20Sep2013

 Hard Science: Action At A Distance

hard science

Did you know that Albert Einstein the greatest physicist who ever lived was proven wrong by James Bell an Irish physicist? Unfortunately he did not live to see the day.

The story goes back to the 1930’s and the hot topic those days among physicist was the emerging quantum mechanics. The basic principle that underlies quantum theory is the requirement that probability be recognised as a fundamental feature of atomic reality that governs all processes, and even the existence of matter.

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Neils Bohr and Albert Einstein

In quantum theory, subatomic particles do not exist with certainty at definite places but rather show “probabilities to exist”. And atomic events do not occur with certainty at definite times and in definite ways bur rather show “probabilities

to occur”. Furthermore, where a subatomic particle will be at a certain time can never be predicted with certainty, because at the quantum level, the position and momentum (mass x velocity) of a particle cannot be determined with certainty. They can only be calculated in terms of probabilities. All that can be done is to predict the odds.

Einstein’s battle with the Danish physicist Neils Bohr, the acclaimed father of quantum mechanics culminated with Einstein producing a paper to counter quantum theoretical ideas with two younger collaborators, Boris Podolsky and Nathan Rosen, which came to be known as the “EPR Paradox”. The EPR paradox was a thought experiment where you consider some hypothesis, theory or principle for the purposes of thinking through its consequences without actually performing the experiment. It has been used successfully on many occasions to explore the principle in question.

EPR devised the thought experiment to argue against a quantum phenomenon Einstein famously called “spooky action at a distance”. Also known as entanglement, the theory implied that something happening over here can be instantaneously linked to something happening over there regardless of distance. He said that was ludicrous. Furthermore they argued that the Copenhagen Interpretation suggested by Bohr and Werner Heisenberg was incomplete.

The underlying theory of entanglement is this. There were a number of ways to produce pairs of particles so that they were effectively twins. When you measured a particular property of one particle, for example its speed or its position you also knew the same piece of information for the other particle; depending on what the property was, it would either have the same value or the diametrically opposite value. According to quantum theory, the particles didn’t have a set value for this property until you measured it.

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Repeatable experiments reveal that quantum connections between two particles can persist even if the two particles are on opposite sides of the universe (NASA/JPL)

It wasn’t just that you didn’t know the value until then, but it didn’t exist. It’s only at the point of taking the measurement that the value was fixed – until then it could have a whole range of values, each with a certain probability. It seemed as if, in the quantum world, the measurements we make on a particle here affect its partner there, in violation of causality, an instantaneous communication travelling across space – action at a distance.

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Quantum entanglement brings to mind voodoo (voodoo idol from Benin, West Africa)

Einstein and co pointed out that according to special relativity this was impossible, therefore quantum mechanics must be wrong, or at least incomplete. They used the example of an initial particle should disintegrate into two particles of equal mass that fly-off back-to-back, the velocities of the two particles will be equal and opposite and the position will also be closely related.

Hence they argued that each of the particles has a definite position and a definite velocity at every given instant of time. EPR reasoned that nothing in your act of measuring the right moving particle could possibly have any effect on the left-moving particle because they are separate and distant entities. Their conclusion was that quantum mechanics is an incomplete description of reality.

The EPR paradox stumped Bohr and was not resolved until 1964, long after Einstein’s death. John Bell resolved it by thinking of entanglement as an entirely new kind of phenomenon, which he termed “nonlocal”. But at that time the technology did not exist to undertake the required experiments.

By the early 1970’s it did. In the early 1980’s Alain Aspect and his collaborators working in France more refined and impressive versions of the experiment was carried out and proved beyond doubt quantum entanglement.

The feature of the universe called locality means that you can influence someone or something over there by someone or something travelling from here to there. Example of a non-local activity is Voodoo (spooky) which contravenes locality since it involves doing something over her and affecting something over there.

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Here is an experiment using an intensified charge coupled device (ICCD) to image in real-time the effect of the measurement of one photon on its entangled partner (Nature.com).

The study of entanglement was ignored for thirty years until John Bell reconsidered and extended the EPR argument. By considering the spin and polarisation of a particle instead of the position and speed, John Bell convincingly showed that Einstein was wrong. Follow these links for a video from Scientific American: http://bcove.me/ab5ew3fx ; http://www.scientificamerican.com/video.cfm?id=quantum-entanglement-the-movie-2012-01-30

Article supplied by our ‘Hard Science’ writer Rohan Jayawardene. Website: www.jjtraffic.com  Email: rohan@jjtraffic.com  

 

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