# History of the use of the concept of phase space in engineering

Engineering textbooks constantly use the concept of 'phase space' (see e.g. http://www.cs.cmu.edu/~baraff/sigcourse/notesc.pdf). That is, they think of the state of a mechanical system as a high-dimensional geometric space. For example, for a system of particles, the phase space has $6n$ dimensions, where $n$ is the number of particles.

My understanding is that the notion of phase space initially developed among theoretical physicists (possible in conjunction with quantum mechanics? or possibly before that point?). Therefore, I'm wondering when and how engineers began to adopt this terminology and concept from physicists.

It might have something to do with differential equations, since the concept of representing the state of a mechanical system as a point in a space becomes obvious when you view ODE's in terms of flows on euclidean spaces.

NOTE: The phase space concept was invented by theoretical physicist and mathematician Willard Gibbs.

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I'm not sure I see an actual physics question here. Engineers are often trained partly by physicists and often work with them in research fields. Of course the cultures mix and useful concepts are carried across discipline boundaries. – dmckee Jun 4 '11 at 1:12
I mean, it's a physics history question and not a physics question, but this website has history questions, no? – Davidac897 Jun 5 '11 at 6:07

The concept of phase space was probably floating around for ages, but it only really became central to modern physics in the mid 19th century, with Boltzmann and Maxwell's formulation of statistical mechanics, Hamilton's reformulation of mechanics, and Liouville's theorem.

The concept of phase space is already implicit in the work immediately following Newton, when Maupertuis and Euler studied variational principles. The trajectory in a variational formulation is never isolated--- the neighboring trajectories are important for determining that the solution is extremal. So Euler and Maupertuis were formulating the condition of Newton's laws as a constraint on the space of all trajectories.

The space of all trajectories is the same as phase space, once you impose the equations of motion. But the space itself is not used in these formulations, because it is not explicitly given a full geometrical symplectic structure. All the structure of phase space left implicit in the 18th century work became explicit in the 19th century.

In order to describe a gas, Boltzmann introduced a function $\rho(x,v)$ which described the probability density for a particle to be at position x and have velocity v. This function obeys a dissipative equation, known as the Boltzmann equation, and this is a paradox, as noted by Lorschmidt and others. The paradox is that the irreversibility cannot emerge from time-reversible laws, because you could reverse all the molecular velocities, and make things go back to how they started. So why is Boltzmann's equation irreversible?

The reason was understood quickly--- the equation is assuming that there are no correlations between molecules that enter collisions, so that each one is described by an independent pick from the same distribution $\rho$. This is clearly not right for dense materials, since the collision will imprint the first particle's position and velocity on the second, leading to at least two-body correlation, which might come back to affect the original particle after a few more collisions. In order to incorporate correlations, you have to think of the density $\rho$ as a simultaneous function of all the particle's positions and velocities.

Hamilton's formulation of mechanics shows that it is the momenta that are the correct variables, not the velocities, and this was made clearer with the result that the phase-space volume is conserved in collisions if you use momenta and not velocities as your coordinates. The Liouville theorem of conservation of phase-space volume was a central result in making plausible the derivation of thermodynamics from mechanics, which was most active at the end of the 19th century, but wasn't fully accepted until the 1920s.

Poincare analyzed phase space motions in his famous analysis of the three-body problem and related issues of non-integrability. I do not know this history of engineering, but I assume that by the turn of the 20th century, when Poincare was writing, these results were standard lore.

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Getretener Quark wird breit, nicht stark. – Georg Oct 6 '11 at 11:23
Ich bin ein Berliner – Ron Maimon Oct 6 '11 at 18:11

By the way,there was a nice article in Physics Today about the history of phase space in physics.

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A free PDF version is available at the author's homepage physics.purdue.edu/nlo – Qmechanic Oct 6 '11 at 13:57