The music is reversible…

Two photons have danced together.

And forever afterwards, they no longer act independently of each other.

To use the term from Quantum Mechanics, they have become Entangled. Disturbing one of them will result, in what Einstein referred to as “spooky action-at-a-distance”, in a disturbance to the other. Certain experiments – known collectively as the Bell Tests – have demonstrated that this really happens.

But this can make it seem as if information is travelling between the two at speeds faster than light.

Which, as everyone knows, is impossible in real life.

So what’s going on?

Well, Huw Price and Ken Wharton came up with a possible answer. In 2012 they suggested, wait for it, ‘retrocausality‘: effect, then cause. The experimenter, on configuring the experiment, influences things that have happened in the past.

It’s as if Schrodinger’s Cat got into a Time Machine.

Now Matthew S. Leifer at Chapman University and Matthew F. Pusey at the Perimeter Institute for Theoretical Physics have lent this idea some mathematical weight.

Nothing in Quantum Mechanics, they point out, has an ‘arrow of time’: it’s not Thermodynamics; there’s no Entropy, the measure of disorder – or information – which must always increase as time goes by.

Everything in Quantum Mechanics is reversible in time, so why not cause and effect?

Everyone agrees on one thing, though: it’s impossible to send information back in time: only the shadowy influence

The music is reversible, but time isn’t.