Quantum mechanics is one of the most precisely tested theories in science, but it has never explained what it means for reality. Now a professor of theoretical physics wants to change that.
Jonathan Halliwell, who teaches at Imperial College London, has spent his career studying the foundations of quantum theory. In a new episode of The Joy of Why, he tells host Steven Strogatz why quantum behavior turns classical. The episode is called Where Does the Quantum World End and Ours Begin?
The Problem With Observers
Halliwell started his research in quantum cosmology, where quantum mechanics applies to the universe as a whole. That raised a strange question: if the entire universe is a quantum object, there is no outside observer to measure it.
Strogatz asked Halliwell directly why this made foundations irresistible. Halliwell answered that the original push for quantum cosmology came from the Big Bang model. That model mixes quantum matter with classical gravity, and as you move backward in time, theorems from Stephen Hawking and Roger Penrose show the early universe hits a singularity — a region of infinite gravity and curvature.
To avoid that, scientists need a quantum theory of gravity. Halliwell’s work took a few elements of the gravitational field, like the size of the universe and some distortion parameters, and quantized them.
Decoherence Explained
The key idea Halliwell discusses is decoherence. It is the process by which fragile quantum behavior becomes dispersed through interactions with the surrounding environment. Decoherence is what turns quantum wave-like behavior into the solid, macroscopic world of objects and the universe at large.
Halliwell describes the “histories” approach to understand this transition. He also tackles some deep paradoxes along the way.
No Need For A Conscious Observer
One of those paradoxes is the question of whether the quantum-to-classical transition requires a conscious observer. Halliwell argues it does not.
Another is Einstein’s famous question: is the moon really there when no one looks? Halliwell addresses that too.
Yoga And Meditation
The conversation ends with Halliwell explaining how yoga and meditation help him sit with the mystery of competing perspectives in science.
A Question That Refuses To Be Answered
The episode is available on Apple Podcasts, Spotify, TuneIn and other podcast apps, or you can stream it from Quanta. It is a deep dive into a question that has resisted answers for decades.
The transcript shows the hosts themselves admit there is no coherent interpretation of quantum mechanics, despite it being tested to astonishing precision. They joke that claiming to understand it is a sign you actually don’t.
That is the honest position. The theory works, the experiments confirm it, but the meaning remains unclear.
Key Facts Box
- Host: Steven Strogatz
- Guest: Jonathan Halliwell, professor of theoretical physics at Imperial College London
- Episode title: Where Does the Quantum World End and Ours Begin?
- Available on Apple Podcasts, Spotify, TuneIn and Quanta
The episode is a reminder that science often knows how to calculate without knowing what it calculates. Quantum mechanics is a machine that works, but the machine itself is silent on why it works.
Halliwell’s approach — histories, decoherence, quantum cosmology — offers a path forward. The question of whether the moon is there when no one looks is a hard one, and Halliwell does not settle it here.
What is clear is that the question is worth asking again.
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