The non-squeezing theorem, also called ''Gromov's non-squeezing theorem'', is one of the most important theorems in
symplectic geometry
Symplectic geometry is a branch of differential geometry and differential topology that studies symplectic manifolds; that is, differentiable manifolds equipped with a closed differential form, closed, nondegenerate form, nondegenerate different ...
. 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
In mathematics, a measure-preserving dynamical system is an object of study in the abstract formulation of dynamical systems, and ergodic theory in particular. Measure-preserving systems obey the Poincaré recurrence theorem, and are a special cas ...
transformation: just picture
squeezing the ball into the cylinder (hence, the name non-squeezing theorem). 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
We start by considering the symplectic spaces
:
the ball of radius ''R'':
and the cylinder of radius ''r'':
each endowed with the
symplectic form
:
Note: The choice of axes for the cylinder are not arbitrary given the fixed symplectic form above; namely the circles of the cylinder each lie in a symplectic subspace of
.
The non-squeezing theorem tells us that if we can find a symplectic embedding ''φ'' : ''B''(''R'') → ''Z''(''r'') then ''R'' ≤ ''r''.
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
Maurice A. de Gosson (born 13 March 1948), (also known as Maurice Alexis de Gosson de Varennes) is an Austrian mathematician and mathematical physicist, born in 1948 in Berlin. He is currently a Senior Researcher at the Numerical Harmonic Analy ...
states:
Similarly:
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
In mathematics and classical mechanics, canonical coordinates are sets of coordinates on phase space which can be used to describe a physical system at any given point in time. Canonical coordinates are used in the Hamiltonian formulation of ...
and ''var'' and ''cov'' the variance and covariance functions.
[Maurice de Gosson: ''How classical is the quantum universe?']
arXiv:0808.2774v1
(submitted on 20 August 2008)
References
{{reflist
Further reading
*
Maurice A. de Gosson
Maurice A. de Gosson (born 13 March 1948), (also known as Maurice Alexis de Gosson de Varennes) is an Austrian mathematician and mathematical physicist, born in 1948 in Berlin. He is currently a Senior Researcher at the Numerical Harmonic Analy ...
: ''The symplectic egg''
arXiv:1208.5969v1 submitted on 29 August 2012 – includes a proof of a variant of the theorem for case of ''linear'' canonical transformations
*
Dusa McDuffWhat is symplectic geometry? 2009
Symplectic geometry
Theorems in geometry