4:52 AM
a partial lunar eclipse will occur at 21:38 UTC today earthsky.org/tonight/partial-lunar-eclipse-on-july-16-17, but I guess it can't be seen here because it will have been 5: 38 am on July 17 here, when it may have been at dawn here.

5:25 AM
maybe peanuts really have particular lasting function of curing hunger. Yesterday afternoon I ate a cup of peanut ice cream after a meal and didn't feel so hungry until wee hours. Usually the hunger curing function of that kind of meal can't last that long.
but now I am hungry again, needing to look for food to cure hunger again. If I can find cheap food which can stave off hunger over 24 hours, that will be nice.

5:40 AM
Faraday cage math hurts my brain
I'm trying to figure out how thick of a steel barrier I'd need to block out a specific (very wide) range of radio waves and just... ouch, my brain
yes, steel, not iron, it's a thought experiment

5:59 AM
@NicHartley I would have guessed anything thicker than a few microns would do. EM waves are absorbed pretty effectively by conductors.

@JohnRennie I figured something similar (that basically any plate of metal I could build with would do) but I'm trying to actually figure out the thickness I need, to be sure.
I'm also trying to do similar math for more cage-y cages, like what's described on this presumably completely unbiased site, and... I dunno if I'm just missing something or it's too late at night for me to be trying to do math of any kind, but I'm hitting a wall

6:35 AM
@NicHartley , check out the wikipedia article on "skin depth", then you can calculate the thickness you need for a given attenuation
unless were talking about a mesh, then stuff probably gets more involved haha
Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor. The electric current flows mainly at the "skin" of the conductor, between the outer surface and a level called the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor. The skin effect is due to opposing eddy currents...

@NicHartley phas.ubc.ca/~phys409/manuals/skindepth.pdf cf. pp.14-18 may be useful

@Gyromagnetic I've been missing that term this whole time ;-;
Thank you! Now I know what to Google for :D
@kylecampbell Thanks! Likewise, I somehow never turned up "skin depth". That's really helpful.

6:59 AM
@NicHartley Happy to help ;)

7:28 AM
2

I need help understanding the physics behind this insightful comment below the question Does water really have strong EM absorption at 3 kHz in solid and 2 GHz in liquid? Why the huge shift?: Please note: This is a relaxation spectrum. The transitions do not come from a resonance (absorption,...

1 hour later…
8:42 AM
120

Do you know properties which distinguish four-dimensional spaces among the others? What makes four-dimensional topological manifolds special? What makes four-dimensional differentiable manifolds special? What makes four-dimensional Lorentzian manifolds special? What makes four-dimensional Riema...

why do we always live in the worst possible dimension

4 hours later…
12:38 PM
Dear hbars, anyone know of a quick n dirty way of simulating a wave with finite phase coherence?
I tried making a string of random numbers from 0 to pi, and then convoluting it with the memory kernel I want as a phase autocorrelation function (exp(-t/T_coh))
and then adding this as a phase argument
but I think I'm doing it wrong, because by doing this, I get more phases close to zero (effectively doing a weighted moving average)
Not sure if I should open a legit question
string -> array, pi -> 2pi

1:05 PM
0

Does Fisher Information quantify only the precision of the instrument? I had asked a question recently. There were some comments by someone named Solar Mike. On repeatedly asking him/her on elaborating what he meant, there was no proper reply. I realized most of his answers are similar, on phys...

3 hours later…
3:35 PM
@Gyromagnetic is skin depth all that relevant to how thick steel needs to be to stop the waves? I’d have thought the amplitude decay coefficient would have been more useful, and as such the relative permeability etc

@JakeRose , skin depth takes relative permeability into account. And in some ways, can be thought of as an "amplitude decay coefficient". Can you send me a link of what you're thinking of? maybe we are talking about the same thing.

4:12 PM
@knzhou sup man. i've quickly checked some of your PDF, about solid state. page 9 at some point , it's written "photon" but I think you meant "phonon"

4:39 PM
@thermomagneticcondensedboson Thanks for the catch, I'll fix it!
Also I apologize in advance for future issues, these notes are all very rough and Solid State is the roughest of all.

5:30 PM
@Gyromagnetic but he’s talking about waves. With skin depth the waves still propagate into the material. The currents and the waves are not the same phenomena.

@JakeRose Skin depth can be obtained from the amplitude decay coefficient.
@ACuriousMind Hi!
Ever heard of [this](https://de.wikipedia.org/wiki/Lavaschak)? In Germany?

@Mo_ Nope. Is it good?

@ACuriousMind It's actually fantastic :)
One of my friends that's going to Germany wants to know how much to bring with himself
for a year

As much as he can carry, I guess :P
I've never seen it for sale anywhere
Though I guess some Persian restaurant or something might have it, but those are also rather rare.

The problem is that some items (such as seeds, some foods, etc.) cannot be brought in from the airports.
At least in the US you can't bring Lavashak :/
(that's what I've heard)

5:43 PM
no idea how that's here

6:13 PM
@Mo_ Bringing any sort of food is generally frowned upon. They can carry parasites that can devastate the local ecology.
depends on the type of food, though, and idk if that applies in this specific case

6:38 PM
@NicHartley This one is made from cooked (boiled) fruit pulp which is then salted and dried (but not completely). I don't think it can really carry any parasites that seeds or fresh fruits can.

7:03 PM
I talked about this in chat several days ago, unfortunately I do not remember exactly the answers and I kind of dropped the subject for a while so excuse me if this sounds familiar.
Considering an elongated spring fixed to a wall from one side, should it be elongated, will the force at each point be different (increasing to infinity at the wall)?
Oh, a infinite force at the wall will make the spring break free, is the force then the same at the origin and from where we're pulling?
How can we explain then the fact that each point is displaced less and less as we reach the wall?
Perhaps the stiffness is not constant?

I don't understand what you mean by "the force at each point"

@knzhou thanks knzhou. I'm trying to learn solid state physics. but i think your notes have some insights i have never read elsewhere, about the crystal momentum vs momentum. that's a part i have trouble to understand overall, so i think your notes will help

@Luyw Why would there be a force at $F_2$ pointing in that direction? What's exerting it?

For some reason I was considering $F$ being inversely proportional to the elongation.

7:12 PM
To the elongation, yes, not to the length
Let's say you mark off 10 equal-length segments

I need to go for now sorry <-<

They all stretch the same amount, right?
So -- barring any material or structural changes -- they'll all be pulling as strong as the others

if we have two resistors each of resistance less than 1 ohm then will have a higher equivalent resistance if we put them in parallel then in series?

@Mathphile No. Why would 1 $\mathrm{\Omega}$ be special?

@Mo_ it can be but I don’t think it relates the same information

7:22 PM
@Luyw I did respond to your comment on that answer. Might as well ask me here if you need to clarify any more (though it seems like you might have already gone through it in chat)

@ACuriousMind not 1 ohm
less than 1 ohm

@Mathphile I understood that. I meant what is special about 1 Ohm as the cutoff point, given that the formula for the resulting resistance isn't specific to the unit Ohm at all?

@ACuriousMind sorry idk what i was thinking

@Mathphile I've thought the same thing multiple times. I find doing the math, or just considering that "1" changes based on the units, makes the thought go away pretty fast. It's also pretty easy to believe since it keeps math consistent.
0

How certain is it that our universe will undergo heat death? Will there be a new universe after the heat death of the universe? Is there a God? Will parallel universes exist after heat death?

"Is there a God?" smuggled in haha
just another casual physics question

Culmination is for the highest point (in literature or else). What was when it is ending?

7:36 PM
@JMac for some reason I was imagining that the parts of the spring do not displace uniformly.
I want to hide.

What is a hidden Markov model? I mean, I know it applies in machine learning and AI but where exactly? Is it like a type of neural network or?

@Luyw Springs can be a bit weird until you really understand what they are doing (but I got really into them and like trying to explain the principles, so bear with me here). It makes sense to picture what a spring is actually doing. It's really just a long wire that gets twisted when you apply a force to it. The coils getting closer or further are really just a result of the entire coiled wire twisting one way or another.
The arrow showing the twisting there does a really good job illustrating it, you just have to imagine that every cross-section of the coil that you take will be doing that exact same thing. Each piece just twists slightly to bring itself closer together or further apart.

@JMac The OP has edited it down to the last question: Will parallel universes exist after heat death? So it's no longer too broad, but I can't think of a good reason to reopen it...

@PM2Ring Yeah... I'm not sure how to really deal with that now. Would "parallel universes" fall under "non-mainstream" in this context?

So I guess If I consider a free spring and pull from each side with the same force, I will have to imagine it as two fixed springs?
It seems tempting to imagine, in this case, the middle part with no force applied to it.

7:51 PM
How are HHMs used in AI?

@Jmac quick question
is you play a song faster then does the frequency of it's sound increase?

For two different forces I could consider a frame of reference in which I could disregard one of the forces and it would boil down to a fixed spring.

@Luyw Nope, treat it the same as a fixed spring with that force acting on it. A fixed spring has the same forces acting on both sides, just like one being pulled from each side does; just that a fixed spring is a "passive" pulling. The spring can't really tell the difference between a fixed and applied force

@JMac Maybe. Some kinds of parallel universe are fairly mainstream, eg the branches in MWI. Or the separate Big Bangs in an eternal inflation cosmology.

@Luyw In the case of a fixed spring, the opposite force is always just going to be equal to the applied force, whereas with two applied forces you have to make them equal, or else you will just develop some internal force in the spring, and the rest will accelerate the spring in the direction of net force

7:55 PM
if* you play a song faster then does the frequency of it's sound increase?

Thank you @JMac :D

@Mathphile Yup, you should be able to try it out with audio programs like audacity.
@Luyw No problem. Springs are one area where I feel like I can actually explain things clearly, so I usually don't mind trying.

does the wavelength remain the same?

@Mathphile No. Higher frequency would be a smaller wavelength.

okay
this makes me wonder if there is some relation between length contraction in relativity and wavelength reduction when we speed up sound
although i don't know much about relativity :P
only some basic formulas i just learned in HS
@Jmac do you think such a connection might exist?

8:05 PM
@Mathphile possibly similar in the sense that the relationship with sound comes from the fact that the speed of sound in air is approximately fixed. I'm not sure if they're that analogous though, or if that analogy is actually bad because it relates sound to light in a incorrect way. I probably one of the worst people here to ask about that though. I studied engineering; quantum mechanics and relativity were basically ignored.

@JMac okay
@JMac do you anyone i can ping here who may know more?

I would just be guessing TBH. I can't remember many details of who is familiar with what here. I'm pretty sure most active members understand relativity better than me though.

well who are some of the active members here?

@Mathphile Please don't ping people you don't know with questions. Anyone who's here and wants to answer your questions will do so on their own.
I, for one, don't understand why you think there'd be a relationship at all.

@ACuriousMind okay i am new here so i don't know the rules
@ACuriousMind well both soundwaves and oter object seem to contract when are sped up
i know it may sound silly
and maybe it is

8:13 PM
Objects don't "contract" when they speed up.
They seem contracted to an observer who's accelerating relative to them.

I need some help understanding HMMs and HHMMSs :P

@ACuriousMind yes forgot to include that in

@NovaliumCompany And how is anyone here supposed to know what these letters stand for?

@Mathphile I don't even know if you'd call it an "analogy"; but light has the same type of relationship between wavelength and frequency as sound in air does, because in both cases the velocity is fixed (speed of light or speed of sound in the medium).

@Mathphile But the wavelength of the sound wave really does change, it' doesn't just "seem" like it. So I don't understand how that's supposed to be at all the same.

8:17 PM
@ACuriousMind Sorry, HMM - Hidden Markov Model and HHMM - Hierarchical Hidden Markov Model. I'm reading about them in a book (How to create a mind) and the book uses those terms pretty often. They are some sort of models for pattern recognition. I don't know exactly what they are and where they are applied. I don't know if they are worth learning but the books says they were pretty big breakthroughs in AI.

@NovaliumCompany I guess you aren't familiar with Markov chains

@PM2Ring I'm not. I guess I'll try to learn them first.

@NovaliumCompany Good idea. ;)

But are those (Markov Chains, HMMs, HHMMs) really important in AI (or pattern rec) anyways?

@ACuriousMind but how do you know the wavelength really does change? After all we are observers of sound right?
it could be possible that wavelength of sound only changes relative to us observers

8:24 PM
@Mathphile But all observers agree on the wavelength!

@ACuriousMind who are the other observers in this case?

Its definition is just the distance between two troughs of the wave, and that distance doesn't depend on our speed (unless we include length contraction here, but let's not get confusing)
@Mathphile All other possible observers
With length contraction, you can find different observers with different speeds that disagree on the length of another object. But with the wavelength of sound, you won't find two disagreeing observers.

@ACuriousMind okay

@JMac I remember John Rennie giving a formula for Hooke's law involving $\Delta l/\ell_0$, do you happen to know it?
It's not $k\frac{\Delta l}{\ell_0}\ell_0$ of course

@ACuriousMind We have two observers A and B. Observer A holds a speaker and plays some music while orbiting around stationary observer B at a speed of 0.9c. How will the music sound for both observers?
i think the music will seem to be playing slow for B
or will there be any other effects?

8:36 PM
@JohnRennie please see my message above!

@Mathphile Yes, because of time dilation. (though the enjoyment of the music would be rather brief, as air friction burns A to a crisp)

@ACuriousMind Observer A will hear the music at the same speed thought right?

same speed as who?

the speed the music is really playing

8:41 PM
@Mathphile yes

From what I know (which is less than you rn) I think machine learning can be quite mathematical when you get down to it

hmm now that i think about it observer A won't hear the music at the same speed

0

Several questions that I am asking at this time are either bad, or they are beautiful, or they are correct and are deleted (as this morning) and I have the following message. Everyone will make the assessments they deem appropriate, but I blindly believe that if someone really needs help you c...

@Luyw give me some context and I might
I’m actually in a car with 4 graduate physicists from Cambridge rn
It’s humbling to be the stupidest in the car

as the air particles that are carrying the sound waves would not be always moving at the same speed relative to the speaker and observer A

8:46 PM
Oh btw @NovaliumCompany not many of us here are familiar with machine learning. It might be worth going over to the machine learning stack exchange because they will definitely have a better conceptual grasp that they could explain to you

@ACuriousMind is is possible that observer A may not hear any sound at all

@Mathphile Well, this entire scenario has to assume some unrealistic air with no friction to begin with, so it is not clear whether 0.9c is more or less than the speed of sound in this case :P
Don't try too hard to apply physics to an impossible scenario, it rarely ends well

@ACuriousMind i guess you're right
but if he is moving in a continuous orbit, i think he should be able to hear the sound at some point

hbärs <3

9:05 PM
<3

@ACuriousMind off topic q: what's your favourite math topic if you had to choose one?

Probably group & representation theory, though I also have a soft spot for abstract nonsense (category theory)

haha interesting - i've heard category theory is probably ~ as abstract as you could go
@ACuriousMind have you used ideas from group & representation theory in physics at all?

@kylecampbell Sure, groups of symmetries/transformations are very common after all!
We even have a tag:

in particle physics or what specifically?

9:17 PM
It's everywhere, really, but yes, also in particle physics

really hey
interesting
I didn't know it was that common

Symmetry is everywhere, and symmetries are usually groups or algebras that are represented on the space of state of your physical system
that's a general principle rather independent of the specific physics. What varies is what the representation theory tells you. Sometimes its full machinery is overkill, sometimes it's not.

that seems pretty intuitive - even in a classical physics problem, you could have a time-translational symmetry
ah interesting
what's an example where it might be overkill?

9:32 PM
I don't actually have an example off the top off my head :P Maybe "overkill" was the wrong word. It's more like...sometimes you can arrive at the same conclusion without ever mentioning the words "group" or "representation"

right

E.g. compare physics.stackexchange.com/a/265646/50583 to physics.stackexchange.com/a/46663/50583. The latter answer does give a reason why photon have no spin-0 state, and it doesn't use representation theory at all.

ah I see

another off-topic q: if you had to relearn your favourite math topic, how would you do it differently?
:^)

i feel like we've now made it on-topic

10:09 PM
@ACuriousMind what kind of symmetry do energies stem from?

I'm not sure I understand the question, but energy is the charge of time translation symmetry.

charge?

charge in the sense of Noether charge, i.e. the conserved quantity associated with the symmetry through Noether's theorem

10:24 PM
Noether should have been the one to prove the ether theory wrong
@JakeRose Thanks for the advice. I won't dive too deep into ML. I just want to learn the basic concepts.

Sigh I dropped my coffee right after buying it

@NovaliumCompany why? i feel like michelson-morley did a fine job
the aether specifically was something that was only going to be shown (or not shown) experimentally anyways i think. Noether was quite far from experimental work.

it looks to me like some new experiments (QM level) are shedding new light on the concept of an ether. think science is going to return to the idea. roughly, it already has, using different terminology.

explain?
special relativity eliminated the need for an aether to explain wave propagation

10:40 PM
was just reviewing a document that quoted einstein saying SR/GR do not preclude the concept of an ether. this is a small minority view. but agree with it from my own research & think new experiments are resuscitating the idea. en.wikipedia.org/wiki/…

interesting
in that case, I learned something new today
do you have any examples of recent experiments?

:) glad you think so. caution that almost nobody else feels this way. have been posting experiments here over the years, they generally get little reaction. but new/ recent ones are even more dramatic. let me dig up again at least 1 for you.

well, I'm always open to hearing new ideas

lol then you are not really like most scientists/ physicists/ phds :P

I would like to see this also please.

10:45 PM
Aw crap, coolmathgames is getting shut down in feb 2020 since flash will die
how are people going to learn math

the old fashion way?

@skullpetrol The boring way you mean...

@vzn could you point me to where they're referencing the idea of an aether?

In seriousness I never really understood how HTML5 is supposed to replace flash. I mean nowadays there are few "HTML5 games" compared to the Flash games that were everywhere in the early internet

10:49 PM
@SirCumference for most, yes

@kylecampbell ok, it takes reading between the lines. am not an expert myself in this and even experts would argue over this experiment. however, it sounds to me like they are manipulating space waves independent of atomic measurements so to speak, both generation and "reception". but this reception is subtly different than what is specially called measurement in QM.

> a document that quoted einstein saying SR/GR do not preclude the concept of an ether...

@skullpetrol Am I the only one who found those games fun in 5th grade tho?
We just need them for higher math

@skullpetrol the wikipedia link has multiple refs. have another one came across recently, will dig it up for anyone with serious interest. its a deep topic.

@SirCumference now that would be a huge leap forward

10:56 PM
indeed. textbooks are for nerds, math games are cool
imagine defeating a dragon by remembering the hundreds of topology definitions

there's a lot of programming work to be done before that will happen