The h Bar

Secret

00:00

Last night dream involves a time machine, where food is sent 5 minutes back into the past and then catch up with their present again via a relativistic boost, resulting in duplicates.

It may be possible that the simplest of the geodesic CTCs will not result in duplicates of an item to be visible all at once except at the edges of the loop (in a similar fashion as Tippett's warp drive time machine) as taking any space like slides, only a maximum of two instance of the object (going back to the past and going to the future) is going to fall onto it

But what if, one of the objects located in the past of another undergone a Lorentz boost. Can it potentially catch up with its present self and thus appearing in the same spacetime slide, and still remain self consistent...?

PranshuKhandal

02:16

Hello everyone!!

I am new, I don't know how to start, but can anyone tell me what does ♦ means, after name of users?

Am I allowed to chat here?

danielunderwood

Those are mods. They have blue names here in chat

PranshuKhandal

What do they do?

@danielunderwood do they have some special powers?

or it something else

dmckee

03:08

@PranshuKhandal Lots of special powers. Bhahahahahahaha! ::begins dry washing his hands::

Of course most of the time we have no cause to use them. Which is good. Very good. I don't want to level cities. Or even sulk in my tent. Really.

2

danielunderwood

Is sulking in a tent one of your many powers?

dmckee

Well, all the best, invulnerable, mythic heroes do it. Don't they?

danielunderwood

huh Google does seem to think that's the case

Clearly my culture knowledge is too low

dmckee

03:26

It was attempt to reference The Illiad.

danielunderwood

Though speaking of cultures, quark is apparently a type of cheese that companies are trying to make into a yogurt-like substance

PranshuKhandal

@dmckee you also have it, how do you get that?

danielunderwood

I need to read more so I actually get references

PranshuKhandal

is it based on reputation?

danielunderwood

At least one way is to be elected. Though I don't really know when a site decides to have an election or what happens with a new site

dmckee

03:31

@PranshuKhandal You get elected as moderator (or appointed as pro tem moderator) on any site (beta site) on the network. Or you work for Stack Exchange in the right role. All site moderators are also chat moderators.

PranshuKhandal

thank you friends

dmckee

There is no direct dependence on reputation, but most moderators have a non-trivial amount of rep for their site: it's one of the ways of getting well enough know to get elected.

danielunderwood

Are you guys moderators across all chat rooms or just the ones associated with the site you're a mod on?

dmckee

@danielunderwood All of chat. a fact that causes some friction from time to time.

PranshuKhandal

can you see the internal working of the site also?

like how it is made or something else like that

dmckee

03:35

We don't have access to the raw code, or the raw data about user votes and the like.

We do have access to some tools which help us to determine which users might be misbehaveing in ways that are not visible on the surface.

On Stack Exchange sites most of the "moderation" is done by users ordinary users exercising powers that are linked to representation. The "diamond mods" are there to deal with unusually situations.

When things are going smoothly it takes very little time because there just isn't much to do.

When things are not going smoothly we spend a lot of time looking at user profiles and reading troublesome comment threads and discussing options among ourselves.

PranshuKhandal

may be i should not ask this, but are you paid for this?

dmckee

So there is usually consensus before we take any strong action.

@PranshuKhandal Only in warm fuzzy feelings. And I think I got a t-shirt and a couple of stickers, once.

PranshuKhandal

that's sounds superb

can you see the comments made by an user on their profile

dmckee

Even ordinary users can see the on-site comments of other users. Moderators can also see deleted comments (for a limited time in the user profile and forever if we look one post at a time).

PranshuKhandal

@dmckee thank you sir for the help

amanuel2

04:43

I cant seem to understand Question 5C

Im so confused on how the friction is MbA and to the right

Zerix

@amanuel2 since the only force that can act on the lower block is friction and it is stated that they don't move relatively then it makes sense that friction acts towards right for it

@amanuel2 chat.stackexchange.com/rooms/54160/problem-solving-strategies

It would be better if you ask that in the problem solving room

amanuel2

@Zerix Isnt Netforce for the bottom block zero? Therefore the Friction is 0 since there is no other force?

dmckee

The bottom block is stationary with respect to the top block, but not with respect to the floor.

If the two blocks were glued to gether then it would be clear that the net force acting on the is to the right and they would accelerate.

Static friction at the block-block meeting surface is doing the job of the glue.

David Z

05:10

@danielunderwood For well established sites, moderator elections are scheduled roughly whenever the existing mod team asks for them, or when one of the existing mods steps down. If the community thinks more mods are needed on a site, someone can make a meta post to start the conversation about getting an election scheduled.

John Rennie

06:14

@Zober the Einstein-Rosen bridge appears in the maximally extended Schwarzschild black hole. However this is a mathematical structure that cannot exist in the real universe because it takes an infinite time to form and the universe isn't infinitely old. So none of the black holes we might observe will contain ER bridges.

user351417

06:38

@Blue Neat, but you need a userscript to view that stuff when there haven't been any edits, which is kinda annoying. Better than nothing though.

user351417

27

Q: Now you can know if a question really hit the HNQ

I noticed this new addition. It is now stated in the revision history if a question has made it to the Hot Network Questions, example:

user351417

Party time!

user351417

(Though I see Emilio Pistanty got there first :P)

Slereah

07:40

Oh look at @JohnRennie, knowing when the universe appeared!

He's that old

John Rennie

@Slereah I remember being massively disappointed when I realised that the maximally extended versions of the Schwarzschild and Reissner-Nordstrom metrics didn't actually exist.

bolbteppa

pZombie

when considering the temperature of some low pressure gas in a box, what is the temperature between the gas molecules, as in space not occupied by any gas molecules?

surely, if i put some nano sized thermometer in-between and let it measure the temperature for some time, it would eventually give us the same temperature as a big thermometer would. But for short periods of time, it appears that the temperature measured might be way lower. Is measuring temperature defined in a way which requires you to measure for an "infinite" time-interval?

John Rennie

08:10

Temperature is a collective property. It isn't defined on scales small enough to make individual particles significant.

bolbteppa

It seems that one can show $GL(n,C)$ has no finite dimensional spinors because restricting to $SL(2,C)$ transforms of two coordinates the rotations $\begin{bmatrix} \cos \theta & - \sin \theta \\ \sin \theta & \cos \theta \end{bmatrix}$ living in $SL(2,C)$ would send a spinor $\psi$ to $- \psi$ under a $2 \pi$ rotation yet $SL(2,C)$ is simply connected so this is impossible. But how does one modify this to hold for $GL(n,R)$, since $SL(2,R)$ has $Z$ as it's covering group

Maybe the fact $Z$ is the covering group means the spinors simply have to have an infinite number of components and this is the proof?

Slereah

@JohnRennie Most people having troubles understanding black holes really stem from that

bolbteppa

(We've entered real physics territory when we use $Z$ instead of $\mathbb{Z}$)

Slereah

08:25

Also @JohnRennie I think you're confusing the Einstein-Rosen bridge with the maximally extended Schwarzschild solution

Einstein-Rosen is two exterior Scwharzschild solutions glued together

quite similarly to the Morris-Thorne wormhole, except with horizon

John Rennie

Hmm, that's not the definition of the ER bridge I'm used to, though I'm willing to believe different people define it in different ways.

Slereah

It's not a super popular solution

It's one of those weird pre-60's solution that was just "Help how do I get rid of those weird singularities"

bolbteppa

Apparently Wald gets this spinor point wrong saying they don't exist

pZombie

if the energy of the space between a casimir cavity turns out to be negative then shouldn't ER bridges be possible?

Slereah

@pZombie Slight issue

Have you seen the actual energy scale of the Casimir effect

and the size

if possible that would not make for great wormholes

pZombie

08:30

it's enough if you could just send information back in time for now

Slereah

Even assuming something ideal like domain wall casimir effect

Not even sure

Especially the back in time part

making wormholes with a time shift makes them collapse

and sending particles through it also probably will

pZombie

i am quite confident there is no way to send anything back in time. There are too many paradoxes or a lot of weirdness. Still, according to Hawking, the energy in a casimir cavity should be negative.

Slereah

It should be yes

pZombie

And negative energy is closely related to negative mass i assume, going by E=mc^2

Slereah

it's not a terribly good source of it though

pZombie

08:36

if a lightbeam running through a casimir cavity would travel only a tiny bit faster than light travels through a vacuum then all you need is SR to show that you could use it to send information back in time

Slereah

Lightbeam would make the wormhole collapse though uwu

very sad

pZombie

you don't need any wormholes

John Rennie

Sending info back in time is more complicated than just moving faster than light.

Constructing a CTC using FTL travel requires accelerating between different inertial frames as well as FTL travel.

Slereah

Also "faster than light" is a tricky thing if you're talking about the hypothesis that maybe light propagates faster in a casimir cavity

Because you're confusing the speed of light with the spacetime proportionality constant

pZombie

anything that according to your reference frame travels at even a tiny fraction faster than light, can be observed by a large group of reference frames as traveling not only FTL, but traveling some distance at negative time

@Slereah No, i am not confusing anything. Allow me to send any signal within my reference frame from point A to B at FTL, even if it is a tiny fraction and i can show you how to build a machine to send information back in time

Slereah

08:41

Trust me I know that shit :V

pZombie

well in principle i could, but i doubt we would have the technology to build such a thing

@Slereah then you agree that if i could send any signal at FTL from A to B within my reference frame, i should also be able to send information back in time, i assume

Slereah

If you can send it along a spacelike path, then yes.

Whether or not that's the case here, that's a different matter

pZombie

it does not matter how it arrives from A to B as long as the signal reaches B at FTL

it could teleport for all i care

Slereah

See what I said above :p

Anything related to FTL is very tricky to decide

The point I'm trying to get across is

If you only consider light by itself

You can show that the front velocity is the speed of light

But if you consider the quantum vacuum along with it

Where things may interact

I'm not sure this is still true

So the question becomes

Does light actually propagates at $c$

in those circumstances

$c$ being the proportionality constant of space and time here

pZombie

@Slereah i.imgur.com/Dy59y0e.png this is what i mean in pictures. If a source which is traveling at 0.5c in this case was to emit a lightbeam at FTL( red line) just when it was to pass me, i would observe this beam to be crossing a distance at negative time (left diagram). All i would have to do next is to get another source do the same in the opposite direction and voila, back in time signaling

Slereah

08:52

I give up

pZombie

@Slereah me too. Your objections seem beyond the point. All one should care about is the front and exit of the casimir cavity. What exactly the photons would do inside the cavity is irrelevant. They could do a dance or whatever. If i get a signal at B and simply divide distance by time interval, then if the result is any value higher than c, if SR is correct, then signaling back in time is possible

Slereah

(My point is that they don't actually go faster than $c$)

Well, maybe

It's complicated

pZombie

09:25

I never understood what some mean by "we move through time at one second per second" - How would it then look like if we moved at two seconds per second through time?

What i can say is that while i move 1 unit towards my path, light moves 1 unit in another dimension and 1 unit in the dimension i am moving towards. Basically light is sqrt(2) times faster than me

if we were not to consider time and space to be different but just consider 4 space

also, how does one consider himself at rest when he is always moving "through time"?

John Rennie

10:08

@pZombie the flow of time does not exist in GR. Time is just a coordinate, like the three spatial coordinates. You can compute $dt/d\tau$ but this is no different from $dx/d\tau$ and does not mean time is flowing.

Slereah

"If each of two spacetimes can be embedded isometrically as an open subset of the other, then the spacetimes are isometric. (FALSE)"

How is that even possible

Damn you Geroch

It's apparently true if the manifolds are compact

So I'm guessing this involves some non-compact shenanigans

user8718165

11:08

hello sir @JohnRennie

John Rennie

@user8718165 hi

user8718165

Sir could you please look at my question physics.stackexchange.com/questions/463691/…

John Rennie

11:29

@user8718165 the answer to your question looks fine to me

user8718165

11:44

yes sir @JohnRennie answer to the first question is fine. But I want know how would be the resultant magnetic field lines.

in the 2nd and 3rd question

user351417

12:30

@EmilioPisanty Um, that's a bit of a random request, but sure, when do you want your alarm? :P I haven't gotten whatever part that is yet.. i'm just about done with the second chapter (It's called JPL, if I recall correctly)

Emilio Pisanty

12:57

@Chair now is good

0

Q: Relationship between structure and shock sensitivity for branched vs straight hydrocarbons and ammonium ions

In p. 137 of John D. Clark's Ignition!, Clark reports an interesting observation regarding the shock sensitivity of different isomers of alkyls and ammonium salts. Specifically, he says that he compared the sensitivity of tetramethyl ammonium nitrate ("Tallulah") in $\ce{N2O4}$ with the isomeric...

hydrocarbons amines

pZombie

It looks like the next big thing will be nano/optical rectennas as they are approaching the stage where they can manufacture them close enough to the properties required to be useful and also mass produce them.

Imagine walking around in clothing that has those nano rectennas weaved into, turning your body heat (roughly 1kwh emitted IR radiation daily) into useful energy to be used to charge your electronic gadgets like your smartphone, maybe also drive assist your bicycle?

Solar panels that make use of sunlight at daytime and also have a layer of rectennas below to make use of IR earth emits at both day and nighttime?

JMac

15:55

@pZombie What % of that body heat do they expect to recover as usable energy?

Akash. B

I made an invention

Slereah

16:21

GR paper from the 70's with 4 authors

and I don't recognize any of them

I guess I don't know them all yet~

Secret

16:43

Timescape

Timescape is a 1980 science fiction novel by American writer Gregory Benford (with unbilled co-author Hilary Foister, Benford's sister-in-law, who is credited as having "contributed significantly to the manuscript"). It won the 1981 Nebula and 1980 British Science Fiction Award, and the 1981 John W. Campbell Memorial Award for Best Science Fiction Novel. The novel was widely hailed by both critics of science fiction and mainstream literature for its fusion of detailed character development and interpersonal drama with more standard science fiction fare such as time travel and ecological issues...

> The resulting message is made of staccato sentence fragments and jumbled letters, due to the 1998 team's efforts to avoid a grandfather paradox. Their aim is to give the past researchers enough information to start efforts on solving the pending ecological crisis, but not enough that the crisis will be entirely solved (thus making a signal to the past unnecessary and creating a paradox).

Actually, one does not need to do that. One only need to ensure at the time the message gets send back, it is not resolved, but the crisis itself can be solved right afterwards

Reading time travel novels really trains you to think in terms of temporal mechanics

John Rennie

I read Timescape when it was first published (in 1980) and absolutely loved it. At the time I was cycling to the Cavendish lab several times a week for lectures and the novel felt very close to home. However I reread it a few years back and I didn't think it had aged well.

ACuriousMind

...and that also felt close to home? ;)

John Rennie

@ACuriousMind oooooooooooooooooh! :-)

ACuriousMind

I jest, of course - at least your arteries must have aged well to cope with the meals you regularly present to us here ;)

John Rennie

Both physicists and cheese age well provided you don't mind the smell.

ACuriousMind

16:57

How far does the analogy stretch...both also go well with wine?

John Rennie

Overexposure to both causes nightmares!

ACuriousMind

That's not true - I've never had nightmares from cheese!

John Rennie

I must that I haven't either despite the reputation cheese has for such things. I suspect the alleged nightmares arise because the cheese causes indigestion, which then causes broken sleep and hence the dreams. But since I never get indigestion the problem never arises.

I don't remember ever having had nightmares about physics or physicists.

Secret

I think looking back at that story, and our current climate situation, we are basically just like the 1998 of Timescape. Except unlike in fiction, we do not have the assistance of our future selves to guide us to fix it

thus we have to rely on our present day selves

pZombie

17:35

@JMac not sure yet. Some papers on rectennas i read claim to have reached 84% efficiency and higher but until i see a square meter panel made up of rectennas and the actual output i remain sceptical. If the 80+% value is correct though, given that we could weave cloth that is covered almost fully with rectennas, then i except around 500 wh of energy per day which could be recovered. Enough to charge your smartphone 15x over.

John Rennie

18:16

@pZombie you don't radiate a kilowatt hour a day. You do indeed produce a kWh per day, but it is mostly lost to convection or of course evaporation of sweat. You produce very little in the way of EM radiation.

pZombie

@JohnRennie last time i checked it is 60% EM radiation vs convection and evaporation being 40%

JMac

that seems unreasonable considering how little the temperature difference

okay i see somehwere else saying it too

pZombie

if this is accurate oerpub.github.io/epubjs-demo-book/content/m46456.xhtml then my 60% value for radiation is correct

15% convection, 60% radiation, 5% conduction and 20% evaporation

JMac

where does that come from though?

pZombie

the internet :D

but i remember reading about the same from another source just don't have the link anymore

bolbteppa

19:02

@Slereah you should fight for the Antoniadis one (still available), a better one hasn't come up all year

GR is not even quantum :p

OxCam are probably the only/main places you'll get to do that stuff without touching the dreaded experiment or handwaveosmology

enumaris

weez

thermomagnetic condensed boson

19:25

I think John Rennie is a bit wrong about radiation

any body near 300 K with a surface of over 1 m^2 radiates a lot of power. as far as i remember, it was huge

the extra net heat that's radiated thanks to calorie intake is much less, but still more than convection and conduction

i think the total power radiated by a human body near 300K was about 20 kW

(and absorbs a bit less)

Loong

@thermomagneticcondensedboson 459 W according to the Stefan–Boltzmann law

thermomagnetic condensed boson

what value of emissivity?

Loong

1

thermomagnetic condensed boson

you're right. about 1 kW (because 2 m^2)

Emilio Pisanty

19:49

U. of California System Cancels Elsevier Subscriptions, Calling Move a Win for Open Access

7

nice!

@DanielSank Congratulations on having a forward-thinking alma mater =)

DanielSank

20:51

@EmilioPisanty :-)

Mo_

@EmilioPisanty "On a system web page, UC has listed possible ways scholars in need of Elsevier articles could get access to them, including through an interlibrary loan, in online repositories like PubMed, and by simply contacting the author and asking for a copy."

They forgot Sci-Hub

Will SciHub break down for a journal if every university cancels its contract with that journal?

Emilio Pisanty

21:14

@Mo_ you know sci-hub is illegal, right?

Novalium Company

(Warning! A philosophical question ahead) What does it mean to "know" something. I know Epistemology studies this but is there a quick and short/easy answer that doesn't involve complicated terms and graphs...

Mo_

@EmilioPisanty Sure. But everybody knows sci-hub and when you cancel your legal contract, then you're implicitly telling everyone to use sci-hub

Novalium Company

I need help understanding the Black Raven paradox. I don't understand what's the confusing/paradox part. What's the problem with coming to the conclusion that all non-black things are non-ravens from looking at a red bus for example?

Novalium Company

21:44

Also the Monty Hall Problem makes no sense to me. :( Help.

danielunderwood

22:18

@enumaris wee

Emilio Pisanty

@Mo_ I mean, are you seriously suggesting that UC should have explicitly included "Use Sci-Hub" in an official document?

knzhou

@NovaliumCompany It's probably useless to think about what it means to know something, that's the kind of thing that gets you in a classic philosophy infinite loop.

To convince yourself that you know X, you have to convince yourself that you know that you know X, but to do that you must show that you know that you know that you know, and...

Might as well just stop at the first level and say you know, you know.

@NovaliumCompany There is nothing paradoxical about the black raven paradox. From the Bayesian perspective, seeing a non-black non-raven is evidence that all ravens are black. It's just extremely extremely weak evidence.

pZombie

22:33

that would make you quite stubborn if someone came along and told you that what you know you know is not right

knzhou

@pZombie I don't negotiate with terrorists. :P

@NovaliumCompany But I think the reason people think it's a paradox is that, since it's such weak evidence, some would say it's intuitively inconceivable that there should be any evidence. Or, in the absence of a way to quantify evidence, they think that observing a black raven should be equally strong evidence to observing a non-black non-raven because they're both "one observation", which also leads to paradoxical results.

As far as I'm concerned like 75% of epistomology questions are solved by just plugging the numbers in.

2

pZombie

me thinking i know i know

i think that would be what descartes would say

for it is not possible to just think you think yet not be thinking

enumaris

@danielunderwood enumaris is busy right now, please leave a message after the beep

beeeeep

danielunderwood

Sorry don't have time to leave a message. I'll put it in my schedule to call back next week

vzn

23:29

@enumaris @danielunderwood thought you guys might like this/ wondering what you think of it... think the media is gonna react to it at some pt... re all the commotion over "deepfakes" etc...

in theory salon, 24 hours ago, by vzn

https://thispersondoesnotexist.com/ https://www.lyrn.ai/2018/12/26/a-style-based-generator-architecture-for-generati‌ve-adversarial-networks/

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