p(1)=7 and p(2)=32 and p(0)=8 but it take long time is there any other way to think about it

Aug 15 at 15:28, by PM 2Ring

There's a nice discussion on clock synchronization which I posted in the comments on that question (which have now been moved to chat). Here's the link for your convenience: https://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/significance_conv_sim/index.html

@yuvrajsingh No, not after your rude behaviour yesterday.

@PM2Ring SR is positively trivial compared to GR, alas

You can even use geometry to solve it

Lorentzian geometry still, but not that hard

@Slereah I posted a comment: physics.stackexchange.com/questions/500807/…

9:02 AM

@PM2Ring sir posting my answer,yesterday my internet connection ,any how stopped,i am sorry for that ,

Obviously the theory should be something along the line of "For a mass $m$, the number of fairies pushing the object is proportional to the force applied"

@Slereah True. I'm just saying Norton might have some info about it.

@PM2Ring i lost my internet connection

@yuvrajsingh Oh, ok.

@PM2Ring will you hep me ,i need only hint

9:08 AM

@yuvrajsingh You can use MathJax in chat. In the Samsung browser on my Android phone I use a bookmarklet from the link here: meta.stackexchange.com/questions/97938/… It's not perfect, but it works.

@PM2Ring i have xiamoi redmi note -3

@PM2Ring quite old.version ,so ......

@yuvrajsingh A bookmarklet should work if your phone can do JavaScript, and if it gives you a Favorites button or menu. And if your phone browser can't do JavaScript, then it will have problems on many web pages. But I guess your phone might not let you edit the URL of a Favorites menu item, so it will be hard to install the bookmarklet.

@yuvrajsingh p(1)=19, not 7

@PM2Ring yes sir p(1)=19

@PM2Ring i cant see any other way to find find what is p(x).

@yuvrajsingh "it take long time is there any other way to think about it" You didn't tell us how you are solving it, so how can we know if there's a better way? ;) You can solve 3 simultaneous linear equations to get a, b, c. It may be faster to use Vieta's rules, if you know the coefficients are integers (or at least rational).

are finding p(x) or a+b?

9:21 AM

Vieta's formulas

In mathematics, Vieta's formulas are formulas that relate the coefficients of a polynomial to sums and products of its roots. Named after François Viète (more commonly referred to by the Latinised form of his name, Franciscus Vieta), the formulas are used specifically in algebra. == Basic formulas == Any general polynomial of degree n P ( x ) = a n x n + a n...

@PM2Ring i already try using vieta rule with product root let say x,y,z,w are root hence xy+yz+zw+wx=b ,

Good point, Fawad. @yuvrajsingh Do you want to find a, b, c. Or do you only need to find a+b+c ?

@PM2Ring yes i need to find a+b+c

@yuvrajsingh You said the poly is quadratic, so it only has two roots, not four!

p(1)

9:25 AM

@Fawad p(1)=19

its your a+b+c

P(1)=a(1)^2+b(1)+c=a+b+c=19

@PM2Ring i got p(x) it is x^2+10x+8 thank you

9:40 AM

@yuvrajsingh Correct.

@Fawad Well spotted!

9:56 AM

Oh man

I even had the perfect quote for that fairy question

"One must be able to say at all times--instead of points, straight lines, and planes--tables, chairs, and beer mugs"

- David Hilbert

Whether fairies or particles are responsible doesn't matter to physics as long as they give us the same observables

@PM2Ring can u help me with these ubuntu terminal messages.

Q: Warnings while launching gedit from the Ubuntu terminal

10:35 AM

@Fawad It's probably safe to ignore those warnings. It looks like the CSS file is for a different version of GTK to what gedit is using.

It's similar to this problem:

8

I am facing these warnings while launching gedit from my terminal. Eventually gedit starts, but with these lines of warning every time when I launch gedit: (gedit:3830): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:138:20: Not using units is deprecated. Assuming 'px'. (gedit:3830): Gtk-...

command-line gedit

@Slereah Certainly! OTOH, if you claim (for example) that fairies inside entangled electrons ensure that their spins are correlated, it doesn't bypass the Bell inequalities. So invoking fairies doesn't actually achieve anything.

Q: Why does nobody ever consider the possibility that the universe is not smooth?

10:56 AM

@Slereah FWIW, there's a new question on Astronomy about the smoothness of spacetime.

1

Disclaimer: I'm not an astronomer, physicist, mathematician, etc. so this is a question from a complete newbie. One of the greatest mysteries of our age is "where is the dark matter?" The universe can be observed to be curved (aka have gravity) in many places where we cannot detect any sufficien...

dark-matter

@PM2Ring I put a solution of TiO2, NaOH and H2O (in between liquid and wet dirt form) into a solution of baby oil and liquid soap mixed. I shook it and the TiO2 seems to form kinda like islands in the sea. The particles want to stick together.

@PM2Ring Are there any more things I can try? Should I ask JohnRennie?

@NovaliumCompany Oh well. That's a shame. Maybe John Rennie has some suggestions. I guess you could try to remove the water from the final mixture, but that won't be easy.

@PM2Ring No water = No charge :(

11:13 AM

Yep. John says we need the water so that the TiO2 gets charged. But the water seems to cause the clumping. I was hoping the detergent would prevent that. Maybe we just need a different surfactant... but I don't know what to suggest.

@PM2Ring no.

In water the TiO2 is charged, but if you put water in with oil then the water just forms a shell round the particles because water and oil aren't miscible.

Then when two TiO2 particles touch their water shells fuse and in effect glue the particles together. That's why when any water is present the TiO2 particles clump together.

@PM2Ring I never said they weren't contextual fairies

Sometimes a fairy can be mischievous and change her previous answer

and as fairies are telepathic, the other also follows suit

To get a stable dispersion in oil you have to exclude all water, and use an oil soluble surfactant.

But I don't know what surfactant would work to stabilise TiO2 in oil. Liquid soap won't work because it contains water.

And how you get a charge in oil I have no idea because as a general rule dispersions in non-aqueous liquid are not charged.

Some special trickery must be in play.

:-<

@NovaliumCompany you'll have to try and Google to find out how they charge the particles in e-Ink displays.

Though it may well be a secret since it's of obvious commercial value.

11:21 AM

Can I call one of the companies and ask them for the secret? :P

You can try :-)

I might actually try that, but as a last option. Will that powder that my mum puts in the laundry machine to spin and wash the clothes work as a surfactant?

@Slereah Telepathic fairies are just non-local spooky action at a distance. And if fairies capriciously change their minds, then you lose predictability. So once again, the fairies don't actually buy you anything that the non-fairy model already gives you.

Pilot wave fairies

@PM2Ring Well sure, I never said they were better than QM!

Although they are because fairies are whimsical

@NovaliumCompany no, because that is a water soluble surfactant (assuming your mum washes the clothes in water :-)

11:25 AM

although of course, as the fairies are non-observable and essentially not useful for the theory, you can perform the...

@PM2Ring accepted answer worked for me

There's a special name for the reaxiomatization of a theory to remove useless terms

But I forget what it is

@JohnRennie So are these the two main problems I'm trying to solve here are: 1. I need to find a surfactant that contains no water so I can disperse it properly in the oil without forming micelles. 2. I need to somehow charge the particles without water.

Yes

:% <-- This is my reaction to your yes

Well, I'm not giving up. It's gonna be a long search so I better get started.

11:31 AM

For example see this

(the PDF is a bit messed up but it's still readable)

@NovaliumCompany Maybe the people who do carpet dry cleaning have some useful chemicals en.wikipedia.org/wiki/Dry_carpet_cleaning

@Fawad Excellent

@JohnRennie Thanks

@PM2Ring Thanks

@NovaliumCompany the term to Google seems to be charge control agent

D-limonene might be interesting

D-limonene isn't charged (though it smells nice :-)

DIY electrophoretic display

11:45 AM

►

:O

Sir Cumference

Morning

@NovaliumCompany Ok. So it's do-able. But how the hell does it work? There's no info or links.

@PM2Ring I'm going to do anything in my power to contact this guy

we.easyelectronics.ru/HomeTech/… :O

...

He literally cuts a broken display

12:14 PM

@NovaliumCompany Well, that's pretty boring.

@PM2Ring Yep

@PM2Ring The wiki page says that an electrophoretic display uses a hydrocarbon oil. Is it a problem I use baby oil?

12:31 PM

@NovaliumCompany Baby oil is a hydrocarbon oil. That's why I suggested it.

@PM2Ring ... Sorry for being stupid

It might have small amounts of plant oils in it, to make it smell nice, but that shouldn't matter.

So I need to find out what they use to charge the TiO2 particles and what they use for a surfactant to spread it in the oil

Yes

Actually, what will happen if you just put charged TiO2 particles (no water) in baby oil, no surfactant?

What's the role of the surfactant here at all?

12:35 PM

@NovaliumCompany How do you propose to charge them?

Ohh, so I need a surfactant that can charge the particles?

@NovaliumCompany To encourage the TiO2 to mix nicely with the oil. I can't tell you any more than what the Wiki article says.

@PM2Ring I charged the particles with H2O and NaOH, but since I don't want any water in this, I need to change the method.

@PM2Ring sir i am going to computer science engineering next year,but i do not about except basic,i mean i have no knowledge about coding or something,can you please basic course should i do before starting of the classes of first year

@PM2Ring I'm sorry for constantly asking you questions, it must be pretty annoying. I'm going to research a lot, and then come back.

12:56 PM

One-way speed of light

When using the term 'the speed of light' it is sometimes necessary to make the distinction between its one-way speed and its two-way speed. The "one-way" speed of light, from a source to a detector, cannot be measured independently of a convention as to how to synchronize the clocks at the source and the detector. What can however be experimentally measured is the round-trip speed (or "two-way" speed of light) from the source to the detector and back again. Albert Einstein chose a synchronization convention (see Einstein synchronization) that made the one-way speed equal to the two-way speed. The...

The horror

"The constancy of the one-way speed in any given inertial frame is the basis of his special theory of relativity, although all experimentally verifiable predictions of this theory do not depend on that convention"

it's not a very convincing theory but it works out the same certainly

What are they trying to say, that it can be variable on the first path but on the way back has to be constant?

The point is that if it's a certain speed one way, it has to be a specific speed the other way so that their sum average to the speed of light

It's the Reichenbach synchronization shenanigan

It would probably apply to all interactions then

1:06 PM

I mean y'know

this is mostly one of those philosophical considerations

I don't think it was ever seriously considered for physics

This is variable speed of light stuff basically

@yuvrajsingh Sorry, I don't know what to suggest. I learned to program several decades ago, and I'm not that familiar with modern courses and tutorials. In the SO Python room we have a small list of tutorials and books, but they are specific to Python.

@bolbteppa Not really

Variable speed of light usually means it varies for everything with time

ie it's not the same speed for 2-way trips

But in one direction it's one speed, in the other direction it's another

Or different at all points but averages out

Sure, but "variable speed of light" usually means something specific

1:09 PM

If it's the same in both directions I don't see the issue

and it's also actually considered as a real physical theory

I think the main issue is that they just can't test the speed without it bouncing off something and coming back right

yes

aren't there experiments where the bounce forward and back are done unequal times and hence account for that issue of directionality?

It's one of those things that are like... This also applies to classical mechanics, but nobody thought about any of this until SR came around

Simultaneity isn't a trivial notion in any theory

it's just that it worked out to assume it in classical mechanics

1:16 PM

I mean I guess that's a good point that they can't measure it without it bouncing right

yeah, as otherwise the whole thing will stretch a galaxy

@bolbteppa Norton discusses this stuff on his site. Here's the link I posted earlier. pitt.edu/~jdnorton/teaching/HPS_0410/chapters/… My attitude is that we should just take it as axiom that the one-way speed is the two-way speed. As Einstein said: "Subtle is the Lord, but not malicious". A universe where the two-way speed isn't equal to the one-way speed is unnecessarily malicious. ;)

> A universe where the two-way speed isn't equal to the one-way speed is unnecessarily malicious. ;)

We cannot assume the universe is benevolent given all that it has subjected us through

yeah that is the Einstein convention

Right, the fact that the light has to interact with something first is problematic though

1:20 PM

@bolbteppa what if the universe has a non-trivial fundamental group

and you can just shine a light on yourself

I'm surprised they haven't measured it without it first bouncing off something

@bolbteppa The point is that you can't

A universe with nontrivial fundamental group will make light sails very easy

Like whatever system you may devise will require synchronization of the two devices at some point

you shine light on yourself, and then you start get propelled, no reaction needed

1:21 PM

It's the basic idea of the synchronization problem

From a quantum perspective are you not emitting light at an object, the object is then absorbing them and emitting new different photons

Going to take some time for the absorption and emitting process to occur which will affect the measured round trip right

Well you know

This problem is added on top of the usual problems of measurements

It's not just balls bouncing off a wall and coming back on a microscopic level is it

Then you just do time for first trip + time for light to be re-emitted + time for the second trip

It doesn't fundamentally change the issue

that delay in absorb emit is taken account as an experimental error

so it changes nothing except increasing the roundtrip time

The need for bounce back does make one wonder whether it is in fact possible for the one way speeding two way speed to differ and the speed of light we measure is really an average

I cannot imagine how the Lorentz group be modified if such non commutative directionality is important

1:28 PM

A stupid question came up in my mind: If I create a magnetic field around my ereader with magnets, will it mess up the display? Since a moving magnetic field will move charged particles.

The fundamental group of the universe will probably be at least $\Bbb{Z}_2$

@NovaliumCompany If the magnetic is strong enough, it will, and it may damage it

@Secret Oh, oks

The main SR claim is that all interactions, light gravity (strong weak force) etc travel at some fundamental speed which we call $c$ at the most fundamental scale and everything else sits on top of that, if it's going to change in any way in this round trip stuff it's not really SR

@bolbteppa yeah as I said

Not really a serious scientific proposal

Just pondering on the difficulty of measurement

Yeah

1:33 PM

True, but I am wondering how big the change from SR has to be to accomodate that non commutativity because of this conversation

it probably wouldn't make for a pleasant theory

I see

Since there's not really any mechanism to say what light ray should have what speed

Once the light travels to the object and just before it interacts, that's probably all you can talk about on a fundamental SR level regarding speed, once it then interacts with the object you're now really analyzing a QFT problem of a photon interacting with say an electron (or proton or rather it's constituents) on a fundamental level, but the whole thing is the interaction occurs at the speed of light too :p

also it's like

The fact that light going one way and has a certain speed, and then the light going the other way has to go back to the original observer at another speed is like...

non-local

because otherwise there's no reason why the light would have a specific speed since it's nowhere near the observer

I guess it could work in 1+1D though

just have left-going light and right-going light have their own speed

in 3+1 it seems a bit more arbitrary

2:06 PM

"The existence of Quine atoms (sets that satisfy the formula equation x = {x}, i.e. have themselves as their only elements) is consistent with the theory obtained by removing the axiom of regularity from ZFC. Various non-wellfounded set theories allow "safe" circular sets, such as Quine atoms, without becoming inconsistent by means of Russell's paradox."

2:24 PM

also, when you break wellfoundness, you no longer have cardinality since all injective functions will get screwed up. Instead, you have some kind of isomorphism between two non well founded sets such that they are isomorphic iff they graphs matches

@JohnRennie I read the article you sent me. I can use quaternary ammonium cation (polyisobutylene quaternary amine sulfate salt) as a surfactant, that contains no water and can charge the particles?

sciencedirect.com/science/article/pii/S1567173905000684 I found this, but there are a lot of dangerous chemicals.

3:23 PM

Unbelievable (click to see, sidenote)

@NovaliumCompany it all looks a bit complicated :-(

@JohnRennie So I won't be able to do it at home? :(

If a matrix is nilpotent, it's eigenvalues are zero, but of course $P_{\mu} \gamma^{\mu}$ for $p^2 = 0$ is nilpotent but only half of them are zero because physics...

@NovaliumCompany looking at the paper you linked that would need to be done in a lab with appropriate safety precautions. That's not something I would attempt at home.

@JohnRennie Does this mean there is no way I can do the project?

3:30 PM

@NovaliumCompany unless you can find some safer alternative ...

@JohnRennie I emailed a lot of people and companies. Any suggestions what I can google?

3:54 PM

@JohnRennie What about this one: citeseerx.ist.psu.edu/viewdoc/… (read Abstract section)

@JohnRennie Also this one: pdfs.semanticscholar.org/eecc/…

@NovaliumCompany I guess all I can say is give it a try.

If you manage to find a surfactant that will work and not kill you then go for it.

I suspect you'll struggle to buy it though as only specialist chemical suppliers will have it.

@JohnRennie Alright, thanks.

@NovaliumCompany also the surfactant will only work if you use the same solvent as them.

They are probably using something like toluene not baby oil.

Q: Proposal: Allowing problems on physics to be allowed

4:09 PM

@JohnRennie Ok. I thought the surfactant is not the main problem. I don't know how to charge the particles without water (and NaOH).

Physics Meta

-1

I see that there are many questions nowadays about specific problems in physics rather than "conceptual" questions. I feel that times are changing and people here would also like to solve some interesting physics questions. I believe that only when you solve questions, does one learn on how to ...

discussion feature-request questions homework-and-exercises faq-proposed

@PhysicsMeta Oh no, no, no, no, no!

@JohnRennie Imagine if it responds

I'll give you my close vote when you pry it from my cold, dead hands.

4:30 PM

Pff, I'm slowly starting to lose hope. I found quite a few articles which reveal details on how the TiO2 is charged and the overall technology but it's very complicated and requires machines such as planetary mill and so on... I'm gonna get a doner kebab and watch a movie.

@JohnRennie Btw I watched The Godfather and it was badass as hell.

@NovaliumCompany it's a good film!

Q: Local fermionic symmetry and GS action

5:19 PM

2

I have a trouble understanding an argument which I think has a simple answer but I am not getting it. The question is that if you don't impose local fermionic symmetry the GS action has only one term and one of the equation of motion looks as $Γ\cdot p\partial_0\psi=0$, where $\psi$ is a Majoran...

string-theory supersymmetry fermions linear-algebra

5:45 PM

How do you make $\leq$ into $=$ in that :\

6:06 PM

"Simon B. Kochen and Ernst Specker, and separately John Bell, constructed proofs that quantum mechanics is contextual for systems of Hilbert space dimension three and greater."

"The Kochen–Specker theorem proves that quantum mechanics is incompatible with noncontextual hidden variable models."

heh

heyyy guess who has the Kochen-Speckler theorem

ncatlab.org/nlab/show/Kochen-Specker+theorem

enumaris

6:29 PM

hmmm

vzn

@bolbteppa thx for sharing. the view expressed is very similar to the new Smolin book & think they both have excellent pts/ are on the right track. Carroll is very popular as a popsci writer, havent read much of his stuff coincidentally his wife Ouellette writes on popsci physics also. looking further, hes a MWI proponent. ugh

This is also a cool physics theorem:

"When a symmetry of a classical theory is broken by radiative corrections, so that there is no choice of local counterterms that can be added to the low-energy effective action to restore the symmetry, the symmetry is called anomalous. Anomalies arise from divergent Feynman diagrams, with a classically conserved current attached, that do not admit a regulator compatible with conservation of the current. Anomalies only arise at one-loop order (Adler-Bardeen theorem)

in diagrams with a chiral fermion or boson going around the loop. Their origin can be traced to the behavior of Jacobian factors in the path-integral measure."

@vzn the mwi stuff accounts for 'bos's attitude to him - his amazon review of one of Carroll's books (arrow one iirc) is pretty funny/ott

6:59 PM

May 3 at 20:14, by PM 2Ring

I find it fascinating how polarising QM interpretations are. "My interpretation is mostly satisfactory, it just has 1 or 2 problem areas. But your interpretation is patently absurd!" ;)

7:24 PM

@PM2Ring the other ones are absurd :p

These interpretations basically become irrelevant in qft because none of them can say anything about it, but they absolutely have to be able to