The h Bar

TanMath

00:00

Our phones must have great angular momentum sensors because the compasses really suck.

dmckee

00:27

Out of the dozen or so comics I've put on my office door that one has drawn the strangest response.

The physicists, chemists and mathematicians get it, smile once and move on. Everyone else either reads it then ignores it or reads and looks painfully baffled for a long time before saying something like "Oh I get it now!".

Only the one's who've gone on to explain have all been way off the mark. Several have thought it had to do with internet security.

0celo7

00:39

The mathematicians get it?

dmckee

@0celo7 Most of the time. Their major doesn't require a huge amount of physics, but the be present in the background of their culture.

0celo7

@dmckee So the prof did find the startup money to keep us all on. I'll probably get reduced hours because I'm a Freshman.

Was the whole "oh shit there's no money" just for show?

Why would he tell us that before really analyzing the finances...

Bernard Meurer

Maybe he's into emotional rollercoasters?

HDE 226868

01:01

Has anyone here ever built a cloud chamber?

0celo7

Yes. Get a can, put a cloud in.

It's pretty simple tbh

HDE 226868

Har har.

0celo7

??

HDE 226868

^^ Sarcastic laughter.

0celo7

:/

What else would a cloud chamber be?

HDE 226868

01:05

Cloud chamber

The cloud chamber, also known as the Wilson chamber, is a particle detector used for detecting ionizing radiation. In its most basic form, a cloud chamber is a sealed environment containing a supersaturated vapor of water or alcohol. When a charged particle (for example, an alpha or beta particle) interacts with the mixture, the fluid is ionized. The resulting ions act as condensation nuclei, around which a mist will form (because the mixture is on the point of condensation). The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path...

In case you weren't joking just now.

0celo7

Huh

Bernard Meurer

@HDE226868 I built one for a science fair last year; why?

HDE 226868

@BernardMeurer A friend and I are planning on building one for our AP Physics E+M final project.

0celo7

wtf

Bernard Meurer

@HDE226868 my take on it is that it is a massive pain in the ass to build, but when it works it is truly amazing

I almost cried, but I'm weird

0celo7

01:09

you people are too smart

smh

Bernard Meurer

what's smh?

0celo7

shake my head

ACuriousMind

"so much hate"

0celo7

that works too

HDE 226868

We'll officially have about a month to work on it (i.e. starting in mid-May), but we'll most likely start around late March or early April. I'd like to maybe work on creating a source of alpha particles from some old smoke detectors and maybe play around with that.

Bernard Meurer

01:11

@HDE226868 I built mine using Peltier plaques on a beefy PSU

0celo7

I would probably kill myself if I tried to make that

Bernard Meurer

I'd show you pictures but I converted my cloud chamber into a mean beer cooler

HDE 226868

@BernardMeurer We're thinking of doing something basic, and then upgrading it.

ACuriousMind

@0celo7 0celo7 the nuke engineer in spe saying that makes me feel slighty unsafe

0celo7

@ACuriousMind spe?

Bernard Meurer

01:12

@HDE226868 Have you ever built something of the kind before?

HDE 226868

Compression was our original plan, but I've been considering changing it to a diffusion-based apparatus.

@BernardMeurer Nope. That should make it interesting.

ACuriousMind

@0celo7 Oh, replace that by "to be", I see that that idiom isn't common in English

0celo7

Some Latin word I've forgotten?

Bernard Meurer

@HDE226868 The single, most important tip I can give about project development of this kind is STICK TO YOUR BUDGET. And make a budget ofc

Because I used ot get excited waste all my money on parts and bam, no more money for lunch that month

ACuriousMind

@0celo7 Yes, spe means "hope"

HDE 226868

01:15

@BernardMeurer Definitely.

Bernard Meurer

And a compressor is powerful, but tough to maintain and noisy. It does make good beer coolers as well

I used peltiers because I always loved how they work and wanted it to look neat

0celo7

In hope?

HDE 226868

@BernardMeurer Is that also electricity-guzzling?

Bernard Meurer

and since i'd be carrying the project around a compressor would be bulky

HDE 226868

If we can get it and work it safely, dry ice may be our best option.

Bernard Meurer

01:17

@HDE226868 Depends on the ones you get, there's no way to cool things without crying upon receiving the electricity bill

Dry ice works great, my first prototype was based on it

HDE 226868

Well, we'd be running it in the school, so we'd be hogging their power. . . I'm not sure the administration would be too pleased. Although the design lab has a frigging laser cutter.

That eats up a lot of power.

ACuriousMind

What is a "design lab" and what is it doing in a school?

Bernard Meurer

@HDE226868 so do peltiers and compressors. But the good thing is that Peltier Plaques are built with pure magic

aka semiconductors

Thermoelectric cooling

Thermoelectric cooling uses the Peltier effect to create a heat flux between the junction of two different types of materials. A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side of the device to the other, with consumption of electrical energy, depending on the direction of the current. Such an instrument is also called a Peltier device, Peltier heat pump, solid state refrigerator, or thermoelectric cooler (TEC). It can be used either for heating or for cooling, although in practice the main application is cooling. It can also...

Thermoelectric is amazing

HDE 226868

@ACuriousMind 1) Laser cutter + 4 3D printers + enough power tools for a small workshop + other things being added for a makerspace. 2) We're a STEM school, and we have no sports teams. Therefore, we have a decent amount of extra money.

@BernardMeurer I do like semiconductors. . .

0celo7

@ACuriousMind Either I blow something dangerous up or @ChrisWhite's prediction comes true

But that's unlikely

Bernard Meurer

01:23

@0celo7 Blow a capacitor up, I promise you it's fun

ACuriousMind

@HDE226868 Uh...I'm not asking about the money. What's a "makerspace" and what's it doing in a school?

Bernard Meurer

today when I had the courage to enter the room I found the capacitor's metal cover encrusted on the ceiling

HDE 226868

Hackerspace

A hackerspace (also referred to as a hacklab, makerspace or hackspace) is a community-operated workspace where people with common interests, often in computers, machining, technology, science, digital art or electronic art, can meet, socialize and collaborate. Hackerspaces have also been compared to other community-operated spaces with similar aims and mechanisms such as Fab Lab, Men's Sheds, and with commercial "for profit" companies such as TechShop. == Activities == In general, hackerspaces function as centers for peer learning and knowledge sharing, in the form of workshops, presentations...

It's in a school by popular demand from just about everybody, teachers and staff included.

ACuriousMind

I see

HDE 226868

The robotics team needed some extra space to work, so the old design lab got refurbished and it expanded pretty quickly.

ACuriousMind

01:29

The "robotics team"? Your schools are different from ours, I think

Very different, apparently

Bernard Meurer

@HDE226868 I built mine on my room and, although I did short circuit and it caught on fire along the way, it turned out nicely. You should be able to build a beautifully working equipment

HDE 226868

@ACuriousMind It's really just our school that's weird like that.

@BernardMeurer That's impressive.

ACuriousMind

@BernardMeurer "It caught fire, but it turned out nicely." You sound badass.

Bernard Meurer

@HDE226868 Thanks man. Always built stuff in my room, only this year I started creating a makerspace with some friends because I couldn't fit an electric saw on my bedroom

@ACuriousMind Hahahaha, hardly that, more like a salvage master

0celo7

...

serious question: are your parents ok with that?

Bernard Meurer

01:43

@0celo7 My mom is okay with it, she's mostly very glad that I'm into stuff the way I am. She doesn't like it when I set stuff on fire tho, but she's comprehensive enough

dmckee

@HDE226868 Not a particular successful one. But we did it on less that thirty dollars. At a party.

4

ACuriousMind

^Now that's engineering

dmckee

I'm looking for a student to refurbish one we own and which allegedly worked pretty well at one time.

Bernard Meurer

True

dmckee

It was a party of physics grad students. What else were we suppose to do, given that you can't have beer in the grad student computer room (which was where we played Duke Nukem death matches)?

@0celo7 Hmm. Would you rather get the email from him saying "I noticed two weeks ago that we had money trouble and I've kept it under my hat while I figured out that I'm flat broke and you're all fired. Sorry."

That's be kind of an asshole thing to do, so he tells you as soon as he's pretty sure there is a problem.

0celo7

01:48

Well when you put it that way...

HDE 226868

@dmckee See, that's impressive.

dmckee

@HDE226868 Meh. We got maybe one track (rather short) a minute which means we were having trouble maintaining saturation, because it should have be more than one a second.

ACuriousMind

Now I'm wondering what the physics grad students used as radiation source

HDE 226868

@dmckee What was the issue there?

dmckee

@ACuriousMind Cosmic rays. You have to do paperwork to take a formal source out of the building.

Bernard Meurer

01:57

@ACuriousMind I used some beauty thing that worked greatly

never knew what that had in it but it worked

dmckee

@HDE226868 At a guess: we were using isopropyl alcohol from the drug store (consumer grade), I think it has too much water in it to work really well.

HDE 226868

@dmckee I actually read about issues with that. Makes sense.

ACuriousMind

@BernardMeurer A..."beauty thing"?

dmckee

Next time I'll get some from a chemical supply house.

Bernard Meurer

@ACuriousMind yeah found it in the bathroom, brush thingy

dmckee

02:00

These days I own a original Fiestaware salt shaker.

ACuriousMind

I don't get it. How do you use that as source?

dmckee

More than 100 counts per second on a survey meter at 5 cm.

0celo7

We have a positron emitter in the lab

Bernard Meurer

@ACuriousMind https://en.wikipedia.org/wiki/Cloud_chamber#/media/File:Radioactivity_of_a_Thorite_mineral_seen_in_a_cloud_chamber.jpg
just chip part of the brush and use it like this

0celo7

@ACuriousMind Was AssCreed 1 such an amazing new thing at the time of its release that they needed all of these tutorials?

NeuroFuzzy

02:59

It was pretty awesome and new!!! So maybe yes

FenderLesPaul

03:23

yeah I remember when AC1 first came out

I was insanely hyped for it

almost as hyped as I was for TES4

0celo7

It's not that good

The combat is crap

I can't even counter? Wtf?

NeuroFuzzy

03:35

You get that ability later iirc.

DanielSank

04:09

What do you guys think: can the value of Boltzmann's constant change?

What about the speed of light?

In particular, are these two constants on equal footing?

Bernard Meurer

@DanielSank I have no clue, but from an educated guess point of view wouldn't the fact that it's called a constant imply it doesn't change?

DanielSank

@BernardMeurer Let me put this another way, if Boltzmann's constant were to double, would the world look different?

If the speed of light were to double things would change for sure.

However, I claim that this is not true of Boltzmann's constant.

Bernard Meurer

If I recall Boltzmann's constant serves for entropy correct?

DanielSank

Note that I'm not talking about using units in which the constants have different values. That's uninteresting. I'm talking about the physical constant actually changing in the sense that a wave of light moves twice as many atom-widths in the time it takes for a neutrino to oscillate, for example.

@BernardMeurer Yes, but I claim this was a historical mistake.

Entropy should be dimensionless. It just counts the number of microstates in a system.

Bernard Meurer

Well there's the equation for average translational kinetic energy (which I think only goes for an ideal gas like this) that is $\frac{1}{2}mv^2 = \frac{3}{2}kT$ for $k$ being Boltzmann's constant

I think that's correct

so if $k$ were to double we'd have much more agitated gasses for one? I think?

DanielSank

04:24

@BernardMeurer I think it's important to note that $k_b$ and $T$ always appear together.

Always.

This indicates that the we we define T is sort of wrong.

The statistical definition of $T$ is $1 / T = k_b \partial \ln(W) / \partial E$.

If we just leave out the $k_b$ we get a temperature which already has dimensions of energy.

and then you'd have $(1/2)mv^2 = (3/2) T$.

You'd also get $S = \ln W$, which is dimensionless as it should be.

I think $k_b$ only exists because historically we defined temperature as its own dimension independent of energy. This is entirely not needed, as is obvious from the statistical definition of temperature.

Bernard Meurer

Yeesh, the variables/constant letters in english are so different; I have a hard time ahead of me

DanielSank

Uh oh. Sorry.

Which one?

Bernard Meurer

W, S, and what comes after $k_b$

DanielSank

Oh ok

Some of this is not standardized.

Bernard Meurer

The names are all different over here, that has always pissed me off

DanielSank

04:29

$S$ is entropy. That's pretty standard.

$W$ is the number of microstates available to the system. It is also denoted $\Omega$ in many places.

Even $N$, I think.

$N$ is a bad choice, in my opinion, because it looks like particle number.

The $\partial$ symbol means "partial derivative".

Many people including me use $\partial$ without thinking too hard about whether we really mean "partial" derivative.

Bernard Meurer

Oooh, right, had totally forgotten about partial derivative

the other ones we just tend to use differently. Dully noted, thanks man!

DanielSank

Sure. Notation and terminology are, in my opinion, incredibly important and unfortunately abused things in physics and math.

Clear notation solves soooooo many problems.

Also, $\partial$ just looks so cool.

Bernard Meurer

Could one say temperature is just a result of kinetics on an atomic scale?

DanielSank

$\partial$aniel

hahaaha

Bernard Meurer

Hahahaha

Fancy

DanielSank

04:33

@BernardMeurer I personally would not do that.

Temperature makes sense in systems without kinetics!

In something like an ideal gas then you can think about kinetics, but to me that's a bit limiting.

To me, temperature means this:

$(1/T) \equiv k_b \partial \ln(\Omega) / \partial E$

That definition is bullet proof and works in all kinds of interesting systems.

I went into this a tiny bit in my answer to the recent post about Boltzmann's constant.

...well actually I guess I didn't. Never mind. Maybe I should have...

Bernard Meurer

Dude you always give these super canonical answers, I just love it

DanielSank

@BernardMeurer Canonical?

Bernard Meurer

Yeah, I use it in the sense of well-based, complete and throughout answers/questions

DanielSank

Oh, thanks!

:D

I like to write answers like that. It helps me remember/solidify things for myself.

I had never actually worked out the thing about temperature being a Lagrange multiplier before, but I had heard about it.

By the way, pressure is the Lagrange multiplier you get from the constraint of fixed volume, and chemical potential is the one from fixed particle number.

Bernard Meurer

I had never though about any of this on those terms

DanielSank

04:41

@BernardMeurer Neither had I until I saw it in a book somewhere.

It's pretty useful though because it helps you understand that "temperature" can show up in any optimization problem!

dmckee

Hmmm ... @DanielSank concerning the comments under your Boltzman's constant answer: I prefer to think that $c=1$ means measuring time in meters, which is logically the same thing.

But I suppose a metrologist would note that we have a precise definition of the second and the meter takes it's definition from the that and $c$, so perhaps your way is more defensible.

DanielSank

If you try to optimize a system where there's a "cost" for each state, and you have a fixed amount of "money", then you will have a temperature.

Bernard Meurer

The second answer to that question gives an insight on changing Boltzmann's constant, it would alter degree thresholds for states of matter according to the guy

DanielSank

@dmckee Ack! But when I write $F=ma$ I have not chosen any system of units.

Bernard Meurer

Optimization problems are the one thing I want to learn but never really picked up to study

dmckee

04:44

@DanielSank Yeah. To some degree that's the difference between theory and experiment. We theorize about beautiful pure thingies. Then we go in the lab and muck about with rather less beautiful real stuff.

And somewhere the twain must meet.

DanielSank

@dmckee Indeed. I must say though that I live in the brain of an experimentalist and I was always confused by the statement "...and then we set $\hbar$ to 1".

I could never understand that.

If $\hbar$ is 1 then it's dimensionless.

So then I write $i (d/dt)\Psi = H \Psi$. Now I want to write down some terms for my Hamiltonian, say for my superconducting circuit I have here in the lab.

I know that there's a term from the capacitor that goes $Q^2 / 2C$ (that's charge squared over twice the capacitance).

But wait! That's got dimensions of energy and the left side of my equation has dimension of 1/time.

Something's wrong!

If I had instead written $i(d/dt)\Psi = (H/\hbar) \Psi$ it is completely obvious what to do.

I could be lazy and declare that $H$ means what people would normally write as $H/\hbar$, i.e. when I say "Hamiltonian" I mean "Hamiltonian over $\hbar$", but I really ought to be explicit about that.

It's not the same thing as setting $\hbar$ to 1.

Thoughts?

dmckee

I do OK in particle physics with distance having dimensions of inverse energy. But when we start to get into stuff with charge and so on I forget how to cope.

DanielSank

@dmckee That's likely because the electro-static-unit doesn't have the same dimensions as charge.

dmckee

So I do them the messy way (with constants) then remove the constants after I've checked. But I'm a dummy.

DanielSank

You're not a dummy. You're a victim of people abusing notation and terminology.

0celo7

04:49

bah

did Einstein set $\hbar=1$

dmckee

I never fully grokked Guassian units. I made do well enough to fool my E&M instructor, but I was never really happy.

DanielSank

@dmckee The electrostatic unit has dimensions of energy^1/2 times length. That's not charge.

dmckee

@0celo7 This just proves that even the greats will abuse notation if it makes their life enough easier.

2

DanielSank

@dmckee That's because it's not a question of units. It's a question of dimensions. This confusion is so pervasive in physics it hurts.

I boggles the mind how badly our community confuses units and dimensions, and always to the ultimate suffering of the student.

dmckee

@DanielSank Oh, I get it. I even do dimensions first and units second in my intro classes.

0celo7

04:51

unless the student just accepts it like a good boy

DanielSank

@dmckee Good for you, seriously. I wish that were drilled into the head by all professors.

dmckee

But getting it in the abstract doesn't translate in to mastering the transitions.

DanielSank

@dmckee That's certainly true.

dmckee

@DanielSank For the most part the students don't get it.

0celo7

what is there to get?

dmckee

04:52

I'm still learning how to prepare the teaching materials, and some work better than others.

DanielSank

@dmckee At least they've heard it so when they get confused later they might realize that they're confused for a good reason and not because they're lacking mentally.

That's really important.

Bernard Meurer

As a student @dmckee is totally right.

DanielSank

@dmckee I think one of the most important transitions for me in physics was when I started to realize that a lot of standard explanations, notation, terminology, and practices are plain stupid.

One of the worst is the confusion about what a tensor is. Won't get into that now...

0celo7

I'm still not convinced you know what the tangent space is, @DanielSank

dmckee

Yes. Deciding what parts of the cargo cult teaching to throw out.

I'm still working on that.

DanielSank

04:54

@0celo7 Possibly not to the level of mathematical rigor you'd like :)

dmckee

@DanielSank It's a thing that transforms like this ...

DanielSank

@dmckee Grah!

0celo7

that's not even wrong you know

DanielSank

Were you trying to needle me with that? ;P

dmckee

I actually think that is deep, but I haven't found the example that exposes the depth.

0celo7

04:55

it's completely equivalent to the mathematician's definition

DanielSank

@0celo7 Yes.... it is.... but it's also a lot harder to understand the meaning.

0celo7

which is?

DanielSank

This is exactly the same issue as thinking of a linear operator as its matrix representation or as a thing apart... a thing which exists without expression in a particular basis.

@0celo7 A multilinear function on a vector space (plus covectors, etc. etc.)

0celo7

@DanielSank so what?

who needs that

Bernard Meurer

Oh, oh I have a question that I thought on the bus today

DanielSank

04:58

@0celo7 Uhh, I dunno, every sub-field of physics?

0celo7

@DanielSank not in that sense

what does the multilinear map definition do for me

(and btw it turns out to be more convenient to define it as a function on (co-)modules into another module)

DanielSank

@0celo7 Tells you a lot about physics! For example, the voltage on these pieces of metal due to some charge is a superposition of the voltages you'd have from each chage separately.

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