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A quantum head scratcher

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Steve Conner

Thu Mar 15 2012, 02:24PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Sulaiman wrote ...

From the above posts I see that we know nothing !
(but I'm not sure ;)

That's Heisenberg's law of the Internet. The product of complete ignorance and invalid assumptions is equal to the intertubes constant, which is approximately 125,000 tweets per day, or 1.5 politicians in American units.

Best explanation I've seen so far is that the observed experimental behaviour is the reality, and "wave" and "particle" are figments of our imaginations that only partly fit the reality.

Bjørn

Thu Mar 15 2012, 04:06PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

...how do you generate single photons?

One way is to take a laser and attenuate it with filters until you get get a stream of single photons at random intervals. For large intervals the chance of getting two photons at once is very small and can easily be taken into account for most experiments. The downside is that you don't know when a photon is emitted.

Quantum mechanics, the more you think about it, the worse it gets.

To discuss what really happens at a quantum level (as opposed to a mathematically approximate level) we need a definition of reality. There are many competing definitions of reality and all seems to have the flaw that they are not rooted in reality (no way to check if the definition is correct).

There is one explanation that I cant see here, that a cloud of virtual particle photons are emitted towards the slits, when they hit something they return back in time through their own path to the source and interfere to create a real photon that knows everything about what it will encounter and behaves accordingly.

Since we don't have a working definition of reality at a quantum level it may turn out that many of the explanations are equivalent even if they sound very different.

IntraWinding

Fri Mar 16 2012, 08:15PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Sulaiman wrote ...
I bought a Hamamatsu photomultiplier that is supposed to be able to count individual photons
with the intention of doing the single-photon double-slit experiment
- I failed
...how do you generate single photons?

I assume they use a very dim light source (or source + filters) so that statistically it is extremely unlikely that more than one photon will ever be in flight between the source and the screen at the same time. In practice I guess that it's good enough if single photons far outnumber multiple photons. I assumed that's why the experiment always runs so slowly, because 'the dimmer the better'.

Ash Small

Sat Mar 17 2012, 11:05AM

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

I've been pondering this for years (hence some of my comments above).

A photon has no 'centre of gravity' (as far as I'm aware), so Newtonian physics doesn't apply. (does this imply that a photon is 'everywhere?).

A photon has a finite amount of energy, so 'normal' wave mechanics doesn't apply. (all the energy used to 'create' it is 'harvested' at the 'receptor'.)

Photons appear to obey Heisenberg. (they are 'everywhere' until 'detected')

I'm tempted to believe that we should think of photons as 'energy fields', which 'emanate' from one point, and 'collapse' about another.

Steve Conner

Sat Mar 17 2012, 01:25PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Ash Small wrote ...

I'm tempted to believe that we should think of photons as 'energy fields', which 'emanate' from one point, and 'collapse' about another.

That's kind of what I was getting at in my original post. But it bothers me. In order for a field to collapse about a point, you need non-locality. The field has to coordinate itself so that all the energy propagates inwards towards the point that it's supposed to be collapsing around. And if the field is propagating at the speed of light in the meantime, doesn't the co-ordination have to happen faster than light?

Maybe this is what actually happens, and it's just another way of expressing the paradox of quantum information.

Ash Small

Sun Mar 18 2012, 10:22PM

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

Yes.

The best analogy I've been able to come up with is as follows:

When money is transferred from one bank account, the transaction is supposedly instantaneous.

The money goes out of your account, an electronic message is sent to the bank holding the receiver's account, and the money appears in their account.

How long does it take for an electronic message to get to the payee's bank?......a few nanoseconds?.....

Suppose the payee's bank is on a planet in a solar system several light years away..........

The electronic message takes several light years to get there, but when it does, the money instantaneously appears in the payee's account as well.

Where was the money in the meantime?

How did it know which direction to go in, so that it could be there as soon as the message arrived?

It must have been everywhere (photons don't have to adhere to the Pauli exclusion principle, and Heisenberg implies that they are everywhere until detected).

The 'information' only travels at the speed of light, but the energy forms part of the energy of the universe.

(I eagerly await a response

)

hboy007

Wed Mar 21 2012, 11:04PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 373

Steve Conner wrote ...

In order for a field to collapse about a point, you need non-locality. The field has to coordinate itself so that all the energy propagates inwards towards the point that it's supposed to be collapsing around.

You have to start somewhere. Actually obtaining an observable of a quantum mechanical system by means of an operator causes the collapse of the wave function as far as the Copenhagen interpretation of QM goes

It has implications that are hard to swallow: the non-locality issue could be resolved by representing the initial state as a superposition of multiple or an infinite number of parallel universes from which you pick a subset that matches the subspace to the observed eigenvalue, or in other words:
Guy

Proud Mary

Thu Mar 22 2012, 10:44AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Note by RSPCA inspector: If we find Mr SchrÃ¶dinger or anyone else keeping a cat in a sealed box for any purpose whatever, we will take legal action under cruelty to animals legislation.

Ash Small

Thu Mar 22 2012, 12:37PM

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

Thanks for posting that link, Hboy.

I think this paragraph from it probably sums up the point I was making above:

"Second, many physicists and philosophers see the reduction of the wave function as an important part of the Copenhagen interpretation. But Bohr never talked about the collapse of the wave packet. Nor did it make sense for him to do so because this would mean that one must understand the wave function as referring to something physically real. Bohr spoke of the mathematical formalism of quantum mechanics, including the state vector or the wave function, as a symbolic representation. Bohr associated the use of a pictorial representation with what can be visualized in space and time. Quantum systems are not vizualizable because their states cannot be tracked down in space and time as classical systems'. The reason is, according to Bohr, that a quantum system has no definite kinematical or dynamical state prior to any measurement. Also the fact that the mathematical formulation of quantum states consists of imaginary numbers tells us that the state vector is not susceptible to a pictorial interpretation (CC, p. 144). Thus, the state vector is symbolic. Here â€œsymbolicâ€ means that the state vector's representational function should not be taken literally but be considered a tool for the calculation of probabilities of observables."

Conundrum

Thu Mar 22 2012, 07:47PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Interesting stuff...
QM is totally counter intuitive it seems.

-A

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