The h Bar

0celo7

19:00

@ACuriousMind Ok

I'll take a look

Physics Guy

@dmckee I know

But I want to hear what he says about it

Or not "hear", maybe........read.

John Duffield

@PhysicsGuy : vacuum fluctuations are not the same thing as virtual particles. Vacuum fluctuations are like the random little ripples you see an the surface of the sea. Virtual particles are like you divide the sea up into abstract chunks and say each is a field quantum.

dmckee

I've been part of experiments where we made pions by whacking nuclei with glancing impacts of high energy electrons. The process is dominated by a single feynman diagram in which one of those "not real, just in that math" lines is put on shell by the interaction.

They are real enough to dig out of the their usual context and play with in the laboratory.

Physics Guy

@JohnDuffield......man.....

Im sorry, but........

dmckee

And vacuum fluctuation are just one application of the "just math" of field theories. They are pretty useful when you want to use Drell-Yan as a probe of the anti-quark content of the nucleon sea, which is another thing that I had a very small part in.

0celo7

19:05

@dmckee 's finally snapped

John Duffield

@PhysicsGuy : if you don't like what I said, read what Matt Strassler said: "The best way to approach this concept, I believe, is to forget you ever saw the word “particle” in the term. A virtual particle is not a particle at all."

Also see the Casimir effect re vacuum fluctuations.

Physics Guy

I know the Casimir-Effect, thank you.

John Duffield

Sadly the article refers to virtual photons a lot.

0celo7

@JohnDuffield Are you doing to respond to David?

John Duffield

@0celo7 : respond to what? Please provide a link.

0celo7

19:09

Do you have @dmckee blocked?

John Duffield

No.

0celo7

7 mins ago, by dmckee

I've been part of experiments where we made pions by whacking nuclei with glancing impacts of high energy electrons. The process is dominated by a single feynman diagram in which one of those "not real, just in that math" lines is put on shell by the interaction.

do you not see this?

Bass

@ChrisWhite I think this is not just about the professionals, but also about students and other learners, who might be the silent majority on the site. Those "harmfully wrong" answers can be utterly confusing to students if one does not recognize them as wrong / non-mainstream. You read the answer and you try to understand and learn the (wrong) stuff in it. Then some time later, somewhere else, you read contradicting (but correct) content.

4

John Duffield

@0celo7 : of course I did. But he didn't ping me.

0celo7

@dmckee Hey, I'm trying to hook up some legacy hardware at work

any idea what this is?

Bass

19:12

Maybe you don't remember where you've read the "harmfully wrong" ideas, so it's difficult to go back and challenge them. All you get is a big, big confusion. Of course, you think the error must be yours. Hours and days are wasted.

Physics Guy

@JohnDuffield Ok, I think we talk past each other. And I dont care at all.

dmckee

@0celo7 Parallel port, probably.

Bass

I understand that there is already a censorship system for this problem, the downvoting system. But let's be honest, it seems a little bit ineffective if the "free" upvotes generate five times more rep than the downvotes which cost rep to the voter. Of course everyone wants to earn rep for their time devoted to this site, but if people with 11.5k rep are reluctant to downvote, then you surely have to agree that the downvoting system does not work as intended.

6

dmckee

Used to be used for printers a lot, so some people will call the cable you need a "printer cable".

Physics Guy

@Ocelo7 At the first look i thought it was a car radio

John Duffield

19:13

0celo7 : it looks like a Scart socket.

dmckee

They used to be the "fast" way to connect things. But that was when more than 280 kBaud was "fast".

0celo7

No, I don't think it's SCART

Bass

My personal opinion is 1) let the up-/downvoting system, i.e. the big community, decide which answers are helpful. 2) At the same time give the +3k community a way to remove really bad content. If five +3k users agree that an answer is VLQ, then why should we keep it?

3

0celo7

And parallel port is sufficiently vague to include everything from that era @dmckee

dmckee

@0celo7 There was one predominant connector standard at the time. THey were very common.

Physics Guy

19:15

@JohnDuffield I know that virtual particles are spontaneous excitations in a quantum field, but..youre saying it so strangely...

0celo7

google.com/…

It doesn't look like these though

it's got that strip in the middle

John Duffield

@PhysicsGuy : they aren't. Sorry.

Physics Guy

„Virtual particles are spontaneous excitations of a quantum field.. Real particles are excitations of a quantum field with a resistance which is useful for observation. . Virtual particles are transients that appear in our equations, but not in gauge. Through power supply......, virtual particles can become real" - Frank Wilczek".

@JohnDuffield

Well, lets not call it spontaneous excitation, rather spontaneous fluctuation.

0celo7

@ACuriousMind I think it's beyond me to verify if your counterexample is correct or not

I'm just confused now

and it seems like you have to check all the $d$ cohomologies

John Duffield

@PhysicsGuy : that's wrong. Sorry. Virtual photons are said to be responsible for the electromagnetic interaction between an electron and a proton in the hydrogen atom. But there are no actual spontaneous excitations going on. Photons are not popping into existence. And as for fluctuations, the Coulomb force is not the Casimir effect. The latter is very weak. The former is not.

And now Frank Wilczek will doubtless delete my VLQ post.

Physics Guy

19:28

@JohnDuffield Good, lets stay by the electromanetic interactions. Two electrons moving towards each other, exchange a virtual photon and turn around and go the other way. Imagine it as a Feynman-diagram.

Now, what is happening there, quantum-field-theoretically ?

The virtual photon does not exist anymore, after a time, when the interaction is over.

It was a mathematical way to describe the electromagnetic interaction.

So how do you describe it with a field, if not as a spontaneous fluctuation ?

0celo7

@ACuriousMind And now I'm confused by what we mean by the $d$-cohomology

is it the vertical cohomology on each $K^{p,q}$?

Or is it for each column

ACuriousMind

@0celo7 Well, I thought you meant the cohomology you get when you consider each of the columns as a simple complex in their own right, i.e. $H_{d}^p(K^q)$ is the cohomology of $K^{q,p-1}\to K^{q,p}\to K^{q,p+1}$.

John Duffield

@PhysicsGuy : replace one of the electrons with a proton. Each has an electromagnetic field. They attract one another to form a hydrogen atom. This doesn't have much in the way of an electromagnetic field. Because the electron and proton have "exchanged field". But they haven't been throwing photons back and forth. The exchange mechanism "works". But those virtual photons are not spontaneous fluctuations. They are not short-lived real photons. They are abstract chunks of electromagnetic field.

0celo7

@ACuriousMind I think I agree that that's what I (Bott & Tu) meant :D

Physics Guy

@ACuriousMind Please say something to our discussion.

ACuriousMind

19:35

@PhysicsGuy No. I do not contribute to discussions with JohnDuffield, that has proven exceptionally futile in the past.

Physics Guy

@JohnDuffield Then please define a virtual particle now, you can use every jargon word that you want. Tell me, how would you define a virtual particle ?

@ACuriousMind Oh, okay.

John Duffield

@PhysicsGuy : as an arbitrary abstract part of a real field.

0celo7

lol

In what sense are quantum fields "real"

I don't think John knows what a definition is

Physics Guy

@Ocelo7 I dont know. I am confused, now.

@Ocelo7 Okay, well, there are real and complex fields, thats clear. But that definition is new to me.

John Duffield

@0celo7 : the photon field is real. Photons are real. Light is real.

0celo7

19:38

@JohnDuffield Photons are not a quantum field

Physics Guy

Real in a mathematical or a physical way ?

John Duffield

@PhysicsGuy : real in a physical way.

0celo7

And giving examples does not define it

This is elementary

@ACuriousMind So what are the $d$-cohomologies of your complexes

Physics Guy

What do you want with an "arbitrary abstract part". Thats just a....sentence. What does this mean to you ?

@JohnDuffield

ACuriousMind

@Bass Well, but "it gets deleted whenever 5 3+ users agree it's useless" without any further policy on what constitutes "useless" is bound to generate confusion and inconsistent review behaviour. If I get five hardliners reviewing my flags they get marked as helpful, but flags on the same type of post reviewed by those who want to keep everything that tries to answer get declined. And the problem is that trying to institute an objective policy very possibly leads to what ChrisWhite fears

Physics Guy

19:42

@JohnDuffield Do you mean that there is a quantum field which corresponds to a photon and a mathematical part of it is defined as virtual ?

If yes, you are wrong.

John Duffield

@PhysicsGuy : imagine a magnet. It's got a magnetic field. If you could divide the space where this field is, into little cubes 1 nanometre across, you could claim that each is a virtual photon. But magnets don't twinkle.

@PhysicsGuy : no.

ACuriousMind

@0celo7 $K$ and $L$ have zero cohomology in the first non-zero column, and $K^{1,0}$ as the 0th cohomology

Physics Guy

Okay, I end this discussion now.

John Duffield

@ACuriousMind : that's not very nice.

ACuriousMind

Let's say all non-zero objects are $\mathbb{R}$. Then the first column is $0\to\mathbb{R}\to\mathbb{R}\to 0$ with the arrow the identity, so it's exact. The second column has $0\to\mathbb{R}\to 0$, so it has $\mathbb{R}$ in the non-zero cohomology.

The $D$-cohomologies are computed from $0\to \mathbb{R}\to \mathbb{R}^2\to 0$ in both cases. In $K$, the map is the diagonal map, in $L$, it is the inclusion of into the left summand.

So in both cases you get $\mathbb{R}$ as the $D$ cohomology

dmckee

19:47

John's notion of what a virtual photon is as expressed in here has no relationship to the math what so ever. But John never bothers with the math.

3

John Duffield

@PhysicsGuy : hydrogen atoms don't twinkle either. Nor do electrons, nor do protons. The notion that they attract because virtual particles pop into existence spontaneously like worms from mud is what's called a lie to children. If you don't believe me, ask somebody else, or ask elsewhere. In fact, ask dmckee.

ACuriousMind

Oh, shoot

My map doesn't induce isomorphisms on the $d$-cohomology in the second column

Okay, so I can't come up with a counterexample, but I can't tell you whether your claim is correct or not

0celo7

@ACuriousMind Is this question too involved for MSE

dmckee

And that's the basic problem with all of John's attempts to explain physics: without the math he's left flailing around with fuzzy word picture that might sound good to people raised on pop-sci but can not make any prediction.

5

ACuriousMind

@0celo7 I don't know. Try asking it there, if it turns out it is too involved try asking at MO after a few days

0celo7

19:50

At MO?

This is an exercise

My flow question, however, is prof emeritus level research

Can you please rubber duck for it

Physics Guy

@John Duffield But still you couldnt define what a virtual particle is. I get to hear: " an abstract arbitrary part..."

0celo7

Magnets don't twinkle :D

Oh my gosh

Emilio Pisanty

@DavidZ That's odd. I'm fairly sure I'd flag that one as VLQ (more through being garbled than wrong) but it won't let me raise that flag.

John Duffield

@PhysicsGuy : it's an abstract portion of say an electron's electromagnetic field. This field is real. But it doesn't consist of a seething maelstrom of photons popping in and out of existence.

0celo7

@JohnDuffield Do you think equations are unhelpful, or wrong? Why do you refuse to use them

@JohnDuffield Einstein wasn't afraid of using equations

dmckee

19:59

@Bass I have to say that it isn't really fair to apply the word "censorship" to downvotes. That is an evaluation of some content rather than a blocking of it.

John Duffield

@0celo7 : no, I don't think equations are unhelpful or wrong, and I do use them. You know I use them. I've referred you to this twice already. But an equation cannot explain what a virtual particle is.

Physics Guy

@JohnDuffield Look, I never said that. Virtual particles exist for a limited time. Thats a fact. Such a particle has every property of a normal particle, its just massless. It can even be derived from the second quantization that there is a non-zero vacuum energy at (let us choose an example) the electromagnetic field, which causes the Casimir effect.

peterh

@Bass This is why you can laugh on the downvoted posts, if you are at least a little bit diligent to write upvoted posts as well. Thus, a downvote mafia doesn't really exist, currently there is relative peace also on the meta. What is much worse, if they close your question. And they close.

John Duffield

@dmckee : please explain to PhysicsGuy that virtual particles are not short-lived real particles that obligingly pop in and out of existence like magick. And that the Coulomb force is not the Casimir effect.

ACuriousMind

@PhysicsGuy Okay, I can't keep quiet about this after all since "virtual particles" are possibly the notion in QFT whose misuse annoys me the most. Virtual particles are just lines in a Feynman diagram. There are no states associated to them, and they don't have a lifetime. (What has a lifetime and is sometimes confused with virtual particles is a resonance, a temporary bound state) If you don't use Feynman perturbation theory to do QFT, you won't encounter the notion of "virtual particle".

The Casimir energy can be derived without any reference to virtual particles whatsoever

0celo7

20:04

@ACuriousMind Are you agreeing with John?

Bass

@dmckee I should have put "censorship" in quotes. It's just that every time someone asks "why don't we remove the bad content", the answer is "we have downvotes". So it is meant to make bad content go into the background. But of course I see the difference.

@ACuriousMind Do 3k users see deleted posts? In other words, are they hard-deleted or just hidden from the normal users?

peterh

@Bass No, only from 10k. But also they can't search for them.

dmckee

A lot of what goes on in these discussions is epistemology. Peolpe what to know what the quantum world "really is". Well, so do I, but I've had to come to terms with the lack of cog-wheels in there.

2

ACuriousMind

@0celo7 I don't know, and I don't care. What I'm saying is that virtual particles are computational tools that are useful, but are often endowed with too much ontological weight. You have to first do the math and then you can tell stories about virtual particles, but reasoning that starts from "virtual particles" tends to go wrong.

John Duffield

I will not settle for shut up and calculate. We do physics to understand the world. We don't do it to be told quantum physics surpasseth all human understanding.

dmckee

20:08

Virtual particles are a a feature of one way of organizing a model; they're "real" enough to use for designing certain experiments. Quarks are also a feature of a model; they are more concretely real and we can measure their mass, charge and spin.

Bass

@ACuriousMind I see, but I still think it's better to agree on a deletion policy (which might be a difficult task which needs some experimentation) than to just let those bad answers sit around with a score around 0.

dmckee

@JohnDuffield On the contrary John, it is entirely comprehensible to humans. That's what the math does for us: it tells us how the world is.

Physics Guy

@ACuriousMind Ok. Do virtual particles arise in all field-quantizations (path-integral, canonical ) ? Please watch that video if you have time. There is the derivation of the non-zero vacuum energy and Casimir-effect (ger) youtube.com/watch?v=dJEinYmcRkk

dmckee

And we know the math tells us that because the math makes testable prediction.

ACuriousMind

They're not the only things that's overused as a crutch to tell stories in QFT. Saying that the path integral means "the particle takes every path at once" is similiarily devoid of meaning if you don't know what formula exactly that is supposed to encode

@Bass Seeing deleted posts is a 10k privilege

John Duffield

20:10

@dmckee : oh yeah? Then I look forward to reading your answer here.

@ACuriousMind : it isn't devoid of meaning. The photon takes many paths.

Bass

@ACuriousMind Too bad. If the community could get SE to change that to 3k, it might be a solution. Five 3k users could always delete a question, but other 3k users can see the deletion and challenge it if they think it violates the policy.

ACuriousMind

@PhysicsGuy Virtual particles are what one calls the internal lines in Feynman diagrams, which in turn are a visual tool to organize the standard perturbation series. I've never seen anyone give any other coherent definition of "virtual particle".

So to understand what a virtual particle really is, you have to learn how either the path integral or the LSZ formalism derives the perturbation series, and how the diagrams encode it.

dmckee

@JohnDuffield My comment on the matter was moved to chat, but I stand by it: your dismissal of the math is a refusal to accept the answer. A photon is an excitation of the field.

ACuriousMind

And at the end of the line, you find that you can't really see in the formalism any of those things that virtual particles supposedly do according to pop-sci descriptions#

Physics Guy

@ACuriousMind Okay, whats with the video, Casimir effect and vacuum energy ?

dmckee

20:13

It has, in principle, a well defined energy, momentum, and helicity, but in practice you have to integrate over the internal lines (to all orders) to get predictions for the outcome of experiments, but once you've done that integration, and fudged the renomalization you get testable prediction that are borne out in experiment.

peterh

@ACuriousMind They have exactly the same parameters as the real ones, maybe it is a not negligible argument that they exist.

John Duffield

@dmckee : I don't dismiss the math. Answer the question. Any way you wish.

ACuriousMind

@PhysicsGuy Already the beginning uses the common misconception of "vacuum fluctuations" being pairs of particles being "created" and "destroyed".

Physics Guy

What are they, then ?

ACuriousMind

This comes from not understanding what the Feynman diagrams from which the vacuum energy is computed actually are - just terms in a perturbation series, and the formalism does not associate any actual particle states to any of those terms

@PhysicsGuy I have yet to see a definition of "vacuum fluctuation" that is not either trivial or meaningless.

The trivial one is that it's the fact that standard deviations of operators may be non-zero.

The meaningless ones are all the others.

Physics Guy

20:17

@ACuriousMind Yeah, thank you. that sounds familiar.

ACuriousMind

Physics would not lose anything if we just all agreed to never use the terms "virtual particle" or "vacuum fluctuation" again

dmckee

> "Others say things like "the photon is an excitation of the photon field". That tells me nothing"

Physics Guy

@ACuriousMind How to call it then ?

ACuriousMind

@PhysicsGuy How to call what?

Physics Guy

Just non-zero vacuum energy ?

dmckee

20:18

You dismiss the theory that explains the meaning of the word in the context of asking for the meaning of the word. What else is there to say?

ACuriousMind

My point is that there's no actual phenomenon that needs to be named. Zero point energy already occurs in the quantum harmonic oscillator of ordinary QM, but you don't hear anyone talking about "virtual vibrations" there

Physics Guy

@ACuriousMind How does Casimir-effect occur then ?

ACuriousMind

@PhysicsGuy What do you mean, "how"?

Physics Guy

Why does Casimir-effect happen ?

John Duffield

@dmckee : just answer the question. If you want to say the photon is an excitation of the photon field, no problem. But don't describe something in terms of something that is not described. That's a non-answer pretending to be an answer. Explain what an excitation is and what the photon field is.

Physics Guy

20:20

What is quantum foam, if its not caused by vacuum fluctuations ?

@ACuriousMind

0celo7

Quantum foam is what's on my soda

Physics Guy

funny.

;)

ACuriousMind

@PhysicsGuy that's not a question QFT (or QM) are designed to answer. It happens because that's what falls out when you put the theory in a box and compute the vacuum energy compared to the outside.

Again, look at the ordinary QHO - "why" does it have a zero point energy?

0celo7

@ACuriousMind Ricci flow.

Physics Guy

What ? What has Ricci-flow to do with this ?

0celo7

20:23

I claim that Ricci flow explains this

ACuriousMind

I get that it is somewhat dissatisfying that physics doesn't answer every "why" question you can think of, but that's, after all, not what it's supposed to do. It's supossed to predict nature, to give us a model we can work with.

8

John Duffield

Vacuum fluctuations are not the same thing as virtual particles. Vacuum fluctuations are real. Virtual particles are virtual. The clue is in the name.

ACuriousMind

@PhysicsGuy What is "quantum foam"?

that's another one of those things I've never gotten a good definition of

0celo7

@ACuriousMind I think that's an LQG thing.

Physics Guy

The result if you put QFT and GR together. Quantum effects on microscopic scales make space-time bubbling, like foam.

ACuriousMind

20:25

@0celo7 Yes, but what has LQG to do with QFT or the Casimir effect or "vacuum fluctuations"?

Physics Guy

The LQG thing is spin foam

0celo7

@ACuriousMind Dunno, I'm not a physicist.

Just trying to help ;(

John Duffield

OK chaps I have to go. Good night.

0celo7

Night

ACuriousMind

@PhysicsGuy We don't know what happens if you put QFT and GR together. Might be string theory, might be LQG, but "space-time bubbling" is another one of those phrases that don't mean anything without a proper formalism behind them.

Physics Guy

20:27

@ACuriousMind It is just a theoretical effect, with occurs when you try to calculate general relativistic equations on a microscopic scale. But I have never heard an exact definition of it, too.

Not related to QFTs on curved spacetime.

But I havent done QFT for a long time. And I never studied it very deeply. Thats why I have problems with it, now.

0celo7

@ACuriousMind Is the proper notation for $\bigoplus K^{p,q}$ $K^{\bullet,\bullet}$?

To distinguish it from the complex $K^\bullet=\oplus(\oplus_{p+q=n}K^{p,q})$?

ACuriousMind

@0celo7 $K^{\bullet,\bullet}$ is what I would denote the double complex itself if I wanted to stress it's a double complex

Sanya

@ACuriousMind This should be written in big letters on the blackboard of every beginner's physics lecture

0celo7

@ACuriousMind Yeah, so the $D$-cohomology is really the cohomology of the single complex $K^\bullet$ though

And the $d$ cohomology is the cohomology of the single complex $K^{p,\bullet}$, right?

ACuriousMind

@0celo7 Yes. Although I would denote that simple complex by something like $s(K)^\bullet$

@0celo7 Yes

Physics Meta

20:31

1

Q: How to make a major correction to your own answer?

After posting an answer I received good comments highlighting an error in my understanding. To correct it would be a major edit, drastically and conceptually changing around half the text of my answer. But when I go to edit it, I am presented with a text box telling me to ► fix grammatical or sp...

0celo7

@ACuriousMind Is it proper to write cohomology of $C^\bullet$ as $H^\bullet(C^\bullet)$?

ACuriousMind

@Sanya Yes, it should!

@0celo7 Matter of taste, but I would write $H^\bullet(C)$ since you only have one free slot.

0celo7

Yeah but I want to write the $d$-cohomology of $K^{p,\bullet}$

Sanya

although I of course had professors quoting the good old "will verstehen was die Welt, im Innersten zusammenhält" and answering the most nonsensical questions @ACuriousMind so maybe the right place for it would rather be the entrance door of the physic's institute ...

0celo7

So that would be $H_d(K^{p,\bullet})$

ACuriousMind

20:36

@Sanya The only people who think that Faust quote works as a program for science are those who know nothing about philosophy of science or epistemology. Which, incidentally, are also things that should be a small part of any scientific education.

0celo7

@ACuriousMind I'm stupid, what is the $d$ thingie called in cohomology

is it just a cochain map

differential operator?

Sanya

@ACuriousMind well, good old experimental physic's chaps ... but yeah, I agree, doing a bit of philosophy of science has proven much more valuable than many other things I did during my studies

0celo7

20:49

@ACuriousMind Does this make sense?

vzn

@PhysicsGuy re virtual photons its interesting youve zoomed in on a point of contention that "somewhat" splits this site into different ideologies; is also a tricky part of the theory that even experts nearly debate... am now wondering, did feynman himself have anything to say about it? probably...! maybe his answer is as good/ slightly )( better than anyone elses? must say do agree with the antiJD sentiments asking for math eqns/ rigor. hopefully that rescues me out of the black sheep bin... o_O

0celo7

0

Q: Induced isomorphisms on cohomology of double differential complexes

Let $K^{\bullet,\bullet}=\bigoplus_{p,q\ge0}K^{p,q}$ be a double differential complex, i.e. we have differential operators $$\cdots\stackrel{d}{\to} K^{p,q-1}\stackrel{d}{\to} K^{p,q}\stackrel{d}{\to} K^{p,q+1}\stackrel{d}{\to}\cdots $$ and $$\cdots\stackrel{\delta}{\to} K^{p-1,q}\stackrel{\delt...

algebraic-topology homological-algebra

vzn

@JohnDuffield what does the eqn have to do with virtual particles?

@JohnDuffield what are "real photons"? isnt that a pleonasm?

0celo7

21:05

that's a big word

John Duffield

21:32

@vzn : nothing.

@vzn : no.

@vzn : shrug. Mathematics won't tell you what a light wave is.

Physics Meta

0

Q: Would it be useful to link the engineering SE on closed engineering questions or not?

On some SE sites, it is normal that the closing reasons contain also a link to another SE site, where the question could have a better chance to survive: On other SE sites, it is unthinkable and even suggesting this results around -20 on the meta. P.s. this question is not a duplicate of this...

discussion

peterh

22:25

@ACuriousMind If you see physics is applied mathematics on experimental data collected far away & long ago, I think it harms the intuition. This will result a brilliant physicist with very strong math - and who never discover anything. The formulas aren't the reality, the reality is behind them. You have to find the reality behind them.

WillO

Ocelo7: Yes, your question makes perfect sense. What you've got is an isomorphism between the $E^2$ terms of two spectral sequences, and you want to know whether this induces an isomorphism on the abutments. The two potential obstructions are: 1) Is the isomorphism compatible with differentials? (Both at $E^2$ and above) and 2) If so, so that you have an isomorphism at $E^\infty$, there's still an extension issue.

vzn

@peterh (actually agree but...) who is doing that? any volunteers?

dmckee

@JohnDuffield That's a purely philosophical position that values some ill-defined "knowing what it is" over the ability to actually make use of your alleged knowledge.

peterh

@vzn No, we only see the news and hope that once we will fly to the stars.

vzn

@peterh physicists who really want to find the reality behind the math are increasingly rare ever since birth of atomic physics in the early 20th century (after everyone approaching that description banged their head against very hard walls for decades). one might say that einstein was such a breed and vanished in his era. and look where it got him in the 2nd ½ of his life. but, yes, have collected a small list of iconoclasts myself...

dmckee

22:35

@peterh The formulas and the associated interpretation are knowing. It's knowledge that give actual results. You can chase your banner of Truth-with-a-big-tee all your life, but models that make predictions are what gets things done.

Sir Cumference

So, uh, is anyone here good with degenerate gases?

dmckee

@SirCumference A couple of people know neutron stars pretty well. I'm not one of them.

Sir Cumference

All right, new question

Anyone know much about gas giants?

Specifically if Jupiter is supported against gravity by thermal pressure

peterh

@vzn I think he had also luck. In his second half, he had not. Maybe a young string theorist.

ACuriousMind

@dmckee Can't help but think of this Abstruse Goose

dmckee

22:46

@ACuriousMind Also abstrusegoose.com/105

WillO

Ocelo7: I suspect that if your double complexes are not bounded, you are going to have problems with falling off the edge of the world ---

peterh

@dmckee Models have to be constructed, somehow. Bohr used a classical mechanical analogy to find the nucleus.

ACuriousMind

@WillO If you want to notify someone in chat, you need to put an @ in front of the name. Also, note that the first letter in the name is an 0 (zero), not an O (letter O).

dmckee

@peterh Er ... Geiger and Marsden found the nucleus. Bohr bodged together an adhocery to explain why the spectrum might be discrete and the electrons didn't fall in.

That was good work, but it sure as heck ain't Truth-wht-a-big-tee.

Sir Cumference

Wait, dmckee

You know about gas giants, right?

dmckee

22:49

Just a little. I wanted to be an astrophysicist at one point, but I ended up going a different way.

WillO

@ACuriousMind: Thank you on both counts!

Sir Cumference

Oh...

peterh

@dmckee Right, Rutherford was it, and Geiger-Marsden made the gold foil experiment

WillO

Maybe before notifying him I will think about this a little more....

dmckee

@WillO If yu click the little down-then-right arrow that appears on a chat message when you hover your mouse, the system will insert the @-tag and point at that message.

WillO

22:51

@dmckee: Thank you also.

peterh

@ACuriousMind The reality is one, the aliens would have maybe different math to deal with it, but they won't have a different physics

Sir Cumference

@dmckee Wait, wasn't it you who explained Virial theorem and why protostars contract?

dmckee

@SirCumference Yeah. But my knowledge is patchy. The Virial theorem stuck, the formalism of radiative heat transport didn't, and so on.

Sir Cumference

@dmckee Didn't you also explain that gas giants are supported by an equilibrium between gravity and thermal pressure?

0celo7

@ACuriousMind back

flow time?

Sir Cumference

22:59

@dmckee Er, you there?

0celo7

@SirCumference He probably died

He's at that age

RIP

dmckee

Sorta. I'm writing stuff for my classes this fall.

0celo7

You sorta died or you're sorta there :P

Sir Cumference

All right, didn't you say gas were supported by an equilibrium between gravity and thermal pressure?

dmckee

@0celo7 I actually had a colleague die on the job once. He went up to the electronics cage to do the shift-checklist and had a heart attack.

0celo7

23:01

@dmckee We had a chem prof who dropped during a lecture

Sir Cumference

You mean heart attack?

dmckee

No one else up there to notice and help. Very sad and it makes it hard for me to appreciate than kind of joking.

Sir Cumference

Or an actual heat stroke?

dmckee

Heart attack. He was working insane hours. I suppose to keep his mind off his divorce.

Sir Cumference

Damn, sorry

0celo7

23:03

@dmckee My prof said he "might" come in tomorrow and help me with Ricci flow/Čech cohomology

dmckee

@SirCumference That's how it goes, sometimes. Academia is hard on marriages. Like a lot of highly competitive jobs.

0celo7

Should I take it as a "I'm not coming in" since he didn't email me

Should I remind him?

@dmckee I have a prof on marriage 3

dmckee

@0celo7 An optimist, perhaps.

Sir Cumference

@dmckee Did he die young?

dmckee

@SirCumference Early fifties. Not young by historical standards, but too young none-the-less.

Danu

23:16

@dmckee :(

vzn

23:37

@0celo7 tasteless... but also maybe utterly nihilist o_O :(

0celo7

I'm not a nihilist

vzn

@0celo7 sure, deny it lol (you did admit it once in the transcript)

bolbteppa

@WillO what is a spectral sequence, how should one think og them? Is there an easy baby example?

0celo7

0

Q: Differential of flow related to gradient Ricci soliton on $S^2$

I'm confused by a claim made in Brendle's Ricci Flow and the Sphere Theorem on page 37. Let $(S^2,g)$ be a gradient Ricci soliton with soliton function $f:S^2\to\Bbb R$. Since the sphere is $2$-dimensional, we have $$\nabla^2f=(\rho-\tfrac{1}{2}s)g,$$ where $\nabla$ is the LC connection, $s$ i...

differential-geometry riemannian-geometry

DanielSank

@dmckee Academia is hard on more than marriages.

When's the last time you met a lab full of students most of which weren't miserable?

dmckee

23:54

@DanielSank There is something to that. In any non-trivial group of grad-students and posts docs a few are always facing a looming threat of one kind or another.

Experimental particle physics at least gives you a decent sized peer group to commiserate with, which is like a support group: they help you find context and coping skills, but it doesn't make the problem go away.

DanielSank

@dmckee Yes, combine that with the blatant lie that any reasonable fraction of those people will get faculty positions and you've got a recipe for misery.

One PI, ten students... do the mat.

@dmckee Sure but then what do you do after you graduate?

dmckee

@DanielSank Graduating is one of those looming threats.

It's approach was one of my two worst periods in grad school.

There is a market of sorts for discarded Ph.D. physicists in industry, but it is diffuse and mixed with hundreds of openings which will bin your resume before a responsible party every sees it.

My post-doc years were haunted by endless job hunting with so little feedback that it sometimes felt like I was e-mailing my packet into a howling wilderness.

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