The h Bar

ChoMedit

01:48

Hello. I've been finding good reference for understanding representation theory of Lorentz group

Could someone suggests?

yuvraj singh

06:02

@ACuriousMind are you and @JohnRennie are in same batch.

John Rennie

@yuvrajsingh no, he's a mere youngster :-)

yuvraj singh

Is he is till in university or passed out. @JohnRennie

John Rennie

08:19

@HoomanNeshat Agreed. It would have to be a negative CC. I had obviously turned off my brain for the weekend :-)

Emilio Pisanty

11:35

@yuvrajsingh ACM finished his masters degree about a year ago.

John's PhD is listed at ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.383872

So about a 35-year offset, or thereabouts

John Rennie

They don't make scientists like they used to :-)

6

Emilio Pisanty

Though then again, if you zoom-out and coarse-grain enough, they do indeed fall in the same batch

@JohnRennie indeed

Now they make them better ;-)

John Rennie

:-)

Secret

The answer to life, mystery of the universe, and everything

yuvraj singh

11:58

@EmilioPisanty OK.

Slereah

12:50

I should make another section of my site

for like

small proofs

It would unclutter a bit some of my articles

So that I don't have to prove a bunch of weird topology in the middle of an article

Slereah

13:15

samuel-lereah.com/db/proofdb/1%20plus%201%20equals%202

There's the first

Please cite me if you use this theorem

Luyw

13:28

hello guys, chemistry rooms are silent so do excuse me for asking here.
"10.0 g of gaseous ammonia and 6.50 g of oxygen gas are introduced into a previously evacuated 5.50 L vessel" they go on to tell me that they react and they give me temperature and I need to find pressure.

the actual way of doing this is by figuring out the chemical formula and add up the number of moles

but why can't I just find the number of moles from the masses? isn't everything conserved?

10/molar mass of ammonia + 6.5/molar mass of oxygen

PM 2Ring

13:50

@Luyw Because the number of moles isn't conserved. Eg 2 moles of hydrogen react with 1 mole of oxygen to give 2 moles of water.

Slereah

14:06

⊢ ∏𝑘 ∈ 𝐴 𝐵 = (℩𝑥(∃𝑚 ∈ ℤ (𝐴 ⊆ (ℤ≥‘𝑚) ∧ ∃𝑛 ∈ (ℤ≥‘𝑚)∃𝑦(𝑦 ≠ 0 ∧ seq𝑛( · , (𝑘 ∈ ℤ ↦ if(𝑘 ∈ 𝐴, 𝐵, 1))) ⇝ 𝑦) ∧ seq𝑚( · , (𝑘 ∈ ℤ ↦ if(𝑘 ∈ 𝐴, 𝐵, 1))) ⇝ 𝑥) ∨ ∃𝑚 ∈ ℕ ∃𝑓(𝑓:(1...𝑚)–1-1-onto→𝐴 ∧ 𝑥 = (seq1( · , (𝑛 ∈ ℕ ↦ ⦋(𝑓‘𝑛) / 𝑘⦌𝐵))‘𝑚))))

Metamath defining the product

Emilio Pisanty

14:47

0

Q: Why is $\sum_{r =1}^3 \epsilon^r_\mu \epsilon^r_\nu $ a projection operator?

It is mentioned in this answer that the completeness relation for the polarization vectors of a (massive) electromagnetic field $$ \sum_\lambda \varepsilon^\mu(\lambda,k) \varepsilon^{\nu*}(\lambda,k) = - \eta^{\mu\nu} + \frac{k^\mu k^\nu}{M^2}. $$ can be understood since $P_\epsilon^{\mu\nu} := ...

@ACuriousMind tailor-made for you ;-)

skullpetrol

@EmilioPisanty how are they better?

Abhas Kumar Sinha

The answer to life, mystery of the universe, and everything

42

skullpetrol

41.999...

Abhas Kumar Sinha

@skullpetrol no only 42

skullpetrol

They are equal :-)

Abhas Kumar Sinha

15:00

@skullpetrol no. en.wikipedia.org/wiki/Hyperreal_number

skullpetrol

Keep it real, my friend.

Abhas Kumar Sinha

@skullpetrol hehehehe....

can anyone help me a bit?

Abhas Kumar Sinha

15:15

does anyone here know a bit transistors? BC 547?

@NovaliumCompany

sparkfun.com/datasheets/Components/BC546.pdf

Here the Emitter-Base Voltage < Collector-Emitter Voltage for NPN Transistor, it confuses me.

@JohnRennie sir do you know about it?

Abhas Kumar Sinha

15:41

@Secret need help

John Rennie

15:59

@AbhasKumarSinha that does seem a bit weird. It's hard to imagine a situation where the collector base voltage could be greater than the collector emitter voltage. In a common base config maybe?

Ryan Unger

@Slereah gravity.princeton.edu/events/…

Maybe you recognize some names

Slereah

That Unruh fellow seems familiar

Abhas Kumar Sinha

@JohnRennie How do you calculate the threshold voltage for the current to pass from collector to emitter, it seems 6 volts here instead of 0.6 volts

Slereah

Also Strominger

Let's see what he did

Abhas Kumar Sinha

@Slereah what?

Slereah

16:06

Also Gonzalez... didn't he write some causality paper

@AbhasKumarSinha This is directed at @RyanUnger

Abhas Kumar Sinha

@Slereah he's nah talking to you now, seems inactive for now.

John Rennie

@AbhasKumarSinha Figure 1 presumably.

Slereah

@AbhasKumarSinha This is a chat, he can read it later at his leisure

Abhas Kumar Sinha

@JohnRennie didn't get what it says...

@Slereah I thought you were live to him

John Rennie

It's showing the collector emitter voltage you need to move out of the saturation region. i.e. you need at least a volt at low currents and 2 volts at higher currents.

Ryan Unger

16:10

@AbhasKumarSinha chat is an asynchronous communication protocol

Abhas Kumar Sinha

@JohnRennie So, I need at least 1 volt to activate the transistor to work for the collector to emitter transmission path?

John Rennie

@AbhasKumarSinha yes

Abhas Kumar Sinha

@RyanUnger hello, slreah has something for you there^ XD

@JohnRennie ah okay.

John Rennie

But you wouldn't operate at such a low voltage anyway.

That sort of transistor would be used at more like 6 or 12V.

Abhas Kumar Sinha

@JohnRennie But, it shows collector - emitter graph not the voltage graph at base...

@JohnRennie np.

@JohnRennie I think that it's not 1 volt, rather 0.6 volt, as in figure 2

specifically 0.8 volt

John Rennie

16:14

Figure 2 is showing you the bias voltage you need (in a common emitter config). The collector current is $I_c = \beta I_b$ so effectively it's showing you the base current as a function of the base voltage.

So the base-emitter junction potential is about 0.6V.

Abhas Kumar Sinha

@JohnRennie so, in simple words, to make 3 volts pass from C to E. I need to apply 0.8 volts additional in B to E?

John Rennie

@AbhasKumarSinha I'm not sure what that means ...

Abhas Kumar Sinha

I need an additional of 0.8 volt battery between Base and emitter to make 3 volt current flow from collector to emitter to complete the circuit?

John Rennie

The collector current is $\beta$ times the base current, so what fig 2 shows is that above 0.6V a base current starts to flow so a collector current starts to flow. That's a current not a voltage.

"3 volt current" ? You mean with a collector-emitter voltage of 3V?

Abhas Kumar Sinha

@JohnRennie heheh... I think I've screwed a bit. I need some time to chew some theory in order to ask. I forgot what to ask.... sorry

John Rennie

16:20

OK :-)

Abhas Kumar Sinha

Anyways thanks :)

John Rennie

I was talking about transistors early today in the problem solving room as it happens ...

Abhas Kumar Sinha

@JohnRennie ah... They are the best thing. I plan to design a 4 bit processor using adder chain to calculate binary numbers.

School projects in middle of exams are a pain...

But, no problem, I'll do it, there are a lot of logic resources all over the internet.

John Rennie

I used to like messing around with transistors. These days everything is digital.

Abhas Kumar Sinha

@JohnRennie I wished the school authorities let me use raspberry pi

they are microcontrollers and much faster.

John Rennie

16:24

If Raspberry Pis were around when I was a teenage hacker I would have spent all my time playing with them and done no school work.

Maybe it's just as well that they weren't :-)

Abhas Kumar Sinha

@JohnRennie that's how I spent my 8-9th grade... I used RPi(s) and Aurodinos. Those days were coooool!

John Rennie

Why oh why do we have to grow up? It doesn't seem fair :-)

MartianCactus

hey so in Compton effect

how can we assume that a photon actually "collides" with the electron

Abhas Kumar Sinha

@JohnRennie hahaha... I can understand.

@JohnRennie what's the time there?

MartianCactus

since photon is actually just a packet of energy without mass...is it even possible for it to colllide with another particle and get deflected like particles with mass do?

John Rennie

16:27

@AbhasKumarSinha the UK s 5.5 hours behind India. It's 16:27 here.

@MartianCactus photons are more complicated objects than you might think.

Abhas Kumar Sinha

@JohnRennie Do you go for an evening walk? ...

MartianCactus

@JohnRennie can you elaborate?

John Rennie

@MartianCactus have a look at:

47

Q: Do photons truly exist in a physical sense or are they just a useful concept like $i = \sqrt{-1}$?

Reading about photons I hear different explanations like "elementary particle", "probability cloud", "energy quanta" and so forth. Since probably no one has ever seen a photon (if "seen" it supposedly - and rather conveniently - ceases to exist) but many experiments seem to verify its properties ...

quantum-mechanics quantum-field-theory photons elementary-particles

@AbhasKumarSinha no. I go for a cycle ride mid afternoon to get my exercise. Then I eat a late lunch then slump into my armchair with a book.

MartianCactus

ok thanks

Abhas Kumar Sinha

@JohnRennie I usually move here at night. The cold air under night stars seems a perfect combination to cure busy mood.

ah okay, it's dinner time here. I'll be back... :)

John Rennie

16:32

@AbhasKumarSinha enjoy dinner! :-)

Secret

It has been a very long time I last touched transitors. Glad that JohnRennie solved it for you

Physics Meta

0

Q: Can I present an answer in the form of a leading question?

Like "Jeopardy" presents answers in the form of questions can I post a physics question and post my own answer to it? The presentation I want to make is a new type of math method to calculate and compare worldlines in relativity. My question would be, "Are there any problems with using this math ...

discussion

Abhas Kumar Sinha

16:49

I'm back...

@Secret Yes... :)

Abhas Kumar Sinha

17:00

Can anyone here help me to choose the best haircut for me?

I'm thinking to change my haircut now...

peepingmoon.com/wp-content/uploads/2018/07/kartik-arryan.jpg vs bizasialive.com/wp-content/uploads/2018/03/… vs biographywiki.net/wp-content/uploads/2018/02/…

@Secret @Slereah @JohnRennie ..?

@NovaliumCompany

John Rennie

I am not the person to ask. I get my hair cut when it gets too long for me to see out and I get it cut at a traditional barber not a salon.

You're asking physicists for fashion tips????

3

Abhas Kumar Sinha

@JohnRennie ah okay... Physicist for fashion tips for another future physicist, probably... XD :-) :D

Okay, there's a lot of chemistry I've to cover tomorrow. I'll go to sleep early. Bye, everyone. Goodnight :)

Novalium Company

17:27

@AbhasKumarSinha The first one looks nice (and maybe the second one)

I was wondering, how people like Bill Gates and Mark Zuckerberg managed to build and control those huge companies at such a young age. I mean, how did they manage the people when they were perceived as young and inexperienced? How did they, I don't know, assert dominance where it had to be asserted. How did they manage teams of people which are older than them...?

Physics Meta

0

Q: How to navigate answers

I have been using/contributing to StackExchange for about a year. I have thus seen what must be a common phenomenon: the same, or very similar, questions coming up again and again. I know there is already some effort to avoid this, but it seems to me the site is in danger of just sprawling ever o...

support feature-request

JMac

@NovaliumCompany You don't need to be older than someone to be in charge and manage them. It depends on who they hired at first, what leadership experience they have, etc. You can get leadership experience even as a youth. One way to not appear young and inexperienced is to be really good at the job and make everyone a bunch of money; especially the money part. It's not that hard to get people to listen when you can show them that following you will make them money.

Novalium Company

17:42

@JMac yeah makes sense.

Aaron Stevens

19:12

@JMac sigh

JMac

@AaronStevens How dare you try to edit question titles so that they are more appropriate!

Aaron Stevens

@JMac Yet another example of a user taking this site way too seriously. Do they think someone from the journal will see this post and be like "Oh wow we really messed up on that one! Physics SE is definitely where relevant physics research is born."

Also I was referred to as "boy".... I didn't think it could get any worse than "sir".

JMac

@AaronStevens physics.meta.stackexchange.com/questions/12492/… I'm not surprised after this question. They strike me as the type of user who thinks everyone is acting against them; when really they're just using the site for things it isn't designed for.

Aaron Stevens

@JMac Yes I completely agree. That meta post is now the first thing I think of whenever I see anything by that user on this site now

JMac

Well, at least he managed to change the title to "Scientific Reports" after like 8 edits...

Aaron Stevens

19:24

@JMac Yeah, and only in the title. Unfortunately the damage has already been done. I highly doubt this question will be reopened.

tpg2114

@AaronStevens You could have flagged it as unfriendly/unkind -- it's useful for condescending comments like that

Aaron Stevens

@tpg2114 Yeah possibly. I didn't find it as unfriendly though, just odd. I figured it was just a sort of translation issue rather than an insult. But maybe they were trying to be mean. oh well

tpg2114

No worries. It struck me as insulting, but it wasn't directed at me... heh.

JMac

Yeah I kinda just assume "boy" is kinda like a "non-sir"; but I could be totally wrong about that; since it's basically just a guess.

Aaron Stevens

@tpg2114 Yeah I suppose I don't tend to feel insulted haha. At least not by people I don't know on the internet.

It has one reopen vote haha

JMac

19:31

@AaronStevens Had it almost right away. I guess someone thought it was a pretty decent question.

Aaron Stevens

@JMac Yeah well that question requires too much critical thinking, which this site is obviously against.

JMac

@AaronStevens Just your classic mainstream dictatorship.

Aaron Stevens

@JMac Based on what seems to be the majority of "conspiracy theories" discussed on this site, it seems like Physics SE's main goal is to hide the fact that SR is actually a contradictory theory that doesn't describe the universe at all

JMac

@AaronStevens Wow way to blow our cover.

Aaron Stevens

@JMac I mean have you ever gotten into a space ship and traveled near the speed of light to see that you still observe light to move at the same speed? Checkmate

JMac

19:44

@AaronStevens Have you ever seen the big bang occur? Didn't think so.

Novalium Company

@JMac "How dare you", sorry I'm pretty sure Greta Thunberg already registered that phrase. It would be a copyright issue so please stop using her phrases.

JMac

If you weren't there you couldn't possible know.

Novalium Company

@JMac Oh, I was there. It was pretty cool. Kinda like fireworks but in a larger scale.

Aaron Stevens

@JMac Have you even seen a wavefunction travel through two slits and interfere with itself? Didn't think so

Have you ever seen the curvature of the Earth? Didn't think so.

JMac

@AaronStevens Have you ever personally fallen in a vacuum tube? Didn't think so. Gravity is actually just due to density, super obvious.

Novalium Company

19:46

@AaronStevens I was there, during the Big Bang. It happened like you think it did. Calm down and have some choco ice cream.

Aaron Stevens

@JMac Have you ever actually seen a block sliding down an incline? Didn't think so.

Novalium Company

I can safely conclude that if you haven't seen it, it didn't happen. Now let's stop and have some choco ice cream. yaay

JMac

@AaronStevens Everyone knows blocks just slide down inclines because blocks are more dense than air, and therefore the buoyant force pushes down more than it pushes up. Gravity was just made up by NASA for an easy paycheck.

Aaron Stevens

@JMac They also call it the "normal force" to hide how it actually isn't normal at all.

Abhas Kumar Sinha

How dare you? We are very close to mass extinction

JMac

19:53

@AaronStevens "Normal force" really does a good job to mask the true Lovecraftian horror that the physics cabal controls.

Abhas Kumar Sinha

We are in the beginning of a mass extinction. And all you can talk about is money and fairytales of eternal economic growth. How dare you!

“For more than 30 years the science has been crystal clear. How dare you continue to look away and come here saying that you are doing enough, when the politics and solutions needed are still nowhere in sight

Klol

XD

These kids deserve applause...

Most intelligent species found on earth..

Aaron Stevens

On a somewhat related yet unrelated note; I have always had a desire to go to some sort of political rally, function, etc. and ask a candidate what they are planning to do about the rampant increase of entropy throughout the universe as dictated by the tyrannical "second law".

It is a serious problem we are facing, and it threatens to destroy us all.

Abhas Kumar Sinha

@AaronStevens what serious problems? Climate change?

JMac

@AaronStevens Tackling the true issues. You would have my vote if you ran.

Aaron Stevens

@AbhasKumarSinha No, I am not talking about climate change. I am talking about entropy continually increasing throughout the entire universe

JMac

19:56

I'd probably just run on a platform of something a bit more short term. I would try to put a stop to the tyranny of the rocket equation.

Aaron Stevens

@JMac Please go further

JMac

It's been going on for around a century now, and it's about damn time someone did something about it.

Abhas Kumar Sinha

@AaronStevens you are also eligible to give speech about that at UN and say we are very close to mass extinction and how dare you. You'll soon become famous.

JMac

I don't get it. Do you have like a problem with people speaking up against climate change?

Aaron Stevens

I think instead of the issue being between climate change / denying climate change that it should be between is climate change good or bad.

Abhas Kumar Sinha

20:00

@NovaliumCompany when it comes to technology, the younger you are, the more experienced. What zuck made was beyond the interest of old people but young people liked it, why? Mark knew what youngers want/like.

Aaron Stevens

I can imagine a group forming that want to push the climate to its extreme and start a new world

Abhas Kumar Sinha

@AaronStevens bad ofc

Novalium Company

@AbhasKumarSinha mmmhm smart

Abhas Kumar Sinha

@JMac yes

Aaron Stevens

I mean it is the motivation behind some movie villains I cannot think of at the moment. "Humans are bad for the environment. So we need to destroy humans."

Ah, Recyclops. That is one of them

JMac

20:02

@AaronStevens Quite frankly, I don't even think it matters that much. The push towards renewable infrastructure seems like an all-around good thing. The biggest hurdles seem to be economic; not even technical limitations or anything. The way I see it, a short term investment to get our infrastructure geared towards renewables will have long term benefits, for everyone besides oil tycoons.

@AaronStevens Polluticorn wishes.

reclyclops is one of the better opening gags.

Aaron Stevens

@JMac Oh most definitely.

Abhas Kumar Sinha

Greta has no contribution towards the prevention of climate change. You speak lines and you are a great actor, just like greta. Climate change can only be tracked by innovation, design, and skills, not by hate speeches. That's very immature. People ain't aware of actual climate change contributors, but these cranks. I convey my utmost disapproval of hate speeches to get publicity

Aaron Stevens

@AbhasKumarSinha Are you talking to me?

Abhas Kumar Sinha

@AaronStevens no, that's@JMac 's reply to why I've problem with them.

JMac

@AbhasKumarSinha Education and promoting activism are actually pretty important aspects about climate change right now. Things like innovation, design and skills are all irrelevant if the politicians ignore it because oil companies have a lot of lobbying sway in many countries. Thee fact is, the technology could be there; but it takes investments that are overshadowed in the short term by non-renewable resources. Condemnation of harmful actions is actually a pretty major promoter of change.

That's how you get things like civil rights movements in the US. From people not taking it anymore and being vocal, getting others to express displeasure, etc.

Aaron Stevens

20:09

@AbhasKumarSinha I don't think JMac is pushing for hate speeches though.

Abhas Kumar Sinha

@JMac so you think pulling kids in politics is a way to influence politicians?

Aaron Stevens

Well this conversation has taken a turn

Abhas Kumar Sinha

@AaronStevens I'm against Greta not Jmac

JMac

@AbhasKumarSinha I think speaking up when you think something is wrong is totally normal. Greta happened to be in a position where her activism resonated and brought her onto the world stage. I don't see how that makes her speaking up a bad thing. I think allowing concerned citizens to express their concern is something that should influence politics, as it clearly did in this case.

It's not like I'm an avid supporter or anything. (As far as I know I only first heard about her a couple months ago) I just think it's weird to paint someone a villain for it; when they are speaking up about things that others clearly agree with.