Jim Al-Khalili: The Ontology of Time. We Had It Backwards

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Jim Al-Khalili (0:30)

There's a lot of quantum physics that's mis-explained. Time itself doesn't flow in any meaningful sense according to physics. The arrow of time is different.

Curt Jaimungal (0:38)

This is Jim Al-Khalili, theoretical physicist, fellow of the Royal Society, host of BBC's Radio 4's The Life Scientific and author of the new book On Time.

Jim Al-Khalili (0:48)

Time is real and the directionality of time is real.

Curt Jaimungal (0:51)

Please dispel my misunderstandings.

Jim Al-Khalili (0:53)

Now you've got me worried now. Have I been teaching the block universe wrong all these years?

Curt Jaimungal (0:57)

On this channel, I, Curt Jaimungal, interview researchers regarding their theories of reality with rigor and technical depth. Today, the professor plunges deep into the mysteries of time, the interpretations of quantum mechanics, and even where he parts ways with Carlo Rovelli. What is the electron doing when no one's looking?

Jim Al-Khalili (1:14)

You've given me a challenge now.

Curt Jaimungal (1:16)

Professor, what is most mis-explained in quantum physics?

Jim Al-Khalili (1:22)

Oh, I mean, there's a lot of quantum physics that's mis-explained and told badly and give people the wrong impression. I would say probably what we call the measurement problem.

The idea of Schrodinger opening the box with the cat that's dead and alive. There are lots of different weird and wonderful exotic explanations like the need for conscious observers and so on that are completely outmoded now. We have sensible ways of explaining it, but I think still a lot of people get confused about what it means when you make a measurement and collapse the quantum state. It'd be great if we taught our students the measurement problem more correctly.

Curt Jaimungal (2:04)

Great. What is the measurement problem?

Jim Al-Khalili (2:07)

Well, the measurement problem is essentially whenever a quantum system, be it an atom or collection of atoms or an electron or photon, interacts with its surrounding environment. The old idea was, the founding fathers explained, was that it had to interact with a classical system and there was this irreversible act of amplification. It had to be a measuring device that was behaving non-quantum mechanically.

Then that became confused with a human had to do the measurement, and then it became you had to be a conscious observer. Then the question was, well, why does Schrödinger have to do the measurement? Why can't the cat measure itself and so on?

We now know that the right way to explain it is that the quantum system becomes entangled with its surroundings. When those surroundings are very big and made up of many particles, then that entanglement becomes decoherence. It's a bit like in thermodynamics, when you say you look at individual molecules of gas bouncing around in a box, there's no concept of temperature there or direction to time, but you zoom out looking at lots of molecules and suddenly you can assign things like temperature and pressure and the directionality to time, according to the second law of thermodynamics. The same thing with quantum measurement. When does decoherence set in? When a quantum system interacts with an environment surrounding it that's very complex and big. The more complex and the bigger it is, the quicker decoherence takes place.

That's essentially what the measurement problem is. Beyond that, it's down to what's your favorite interpretation of quantum mechanics? If you open the box and see the cats alive, what happened to the version where it's dead? Well, if you subscribe to one version of explaining quantum mechanics, the many worlds interpretation, you say, well, it's in another universe instead. There are other versions that don't require more than one universe, but it's a well-understood. Measurement problem is well understood now, but there's still a lot of confusion when people try to explain it to it in non-technical language.

Curt Jaimungal (4:18)

Wouldn't DecoHarris explain that you don't have the off diagonal? So I know this is a bit technical, but it still doesn't tell you about why one outcome is selected.

Jim Al-Khalili (4:27)

Exactly. Yes, exactly. So I always think of it, the off diagonals are killed off. That's what DecoHarris does. So it kills off the possibility that the cat is dead and alive at the same time. But the option of live cat and dead cat are still there.