Jim Al-Khalili: The Ontology of Time. We Had It Backwards
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What is the quantum measurement problem and why is it often misunderstood?
There's a lot of quantum physics that's misexplained. Time itself doesn't flow in any meaningful sense, according to physics. The arrow of time is different. 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. Time is real and the directionality of time is real. Please dispel my misunderstandings. No, you got me worried now. Have I been teaching the blocked universe wrong all these years? On this channel, I, Kurt 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? You've given me a challenge now. Professor, what is most misexplained in quantum physics?
Oh, I mean, there's a lot of quantum physics that's misexplained and told badly and give people the wrong impression. I would say probably what we call the measurement problem. You know, the idea of Schrodinger opening the box with the cat that's dead and alive. And there are lots of different weird and wonderful exotic explanations like the need for conscious observers and so on that are completely outmoded now. And 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 would be great if we taught our students the measurement problem more correctly. Great. So what is the measurement problem?
Well, the minimum problem is essentially whenever a quantum system, be it an atom or collection of atoms or an electron or photon, whatever, interacts with its surrounding environment. So the old idea was, you know, the founding fathers explain... 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. And then that became confused with a human had to do the measurement and then it became you had to be a conscious observer. And the question was, well, why does Schrodinger 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.
And when those surroundings are very big and made up of many particles, then that entanglement becomes decoherence. So 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 a 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. And the more complex and the bigger it is, the quicker decoherence takes place.
And that's essentially what the measurement problem is. Beyond that, it's down to what's your favorite interpretation of quantum mechanics? You know, what happens if you open the box and see the cat's 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 the measurement problem is well understood now, but there's still a lot of confusion when people try to explain it in non-technical language.
Wouldn't Decoherent 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.
Exactly. Yes, exactly. So I always think of it, the off-diagonals are killed off. That's what Decoherent does. So it kills off the possibility that the cat is dead and alive at the same time.
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Chapters
7 chapters
1
What is the quantum measurement problem and why is it often misunderstood?
0:00–17:08
2
How do realist interpretations of quantum mechanics differ from Copenhagen and many‑worlds?
17:08–34:45
3
What does it mean for an electron to have an ontological state when no one is observing it?
34:45–50:27
4
How does Minkowski’s block‑universe picture reshape our understanding of space‑time?
50:27–1:07:32
5
Why is the flow of time considered a psychological illusion while the arrow of time is fundamental?
1:07:32–1:26:49
6
How does the past‑hypothesis act as a ‘cheat’ in explaining entropy’s increase?
1:26:49–1:42:20
7
In what ways does Carlo Rovelli’s emergent‑time view clash with the block universe?
1:42:20–1:45:23
Speakers
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