Posted Aug 20, 2000 by yossiv
I suggest you review your knowledge quantum physics. The uncertatinty comes from the Duality and not vice versa. Plus even If we made a measurement which wouldn't affect the momentum(not the velocity,)we still wouldn't be able to measure both position and momentum, or energy and life time, or more then one angular momentum of the same particle, because the uncertainty principle is derived from the fact that we can assume a wavelength to anything and everything, and not becuase of techniqualities like hitting the particle with a photon.
read about EPR experiment I believe you'll like it.
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Posted Apr 15, 2004 by SgtWilko
Yossiv has got it right. I'm afraid the main article has fallen into the usual trap. This is partly the fault of Heisenberg himself, who famously illustrated uncertainty with a thought experiment involving a gamma ray microscope. But of course, if you bounce a photon (or anything else) off a particle in an attempt to measure its position, it's going to move, so you can never know its position and velocity.
However this would be a feature of ordinary classical mechanics, not just of quantum theory. The analogy is a poor one, and unfortunately one that is even repeated in many textbooks.
The difference is that in classical mechanics you can see where the photon and the particle go after the collision and deduce their position and velocity at the time of the collision. The uncertainty principle is much deeper, and says you can't do that.
Uncertainly cannot be demonstrated with any analogy that looks like colliding particles. It is more to do with wave / particle duality and the amount of information that can be extracted from a system. It says that the more information you extract about a particle's velocity, the less you can extract about its position. This is not due to any weakness of your apparatus, or the energy or momentum of the photon you use to do the measuring. It is due to the very nature of matter at the smallest scale. It would apply equally if you had some as yet unthought of means of measuring the velocity of a particle without interacting with it at all.
What it is really saying is that the very act of pinning down a particle's velocity increases the amount of wave type (as opposed to particle type) behaviour that the particle displays in its position. The position is not simply becoming more difficult to measure; the particle actually ceases to have any fixed position that can be measured at all, by whatever means, just because you measured its velocity.
The article should really be deleted or amended because it is perpetuating the urban myth that uncertainty is caused by the recoil of a particle we are trying to measure from the photons we fire at it in attempt to do the measurement. Not at all! We are educating people with duff information! Quantum mechanics is far more mysterious than that...
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