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Entangled in photons: the spooky behaviour of light particles

If you’re after science that makes you question your place in the universe, focus on that phrase “light years”, one that astronomers use so casually.

A photograph of the Large Hadron Collider in the Science Museum. Photo: Getty
A photograph of the Large Hadron Collider in the Science Museum. Photo: Getty

We are constantly making discoveries that reveal new wonders of the universe. Research presented in June, for instance, shows that some of its most spectacular features, such as the vast towers of gas and dust known as “the Pillars of Creation”, are a result of the way massive stars emit radiation that sculpts nearby gas clouds.

Those pillars are roughly four light years high and 7,000 light years away – which is close, compared to another discovery made recently. Scientists have found three black holes orbiting each other just over four billion light years away.

It’s an extraordinary thing to see things that distant, but in many ways this is just cosmic stamp collecting. These discoveries are informative – breathtaking, even – but they don’t cause you to question your place in the universe.

If that’s what you’re after, focus on that phrase “light years”, one that astronomers use so casually. Herein lies a truly discomfiting mystery.

Light years are a measure of the distance a photon – a packet of light energy – travels in a year. It’s a useful measure because light is the fastest thing in the universe. Yet we are still getting to grips with the properties of photons and it seems that they don’t experience distance in the same way as we do.

Fifty years ago, a Cern physicist called John Bell outlined the weirdness of photons. In a 1964 paper that built on some of Einstein’s work, Bell showed that they defy all ordinary notions of time and space. The phenomenon Bell explored is popularly known as “quantum entanglement”. It involves what Einstein once termed “spooky action at a distance” occurring between two particles. The spookiness begins when we make two photons interact in a way that leaves them entangled – the information about one is partly held in the other. The particles are “complete” only as a pair. Then we keep one on earth while sending the other to, say, the Pillars of Creation. It turns out that we can instantaneously influence the distant photon’s measured properties, such as its direction of spin.

That influence occurs because the spins of an entangled pair of photons are random but linked. You can think of it rather like knocking over two coins that are spinning on their edges. If we poke the one on earth, it might come up heads (entirely at random). If it does, we find, weirdly, that an immediate knock to the other one out there at the Pillars of Creation will give us a tail.

This cosmic connection can’t involve any signals passing between them: it would have to be quicker than light. The only explanation is that photons inhabit a reality beyond the space and time in which we live out our existence.

Entanglement’s delicate nature makes it a kind of tamper-proof seal. In the emerging science of quantum cryptography, entangled photons provide security guaranteed by the laws of physics. Financial institutions already use such measures and we are about to extend the network into space. A team of Italian researchers announced last month that they had bounced photons between satellites and earth without disturbing their quantum properties, laying the groundwork for “quantum communications on a planetary scale”. Here’s the wondrous fact: we are engineering a cosmic network that we may never fully understand. 

Michael Brooks’s “At the Edge of Uncertainty: 11 Discoveries Taking Science by Surprise” is published by Profile (£12.99)