Quantum Entanglement and Observation: How Does It Work?

[ealbers]

Now please forgive the question, and thank you in advance for your patience...

I am a little confused by the wavefunction collapse upon observation thing (yes, thing is a technical term :-)...

So, is it the same as, say I have a 'penny splitter', i.e. 'beam splitter'...I shoot my penny towards the penny splitter, it splits the penny down the middle, sending the Heads one way, and Tails another (but does so RANDOMLY)

Upon observing one of the directions, I see say HEADS and instantly know what the Other Direction is, even though it may be a light significant distance away...of course before my observation, there is no way for me to KNOW which I would observe, as the heads part and the tails part were sent in truly random directions at the penny splitter...


Is this what is going on?? (Trying to understand this observation collapse thing)...

Thank you again for the patience with such questions...

Eric

 

[StevieTNZ]

Now please forgive the question, and thank you in advance for your patience...

I am a little confused by the wavefunction collapse upon observation thing (yes, thing is a technical term :-)...

So, is it the same as, say I have a 'penny splitter', i.e. 'beam splitter'...I shoot my penny towards the penny splitter, it splits the penny down the middle, sending the Heads one way, and Tails another (but does so RANDOMLY)

Upon observing one of the directions, I see say HEADS and instantly know what the Other Direction is, even though it may be a light significant distance away...of course before my observation, there is no way for me to KNOW which I would observe, as the heads part and the tails part were sent in truly random directions at the penny splitter...Is this what is going on?? (Trying to understand this observation collapse thing)...

Thank you again for the patience with such questions...

Eric

You have a few misunderstandings:

1. the penny would not split into two. it would be considered a superposition of being reflected and transmitted.

2. the beam splitter would not reflect heads and transmit tails.

I suggest 'Dance of the Photons' by Anton Zeilinger, and the appropriate pages on observation and beam splitters.

 

[ealbers]

Its a metaphore, as I try to explaine some of this to my young sons...who are full of questions about quantum mechanics (even at 10)...

When you shoot the penny towards the penny splitter, and it gets split into heads and tails, it results in heads one random way, but tails by default the other.

IS this what goes on with light? When you have a light beam and 'split it' is it the same as carving off a 'heads' and a 'tails'?? Thus making the future observation one of looking at weather you have Heads and thus knowing the other is Tails?

Or in the case of 2 electrons, and a light wave entangling them, does the light wave simply put one in spin up and one in spin down randomly, and upon observing one, you 'instantly know' what the other is, simply by default??

Thanks again, and please try to understand, I'm trying to relate this to some rather young kids who keep asking infernal questions (and which I encourage to do so :-))
Eric

 

[lugita15]

This is what lots of people naturally assume quantum entanglement to be when they first hear about it. They think that the two particles have some shared piece of information, like a common set of instructions, that is determining how the two particles behaves and thus is responsible for the correlations observed between the two particles. This is even the conclusion Einstein himself came to in his famous EPR paper.

But then JS Bell proved his famous theorem, demonstrating that such a straightforward "local hidden variables" interpretation is insufficient to explain entanglement. It seems to be a stranger phenomena than that. You can read about Bell's theorem in "quantumtantra.com/bell2.html" by Nick Herbert.

 

[San K]

When you shoot the penny towards the penny splitter, and it gets split into heads and tails, it results in heads one random way, but tails by default the other.

Eric

hello eric/ealbers, welcome to the forum.

no one knows what exactly happens. so we imagine...:)

these imaginations have a wide range...from many worlds theory...where the universe splits into various universes...to something going through both slits...

these imaginations/hypothesis are commonly labelled as "interpretations" in QM.

there are many guess/interpretations/hypothesis of what exactly is happening.

the above idea is part of one (or more) interpretation (and its variants).

personally i think the above interpretation is a decent one however it has its own challenges and explains only a part and thus we have various variants (with more guess/assumptions added) of it.

 

[DrewD]

I don't want to start an interpretation battle in this thread, but since it sounds like ealbers is new to the ideas of QM I would like to point out that, while San K is absolutely correct, the interpretation that you (ealbers) presents is not the most widely accepted. Popularity does not mean correctness, but it is worth noting that the more common interpretation is that a particle (or penny, which is just many particles) doesn't actually travel in one direction or the other. In the standard interpretations, a particle does not have a well defined position until it is measured.

To the best of my knowledge, there is no experimental evidence that supports one interpretation over the other. All of the interpretations have some weird ideas, so it just depends on what you think is weirder. If you are explaining it to your kids though, I would certainly let them know that the most common interpretation is not that a particle has a specific attribute and we just don't know what it is, but instead that the particle has a little bit of both attributes until it is measured. It is possible that someday experiments will prove one interpretation over the other, but currently the predictions are all the same.

 

[Nugatory]

I don't want to start an interpretation battle in this thread, but...

If you are explaining it to your kids though, I would certainly let them know that the most common interpretation is not that a particle has a specific attribute and we just don't know what it is, but instead that the particle has a little bit of both attributes until it is measured. It is possible that someday experiments will prove one interpretation over the other, but currently the predictions are all the same.

How about we go for a twofer and start a Bell's Theorem battle in the thread too?

That most intuitive of all possible interpretations, the one that says that the particle "really" has some attribute and its entangled partner just as "really" has the opposite attribute, and we just don't know which until we look at one member of the pair... That s the one that is not supported by experiment.

 

[lugita15]

To the best of my knowledge, there is no experimental evidence that supports one interpretation over the other. All of the interpretations have some weird ideas, so it just depends on what you think is weirder. If you are explaining it to your kids though, I would certainly let them know that the most common interpretation is not that a particle has a specific attribute and we just don't know what it is, but instead that the particle has a little bit of both attributes until it is measured. It is possible that someday experiments will prove one interpretation over the other, but currently the predictions are all the same.

There are certainly several different interpretations of quantum mechanics out there that are all experimentally indistinguishable from the predictions of QM. But it has been proven that a local hidden variables interpretation, i.e. one in which the correlations between the two particles is explained by the particles having certain definite attributes agreed upon in advance, can NOT reproduce the predictions of quantum mechanics concerning entanglement. This result is known as Bell's theorem, and you can read about it in the link from my earlier post.

 

[DrewD]

True, but non-locality is not a necessary assumption for our universe. I absolutely accept the orthodox interpretation (as do almost all actively publishing physicists), but there are some that do not. Since Bell's inequalities only show that there cannot be a local hidden variable theory, strictly speaking, San K was not wrong. I'm not going to respond again to this thread, so if you think that I have somehow misrepresented this, PM me.

 

[ealbers]

Thanks for the link, I showed/described this to my son (10) and you all will be very relieved I'm sure to know that he agrees, "That's very weird"...there you have it...odd stuff for sure...though he did ask if maybe the detectors were connected somehow :-) Which I thought was a pretty good thought.
Thanks again!
Eric

 

[Darwin123]

Now please forgive the question, and thank you in advance for your patience...

I am a little confused by the wavefunction collapse upon observation thing (yes, thing is a technical term :-)...

So, is it the same as, say I have a 'penny splitter', i.e. 'beam splitter'...I shoot my penny towards the penny splitter, it splits the penny down the middle, sending the Heads one way, and Tails another (but does so RANDOMLY)

Upon observing one of the directions, I see say HEADS and instantly know what the Other Direction is, even though it may be a light significant distance away...of course before my observation, there is no way for me to KNOW which I would observe, as the heads part and the tails part were sent in truly random directions at the penny splitter...


Is this what is going on?? (Trying to understand this observation collapse thing)...

Thank you again for the patience with such questions...

Eric

In your example, there is only one penny. Therefore, the penny can't exist in two channels. Once you determine the state of the penny in one channel, the penny can't exist in the other channel. Let me put it another way. Your beam splitter can not work unless the penny has been modified to be in a coherent state.
The normal condition for pennies is that the atoms that make up the penny is not entangled. If the atoms in the penny are not entangled, the beam splitter will shatter the penny into two sets of atoms each going down one or the other channel. There is no head or tails once the penny has been shattered. Therefore, your penny splitter can't work for normal pennies.
The penny can be altered so that all the atoms that make up the penny are entangled. This would involve cooling the penny to a temperature just a few microKelvin. You would also have to damp some of the vibrations of the penny and place it in a dark room devoid of radio waves for a long time. It would also help to slow down the penny relative to the penny splitter so that the wavelength of the penny is longer than the thickness of the splitter.
If all this is done, all the atoms of the penny will move together. The penny that hits the splitter will move in either one channel or the other, but not both. The penny would not shatter, but it would not split. Detection of the penny in one channel would automatically mean that the penny is not in the other channel. Therefore, your statement about the state of the penny in the other channel is meaningless.
The question that you really would like answered is what happens if one somehow entangles two such pennies in an initial state where each penny is in the same state. The state of each penny is unknown, but you somehow arranged for the two states to be the same.
If each atom in each penny entangled, then the pennies won't shatter when they hit the beam splitter. However, the pennies could end up in the same or different channels. The statistics of such an experiment would be very interesting. My guess is it would depend on the angular momentum state of the pennies. The statistics would not be classical.
The critical point that you are missing is that the penny has to be prepared in such a way that the beam splitter doesn't shatter or shake the penny. This is a lot easier for C60 molecules than for pennies. Preparing an electron this way was a lot easier then preparing the C60 molecule that way.
The C60 experiment was so cool for one thing because it was so cool.

 

[yuiop]

Is this what is going on?? (Trying to understand this observation collapse thing)...

Hi Ealbers, here is a slightly different physical model that better illustrates the mysterious nature of of entangled particles. First make two rectangular boxes, each divided into 3 compartments. For a given box, each compartment is identified by a different colour, say Red, Green or Blue. To simulate the experiment one person is the emitter and places 6 pennies in the total of 6 compartments of the two boxes. Another person is the receiver and is only allowed to open one compartment from each box and note the colour of the compartment and whether there is Heads or Tails in a given compartment.

To simulate a quantum entanglement result, the following results must be obtained:

1) If the receiver opens compartments with the same colour, then 100% of the time there must opposite sides of the coins in those compartments. eg, if they finds Heads in the Red compartment of box 1 then they must find Tails in the red compartment of box 2 and vice versa.

2) If the receiver opens compartments with different colours, then they must find that both compartments contain Heads or both contain tails (correspond) 75% of the time.

3) The average number of Heads for a given colour compartment must be 50%.

No matter how the emitter places pennies in the boxes, he will find it impossible to ensure all 3 conditions above are satisfied, if he cannot predict which compartments the receiver is going to open.

Here is another physical model illustrates the mystery. Take a disk and divide it into 4 equal pie slices. Cut out two slices opposite each other. Cut out a small part between the two gaps to make one large bow tie shaped gap. Call this cut disk the polariser and make a duplicate. Now make some slightly smaller thin disks that represent the photons. When a given polariser is horizontal, photons with random orientations, but edge on to the polariser, should pass through the polariser 50% of the time. To simulate the experiment, disk photons are dropped with random orientations, but because they are entangled, one photon should always be orientated at 90 degrees with respect to the the other. When we carry out the experiment we make the following observations.

1) When the polarisers have the same orientation, only one of each photon pair passes through its polariser.

2) When the polarisers are orientated at 45 degrees to each other, both photons of each pair pass through the polarisers 50% of the time.

3) When the polarisers are orientated at 90 degrees to each other, both photons of each pair pass through the polarisers 100% of the time.

4) When the polarisers are orientated at 120 degrees to each other, both photons of each pair pass through the polarisers 66.66% of the time.

5) When the polarisers are orientated at 30 degrees to each other, both photons of each pair pass through the polarisers 33.33% of the time.

Now the quantum prediction agrees with 1), 2) and 3) but for 4) the quantum prediction is 75% and for 5) the prediction is 25%.

In short, the physical models fail to accurately reproduce the quantum result that we observe in real quantum entanglement experiments. Bell's inequalities predict that any physical model of photon will satisfy the inequalities, but actual quantum experiments demonstrate that Bells' inequalities are violated.

Any realistic model that excludes faster than light communication, multiple parallel universes, pilot waves that travel forward and backward in time or other exotic explanations that our outside our normal experience cannot duplicate the results that are observed in actual quantum experiments.

 

[yuiop]

Upon observing one of the directions, I see say HEADS and instantly know what the Other Direction is, even though it may be a light significant distance away...of course before my observation, there is no way for me to KNOW which I would observe, as the heads part and the tails part were sent in truly random directions at the penny splitter...

The point is in quantum observations you cannot have simultaneously have full information about a photon. In the first model I gave in #12, one of the rules was that you could only open one of the three compartments of a given box. To refine the model let's say when you open one compartment it sets of detonations in the other two compartments that destroys the contents of the hidden compartments. For example if you open a red compartment and find heads, then you instantly know that the corresponding red compartment of the other box contains tails, but you have no idea what you will find in the green and blue compartments of the other box, because that information has been lost when the pennies in the green and blue compartments of the first box where destroyed when you opened the red compartment. Now if you open the green compartment of the other box and find heads you can presume the green compartment of the first box contained tails, but opening this compartment destroyed the red and blue pennies in the second box. Because the blue pennies have been destroyed in both boxes, you will never know what the emitter put in the blue boxes and you do not have complete information about what was in all the boxes, even much later, when you put all your observations together.