The h Bar

dmckee

00:01

I should have known better, of course, I'd met about twenty physicists-by-training with advanced degrees at various adult functions my parents dragged me to. Exactly two were still doing physics, and most were not even in obviously technical fields.

0celo7

@dmckee Thats a nice prospect for the physics students in here

dmckee

But I was such a snot nosed little twat that I figured that was a sign that they were defficient somehow.

@0celo7 The market for BS's is rather more traditional. You can more fairly smoothly into engineering or into tech.

Taking the grad school plunge is entering your future into a lottery for the pleasure of pure learning, and a big hit in accumulated compensation.

0celo7

I'm not a physics student

I wonder if the misery in this chat had anything to do with that

dmckee

@0celo7 You keep saying that.

0celo7

@dmckee I do, because it's true

I've done a good job of convincing myself of it

I really don't like physics

It's math I now have to rid myself of

DanielSank

00:10

@dmckee Heh.

@dmckee You have to know people.

@dmckee Not your fault. Academia spreads that lie around and seems to be proud of it.

dmckee

@DanielSank And the people I knew had retired, so they had little swing.

DanielSank

@dmckee ah

WillO

@0celo7 @0celo7: I posted an answer to your question at MSE.

0celo7

00:26

You missed the part in the OP where $p$ and $q$ are nonzero

WillO

@0celo7 You are absolutely right; I missed that.

You meant to say non-negative, not non-zero.

Anyway, yes, with that additional condition I think what you're claiming is likely to be true.

0celo7

You're right

Strange, I distinctly remember thinking "nonnegative" when I typed that

(I get those two mixed up a lot)

@WillO I hope it is!

WillO

Well, it's much less of a blunder than my missing the fact that there was any condition in the first place!

0celo7

I'd like an answer that doesn't use big machinery of homological algebra though

I'm afraid if I ask on MO they'll tell me to use a triple filtered Ext functor filter

or something stupid

WillO

:)

0celo7

00:30

And my (geometric analysis) advisor will tell me to ask an algebraist

WillO

More likely they'll tell you it's off topic because the answer is obvious --- but it's not 100% obvious to me.

0celo7

This Ricci flow thing WILL kill me

WillO

Are there any other conditions on the $K^{\bullet,\bullet}$? E.g. are they projective?

0celo7

I don't know what that means

WillO

Or better yet, are they vector spaces?

0celo7

00:34

Indeed they are

WillO

Oh!

Then it's just about got to be true.

0celo7

I eagerly await your proof :D

WillO

I have no proof, but here is how a proof will go:

Break up your double complex into a direct sum of much simpler double complexes, all concentrated in just two total degrees. If all the $K^{\bullet,bullet}$'s are vector spaces, you can do this.

0celo7

Concentrated in just two total degrees?

WillO

Do the same for $L$. Then you can break up your map $f$ into pieces that take one piece of $K$ to one piece of $L$ and analyze these....

Yes.... Total degree means p+q

0celo7

00:38

I still don't know what you mean by that

WillO

So---- pick your favorite $p$.

0celo7

ok

WillO

Your life would be much easier if all the non-zero terms of K were one-dimensional vector spaces that live in K^{a,b}'s where a+b is either p or p+1

0celo7

I...don't know

WillO

Call those complexes "simple". (That is not standard terminology.)

You can write your complex as a direct sum of simple complexes.

I could be wrong, but I believe this will lead to a proof

0celo7

00:41

I'm sorry, I don't see what that has to do with vector spaces

WillO

I am invoking a theorem that says that if the K^{pq} are all vector spaces, then your double complex is a direct sum of simple complexes

0celo7

uhhh

Can you describe this in detail please?

I don't know much homological algebra at all

WillO

I wish I had a way to draw a picture here. Let me go draw one elsewhere and I'll be right back with it.

0celo7

ok

WillO

okay---is there a canonical place to post a picture, or is it possible to post one into this chat window? or should i put it elsewhere for you?

0celo7

00:52

imgur

see the upload button at the bottom

WillO

imgur wants me to create an account. try panix.com/~wotw/ocelo.gif

There you see four different double complexes (all non-pictured spaces are zero).

0celo7

what kind of imgur are you using

WillO

These are even simpler than they look, because all of these vector spaces are 1-dimensional.

0celo7

uhhh

WillO

Theorem: Any bounded double complex is a direct sum of double complexes of this sort (and their obvious longer generalizations).

I'm not sure what "uhhh" means :)

0celo7

00:56

I don't know what this picture is telling me

WillO

You mean you don't understand what I'm saying, or you don't understand why it might be useful?

0celo7

Either

WillO

Okay....the first is probably a prerequisite for the second....

0celo7

What is this picture supposed to be

@WillO Usually, yeah

WillO

It is not one picture. It is four completely separate pictures.

0celo7

00:58

oh, that's a helpful hint

WillO

The first picture is a double complex in which all but four entries are zero, and the other four entries are one-dimensional.

The second (to the right) is a double complex in which all but three entries are zero and the other four are one-dimensional.

The third is another double complex with just four non-zero entries. The fourth has just five, and other "staircase-like" double complexes can have six, seven, eight, etc.

If you knew that your double complex was one of these, it would be very easy to calculate with. It's easy to compute the horizontal, vertical, and total homologies.

0celo7

Hmm

But in the third picture

don't you get things from three total degrees

WillO

You don't know that, but you do know something along those lines: Your double complex is a direct sum of complexes that are this simple.

0celo7

nevermind that

WillO

My total degrees are fixed along 45 degree lines that run NW to SE

0celo7

01:01

@WillO now that I do not see

WillO

you might be in the habit of indexing differently

you do not see.....what?

0celo7

not all of the thingies in the complex are the same vector space

I do not see that $K^{\bullet,\bullet}$ is the sum of such complexes.

WillO

But they are all direct sums of one-dimensional vector spaces.

I am not claiming that it's obvious that your complex is a direct sum of such things. I am claiming only that it's true.

0celo7

Ok, let us suppose that this is true

WillO

I am also claiming that I've often found it a useful thing to know, and I suspect it will be useful here, though I make no absolute guarantees.

0celo7

01:03

why is it useful?

WillO

Because it breaks big complicated problems up into much smaller uncomplicated problems (though at the cost of moving the complication over to how the solutions to the smaller problems fit together)

0celo7

ok

WillO

In the other direction, if you want to look for a counterexample....you can compute, from the structure of the double complex, maps $H^q(K^p,{\bullet} \rightarrow H^{q-1}(K^{p+2},\bullet}$$. You've got (by assumption) that the homologies are the same when you replace $K$ by $L$, but not necessarily that these maps between homologies are the same when you replace $K$ by $L$.

These are important invariants, and if they differ in some example, then that's a good candidate for a counterexample.

Okay, what you want is true. I have a proof, but it will take a little time to write it up.

But in brief (and recognizing that you might or might not know all these words):

Let C be the mapping cone of f. Then the first spectral sequence for the homology of C collapses at E^2. The boundedness condition says that the spectral sequence converges, and voila.

Secret

01:50

6 hours ago, by 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.

5 hours ago, by 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.

One way to work out the why is allowing two "whys" of the same nature to interact with each other

As an example, before spectroscopy involving polarised light, we only knew something is an enantiomer because it interacts with another enantiomer

In the end at such level, we don't know what the "whys" are and why they act this way, but from these indirect experiments, we can identify what "why" we are dealing with

Another example is the stern gerlac experiment which told us an electron has a angular momentum like quantity that spans a 2 dimensional hilbert space. The maths then organise these findings into a systematic fashion so that prediction can be made from it. But what it "actually" is we don't know, other than it is some label of the representation of an more generalised notion of changing the orientation when looking at th…

So I argue that most of the time, because most things cannot be directly probed, most of the whys have to be inferred from the hows, just like how daniel mentioned about the electromagnetic vector potential is a nice bookeeping device to predict motions of charged objects under electromagnetism

@Bass You nailed what I am trying to say back there

> 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.

DanielSank

03:41

I'm annoyed that there's a featured question asking what a photon is.

It's good that the question "what is a photon" is getting attention, but it's less good that it's a duplicate which got away with being a duplicate because it 1) has a bounty, and 2) is (unnecessarily) long.

Also, @JohnDuffield, a question not having an answer is not an excuse to post a duplicate question. Instead, edit the original, put a bounty on the original stating that you want a better answer, campaign in chat for the original to get attention, or do nothing.

Don't post duplicates because you feel like it.

John Rennie

05:02

@PhysicsGuy You have to use lattice QCD to study glueballs because the perturbative approach doesn't converge. That's true for all QCD bound states and doesn't signify anything special about glueballs.

Bernard Meurer

@jumbo Hey

@JohnRennie Isn't it late there?

John Rennie

@BernardMeurer Early not late :-) It's six a.m.

Bernard Meurer

@JohnRennie Any servers turning evil?

John Rennie

I start work around 05:30 checking which of the 600 or so servers I look after have died overnight.

So far no problems.

It's the Windows update this week - it starts the second Tuesday in the month - and when 600 servers update and restart something usually goes wrong. But so far no problems.

Bernard Meurer

Keeping my fingers crossed :)

John Rennie

05:07

Most of the servers are configured to update on Saturday night.

We do that so I have all of Sunday to fix any problems.

Sunday morning (at 05:30!!!) is usually a bit stressful.

Bernard Meurer

Velvet Underground-"Sunday Morning" from "Velvet Underground and Nico" LP

►

Might be helpful :v

John Rennie

The Velvet Underground - that's practically musical archeology

Bernard Meurer

I really like Velvet Underground

John Rennie

@0celo7 That's a GPIB / IEEE 488 connector - see radio-electronics.com/info/t_and_m/gpib/ieee488-connectors.php

Bernard Meurer

@JohnRennie Holy shit how did you know that?

He asked me and I guessed it must've been some weird serial connector from the early 2000's

John Rennie

05:16

Back in the stone age IEEE busses were quite common for hooking equipment together. We used to use them for running measuring kit from our BBC micros.

There you go, there are advantages to being old :-)

Bernard Meurer

Could you daisy chain?

John Rennie

Yes

Bernard Meurer

Badass

USB not being able to daisy chain sucks balls

RIP FireWire

John Rennie

They were actually a very good way to integrate stuff. Do a Google on it some time and you'll see how well designed the protocol was.

But it never caught on outside specialist applications and it was always expensive

@PhysicsGuy a virtual particle is a mathematical fiction. It's used as a description of a term in a perturbative calculation. the interaction described by the virtual particles is real, but the particles themselves are not.

@PhysicsGuy but in some circumstances the interaction is dominated by a single VP term to such an extent that the VP is nearly real. Push things a bit further and you get a real particle not a virtual one. This is what dmckee was referring to when he talked about boosting VPs to be on shell.

So the distinction between a real and virtual particle can sometimes be so fine that it's almost just a matter of semantics to say whether the VP is real or not.

Secret

05:44

Is it correct to say that a real particle in the context of perturbation theory is an interaction term that dominates in the system?

John Rennie

@Secret No. Well, I suppose a virtual particle becomes real in the limit of infinite lifetime.

Which was what dmckee meant.

@Secret have a look at this Wikipedia article. This is quite a nice description of the difference.

On shell and off shell

In physics, particularly in quantum field theory, configurations of a physical system that satisfy classical equations of motion are called on shell, and those that do not are called off shell. In quantum field theory, virtual particles are termed off shell (mass-shell in this case) because they don't satisfy the Einstein energy-momentum relationship; real exchange particles do satisfy this relation and are termed on shell (mass-shell). In classical mechanics for instance, in the action formulation, extremal solutions to the variational principle are on shell and the Euler–Lagrange equations give...

It's when you get particles that are right on the borderline between on shell and off shell that the distinction becomes fuzzy.

Secret

06:03

Hmm, so if I had a field whose interaction is "90% on shell (not sure if there's a way to formalise this, but what I have in mind is it is located very close to one of the mass hyperbola)", then I will find its interaction will be 90% like that of "classical" relativistic field while the rest of the 10% will be more quantum like such that conservation laws don't hold?

John Rennie

Suppose we consider some scattering process where some particles come in from infinity and other different particles head out to infinity. The way QFT describes this is by using creation operators $a^\dagger$ and annihilation operators $a$.

So the incoming particles get annihilated by $a$ an the outgoing particles get created by $a^\dagger$. The task is to calculate the probabilities for these operators.

David Z

@EmilioPisanty because it's too old. You can only cast VLQ flags on posts posted in the past week, I believe.

John Rennie

For an interacting field we have no way to calculate these operators precisely, but we can express them as a sum of various combinations of the free field operators.

So we end up with a sum of lots of different combinations of the free field $a^\dagger$ and $a$.

Each combination describes the creation and annihilation of the corresponding particles, so each term is like various different combinations of particles being created and annihilated. And these are the virtual particles.

NB I'd be cautious about starring these statements until ACM has had a chance to check them. This is my understanding but I would not swear under oath that this is a correct interpretation.

Secret

I see, Btw, it wasn't me who star them

peterh

I was it, because I found it wonderful, but now I am staring only the first sentence to find it laterf.

John Rennie

06:12

So the point is that the real interaction is expressed as a combination of operators that look like they are creating particles, but they aren't really. That's why we say virtual particles don't really exist.

As we approach the mass hyperboloid we find the real operators are increasing dominated by a single term, and that's what we mean by the virtual particles becoming more and more real. But it isn't meaningful to say a particle is 90% real.

peterh

@JohnRennie These ops are playing not on a normal wave function, but on a Fock space. It is essentially an union of the 0 to infinite dimensional wave functions. Until this could I understand the things in the QFT books. What is not clear: do these operators have eigenfunctions? Hamiltonians, whose eigenvalues would be the observable energies?

John Rennie

Note that the Fock space only exists for free fields so it is an idealised structure. We don't know the equivalent of a Fock space for an interacting quantum field theory.

A creation operator creates a plane wave, but exactly what that plane wave is I have to confess I don't know. From the little I understood of ACM's attempts to explain this to me it is not simply the free particle solution to the Schrodinger equation.

@ACuriousMind: is this anything near the truth, or am I as usual wildly misunderstanding QFT?

12 mins ago, by John Rennie

Suppose we consider some scattering process where some particles come in from infinity and other different particles head out to infinity. The way QFT describes this is by using creation operators $a^\dagger$ and annihilation operators $a$.

peterh

@JohnRennie As I know, the creation operator creates a free particle, which is a plane wave with a beautiful $e^{j\omega t}$-like wavefunction.

John Rennie

@peterh See:

Jul 28 at 17:48, by ACuriousMind

The thing the creation operator creates is in no precise sense a plane wave because it is not a function of spacetime

and the associated discussion.

peterh

Uhh, it is harder as I thought...

John Rennie

06:22

@peterh that's what I think too :-)

Basically there's no shortcut. To understand this stuff you have to sit down and learn QFT properly.

peterh

@JohnRennie I am whining the "simplicity" of the old eigenvalue-based QM

John Rennie

But there's such a high barrier to getting started. Every time I sit down with my QFT for Beginners books I stall in the first chapter due to my poor grasp of the maths required.

@peterh I suspect the simplicity of the Schrodinger approach to QM is actually not helpful if you want to go on and study it in depth. It encourages you to oversimplify then think that's all there is to QM.

peterh

@JohnRennie It uses a lot of abbrevations with a relative simple meaning, but it is not clear to me, what they really mean. It would be clear probably after I learned a lot.

DanielSank

06:47

PSA: we should fix the homework policy

::looking for DavidZ's post::

Secret

07:12

Before that discussion about creaton operator creating a plane wave, I was almost tempted to make a PSE question asking why all physical models can always approximate something either by integrals or series of some idealised things (It seemed to be unique to physics, as chemistry and biology and maths don't have such widespread use of such things) an why experimetnal result always seemed to agree with these approximations

The only case where I ever seen that some formalism becomes "discontinous" is when it requires a leap of the conceptual framework, such as you cannot derive any relativis…

To elaborate what I am trying to say, recall how back in undergraduate physics when solving electric fields of a configuration of charges, all you need to do is integrate all the charge densities.

Another example is entropy, which is basically a counting problem in some sense

We also have quantum states which are superposition of some base state, vector spaces made of basis vectors

Even on the abstract level of groups, one can define some continous map from one transformation to another

And in differential geometry, you have continous deformation of something

Putting the above paragraph in "psedomathematics" form. why is the problem solving approach

$\text{Actual thing}=\lim_{n\rightarrow \infty} f(\text{Idealised things}_n)$ works so well...?

Secret

NB: "Pseudomathematics" sentences is a (desperate?) attempt used by me in order to communicate a thought that I cannot communicate coherently to most people due to the lack of vocabuary in my disposal. Therefore I sometimes "hijack" the properties of some mathematical object and use analogy to reduce their rigor into something such that it helps me to describe what I am trying to say

These sentences are known to cause mathematician to cringe due to the vagueness involve in them

The tendency for me to hijack the properties of technical terms and objects to communicate my thoughts is one reason I can easily make physics jokes

On the other hand, it is one reason why I easily fall into misconceptions in some cases because two different concepts became too similar under this framework and thus they start to mix together

Thomas Klimpel

08:00

in theory salon, 1 min ago, by Thomas Klimpel

Surprises of the Faraday Cage, by Lloyd N. Trefethen. Surprising indeed!

John Rennie

And now for my third (pint¹) mug of coffee. Some mornings ...

¹ 0.568 litres

Secret

08:23

> sinusoidal oscillation in one direction corresponds to exponential decay in the direction at right angles in the complex plane

i think we have seen something similar in the WKB approximation

probably because momentum and position operators are related by a fourier transform

Therefore, compelx mommentum becomes an exponentially decaying term

>L1. There are gaps out there. If you find something fundamental that nobody seems to have figured out, there’s a chance that, in fact, nobody has.

L2. Analogies are powerful. I would never have pursued this problem had I not been determined to understand the mathematical relationship between the Faraday cage and the trapezoidal rule.

L3. Referees can be useful. Thank you, anonymous man or woman who told us the Faraday cage section in our trapezoidal rule manuscript wasn’t convincing! We removed those embarrassing pages, and proper understanding came months later.

Anyone who think they have got something alogn the lines of L1, should check carefully with their collaborators on whether every steps that lead to that conclusion is not laden with mistakes.

Just as all the good ol' LIGO and LHC guys check their results very carefully in order to not have things like the "OPERA superluminal neutrino faulty cable incident"

It seems chapman and co. are doign a good job on that

As for L2, let us go to the next paragraph...

Secret

Analogies are one of the ways to related two seemly unrelated things together. Once again, if we "hijack" mathematics to describe this property you will get a picture that look like this:

Imagine there's a space (set+topology) consists of concepts as its elements. There's some concept A and some concept B somewhere in this space (we don't know where, because we only have a topology not a coordinate chart, we only know it is there).

Now imagine an analogy is like some kind of map f that maps A, B into some C such that they are 'somewhere in between in properties to' A and B. This map has a…

Secret

08:42

However, analogies can be dangerous, because not everything is always being truthful to you. for example, while coloumbs law and newtonian gravitation might look mathematically similar, it does not mean they are related. As our experience in physical intuition grows as we solve more problems, then we became better at seeing through these illlusions

Secret

Back in my chemistry demonstrator training session, we are required to watch this old documentary, which talks about the issues of pet theories
https://www.learner.org/resources/series28.html#

Since then, it caused me to became more aware of my gazillions of pet theories living in my mind. Thus my questions become sharper (although may not look so because my communication skill is incoherent, as most h barer noticed)

A long time back in my high school period, it is actually a biology teacher, not a physics teacher that taught me and my classmates the principle of solving problems by elimination

this method set up a chain of logical reasoning in biology, best reputed for its memorisation, so that it decomposes into something as systematic as physics

over time, the idea grows as the concepts in mathematics added to the mix as I study

Eventually, after reading susskind, the concept cemented itself into a perculier way of asking questions

and studying became complicated with the constant need to fight my gazillion of pet theories

One recent example had occured yesterday, involving the discussion about photons. Some long time ago in the history of h bar, I heard from some users on a pet theory that talks about photons being a bound state of electrons and positrons

Fast forward to the present day, for some unknown reason, the curiosity on that grew. The alamr system then said there is a high chance if left unchecked, it will become a pet theory and corrupt my thoguth process

Because of the concept space analogy mentioned above, combined with the increase in awareness of the effects of pet theories after watching A Private Universe, and then things become a bit "chunky" after finished reading susskind, the following action is carried out

> For a person who is not fully sucumbed to his/her pet theory, the best way to combat it is to ask that pet theory on what are the predictions, and then check if these predictions matches what we know in real life experiments and theories. If the pet theory fail the test, then it is incorrect

So in order to combat that "photon as a kind of positronium" pet theory that is suddenyl recalled for reasons unknown, the following question was directed to acuriousmind

But first, the pet model need to be generalise as much at it can to reach the limit of the scope of "predictions" it can offer, so that a question that can bring it down become most effective by blocking all possible loopholes that can make the theory survive

Therefore, "photon as a kind of positronium" generalised into something more plausible as "photon as a composite of massless electric charges so that it is overall neutral"

The next step is to ask this pet model what are its predictions: We then get "nth order multipole moment may be detected"

However this is not enough to kill the pet theory thus

18 hours ago, by Secret

What would instead would have been observed if photons somehow has a composite structure?

18 hours ago, by ACuriousMind

@Secret You're asking the question the wrong way around: Our current theories have the photon as non-composite, and QED matches experiment perfectly. What exactly you expect for a composite photon would heavily depend on what it is composed of

After some discussions in making that question use to bring down the pet theory clearer, we reached the answer

18 hours ago, by ACuriousMind

Considering that glueballs are massive, though, I think you're right that massless composite particles can't occur in ordinary QFT

17 hours ago, by ACuriousMind

@Secret What "vacuum solution"? A massive photon would just wreck the inverse square law, and make the force decay exponentially

17 hours ago, by ACuriousMind

Yeah, I'm not convinced that there are consistent QFTs with composite massless particles

17 hours ago, by Herr_Mitesch

well, of course a photon could have intrinsic higher multipole moments, which would couple it to itself in more complex ways, but those are non-renormalizable anyway, I guess.

The answer is more than enough to break the foundation of that pet theory thus it is no more

Secret

09:10

One more thing, a possible explanation on why I seemed to always ask questions the wrong way around (in addition to that incoherency), might be because I use the answer of the question to falsify something, therefore, when I ask a question, it often has a purpose of using the answer to eliminate one of the many possible pathways in the thought process, so that the thoght process will stay in the correct path and lead to the correct conclusion.

This unusual thought process of mine (and others) might have something to do with that logic chain approach by our biology high school teacher

but so far it serve me well

Secret

Another interesting observation (probably might be just me, but maybe some others as well). When people ask questions where the answers help knock down his/her pet theories, unusual topics in the mainstream will be discussed. This often have the effect of making the overall discussion meaningful

Put this in the concept space framework, knocking down pet theory appraoches often result in us exploring rarely explored regions of concept space, and hence fresh insights or just a highly meaningful discussion

An interesting consequence of my thought process flow in a way similar to that concept space framework is the tendency to think about problems of this class in context where such questions are not comonly conceived about

Hearing the shape of a drum

To hear the shape of a drum is to infer information about the shape of the drumhead from the sound it makes, i.e., from the list of overtones, via the use of mathematical theory. "Can One Hear the Shape of a Drum?" was the title of an article by Mark Kac in the American Mathematical Monthly in 1966, but the phrasing of the title is due to Lipman Bers. These questions can be traced back all the way to Hermann Weyl. For the 1966 paper that made the question famous, Kac was given the Lester R. Ford Award in 1967 and the Chauvenet Prize in 1968. The frequencies at which a drumhead can vibrate depends...

This has been said to be one reason why when I do mathematical proofs, I often end up proving the converse without realising

hence the reason why people often said I think "the wrong away around"

Mikael Kuisma

09:28

@JohnRennie I don't believe in virtual particles. Let's say in scattering theory, by definition virtual particles are inner vertices of a diagram and real ones the asymptotic states. So, the definition of a real particle is that it propagates in free space without interactions. Why does a free space propagation suddenly make a particle real? I don't believe in real particles either, since pretty much everything is always a quasiparticle. :)

Particle for me is a choice of a convenient 'basis'. It is usually

wanted that it resembles classical analogies though, but that does not make it any more real.

But then again, I don't know QFT that well...

John Rennie

@MikaelKuisma Hi Mikael. Did you look at my subsequent posts where I try to explain what virtual particles are in terms of a sum of creation and annihilation operators?

Mikael Kuisma

@JohnRennie I did now. I agree on that, because that was math. But somehow it seems that I always have quite strong opinions on interpretations.

I'm sort of interested into this QFT thing now. I think I will on some weekend sit down, and derive some lattice QED variant and code that with Matlab etc.

But QFT, it is such math. Everything is infinite etc.

ACuriousMind

10:21

@JohnRennie It is near the truth

John Rennie

@ACuriousMind Phew :-)

ACuriousMind

Although it doesn't contain the hard part of how to actually get the amplitude from that ;)

John Rennie

I wonder if it worth writing up in a more coherent form as an answer to one of the many "what are virtual particles?" questions on the site.

ACuriousMind

@DanielSank the part here that worries me is that I was the only one of four reviewers who voted to close this obvious duplicate. I'm less baffled that this question was posted than I am how it was not immediately closed. Come to think of it, not tagging it quantum-mechanics or special-relativity probably was a conscious decision to not give the gold badge holders the opportunity to close it single-handedly :P

2

K. T.

hey guys

ACuriousMind

10:33

hey guy

(or gal)

K. T.

wow.....44.2k reputation :O

you're a physics stackexchange legend

ACuriousMind

Just been here for a while and got too much free time on my hands :P

John Rennie

@ACuriousMind that question is a good example of the havoc wreaked by the hot network questions list. Bosoneando's answer is the only one that makes a serious attempt to answer the question and it's lagging at 11 upvotes. The top voted answer isn't an answer at all.

K. T.

Haha

John Rennie

If Bosoneando extended the answer to show how we get experimental observations like $E=h\nu$ I think it would be verging on the awesome.

Hi K. T. Did you want to ask something or is this a social call? :-)

K. T.

10:37

Hey John...I didn't want to ask anything in particular just checking the chat out :D

ACuriousMind

@JohnRennie Well, JanLalinsky's answer is also a better attempt to answer the question, I think, but yeah, the good answers are at the bottom of the list because the average HNQ visitor can't understand them :P

John Rennie

@peterh it now has the ACM seal of not total disapproval :-)

17 mins ago, by ACuriousMind

@JohnRennie It is near the truth

David Z

@ACuriousMind I don't know, I wouldn't have applied those tags either.

K. T.

Actually, I do have a question. But it's a bit broad and not really worthy of a SE post

David Z

Out of curiosity, you can't re-tag it and then VTC as duplicate, can you?

ACuriousMind

10:41

@DavidZ nope, the dupe hammer acts on the initial tags

K. T.

could I ask it anyway? it's regarding qft I guess

John Rennie

@DavidZ no, I've tried that before :-)

You get a message telling you to bog off.

David Z

@K.T. anything goes here, pretty much

ACuriousMind

Which has puzzled me before at other questions because Qmechanic added the QM tag but I just cast an ordinary dupe vote

John Rennie

@K.T. ask away. The worst that will happen is that we don't answer.

K. T.

10:42

Sweet @david @john

David Z

@ACuriousMind Ah, yeah, I thought so. I vaguely remembered hearing about that in the mod chat room or on meta some time ago.

ACuriousMind

@DavidZ Well, I would have chosen either the QM or QFT tag at least, seeing how the photon is a quantum mechanical object.

K. T.

Okay so....If you had to explain the Higgs mechanism in quite simple terms (I know, sort sort of contradictory) then how would you do so?

Secret

0

Q: Entropy - State Function but not conserved?

By the second law of Thermodynamics we can define the state function Entropy up to an additive constant via $ \Delta S := \int{\frac{\delta Q_{rev}}{T}} $ At the same time the second law of TD gives us the inequality: $dS := \frac{\delta Q_{rev}}{T} > \frac{\delta Q_{irrev}}{T}$ Hence when t...

Entropy is a nonconserved state function. How to explain that in nitty gritty details I am not sure

John Rennie

2

Q: Bug in closing as duplicate?

If you have a gold badge in a tag then you can immediately close as a duplicate any question that has that tag. So for example I have a gold badge in [general-relativity] and [newtonian-mechanics] and should be able to close as duplicates any questions tagged with these. However I just VTC'd as ...

bug status-bydesign

ACuriousMind

10:43

May 24 at 22:15, by knzhou

Hey everybody, I'm trying to explain the Higgs mechanism to high schoolers, without using any 'fancy physics'.

David Z

@ACuriousMind I'd definitely go for QFT. I think quantum-mechanics on this site is for nonrelativistic QM.

@K.T. probably like this

K. T.

@ACuriousMind how do I can only see the first line of that post

@DavidZ will check it out thanks!

John Rennie

@K.T. click on the blue text "May 24 etc" - it's a link to the original message

ACuriousMind

@K.T. Ah, I linked an old chat message to show you we've talked about that before here and didn't really come up with a good thing. To see the rest of the chat click on the date

However, now different people are here, maybe someone has a good idea

K. T.

oh cool got it thanks curious and john :D

David Z

10:46

Ah, well if we're targeting high schoolers, I would not do it the way I posted.

John Rennie

@K.T. the best explanation I've seen is by Matt Strassler but this is probably above high school level.

K. T.

Oh, well that's what I am

Matt strassler!! I love his blog! Thanks John

John Rennie

Then trouble is that it's impossible to give a simpler explanation without it being wildly inaccurate.

Have a look at that article and see how you get on.

K. T.

yeah exactly...will do, thanks!

John Rennie

@K.T. He's written some absolutely awesome articles. I wish I could write anything half as good.

K. T.

10:49

yeah, he has! And he's extremely nice too! He went to the trouble of personally answering some questions I had for him (through facebook), and answering them very thoroughly

John Rennie

@DavidZ Amusingly, even if the initial tags have been deleted the dupehammer is still possible.

K. T.

hey guys sorry while reading one of the answers I don't quite understand something....When describing the higgs field potential, the writer writes "that Phi is a complex valued variable"

oh sorry nevermind I get it ignore my previous message

Secret

It still amaze me how the observable spin has a dependence on orientation in spacetime. I wonder if there are other observables that has no classical analog (e.g. can only be described by livinig in a hilbert space) that has such orientation dependent property?

ACuriousMind

11:07

@DavidZ @dmckee @Qmechanic What do I do if I notice a consistent pattern that certain users vote to leave open obvious homework-questions and very clear duplicates? Especially voting to leave open the obvious duplicates is very disruptive to the site in my opinion, since people tend to stumble upon the newer questions, scattering good answers to the same question all over the place

John Duffield

@DanielSank : no.

@ACuriousMind : it isn't an obvious duplicate. It's just a question that you can't answer.

@JohnRennie : that's sometimes how it is even when HNQ isn't involved.

David Z

11:24

@ACuriousMind Flag one of the user's posts with a custom mod flag explaining the situation.

John Duffield

@DavidZ : people who've taken a lot of time and trouble to answer a question aren't going to be too happy that it's been closed and the bounty removed. Especially when the top answer on the alleged duplicate is yours. And there was I saying this is physics stack exchange at it best.

David Z

Yes, I'm aware of that.

John Duffield

@DavidZ : but are you aware that your answer is wrong?

Emilio Pisanty

11:45

@ACuriousMind Yeah, I've seen some of that.

John Duffield

@DavodZ : see Arnold's answer and his reference to The Nature of Light: What Is a Photon? edited by Chandrasekhar Roychoudhuri and Rajarshi Roy. I know the former. This is worth a read : arxiv.org/abs/0803.2596 . There is no n in E=hf. There is no evidence that a photon is some “infinite set of component sinusoidal waves”.

0celo7

12:07

@BernardMeurer Our IT person knew instantly

Yashas Samaga

@JohnRennie physics.stackexchange.com/questions/273578/… check comments - the OP wants the Q's duplicate status to be removed - he has edited the Q

K. T.

Hey @da

DavidZ* I read your answer it was super helpful thanks!

but I still have gaps in my understanding about how it works, in the physical sense

For example where do condensates come into play in the higgs mechanism? And also I've heard that a very similar thing happens in superconductors, but I don't understand how

David Z

12:52

@K.T. You'd have to ask someone with more of a condensed matter background to understand how it relates to superconductors and condensates

K. T.

oh ok

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