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Q: Should Stack Overflow be a "last resort" resource?

1:48 AM

If physics doesn't make sense, it's just advanced.

dmckee

@DavidZ I didn't chip in on that one, because I felt I'd said my piece on the subject in

12

I recently left this comment for a poster who was insulted by someone asking him if he could use Google: He does make a valid point, though. You'll find on StackOverflow that we, as a community, place a heavy importance on doing your own research, and basically seek to use SO as "last chance,...

discussion

user54412

@tpg2114 I see where you're coming from, but I feel using just translational degrees of freedom is too restricting

user54412

for instance there are rotational degrees of freedom for various molecules, as well as vibrational

@dmckee So having read that -- how do you view questions where I or you might find the answer in 10 seconds on Google, but it's because we know the words to search for while OP may not? Obviously if you just take the adjectives and nouns in the question and Google it and find the answer, they didn't do enough work

user54412

and even classic heat capacity changes depending on the number of such degrees of freedom (monatomic vs. diatomic ideal gasses for instance)

1:55 AM

@ChrisWhite I know, but when we (I say "we" from a gas dynamics standpoint) mean the other forms, we explicitly say rotational temperature, vibrational, electronic, etc.. If we say "temperature" it is either assumed all the modes are in equilibrium (and so translational temperature is fine) or we are only talking about translational energies

And yes, heat capacity can change (unless you are assuming gases are frozen). But I have only studied all of this from the perspective of gas dynamics so we always have something with mass. I've never looked at what happens with photons

I really don't know how photons are considered w.r.t. temperature/energy. I would ask a question about it but I'm worried it is clearly defined in some elementary text somewhere that is just outside of my area and it would be a bad question with an obvious answer

user54412

I too only work with translational temperature (in perfectly conducting, adiabatic, perfectly collisional plasmas!)

In the vicinity of mass-less spherical cows too

user54412

but for instance interstellar dust temperature is understood to basically be the temperature of the phonons excited by the last photon to strike the dust

We do actually have a multi-temperature capability for non-equilibrium flows and plasmas, but we treat each temperature as it's own equation

@ChrisWhite Which is what I was trying to argue in my point 3 and the comments under it -- the "temperature" of a photon is only measured when it interacts with something. So if we have a volume with photons passing through it and no matter with which to interact, the "temperature" is undefined

And so if we measure the temperature of the CMB, the "volume" that has that "temperature" is really just confined to the sensor that detected it. Not the finitely-infinite volume the photon passed through to get to the sensor.

Or it's the temperature of the volume of mass that released the photon to begin with

user54412

@tpg2114 It should fall out from $1/T = \partial S/\partial U$ in the statistics of an ideal gas of massless bosons. How much that's just pushing the question back is another issue. Also, that wouldn't be a bad question at all.

2:05 AM

Maybe I will go ahead and post the question then. It seems like something that might actually spark some different opinions

user54412

I guess there's the stat mech definition: write down S and U (somehow -- that was always the least clear thing in stat mech) and differentiate. Then there's the old-school thermodynamics definition: place a thermometer in equilibrium with the thing you want a temperature for. Asking how these two are equivalent (both mathematically and intuitively) seems pretty constructive -- I know it was glossed over in my courses.

Is there such a thing as "equilibrium" between photons?

Like, if I put a bunch of particle A in a box with a bunch of particle B, eventually the two would be in equilibrium with one another

But what if A was photons instead. The total energy would be the same, but the photons would become "absorbed" into B

Can there even be photon-photon interactions? If both A and B were photons at different "temperatures" (or energies), no equilibrium would be reached right?

I'm pretty... confused

David Z

2:22 AM

@tpg2114 (re: last sentence) you'd think, but remember this recent case in which that was far from obvious

thermodynamics statistical-mechanics photons temperature

And photons have 2 degrees of freedom, so if we go with the "temperatures based on D.O.F" naming, it wouldn't be translational temperature but something else. I'd call it "spin temperature" but I don't know if the D.O.F's are actually called spin

@ChrisWhite Question is posted (had to have fun with the title) -- feel free to edit if something isn't clear (or make suggestions)

0

Q: How hot is your photon?

This question comes from my answer to the question Can a cubic meter of space at absolute zero have any object with mass inside? and the related discussion under it. To summarize, I stated that the temperature within a volume is related the ensemble average of the kinetic energy of the particles ...

user54412

In other news the next batch

user54412

[of](http://arxiv.org/abs/1502.01588) [Plank](http://arxiv.org/abs/1502.01589) [papers](http://arxiv.org/abs/1502.01590) [came](http://arxiv.org/abs/1502.01591) [out](http://arxiv.org/abs/1502.01592) [today](http://arxiv.org/abs/1502.01593),

user54412

[including](http://arxiv.org/abs/1502.01594) [this](http://arxiv.org/abs/1502.01595) [one](http://arxiv.org/abs/1502.01596) [and](http://arxiv.org/abs/1502.01597) [this](http://arxiv.org/abs/1502.01598) [one](http://arxiv.org/abs/1502.01599).

user54412

Great, I broke chat

2

2:26 AM

I haven't asked many questions on this site, but it's a pretty odd collection

user54412

@tpg2114 polarization is probably a better word. But it's not quite the same as a spin lattice I suppose

Possibly. I don't know what the +1/-1 modes are called. I assume they have a name

user54412

it's not like it takes energy to rotate a light beam's polarization

That would imply there is no energy-storing D.O.F then

And we're right back at "what the heck kind of energy is it really"

I kind of want to post a question asking what the heat capacity of a photon is now :)

@DavidZ I would agree with your opinion on the matter -- the question itself is answered obviously by searching. I think what Anna was arguing is more about the OP not understanding the result, but OP didn't say "Hey, I found this and I'm not sure how to interpret it". That, to me, would be sufficient

user54412

--> More seriously on Plank, this paper is the summary. Table 9 has the cosmological parameters, and there are some very pretty sky maps too.

2:32 AM

I'm in for the sky maps. I won't understand anything else

user54412

The big CMB cosmologist in my department informally declared that we've reached the end of the era of measuring the CMB temperature

user54412

there's stuff worth doing in polarization, but we've pretty much exhausted the useful information from just self-correlating temperature

user54412

Figure 20 from that paper struck me as something that could come from an MHD simulation. Then I read the caption and realized that it's the galactic magnetic field, so that makes sense.

Fig 20 and 22 are pretty awesome looking

user54412

and people think space is "empty"

user54412

2:36 AM

actually... I wonder if one could sensibly define a magnetic field temperature...

So I just had a thought... what if the energy of a photon (or a magnetic field) is really just a potential energy?

Since photon number is something not conserved, the energy contained in the field can be absorbed by things (PE->KE) or emitted (KE->PE)

And the same could possibly be said about magnetic fields

Searching for magnetic field temperature is too polluted with how magnets work at different temperatures...

@ChrisWhite Changing topics, how's the thesis coming along?

user54412

Not too bad. I'd say there's a >50% chance I graduate.

You should have reported it as a p-value against the null hypothesis that you won't graduate

user54412

(We have a strict 5 year limit, and I have 18 months left. It's not a question of how long the degree takes, but whether or not it happens at all.)

Our department is starting to implement strict time limits

But grandfathering us older folks in gently

user54412

2:45 AM

it definitely cuts both ways

The motivation in our department is that professors keep us around far too long working on industry projects since we are ridiculously cheap labor

user54412

it's nice to have everyone trying to get me to finish for my own good, but I'm reasonably sure I'd have a stronger career start (in academia) with an extra year

Our funding agencies have actually started telling our boss to graduate us or they'll stop funding him

Is it possible to stay on after you finish for ~1 year as a research engineer/post doc?

user54412

wow -- when funding agencies notice that, that's when it gets serious

user54412

@tpg2114 nope -- we're unceremoniously kicked out the door, not welcome to return except in a second post doc (maybe) or faculty position

2:48 AM

Yeah... it's not great. We've had several people over 10 years where they couldn't work on their thesis because of industry projects

That's not good. But I guess it is good at the same time. I mean, it's better than our... incestuous? practices. We have people who graduate, work for another 6 months, move to another lab in the department for 6 months, and maybe leave, maybe just stick around as a research engineer

There's surprisingly little turnover

user54412

So every week there's a lunch discussion at the Institute for Advanced Study, attended by all the local big shots in astro -- the sorts who can (or think they can) determine all sorts of policies and such. This past week was particularly political, with a former congressman (and physics PhD) attending.

user54412

A good deal of discussion revolved around restructuring of academia in the not-too-distant future

user54412

certain fields are really starting to be hit hard by the leveling off of the exponential growth they've experienced for several decades

user54412

and the faculty/postdoc/phd ratios are really unsustainable when taken together with the "get a phd and one day you'll become a professor" attitude

The perk of being funded by primarily defense-sector spending is our money is pretty secure and plentiful.

Although most of us don't even go the academia route. I think we (aerospace) tend to break the PhD to be a professor mold

I know in computational work, if you have a BS you really can't do it. An MS will have you making the grids for the guy with the PhD to actually use and analyze.

user54412

2:59 AM

I think there's a consensus that more PhDs outside academia is probably a good thing, but it would help if PhD programs accept this role and don't deceive students about job opportunities.

user54412

(That's not necessarily a complaint about my department in particular, just a general observation)

I actually don't have any idea what that is like. Our department doesn't let us teach, doesn't let us TA, and our ties with military research labs and industry all but hands us industry jobs. And makes it very, very hard to get academic jobs

It's always interesting how perspectives differ

My brother (at Pitt in the math dept) said he went to a seminar by a professor in the department who claims to have proven the Boussinesq hypothesis.

I want to see that paper

Not published yet though. So I'm skeptical.

user54412

interesting -- ping me if you hear more about that

I will. That's a pretty big deal if true. I'm sure he proved something, but I'm worried it's under some extremely restrictive conditions that give it very narrow scope

The turbulence modeling field is really divided on the validity of the hypothesis

user54412

@tpg2114 A continuum of incompressible, spherical cows?

3:08 AM

Heh, exactly

Spherical cows in an axisymmetric, periodic domain

dmckee

3:30 AM

@tpg2114 I tend to be pretty forgiving of those kinds of things the first time I see. Even the second. But after a while ...

I'm aware that this isn't always a fair reaction because if the previous ones aren't coached in the same language the OP is using they still might have looked and not found it.

But I'm human, too.

dmckee

On the spherical cow front, I'm collecting spherical cow models that get things more right than they have any right to.

I have a uniform density model of a star I use with my Virial Theorem lessons that can get the temperature of the sun more or less right. At least it hit about the mean of the core temperature and the surface temperature.

user54412

Impressive. Looking up the real numbers it seems the central-to-average density ratio of the Sun is actually over 100.

I think the most mindblowing statistic about the Sun is how long it takes for a photon from the core to actually reach space

user54412

@dmckee I'm trying to recall some good ones from my order-of-magnitude undergrad physics course (one of the best courses I've ever taken).

user54412

We estimated things from pretty much every branch of physics, some of them probably unreasonably well.

3:42 AM

@tpg2114 are you attending GT?

@AlfredCentauri Yes, for entirely too long

I did my undergrad here and just kept right on going

@tpg2114 I was there, on and off, from '91 through '10 over in the EE building on the corner.

I don't get over there very much. The only EE class I ever had was the intro to circuits for non-majors

I'm in the AE buildings, across from the hotel on North ave

Were you teaching?

@tpg2114 I taught that class for several years.

Hrm... let me hop in OSCAR and see when I took that

Summer 2006

That was the class (ECE 3710) and the lab I took in Fall 2007 (ECE 3741)

I dated a girl who was an ECE major. She got her MS there also, was working on some DSP. Voice recognition I believe?

Back in 2004-2005

3:53 AM

@tpg2114 I'm almost certain I taught a section of that class SU06. There were always several sections so it's possible you had my section. Actually, in almost every semester, I found that some of my 3710 students were quite a bit more capable than many of the EE majors I taught. Of course, I tried to recruit them...

@AlfredCentauri My instructor was... Erik I Verriest

(which if that is you and you don't want your real name known, I can delete that)

@tpg2114 No worries, you had a different section. Erik's name is familiar to me but I can't say that I remember meeting him.

So have you moved on to another institute?

@tpg2114 No wonder his name is familiar - he's a professor there. That's actually unusual for a prof to teach 3170 - it's normally 'non-traditional' grad students.

I can't say that he was the one who actually taught us. I want to say he came in day 1 and then handed us off to a grad student

But I wasn't really all that into the class so I have sort of repressed it

4:03 AM

@tpg2114 I'm no longer residing in GA and I'm now pursuing admission into a physics graduate program in another state. I find physics more interesting now.

What did you work on at Tech? And what aspect of physics are you interested in now?

Side note -- surely we have a question that explains refraction in water somewhere, right? This one can't be the first...

0

Q: Chopsticks in Water

When you put chopsticks (or any other object) in a glass cup of water, why do the chopsticks seem curved or bent? Is it because of the shape of the cup, the water, or the chopsticks itself? Note: I'm sorry I currently don't have an image of it. You can try this at home. Just put a pair of chopst...

reflection

David Hammen

4:27 AM

General question: Does anyone other than me find it ridiculously hard to log into chat? Logging into any of the SE networks is easy (my computer remembers). Once I've logged into any one, I'm generally automagically logged into every SE site in which I participate (SE remembers). Logging into chat? That is not easy. Sometimes the magic works, sometimes it doesn't.

user54412

@dmckee I'm apparently too tired to do it now, but I was trying to reproduce an argument that derives the radius of Jupiter from hydrostatic equilibrium (equivalently the virial theorem), the ideal gas law, and the Schroedinger equation (without any actual quantum mechanics). All you need is G, hbar, m_p, and k_B, no Jupiter mass.

@DavidHammen I usually don't have problems. I seem to have the opposite issue -- I can never seem to leave any chatrooms I join. I see my name in rooms for far longer than I have the tab open

@ChrisWhite $R_j \propto f(G,\bar{h},m_p, k_B)$ You can thank me for doing your math work later ;)

I am completely side tracked by looking up awesome videos and images from the GOES satellites now

8:41 AM

I made it through the exam period! :D

@ChrisWhite There are courses on that in the US? Great

JamalS

9:06 AM

@Danu: How were they? Did you make up for the bad oral?

9:56 AM

Yeah, pretty much

I barely passed diffgeo but that's okay since I was convinced I was going to fail. Pretty sure I'm near the top of the class on my QED/GR exams.

JamalS

How come you didn't do well in diffgeo?

very hard course

plus I came in with 0 math background (topology & strong linear algebra are.... recommended to say the least)

user54412

@Danu Good. You're still a real physicist, not a mathematician ;)