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12:17 AM
Q: Can a new Pysics SE 093 for beginners be created?

MuzeCan a simple physics SE be designed for people who have little or no training who are enthusiasts none the less? That way moderators can instant migrate lessor questions there instead of instant close? Consisting Lamond of answers and questions it would provide a new Physics093.beta for beginners...

@ACuriousMind I need your input on something
I feel like I learn better when I take notes in latex versus with pen and paper but all these modern studies claim that writing notes down (instead of typing) is better for learning.
dunno which to choose
do I listen to scientific studies or my own experience?
@obe Your own experience, of course. Consider whether your feeling is actually backed by facts, though (you might feel that you learn better but not actually produce better results, for instance).
I think the reason is because when I type things I'm constantly re-reading them to see if I made any typos or mistakes, which is a form of repetition (helping me retain the information better) where as when I write I don't even re-read except for when I'm reviewing for a test or something.
also because my handwriting is the worst xD
I suppose those are facts to back up my experience so I'll type. Thanks :)
12:34 AM
Does anyone here know black magic? I have an Analysis exam tomorrow
@BernardoMeurer Relax :)
@KyleKanos If I might say, I don't know the class of instruments at issue in physics.stackexchange.com/questions/308379, but my impression of the question is that the OP wants someone to give him step-by-step instructions for the particular tool in front of him.
In other words that this is a no-effort 'do my thinking for me' question.
Perhaps I'm wrong about that, but in that case the OP is doing a fairly poor job of communicating.
@ACuriousMind On a realist scenario I get 13/20, which is a passing grade
So i think I will be okay
Unless my asshole prof throws in some black magic question, like one year where to solve a primitive you had to know the derivative of the arctanh x
@BernardoMeurer Using the exponential forms you can recover the trig and hyperbolic primitives long hand if you have to. But it burns up a lot of page and clock time.
@dmckee it was basically $$\int\frac{1}{1-u^2}du$$, that's $\tanh^{-1}(x)$
That was after you did some usub that I no longer remember, but that shouldn't matter
Problem is, we didn't even learn hyperbolic trig functions, lol
12:43 AM
I never learned them either.
What are they useful for anyway?
So I was reliant on those exponentials forms the few times they came up.
Can confirm that up to calc 3 we saw and glazed past them. Would probably learn them later if I kept going.
@dmckee How would I solve that having no clue that is has anything to do with the arctanh?
@Anon234_4521 They seem to be this hazy mathematical tool that everyone knows about yet no one uses :P
@BernardoMeurer They show up in relativity (the Lorentz transform can be written that way) and as the general form of the wavefunction in classically-forbidden regions in SE problems.
12:45 AM
@BernardoMeurer You wouldn't. solving integrals is all about memorizing solutions, it's boring and I'll never understand why the people at math.SE seem to think it's an art :P
@ACuriousMind Lol :P
@dmckee I see, that's cool
@ACuriousMind The art, such as it is, lies in solving as many integrals as possible after having memorized as few primitive forms and methods as possible. AKA the art of being a lazy, but clever bum.
In their defense, if you're solving the integral then you're not memorizing and using the solution. Some people like the process of determining how they should go about solving an integral, taking a derivative, etc. It's one of the reasons people find math fascinating; i.e. they get to feel smart, I guess.
Integrals are like picking wheat
No one likes it, but without it we can't have shit beer
Hello! In my physics class, we are beginning to learn about dark matter, and I asked whether or not dark matter existed only in the vacuum of space or if it was present in other parts of the universe, like on Earth. He said that it acts kind of like a ghost in a way and moves through Earth. Is this a good description? And if so, why does it do this?
12:58 AM
@ACuriousMind Since your heart is made of dark matter why don't you answer this one?
@KristinaL Since we don't really know what dark matter is yet no one can answer that.
Awh dangit. How do we know that dark matter exists in the first place? Is it just so the math adds up?
@ACuriousMind You need new facebook pictures man, stalking you is boring
1:14 AM
Q: Is there evidence of dark matter in our galaxy?

Arpad HorvathIs there evidence of dark matter in our galaxy? How can we measure this, say, how many percent of the center of our galaxy is dark matter? I did not find the answer in the question What's Dark Matter and Who Discovered it.

@ACuriousMind Can you bring Chris White back please?
@BernardoMeurer I'm afraid Summon Chris White is not on my spell list
@ACuriousMind :(
You know, the other day I was doing something with Boost's Multiprecision library
and it was hell on earth to work with
and I just missed having Chris White telling me I was a shithead for using Boost at all
So then I just switched to GMP on pure C to make him proud
Thank you @ACuriousMind
but then the project got too boring even for me and I went to have a beer
2 hours later…
3:03 AM
@JohnRennie I'm more surprised that no one in the history of this chat has suggested that...
3:31 AM
@thoughtforfood Please don't bring politics here
Jan 12 at 8:37, by John Rennie
In the mean time please restrict discussions of politics to the Haven't You Got Anything Better To Do But Talk About Frakking Trump Stack Exchange
3 messages moved to Trash
@ACuriousMind I suppose we can consider it official policy to keep politics out of this chat room, then
as of now
@DavidZ At the very least the sort of politics where the first sentence involves the word "troll", yes.
3:37 AM
@ACuriousMind good call
Ok, sorry.
Have you guys seen this?
Feel free to move it to the Trash :P
> This might be described as the probability a qubit is either 1 or 0, but it's actually more subtle than that and relies on a property intrinsic to quantum physics that is impossible to emulate using simple probabilities.
(!) lol cites smbc/ aaronson cartoon, talk about "pop science"...! o_O
Please cast flags only on actually offensive content.
3:57 AM
@ACuriousMind the system sent me an invitation to the Trash room?
Better to error on the side of niceness? @DavidZ
@thoughtforfood I'm not sure what you mean
You said it was a "good" call.
"Good" in what sense? @DavidZ
@thoughtforfood In the sense that discussions of politics where the first sentence involves the word "troll" are not appropriate here
Like I said
4:15 AM
@DavidZ science is made by humans. (hence) sometimes theres a fine line )( between science & politics.
That's exactly what this president intends to test.
For the next 4 years.
@thought hi welcome, havent seen you around in here, what brings you to the room?
Just browsing :-)
@thoughtforfood noticed you cited notoimmigration.com. & mentioned 0celo7. howd you notice him?
4:31 AM
He's a legend.
@thoughtforfood really? how so? did you ever chat?
Yup..he's out spoken and generally misunderstood.
In real life I know many people like that.
Oooh, neat, a superstar :D
Howdy @Kaumudi.H
@SirCumference Hi :-) Wadup?
Nothing much, just studying right now
4:42 AM
Last night dream (h bar section):
In physics stack exchange, I asked a question concerning the physics of powder game. I then get banned from h bar for 60 days by N(forgot). Later on, a mistake was found. David Z then commented I should wrote a small infographic explaining the intent of my question and N(forgot) will understood and unban me. Meanwhile, it seems some users are taking advantage of the situation. They (a user named Doctor and (forgot)) impersonate as me by changing their display name to Secret, and wrote some trollish comments. N(forgot) was initially fooled and responded that it is my choice to (forgot). Late
Clearly, you spend too much time hereabouts :-P
I remember a weird dream I had, where this was an actual bar and 0celo was the bartender
But he wouldn't serve me a drink unless I solved one of his riddles, which was a math problem involving hieroglyphics.
If anything, Bernardo should've been the bartender.
4:50 AM
@SirCumference I had a similar dream too.
how did he look in your dream? lol
@obe Like revolver ocelot :P
I do recall that me and a few others here had to escape from the stack exchange network. HDE was there and had some weird portal to space, so someone else and I used it to escape to Jupiter
in the dream?
John Rennie and ACM tried to strike us down with a giant laser
I guess that's what happens when I'm on this chat too late
4:52 AM
was I in it?
Don't recall the dream too much :/
I think ACM was trying to show me that it was physically impossible for me to survive, but I escaped anyways
I am also annoyed that I may not have been present--you should've seen me eating or cooking or something. Food.
reading through old chat logs, some conversations I bookmarked from before are really funny xD
4:56 AM
I had this dream that my professor was a snake, but because I couldn't speak Parseltongue I failed the class
did the other students know how to speak it?
and that must have been a long dream since a class would be like 3 months long.
@obe Yep
Dreams spanning more than one day (in the dream's perspective) is quite common for me. It is quite interesting how in real life only a few hours have passed
I remember the last time we had a dream conversation
You ever have a dream where something really good happens, but then you wake up and realize it wasn't real?
5:02 AM
Jan 13 '16 at 22:57, by FenderLesPaul
I had a dream that @0celo7 was touching me relatively
@obe What the hell?
taken out of context ofc. xD
oh wtf lol
Jan 13 '16 at 22:58, by FenderLesPaul
@0celo7 just told me it wasn't a dream
Very rare for me nowadays. My real life is quite steady atm, thus via Mechanism 1, my dreams tend to do things opposite to it.

Sometimes, I am aware I am dreaming thus I know whatever too good to be true thing is not real

Having said that, I once have a buffet dream where I have ate the best ever buffet (I cannot believe how my brain managed to simulate all the taste. The oxtail soup is the best one I ever had)
@SirCumference or the opposite
most of the time it's something really bad for me.
but unreal ofc.
5:19 AM
Last night dream (Harry potter section):
In the Warner Bros Studio set for the Hogwatz grand dinning room (where there are now beds filled in it). A female professor (resembles professor Minerva McGonagall) taught the class some spells. Later on, professor Remus Lupin entered the hall to taught the kids some other spells. Having been through this scene before, I knew shortly after, some dark wizard will enter and terrorise the place. The dream, however played out differently this time. While yes there is still something bad is going to happen, this time it is the full moon and Lupin becomes a werewolf. He end up attacking the clas
To understand why the 1st girl will not survive, the dream give an explanation that dark magic wound never heal. This is kinda surprising to me as in the dream I thought they can put a prosthetic on her to replace her lost leg. This suggests the werewolf's attack has some kind of toxic dark magic component in it, which will become irreversible if the wound is too deep.
Level 1: x^2 y^(x+1) y' - y^2 x^(y+2) lnx y' = y^3 x^(y+1) - x^2 y^(x+2) lny
good luck solving it
and hope that I didn't make any mistakes
@anonymous @Secret ^
How Complex Can ODE Get? part 1
1 hour later…
6:27 AM
@DHMO I've forgotten all about differential equations :P I'll revise next week and solve it then:D
@BernardoMeurer You don't need to know hyperbolic functions to solve that. Just use logarithms!
@DHMO Are you sure you get all the terms correctly? wolfram alpha failed to provide a closed form solution despite it seemed to be able to plot something
6:45 AM
@thoughtforfood thanks for posting the link. Amazing how quickly this got organized.
also interesting to note the order in which signatories are listed.
@Secret Wolfram alpha isn't particularly good at solving differential equations. It failed on many occasions before even though I could find the solution from other sources.
For that equation, I tried to reverse the differentiation of $x^y$ and $y^x$ but there are extra powers of x and y that stopped me from doing so.
It is also quite intwined that I am not sure whether it is separable
7:07 AM
Question, is metallic hydrogen synonymous with degenerate hydrogen?
The pressure achieved in the recent experiment is far, far lower than the pressure needed to create degenerate matter.
Hmm, Wikipedia says it is a type of degenerate gas
Indeed it does. I'm not sure what it means by that but it is most certainly not degenerate matter in the sense of the matter fromed in neutron stars.
@JohnRennie It doesn't need to be neutron degenerate
Ah, OK, I assumed that's what you meant.
7:14 AM
So I guess metallic hydrogen refers to electron degenerate hydrogen?
Q: Why is metallic hydrogen degenerate matter?

zergsomgWhy is metallic hydrogen considered a form of degenerate matter, akin to neutronium and electron-degenerate matter? I can understand that for the other two, degeneracy pressure is the only force countering inward gravity for very massive stars, but how does this concept also apply to super pressu...

Ok, on Googling I see the phrase degenerate matter widely used in connection with metallic hydrogen.
I just really wish Wikipedia would cite its sources more often...
It says gas giants like Jupiter contain metallic hydrogen. I do know that gas giants like Jupiter are electron degenerate, due to their low temperatures
I guess I'll just ask a question
Is it possible to have some substance with a ridiculously high energy barrier that only high pressures can overcome, but a ridiculously low final state such that once it forms, it will stay stable even if you depressurise it?
Wait, can degenerate gases become liquids?
@JohnRennie Am I the only one in favor of this change?
7:34 AM
@SirCumference I would guess most people don't really care what the chat room is called and see no reason to change the name. However I like the idea and ten pople have starred my comment suggesting it.
I was going to propose it at the next chat session.
7:51 AM
Q: Are there reactions (physical or chemical) such that the product remains stable even after it is depressurised?

SecretIn transition state theory, the kinetics and thermodynamics of most reactions are controlled by the height of the transition state barrier and the difference in energy level between the reactants and the products. Some reactions have very high energy barriers such that extreme conditions are nee...

8:07 AM
@KyleKanos Commented on the post.
8:39 AM
(x^(y+1)xlnx - x^2 y^x)y' = y^(x+2)lny - x^y y^2
@Secret @anonymous ^ corrected
Level 2: 4x^2 y'' + 5x y' + 15y = 0
sorry, 5 -> 10
Hey, everyone :-)
Morning :-)
8:56 AM
Good morning :-)
@Secret Pressurise and heat graphite and it turns to diamond. At STP diamond is metastable but with a long lifetime.
@ZeroTheHero you're welcome
@JohnRennie I know, but the question is trying to ask about a product formed from high pressure that end up being thermodynamically more stable than the starting material, analogous to products with exothermic reactions formed by heating and other means
Chemists have always been interested in diamonds but their wives have been more interested :P
The energy profile of such reaction will have a very high energy barrier for the transition state, but the product is at a lower energy than the starting material
I am wondering why we seemed not able to have pressure driven reactions with this sort of energy profile, given how most pressured form products tend to explosively dcomposes after depressurisation (and for the case of diamond, it is actually thermodynamically less stable despite having a long life time)
9:08 AM
@Secret Start with diamond, heat it to not quite such a high temperature and it will turn to graphite. Then cool back to room temp and the graphite will be stable.
@JohnRennie Really? Diamond turns to graphite on heating ? I thought diamond is inert even at high temperatures..
But that's heat, or is it basically the same as applying pressure in a thermodynamic point of view (NB its not ideal gas, thus I am not very certain on how the pressure will be related to energy input other than it will be in the form of work)
@anonymous No, it is actually possible to combust diamond at high temperatures
so diamonds are not forever, it just happens it is so metastable that it takes a long time (millions of years?) before it convert slowly to graphite at ambient conditions
@Secret How high? Does combustion of diamond form graphite ?
@anonymous I forgot, it's more than 4000C, and if you combust it under oxygen, it forms $\ce{CO2}$ and all that usual stuff when you combust any carbon rich compound.
@Secret What if I heat it in absence of oxygen ? Will it form graphite at 4000 C ?
9:15 AM
@anonymous Well, it should rearrange it to graphite
Graphite to Diamond is an exothermic process. I don't see how heating helps here.
Maybe it needs some activation energy? I still can't believe that you can go from diamond to graphite by heating it. Do you have any source for it?
Q: How covalent bonding lower the energy of potential energy?

Jimenez KimConsidering the potential energy of interacting particles, how does covalent bonding lower the energy of the system?

that is so difficult to answer
the abstract is more than enough for the question
@Secret Thanks. I think the heat needed is to provide sufficient activation energy for the reaction. The reaction is spontaneous but would normally take a long time. I see now!
9:26 AM
The question I asked in CSE, is then to find a spontaneous reaction which requires huge pressure instead of heat to activate it, so that the resulting product will be stable under decompression
9:39 AM
@Secret Do you know about fullerenes ? Are fullerenes less stable than graphite or more stable ?
@anonymous most carbon allotropes are less stable than graphite at ambient conditions
@DHMO The best I can get it down to is $x(xx^y(y'\ln x)+yx^y (yx^{-1})))+y(yy^x(ln y)+xy^x(xy^{-1}y'))=0$ However, there are some extra x and y thus I still cannot reverse the product rule. I think I need a hint on what I need to do. Do I need some fancy substitution?
@Secret So, what is it that makes graphite so stable? Is there anything special about its structure other than the aromaticity ?
@anonymous I have not read into much detail about that. However for the case of fullerenes, the ring strain could be a factor
@Secret I have no idea how one would go about doing it straightforwardly
I constructed it from the solution
What does the solution look like. My final suspicion is that this might be an exact ODE, otherwise we pretty much exhaust most of the methods in analytically solving nonlinear ODEs
9:50 AM
the solution is in the form of f(x) = g(y)
when I see ln x, I substitute it away
(I haven't tried. I'm just talking)
@DHMO Where is your problem from? Any book ?
@anonymous I created it
10:06 AM
@DHMO In that case it might not be able to solve it...
10:17 AM
The ODE is inexact, just checked their mixed derivatives they don't match up in any extent
There's still one more thing can be tried (which is too time consuming for me atm thus I will deal with it later)
An inexact differential equation is a differential equation of the form M ( x , y ) d x + N ( x , y ) d y = 0 , where ∂ M ∂ y ≠ ∂ N ...
@anonymous I mean, I have an answer
and I differentiated each side
@DHMO You solved it ?
Tell me how :)
@anonymous no, I created the question from the solution and I do not know how I would solve it straightforwardly
@Secret if you want a big hint: answer is f(x) = f(y)
@DHMO Oh I see. In that case one needs to able to spot the exact differential which is sometimes really difficult.
11:20 AM
Isn't this answer rather problematic? It sort of implies that anything not follow that equation is "not exactly a wave". Am i misunderstanding it?
And since the Q title is "Do all waves of any kind satisfy the principle of superposition?", the answer should be a clear "No". Instead it feels like a "yes, unless it's not exactly a wave".
11:40 AM
@Secret corrected: x (x^y ylnx - x y^x)y' = x y^(x+1) lny - x^y y^2
I am sorry for the confusion
@Secret wolfram alpha confirmed it, so please don't cheat.
@anonymous ^
11:54 AM
@JohnR: Interested in an embarrassing (if u get it wrong) tongue-twister?
Go on ...
"I'm a pleasant pheasant plucker and nobody's ever heard a pleasant mother pheasant plucker like me say the F-word"
I'm afraid I learned that one as a schoolboy decades ago :-)
Oh, is that one famous?
There's a song about it made by one of the vlogbrothers many years ago:
I'm not a pheasant plucker
I'm a pheasant plucker's mate
I'm only plucking pheasant
Cos the pheasant plucker's late
Oh, well, it's not the same. This one contains the word "mother", which will screw you if you get it wrong.
Try to say it 5 times faster than normal. You will find that u'll run into trouble soon enough... :-)
I know a Thai tongue twister (not obscene)
11:59 AM
@Rumplestillskin:hi :-)
@Kaumudi.H kai kai kai kai
@JohnRennie hi there!
Are the pronunciations different?
I take it this is the main room? :) @JohnRennie
@DHMO Any idea why H- attacks carbonyl carbon rather than abstract the acidic hydrogen ?
12:00 PM
@Kaumudi.H Thai is a tonal language so the meaning depends on the tone of voice you use. The four words have (a) a rising tone (b) a falling tone (c) a high tone and (d) a low tone.
During reduction of amide by LAH
But unless you know to listen out for the tone the four words sound identical.
@JohnRennie Oh, that's interesting, I haven't heard a tongue-twister involving tones before.
@anonymous the hydrogen isn't acidic
the nitrogen is basic
@DHMO Why not? It is attached to electronegative nitrogen. Isn't it ? The hydrogen gains a partial positive charge
12:02 PM
It's not really a tongue twister - more of a test for non-native Thai speakers to check how well they can speak Thai.
Is anyone interested in Kruskal Szekeres coordinates and how to obtain a Kruskal Szekeres diagram using CAS? I've updated my question with a poor effort of mine and would really like to see how it is done.

Any and all help is a welcome gift!
@DHMO Finally, I can reverse the product rule of this one: The answer is $x^y=y^x$
@JohnRennie Ohh, nice.
@Rumplestillskin the problem with your question is that it seems to be about plotting software i.e. how you would get a computer to draw the diagram. It isn't really a question about physics.
I see what you are saying John! It is quite the conundrum!
12:04 PM
@anonymous interesting question...
I guess N is not so electronegative anyway
@Secret how?
@Rumplestillskin I have no idea what software you would use to draw the diagram or how to program it. But why would you bother? The KS coordinates are very useful in understanding the causal structure of the Schwarzschild metric. Questions on that are on-topic, but just how to draw it?
That doesn't seem very interesting.
@DHMO oops sorry I mean: $y^x=x^y+C$, I forgot the constant.
@Secret how?
The trick is to notice that the derivative of $x^y$ and $y^x$ is buried in the ODE. Therefore, after rearranging the 4 terms in the ODE, you get basically $(x^y)'=(y^x)'$
12:10 PM
That is, I am basically doing the product rule backwards
I see
@JohnRennie It's just one of them things that I would like to be able to do. There is a lot to be learned from the KS Diagram and some peculiar phenomenon going on. Also, I would like to give a short talk this year on Schwarzschild geometry and it would nice to display a cool image of KS diagram and show people how to do it! I guess I will have to keep battling!!
2 hours later…
1:51 PM
Lol Wolfram alpha
2:29 PM
@anonymous QUICK
last help before exam
starts in 30 minutes
do you know about series?
@BernardoMeurer @DHMO may help you quickly!
2 hours later…
4:01 PM
@BernardoMeurer Dammit! Sorry man, I was outside home. Didn't see your message earlier :/. Hope your exam goes well.
4:31 PM
Can anybody help me in this
I want to know what would be the angle between its velocity and string when it becomes taut
After that I will try myself
Well, if you show what you tried first, perhaps someone can help you work out what's keeping you from being able to do it yourself
@DavidZ I am not getting anything
Oh, I got it :-P
4:52 PM
Ah, good
Jan 13 '16 at 22:57, by FenderLesPaul
I had a dream that @0celo7 was touching me relatively
1 hour later…
5:59 PM
I'm doing some experiments on resonance frequencies of buildings and I just graphed one amplitude/frequency graph and it some how has 2 peaks?
Anyone have any idea why this might be happening?
6:14 PM
@Wdoctor123 Can you show your graph and explain your procedure of graphing ? Remember that resonance occurs at integral multiples of fundamental frequency too.
how do i send images?
well that's the link to the graph
@Wdoctor123 The "upload..." button right next to the "send" on.
I have to admit that I got to where I didn't see it anymore and had to ask, too.
I took 20cm ruler, and attached it to the vibrating machines and increased frequency little by little, each time recording the amplitude
there's no upload button
@dmckee Wdoctor doesn't have enough rep to use upload option.
6:25 PM
Hm. Forgot about that.
@anonymous how much does it take?
did you see the image?
@DavidZ 100 rep.
Ah, I forgot. Thanks.
@Wdoctor123 See, notice that the peaks occur at 3 and 9 Hz approx
They are integer multiples
I'm not sure how accurate your values are
6:28 PM
0.01+/- cm
I should I explain that?
got other result with different height
0.1 +/- cm *
@Wdoctor123 It seems like 3 Hz is your fundamental frequency from your first graph and 2Hz is the fundamental from your second graph
Also remember that the fundamental resonating frequency does not have the highest amplitude
(at least not always)
I think this could be a good question
for the main site
I am not an expert on this topic...
wait so height amplitude isn't the resonance point?
@Wdoctor123 Height amplitude is at the resonance point but there are multiple resonance frequencies.
I thought resonance frequency is when the thing vibrates most violently
so i guess it was just one of the harmonics i guess
@Wdoctor123 It does vibrate "most" violently in a small region. There are several maxima present in the graph at integer multiples.
@Wdoctor123 Exactly.
6:36 PM
So explanation would be
There are more than 1 resonance frequencies
but there's one dominant one
@Wdoctor123 And the dominant one may not be the fundamental frequency :) Anyway I would advise you to ask this question on the main site again. You may get much better answers!
ok thanks :)
1 hour later…
8:48 PM
@vzn Sounds like a string theory thing
9:07 PM
@obe Just asking, does the background you're making include any public domain or creative commons graphics?
@SirCumference its been discussed for a few years in mostly theoretical terms. dont really think it originates in string theory much. it seems to be more an exotic area called "digital physics" that goes back decades... possibly roots more in CS...!
9:24 PM
@SirCumference all original as of right now except the font for the title and other texts
@obe K, do you know the name of the font?
alien league
or something like that
it's not official so who cares xd
Yeah but still, copyright is a little scary :/
@ACuriousMind Could you answer a question?
9:49 PM
i thought everyone has been over this already
@obe Ok
Btw can I ask you something?
@SirCumference Dude, what about "just ask your question" is so hard to grasp that we have to do this routine of "Can I ask you something?" every time you have a question :P
@ACuriousMind Well, if no one here knows the answer, then I wasted my finger muscles typing it :P
...and how are you going to tell if they know the answer before you asked them the damn question?
Well, anyways, the question is about more cosmology...wooo
Specifically, can the observable universe ever shrink?
10:05 PM
@SirCumference Define "shrink"
@ACuriousMind Its radius decreases
Its radius as measured by whom?
By the person at the center of the observable universe
10:08 PM
@SirCumference Eh, I'm not convinced that's a well-defined question, but depending on what weird shit the scale factor does, it may well be that the observer at one time will measure a larger radius than an observer at later times
@ACuriousMind The confusing thing is what wikipedia says in its Big Rip article
> A universe dominated by phantom energy is an accelerating universe, expanding at an ever-increasing rate. However, this implies that the size of the observable universe is continually shrinking; the distance to the edge of the observable universe which is moving away at the speed of light from any point moves ever closer.
> When the size of the observable universe becomes smaller than any particular structure, no interaction by any of the fundamental forces can occur between the most remote parts of the structure. When these interactions become impossible, the structure is "ripped apart". The model implies that after a finite time there will be a final singularity, called the "Big Rip", in which all distances diverge to infinite values.
This has a bunch of [why] tags on wikipedia, btw
Now I'm really confused. First, I thought the Big Rip occurs once the scale factor reaches infinity, rather than when the observable universe becomes too small?
Or do these occur at the same time?
10:24 PM
@SirCumference Thinking on it, the observable universe cannot shrink - its comoving distance is just $\int_0^t \frac{\mathrm{d}t}{a(t)}$, which is monotonically increasing.
See Wiki if you like Wiki quotes better :P (In this case, the statement "The observed size of the universe always increases" is sourced to two different sources")
@ACuriousMind Are you sure that isn't only for a universe with $w \geq -1$?
@SirCumference Why would the comoving distance of the horizon care for what value $w$ has?
No idea, otherwise Wikipedia is just flat out wrong
$w$ determines how $a(t)$ behaves, but once you have $a(t)$, the distances don't care what $w$ is.
@SirCumference Dude, the "particle horizon" article contradicts the Big Rip article. So one of them is flat out wrong either way, so don't get hung up on it :P
@ACuriousMind Oh fun...
@ACuriousMind Yeah, that feels kind of obvious...
Well, guess you can't trust anything these days
10:29 PM
In general, I tend not to trust Wiki articles with multiple [why?] or [citation needed] :P

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