Non-squeezing theorem


The non-squeezing theorem, also called Gromov's non-squeezing theorem, is one of the most important theorems in symplectic geometry. It was first proven in 1985 by Mikhail Gromov.
The theorem states that one cannot embed a ball into a cylinder via a symplectic map unless the radius of the ball is less than or equal to the radius of the cylinder. The theorem is important because formerly very little was known about the geometry behind symplectic maps.
One easy consequence of a transformation being symplectic is that it preserves volume. One can easily embed a ball of any radius into a cylinder of any other radius by a volume-preserving transformation: just picture squeezing the ball into the cylinder. Thus, the non-squeezing theorem tells us that, although symplectic transformations are volume-preserving, it is much more restrictive for a transformation to be symplectic than it is to be volume-preserving.

Background and statement

Consider the symplectic spaces
each endowed with the symplectic form
The space is called the ball of radius and is called the cylinder of radius. The choice of axes for the cylinder are not arbitrary given the fixed symplectic form above; the circles of the cylinder each lie in a symplectic subspace of.
If and are symplectic manifolds, a symplectic embedding is a smooth embedding such that. For, there is a symplectic embedding which takes to the same point.
Gromov's non-squeezing theorem says that if there is a symplectic embedding, then.

Symplectic capacities

A symplectic capacity is a map satisfying
  1. If there is a symplectic embedding and, then,
  2. ,
  3. and.
The existence of a symplectic capacity satisfying
is equivalent to Gromov's non-squeezing theorem. Given such a capacity, one can verify the non-squeezing theorem, and given the non-squeezing theorem, the Gromov width
is such a capacity.

The “symplectic camel”

Gromov's non-squeezing theorem has also become known as the principle of the symplectic camel since Ian Stewart referred to it by alluding to the parable of the camel and the eye of a needle. As Maurice A. de Gosson states:
Similarly:
Although the term "symplectic camel" is sometimes used loosely to describe Gromov's non-squeezing theorem in its static form, experts in symplectic topology reserve it for the parametric version, which concerns the impossibility of moving a symplectic ball from one side of a hyperplane to the other via a one-parameter family of symplectic embeddings, in such a way that the symplectic reductions of the intersections of the balls with the hyperplane are always contained in a cylinder of smaller capacity.

Further work

De Gosson has shown that the non-squeezing theorem is closely linked to the Robertson–Schrödinger–Heisenberg inequality, a generalization of the Heisenberg uncertainty relation. The Robertson–Schrödinger–Heisenberg inequality states that:
with Q and P the canonical coordinates and var and cov the variance and covariance functions.