Talk:Phase space

Work in Progress
To make Phase Space the main article on the (p, q) space of dynamical systems, and Phase diagram mainly on the physical chemistry uses of that term Linuxlad 23:48, 19 Feb 2005 (UTC)

Speaking of that, it looks like someone erroneously linked the English article on Phase Space with the French article on Phase Diagrams... Ed Sanville 18:56, 26 July 2005 (UTC)

The Images
The images require labels on their axes. The first image has none. The second image's resolution is too poor to decipher the axis labels.

quantization
The statement on quantisation needs a tidy - it's the product p.q which has the dimensions of action surely. (so from memory, dp.dq = h) Bob aka Linuxlad


 * Done. Howeever, I was sloppy with the links, some may be disambig pages. I have to run ... linas 16:24, 13 March 2006 (UTC)

Now, the statement on quantisation is better: no one will understand, who not already knows! It's a pity! ErNa 07:48, 14 March 2006 (UTC)

State Space
Isn't "Phase space" = "State space"? They both are the space of all possible states, aren't they? --Javalenok 19:48, 3 May 2006 (UTC)


 * No, "state space" is a synonym for Configuration space, which is a related but different concept. -- Four Dog Night 02:48, 25 August 2006 (UTC)


 * for a classical system with N particles, each point in the 3N + 3N dimensional phase space is a possible configuration of the system. in statistical mechanics, a "state" is then a probability distribution (in mathematical terms, a probability measure) on the phase space. so the state space is the family of probability distributions. for example, the microcanonical ensemble is a state; it corresponds to the distribution that is constant everywhere on some constant-enery surface of the phase space. Mct mht 04:47, 25 August 2006 (UTC)


 * ... and zero everywhere else. Ed Sanville 10:29, 25 August 2006 (UTC)


 * Another related category: "Dimensional analysis" Vugluskr 11:30, 29 December 2006 (UTC)

I meant the State space (controls). By (1) containing all possible states of a system and (2) being visualized by phase portrait, it is essentially identical to the phase space. You claim that the 'state space' is a configuration space, which represents only particle coordinates. Yet, I see the opposite: Instead of being dedicated to the particle potential coordinates, the abstract dynamic system state space permits any state variables. That is, by representing any system, not only particles, state space is more general than your phase space. --Javalenok (talk) 11:47, 31 July 2010 (UTC)

Phase space and Parameter space
"In a phase space, every degree of freedom or parameter of the system is represented as an axis of a multidimensional space" I think this phrase is misleading. There is a difference between phase space as defined here and parameter space. The phase space is all the possible states of a given system (usually over time for different initial conditions). Changing the parameters of a system (for example the stiffness of a spring) can give a different phase space. If we allow the parameters to be changed then we obtain a family of phase spaces. Usually, if the system has fixed points one can study the evolution of this fixed points when changing the parameters of the system, leading to the bifurcation diagram in which the parameters of the system are the axes of the plot. To summarize, in the phase space you do not represent the parameters as an axis. If the parameter has dynamics then ti can be considered a degree of freedom of the system. JuanPi —Preceding comment was added at 12:40, 20 February 2008 (UTC)

WikiProject class rating
This article was automatically assessed because at least one WikiProject had rated the article as start, and the rating on other projects was brought up to start class. BetacommandBot 10:01, 10 November 2007 (UTC)

Contrast with configuration space
I think it might be helpful to contrast phase space with configuration space, or at least link to that article at some point. Steve Avery (talk) 02:20, 14 January 2008 (UTC)

dynamical
I assume all of those "dynamical" and "thermodynamical" should actually be "dynamic" and "thermodynamic"? Didn't change them in case it's some weird mathematical usage. —Preceding unsigned comment added by 196.30.245.149 (talk) 14:22, 4 March 2008 (UTC)

Use outside of physics and mathematics
I've noticed the use of the phrase "phase space" used in the context of ideas. It describes all the possible outcomes of a given scenario. Since this use is not widespread, does it merit mention in this article? —Preceding unsigned comment added by Scochran4 (talk • contribs) 21:23, 10 February 2009 (UTC)

are the x & y axes of that first illustration defined?
they are not labeled in the graphic. I fancy myself smart and interested in heady stuff like this, but I think I need to knwo what those axes are to understand the illustration. right? skank-L juice 13:58, 21 August 2010 (UTC)

If x is position & y is momentum, (commom) then the diagram is flipped - should be clockwise... —Preceding unsigned comment added by 74.70.151.113 (talk) 23:25, 17 October 2010 (UTC)

Even if the axes were dimensionless (and in this case, they can't be as momentum is expressed in units), they should be labelled. Ross Fraser (talk) 00:29, 13 August 2011 (UTC)

Thermodynamic Phase Space

 * "a point in the 6N-dimensional phase space describes the dynamical state of every particle in that system, as each particle is associated with three position variables and three momentum variables"

Really? 6N-dimensional space? So for a system of 10^20 gas particles, we'd have a 6*(10^20)-dimensional phase space? It seems to me that this is a typo and should read 6-dimensional phase space. I'll change it in the next couple of days if nobody objects. Larryisgood (talk) 18:38, 12 October 2010 (UTC)


 * Really. Cf. Decoherence  .  Cuzkatzimhut (talk) 21:02, 23 November 2010 (UTC)

Cotangent space or cotangent bundle?
The article states: For mechanical systems, the phase space usually consists of all possible values of position and momentum variables (i.e. the cotangent space of configuration space).

It seems to me the expression "cotangent space" should be replaced with "cotangent bundle". As I am not 100% sure, I open a talk page. Anderstood (talk) 20:21, 7 July 2014 (UTC)


 * I agree that there's a problem. The prose is ambiguous -- if the "i.e." comment is taken to be explaining only the preceding term "momentum variables", then maybe it's not incorrect, but that's not the natural reading, and in any case, the comment (and the term "cotangent") is out of place in the lede, so the best approach is just to delete it all, which I will now do... Eleuther (talk) 20:18, 24 October 2014 (UTC)

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Duffing oscillator phase path
The animated image showing the evolution of a system's phase claims that what is shown is the phase path of the Duffing oscillator. However, the "phase" crosses its own path several times, which it cannot do. Since all information needed to determine the evolution of a system's phase can be determined from the phase itself, the time derivative of the phase must also be completely given completely by the phase itself. Hence, a system's phase cannot cross its own path because that would mean that at the crossing point, the phase has multiple time derivatives, which it cannot have in a single point. Therefore, what is shown in the image cannot be the phase path of any system, for the simple reason that the path crosses itself.

So, what exactly is it that is shown in the image? —Kri (talk) 17:05, 3 January 2021 (UTC)


 * I agree; the figure also had me baffled. I think that it should be removed since it confuses more than it helps, and as was pointed out above, the path intersects itself.  Hmmm... Thinking about this a bit more leads me to believe that the time-dependent forcing term could cause the paths to cross. 72.93.238.65 (talk) 18:11, 23 January 2021 (UTC)


 * Since the oscillator is being damped and/or driven by a non-constant force, there doesn't seem to be any reason to expect that the entire dynamics of the system would be captured by position and momentum alone. Solemnavalanche (talk) 22:08, 3 February 2021 (UTC)


 * Non-intersection of phase curves is a property of autonomous systems. Since the Duffing oscillator is not autonomous, it will not have that property. One can, of course, introduce $$\dot \tau = 1$$ to have a non-intersecting phase space $$(\tau, x ,\dot x)$$. However, any image of this phase space will appear to have intersecting phase curves. Malexj93 (talk) 08:56, 16 March 2021 (UTC)


 * Ah, that's right! I removed the dubious template instance. —Kri (talk) 12:58, 10 April 2021 (UTC)

Number of dimensions of the configuration space and the phase space
In the text is written "The phase space of a two-dimensional system is called a phase plane, which occurs in classical mechanics for a single particle moving in one dimension, and where the two variables are position and velocity. In this case, a sketch of the phase portrait may give qualitative information about the dynamics of the system, such as the limit cycle of the Van der Pol oscillator shown in the diagram."

I think it should be "The phase space of a one-dimensional system is called a phase plane..."

Phase Space Normalisation
"Once the phase integral is known, it may be related to the classical partition function by multiplication of a normalization constant representing the number of quantum energy states per unit phase space. This normalization constant is simply the inverse of Planck's constant raised to a power equal to the number of degrees of freedom for the system."

I think this is misleading, because if we have a N-particle System in 3 dimensions with coordinates (p,q) it has 6N Dimsnesions but the power of h of the prefactor of the Phase Space Integral is still 3N. XNR32 (talk) 10:49, 18 February 2023 (UTC)