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Old Jun 29th 2014, 06:20 AM   #11
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I've seen that video before. He hasn't even kept up to date with what's on physicsworld. Have a look at this:

http://physicsworld.com/cws/article/...se-of-weakness

Check out "weak measurement" work by Aephraim Steingberg et al and Jeff Lundeen et al:

http://www.physics.utoronto.ca/~aephraim/

http://www.photonicquantum.info/

See for example this: http://www.photonicquantum.info/Rese...efunction.html

"With weak measurements, it’s possible to learn something about the wavefunction without completely destroying it. As the measurement becomes very weak, you learn very little about the wavefunction, but leave it largely unchanged. This is the technique that we’ve used in our experiment. We have developed a methodology for measuring the wavefunction directly, by repeating many weak measurements on a group of systems that have been prepared with identical wavefunctions. By repeating the measurements, the knowledge of the wavefunction accumulates to the point where high precision can be restored.

So what does this mean? We hope that the scientific community can now improve upon the Copenhagen Interpretation, and redefine the wavefunction so that it is no longer just a mathematical tool, but rather something that can be directly measured in the laboratory."
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Old Jun 29th 2014, 06:35 PM   #12
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For a thread on QM I have remarkably little to say. For the moment I want to go back to a post that mentioned how the electrons "move around" in the sense of what happens to the E field of an atom.

The wavefunction of the electron is the most crucial component...the wavefunction tells you everything about the particle. By summing over all the electrons then you have the wavefunction for the electron "cloud." (By the way, to get a reasonably sensible function for this is quite "non-trivial.") Since the wavefunction only gives you the probability function of what shape the cloud will have we can pretty much only calculate the time average of the E field. If you actually measure the E field of the cloud you will find that it's pretty well constant...the variation of the electron cloud is very much shorter than the equipment we are using to measure it with can handle. Or at least on any equipment that I've heard of.

Now, if you want to get a real pain in the cerebellum you can calculate this for each electron in the cloud (again, "non-trivial" to say the least!) and you should be able to find fluctuations in the E field.

This is a long-winded way of confirming that yes, the E-field varies.

-Dan
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Old Jun 29th 2014, 10:32 PM   #13
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Originally Posted by Farsight
I've seen that video before. He hasn't even kept up to date with what's on physicsworld.
So what? Most physicists haven't heard of that site and just because a physicist hasn't seen an article it doesn't mean that he's ignorant of the physics. Any scientist would be very poor scientist if he changed his views based on what he read in one paper. It takes a great deal of observations and experimental work to change the orthodox view of mainstream physics. If that wasn't the case then science would be chaotic with everyone coming to different conclusions so often. It takes years for something to be thouroughly tested, analyzed and studied carefully and only then when it is able to pass all muster is it taken as a change in view.

Originally Posted by Farsight
See for example this: http://www.photonicquantum.info/Rese...efunction.html

...We have developed a methodology for measuring the wavefunction directly, by repeating many weak measurements on a group of systems that have been prepared with identical wavefunctions.
This is something I'd actually be willing to read. However I don't see any references there or in the root website. Find the published work and I'll read it. If I think there's anything there I'll raise the subject with some of my friends who are experts in the field.

However I want to stress that this forum is more about learning orthodox physics and not about the fancy theory of the day because those change quite often. So please, let's keep it current, i.e. mainstream please.

Last edited by Pmb; Jun 29th 2014 at 10:41 PM.
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Old Jun 30th 2014, 10:18 AM   #14
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I've been reading this thread more carefully and have a couple more comments.

Originally Posted by Farsight View Post
"The electrons do not orbit the nucleus in the sense of a planet orbiting the sun, but instead exist as standing waves."
The electrons in the Bohr theory, reinforced by the wave concept from de Broglie, were standing waves. Modern Quantum Mechanics gets rid of that concept. The electron wavefunction defines the shape and size of the orbital that it is in. But the wavefunction is not a standing wave. For example, the electron wavefunction in the Hydrogen atom is not even close to being a standing wave. See here. Note that these functions do not include the part of the wavefunction involving spin.

Originally Posted by Farsight View Post
...This is the technique that we’ve used in our experiment. We have developed a methodology for measuring the wavefunction directly, by repeating many weak measurements on a group of systems that have been prepared with identical wavefunctions. By repeating the measurements, the knowledge of the wavefunction accumulates to the point where high precision can be restored.

So what does this mean? We hope that the scientific community can now improve upon the Copenhagen Interpretation, and redefine the wavefunction so that it is no longer just a mathematical tool, but rather something that can be directly measured in the laboratory."[/I]
The wavefuntion of an electron cannot be measured directly as the experiment above even admits. The problem is that the electron exists in all possible states until it is measured. The electron in Hydrogen, for example, is in all possible states, ie. all n, l, m, and s. There are states that are more probable than others, but you can't get a complete picture as there are too many states to measure. And even then all you can get is the probability amplitudes and not the wavefunction itself. The electron cloud measurement is much easier to tackle and, I believe, has been measured for many elements.

And that's just the bound states. I can't think of any way to measure the wavefunction of a free particle. You can do scattering experiments to find out some of the details about the incoming and outgoing wavefunctions but you are never going to get the actual wavefunction here either, just probability amplitudes.

Whoever is giving you this information probably needs to review their Quantum Mechanics.

-Dan
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Old Jul 1st 2014, 01:44 PM   #15
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Originally Posted by topsquark View Post
Whoever is giving you this information probably needs to review their Quantum Mechanics.
It's the Institute of physics in the UK. See this:

http://physicsworld.com/cws/article/...oughs-for-2011
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Old Jul 1st 2014, 03:30 PM   #16
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I read a number of different experiments but in reality I stopped taking it seriously when I read this:
Steinberg says that physicists have been taught that "asking where a photon is before it is detected is somehow immoral".
"Immoral?" Really?

Then we have this, for "weak measurement"


where the initial and final states are preselected. The weak value of the observable becomes large when the post-selected state approaches being orthogonal to the pre-selected (initial) state. In this way, by properly choosing the two states, the weak value of the operator can be made arbitrarily large, and otherwise small effects can be amplified.
I have two issues with this definition of weak measurement. First, this is simply the definition of how to compute the matrix representation of the operator A. Nothing has changed by calling it the result of a weak measurement. This is standard for QM. Second: If we have the complete set of orthogonal vectors (ie. the vector space is spanned by the set {| phi _ i >}, then we can choose any linear combination of final states | phi_f > that is orthogonal to | phi _i > as the initial state is orthogonal to the space of the remaining vectors. For example, if we have three vectors in Euclidean 3-space, the usual i, j, and k unit vectors, any choice of linear combinations of i and j are orthogonal to k. I don't understand what they mean by "approaching."

Obviously I don't understand weak measurements, but I have two comments to make. First: How can they a priori chose the final state in the double slit experiment? It seems to me that the Stern-Gerlach experiment would be a more fruitful approach since you are choosing the final state just by picking a point on a screen to measure at. Second: If you are talking about a set of mutually orthogonal base vectors (as is mentioned) then all you are doing is diagonalizing A. That means you are measuring the eigenvalues of a diagonal matrix, which means in turn that you are measuring the likelihood of the system to be in a particular state, which means in turn you are measuring the probability a certain final state will be in.

I'm not going to say that the method is bogus, but I am not going to be changing my mind as to whether this method will "measure" the wavefuntion of a particle...It will measure the probability the particle is in a given state.

-Dan
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