# The Naked Scientists Forum

### Author Topic: In quantum mechanics, does the observation preceed the event?  (Read 16890 times)

#### sciconoclast

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##### Re: In quantum mechanics, does the observation preceed the event?
« Reply #50 on: 18/06/2015 23:50:41 »
The experiment you described would not work but an experiment to achieve the results you are looking for has been done by Aephraim Stienberg at the University of Toronto.

"They succeeded in experimentally reconstructing full trajectories which provide a full description of how light particles move through the two slits and form an interference pattern."
http://phys.org/news/2011-12-toronto-breakthrough-physics-world.html

This experiment is a problem for the more wilder claims of quantum theory and as it relates to this thread for the requirement of observation.

In 2011 this was voted the experiment of the year in several publications. How did you guys miss it.

#### lightarrow

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##### Re: In quantum mechanics, does the observation preceed the event?
« Reply #51 on: 20/06/2015 12:39:37 »
Further explanation:
Figure 1:  The photon has passed through the slits (located at the left and out of the picture), and is described by its "waveform", by which I mean its state function Ψ(x,y,z,t) whose absolute square at any point is the probability that a photon would be found there,
No, such a function doesn't exist for a photon.
For electrons or others non-zero-mass particles it exists, but not for photons.

If you want to modify your question using other particles, e.g. electrons, make us know.

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BlueRay
« Last Edit: 20/06/2015 12:45:18 by lightarrow »

#### PmbPhy

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##### Re: In quantum mechanics, does the observation preceed the event?
« Reply #52 on: 20/06/2015 13:20:42 »
Quote from: lightarrow
For electrons or others non-zero-mass particles it exists, but not for photons.
Where'd you get such a notion from? See: The Photon Wave Function by J. H. Eberly, L. Mandel, Coherence and Quantum Optics VII, Eds., Plenum, New York 1996, p. 313. This is available online at http://www.cft.edu.pl/~birula/publ/CQO7.pdf

#### lightarrow

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##### Re: In quantum mechanics, does the observation preceed the event?
« Reply #53 on: 20/06/2015 19:44:10 »
Quote from: lightarrow
For electrons or others non-zero-mass particles it exists, but not for photons.
Where'd you get such a notion from? See: The Photon Wave Function by J. H. Eberly, L. Mandel, Coherence and Quantum Optics VII, Eds., Plenum, New York 1996, p. 313. This is available online at http://www.cft.edu.pl/~birula/publ/CQO7.pdf
See post 42. You have written:
"And apparently you're under the wrong impression that photons have a wavefunction"

Anyway, the document you linked doesn't show that exists a wave function which square modulus is the probability to find the photon, as Atomic wrote.

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lightarrow

#### PmbPhy

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##### Re: In quantum mechanics, does the observation preceed the event?
« Reply #54 on: 20/06/2015 19:58:23 »
Quote from: lightarrow
For electrons or others non-zero-mass particles it exists, but not for photons.
Where'd you get such a notion from? See: The Photon Wave Function by J. H. Eberly, L. Mandel, Coherence and Quantum Optics VII, Eds., Plenum, New York 1996, p. 313. This is available online at http://www.cft.edu.pl/~birula/publ/CQO7.pdf
See post 42. You have written:
"And apparently you're under the wrong impression that photons have a wavefunction"

Anyway, the document you linked doesn't show that exists a wave function which square modulus is the probability to find the photon, as Atomic wrote.

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lightarrow
You're right. That topic falls under quantum electrodynamics and relativistic quantum mechanics, neither of which I've studied yet. I looked them up. Unfortunately I got conflicting answers in both cases that I looked them up and forgot what my answer was the first time when I posted the second. I'll contact someone I know and ask him.

The problem with the Wikipedia article is that I didn't read it closely enough. Had I done so I'd have seen this
Quote
More generally, the normal concept of a Schrödinger probability wave function cannot be applied to photons.[57] Being massless, they cannot be localized without being destroyed; technically, photons cannot have a position eigenstate |\mathbf{r} \rangle, and, thus, the normal Heisenberg uncertainty principle \Delta x \Delta p > h/2 does not pertain to photons. A few substitute wave functions have been suggested for the photon..
So what I misunderstood was that the "normal" concept of a wave function doesn't apply but "A few substitute wave functions have been suggested ..."
« Last Edit: 20/06/2015 20:03:30 by PmbPhy »

#### Atomic-S

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##### Re: In quantum mechanics, does the observation preceed the event?
« Reply #55 on: 23/06/2015 05:12:24 »
Quote
Atomic-s I've only skimmed trough thread, is this the experiment you are trying to explain?

No, but it sounds interesting. More details would have been helpful.

#### Atomic-S

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##### Re: In quantum mechanics, does the observation preceed the event?
« Reply #56 on: 23/06/2015 06:43:29 »
Quote
So what I misunderstood was that the "normal" concept of a wave function doesn't apply but "A few substitute wave functions have been suggested ..."
May I suggest as the wave function, the classical electromagnetic field equation that would describe the light at high intensity passing through this region, times some very small constant.  After all, such an equation describes, in the case of full-strength radiation, the intensity we would observe if we took intensity measurements at various points, and thus corresponds, in the case of very weak radiation, to the probability that we would detect a photon at any particular place and moment of time if we tried.

#### PmbPhy

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##### Re: In quantum mechanics, does the observation preceed the event?
« Reply #57 on: 23/06/2015 16:51:15 »
Quote from: Atomic-S
May I suggest as the wave function, the classical electromagnetic field equation that would describe the light at high intensity passing through this region, times some very small constant.
You don't need to suggest it. That's the way that it's done. It's not a real wave function though since that's defined as the inner product of the bra <r| with the state ket and no such bra exists for photons. The constant has to be chosen to normalize the resulting function.

#### The Naked Scientists Forum

##### Re: In quantum mechanics, does the observation preceed the event?
« Reply #57 on: 23/06/2015 16:51:15 »