The h Bar

roobee

00:13

@JMac thanks. it looks like you can make a water stream appear to be frozen using a trash bag. hoping i'll have the time to try it out

JMac

@roobee Yeah I was actually pretty surprised when I first saw that video. I never really thought about or realized how easy it could be to make such cool streams.

DanielSank

00:50

Plz upvote

3

Q: Cannot confirm email subscription

I am trying to subscribe to this filter. It seems that I am in fact subscribed, but I'm not getting mail. Now, I used to get email from this filter, but around January of 2018 it just stopped sending me mail. When I view the filter page now, there's a box on the right side that says "Confirmation...

Physics Meta

04:45

0

Q: Reason behind my negative user experience?

Let, me first begin by saying my overall user experience in Physics StackExchange has been wonderful. According to this it is okay to downvote poorly written questions. Now in the past I've had an interesting experience where this was cross-posted to physics stack-exchange and severely downvoted...

discussion

More Anonymous

So I wanted to ask something: For those who believe that Von Neumann entropy is not thermodynamic entropy. Is there heat, temperature and a 2'nd law (entropy of an isolated system always increasing) for this Neumann entropy? @ACuriousMind Any thoughts?

PM 2Ring

@SirCumference That's a Lorentz factor of a little over a trillion. You need to go a lot faster than .9999c for that.

Sir Cumference

@PM2Ring yeah i expected, i just wanted to convey a point :P

PM 2Ring

@SirCumference No worries. I figured you weren't trying to be precise.

PM 2Ring

05:10

There's a simple way to figure out the speed corresponding to a (roughly) integer Lorentz factor, gamma. It uses a Pythagorean triad. Let n be the desired gamma. Then $(4n)^2 + (4n^2-1)^2 = (4n^2+1)^2$. So $v=(4n^2-1)/(4n^2+1)$ and $\gamma=(4n^2+1)/4n=n+1/4n$. Eg, let n=5. 20^2 + 99^2 = 101^2, so v=99/101, gamma= 101/20 = 5+1/20

So to get a gamma of near $10^{12}$ we need $v=(4\times 10^{24}-1)/(4\times 10^{24}+1)$

yuvraj singh

05:36

@dmckee astronomy.stackexchange.com/questions/33808/…

can you elaborate it more.

PM 2Ring

06:00

@yuvrajsingh It is not clear what you are having difficulty in understanding. You can choose any point in time to be t=0, events after that point in time have a positive time coordinate, and events before that point in time have a negative time coordinate.

yuvraj singh

06:17

How do I define origin.

Then @PM2Ring

PM 2Ring

@yuvrajsingh You can choose any origin which is convenient for your purpose. If you're discussing the trajectory of a rocket, the moment that the rocket takes off is a good choice for t=0. If you're discussing Big Bang theory, then the moment that the Big Bang started is a good choice for t=0.

user157860

08:01

@JohnRennie, hi, John, I tried to find a physical formula containing a subtraction in the denominator, to no avail. You were right, no chance to get zero. Actually I did not find any subtraction anywhere: only divisions, nultiplications and sums. Any guess why it is so?

then there is the funny unit question: again, one eV is J/10^19 and h is J/ 6x10^33. why the units of h are eVxs? also h can be considered KE,like eV, the KE whe an electron is hit by a photon, isn't it so?

John Rennie

08:34

@user157860 The units of $h$ are joule seconds ...

$h$ isn't a kinetic energy.

user157860

@JohnRennie, yes, I wrote that, but also eV can be considered something else. I said that h can be considered KE in the same way as eV is considered KE taking the example of an electron moving from a to b, but that is coulomb force, isn't it. Likewise h is J x s if you consider it as a frequency, but it too can be considered the KE of an electron hit by such and such photon. Also heat is frequency, yet it is expressed in J , isn't it?

John Rennie

08:56

I have to confess I struggle to see what you are getting at. Can you give an example where $h$ is considered KE?

user157860

@JohnRennie, I gave it: when hv hits an electron it becomes KE

John Rennie

I guess you mean that for a photon with frequency $\nu$ the energy is $h\nu$.

So for example in the photoelectric effect the energy transferred to an electron is $h\nu$.

Slereah

Morning

Loong

10:04

Happy Halloween

Slereah

10:33

I am spooked

user157860

10:43

@JohnRennie, isn't that energy transferred KE?

Novalium Company

11:38

happy halloweed

halloWEED

jk

I <3 u all

happy halloween to all beautiful creatures here.

@JohnRennie @AaronStevens @ACuriousMind @AbhasKumarSinha @AvnishKabaj @bolbteppa @CaptainBohemian @DanielSank @dmckee @danielunderwood @JMac @JakeRose @Loong @MoreAnonymous @PM2Ring @Rudi_Birnbaum @RyanUnger @rob @Slereah @SirCumference @skullpetrol @Semiclassical @Secret @skillpatrol Happy Halloween to each and every one of you! Thanks for helping me through the years despite the stupid questions I asked. Thank you for never giving up on me even when I was annoying and too lazy to google. <3

I'm from Bulgaria, we don't celebrate that here, but still.

PM 2Ring

12:04

@yuvrajsingh Your black hole answer on Astronomy consists of other people's work without attribution. Eg, John Rennie's physics.stackexchange.com/a/252236/123208 and Anders Sandberg's physics.stackexchange.com/a/476896/123208

@yuvrajsingh That is called plagiarism, which is unethical and a serious offence on Stack Exchange. You are certainly permitted to quote other people, but you must mention their name, and preferably link to the original.

Aaron Stevens

Well that's just frustrating

yuvraj singh

@PM2Ring sorry, how should I metion.

I just deleted the answer and I will add the link, and rewrite the answer.

John Rennie

@yuvrajsingh hi

PM 2Ring

@yuvrajsingh You can put the quoted material in a yellow quote block, and give the author's name & link either before or after the quote block. Eg, [John Rennie said](the_link): QuoteBlock.

yuvraj singh

Hi , it is because of me, I apologize, I should ask and mention your link.

@JohnRennie

@PM2Ring I will take this, and I assure you all in future u will do this mistake.

Aah, that was type, I write it again, in future I will not do this mistake

PM 2Ring

12:19

@yuvrajsingh That's better. :)

@NovaliumCompany Hi. We didn't celebrate Halloween in Australia when I was a kid. But it has become a little popular here in the last couple of decades. At least, supermarkets sell Halloween stuff, some kids go trick or treating, and some people throw Halloween parties.

Novalium Company

@PM2Ring Exactly the same here

yuvraj singh

@PM2Ring I updated is it look fine

PM 2Ring

12:36

@yuvrajsingh That's ok. But it would be even better if you put the quoted material in quote blocks, so that readers know exactly which words come from John, which words come from Anders, and which are your own words.

yuvraj singh

Sure.

Physics Meta

0

Q: Anonymity Feature

Recently my questions were downvoted, though the reason for such is unknown to me (also it's not quite a big deal for me) but I believe that if the downvoter had given me some suggestions for improvement then I would be able to improve them. I believe that thee sole reason for this behavior found...

discussion feature-request

Beta Decay

13:22

How do you put a drag force into a Laplacian?

I'm not sure how to do the integral $V_d = \int F_d \mathrm{d} x = \int -k\dot{x}\space \mathrm{d}x$

Emilio Pisanty

@BetaDecay you can't

@BetaDecay do you mean Laplacian or Lagrangian?

(they're very different things)

Beta Decay

Haha yes, Lagrangian

Ffs I wondered why my Google searches were returning nothing of use :P

Emilio Pisanty

they other key search term is 'dissipation'

2

Q: Lagrangian formalism and dissipative systems

Why the central concepts of classical mechanics, viz. Lagrangian and Hamiltonian formalisms cannot address constraint forces like friction and others in dissipative systems?

classical-mechanics lagrangian-formalism friction hamiltonian-formalism dissipation

(for instance)

Aaron Stevens

1

Q: Lagrangian formalism application on a particle falling system with air resistance

I have this problem, with a first-step resolution: $$...$$ So, I just don't know why they put the term $\frac{\partial F}{\partial \dot{z}}$ in Euler-Lagrange's equations. Why? I know that the dissipation function isn't a conservative force, but I don't know why the partial derivation. For hol...

forces lagrangian-formalism drag dissipation

Beta Decay

I see, so $-k \dot{x}$ goes into the E-L equations

Cheers

Emilio Pisanty

13:31

Lagrangian mechanics

Lagrangian mechanics is a reformulation of classical mechanics, introduced by the Italian-French mathematician and astronomer Joseph-Louis Lagrange in 1788. In Lagrangian mechanics, the trajectory of a system of particles is derived by solving the Lagrange equations in one of two forms: either the Lagrange equations of the first kind, which treat constraints explicitly as extra equations, often using Lagrange multipliers; or the Lagrange equations of the second kind, which incorporate the constraints directly by judicious choice of generalized coordinates. In each case, a mathematical function...

(direct link to the Extensions to include non-conservative forces section)

ChoMedit

14:31

It's a good day to know what is a ghost particle.

Slereah

There are many spooky physical concepts

Ghosts, phantoms, demons, djinns, skeletons

Slereah

15:25

@ACuriousMind Is there a simple way to find a "good" Hilbert space upon quantization

(with finitely-many degrees of freedom)

I know there is the Stone-von Neumann theorem for the basic case, but is there a more general case

for the BRST quantization of point particles they argue that it's $L^2(\mathbb{R}^n) \times \mathbb{Z}_2$

Which I'd believe but is there a systematic way of doing it

If you have a set of commutation and anticommutation rules, what's a good guess at the Hilbert space

More Anonymous

Anyone here knows about this https://physics.stackexchange.com/questions/511209/on-deducing-states-which-were-once-never-measured?noredirect=1#comment1151956_511209 ?

what are "classes of observables" ? Someone put that I should specify that but I didn't. I use someone but it was deleted too soon. I hope it wasn't Shor :/
I think the comment was to specify which class of observables :/ Is it the dimensionality of the eigenvector involved?

FOund it: johnboccio.com/research/quantum/notes/Jauch.pdf

yuvraj singh

15:57

Can anyone help me. i.sstatic.net/3iE3w.gif

Abhas Kumar Sinha

@NovaliumCompany Hello, we don't celebrate Halloween here in India. But, you'll see plenty of stuff in public places like the supermarket. Personally, I don't know much about this.

@yuvrajsingh not JEE, I think...

yuvraj singh

It is of IRODOV actually I am asking answer of this question.

It is for JEE ONLY.

And other exam too.

@AbhasKumarSinha

Abhas Kumar Sinha

@yuvrajsingh IRODOV has a lot of things that aren't in JEE Syllabus. That question for example.

yuvraj singh

Actually I am giving kvpy, so I do not know how to approach that.

Abhas Kumar Sinha

@yuvrajsingh Sorry, haven't completed that chapter, can't answer your questions.

@yuvrajsingh You can appear KVPY only if you are in 12th or less.

yuvraj singh

16:06

Actually I am In IIT roorkee @AbhasKumarSinha

Abhas Kumar Sinha

@yuvrajsingh IIT R? Then why reapper IIT?

yuvraj singh

no that, s not the only criteria there is other way too.

Abhas Kumar Sinha

@yuvrajsingh oh okay, not sure about that then...

yuvraj singh

Actually I want to get in IISC bangalore.

Anyway by. Thanks for your time.

Abhas Kumar Sinha

@yuvrajsingh That's everyone's dream. But IISc is very difficult...

@yuvrajsingh Thanks for that :)

DanielSank

17:09

@NovaliumCompany You are most welcome.

Aaron Stevens

18:13

I often see an explanation of the double slit experiment as the particle "interfering with itself". I really do not like this explanation because it seems to give the wavefunction more physical meaning than it ought to. But is there any reason that we really should describe the double slit experiment in this way?

ACuriousMind

@AaronStevens Well - the interference bands are due to the "two parts" of the wavefunction (one from each slit) interfering. So if "wavefunction = particle" then "double slit = particle interfering with itself".

I.e. this phrase is meaningful if and only if you believe that the wavefunction "is" the particle

Aaron Stevens

@ACuriousMind Right, and I suppose that is what my issue is, since I don't like thinking in that way

bolbteppa

Researchers uncover an anomaly in the electromagnetic duality of Maxwell Theory

Aaron Stevens

I mean in classical mechanics we don't say the object is its position vector.

We don't think there is an actual arrow pointing from some place to where the object is

ACuriousMind

@AaronStevens I agree with you. But "the wavefunction" is the easy answer to the question "Well, if a particle is not a point, what is it?", so it is what many latch on to

Aaron Stevens

18:26

@ACuriousMind That's true... I am probably being a bit picky anyway. I just feel like it is such a pop-sci way to describe QM which leads to other incorrect conclusions like the particle being smeared out, or the particle existing at all points in space at once.

ACuriousMind

@AaronStevens I think the "smeared out" way of thinking has some merit, e.g. in the context of atomic orbitals. It just isn't the best way to think about arbitrary wavefunctions

Aaron Stevens

@ACuriousMind Yeah. They are expressions that need some serious qualifications that everyone agrees upon before you decide to use them.

Semiclassical

19:11

I guess I’ll note that the phrase “the particle interferes with itself” isn’t really sensible in the Bohm interpretation either

Aaron Stevens

@Semiclassical Yeah. That's true

Semiclassical

The wavefunction interferes with itself, and the wavefunction determines the allowed trajectories of the particle. But the particle doesn’t act on the wavefunction

(Particles as such don’t interact at all AFAIK: their wavefunctions interact, and these in turn determine the trajectories)

Aaron Stevens

I just don't find the need to describe it as "a particle interferes with itself". You can describe it as just the parts of the wave-function interfere with each other. You still talk about interference without bringing in all the difficulty of explaining what "a particle interferes with itself" actually means.

Semiclassical

Well, within the Bohm interpretation you’re choosing to insist on the particle’s trajectory making sense in the first place, but I agree that there’s no need to invoke the particle to describe the interference pattern

The motivation to have a particle is to account for the fact that only one particular outcome is actually realized

So it’s more to do with the measurement problem

Slereah

I don't have a measurement problem, I can stop whenever I want

11

Aaron Stevens

19:17

@Slereah Nailed it

Avnish Kabaj

19:50

@NovaliumCompany Happy Halloween

vzn

20:08

lol! +1! ... now for contraian pov: copenhagen interpretation is equivalent of drunk looking under streetlight for his keys because ... !

Semiclassical

20:22

Except that drunks usually don’t succeed

ACuriousMind

@Semiclassical I've seen drunks succeed at many things they wouldn't even have attempted when sober!

Semiclassical

If a drunk finds their keys, does their driving constitute success?

vzn

20:38

DUI!!! drunks dont succeed? exactly! ... "shut up + calculate" → opiate of the masses

bolbteppa

The barman clearly took the drunk's key's and stopped them from driving tonight

vzn

Sep 14 at 15:05, by PM 2Ring

For Winning The Nobel Prize, Niels Bohr Got A House With Free Beer ... after he won the Nobel Prize in 1922, the Carlsberg brewery gave him a gift – a house located next to the brewery. And the best perk of the house? It had a direct pipeline to the brewery so that Bohr had free beer on tap whenever he wanted.

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