The h Bar

Aaron Stevens

03:52

republicworld.com/technology-news/science/…

I keep getting notified of articles along the lines of "Newton has failed" or "New experiment disproves Newton".

Like it's breaking news that Newtonian gravity doesn't explain everything? Haha

John Rennie

04:10

@AaronStevens I've seen a few of those too. \All from non scientific sources like mainstream newspapers.

They seem to have been started by the recent observations of orbits around Sag A*.

Aaron Stevens

@JohnRennie Yeah, I have probably trained Google to think I like to read about science from non-scientific sources. I just want to see how wrong things are sometimes

Ryan Unger

john is it a nice temp in the morning

John Rennie

@RyanUnger It's been raining for two days now!

Ryan Unger

wow Britain's weather system is back online

John Rennie

It's about 20°C so the temperature is just right

@RyanUnger Yes :-)

Captain Bohemian

05:25

@JohnRennie 20°C sounds like our winter and is too cold. The most comfortable air temperature is like 26 C.

John Rennie

@CaptainBohemian I suspect what people find comfortable is just what they're used to. I'm happiest when the temperature is in the range 15°C to 25°C. Above that and I find doing anything energetic makes me very sweaty. Below that it get's a bit cold if I'm not doing anything physical.

Captain Bohemian

If you have to take cold shower because there is no warm shower facility, it's better to have the air temperature be at 30 C or above.

Slereah

06:13

Reading about branched manifolds isn't easy because most papers on the topic are about dynamical systems

and I don't know much about that

PM 2Ring

06:54

Allegedly, the Sagnac effect can’t be explained by SR. You need to use a Fluid Aether theory. physics.stackexchange.com/a/493973/123208 :D

Abcd

@JohnRennie Newton's gravity is wrong?

John Rennie

@Abcd only in the sense that it's an approximation that breaks down when we consider relativistic effects.

Abcd

@JohnRennie hmm news people are making a big deal out of it

John Rennie

in the last few days a paper has been published reporting observations of stars orbiting close to the black hole at the centre of our galaxy, and those orbits show relativistic effects that cause them to differ from the predictions made by Newton's law.

They are cool observations, but not anything we didn't expect.

Captain Bohemian

07:21

I wonder which fields of physics that complex analysis finds applications.

Novalium Company

I'm experimenting with rubbing a balloon on a silk (to get the electrons) and then touching the balloon to two small aluminium foil balls to see if they'll repel but they don't (if I do it with aluminium foil sheets, they stick to the balloon for some reason, and again, they don't repel). What's happening here?

My final goal is to charge titanium dioxide powder by rubbing a charged balloon in the powder. (I don't know if that will work though) Is that cheapest way to go about doing this?

John Rennie

I suspect you'll struggle to build up any sizeable static charge on any conducting object.

Static leaks off conducting objects really easily, and it normally takes special precautions to stop it leaing away.

Novalium Company

07:43

I know I'll struggle. What are the special precautions?

Does the charge leak off to the air or what?

What if I put the titanium dioxide over and insulator (wood) and then charge them with the balloon by touching it?

John Rennie

When you say you're trying to charge the dust, what exactly are you trying to achieve? A charged dust would form a kind of vapour i.e. the charged dust particles would repel each other so they separate and form a weird kind of fluid.

Novalium Company

Well, I'm trying to make a bigger e-paper capsule. I'll submurge the titanium dioxide in black dyed water and put that substance in a capsule. I'll use an electric field to attract/repel the titanium dioxide.

John Rennie

If you're putting the powder in water any charge you put on it will disappear the moment it touches the water. However it is possible to charge particles in water using chemistry rather thsn physics.

Particles in water normally develop a phenomenon called an electric double layer.

That happens because ions either adsorb or desorb from the surface to leave a net charge.

Double layer (surface science)

A double layer (DL, also called an electrical double layer, EDL) is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object. The first layer, the surface charge (either positive or negative), consists of ions adsorbed onto the object due to chemical interactions. The second layer is composed of ions attracted to the surface charge via the Coulomb force, electrically screening the first layer. This second layer...

You can control the surface charge by various means e.g. by adjusting the pH.

Novalium Company

Can I do this method safely and cheaply in home?

John Rennie

Yes, though measuring the charge normally requires specialist equipment. However if you make your solution alkaline you'll almost certainly get a negative charge on the particles in the water.

A quick Google found this info that looks useful

Novalium Company

07:58

Why does the charge leak off to the water when the water is not a conductor?

John Rennie

The charge is very small so even the tiniest current will quickly neutralise all the charge.

That means even if the resistance is very large, i.e. the current is very small, the charge will quickly be lost.

And water is a conductor. It isn't a good conductor but it does have a small conductance i.e. a large but not infinite resistance.

Novalium Company

So I should buy some alkaline powder, dissolve it in the water and then put the charged particles?

Slereah

math.leidenuniv.nl/scripties/HommelbergBach.pdf

John Rennie

It isn't clear exactly what you're attempting, but it sounds excessively ambitious to me. My guess is that to make anything even remotely resembling an eInk cell would require a proper colloid science lab and you haven't a chance of doing it at home.

Slereah

Yet more papers to indulge in my evilness

Novalium Company

08:10

@JohnRennie What exactly isn't clear? I'm trying to make a single bigger epaper capsule by putting black dyed water in the small container and putting charged titanium dioxide in it. I'll then close the capsule and use an electric field to move the charged titanium dioxide. I just don't know how to charge (and stay charged) the titanium dioxide. How do I do it?

You said there is a safe and cheap way to charge the TiO2 in home. What is it?

Novalium Company

08:28

@JohnRennie Will putting the titanium dioxide in the black dyed water and then putting alkaline (NaOH maybe?) in the final substance work? The alkaline should give charge to the TiO2?

Loong

@EmilioPisanty ISS

Slereah

09:00

Does integrating over function spaces also have some fundamental theorem of integration?

That is, $$\int_{f_1(x)}^{f_2(x)} \mathcal{D}f(x) \frac{\delta F[f(x)]}{\delta{f(x)}} = F[f_2(x)] - F[f_1(x)]$$

Or something similar

Emilio Pisanty

@Loong still looks like a TIE fighter to me

=P

Loong

:-)

Slereah

@EmilioPisanty <-o->

Although I guess that can't quite be the form of the theorem, since it's not the same functions

John Rennie

09:16

@NovaliumCompany disperse the powder in water at about pH 9 to 10. You need a sodium hydroxide concentration of $10^{-5}$ to $10^{-4}$ molar. Ideally you'd disperse the powder using an ultrasonic bath, but I'm guessing you don't have one so just use whatever mixer you have to hand.

That should give you a stable TiO2 dispersion with a negative charge.

Physics Meta

09:36

0

Q: Nice to be here, serial downvoters

So now getting serially downvoted. 5 questions downvoted in short order, are the answers I gave so bad? See

discussion

PM 2Ring

Here's a nice black hole question to answer, or hammer. The existing answer is sub-optimal... physics.stackexchange.com/questions/493974/… I looked for dupe targets, but I couldn't find a close match, just questions talking about BH collisions.

Novalium Company

10:04

@JohnRennie Thank you!

Novalium Company

10:31

@JohnRennie I hope you haven't given me a formula for a bomb, so I'll research a bit more ;P

ACuriousMind

@PM2Ring 1. Consider downvoting answers you disagree with. 2. Does the following argument seem reasonable to you? "Since an observer at infinity never sees objects crossing the event horizon, these objects cannot come back from beyond the horizon since there simply is no time for them to do so in this observer's frame. Hence, you cannot find anything that can "escape" the event horizon regardless of the details of the situation."

John Rennie

@NovaliumCompany damn, rumbled! :-)

Novalium Company

@JohnRennie I learned my lesson not to trust random internet guys so easily xD

"disperse the powder in water at about ph 9 to 10" you mean the TiO2 powder right? Also how do I get water ph 9 to 10 (I can google that nvm)

John Rennie

@PM2Ring it's a good question. It's asking about the back reaction of our test mass i.e. how the test mass perturbs the geometry. That's not a trivial question. As usual Duffield has comprehensively misunderstood the question and posted balderdash.

@NovaliumCompany $10^{-4}$ molar NaOH has a pH of about 10. NaOH is easily bought as it's used for cleaning drains.

Slereah

hm

Is the worldline formalism obtainable as a limit of string theory

Novalium Company

10:39

So I start with tap water in a cup. I put $10^{-4}$ molar NaOH in the water. I put black paint in the water (to make it black). I add the TiO2 and I mix it with a blender?

Slereah

IIRC it's infinite tension, no?

PM 2Ring

@ACuriousMind 1. I do that, in fact I use a lot of rep casting downvotes, but I also exercise caution when interacting with certain posters... 2. Yes, that's quite reasonable. Even those people who think that Schwarzschild coordinates are the only true coordinates shouldn't disagree with that argument.

skullpetrol

@JohnRennie "rumbled"? sounds like a local slang??

John Rennie

@skullpetrol yes it's UK slang though a bit old fashioned now. "I've been rumbled" means something like "My evil plans have been discovered".

skullpetrol

Thanks for explaining it :-)

John Rennie

10:43

@skullpetrol You would only use the expression in an ironic way these days.

skullpetrol

I see.

PM 2Ring

@JohnRennie I agree it deserves a good answer. I assume that the test mass can only make the curvature worse, but as the OP says, it'd be good to see some actual maths. JD's answer isn't unexpected, he can't help himself with questions like this.

John Rennie

@PM2Ring As far as I know the only answer is that it can't be done except by a numerical calculation. But there might be some tricks that can be used if the perturbation can be assumed very small.

Slereah

Yeah there's no "good" two body solutions in GR

Novalium Company

@JohnRennie I've been going around the whole internet (SE forums and more) searching for a way to charge the TiO2 particles and you are the first on who actually tells me how, thanks! Now I'll research a bit more, see if there are any health risks and dangers and how to actually do it. Thanks!

PM 2Ring

10:49

@JohnRennie And if the perturbation is large, then we're getting into the territory of 2 black holes colliding, for which there are some existing answers.

John Rennie

@NovaliumCompany to be fair if you'd said up front you wanted it charged when in suspension in water the answer would have been a lot simpler.

The only health risks would be from breathing in the TiO2 dust. It isn't toxic, but very fine dusts get right down into the lungs and can cause damage there. NaOH is corrosive (that's why it cleans drains) but isn't toxic. Wear kitchen gloves and you'll be fine.

Loong

@JohnRennie most important risk is probably: NaOH + eyes -> quick blindness

since he plans to put NaOH in a blender

John Rennie

Good point. @NovaliumCompany if you're going to mess around with alkalis wear some form of eye protection. Alkalis are very good at destroying proteins and getting concentrated alkali in your eyes will cause major damage - as well as hurting like hell!

ACuriousMind

@PM2Ring Well, essentially the only way to really answer the question would be if we had a complete GR model of two-body mergers with (at least) one body being a black hole. Then one could show (or not) that regardless of the parameters the two bodies merge after one has crossed the event horizon of the other (though for heavier bodies it might become difficult to determine the location of the horizon during the merger)

PM 2Ring

@NovaliumCompany How much of this stuff do you intend to make? $10^{-4}$ molar NaOH means 4 mg + enough pure water to make the volume up to 1 litre.

John Rennie

11:03

@ACuriousMind for a small but not negligible test mass I feel there ought to be a way of analytically approximating the problem. Maybe some form of linearisation. However I've never seen a calculation like that.

ACuriousMind

@JohnRennie I think no "small" perturbation will yield any interesting effect

You'll probably get that the horizon "wobbles" a bit and nothing more

John Rennie

@ACuriousMind The LIGO team do the full numerical calculation all the time, but numerical calculations tend to leave you with no feel for what is going on.

ACuriousMind

Yeah, that's the problem - we can't run the numerics for all possible scenarios here

Novalium Company

My dad wants me to wear rubber gloves, breathing mask, ski glasses and long sleeve clothing, the window open (for air to circulate)

So I guess I'm secured enough :P

I don't know the stoichiometry of the experiment though

John Rennie

@ACuriousMind Anyway, I've posted an answer. Not a good answer, but I hope at least enough of an answer to reassure the OP that JD isn't representative of the site membership.

How can this not be a duplicate but I'm damned if I can find a duplicate.

Novalium Company

11:12

@PM2Ring I have no idea, about 100 ml. Can you tell me the stoichiometry of the experiment, because I don't know how much of what to put in that 100 ml cup.

John Rennie

@NovaliumCompany do you know what molarity of a solution means?

PM 2Ring

@JohnRennie The OP asked another shell theorem question an hour later, but it got hammered. physics.stackexchange.com/questions/494003/…

John Rennie

But, on to the more important issues. It is weeing down in Chester and I have no food in the house. Do I walk to the local shop, which doesn't have a wide range of food, or gird myself in goretex and cycle to the supermarket?

Novalium Company

@JohnRennie yep, you can calculate other stuff with it, grams and stuff... (I can rewatch a khan academy vid on it)

John Rennie

Well the molecular weight of NaOH is 40, so a one molar solution is 40g in one litre.

PM 2Ring

11:17

@NovaliumCompany Do you have good scales? Like I said above, 4 mg of NaOH plus water to bring the volume up to 1 litre. Add the alkali to almost enough water, then top it up. It's probably easier to make it (say) 10x stronger, then dilute it to the desired strength.

John Rennie

To make the 0.0001M solution you want you need 4mg in one litre.

Novalium Company

I think I'll master my stoichiometry terminology first and come back :P

John Rennie

I suggest you make a 1M solution because 40g is an easy weight to measure using kitchen equipment.

Then you can dilute your one molar solution by a factor of a hundred by putting ten cm^3 of it in a litre of water. Do this twice to get a dilution by a factor of 10,000, which will give you the 0.0001M solution you need.

PM 2Ring

FWIW, dissolving NaOH in water is rather exothermic, but I doubt you'll notice that, at such a low concentration.

John Rennie

But handle with care because 1M NaOH is very corrosive.

PM 2Ring

11:22

And that 1M solution will definitely get hot when you prepare it.

Novalium Company

@JohnRennie Alright, thanks everybody! I'll relearn my stoichiometry and come back, when I better understand what you are talking about. :D

John Rennie

@PM2Ring I've dissolved NaOH crystals in water lots of times and I recall no great release of heat ...

ACuriousMind

I wonder if Bulgaria has extradition agreements with Britain?

Novalium Company

@ACuriousMind Donno, all I know is that if brexit happens, I ain't going to british univerity easily xD (not that I want though)

ACuriousMind

Upcoming Daily Mail headline: "Former soap scientist instructs Bulgarian youth how to blow himself up over internet 'bar'"

John Rennie

11:23

> how to blow himself up

A tyre pump and some vaseline.

Novalium Company

xDDDDDD

PM 2Ring

@JohnRennie It does get noticeable warm, but nothing like mixing concentrated H2SO4 with water. Many washing powders contain NaOH, and get noticeably warm when dissolved in a small amount of water.

John Rennie

@NovaliumCompany I worked for twelve years as a colloid scientist, and what we're describing is colloid science so it's exactly the sort of thing that used to be my speciality :-)

Although I don't recall ever working with TiO2. My work was mostly with silica, alumina and polymer particles.

PM 2Ring

When I was a kid, I used to make hydrogen by reacting NaOH with aluminium. Of course, that reaction also gives off heat, but it's warm before you put the aluminium in.

Novalium Company

Alright guys, I'll return with stoichiometry learned, now I'll go, cya and thanks :----)

John Rennie

11:30

@PM2Ring Me too. I think it's terrible that the children of today aren't given the opportunities to corrode, poison and maim themselves that we enjoyed as kids :-)

PM 2Ring

I also tried the more traditional zinc plus hydrochloric acid, but that reaction's pretty slow. Mind you, my zinc was a bit oxidized, being recovered from old dry cells.

@JohnRennie :) It's still pretty easy to buy caustic soda. In Scandinavia, people use it to preserve fish, and one guy told me that it's used as a condiment on the table... presumably in liquid form. :)

Aaron Stevens

The only thing my in-law side of the family thinks I can do with my physics knowledge is make bombs. Since that is not what my work is focused on it essentially never comes into the conversation.

John Rennie

ACuriousMind

@PM2Ring There's plenty more foods that involve caustic soda, and you should get "food-grade" NaOH preparations pretty easily in most countries

John Rennie

Available from your local hardware store! :-)

Though I doubt it's very pure.

I think I'm going to brave the rain and go buy something for lunch. Better wet than hungry.

Pasta with stilton and cranberries yesterday:

PM 2Ring

11:43

I haven't tried to buy caustic soda for a few years, but it used to be common in any well-stocked supermarket or hardware store, here in Australia. These days, more fancy preparations are available for cleaning greasy drains, that contain extra ingredients, but I expect that good old NaOH is still popular.

Loong

12:14

ACuriousMind

@Loong Jupp :)

skullpetrol

12:25

baking soda?

Loong

@skullpetrol No. Baking soda is sodium hydrogen carbonate.

skullpetrol

TIL, thanks.

PM 2Ring

Note that baking powder consists of baking soda plus a mildly acidic salt like calcium phosphate, and a buffer like cornstarch. See en.wikipedia.org/wiki/Baking_powder

skullpetrol

coolio

Novalium Company

12:57

@JohnRennie And I'm sitting here with my boring ham sandwich :p

PM 2Ring

This question has a nice click-bait title. :) physics.stackexchange.com/questions/493884/… It should attract attention on the HNQ, but probably not a lot of votes, since the answers are a bit anti-climactic.

Ryan Unger

@ACuriousMind so did you play DEMD or not

ACuriousMind

May 20 at 16:13, by ACuriousMind

@RyanUnger Not yet, it renders everything with weird-looking stripes on my machine and I couldn't figure out why

Ryan Unger

oh lol

how do you have such good chat memory

Sir Cumference

Out of the blue question, but is "birthday" a holiday

debating a friend on this

ACuriousMind

13:10

Depends on the person. E.g. Jesus' birthday is commonly a holiday :P

Sir Cumference

well just the idea of a birthday

i mean it's celebrated once a year

ACuriousMind

So?

Sir Cumference

what other requirements are there to be a holiday

Ryan Unger

lol what

ACuriousMind

@SirCumference By "holiday", I would understand a day that is a festive occasion for a large community, such as a country.

Sir Cumference

13:13

the idea of a birthday is adhered to by a large community

ACuriousMind

what

"Birthday" isn't a specfic day, though

Some persons' birthdays indeed are national holidays, most people's birthdays aren't..

Ryan Unger

@ACuriousMind some people's death days too

ACuriousMind

sure

PM 2Ring

@SirCumference Having a day off work?

Aaron Stevens

@PM2Ring Is there a better climax you are expecting?

PM 2Ring

13:27

IMHO, name days have stronger claims on being holidays than birthdays do.

ACuriousMind

@PM2Ring I find both the question and the answers terrible. You cannot really isolate the HUP from quantum mechanics - there is no uncertainty without the rest of QM, and there is no QM without uncertainty. So, arguably, every single phenomenon that is classically inexplicable is an example of the HUP.

PM 2Ring

@AaronStevens No. But I bet a lot of the visitors to that page hope to see stuff in daily life that does directly demonstrate the HUP.

@ACuriousMind Agreed. Do atoms exist? Yep. Therefore HUP. :)

ACuriousMind

That sun that shines on you through fusion of particles, with a fusion rate only explicable by tunneling? HUP. The semiconductors that power your electronics? HUP.

The problem is probably that the HUP is very visible in pop-sci and intros to QM because its statements are so un-classical, so people expect it to have some sort of deep impact

Ryan Unger

I never got an answer to my HUP problem

Sir Cumference

@PM2Ring People get that

Ryan Unger

13:32

Not a reasonable one anyway

Sir Cumference

sometimes

Also people say "happy birthday" like they say "happy [most other holidays]"

ACuriousMind

@RyanUnger which one?

@SirCumference So because people might say "happy last day of work", last days at work are holidays?

Ryan Unger

@ACuriousMind are there physical states outside of $Dom(\hat X)\cap Dom(\hat P)$

Sir Cumference

@ACuriousMind do people say that though

ACuriousMind

@RyanUnger Ah, that just starts silly discussions about what "physical states" are :P

Ryan Unger

13:35

I'm not saying it's a good question

PM 2Ring

@SirCumference Some do, some don't.

ACuriousMind

@SirCumference depends on the work :P

Ryan Unger

But it's apart of my larger philosophical question of why sloppy math doesn't doom quantum physicists

ACuriousMind

Because we've had a few decades to throw out all the sloppy math that didn't work ;)

Aaron Stevens

@ACuriousMind How is tunneling due to the HUP?

Ryan Unger

13:36

Yeah so what sloppy math was completely wrong?

I didn't get an answer to that question either

Everything that was "wrong" (when people were working on foundations) was wrong for a physical reason

ACuriousMind

@AaronStevens See my message before that where I say that I find it silly to try to isolate the HUP from QM. It doesn't really make sense to me to attribute some quantum effect to "the HUP" and others not.

Ryan Unger

@ACuriousMind Pair production happens because of HUP.

Aaron Stevens

@ACuriousMind Can all of QM be formulated from the general uncertainty principle? I think there are instances where the HUP isn't really at play. The HUP just describes a relationship between position and momentum measurements. Certainly that isn't always important

ACuriousMind

@AaronStevens No (it doesn't encode any information about time evolution), but you can't really have QM without it. I also object to reducing the uncertainty principle to position and momentum - it is really a general statement about how measurement and non-commuting observables work

Reducing it to position and momentum also leads to this giant red herring of using classical Fourier analysis to explain it, which one answer there already exhibits.

Aaron Stevens

@ACuriousMind I completely agree with all of that. I just don't think it makes the question invalid.

ACuriousMind

13:49

Well, I don't think it's invalid. It's just not very useful :P

Aaron Stevens

@ACuriousMind Fair enough haha. I feel like the HUP is one of the topics on this site where there are so many interpretations and understandings (true and false) of it. And then the pop-sci explanations of it gets in the way as well. It gets pretty frustrating. For example the car example in one of those answers. Or the "light disturbs particles" explanation.

ACuriousMind

Yes, that's very true

Aaron Stevens

Maybe it's one of those cases where it's better to explain what the HUP is not rather than what it is haha

PM 2Ring

We can blame Werner Heisenberg himself for the "light disturbs particles" thing.

ACuriousMind

ugh, yes, Heisenberg's microscope is one of these things from early quantum theory that we should better forget about

Ryan Unger

14:02

I’m pretty sure Shankar mentions this

Don’t remember if he says it’s legit

Aaron Stevens

@RyanUnger I don't ever remember seeing in in there. But it's been a couple of years since I've cracked that open myself

@PM2Ring it's the thing most YouTube videos latch onto I feel like

Ryan Unger

page 140 in Shankar

he estimates $\Delta X\Delta P$ by bouncing a photon off of an electron

surprise it's about $\hbar $

is that what Heisenberg's microscope is about

Aaron Stevens

@RyanUnger Ah yep you're right. Interesting. Yeah that's the idea behind it.

Ryan Unger

now how I remembered that but not the conversation we had a month ago about Deus Ex @ACuriousMind is truly amazing

Aaron Stevens

The video game? Haha

ACuriousMind

14:15

@RyanUnger I was just about to remark that your book memory seems to equal my chat memory ;)

PM 2Ring

To be fair, Heisenberg was a strong adherent of Logical Positivism, so it's nor surprising that such an operationalist approach to the HUP appealed to him. But I must admit I was a little surprised when I read it in a popular science book he wrote (which I read in English translation).

Aaron Stevens

My crazy QM professor just liked to talk about how Heisenberg was a Nazi

ACuriousMind

Which, unlike with some other German scientists of the time, is actually debatable in his case

JMac

@ACuriousMind So it's uncertain?

Aaron Stevens

@JMac Nailed it

ACuriousMind

14:23

Heh

PM 2Ring

Heisenberg managed to avoid building an atomic bomb without acting in a way that would have got him kicked off the project.

Aaron Stevens

14:48

Perhaps the confusion in the uncertainty principle is it's name? If you think about it, it's really describing a relationship between the standard deviation of measurements of non-commuting operators. It's not giving us the uncertainty of our measurements around a "true" value of that measurement.

ACuriousMind

Yes, uncertainty is certainly not the best name for it

John Rennie

General relativity has a *certainty principle*.
The less people know about it the more certain they are that they understand it.
This is readily proven by experiment.

Loong

I guess that principle is applicable to other disciplines, too.

John Rennie

GR seems to suffer more than most. I suspect it's because QM is sufficiently impenetrable to keep out all but the determined nuts.

Aaron Stevens

15:05

Hey, how hard can it be to learn about something that is "general"?

ACuriousMind

@JohnRennie Well, in QM, you instead get a lot of people insisting that it must be wrong because it doesn't play nice with their intuitions

John Rennie

@ACuriousMind true :-)

Though even the most sober of colloid scientists might sometimes find themselves wondering if we have found the best way to formulate QM.

Of course those sober minded individuals might also find themselves wondering if GR is actually an emergent description with a deeper underlying reality.

bolbteppa

15:21

@Slereah in Greiner's Field book they take $\delta F = \int ...$ as the defining relation of a functional derivative

Ryan Unger

@ACuriousMind ha-ha

@ACuriousMind so I started DEMD and it even messed with my hardware

I had to enable DX12 which normally makes things run poorly

But in this instance it fixed all the stuttering issues

Slereah

15:37

@JohnRennie did you formulate the toothpaste formalism of QM

where reality is made of teeth

ACuriousMind

@Slereah that's only true in my nightmares

Slereah

exchanging colgatons

John Rennie

@Slereah well toothpaste is a non-Newtonian fluid and vzn tells me so is spacetime. So there you go: toothpaste = spacetime.

Slereah

Could be

John Rennie

Cosmic strings are the universe's way of flossing

Slereah

15:52

Let's just giv eout GR licenses

If you can solve the Schwarzschild metric by hand, you get to talk about GR

you know the whole cosmic string business is such a scam

Like if you read pop science, you come away with the idea that cosmic strings are one dimensional objects

When they are not at all

ACuriousMind

???

John Rennie

@Slereah the topological defect strings, or the idea that string theory strings could have been stretched out by inflation?

ACuriousMind

I'm pretty sure both are one-dimensional :P

Aaron Stevens

16:10

@Slereah Make a post on meta. If you want to answer a question with a certain tag you have to show proof of sufficient knowledge of that tag :P

Slereah

Topological defects are not n dimensional

Very thin

But still

I mean I guess you can define an interface that is, but you can do that with everything

ACuriousMind

@Slereah Hm? The topological defect is usually signified by a singularity in some parameter, which is very much localized. E.g. for cosmic strings you get a line of infinite energy density, no?

Slereah

No

I mean you have the idealized topological defect

But the real defects are just very abrupt changes in fields

ACuriousMind

Ah, well, sure. Seems a bit arbitrary to me to demand non-idealization in this case, though

There's a lot of statements we can make both in pop-sci and real physics that depend on some idealization of the underlying situation

Slereah

Well in that case why not say that an idealized sheet of paper is two dimensional

ACuriousMind

16:15

@Slereah It sure is!

And you may well model it as such in some situations

John Rennie

@ACuriousMind that's the way they are modelled in GR because it gives a nice simple solution - a conical defect. But assuming the string is a misalignment of some QFT vacuum structure it's entirely possible the vacuum structure changes smoothly over a non-zero distance.

Slereah

But in pop sci there's the general feeling that it's actually 1D

Domain wall solutions are basically just ~tanh

Not quite the perfect surface

Emilio Pisanty

@JohnRennie how is this not a duplicate?

(I'm genuinely curious)

John Rennie

@EmilioPisanty It was close but it seemed to me the questions are different.

Emilio Pisanty

@JohnRennie how so?

John Rennie

16:27

The later question asks why H&Z carefully define the wave as the real part of the complex exponential then casually drop the $Re$ operator in their subsequent working.

That's a different question to the more general question about the physical significance of the approach.

Emilio Pisanty

@JohnRennie It's still answered in the linked thread, but OK, I see your point.

John Rennie

@EmilioPisanty I agree that if you read, learn and inwardly digest the other thread you will understand the later question as well.

That's why I say it was close, but in the end I flipped a coin and it came up as no duplicate

Emilio Pisanty

@nitsua60 Thanks for your thoughtful & context-aware intervention =).

bolbteppa

16:48

Where do we get these GR licenses

Loong

We posted the order details on meta but the Charcoal guys deleted the post as spam.

Slereah

@bolbteppa the department of general relativity

John Rennie

I'm just one vote from a rep max on the science fiction stack exchange!

(I post on the science fiction SE when I'm bored with posting fictional science here :-)

This is the paper responsible for all those Newtonian gravity is dead articles in the popular media: Relativistic redshift of the star S0-2 orbiting the Galactic center supermassive black hole.

It's good that people are interested enough to make the popular media take an interest, but it would also be nice if they pointed out this is really no different to the inconsistencies in Mercury's orbit first measured well over a century ago. As news goes it isn't all that new.

Loong

17:15

aha

John Rennie

Available on the Arxiv here

ACuriousMind

17:30

@JohnRennie It's nice of them to notice the death of Newtonian gravity 100 years after its passing :P

Slereah

I hope the phlogiston will be alright

Loong

@Slereah I guess, phlogiston is responsible for this: physics.stackexchange.com/q/492709/59991

ACuriousMind

@Slereah No, it was phlogged to death

Slereah

I think the phlogiston gets a bad wrap

It was an entirely fine idea

Loong

blame Lavoisier

Slereah

17:41

@Loong I often do

@ACuriousMind Is there a rigorous proof of the string action reducing to the point particle in some limit

Barbashov has one but it's fairly handwavy

ACuriousMind

@Slereah How could it? It's an action on a 2d worldsheet, no limit can reduce it to an action on a world-line!

Slereah

@ACuriousMind Well Barbashov seems to differ!

Loong

@EmilioPisanty see also xkcd 1830

Slereah

Take the action $$S = \int d\tau d\sigma \sqrt{\dot{X}^2 X'^2 - (\dot{X} \cdot X')^2 }$$

ACuriousMind

I also don't understand what there is to reduce - the NG action for both string and particle (and any other volume) is just $\int_\Sigma \mathrm{d}s$, they only differ by what $\Sigma$ is

Slereah

17:53

Factor it as $$S = \int d\tau \sqrt{\dot{X}^2} \int_{\sigma_1(\tau)}^{\sigma_2(\tau)} d\sigma \sqrt{\frac{(\dot{X} \cdot X')^2}{\dot{X}^2} - X'^2} $$

Call that integral $L(\tau)$, so that $$S = \int d\tau \sqrt{\dot{X}^2} L(\tau)$$

Then the handwaving is that the tension of the string is $\approx m_0 c / l_0$, for $l_0$ the length

So that $$S = m_0 c \int \sqrt{\dot{X}^2} \frac{L(\tau)}{l_0}$$

ACuriousMind

It also seems completely pointless to me to desire such a reduction, since the classical string action really isn't all that important for string theory, except as a starting point for the quantum theory

Slereah

And then something something $$\lim_{\sigma_2 \to \sigma_1} \frac{L(\tau)}{l_0} < \infty$$

@ACuriousMind Well I am also curious about reducing string theory to worldline formalism

Via some retract of the worldsheet or something I guess?

ACuriousMind

No, that won't work

Slereah

why not

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