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Sandoval
2009-Feb-04, 01:39 AM
In IFMs (quantum Interaction-Free Measurements - see "Interaction-Free" Quantum Measurement and Imaging (http://www.npl.washington.edu/AV/altvw101.html), or google the subject) the position of a particle can in principle be determined to any desired level of precision without a single interaction between the measuring apparatus (the world) and the particle itself. Let us cook a thought experiment: a) use IFM to determine the particle's position; then, b) interact with the particle to determine its momentum. Can IFMs thus (in this or any other way) be used to violate HUP (Heisenberg's Uncertainty Principle) ?

papageno
2009-Feb-04, 07:22 PM
The Heisenberg principle works for simultaneous measurements of position and momentum, not for measurements done one after the other.

DrRocket
2009-Feb-04, 08:06 PM
The Heisenberg principle works for simultaneous measurements of position and momentum, not for measurements done one after the other.

I find that a little confusing. Another way to look at it is that the operators corresponding to the measured quantities do not commute. So, if you measure position and then momentum you get a somewhat different answer from what you get if you measure momentum and then position.

Sandoval
2009-Feb-05, 12:08 AM
The Heisenberg principle works for simultaneous measurements of position and momentum, not for measurements done one after the other.

Papageno, you are wrong. You confuse simultaneous values to the position and momentum with simultaneous measurements. Simultaneous measurements to what degree? Down to Planck's time? Or do you mean a single experiment (meaning, a single interaction) that could measure position and momentum at the same time?

papageno
2009-Feb-05, 09:28 AM
Papageno, you are wrong. You confuse simultaneous values to the position and momentum with simultaneous measurements.

You get values only when an operator is applied to the wavefunction. And applying an operator representing a physical quantity is a measurement. Therefore, the only values you get is when you perform a measurement. Before you do a measurement, you only have probabilities, not defined values.
That is the basis of the mathematical formalism of Quantum Mechanics.

The Uncertainty principle applies to operators representing conjugate physical quantities, when they are applied simultaneously to the same wavefunction. That is, it applies to simultaneous measurements.

Simultaneous measurements to what degree? Down to Planck's time?

We don't know yet to what degree.

Or do you mean a single experiment (meaning, a single interaction) that could measure position and momentum at the same time?

The Uncertainty principle is not restricted to a single interaction.

alainprice
2009-Feb-05, 01:27 PM
In IFMs (quantum Interaction-Free Measurements - see "Interaction-Free" Quantum Measurement and Imaging (http://www.npl.washington.edu/AV/altvw101.html), or google the subject) the position of a particle can in principle be determined to any desired level of precision without a single interaction between the measuring apparatus (the world) and the particle itself.

You have yet to proove this.

Maybe you're thinking of interactions that collpase the wavefunction before the particle gets to the detector. The truth is, if you have a detector, you have an inevitable interaction between the apparatus and the particle.

Sandoval
2009-Feb-06, 12:22 AM
You have yet to proove this.
No, I don't. It is now a well established feature of the quantum weirdness.

Maybe you're thinking of interactions that collpase the wavefunction before the particle gets to the detector. The truth is, if you have a detector, you have an inevitable interaction between the apparatus and the particle.
No, I am talking about detection without a single photon interaction. Please, read the article I linked to in the beginning. The following might also be of interest:
Elitzur–Vaidman bomb-tester (http://en.wikipedia.org/wiki/Elitzur-Vaidman_bomb-testing_problem).

Cougar
2009-Feb-06, 02:12 AM
...a) use IFM to determine the particle's position; then, b) interact with the particle to determine its momentum. Can IFMs thus (in this or any other way) be used to violate HUP (Heisenberg's Uncertainty Principle) ?

Apparently not, since by the time you determine its momentum, it is no longer in its previous position.