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Hello, if the most famous fruit in physics is an apple, the most famous animal in physics is a cat.

It belongs to Edwin Sherdiger, a theoretical physicist who in the early 20th century helped to develop the theories of quantum mechanics.

Sherdiger's cat doesn't actually exist. It's the subject of a thought experiment in which the equations of quantum mechanics make it appear both dead and alive at the same time.

The problem of a cat that's both dead and alive illustrates the challenges of quantum physics and that the heart of this apparent absurdity is a thing called the measurement problem.

The measurement problem arises because we don't really understand how the particles that constitute our world behave.

They're fundamentally mysterious to us, even shocking. And they defy our attempts to measure and make sense of them.

Possible solutions range from the existence of multiple realities to the rather more mundane possibility of an error in mathematics.

But a solution you've found could transform our understanding of reality.

We'd be to discuss the measurement problem, assignment saunders, professor in a philosophy of physics at University of Oxford,

Basil Hailey, emeritus professor of physics at Birkbeck University of London, and Roger Penrose, emeritus, raspold professor of mathematics at the University of Oxford.

Basil Hailey, at the heart of the measurement problem is a distinction between how we think of the world at the level of the very small and how the everyday world of large objects seems to be classical physics and quantum physics.

Can you outline the distinction between the two?

Yes, let's start first of all with the classical world.

And one example that I'm going to talk about where we can see Newtonian mechanics actually in action is on the snooker table.

We have little balls moving in straight lines, we have balls which we push against, the cushion and bounce back again, and we're fairly familiar intuitively with the laws of the macroscopic world.

Let's now go down, make the billion more smaller and smaller and smaller until we get to the size of an atom.

Now let's imagine trying to play snooker with an atom.

What we would expect it to do would be to move in straight lines, hit the cushion and bounce back again, and so on, if the laws of Newtonian physics are obeyed.

But we get a surprise if we keep pushing a snooker ball against the cushion it always comes back again.

If we do it with an atom it sometimes doesn't come back again, it's actually gone through the cushion and this is the example of radioactivity.

But there are more surprises and I want to bring the surprise out by doing a crazy type of experiment on the snooker table first.

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