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• CommentRowNumber1.
• CommentAuthorUrs
• CommentTimeSep 15th 2017
• (edited Sep 15th 2017)

At field (physics) I am beginning to write an actual introduction to the topic, now in a new section titled “A first idea of quantum fields”.

This means to introduce the concept with precise detail, but in a simple context (trivial and bosonic field bundles over Minkowski spacetime, perturbatively quantized) that allows to get a quick idea of the idea of the concept of (quantum) fields as such, without being distracted by other details.

So far I made it up to the derivation of the EOMs. Discussion of (deformation) quantization is to follow (maybe by tonight, depending on how much trouble I have with the trains) and I plan to sprinkle in the detailed example from scalar field in parallel with the abstract discussion.

• CommentRowNumber2.
• CommentAuthorDavid_Corfield
• CommentTimeSep 15th 2017

Did some proof reading.

• CommentRowNumber3.
• CommentAuthorUrs
• CommentTimeSep 15th 2017

Thanks, David!

I did some more work in the Sandbox, but not ready yet.

• CommentRowNumber4.
• CommentAuthorUrs
• CommentTimeSep 18th 2017
• (edited Sep 18th 2017)

I have expanded a little more in the Sanbox. But it turns out I need more time than I thought to finish typing this “First Idea of Quantum Fields”. Now I need to interrupt working on it until this Wednesday, and instead meanwhile I need to work further on S-matrix. Eventually the dust will settle…

• CommentRowNumber5.
• CommentAuthorUrs
• CommentTimeSep 20th 2017

Have been further working on it, but still not done.

It finally dawned on me that it is pointless to try to squeeze this material into the “Idea”-section at field (physics). So I have declared it now a stand-alone chapter of geometry of physics. Now here:

• CommentRowNumber6.
• CommentAuthorTodd_Trimble
• CommentTimeSep 20th 2017

Let me say though that I think what you’re doing recently (the Insights article at Physics Forum, the articles on S-matrix and pQFT, etc.) is just great. For me it’s some of the most useful exposition I’ve ever seen for this area – far better than the books by Witten et al., this really is Quantum Fields for Mathematicians. Keep it coming!

• CommentRowNumber7.
• CommentAuthorUrs
• CommentTimeSep 20th 2017

Thanks for the feedback, Todd!

When this chapter “A first idea of QFT” is closer to being stable (hopefully by tomorrow evening), I’d be very interested in hearing of your experience while reading through it, such as at which points the progression of the text seems unclear or maybe too unmotivated, or else maybe too pedantic or repetitive – whatever the case may be.

• CommentRowNumber8.
• CommentAuthorUrs
• CommentTimeSep 21st 2017
• (edited Sep 21st 2017)

I have now made it to the description of the covariant phase space in section 7, and filled in full details on the Poisson bracket Lie $(p+1)$-algebra in section 5. (here)

Using this, next will finally be the discussion of the algebra of quantum observables in section 8. But not tonight.

• CommentRowNumber9.
• CommentAuthorDavidRoberts
• CommentTimeSep 21st 2017

I”m enjoying this too.

• CommentRowNumber10.
• CommentAuthorDavid_Corfield
• CommentTimeSep 22nd 2017

These examples may be combined: the mapping space $[\Sigma, \mathbf{\Omega}^n]$ is a kind of smooth classifying space for differential forms on $\Sigma$

Is ’on’ the right preposition here? Isn’t $[\Sigma, \mathbf{\Omega}^n]$ classifying $\Sigma$-parameterized differential $n$-forms?

• CommentRowNumber11.
• CommentAuthorUrs
• CommentTimeSep 22nd 2017
• (edited Sep 22nd 2017)

Isn’t $[\Sigma, \mathbf{\Omega}^n]$ classifying $\Sigma$-parameterized differential $n$-forms?

What you are thinking of are the plots

$U \to [\Sigma, \mathbf{\Omega}^n]$

which equivalently are differential $n$-forms on $U \times \Sigma$, which one may think of as $U$-parameterized differential forms on $\Sigma$ – since $U$ varies, while $\Sigma$ is fixed.

The subtlety here, which I did not want to get into in this “first idea”-chapter, is that the actual classifing smooth space of differential forms on $\Sigma$ should assign to $U$ not all the differential forms on $U \times \Sigma$, but just those which are plain functions with respect to $U$ (with no “legs” along $U$). This genuine classifying space of differential forms on $\Sigma$ is the concretification

$\sharp_1 [\Sigma, \mathbf{\Omega}^n] \,.$

There is detailed discussion of this point at geometry of physics – differential forms, in the section Smooth moduli space of differential forms

• CommentRowNumber12.
• CommentAuthorUrs
• CommentTimeSep 22nd 2017

I’m enjoying this too.

Thanks. If you have any comments (in particular critical comments) please do let me know.

• CommentRowNumber13.
• CommentAuthorDavid_Corfield
• CommentTimeSep 22nd 2017

I guess that’s a generalized element point of view, like we might have:

In $Set$, we say that $[A, 2]$ is the classifier of subsets of $A$, rather than just the set of subsets of $A$, because for any $X$, $X \to [A,2]$, picks out $X$-parameterized subsets of $A$.

• CommentRowNumber14.
• CommentAuthorUrs
• CommentTimeSep 22nd 2017

$X \to [A,2]$, picks out $X$-parameterized subsets of $A$.

Exactly! But this is not how you said it in #10. In your analogy $\Sigma$ there corresponds to $A$ here.

Anyway, I suppose we perfectly agree on what’s going on.

• CommentRowNumber15.
• CommentAuthorUrs
• CommentTimeSep 22nd 2017

I have now spelled out the example of the free real scalar field alongside the development of the theory. Currently it culminates in a detailed derivation of the “causal propagator”/”Peierls bracket”/”Pauli-Jordan distribution” from first principles – here.

• CommentRowNumber16.
• CommentAuthorUrs
• CommentTimeOct 4th 2017
• (edited Oct 4th 2017)

I have been working on the next section: “Gauge symmetries”.

• CommentRowNumber17.
• CommentAuthorDavid_Corfield
• CommentTimeOct 10th 2017

I imagine you’ll be editing offline, so I’ll note some typos here:

to give first good precise idea; allowed to be globally hyperbolic Lorentzian manifold; family of smooth functions on $\Sigma$ [should be $X$]; differential geoemtry; hence a field) [no opening bracket];

A stray ’s’ appears in the right hand diagram after

such that this system is compatible with the above projection maps, i.e. such that

Not sure I see this:

\begin{aligned} \left(j^\infty_\Sigma(A)\right)^\ast(f_{\mu \nu}) & = F_{\mu \nu} \\ & = (d F)_{\mu \nu} \,. \end{aligned}

Why does $F_{\mu \nu} = (d F)_{\mu \nu}$?

• CommentRowNumber18.
• CommentAuthorDavidRoberts
• CommentTimeOct 10th 2017
• (edited Oct 10th 2017)

That last one might have to be $(d f)_{\mu\nu}$

• CommentRowNumber19.
• CommentAuthorUrs
• CommentTimeOct 10th 2017

Thanks! All fixed now.

($d F$ should have been $d A$ :-)

• CommentRowNumber20.
• CommentAuthorUrs
• CommentTimeOct 11th 2017

There is now some first actual content in the next section: Reduced phase space.

• CommentRowNumber21.
• CommentAuthorUrs
• CommentTimeOct 12th 2017
• (edited Oct 12th 2017)

I am exploring ways to best draw the big picture regarding the quantization of gauge theories. Here is one attempt:

$\,$

$\,$

$\array{ \underline{\mathbf{\text{pre-quantum geometry}}} && \underline{\mathbf{\text{higher pre-quantum geometry}}} \\ \, \\ \left\{ \array{ \text{Lagrangian field theory with} \\ \text{implicit infinitesimal gauge transformations} } \right\} &\overset{ \text{explicate} \atop \text{gauge transformations} }{\longrightarrow}& \left\{ \array{ \text{dg-Lagrangian field theory with} \\ \text{explicit infinitesimal gauge transformations} \\ \text{ embodied by BRST complex } } \right\} \\ && \Big\downarrow{}^{\mathrlap{ \text{pass to} \atop \text{derived critical locus} }} \\ \Big\downarrow && \left\{ \array{ \text{dg-reduced phase space} \\ \text{ embodied by BV-BRST complex } } \right\} \\ && {}^{\mathllap{\simeq}}\Big\downarrow{}^{\mathrlap{\text{fix gauge} }} \\ \left\{ \array{ \text{ decategorified } \\ \text{ covariant } \\ \text{ reduced phase space } } \right\} &\underset{\text{pass to cohomology}}{\longleftarrow}& \left\{ \array{ \text{ dg-covariant} \\ \text{reduced phase space } } \right\} \\ && \Big\downarrow{}^{\mathrlap{ \array{ \text{ quantize } \\ \text{degreewise} } }} \\ \left\{ \array{ \text{gauge invariant} \\ \text{quantum observables} } \right\} &\underset{\text{pass to cohomology}}{\longleftarrow}& \left\{ \array{ \text{quantum} \\ \text{BV-BRST complex} } \right\} }$

Here:

term meaning
“phase space” derived critical locus of Lagrangian equipped with Poisson bracket
“reduced” gauge transformations have been homotopy-quotiented out
“covariant” Cauchy surfaces exist degreewise
• CommentRowNumber22.
• CommentAuthorUrs
• CommentTime4 days ago
• (edited 4 days ago)

I have now brought in the material which I had earlier written into separate entries, such as to have a skeleton for the complete notes (needs to be inter-connected and polished still, but should give an idea of the intended content): here.

There are now 19 sections. I don’t think I could do with less, but also I shouldn’t have much more.

I changed my strategy regarding introducing the required generalized geometry. Previously I had a lightning introduction to “functorial geometry” in the first section “Geometry”, but trying this out on readers like Arnold Nuemaier over in the PO-Insights discussion here I realized that this doesn’t work for the intended audience. So now I am instead trying to introduce the required geometry alongside as the corresponding physics gets introduced: first the “functorial geometry” in section 3 Fields then later the “higher geometry” in section 9 Gauge symmetries.

This still needs a bit more work, I suppose. But maybe one can see if it is going in the right direction now.

• CommentRowNumber23.
• CommentAuthorUrs
• CommentTime3 days ago
• (edited 3 days ago)

David C: I see that you made an edit yesterday evening. To check what you did, I tried to click on “see changes”, but that produced a “502” error. Now I saved another version of mine and then similarly tried to see your changes via the History link here:

https://ncatlab.org/nlab/revision/diff/geometry+of+physics+–+A+first+idea+of+quantum+field+theory/45

but again I get a 502 error.

I’ll report this to Adeel. Meanwhile: Is it easy for you to remember what you edited?

• CommentRowNumber24.
• CommentAuthorDavid_Corfield
• CommentTime3 days ago
• (edited 3 days ago)

Yes, you had an extra curly bracket in

$D_\mu a_\nu^\alpha \;\coloneqq\; a^\alpha_{\nu,\mu} + \tfrac{1}{2} \gamma^{\alpha}{}_{\beta \gamma} a^\beta_{\mu} a^\gamma_{\nu} - (\mu \leftrightarrow \nu)$

so it wasn’t compiling.

• CommentRowNumber25.
• CommentAuthorDavid_Corfield
• CommentTime3 days ago
• (edited 3 days ago)

Oh dear, now I can’t see it compile here. It was the equation after

consider the functions on the jet bundle given by

[edit: silly me! I pasted in the original mistaken code.]

• CommentRowNumber26.
• CommentAuthorUrs
• CommentTime3 days ago

Thanks! I fixed it.

When you say “doesn’t compile” do you mean that the whole page failed, or just the equation? That’s why I keep checking in the Sandbox, to ensure that the page as a whole comes out readably after saving. On my end it alsways did.(?)

1. Just that equation.

• CommentRowNumber28.
• CommentAuthorUrs
• CommentTime3 days ago

I have started bringing in the example of the Dirac field (here). To that end I added to the section “Spacetime” discussion of spin, and to the section “Fields” discussion of supergeometry.

Now there is need to harmonize some notation a bit better. But not today.

Next I should try to sort out that issue with distributions as linear maps in the Cahiers topos (here).

• CommentRowNumber29.
• CommentAuthorDavidRoberts
• CommentTime3 days ago

For $\mathfrak{g} = \mathfrak{su}(2)$ this is a field history for the gauge field of the strong nuclear force in quantum chromodynamics.

This is a typo, no?

Also: “ant-symmetry”, “so smal”

In your “Remark 2.22. (two-component spinor notation)”, you write

denoted $(\xi^{\dagger \dot a})_{\dot a = 1}^2$

which at first I thought was a power, but then I realised you meant $\dot a=1,2$. This might be less confusing as the notation is already rather dense. Similarly for the left-handed spinor.

In “Definition 2.28. (adiabatic switching)”, you have

the vctor space space $C^\infty(\Sigma)\langle g \rangle$ spanned by a formal variable $g$

which apart from the typo was slightly confusing in the sense that you are, immediately before, talking about a cutoff function $g_{sw}$, so at first I guessed that $g$ here might be such a function, but I guess it is the gauge coupling? Might be worth disambiguating, or sign-posting here.

an infinitesimally thickened Cartesian space $\mathbb{R}^n \times Spec(A)$ is represented by a commutative algebra $C^\infty(\mathbb{R}^n) \otimes Spec(A) \in \mathbb{R} Alg$ which

that second $Spec(A)$ should be just $A$, I think.

for each infinitesimally thickened Cartesian space $\matbb{R}^n \times Spec(A)$

\matbb rather than \mathbb

More: “the 1-dimensional even vector sspace”, “an super Cartesian space”

super-commutative algebra $C^\infty(\mathbb{R}^n) \otimes Spec(A) \in \mathbb{R} Alg$

again, $A$, not $Spec(A)$, I think.

That’s down to the start of the section “Field variations”. More later, I hope.

• CommentRowNumber30.
• CommentAuthorUrs
• CommentTime3 days ago
• (edited 3 days ago)

Thanks for catching all this! All fixed now.

In “Definition 2.28. (adiabatic switching)”, […] slightly confusing

I have tried to improve the wording to clarify this. Now it reads like so:

For a causally closed subset $\mathcal{O} \subset \Sigma$ of spacetime say that an adiabatic switching function or infrared cutoff function for $\mathcal{O}$ is a smooth function $g_{sw}$ of compact support (a bump function) whose restriction to some neighbourhood $U$ of $\mathcal{O}$ is the constant function with value $1$:

$Cutoffs(\mathcal{O}) \;\coloneqq\; \left\{ g_{sw} \in C^\infty_c(\Sigma) \;\vert\; \underset{ {U \supset \mathcal{O}} \atop { \text{neighbourhood} } }{\exists} \left( g_{sw}\vert_U = 1 \right) \right\} \,.$

Often we consider the vector space space $C^\infty(\Sigma)\langle g \rangle$ spanned by a formal variable $g$ (the coupling constant) under multiplication with smooth functions, and consider as adiabatic switching functions the corresponding images in this space,

$\array{ C_c^\infty(\Sigma) &\overset{\simeq}{\longrightarrow}& C_c^\infty(X)\langle g\rangle }$

which are thus bump functions constant over a neighbourhood $U$ of $\mathcal{O}$ not on 1 but on the formal parameter $g$:

$g_{sw}\vert_U = g \,$

In this sense we may think of the adiabatic switching as being the spacetime-depependent coupling “constant”.

• CommentRowNumber31.
• CommentAuthorDavidRoberts
• CommentTime3 days ago

Aha, that’s interesting. Both my guesses were wrong.

More:

Their variational derivative uniquely decomposes as 1) the Euler-Lagrange derivative $\delta_{EL}\mathbf{EL}$ which is

is (or defines) a prequantum [Lagrangian field theory]]

and just before “Here $\star_\eta$ denotes the Hodge star operator of Minkowski spacetime.”

one sees an equation where it is given as $\wedge_\eta \star$. Also the sentence following seems to be truncated. Perhaps you were going to expand the example, or just meant to make a comment saying more general Lie algebras were possible.

such that $\delta EL$ is proportional to the variational derivative of the fields (but not their derivatives[…]

missing underscore.

More later.

• CommentRowNumber32.
• CommentAuthorUrs
• CommentTime3 days ago

Thanks again!! All fixed now.