The h Bar

2:04 AM

Last night dream:

talked of a space science project called Project Firefly. In the schematics that is being projected to the audience, is a huge blip like ship which is covered in some material that allow it to withstood the heat from the corona, chromosphere and the photosphere for 10 minutes. Scheduled launch date in the year 4400, the Firefly Ship will travel towards the sun, dip momentarily like a whale into the photosphere to collect a sample there and return earth for analysis.
The schematics also shown all the science machinery are at the tail of the ship, and some confinement unit is being used to co…

0celo7

3:37 AM

@Slereah Yvonne apparently has a book on supermanifolds

0celo7

4:11 AM

@JohnRennie yo

do you have time to read a physics presentation

John Rennie

@0celo7 Read yes, understand ... maybe

0celo7

@JohnRennie writing at the speed of heat

will send soon

s. patroller

4:42 AM

speed of heat

0celo7

4:56 AM

@JohnRennie dropbox.com/s/twrggcmvc3luzpy/Fluids%202%20talk.pdf?dl=0

@EricSilva some physics for you

@Slereah you might be interested as well

John Rennie

Reading now ...

> Special relativity is just GR with the metric

Whenever I say that you tell me off :-)

> GR is "locally Minkowskian" in the sense that any metric can be
brought into the Minkowski form at a point in certain coordinates
(Einstein equivalence principle). This corresponds to being in an
intertial frame.

Whenever I say that you tell me off even more :-)

0celo7

no

you say "locally flat"

I am not saying that

and I clarify in which sense I mean that

John Rennie

Page 33 - minor typo:

> So for M large enough, there is no equilibium configuration. Is there gravitational collapse to a black hole?

0celo7

thanks

John Rennie

You can't say "(duh)" in a presentation :-)

rob

5:07 AM

@JohnRennie the hell you say

0celo7

fineeee

Semiclassical

Just say "it follows trivially" :>

John Rennie

@rob :-)

0celo7

@Semiclassical see the link above

Semiclassical

Hmm. I'm not sure what it says to me that the first thing that comes to mind when someone asks me to say why SR and fluid mech are going to have problems, what first springs to mind is the Doppler effect.

John Rennie

5:11 AM

Page 7 another minor typo:

0celo7

@Semiclassical basically all of stat mech breaks down

John Rennie

> For an isotropic fluid, we have a good description in SR because there are no dissiptative terms

Semiclassical

Hmm.

0celo7

That's of course an oversimplification

I haven't thought about this in great detail, but I know some people have

Semiclassical

I just had in mind that the Doppler effect for sound makes explicit reference to the speed of the medium whereas SR (necessarily) doesn't

@0celo7 I haven't thought about such things at all, but it sounds consistent with something I remember reading so I'll buy it

0celo7

5:14 AM

apparently there's no Lorentz invariant theory of brownian motion

Semiclassical

Hmm.

Well, the reason that sounds right is that I remember one of the problems with trying make dBB compatible with Lorentz invariance is that the whole notion of 'quantum equilibrium', which it needs in order to be compatible with the Born rule, runs into some serious issues

vzn

@0celo7 nice... have you ever thought of/ considered a parallel between speed of sound in a gas and speed of light in the spacetime fabric? apparently nobody else (but me) has... you might be this )( close to working it out in detail... or maybe not... if not you, maybe someone else... o_O

John Rennie

Page 42: minor typo:

> Distributional matter models (Eintein-Vlasov)

Semiclassical

the problem with that analogy is that, if you look close enough at a gas, you can see the motions of individual particles

There's no similar microscopic story for GR.

vzn

@Semiclassical (eg in perfect gases...) particles tend to average out in the limit so to speak (ie consider the behavior as particle sizes become infinitesimal) and the gas can be regarded as nearly a continuum in the limit. in the limit, its like a continuous deformable substance. aka spacetime fabric

0celo7

5:19 AM

@Semiclassical yeah

@JohnRennie understandability on a scale from 1 to 10?

Semiclassical

Only as an approximation. It may be a very good approximation in most contexts, but that doesn't mean it's satisfactory in all contexts

0celo7

@vzn there's no problem with perfect fluids in GR

Semiclassical

I don't doubt that the math is similar, mind.

0celo7

for a certain value of "no problem"

John Rennie

@0celo7 I thought it was great. Is it aimed at reasonably experienced mathematicians?

0celo7

5:21 AM

@JohnRennie engineers and physicists

Semiclassical

But that math is at level of the the macroscopic description, not the microscopics.

vzn

@0celo7 like your continuous/ topological/ pure math intuition here... think you should run with it further... push it to the limit™ (already your philosophy?) :)

John Rennie

I would have guess some of it will be too hard for engineers

0celo7

and applied mathematicians

well it sucks to be them!

John Rennie

@0celo7 :-)

Life's like that sometimes :-)

Semiclassical

5:22 AM

lol

Within the dBB story, the way they try to get out from under this issue is to assume the existence of a preferred foliation...which technically works, but it seems like the cure is worse than the disease

vzn

@Semiclassical think its definitely worth working out & writing up and not just a mere exercise in a toy model. but its hard convincing ppl around here of anything. :( btw think this is a natural way to introduce the concept of lorentz frames etc... all from classical mechanics. and missed by the pioneers. huge oversight there! o_O

0celo7

@JohnRennie Thanks. I'm gonna throw in a derivation of the Lorentz factor at the beginning for the heck of it

it's going to be the most intense 75 minutes of these poor people's lives

Semiclassical

@vzn I think you run the risk of re-doing Lorentz's own work.

vzn

@Semiclassical there is no harm in it. do people not do exercises in books because one is re-doing the work of others? anyway its a very substantial conceptual bridge. possibly pointed out nowhere in the literature. think thats a gaping hole there.

Semiclassical

My point is more that, if one only proceeds as Lorentz did, one runs the risk of missing the reasons why people chose not to go down that route

vzn

5:28 AM

@Semiclassical ppl didnt go down that route because they didnt see it. if its not published, it doesnt exist. the tree fell in the forest and nobody heard it...

Semiclassical

Dude.

Lorentz

vzn

@Semiclassical no ref to ideal gases/ sound speed in his work... prove me wrong... also no ref by anyone else either... o_O

0celo7

night ppl

Semiclassical

Well, to be clear, I really don't know what you're proposing

vzn

lol whatever

10 mins ago, by Semiclassical

I don't doubt that the math is similar, mind.

Semiclassical

5:32 AM

The mathematical analogy is between waves as material oscillations and light as a wave phenomena

Both of them satisfy an appropriate version of the wave equation.

The difference is that with sound waves, you could imagine looking very closely at the air molecules involved and actually see their collective motions

In that scenario, there's both a macroscopic and a microscopic description.

With light, the macroscopic description is classical E&M if you're not worrying about spacetime curvature

For a microscopic description, the suitable analogue would be QED. But once you're doing quantum stuff then the whole notion of well-defined particle trajectories is problematic.

The very short version of this is that we have a physically relevant microscopic description of light waves. That's what QED is. You could presumably write down classical microscopic models that would reproduce classical E&M waves; there might even be some pedagogical utility to that. But one shouldn't expect that classical model to be the real story.

G. Bergeron

6:49 AM

@0celo7 @Slereah Not always. If in Sets, than yeah..

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