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A quantum head scratcher

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Dr. Slack

Fri Mar 23 2012, 08:46PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I think the closest I've seen to an, well not explanation of QM, but a something-or-other of it, was given by Feynman in one of his series of lectures, available somewhere on the net if you Google, 4 off one hour talks. Granted, it doesn't tell you what, or how big, a photon is, but if you grok it, it may stop you worrying.

"Given a system's present state A, and a specified future one B, QM gives you the probability that state B will be observed. Period. Nothing else."

So if you set up the conditions of Young's slits, you get bands on the screen, because you're asking the question of the system where will the energy be detected by the screen. If you now ask which slit the effect went through, it's a different question, and will have a different answer. Asking the "which slit" question destroys the "where on the screen" question.

When I first heard about gravitational lensing by distant galaxies, I thought "whoa, that means a photon has to be 1000s of light years across", so it can maintain coherence as it passes to th eleft or the right of the galaxy. But I guess that's not much more difficult than being 0.5mm accross to get through both of Young's slits, both distances are a damned sight larger than its notional wavelength, which I'd swallowed quite happily in 6th form physics, and it's got to go round corners all by itself to reform on the screen.

The main point is that QM, which is yet to be falsified, says nothing about what happens in between. It's a model for what happens, if we use it consistenly with the rules we have invented for it. Unfortunately, to use this model, rule number 1 says we have to cast a black cloak over all of whatever happens between state A and state B. If we take a peak under the cloak, the model stops working. We don't know why.

There are other models. We observe light refraction and invent the wave model, and see the photoelectric effect and invent the particle model, and these are very successful models in their own contexts.

One member of this group, I forget who and am too lazy to search, uses (or used) as his signature "All models are wrong, but some are useful". The usefullness of the QM model extends only to the results and not to the mechanism. The usefulness of the wave or particle models extend only to mutually inconsistent assymptotic behaviour for large systems.

We don't have the imagination yet to replace QM with something that CAN tell us what happens in between. So until then, relax, and let go of all three models. Choose the one result that most interests you, and pick the model that's going to give you that. Or work on replacements for QM.

WaveRider

Tue Apr 03 2012, 12:09PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Steve,
I think part of the problem is the notion that a photon has a "location" and "dimension". These ideas are difficult to define for the photon.

Attempting to "probe" the position or momentum of the photon (by scattering, etc.) upsets the phase history of the photon. This is what destroys the dual-slit interference experiment interference pattern if you attempt to gain knowledge of which slit the photon goes thru. The act of probing the photon can be shown to require a non-reversible interaction that has the side effect of slightly randomizing the physical quantities that you wish to observe. Randomizing the photon phase (and momentum, etc) destroys the photon coherence. Why this randomization needs to take place is very mysterious, indeed and indicates something fundamental to our universe is going on that we do not fully understand.

Furthermore, if you reject the idea that an unobserved photon has "location", you can see how it may experience "self-interference" in a gravitational lens. Richard Feynman's path-integral theory can be used to model this as can the so-called "many-worlds" idea. The many-worlds idea is particularly weird...

Just my 2c! (or tuppence

)

Uspring

Mon Aug 06 2012, 01:12PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

The idea of collapse of a wavefunction (Kopenhagen interpretation) is certainly strange. Especially if you consider, that the temporal evolution of a wavefunction, which is described by the Schroedinger equation does not involve a collapse. Given an initial state, this equation will tell you the state at any time in the future provided that you don't interfere with the system, i.e. make a measurement. The Kopenhagen interpretation differentiates in an extreme way the observed system from the observer. The observer creates a collapse by making a measurement on the system.

But why can't the observer be considered as part of the quantum system himself? (Which would make the assumption of collapse obsolete)

Look at this scenario: Tell a physicist to make a double slit experiment and put a position detector behind the double slit, so that he can determine, which slit the particle went through. Before he proceeds, you put him and his lab into a box, so that you can't observe what is happening inside. Some time later you open the box and ask the guy inside, what he observed. He'll tell you, that he measured the particle going through one of the slits and that his observations are consistent with assuming a collapse.

Suppose now, that you know the exact state of everything in the box just before you close it. Then run a simulation with the help of the Schroedinger equation up to the time you open the box. The simulation will not involve a collapse up to the time you open the box. Inspecting the simulation at that time should show a wave function of a physicist, who believes, that he has observed a collapse. The simulation on the other hand didn't involve a collapse until the box was opened.

How can that be? This is how it works:

One consequence of collapse is that if you make the same measurement right after the first one, it should come out the same. So if you put a second position detector right behind the first one and measure "left slit" in the first one, you'll measure "left slit" in the second one also and vice versa. In the first detector the wavefunction collapsed to "left slit" and stays left or it collapsed to "right slit" and stays right.

In the simulation the interaction between the particle and the scientist will make the wavefunction split up into a superposition of 2 parts: A scientist, who has measured left with a particle going through the left slit and a scientist who has measured right with a particle going through the right slit. In the second position detector the wavefunction will not split again. The wavefunction will be a superposition of a scientist who has measured left twice with a particle being left and a scientist, who has measured right twice with a particle being right.

The scientist in the box will believe, he has seen a collapse, even though, from an outside perspective, the wavefunction just split. This is much along the lines of Everetts "Many worlds interpretation".

There is an ongoing discussion, whether other parts of measurement theory, e.g. the Born rule, which states, that only eigenvalues of the measurement operator can be measured can also be derived from the Schroedinger equation. My belief is, that this should be possible.

With respect to the OP observing a mile wide photon this interpretation would mean, that the observation does not really happen but instead a superposition of observers is created where each one believes that his particular sequence of events is real.

If that leaves you even more puzzled, you're in good company.

Ash Small

Mon Aug 06 2012, 02:57PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Can I just remind everyone here that we don't actually 'observe' or 'measure' photons themselves.

We only observe the 'result' of their 'interaction' with the 'Higgs field' (or 'Aether', as it was referred to in the 19th century).

Uspring

Mon Aug 06 2012, 04:10PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

AFAIK the Higgs field does not interact with photons since they don't have a rest mass.

The Aether was thought of as a medium or a frame of reference for the propagation of light. The Michelson experiment showed, that there is no absolute frame of reference. The Higgs field does not imply a special frame of reference. It is a relativistic theory.

Ash Small

Mon Aug 06 2012, 05:14PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Uspring wrote ...

AFAIK the Higgs field does not interact with photons since they don't have a rest mass.

As far as I'm aware, it is the 'Higgs field' that is responsible for 'mass'. Without the Higgs field, nothing has 'mass'. It is the 'resistance' of the Higgs field that leads to the 'illusion' of mass. I 'assume' that the Higgs field has very little or no resitance to photons, hence we cannot observe any 'mass' associated with them.

Uspring wrote ...

The Aether was thought of as a medium or a frame of reference for the propagation of light. The Michelson experiment showed, that there is no absolute frame of reference. The Higgs field does not imply a special frame of reference. It is a relativistic theory.

There do seem to be some similarities between the Higgs field and the antiquated theory of 'the Aether', however I don't really know enough about either to be able to quantify or qualify this at the moment. It may be better to leave the aether out of this discussion for the time being. l mentioned it because it was thought to be connected to the 'proagation' of light, and has some similarities with the Higgs field.

I stand by my previous statement that we cannot measure or observe photons, only their effects.

Uspring

Mon Aug 06 2012, 06:31PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Ash Small wrote:

I stand by my previous statement that we cannot measure or observe photons, only their effects.

No doubt about it. It doesn't really distinguish between a photon or e.g. a table, though. When I see a table, my eyes just notice the reflection of light from it, i.e. the tables effect.

I don't understand the bearing of your statement wrt to my earlier post.

Ash Small

Mon Aug 06 2012, 08:30PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Uspring wrote ...

I don't understand the bearing of your statement wrt to my earlier post.

Assuming you are referring to your post regarding the Michelon experiment, I don't think I addressed it directly.

I was just pointing out some similarities between the Higgs field and the Aether.

I will point out that the Michlson experiment ( from what I've briefly read) seems to be based upon a misapprehension (at least as far as I understand the Higgs theory).

In the days of the Aether theory, mass was taken for granted (Newtonian physics prevailed).

The Higgs theory seems to differ from the Aether theory primarily by stating that without 'the field' mass doesn't exist, and mass is 'an illusion' created by 'the field', which 'resists' change, or movement. (ie a body at rest exerts no force)

The table that you 'see' in your previous post resists you 'falling through it' when you lean on it due to the resistance of the Higgs field.

Therefore, if we equate the Aether to the Higgs field, the Michelson experiment becomes meaningless, as it involves masses (the Earth and the sun) traversing the field, rather than 'energies', or 'forces'. It is the field itself that gives the 'illusion' of mass.

My original point was that, in a similar way, the photons themselves (or what we observe as 'photons') don't actually traverse the field, but what we observe is an illusion created by the photon's interaction with the field, which permeates the whole of the Universe.

(I've tried to keep this as simple as possible)

Uspring

Wed Aug 08 2012, 11:22AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Ash Small wrote:

My original point was that, in a similar way, the photons themselves (or what we observe as 'photons') don't actually traverse the field, but what we observe is an illusion created by the photon's interaction with the field, which permeates the whole of the Universe.

The Higgs field is a part of the standard model i.e. quantum theory. The electromagnetic field also belongs to the standard model and permeates all of the universe just as any other wave function. I think, that the puzzle of non locality, that started this thread is not solved by just another field. That hinges on the interpretation.

In Everetts many world interpretation (MWI) a wave function collapse, which is a non local process, doesn't appear. When a photon from a distant star hits the earth, a superposition of a plethora of observers is created, which detect the photon at a specific place for each one of them. Each observer is tied to his particular finding so that it is not possible for him to see the photon also at some other place. That doesn't mean that the other observers don't exist. Their findings are just inaccessible.

There have been experiments to "undo" observations, e.g. the "quantum eraser".

From the point of view of the MWI the 2 observers, which see the the photon going through the right and the left slit, respectively, are coalesced back into one. This one, having no knowledge about where the photon went through, can observe the interference pattern of the double slit.
If the 2 worlds didn't exist in some way, their effects could not be merged.

Ash Small

Wed Aug 08 2012, 12:13PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Uspring wrote ...

When a photon from a distant star hits the earth, a superposition of a plethora of observers is created, which detect the photon at a specific place for each one of them.

I would dispute this.

I'd argue that only one observer 'sees' each photon, and that the energy from that photonis 'recieved' by the eye of that observer.

Other observers 'see' other photons emitted from the same source.

I'd also argue that 'detecting' the photon destroys it (or, at least, absorbs it's energy).

There is an argument that when a photon 'passes' from one medium to another (refraction or reflection) it is 'absorbed' then 're-emitted'.

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