### Hindsight Prediction in the Quantum World

Tuesday, February 10th, 2015OK, we’ve all heard that “hindsight” is always 20/20. Knowing the past can predict the future.

Now we’re learning that knowing the future can predict the past.

Wait, what?

Professor Kater Murch of Washington University in St. Louis was written up in The Daily Mail…

If this proves true in our ‘classical’ world, it would mean that what we’re doing now has been influenced by the decision made by a future version of us.

This all remains theory, but physicists have created devices that has allowed them to measure these fragile quantum systems to see if this really is the case in the quantum world.

Professor Kater Murch at Washington University used this technique to look at the quantum state of two particles at different stages in their evolution.

The quantum state was detected by putting a circuit inside a microwave box.

A few microwave photons – or particles of light – were sent into the box, where their quantum fields interacted with the circuit.

When the photons exited the box they had information about the quantum system.

‘We start each run by putting the qubit in a superposition of the two states,’ Professor Murch said.

‘Then we do a strong measurement but hide the result, continuing to follow the system with weak measurements.’

They then try to guess the hidden result, which is their version of the missing page of the murder mystery.

‘Calculating forward, the probability of finding the system in a particular state, your odds of guessing right are only 50-50,’ Murch said.

‘But you can also calculate backward using something called an effect matrix. Just take all the equations and flip them around. They still work and you can just run the trajectory backward.

‘So there’s a backward-going trajectory and a forward-going trajectory and if we look at them both together and weight the information in both equally, we get something we call a hindsight prediction, or ‘retrodiction.’

The shattering thing about the retrodiction is that it is 90 per cent accurate.

When the physicists check it against the stored measurement of the system’s earlier state it is right nine times out of 10.

This suggests that in the quantum world time runs both backward and forward whereas in the classical world it only runs forward.

Professor Murch told Dailymail.com that it’s as if you left your keys somewhere in the house, but couldn’t remember where.

In the quantum world, they could exist in every room of the house simultaneously.

When you eventually find them in the kitchen, in the classical world it is clear that they were there all along, in the quantum world the uncertainty is intrinsic, but Profesor Murch was able to show that indeed hindsight can be applied to make a better guess about where they were in the past.

In the same way, the improved odds in the current experiment imply the measured quantum state somehow incorporates information from the future as well as the past.

And that might implies that time, notoriously an arrow in the classical world, is a double-headed arrow in the quantum world.

‘It’s not clear why in the real world, the world made up of many particles, time only goes forward and entropy always increases,’ Professor Murch added.

‘But many people are working on that problem and I expect it will be solved in a few years,’ he said.

Very interesting.