The h Bar

@Akash.B morning :-)

Akash. B

@JohnRennie same to you

@JohnRennie what are the mysteries that present day physics try to solve?

@Akash.B that's a very broad question. I'm not sure there is a simple answer.

M.N.Raia

6:03 AM

good morning/afternoon/night for everyone!

@M.N.Raia morning :-)

M.N.Raia

here is my email in case there' a researcher here who wants to leak the paper of saggitarius A* VLBI black hole image mraianeto@gmail.com

hahahhaa

6:26 AM

@M.N.Raia :-) It sounds exciting. It has the same sort of feel as when LIGO released that first detection of a gravitational wave.

Captain Bohemian

7:08 AM

a confirmation mail is of such importance for the receiver though it may be so boring to write.

7:53 AM

Please post questions like this in the problem solving room

Fawad

Sorry I thought this was jee chat

sorry again

user929304

9:08 AM

@Qmechanic Hi, I just came across this QM related post thought you'd be interested in, physics.stackexchange.com/questions/469949/… it seems the derivative is correctly taken, but not sure to write an answer.

Could Black Holes Made Of Light Power Our Spaceships?

10:34 AM

just figured out that my Favorites Questions = Interesting Question I thought I would have read them later but I have never read them.

Maybe one or two exception ... over 100+ questions.

Secret

2:07 PM

►

@Secret Here's a recent question about black holes made of light. physics.stackexchange.com/questions/469345/… Also see the chatroom of comments linked on the question. I seriously doubt it will ever be practical to construct such things, the required energy density is insane.

Slereah

2:29 PM

There's a black hole moving at the speed of light solution

The Aichelin metric or something

Aichelburg–Sexl ultraboost

that's the one

"The ultraboost helps also to understand, why fast moving observers won't see turning stars and planet like objects into black holes."

Mohammad A

energy stored per unit volume in magnetic field is called?

Question: Energy stored per unit volume in magnetic field

2:46 PM

@MohammadA I don't think it has a name. Just energy stored in the magnetic field.

@MohammadA Energy stored per unit volume is the energy density, which for a magnetic field is $\frac{B^2}{2\mu}$

@JohnRennie Can you hammer this, or does it need a tag added first? physics.stackexchange.com/questions/470513/…

@PM2Ring tag it with special-relativity and I'll hammer it

2

3:01 PM

I was watching a MIT professor proving "every attractive 1-dimensional potential has a bound state", and he especially mentioned that does not work for 3-dimensional potential.... I wonder what's its application to real life.

I mean if the proof can give me any real prediction about the physical world.

@JohnRennie Tagged

@PM2Ring BOOM! :-)

Ta!

I tried discussing this stuff with that OP on one of his recent answers (now deleted), but I figured it might be worth one more try. But as you can see from the comments, he's locked in to the idea of universal simultaneity and won't budge.

Of course, I'm not implying he's stupid. Relativity of simultaneity and the Andromeda paradox is counter-intuitive. But if you accept Lorentz transformation then you have to abandon non-local simultaneity.

knzhou

3:22 PM

Given that he called people who accept SR sheep in a different question, I don't think there was anything you could've done.

Q: Non-causality-violating FTL travel. / Can omnipotence beat causality?

0

While building my sifi world I dug into ftl-travel and encountered these mean bullies, causality and relativity. They told me I cann't have faster than light travel, because effect can't precede cause and it is easy to violate that using ftl. I really don't like that. My idea to circumvent caus...

In general, it's not productive to argue with someone who classifies those they disagree with as "sheep".

@Semiclassical there are people who want to learn, and people who want to be proved right.

It's rarely productive to argue with the latter.

what's wrong with sheep? They are cute. that's sth to insult in english?

the implication is that you follow directions from someone else without questioning them

3:37 PM

@Shing have you ever had to look after sheep? I have. I lived in the country as a lad. From direct experience I can tell you sheep are quite astoundingly stupid.

and then there's "sheeple" which is pretty much an immediate "okay bye"

@JohnRennie quite

@JohnRennie I have never... and I was told pigs are smart

are they really smart?

they're pretty clever from what I recall

A professor of mine told me that

@Shing yes. They are at least as clever as dogs and probably cleverer.

sidharth chhabra

3:38 PM

@JohnRennie I saw your answer to physics.stackexchange.com/questions/136860/… I must say it is well written

@JohnRennie to steal from a college humor sketch, the latter are the kind of people who google "proof that vaccines don't work" and pick the one link that supports their opinion

(that is, unfortunately, an exaggeration of reality. there's more than one site peddling that crap)

@sidharthchhabra thanks :-)

I spent a long time on it!

Pigs are interesting critters, but the smell is a bit much. ;)

I wonder a bit about that. Certainly pigs in a farm don't smell great, but is that a reflection on the pigs or on the conditions they're placed in?

i.e. would pigs in the wild be remarkably smelly? I'd guess not.

Some people with a pet pig keep them clean. But under natural conditions they tend to be pretty smelly.

3:53 PM

i guess what I'm wondering is how smelly they are compared to other animals in the wild

The problem is that pig poo is fairly similar in smell to human poo.

sidharth chhabra

@JohnRennie You seem to be very knowledgeable in this field. I came across a paper that claims that Heisenberg UP is violated by empirical evidence based on high precision measurements using tunneling microscopes. link-springer-com.proxy.lib.umich.edu/article/10.1007/… any thoughts? I posted on the stack the other day however, was closed down for asking it.

yeah, but do pigs in the wild tend to live in the same location as their poo?

they're forced to do so in a farm setting

@sidharthchhabra the default response to a paper like that is "extraordinary claims require extraordinary evidence"

also, that link is behind a U-Michigan login

so that's probably not the best one to use

is it this paper ? adsabs.harvard.edu/abs/2012FrPhy...7..218H

(also, that same author has a paper titled "Is quantum mechanics creationism, and not science?". That doesn't exactly lend credibility.)

@JohnRennie Is the following statement good enough to explain what time is? "We perceive increase in entropy as time".

4:09 PM

I think any simple statement that purports to explain "what time is" is likely to be too short :P

@Semiclassical But,why? ...I think it is very simple(at least for my current level of knowledge in physics)....Could you explain :
why its difficult to explain in simple words?

4:22 PM

There are several arrows of time. It would be nice if we could show that they all imply each other.

Also see physics.stackexchange.com/questions/235511/…

That's a big answer!..It's gonna take some Time ,will be back

bolbteppa

@Slereah Introduction to Regge Calculus for Gravitation from today

Have no motivation to learn this stuff right now but the notes look good

4:37 PM

@PM2Ring So,if I consider special relativity then time looses its speciality and its just another coordinate,haa?

@ayc Time is still special in relativity, even though time & space are unified.

@ayc Do we even perceive an increase in entropy? We perceive the effects of that sure, but the effects of the increase in entropy aren't how I would describe us as perceiving time. If we somehow existed in a world with symmetrical time, things still shouldn't occur instantly, so there should still be a time associated with that, no? Even if the arrow of time weren't so straightforward, it doesn't necessarily mean that the temporal distance between events doesn't exist

@ayc time is just a coordinate, but what distinguishes it from space is the sign with which dt^2 appears in the metric.

Re the entropy: everything we perceive about the world is a result of our brains functioning. The brain is just a machine (albeit a rather squishy one) and like all machines it increases entropy as if functions. So the brain necessarily operates in the direction of increasing entropy.

And if A & B are 2 events, all frames agree whether the spacetime interval from A to B is timelike, lightlike, or spacelike. As I said a few hours ago: physics.stackexchange.com/questions/470513/…

My guess is that we perceive time flowing in the direction of increasing entropy because our brains operate in the direction of increasing entropy.

4:47 PM

I like the attitude of the Discworld trolls: we're actually traveling backwards through time, because we can see where we've been, but not where we're going. ;)

@PM2Ring Now if only our brains were superconducting, too, we'd have come up with that on our own ;)

Exactly. :)

That scene with Detritus in the Pork Futures warehouse is one of my favourites.

I really need to get back into the discworld audiobooks, but something about them isn't pulling me in compared to just looking for new series

@JMac there are too many good SFF books out there screaming to be read, and people keep writing more! :-)

There's nothing quite like the Discworld, and there are sadly as many Discworld books as there will ever be :(

4:55 PM

I've read most of the Discworld books at least 4 or 5 times, apart from the final few. I'll probably read them all again, at some stage, but lately I've been getting into old science fiction from the 40s & 50s. I'm currently reading a big collection of short stories by Fredric Brown.

Comedy is really, really hard to write. Many have tried and few have succeeded. Terry Pratchett didn't always succeed, but on balance he succeeded more than most, and arguably much more than most.

@JohnRennie Even the limited amount of Colour of Magic that I've listened to probably has more good humor than the rest of my library. He really does hit the mix of subtlety right in the sweet spot. It's usually not like "LOOK AT THIS JOKE I MADE, YOU SHOULD BE LAUGHING"; which is what usually happens when SFF tries to place comedy in stories (at least in my unfortunate experience)

@JMac agreed

Even when Pterry's humour doesn't quite work for me, I don't mind too much. I just enjoy being in the space he creates. I bet he would've been a great bloke to hang out with.

Douglas Adams is the other obvious British SFF comedy writer, but I rarely find myself actually laughing when reading his books.

4:59 PM

@JohnRennie You one day told me that you think the universe is non-deterministic because wave function collapse is random ..right?

Lesser known comedy authors would be Robert Rankin and Tom Holt, but they pale in comparison to Terry Pratchett.

@ayc did I say that? If so that's an oversimplification.

They both wrote clever humour, but I get a stronger sense of human warmth from Pterry.

I don't think I've really read any Douglas Adams, but I've seen several of his adapted works. I actually really liked Dirk Gently's Holistic Detective agency on Netflix; but I'm pretty sure the plot there never really showed up in the books. Also, the humor was only so-so; I just like time travel shenanigan plots.

@ayc we don't really understand wavefunction collapse. We know what the end results are but not what is actually going on. The trouble is that this road leads us into the swamp that is the interpretations of quantum mechanics. Many physicists have perished there.

> Also, the humor was only so-so

@JMac Precisely

@JohnRennie So,If I ask you :How these random events in the small world, give rise to a very rigid law:increase in entropy of the universe, then would you say Nobody knows?

5:05 PM

Anyhow, I'm off to drink a beer and read a SF book. I haven't decided which book yet - there are about a hundred books on my "to read" list.

I've noticed with your ability to answer Story Identification on SFF.SE suggests that your "read" pile is also pretty chunky :P

@ayc we don't really understand what happens in the wavefunction collapse, but we know that the collapse obeys statistics that we can compute. Thermodynamic properties like entropy emerge from those statistics.

@JMac :-)

My first exposure to Douglas Adams was via the original HHGTG radio plays. I was impressed! The books were great, but they didn't have the same impact for me as the radio version. And IMHO the movie & TV versions were so-so.

@PM2Ring Reminds me of an old line in philosophy: Life can only be understood backwards; but it must be lived forwards.

@JohnRennie Which is not to say that swamp is unimportant or uninteresting, but it is exceptionally perilous

@JohnRennie So,if we want to know the end result of a process then we do some statistics and determine the probabilities ...that's what we do,right?(glorified dice throwing)

5:15 PM

Different interpretations will have different views about the randomness, but at an operational level---i.e. what you can predict about experiments using quantum mechanics---there's not really any debate

just don't take anything too deep..."glorified dice throwing": I picked it up from a movie(The man who knew infinity)

@PM2Ring Pratchett cares for his characters, and especially his villains are often more broken than evil. The humour manages to stay out of the way of stories that would be engaging in their own right. And in my interpretation most of the books have certain themes, like Small Gods being about the institutionalization of religion or many of the Night Watch novels being about the tension between personal morality and law. I couldn't tell you what Hitchhiker's Guide is "about" if I tried.

Not to say that every book has to be "about" something, though

(and whether it's understood as being about something will depend on the reader, etc)

@ACuriousMind Did you read any short stories of pratchett?

I don't think I have

5:29 PM

@Guys,I'm done for today...see y'all later!..Bye

@ACuriousMind Agreed. Small Gods is a great work, beloved by both atheists and theists, a pretty remarkable achievement, IMHO.

There are a few Pratchett short stories floating around, including some Discworld ones. You may be able to find a free version online of Troll Bridge

6:22 PM

@ACuriousMind Have you decided to leave SAP and come work with me yet? :P

@BernardoMeurer Nope, still pretty happy here :)

/me acquires SAP to get ACM

@ACuriousMind I will eventually conquer you :P

Q: Ascending to three dimension from one dimension

I was told when you write paper, you need a sexy title. So I start practicing it by making sexy titles via asking question here.

0

I was watching a lecture video from MITx - the professor proved a pretty cool theorem "every attractive 1-dimensional potential has a bound state"; however, that only holds in 1-dimension, and the theorem doesn't hold in 3-dimension. Then I was left wondering: How do we generate any results ...

quantum-mechanics experimental-physics wavefunction potential schroedinger-equation

is the title sexy enough?

@BernardoMeurer Maybe - but not today ;)

I started learning Rust, btw :P

@Shing Have a look at physics.meta.stackexchange.com/q/6413/50583

@ACuriousMind Oh, nice!

I managed to hire quite a bit of Rust talent, I've been very happy working with them

We're working on some really novel stuff :D

6:32 PM

@ACuriousMind @ACuriousMind thanks for the link!

@BernardoMeurer I'm slowly starting to love the borrow checker instead of hating it ;)

@ACuriousMind Yeah, that's the natural transition

It ends when you have to write unsafe code and then all you want is the borrow checker

@ACuriousMind I have these two discussion logs on something we're implementing that you might be interested in...

Heh.

Cross-process, safe, lock-free channels in the form of ringbuffers

um... based on the views, I suppose my title is kind of sexy, but the reaction that get 0 upvote (especially given another 14 upvotes question asking same theorem but can be found on book or google), there is a lot of room for me to improve.

6:42 PM

I've seen some custom softwares for different electronics but how do they make them? I mean, I've seen 3D printers with custom software, but how and where and in what language do they write the UI and functioning?

"Escaping the bound (states) of one-dimensional potentials"

user76284

Some would say "wave function collapse" isn't a thing at all.

That's more sexy!

@BernardoMeurer Are these intended to be different from lock-free FIFO queues?

sidharth chhabra

@Semiclassical @Semiclassical thank you for taking time to find the paper. I apologize that I didn't give the right URL. The paper is making a radical claim. Since I am not knowledgeable enough to evaluate it, I am asking for your help.

6:46 PM

@ACuriousMind Well, they are FIFO but there's some subtleties involved, being SPMC. In particular there's a requirement that consumers can consume the same data more than once. i.e. the data remains available after consumption, and is only gone once the producer wraps around and overwrites it

The trouble is that, if a paper is wrong and so nobody pays attention to it, then it's hard to find sources which respond critically to it.

Doing that in a safe way is... challenging

But a paper which purports to experimentally challenge the Heisenberg uncertainty principle really doesn't seem likely to sustain much scrutiny

@Shing the amusing thing is that, in writing that out, I was remembering the following (hilariously hyperbolic) sentence from a certain quantum mechanics textbook

specifically, the following list from Amit Goswami's QM textbook:

"Outstanding features: 1) gives a thorough grounding in the fundamental aspects of quantum mechanics; 2) provides a wealth of worked-out examples; 3) offers a balanced presentation of quantum system, which helps students make a bridge to whichever aspect of modern physics they want to pursue; 4) goes into enough depth without exceeding the mathematical level of the beginning student; and 5) liberates physics students from the bonds of classical prejudices."

One of these things is not like the other...

lol

"liberates physics students from the bonds of classical prejudices"

yuuuuuup

as a mathematical text, it's fine

But it's even more valuable as an example of the kind of stuff I can't stand when it comes to QM interpretations

user76284

6:57 PM

A presentation by Sean Carroll on the subject: youtube.com/watch?v=iXRLDatmbgA

Anonymous

Sean Carroll can often be misleading. Take it as pop-science, no more.

user76284

What do you find misleading about that presentation?

I thought it was explained quite well.

bolbteppa

7:46 PM

'Physicists of the world, unite. You have nothing to lose but your [classical] chains!'

8:00 PM

I've seen some custom softwares for different electronics but how do they make them? I mean, I've seen 3D printers with custom software, but how and where and in what language do they write the UI and functioning?

@NovaliumCompany Probably depends pretty much exclusively on how the 3D printer runs or takes instructions

@JMac I'm talking about different hardware in general. How do they design and "put in" the software?

@NovaliumCompany Most hardware contains an embedded system. Programming these is not fundamentally different from programming an "ordinary" computer, although it tends to operate under tighter constraints.

Another question. From my understanding, the stronger the gravitational field, the slower time is compared to somewhere with little or no gravity. So what if we could dig and somehow survive deep into the Earth's core down to it's center of gravity. What will happen then? The gravity will be so strong so the person will be leaping through time?

In this case youtube.com/watch?v=jPU4iv378ig the embedded system is Arduino's software?

@NovaliumCompany Gravity does not get stronger as you move towards the center of the earth. At the center of the Earth, you would be weightless since there's equal mass in all directions from you.

8:15 PM

@ACuriousMind I'm talking very very close to the "perfect center".

Gravity there should be very strong?

@NovaliumCompany Very very close to the perfect center you would be very very close to weightless :P

Anonymous

@NovaliumCompany No, it'd be 0.

wtfff

Going up, gravity weakens, going down, gravity increases, no? :(

Anonymous

(considering the Earth is a symmetrical, of course)

@NovaliumCompany Think about it like this: If you were at the center, what direction would this strong gravity be pulling you to?

Neglecting small imperfections, the situation is spherically symmetric: Since therefore nothing singles out a particular direction, there's no direction you can be pulled towards

@NovaliumCompany Only as long as you stay above the body that causes the gravity

8:18 PM

I don't understand. :\

Anonymous

@NovaliumCompany Do the math, really.

@NovaliumCompany Okay, another try: As you descend into the Earth, more and more of Earth's mass will be "above" you.

Anonymous

The intuition is...well yes...when you're at the perfect center there's an equal amount of mass in every direction.

Q: Asking a question about entropy as a systems more typical states?

But that mass still exerts its gravity on you, pulling you towards itself. Just it now above you, so it counteracts the part of the Earth that's still "below" you.

When the masses "above" you and "below" you are equal, you become weightless since their forces exactly cancel. The point at which this happens is naturally the center of the Earth

Physics Meta

0

I want to ask a question about entropy but am not sure about it and am seeking some general feedback first. I've heard of the metaphor of a finite 2D square grid with each square in one of two states as being (sufficiently in this case) like a universe, or large piece of the universe. Most of t...

discussion questions

8:20 PM

I kinda get it now.

But it still feel counter-intuitive.

Lemme think about if for a sec.

Anonymous

It's kinda important to precisely define gravity in the Newtonian sense, first.

The problem with intuition is that it's frequently wrong until you're trained it not to be :P

So if I found myself at the center of the Earth, where gravity will be 0, time will pass very fast for me, so I'll return very old to the surface of the Earth?

@NovaliumCompany Not really, the effect is miniscule because Earth's gravity is weak to begin with. See physics.stackexchange.com/q/10089/50583

Got it.

What happens if I found myself 10 meters (just an example number) away from the center of the Earth with no momentum?

Where will gravity pull me to and how strong will it be?

And what is the point where gravity is strongest?

From my understanding, going up decreases gravitational pull, going down, decreases gravitational pull. :\

8:40 PM

@NovaliumCompany You will be pulled to the center with a gravitational force the same as if you were just above a ball with 10m radius and there was no earth above you. This is the shell theorem

Anonymous

@NovaliumCompany On the surface, or rather at the poles cf. Why is Earth's gravity stronger at the poles?.

Anonymous

As you ascend from the surface, the gravitational pull will reduce. And so will it, when you descend from the surface.

Anonymous

@NovaliumCompany You'd in fact, in some sense, be time traveling into the future.

Anonymous

8:57 PM

Eh, how did this get 5 upvotes. Weird.

Anonymous

Would serve as a good reference, nonetheless.

I figured it was because people felt it would be a good reference. I kinda found the votes on the question weird too; but it looks like a lot of people looked at the question, so it's not that surprising that it's useful to some.

@Blue Upvotes on meta questions can not only indicate agreement, but simply that people find it useful that we explicitly talk about the matter.

It might even be "Oh, a meta question that isn't a rant" -> Upvote.

Meta: Where the rules are made (up) and the votes don't matter :P

Anonymous

I'd grant that, yeah. At least it's not yet another rant. :P

The rants are usually hilarious. "Everyone needs to stop downvoting. It is ruining everything, and it makes everyone leave the site because everyone else is basically bullying them by clicking the down arrow"

Anonymous

9:10 PM

@JMac You just brought back some memories. I usually quote Robert's answers in these cases. :)

Anonymous

> Conflating the issues of "be nice" with trying to assert that everyone must agree-always creates an uncomfortable environment where everyone is afraid to disagree or express any type of dissenting opinion — whether it's through a down-vote or a close-vote or a comment — until the whole thing ceases to be functional at all.

It's funny how much it comes up. You get users posting like the most incomprehensible stuff, then making a meta thread asking why everyone "being mean" to them doesn't violate the CoC. Then you read the thread and it's just people pointing out technical issues with the question