The h Bar

Dave

03:36

Has anyone else seen Sabine Hossenfelder's physics videos on youtube? Is she a crank? Is she an active researcher? They're showing up in my youtube feed, but I can't tell how much creedence to give her.

John Rennie

04:34

@Dave I think she has left physics now, but she was a respected physicist work in the general area of quantum gravity.

Just recently she has attracted a certain amount of notoriety for her comments on a possible future new collider, but as is so often the case the situation has become rather over dramatised.

Slereah

07:16

morning

Scientist Smith YT

@EmilioPisanty Well then there should be one like that. Because that would change a lot if no one thought of that frame of reference before. It would almost be dumb to not think of that frame of reference if you ask me.

@EmilioPisanty But lets say that doesn't exist for a moment. How would we explain different point of references from different objects? In this case the light is the object.

Slereah

0

Q: Notion of Present

Can't I sync all watches in spacetime and call this time slice the present? In Carlo Rovelli's book he tried to explain that the notion of the present is local only, which I could not follow.

I want to answer the question but the more I write the less sure I am that GR makes sense :V

Bloody synchronization

Gonna have to revise my original ideas for synchronization because it turns out the horismos isn't that trustworthy

Emilio Pisanty

07:34

@ScientistSmithYT light is not an object

@ScientistSmithYT "physics would be simpler if it was simpler"? Yes. But wishing that nature were different won't get you anywhere.

Slereah

@EmilioPisanty I motion that we redesign the universe so it looks like this

One dimensional universe I can deal with

Emilio Pisanty

The relativistic formulas that relate different frames of reference (the Lorentz transformations) become singular when you try to have one of the frames move at the speed of light with respect to an existing inertial frame

It's described in extreme detail in Wikipedia, every textbook on special relativity, and a myriad questions on this site, so I'm not going to - this is the point where it shifts over to being your job to look it up

Slereah

Hm

Emilio Pisanty

@Slereah I'm pretty skeptical of that sketch. "my" view of lineland? The square's?

Slereah

Yes

Emilio Pisanty

07:43

So it's not really a sketch, then, is it?

Slereah

Don't question the line

Emilio Pisanty

It's more of a walk-through model

@Slereah I'm not questioning the line, I'm questioning the plane

This is what the square looking at lineland would look like to the sphere

From outside the plane

Slereah

I think I need to prove $$p_1 , p_2 \in \gamma \wedge p_1 \ll p_2 \wedge p_1 \in N^-(q) \wedge p_2 \in N^+(q) \to \exists p_1', p_2', p_1 \ll p_1' \wedge p_2' \ll p_2 \wedge p_1' \in H^-(q) \wedge p_2' \in H^+(q) \wedge t(p_1, p_2) \geq t(p_1', p_2')$$

Easy peasy

Emilio Pisanty

But I'm skeptical that the square himself would reconstruct that model

=P

Slereah

It would help

in other words I need to show that while a null geodesic may not be on the horismos, but there is such a null geodesic which can be differentiated experimentally

And hopefully differentiated in finite time

I think that's reasonable, but I never trust a reasonable theorem

Also this would work fine for the ideal case where the observer just blasts the spacetime all the time with light rays and massive particles in every direction

But I'm not sure that will help for the realistic case with just sporadic sprays of photons

I mean obviously that theorem is wrong, but it may be correct in a globally hyperbolic spacetime

Yeah I think that is correct

$q \neq \gamma$ so since $\gamma$ has points in both $J^+(q)$ and $J^-(q)$, which are disconnected if $q$ is removed, it intersects the horismos in such a way

Scientist Smith YT

08:07

@EmilioPisanty Well do we classify it as a person, place, thing or idea?

@EmilioPisanty But I'm probably going to move on from that topic and wait a while before I ask my next question.

@Slereah I've been there and done that. :) That's usually when I just stop... Fall asleep ... Then continue where I left off. I end up figuring it out soon after.

@Slereah Didn't work, I was trying to quote a text in a message you sent, but I still have yet to figure that out.

@PM2Ring I just saw your message in my inbox. And the link to the SE Q&A. I'll read that tomorrow. Well... (Its already the next day here so today, but after I get some sleep) thanks :)

Emilio Pisanty

08:34

@ScientistSmithYT I wish you good luck in understanding physics under a worldview that allows only for those five categories.

PM 2Ring

@Dave Yes, according to Wikipedia, Sabine is still active in physics: "she is a Research Fellow at the Frankfurt Institute for Advanced Studieswhere she leads the Analog Systems for Gravity Duals group". She's been turning up in my YouTube feed too, but I haven't watched any of her clips yet, although I have read a few of her blog posts.

Sabine has ruffled a few feathers in the last year or so, for suggesting that there are better things to do with physics research money than building a bigger accelerator than the LHC. Also, her book Lost in Math: How Beauty Leads Physics Astray upset a few people. ;)

@EmilioPisanty Congratulations on getting the Optics gold badge!

Slereah

08:51

Alrighty

I think I have a Proof

Phew

I think if you consider the shortest light trip between an observer and a point, you obtain the horismos trip

NOW

Can you perform a global synchronization from such data???

Dun dun dun

PM 2Ring

@Slereah Ben's answer looks ok to me. On a related note, see physics.stackexchange.com/q/503978/123208

Slereah

It is fine

But synchronization in GR is such an awful problem

I mean it's entirely fine if you assume a normal neighbourhood

But otherwise

Not so good

I mean really, what I'm basically doing is just GR trigonometry

Every light synchronization is just a triangle with one timelike side and two null sides

I know there's some papers on SR trigonometry but I'm not sure that is the case for GR

PM 2Ring

Triangulating the universe is hard. It's hard enough just triangulating the Earth's surface, but surveyors generally don't have to worry about the curvature being dynamic.

Slereah

Well I'm assuming some fairly generous hypothesis

1) global hyperbolicity

2) every point of spacetime has an observer going through it

3) every observer just shoots light beams and massive particles in every directions constantly

4) those beams contain every possible information from the observer

Basically this

Also if things get too tough I may also assume trivial topology because heck

But the one thing I will not assume is LOCAL NEIGHBOURHOOD

Because that is easy mode and also it's been done already

PM 2Ring

09:18

There's a good reason that it's a GR slogan that simultaneity is local. Easy mode is (relatively) easy, hard mode is impossible. ;)

Slereah

Well global simultaneity is fine in GR if you assume global hyperbolicity

BUT

the hard part is getting it exclusively from measurements

Also I think "A spacetime constantly filled at all time with EM radiation from infinitely many sources containing infinitely much encoded data" probably stretches the definition of "test field" a bit

But I'll allow it

I mean assuming a foliation, it's trivial to find a simultaneity scheme, but then that's assuming you know the foliation in advance

PM 2Ring

But even if you succeed in building your synchronized network purely from measurements, it's still of limited value: over sufficiently large distances you still have the Andromeda paradox.

Slereah

Since it's basically $$t = t_1 + \varepsilon(x) (t_2 - t_1)$$

@PM2Ring Well I do make for some fairly generous hypothesis

The next step ideally would be "What subset of the metric phase space is given from a finite set of measurements?"

And perhaps even "How much deviation can you get from predictions given a set of measurements?"

Emilio Pisanty

@PM2Ring thanks =)

It's been a long time coming

It's nice to have gold tag badge that @JohnRennie doesn't have

Slereah

ie if you guess from the set of measurements that the initial data is $g_3(0, x)$, $\alpha(0, x)$ and $\beta(0, x)$, up to some uncertainty due to the measurement

Emilio Pisanty

09:25

It's a super exclusive club, that

Slereah

How much uncertainty do you get at $t + T$

I have no idea if that can even be done, really

PM 2Ring

@EmilioPisanty :) Physics.SE needs all the hammer-wielders it can get.

Slereah

Even assuming very generous conditions, I think the subset of $\Gamma(SM)$ (space of all metrics) that fit a measurement is pretty large

Emilio Pisanty

@PM2Ring honestly, there's not that much movement on that tag. There'll be cases where the hammer is useful, but not all that many

Slereah

You could add arbitrarily much very high frequency variations and you'd still end up with the same measurements up to uncertainty

Although I think you can cut that down a bit with some restrictions on the total variation of the metric

PM 2Ring

09:33

@EmilioPisanty True, and many optics questions (apart from blatant homework) seem to be of reasonable quality, so they're less likely to be closed for other reasons.

Swapnil Das

Can anyone help me out?

Slereah

Optics isn't as mysterious to the layman

Nobody wonders about the mystic mysteries of the lens

PM 2Ring

@Slereah And let's not forget about uncertainty due to HUP... or maybe we shouldn't worry about that until we get a quantum gravity theory. :)

Slereah

Well

I guess there was that

what-if.xkcd.com/145

@PM2Ring This doesn't matter all that much rly

Because it's not fundamentally different from instrument uncertainty

Swapnil Das

Slereah

09:36

Also like

I haven't worked out everything, but just using a clock or rod in GR is fundamentally uncertain

Because the metric itself influences the mechanism

The metric is going to change the frequency of atomic clocks or the interatomic length of a rod

@SwapnilDas First hint : take a bigger picture because some of us are old people

Swapnil Das

@Slereah You have the zoom button on the right top corner :P

PM 2Ring

@Slereah True, but with HUP things get wobbly on a large enough scale, even in a flat Minkowski spacetime.

Slereah

@SwapnilDas Zooming an image doesn't increase the resolution!

@PM2Ring Sure, but the point is that I don't assume anything about the metric

Well

That's a lie

I assume that the variations of the metric are small enough that the uncertainty of our measuring apparatus is bounded to some known quantity

I don't think you can actually do GR with zero assumptions about the metric

@EmilioPisanty please explain

Swapnil Das

@Slereah Oh, I thought the words were clear.

Slereah

it is very blurry

Swapnil Das

09:44

Yup ok I'm trying to magnify and repost,

It's a problem on gyroscopic motion.

PM 2Ring

@SwapnilDas It's a bit blurry on my phone, but it's readable. Unfortunately, I don't know how to solve your problem. My knowledge of gyroscopic stuff isn't as good as I would like

Swapnil Das

@PM2Ring Nvm thanks.

@Slereah Any improvment? :P

PM 2Ring

Sadly, the xkcd forums are still offline. It's been a couple of weeks now. :( forums.xkcd.com/index.php

@SwapnilDas Much better!

Swapnil Das

Oh.

Slereah

Know what's cool tho

You can make ultrastatic spacetimes into optical systems

Since it's basically the same Lagrangian

The metric just corresponds to some anisotropic medium

Though I'm not sure you can simulate every metrics with anisotropic media

PM 2Ring

10:03

Yeah. The first time I heard about that sort of thing was an optical version of the Poincaré disc representation of the hyperbolic plane.

Slereah

10:44

Let's build a glass hyperbolic plane

Slereah

11:07

Hm

I think the global time function is just gonna be constructed from local data

Just find the synchronization in an infinitesimal neighbourhood

The foliation is the surface everywhere tangent to the distribution defined by these local synchronization

That might be the Marzke-Wheeler coordinates, rly, but who can tell since that paper is so vague

Slereah

11:34

"Our academic system is descended from Akademos, a pagan grove near Athens, sacred to the goddess of wisdom. Members claimed ownership of all knowledge, by which they meant stuff they made up by their own powers of reasoning. Practical knowledge was left for tradesmen and servants.
So when you talk about “hard science” you are talking about the output from this bunch of pagan worshippers making up stuff in their own heads and calling it science, while insulting and rejecting people who work by observations and tests, as required by the scientific method. "

John Rennie

@EmilioPisanty :-)

tpg2114

I'm going to momentarily hijack chat for "personal" reasons -- if anybody knows US Citizens with an MS or PhD in Aerospace/Mechanical engineering or related fields (either graduated or soon-to-graduate) and experience with computational fluids, I am looking to hire people on my computational propulsion team. Pass the word, ping me here if needed and I can send out descriptions

3

Okay, back to regularly-scheduled programming

ACuriousMind

11:51

@Slereah that's not quite right - Akademos was a hero, the Grove was called academeia (sacred to Akademos) , and Plato happened to build his school there, which in time came to be called academeia, too

Swapnil Das

@JohnRennie Could you help me with the above question?

Slereah