However, the quantum-mechanical pic

However, the quantum-mechanical picture of time's arrow leads to something deeply peculiar. In some experiments, it looks as though influences can work backwards in time. The future can affect the past.
Take the double-slit experiment, in which a quantum particle such as a photon of light is fired at two narrow slits in a screen.
Suppose we do not measure which way the particle went, and so cannot tell which slit it went through. In this case, we see an interference pattern – a series of light and dark bands – when the particles emerge on the far side.
This reflects the wave-like character of quantum particles, because interference is a wave property. The interference even persists when the particles pass through the slits one at a time, which only "makes sense" intuitively if we imagine each particle passing, wave-like, through both slits at once.
However, now suppose we place a detector by the slits to reveal which one the particle passes through. In this case the interference pattern disappears, and the particles act more like sand grains being fired through holes. Measuring a particle's path destroys its waviness.
Here is the really strange thing. We can set up the experiment so that we only detect which slit a particle passed through after it has done so. And yet we still see no interference.
How does the particle "know" that it is going to be detected after passing through the screen, so that when it reaches the slits it "knows" whether to go through both slits or just one? How can the later measurement seem to affect the earlier behaviour?
This effect is called "retrocausality", and it seems to imply that the arrow of time is not as strictly one-way as it seems. But does it really?
Most physicists think that retrocausality in these delayed-choice experiments is an illusion created by the counterintuitive nature of quantum mechanics.
Detecting a particle "after" it has passed through the slits does not really influence the path it takes, they say. That is just the way we are forced to imagine what is happening when we try to apply our classical intuition to quantum events.
"Post-selection is like a parlour trick that makes it seem like there is backwards causation where there actually is none," says Todd Brun of the University of Southern California. "It's like the guy who shoots at the side of a barn and then goes and draws a target around the bullet hole."
However, others say that backwards effects in time are a perfectly valid way of interpreting such processes.
According to Ellis, we can regard retrocausality as a kind of fuzziness in the "crystallisation of the present". "Quantum physics appears to allow some degree of influence of the present on the past, as indicated by delayed-choice experiments," he says.