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Recently, I have been learning about path integrals and I was wondering, can the probability of a certain path be weighted more in a path integral? Said in another way, can certain paths have more probability in a path integral?

TanMath
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    That's the whole point of the path integral. – Danu Dec 15 '14 at 00:31
  • What do you mean by "probability of a path"? The $\mathrm{e}^{\mathrm{i}S[\gamma]}$, for $\gamma$ a path? And what do you mean by them having "more probability"? – ACuriousMind Dec 15 '14 at 00:33
  • @Danu well, in one of my other questions, it was mentioned that the probabiliy is always the same for each path... – TanMath Dec 15 '14 at 00:34
  • @ACuriousMind I mean that one path has more chance of actually occuring when osberved... – TanMath Dec 15 '14 at 00:34
  • Paths don't "actually occur". – ACuriousMind Dec 15 '14 at 00:35
  • @ACuriousMind then what's the point of a path integral? – TanMath Dec 15 '14 at 00:35
  • Telling you exactly what the chance is that, given you measure a particle at $x_1$ at time $t_1$, you will measure a particle at $x_2$ at time $t_2$ ;) – Danu Dec 15 '14 at 00:39
  • @Danu so I want to increase the chance for one of those paths... – TanMath Dec 15 '14 at 00:40
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    @TAbraham the whole point of the path integral is that the particle takes (in Feynman's words "sniffs out") all paths between its start and end point. Not that it takes only one and chooses with various probabilities. You absolutely shouldn't think of it taking any one path (because this is wrong). – or1426 Dec 15 '14 at 00:43
  • @or1426 "sniffs out" for what reason? Now I don't get the point of path integrals... – TanMath Dec 15 '14 at 00:47
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    @TAbraham Please try to at least reach a minimum level of understanding on what it is you're talking about, before asking your questions. Read about path integrals, here e.g., or grab one among the so-many nicely written basic QM textbooks and get started! You cannot learn everything just by asking questions one by one in a forum... – Ellie Dec 15 '14 at 00:47
  • I am deleting.... – TanMath Dec 15 '14 at 00:48
  • Related question by OP: http://physics.stackexchange.com/q/149953/2451 – Qmechanic Dec 15 '14 at 01:02

1 Answers1

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In general the "weighting" of each path $q$ in a path integral is given by $e^{\frac{i}{\hbar}S[q]}$. Then paths for which the action $S$ is stationary with respect to small deviations from the path are the only ones which really contribute because the contributions from those with non stationary $S$ get averaged out as the phase changes very rapidly (because $\hbar$ is very small).

The number $S[q]$ is defined as:

$$S[q] = \int_{t_0}^{t_1}L(q(t),\dot{q}(t), t)dt$$

Where $t$ is some parameter that varies along the path and $L$ is the lagrangian. The Langrangian will depend on the details of your system but for a free particle it looks like the classical kinetic energy $L = \frac{1}{2}\dot{q}^2$.

or1426
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  • what does S[q] mean ? – TanMath Dec 15 '14 at 00:39
  • @TAbraham it is the action along the path q. – or1426 Dec 15 '14 at 00:44
  • u mean what's calculated by the Euler-Lagrange equations? but isn't that for classical mechanics? – TanMath Dec 15 '14 at 00:45
  • @TAbraham, you use the same action $S[q]$ in classical mechanics and in the path integral formulation of quantum mechanics. The difference is that in classical mechanics, the particle takes the path of stationary action; while in quantum mechanics, the particle takes all of the paths. But it can be showed that the path of stationary action is usually the dominant part of the path integral due to positive interference, which is why the principle of stationary action works so well in classical mechanics. – jabirali Dec 16 '14 at 01:20