So Einstein was wrong - the world is fuzzy

Descartes Cartoon


Of all breeds of scientists,physicists are notoriously the most hard-nosed.Having given the world the likes of electric power,lasers and nuclear weapons,physicists aren't exactly famed for their love of touchy-feely discussions discussions about life,the universe and everything.

Yet dealing with the sub-atomic world,physicists inevitably find themselves having to confront the very touchy-feely stuff they have so little time for. Quantum theory,as the rules of the sub-atomic world are known,may be the crowning glory of 20th century physics,but is as enigmatic now as it was at its birth a century ago.

Back then,physicists merely had to get their heads around the idea that sometimes light behaves like a wave,but other times it is best thought of as a packet - or "quantum" - of energy.

Now a new set of experiments is forcing physicists to accept that these packets of light "talk" to one another,exchanging notes on what has happened to them.More bizarre still,this chatter takes place faster than the speed of light - in fact,it seems to take place instantaneously,no matter how far apart the particles are.

The experiments are based on an idea put forward in 1935 by Einstein to settle an argument he was having with another giant of modern physics,the Danish physicist Niels Bohr.Both won a Nobel Prize for their pioneering work in quantum theory,but their views on what the theory actually means could hardly have been more different.

Bohr's view was decidedly mystical: according to him,the mathematics of quantum theory implied that particles of light and matter have no real existence until they are observed.Until then,their properties -where they are,how fast they move - are entirely fuzzy.

Poppycock,declared Einstein:particles are real,and have perfectly concrete properties whether we observe them or not.And he came up with a "thought experiment" to demonstrate it.Imagine a single molecule,made up of two identical particles.Suppose the molecule explodes,sending one particle to the left, the other to the right.

Now,according to Bohr,neither of the two particles has any definite properties until someone observes them.But hang on,said Einstein:what about Newton's famous law of action and reaction?
According to this,each particle will have identical velocities,so measuring the speed of one instantly reveals the speed of the other -without actually performing a measurement on it.

But maybe the first particle somehow informs its partner "my speed has been measured,so stop being fuzzy,and get yourself a definite speed now"? Einstein blocked off this route by demanding that the measurements are done so quickly that no message could reach the other particle fast enough,even if it travelled at the speed of light.

Much to Einstein's chagrin,Bohr did find a way out of the argument: roughly speaking,he claimed that the fuzziness is so great,it is impossible to know if the two particles really are totally separate from each other.In other words,Bohr was claiming that every particle in the universe is constantly "in touch" with every other,instantly informing one another of what is happening.

Einstein,not surprisingly,rejected this argument.But now Nicolas Gisin and his colleagues at the University of Geneva have performed the experiment Einstein proposed,with amazing results.
 Instead of an exploding molecule they used pairs of photons - packets of light - squirted down optical fibres in opposite directions.After travelling six miles,their properties were measured in special detectors and compared.

And - as Einstein demanded - the measurements were made so fast that there was no chance of the photons "telling" each other what had happened to them, even if the messages went at the speed of light.
 According to a report in this week's New Scientist,the results were unequivocal;just as Bohr had predicted,the photons behaved as if they were in instantaneous communication with each other.

So Einstein had got it wrong after all - but do the results also mean his famous law against faster-than-light travel is also wrong?
 Not quite: as Bohr claimed,the particles aren't really beaming messages from place to place - they're actually "entangled" with each other,forming a huge, fuzzy, cosmic web that keeps them almost literally in touch with each other.

Now there's something to ponder at this weekend's Glastonbury Festival.

Robert Matthews


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