There is a video on YouTube which has Deutsch explained

the famous quantum double slit experiment,

which is about particle wave duality,

is light a particle or a wave.

You pass it through a slit,

depending on whether there's observer

and interference or not,

it ends up in a wave pattern,

it ends up as individual photons.

And this is the famous experiment,

which has baffled people for a long time

and caused them to revise their worldview.

The one that let Einstein to say,

God does not play dice with the universe.

Correct.

Einstein was a realist.

At the time when the founders of quantum theory

were trying to develop a good explanation

of what precisely was going on

with these experiments in quantum theory,

Einstein rejected all of them on the basis

that they weren't realistic.

...