10:00 PM
@ACuriousMind How was Wacken?

I have a 3-digit lock. The current code is 137 because I used to like physics. What should I change it to now?

@WilliamBulmer Ah, the trace must be an actual energy density because the whole EM stress-energy tensor must be massless. The trace of the full tensor is $\rho +\mathrm{tr}(\sigma) = 0$
@0celo7 666
@BernardMeurer Really fucking awesome!

@ACuriousMind It should be math related, and I don't want to kill Reb with satanic stuff

@ACuriousMind Thanks. That looks very much related to what I am studying.
@ACuriousMind Hahah, I am not yet proficient in GR. That one will take me a little while to understand

10:02 PM
@0celo7 How could any three numbers be more "math-related" than any other?

It appears I have much to think about

@ACuriousMind 137 is physics related

@ACuriousMind thank you for your insight

@BernardMeurer I'm going again next year

@ACuriousMind Sounds great! I swear I'm planning on joining :)

10:06 PM
@BernardMeurer Be sure to bring a high tolerance for mud ;)
The weather was actually pretty nice most of the time, but it rained every morning
Tickets for next year are almost sold out already btw

@WilliamBulmer skimmed wikipedia re stress tensors Cauchy/ Maxwell. alas not familiar with this area. are you the 1st to make a correlation? seems interesting on the face. is the "Cauchy Stress tensor of Classical Field Theory" a thing somewhere?

10:22 PM
@ACuriousMind I know :/

How is 137 physics related ?

I just need a couple more days to get the money
I spent all my bucks on bureaucracy for my visa

Well, I'm sure there will be sales e.g. on ebay later
@PhysicsGuy I'm guessing he's referring to the $\alpha = 1/137$ numerology.

Isn't it wacken who permabans people who buy from third parties?

@WilliamBulmer suggest dont give up on dimensional analysis despite other "advice", it is downplayed or dismissed by some theorists but some others have used it to great effect, had this debate with slereah weeks back, could dig up a link of it being used by serious scientist. think there is some key analogy of dynamics of spacetime that is yet to be made (but right on the verge) & it will open up a lot of new vistas.

10:26 PM
@BernardMeurer The tickets aren't personalized
And I think there's also an official way to resell tickets

@ACuriousMind I remember some festival who went back in time and killed your grandparents if you resold tickets

Don't tell @Slereah
He'll try to get their time machine and it will not end well

Dimensional analysis is a useful tool, not because it gives reliable answers (sometimes it does and sometimes it doesn't and it doesn't give any hints to tell the cases apart), but because it gives you a good enough answer for the purposes of Wheeler's absurb-sounding advice to not start a calculation until you know the answer.

@WilliamBulmer isnt it all essentially "acting" on spacetime?

@BernardMeurer Well, they tried that with personalized tickets a while back but legal difficulties in actually forbidding people to tamper with those and banning resellers led to them not doing that anymore

10:31 PM
@dmckee dimensional analysis is very useful for showing "patterns" of energy dynamics throughout different physical systems where the math is equivalent but the milieus are or seem different.

@ACuriousMind Indeed

@ACuriousMind papa bless

@ACuriousMind Ah, got it.
478.
$1,000 if you guess the significance user54412 Why are 5 and 6 afraid of 7? Because 4, 7 ate. @ChrisWhite Burn 10:33 PM How is that a burn @BernardMeurer you're a meme @ChrisWhite Is that Yoda's version of that number joke? ;) @0celo7 I'm telling him to burn you gafster @0celo7 Your mom's a meme Hey I'm a goofster @vzn The thing about dimensional analysis is that you only know it was right in retrospect: after you done the full calculation or run the test. It's a powerful tool that has occasional silent flops. And the sometimes non-signalling nature of the flops is the reason to treat it with caution. actually that's wrong 10:34 PM @JohnDuffield agree the connection is interesting or even far more than merely interesting. in the way the tip of an iceberg is "interesting". this basic analogy is based on dimensional analysis @ACuriousMind 's the spoofster Bob's the real meme On the other hand, it's a great way to make short plausibility argument when you don't want to deal with a complete treatment. I got a lot of those from profs in graduate school. WEATHER STATIONS How is Bob a meme Hmm 10:35 PM @dmckee its different than solving eqns but goes "hand in hand". theres still lots of hard work with eqns. its a sort of parallel or shadow system of physics/ dynamics analysis. it helps devise eqns. aka einstein "imagination is more important than knowledge." arguably many important eqns from physics were devised wrt dimensional analysis techniques. imho it could be said of eg schroedinger eqn. 6 pages later and I've made no progress on this problem @ACuriousMind The Ćech cohomology is the direct limit of what cohomologies? In the context of manifolds, they're called Čech-de Rham cohomologies @0celo7 Of the Cech cohomologies relative to a given cover, I'd say @vzn That cauchy stress tensor is the spatial part of the stress-energy tensor as ACuriousMind said. It appears in classical field theory, and is part of the Cauchy Momentum Equation, which we use to describe non-relativistic momentum transform in a continuum @WilliamBulmer somehow think these eqns surely have some basic link between relativistic and nonrelativistic @vzn I don't think dimesnional analysis ever suffices as a valid physical argument for why something should be true. As I had said, I think it is more helpful as a compass for finding deeper physical/mathematical relationships 10:39 PM @WilliamBulmer agreed! its not any kind of proof of anything. only an xkcd cartoon would be so naive etc... but it can be something like a semiformalized intuition @vzn They do. As I said, the Cauchy stress tensor is the spatial part of the more general stress-energy tensor Though, I won't hazard anymore of an explanation than that because I am still learning GR @ACuriousMind Will you be willing to hear my flow question tomorrow @WilliamBulmer it seems that there is some small scale eqn of spacetime distortion that might apply/ generalize at larger scales, still yet to be uncovered, and that this is part of the difficulty of merging QM+GR @vzn And yet another vague statement without any meaning. When will one of your hunches ever be precise enough to be actually right/wrong? @vzn I personally have much of a distaste for even using it as intuition for why something should be true. It's a bit like reasoning that Antonio Banderas is Hispanic because his name sounds hispanic 10:42 PM You always do this: Hint at something "still to be uncovered" without actually giving any reason one should expect that @ACuriousMind all great new physics starts as intuition... are you interested or not? (rhetorical! usually so far the answer is not) :P @ACuriousMind theres a reason very precise software engineering full of copious amts of airtight logic is also called development... also in crisp photography etc @vzn Physical intuition is different from blind guessing or prophecies that rival horoscopes in their specificity. I might be interested in - although sceptical of - the former, but I'm not interested in that latter. @ACuriousMind try a real comic/ art instead, just watched the movie, its amusing, it has feynman playing bongos at beginning & think it might have been filmed partly at caltech, is anyone else gonna see it? phdcomics.com/comics.php Maybe I need a book on Riemann surfaces That might hold the key @BernardMeurer to more success @ACuriousMind alas, its impossible to tell the difference early on, its a zen question of science :( 10:47 PM Riemannian surfaces are interesting. I read a paper of Ed Witten where he pointed out, that the Hamiltonian for a particle moving on a riemannian surface is equal to the Laplace-Beltrami Operator. I have an idea! Oh god, I need to write it down before I forget @ACuriousMind the top physicists in the world expect a GR+QM unification and of course lose no time or see no contradiction sniffing at/ criticizing einstein for seeking it for nearly ~½ his life without publishable results. Ahhhh @vzn Einstein was a crank, honestly He was just right at the beginning There is a unification of GR*QM Loop Quantum Gravity 10:50 PM Twistor theory String theory Causal Sets @PhysicsGuy Twistor theory might be my next conquest. There are a lot. @Ocelo7 Thats super ! Causal Dynamical Triangulation After Bott & Tu and this small Ricci flow, then on to sheaf cohomology (@ACuriousMind ) and twistors Then CLN Ricci flow next summer Or Li Geometric Analysis, we'll see. 10:51 PM @Ocelo7 Wait wait wait. Twistor theory in physics has nothing to do with twistors in topology. Huh? @0celo7 lol actually maybe some similarity to your other favorite grothendieck in that way I don't know anything about the man Not even his first name @vzn There is a concrete reason to expect a unification of GR and QFT: both describe our world at different scales, and reductionism has so far served us well in physics - by inference, it is reasonable - though not certain - to expect both of these theories can be reduced to a more fundamental theory of which they'll both be a part. What is your reason for expecting a "small scale eqn of spacetime distortion" and what does that even mean? @ACuriousMind it means nothing until someone figures out what it means, as we both are well aware, still working on that, & thx so much for all the help :P 10:55 PM So just meaningless vagueness, then, as I expected. @Ocelo7 But if you want to study (mathematical) twistors, you maybe like these videos (about supersymmetric gauge theories from a mathematical point of view): @ACuriousMind JD/ WB pointed out some eqns, its a start... rome wasnt built in a day @PhysicsGuy I think he would've been able to get to the rest of the playlist from just a single video ;) @ACuriousMind ........... Yeah @ACuriousMind Youre from Heidelberg ? 10:59 PM @ACuriousMind look, am willing to take a small stab at this/ go out on a limb (for you to immediately saw off...) a lot of eqns point that spacetime itself has a dynamic/ elastic property, and lightwaves are apparently just the small scale distortion of it whereas gravity is the large scale distortion of it. so then, the zen question, how do these two mesh? @PhysicsGuy Not originally, but I currently live there @ACuriousMind Im from Frankfurt Hello, everyone. Does anyone know anything about Hamiltonian actions? You mean classical hamiltonian mechanics ? I'd like to know whether there is a weaker condition than$H^1(M)=0$for there to exist a Hamiltonian action on a manifold$M$. Yes I do, but specifically the symplectic-geometry formulation of it. 11:01 PM @vzn I don't know what "elastic property of spacetime" means, or why you would call lightwaves a "small scale distortion" of spacetime when spacetime is not dynamical in the classical or quantum theorie sof electromagnetism that describe light waves. @WilliamOliver Hi. @ACuriousMind did you see JDs eqn/ analogy or not? speed of light, permissivity/ permittivity of free space etc... nearly same as what WB is "poynting" at :P @ACuriousMind (its always so fun arguing with you because you are nearly always a perfect standin/ representative for The Physics Establishment™) @ACuriousMind sounds awfully like JD @vzn I am not saying they are distortions of space time. I'm currently interested in why the eigenvalues of any stress tensor should have units in energy density, other than mathematical coincidence. @ACuriousMind answered part of it, I think @Alyosha I dont understand that question. Why shouldnt it exist ? 11:09 PM @0celo7 "even a stopped clock is right twice a day" :\ @vzn Oh yes, I've seen that "analogy" from JD many times until I began to ignore him. "Elasticity" is a well-defined notion in continuum mechanics of materials, but it's not a well-defined notion w.r.t. spacetime, at least not in GR. It doesn't actually mean anything to say that "spacetime is elastic", and it doesn't mean anything to say that light is "distortion of spacetime" because there are no equations that would allow that interpretation. 4 JD's not a stopped clock, he's transcended clockdom @ACuriousMind he just posted (some of) the equations, the equations exist but they havent been analyzed/ recognized fully for what they are, agreed that work (both analytical/ interpretive) remains to be done These are figures of speech disguised as the usual pop-sci phrasings of an actual physical theory, except that there's not actual theory behind them. Huh, what is that discussion about ? 11:11 PM @PhysicsGuy Specify "that". @ACuriousMind standard SR/GR is referring to deformations of spacetime. distortion, deformation, tomato, tomaTOE I mean: let Lie group$G$act on symplectic manifold$M$to preserve the symplectic form. Then why must there be a momentum map$\mu:M \to \mathfrak{g}^*$? That is, why must it be possible to associate to every element$X$of$\mathfrak{g}$a function$H$so that the vector field$\tilde{X}$on$M$induced by$X$satisfies$\omega(X,-)=dH$? @vzn Further work on what? What equations specifically are you referring to that "haven't been analyzed/recognized fully"? @ACuriousMind surely easily an entire paper could be written expanding on the "analogy" that you just dismissed. easily in the sense there is that much material but very hard that almost nobody alive could do it or has done it... based on convention @vzn SR is not deforming spacetime, and it's clear that the technical meaning of "distortion of spacetime" is a change in the metric from a "standard metric" understood in the context. What is the technical meaning of "elasticity of spacetime"? 11:13 PM @ACuriousMind all those theories/experiments/ideas which about the elasticity/permeability/etc of spacetime It would be cool if someone would explain this as I'm sure it could be interesting or new and might shed light into string theory. If$H^1(M)=0$then you can just integrate (some vector field relating to$\tilde{X}$) along$M$to get$H$. So you're guaranteed a momentum map in this case. But$H^1(M)=0$is pretty strong a condition. Is there a weaker one? I am become troll. @ACuriousMind you just said distortion of spacetime is not defined, and now you say its clear what the technical meaning is. sorry am getting mixed msgs here :( @vzn No, I said it doesn't mean anything to say light is distortion of spacetime. @vzn No, he did not say that. 11:15 PM Light has some effect on spacetime due to the EM stress-energy, but that's plainly a well-known fact that doesn't deserve any additional "attention" @ACuriousMind Light is distortion or light distorts? @vzn How is it you say that an entire paper could be written when you can't even cite the equations you are talking about? @ACuriousMind it appears the math has been long telling us that light waves themselves are ripple in spacetime.... think its the tip of iceberg of large new theory also merging GR+QM. it will take a genius on the level of einstein to work out the details... o_O @vzn What math @vzn "it appears"? To whom? 11:17 PM This is news to me > it appears the math has been long telling us that light waves themselves are ripple in spacetime There is nothing special about light Reference/equation/*something* needed. Can we at least have an Einstein quote? @PhysicsGuy I've solved part of the problem :D @Alyosha There always must be a momentum map. But, sorry, I am not so familiar with symplectic geometry (and Hamiltonian group actions related to your question). 11:19 PM @0celo7 it seems light and space are not actually different, its a flaw in our (human, that is) habitually dualistic thinking. light is rippling space. I've beaten the answer into the form$\alpha(t)v+\beta(t)Jv$, now I have to determine$\alpha$and$\beta$@vzn WHERE DID YOU GET THAT @0celo7 from the eqns. mixed with intuition etc... @PhysicsGuy Do you know where I could find a proof of that? Hmm, I can get$\alpha^2(t)+\beta^2(t)=1\$

@PhysicsGuy You only get the momentum map if you're given a Hamiltonian group action. The question is when such an action can exist (I don't know the answer, unfortunately).

11:20 PM
@Alyosha :/ Im sorry. But good luck.

@PhysicsGuy Thank you.

@vzn "etc."? What is this et cetera that no one but you and JohnDuffield have access to? Stop weaseling around and write down a single equation substantiating any of the claims you made.
2

@Ocelo7 The Ricci-Flow problem or what

Hi! I'm am confused about a comment that I received here physics.stackexchange.com/questions/273032/… "The thing is, there is no photon at all in QM;" What does this mean?

@PhysicsGuy Yeah

11:22 PM
@ACuriousMind lol some of the striking thing is that maybe most of the eqns are already written down... (but ofc nobody will believe it until something new is derived from the "new" assumptions)

@Ocelo7 What do these equations mean ?

@DanielSank Oh, you're vzn's puppetmaster, too? ;P

@Ocelo7 How could you reduce it that far ?

Nooooo

@ACuriousMind Oh you know it.

11:23 PM
@vzn Where are they written down?

@Ocelo7 Huh ?

I mean, surely I can take a Hamiltonian and do stuff like cavity QED with photon wave functions, perfectly well with just regular first quantization. And I think I can call them photons as well.

see what happens when someone tries to be creative/ imaginative around here? everyone gets paranoid :( o_O

Where do you think this conversation is going?
You've clearly asked for concreteness and you're not getting it.

11:24 PM
@DanielSank Nowhere. I'll stop.
I'm just tired to see this room filled with not only one but more people who clearly have no idea what they are talking about yet try to deceive others into believing physics is a great conspiracy.

@ACuriousMind its not a conspiracy its just not finished yet dude, try not to feel too bad/ upset about that :P

@MikaelKuisma I think Gennaro's comment is supposed to mean that you haven't really a reason to connect those "photons" with the electromagnetic field.
Aside from it working as an effective theory, that is

@Ocelo7 Whats the problem ?

Which is a good reason to use it as an effective theory, but not really satisfactory when used as an answer to what a "photon" is. But maybe it just illustrates that the concept of "photon", just as the concept of "particle" in general, becomes rather hard to nail down when you dig deep enough

Yeah.

11:28 PM
From fundamental perspective of course quantum field theory is probably the way to go, sure. But ok, I think I see the point now.

@ACuriousMind good question. there is an answer. the photon is a localized/ probabilistic measurement of the (spacetime) wavefront. by an atom (or other particle)... the only known spacetime-rippling measuring device that we can interface with

I was writing the answer from somebody who does not know QFT to somebody who does not know it either :)

@MikaelKuisma I think it's actually a good reminder to all us QFT purists that there are such "first quantized" approaches that work rather well in their contexts ;)

I have recently become interested in quantization of fields for practical scientific purposes related to plasmons, I think physics SE is a good sandbox for that.

@MikaelKuisma Yeah just be careful because a lot of the really experienced and knowledgeable people here mix relativity into QFT without telling you they're doing that.
It's really hard to ask questions about quantized fields here without everyone immediately getting into complexities that don't exist in non-relativistic situations.

11:36 PM
I woudn't be needing relativity. Probably not even retardation of fields. Biggest system I have is 3nm and that is huge.
But I have a feeling that I will be needing the concepts.

@MikaelKuisma Again, just be careful because folks here will tell you stuff that's only needed in relativity without saying so explicitly :)
You have to be defensive about that.

@ACuriousMind It was certainly a big surprise to me to find that out (like a month ago, when reading some cavity QED paper). I have not studied any QFT and always thought it as super mystical stuff, and that you 'actually have particles' there. But then, it is just plain old boring wave functions it seems (at least for such sand box model I presented).

@PhysicsGuy Hey
Can
*Can't really talk right now haha, I am at work

@DanielSank Thanks for the tip :)

@PhysicsGuy Now I'm trying to find an ODE in the tangent space that will give me the coefficient functions