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Mathematical methods in classical mechanics, classical and quantum field theory, quantum mechanics, statistical mechanics, condensed matter, nuclear and atomic physics.

67 votes
Accepted

Poincaré Conjecture and the Shape of the Universe

In Einstein's theory of General Relativity, the universe is a 4-manifold that might well be fibered by 3-dimensional time slices. If a particular spacetime that doesn't have such a fibration, then it …
Greg Kuperberg's user avatar
28 votes
Accepted

Basic question about differential forms and physics

In both physics and mathematics, there are times when you want a signed multiple integral $dx \wedge dy$, and there are times when you want its unsigned counterpart $dx\;dy = |dx \wedge dy|$. The dif …
Greg Kuperberg's user avatar
26 votes

In what ways is physical intuition about mathematical objects non-rigorous?

As I see it, the situation is a combination of all of the reasons listed, but I would frame the issues differently: There are many derivations and topics in physics that are entirely rigorous in prin …
22 votes

Noether's theorem in quantum mechanics

In hindsight, Noether's theorem is a dramatic hint of quantum mechanics. Mariano is completely correct in his comment that the conserved quantity is $A$ itself, but it deserves a bit of explanation. …
Greg Kuperberg's user avatar
19 votes

What are D-branes, really?

I'm going to attempt a short, partial answer written for pure mathematicians. The word "brane" in high-energy physics means "submanifold". The word is short for "membrane". More precisely, it means …
Greg Kuperberg's user avatar
15 votes
Accepted

Why is every symplectomorphism of the unit disk Hamiltonian isotopic to the identity?

It is a theorem of Smale that the group of orientation-preserving diffeomorphisms of $D^2$, rel boundary, is contractible. If the diffeomorphisms can move the boundary, you can establish a homotopy e …
Greg Kuperberg's user avatar
10 votes
Accepted

What is the physical difference between states and unital completely positive maps?

The main interpretation, which is fundamental in quantum information theory, is that the transpose of a UCP map $E$ is a linear map on quantum states that represents a realistic information channel. …
Greg Kuperberg's user avatar
8 votes

Quantum channels as categories: question 1.

As I see it, this posted question and some aspects of the answers turn an important but straightforward fact into something needlessly complicated and less general. Let $\mathcal{A}$ (Alice) and $\ma …
Greg Kuperberg's user avatar
6 votes
Accepted

Casson's invariant and the trivial connection contribution to witten's 3-manifold invariant

The Casson invariant is not the same sum or integral over connections that you would derive from the perturbative expansion Cherns-Simons quantum field theory at all flat connections. There is more t …
Greg Kuperberg's user avatar
2 votes

How is Fredkin and Toffoli's Conservative Logic related to Linear Logic?

The question seems to be groping for a fancy, specific answer when, in my view, the most important connection is relatively basic and general. In mathematics, as you say, you have symmetric monoidal …
Greg Kuperberg's user avatar