Date: January 8, 2011

Title: Untangling the Entanglement


Podcaster: Steve Nerlich

Organization: Cheap Astronomy –

Description: Although many a popular science text has suggested we live in a universe full of live/dead cats and spooky action at a distance, you don’t have to believe all this stuff.

Bio: Cheap Astronomy offers an educational website helping you keep your financial situation looking up.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Jesper, Elina and Jonathan from Sala in Sweden.


Hi this is Steve Nerlich from Cheap Astronomy and this is Untangling the entanglement.

Very occasionally, I get an email requesting a podcast topic and Terry, currently based in New Zealand, requested I do one on quantum entanglement. It was a bit hard to think what to say on this subject that hasn’t been said before, so I thought I’d aim broader and try to deliver a grounded view of quantum physics generally and quantum entanglement specifically.

Firstly, a quote from Albert Einstein – and no it’s not the one about the throwing of dice, but instead this: As far as the laws of mathematics refer to reality, they are not certain, and as far as they are certain, they do not refer to reality.

Einstein’s quote about god not playing dice allegedly follows the statement: Quantum mechanics is certainly imposing, but an inner voice tells me it is not yet real. Despite the sound of that, it’s very likely that Einstein thought that quantum mechanics was a fascinating and ground breaking line of enquiry. After all, he won his Nobel prize for work on the photoelectric effect – which clearly demonstrated that light was made of photons – essentially particles of light, even though light clearly showed wave-like properties in other circumstances.

Not long after that it was found that sub-atomic particles also have wave-like properties. This wave-particle duality is fundamental to quantum theory, including Heisenburg’s uncertainty principle which requires that you cannot precisely measure a particle’s position (which assumes you are dealing with a point like particle) and simultaneously measure its momentum (which can be a characteristic of wave-like phenomena). You can determine one or the other of these properties any time you like – but you can’t do both simultaneously.

It’s unhelpful to say that this finding is strange and weird and non-intuitive. What it is – is unexpected. It indicates that at the sub-atomic scale, the fundamental building blocks of both light and of matter are not really particles and not really waves. They’re something else, which we might call wavicles. These wavicles will look like waves if you measure them with things that detect waves – and will look like particles if you measure them with things that detect particles.

Quantum mechanics becomes useful if we just embrace the uncertainty of these wavicle properties – and apply forms of statistical mathematics which define wavicle properties as probabilities rather than as something fixed or pre-determined.

A good analogy to show how quantum mechanics can be useful is to think about radioactive decay. You genuinely cannot predict with any level of certainty whether or not a single isotope of uranium-235 will decay within the next ten seconds or take more than a billion years to decay. However, you can say with almost complete confidence how long it will take 50% of a kilogram-sized chunk of uranium-235 to decay, because that’s just its half-life, which is 704 million years.

Where things get weird is if you decide the very useful mathematics of quantum theory is fundamentally real – something Einstein had warned against in that earlier quote.

In the 1920’s, Neils Bohr and others agreed on the Copenhagen interpretation of quantum mechanics – which essentially said that the mathematics really did reflect reality and hence sub-atomic particles really did exist as a wave functions of probability – and it is only when they are observed or measured that these wave functions collapse into a determined state.

The story of Schrodinger’s cat was introduced by Schrodinger to highlight what he saw as the implausible consequences of applying the Copenhagen interpretation to the real world. The story is way too well known to spend precious podcasts minutes on it. Instead, let me tell you about the less well known story of Wigner’s friend.

A physicist, Eugene Wigner proposed a scenario where he gets a friend to run the Schrodinger’s cat experiment while he (Wigner) goes out for lunch. Upon his return, but before he enters the house, quantum indeterminancy requires that his friend now exists in two states – a happy friend with a live cat or a sad friend with a dead cat. And if that friend had to pop out on an errand and left a message with the housekeeper about the outcome of the experiment then both the friend and the housekeeper exist in indeterminant states. And if Wigner’s friend to post the outcome on Facebook – then there’s a whole online community of people existing in indeterminant states until Wigner finds out the result.

Schrodinger’s cat is just a thought experiment – intended to indicate that if you try to apply the Copenhagen interpretation to the real world – it starts looking not so much weird and strange, as a little implausible.

You can either accept that there a potentially infinite number of multiple parallel universes – or maybe just accept that in our one, and clearly real, universe there is absolutely no way to know what happened to Schrodinger’s cat until you take a look. The standard quotation ascribed to William of Ockham – the guy with the razor – seems entirely suited to this situation. One should not multiply entities beyond necessity.

Einstein added to the Copenhagen interpretation debate with what’s known as the EPR paradox – and Terry this is finally where we talk about quantum entanglement.

Say a pi meson decays to release an electron and a positron and these then shoot off in opposite directions. Since they are a particle and anti-particle pair each must spin in the opposite direction to the other – but the spin of either is a property governed by quantum indeterminancy.

In other words, the spin of either is completely unpredictable and unknown until we actually measure it. But of course all you have to do is measure the spin of one and you will immediately know exactly what the spin of the other one is – even if it is now several million kilometres away – because you already know that it will have the opposite spin.

So – there’s nothing at all weird about this if you tell the story in that way. Where it gets weird, or really a little implausible – is if we apply the Copenhagen interpretation. If we choose to believe that probability wave functions are real – rather than just mathematical – entities, then when you collapse one particle’s wave function of spin by determining it, the other particle’s wave function of spin must also collapse because you have immediately determined what its spin is as well.

Telling the story in this way requires us to believe that the second particle is somehow instantaneously informed that the first particle’s spin state has been determined. Einstein described this as spooky action at a distance, carrying the added implausibility that the transfer of information, from one particle to the other has to happen at faster than the speed of light.

Einstein and his colleagues came up with the EPR paradox – not to debunk quantum mechanics, but just to question the wisdom of trying to scale the Copenhagen interpretation up into the real world.

And even today on the 8th of January 2011, theoretical and experimental physics still leaves you having to make interpretative choices. You can either go with the weird version – or stick with a more conventional view of the world and be OK with acknowledging that there is a bunch of stuff here that we don’t fully understand – at least, not yet.

Thanks for listening. This is Steve Nerlich from Cheap Astronomy, Cheap Astronomy offers an educational website, taking the woo out of astrowoonomy. No ads, no profit, just good science. Bye.

End of podcast:

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