The Secrets of Quantum Physics BBC4

[Pabmusic]

About the gloves and the coins - don't they in fact prove Einstein right, not Boehr? I can't how the card analogy works.

If you have a coin, with a heads and tails, and split it down the middle, one half has the 'head-ness' and the other has the 'tails-ness'. If you spin the bit with the heads, it's bound to fall on its heads side because it doesn't have a tails - or it does, but that's on the other bit which which has a tails but not a heads. Spin both and they will fall, respectively, heads and tails. [The gloves have a 'right-ness' and 'left-ness'.] When the coin is 'whole' it could fall on either side but once it's separated it will fall on both sides when you spin it. Doesn't that make Einstein right?

Or have I missed the point too?

I'm afraid you have, because your rationalisation seems to be 'fixed' before the 'head-ness' or 'tail-ness' has been resolved. Before it is 'fixed' the coin could fall on either side. (I've just re-read this and it doesn't make sense...still. what the...? Richard Feynman said that those who think they understand quantum physics don't understand quantum physics.)

[french frank]

I'm afraid you have, because your rationalisation seems to be 'fixed' before the 'head-ness' or 'tail-ness' has been resolved. Before it is 'fixed' the coin could fall on either side.

I said that - it could fall on either side: the randomness there favours Boehr. But the next bit favours Einstein because the 'fixedness' occurs with the separation of the two parts.

(I've just re-read this and it doesn't make sense...still. what the...? Richard Feynman said that those who think they understand quantum physics don't understand quantum physics.)

Well, I don't think I understand

BUT in JaK's coin analogy, what do the TWO coins represent?

[mangerton]

I've just caught up with the comments here. I recorded the programme and watched it last night, and I found myself rewinding a few bits, which certainly assisted comprehension. It's a fascinating topic, and I'm looking forward to next week's programme.

[ardcarp]

About the gloves and the coins - don't they in fact prove Einstein right, not Boehr? I can't how the card analogy works.

If you have a coin, with a heads and tails, and split it down the middle, one half has the 'head-ness' and the other has the 'tails-ness'. If you spin the bit with the heads, it's bound to fall on its heads side because it doesn't have a tails - or it does, but that's on the other bit which which has a tails but not a heads. Spin both and they will fall, respectively, heads and tails. [The gloves have a 'right-ness' and 'left-ness'.] When the coin is 'whole' it could fall on either side but once it's separated it will fall on both sides when you spin it. Doesn't that make Einstein right?

Or have I missed the point too?

I'm afraid if you split a coin down the middle, the blank side (i.e. the bit you've cut through) of the heads will be a de facto tails...and vice-versa.
So that analogy is maybe a tad too concrete for the Quantum World?

[french frank]

I'm afraid if you split a coin down the middle, the blank side (i.e. the bit you've cut through) of the heads will be a de facto tails...and vice-versa.

I thought you said it was a 'What if'? What if the properties that make it 'tails' have been removed? There is no 'other side' - other than the bit that's been removed. After all, if you did cut a coin down through the middle, one side would have a heads imprint but the other side wouldn't have a tails, would it? It wouldn't have anything. Aren't we talking theory, not how we perceive reality? If you think of the gloves as being an entity when you separate the two, the left hand doesn't automatically have a right hand side, does it?

So, to repeat: in JaK's coin analogy, what do the TWO coins represent?

[vinteuil]

So, to repeat: in JaK's coin analogy, what do the TWO coins represent?

... I think you will find the answer lies in the article on Bell's Inequality I helpfully provided at #17 supra - viz :

http://en.wikipedia.org/wiki/Bell%27s_inequality

... "The theorem is usually proved by consideration of a quantum system of two entangled qubits. The most common examples concern systems of particles that are entangled in spin or polarization. Quantum mechanics allows predictions of correlations that would be observed if these two particles have their spin or polarization measured in different directions. Bell showed that if a local hidden variable theory holds, then these correlations would have to satisfy certain constraints, called Bell inequalities. However, for the quantum correlations arising in the specific example considered, those constraints are not satisfied, hence the phenomenon being studied cannot be explained by a local hidden variables theory."

Next question?

[teamsaint]

I

I thought you said it was a 'What if'? What if the properties that make it 'tails' have been removed? There is no 'other side' - other than the bit that's been removed. After all, if you did cut a coin down through the middle, one side would have a heads imprint but the other side wouldn't have a tails, would it? It wouldn't have anything. Aren't we talking theory, not how we perceive reality? If you think of the gloves as being an entity when you separate the two, the left hand doesn't automatically have a right hand side, does it?

So, to repeat: in JaK's coin analogy, what do the TWO coins represent?

The two coins represent a pair of quantum particles, possibly created at the same time, whose properties are connected through the theory of Entanglement,which says that what happens to one, even if they are very far apart,affects the other.
About 32 mins in.

Or is this a different set of two coins you are discussing?

[french frank]

Next question?

Oh, so, it's a quantum entanglement prediction, is it? Clockwise-anticlockwise v head-tails?

Add: Isn't this what team is saying?

[Gordon]

....BUT in JaK's coin analogy, what do the TWO coins represent?

Although the programme was very good in introducing the story of the quantum it didn't deal with entanglement well - or at least the explanations and analogies were almost as confusing as the phenomenon itself!! He could have saved all that wasted time in the night club which added nothing of value and spent it on better preparation for his demonstrations. JaK did not explain entanglement itself well enough neither did he explain Bell's Inequality well enough - he should have talked us through the 4 probability/correlation terms and what they represented then the addition of outcomes from his demonstration would have made more sense and the origin of the "2" on the RHS would have made more sense too. Maybe next week will clarify it all!! Then again.....let's hope that the two parts of the programme series are indeed entangled!!

The TWO coins are supposed to represent two entangled states of a system of particles [which could be a group of them]. When they are both spinning they are in a raw state of Bohr's probability wave, the two sides of each coin representing the two possible "real" states of the particles in question but which coin is which at this stage is not "known", it is undetermined and their "real" observed states only have potential. The analogy breaks down if you try to use the coins to verify Bell's Inequality unless you can find a way of "entangling" the coins. Better use something else!!

Coins with a heads but without a tails ie blank do not exist in this quantum world - blank must be opposite of heads [analogy breakdown]. The whole point is that the particles [coins] MUST have two [or more] complementary properties or states [analogy: heads/tails] that may be revealed in measurement or observation otherwise there is no basis for entanglement, there is nothing to entangle. A coin is useful for properties that have two complementary states eg spin or polarization - "right/left", there is no blank for spin - and so the blank must always be a "tails" [or whatever the opposite of "heads" is!!]. IOW if the property being measured is polarisation then "heads" is "Left" and "tails/blank" is "Right" [or vice versa].

Particles are entangled only under special circumstances of their generation ie they are generated as a connected pair so that they arise from the same event and so each has "knowledge" of the other OR the pair share a common but complementary set of properties. Thus when one is "measured" [ie called upon to react in another energetic system and so declare its properties] the other is somehow aware [due to the entanglement] and will reveal those complementary properties when it is measured. The measurement systems must in each case therefore be able to respond equally to BOTH pairs of complementary properties ie it's no good a detector seeing the "left" polarisation of it can't see the "right". The mystery is how does the unmeasured particle [ie still in a state of Bohr-ean probability] know that the other has been measured and so set its own state to be opposite? OR is it that it isn't the particle but the measurement system that "knows" to look for the opposite? There are more questions than answers.

In the glove analogy the gloves are meant to be produced as a connected pair [as are the coins] - humans [nearly] always come with symmetry ie left and right hands need left and right gloves. That's nature saying there are no coins without tails, no lefts without rights, ups without downs. [If people were not so symmetrical and had only one type of hand then there would not be two types of glove that must come in pairs.] If the boxes were full of gloves of random hands [as the normal world of non-entangled gloves would be] then the effect does not work - so must appear with random handedness when inspected separately. The essential point is that these particles/gloves/coins have to be prepared specially in order to be entangled. The experiment with the lasers and polarisation showed what happens but wasn't explained well enough.

EDIT my use of the word Symmetry reminds me that the quantum theoretical merging of the forces of nature - Electro-Magnetism and the Week and Strong nuclear forces [but not yet gravity] has involved invoking symmetries. The breaking of those symmetries in the long past separated out these forces so we experience them separately. What is entanglement but a kind of shared symmetry where one particle of a pair takes up the opposite character of another?

[teamsaint]

thanks for that fabulous post, Gords.

Couple of questions:

Is this entanglement theoretical, or has it been observed, (very tricky are obviously) or measured, or whatever?

also, are these entangled pairs common, or rare, and do they have particular functions?

(incidentally, re the night club, I did think the use of the song and its lyrics was quite funny !)

[Gordon]

thanks for that fabulous post, Gords.

Couple of questions:

Is this entanglement theoretical, or has it been observed, (very tricky are obviously) or measured, or whatever?

It is most certainly theoretical and has seen vast quantities of paper published in the scientific journals over many years now. As to experimental proof I'm not sure, not being an expert in this [where is Vile Consort when you need him?] but that business that JaK did with the polariser demonstrating Bell's Inequality could be seen as proof IF you accept that Bell is right!!

I think that physicists believe that there is an entanglement phenomenon and that the predictions [hard thing to talk about given the weirdness of entanglement] of theory are being explored. The promise of harnessing the process is great eg Quantum cryptography and one aspect of it is that time itself may be an aspect of or even a consequence of entanglement. Try some googling to see what comes up. I've just looked up the Wiki article and it seems worth reading.

also, are these entangled pairs common, or rare, and do they have particular functions?

Don't know enough to say but one might expect that everyday high energy particle interactions might well produce such things. Unless I misunderstood him JaK managed to produce them to order in his demonstration of Bell's Inequality - as he seemed to be using lasers that means the particles in question were photons which do have the property of polarisation, ask any radio aerial!! How to assure yourself that they are indeed entangled is another question - again JaK did not say what he was doing to generate these entangled photons.

The debris from collisions of sufficient energy to break down the strong nuclear force might result in some of the debris particles being entangled. What JaK did not mention [and may do in the next programme] was antimatter predicted by Dirac [A Bristolian by birth and early education and one of the handful of people that may have actually understood quantum mechanics] in the mid 1920s and found not long afterwards and Dirac's surmising that space itself is, at quantum scales, a seething cauldron of particles being created and destroyed all the time using Heisenberg's Uncertainty principle to sneak under the rules of macroscopic physics. Maybe such releases of matter/antimatter quanta are entangled?

(incidentally, re the night club, I did think the use of the song and its lyrics was quite funny !)

I agree about the night club being entertaining and JaK's relating the other events of the time gave it a bit of context!! I didn't think it added much to the science though.

[french frank]

AIn the glove analogy the gloves are meant to be produced as a connected pair [as are the coins] - humans [nearly] always come with symmetry ie left and right hands need left and right gloves. That's nature saying there are no coins without tails, no lefts without rights, ups without downs.

But the glove analogy was saying that you could separate the two gloves into a left and a right. But you can't separate even a hypothetical 'coin' into heads and tails?

[clive heath]

I've now watched the programme and found it fascinating and challenging in respect of the Bell Theory and subsequent laser light experiments which I shall have to read up on.
It occurred to me that awareness of the different colour of heated metals precedes the light bulb by several thousand years of metallurgy. I googled "Helium the three body problem" which is I think a reference to the mechanics of any 3-body problem compared to the relative simplicity of the hydrogen atom, whose emission lines were a factor in the acceptance of Planck's ideas.
One thing though, completely wrong, was the shadow of his hand, NOT diffraction ( he said "bending") of the light waves as they pass your hand, that only applies if the shadow-casting object is of similar wavelength to light e.g. an eyelash. The fuzziness of the shadow is because the sun is not a point source of light but a disc. Compare the hard-edged shadows of spotlights in a theatre with the softer-edged shadows of floodlights.

[Gordon]

But the glove analogy was saying that you could separate the two gloves into a left and a right. But you can't separate even a hypothetical 'coin' into heads and tails?

The problem with all this is that the coin analogy isn't very good, it's beginning to get in the way. The glove is a better one [but not ideal either] than the coins.

A box with a single pair of gloves in it, 1 Left and 1 Right is "entangled" BECAUSE there is one, and only one, of each in that box. The state of the pair TAKEN AS A PAIR, which you must do before you reach into the box, is indeterminate. When you reach in and take one out you know it is either a left or a right AND that the other IS the opposite.

That can't be done with a pair of coins, the coin isn't that amenable. Splitting the coin breaks the analogy, the coin no longer represents the state of nature that you need it to. A blank sided coin has no parallel in the experiment because a "real" particle has no state of blankness - unless one can think of one?

Try this instead:

Two passengers arrive at immigration off different flights at different times, each having a British and a French passport [humour me!]. The first offers his British passport and passes through. A second [and many more] passenger may also have these same dual passports and may choose at random which he/she offers and successfully pass through.

IF however these passengers are "entangled" then, regardless of whether they actually know each other at all or when and where they come from, when the first offers his/her British passport the other MUST offer his/her French one. Simples! A blank passport will not do.

One way to give this entanglement is of course to ensure that the passengers do know each other's future actions [perhaps through an intermediary] and so are correlated in their use of passports. The entangled deal is that the first offers British and the second knows then to offer French - this "deal" is what Einstein called the Hidden Variables solution, it's all arranged at the source. The passengers know in advance what their properties are but this is hidden from any observer. Does that work?

[ardcarp]

One thing though, completely wrong, was the shadow of his hand, NOT diffraction ( he said "bending") of the light waves as they pass your hand, that only applies if the shadow-casting object is of similar wavelength to light e.g. an eyelash. The fuzziness of the shadow is because the sun is not a point source of light but a disc. Compare the hard-edged shadows of spotlights in a theatre with the softer-edged shadows of floodlights.

I did wonder about that. OTOH he is a Prof of Physics.......

All I can remember for certain is the experiment at school where parallel beams of light pass through narrow slits and cause wonderful interference patterns (or were they called 'fringes'?).

I thought the night-club analogy was a bit protracted (pointless?) but I suppose they had to hold the viewers' attention in what is a difficult subject (to put it mildly).