What is Quantum? A journey through quantum theory, entanglement, and quantum computing with Rovelli, Krall, and Simmons

This BBC World Service documentary takes a ferry ride into the heart of quantum physics to explore what quantum really is. Guided by host Marnie Chesterton, the episode threads together simple questions, deep ideas, and practical perspectives from physicists Carlo Rovelli and Michelle Simmons, philosopher Elise Krall, and science writer Philip Ball. Through a one-question constraint, the program probes how quantum theory challenges our everyday picture of reality, from entanglement to quantum computing, ending with a thoughtful reframe of what we mean by describing the world.

Framing the Question: What is Quantum?

On a sea-crossing voyage, the host and a ferry full of quantum minds set out to answer a deceptively simple question: what is quantum? The podcast frames quantum physics as a theory that describes how matter and energy behave at the smallest scales, underpinning modern electronics while clashing with ordinary experience. The conversation features Carlo Rovelli, a theoretical physicist and author, who opens with a bold framing: quantum demands a new way of asking about reality, not just a list of what exists. The journey uses Helgeland, a historic island, as a backdrop for a discussion that will blend science with philosophy, tradition with invention, and curiosity with rigor.

"What is quantum? Oh boy, what is quantum? I would say that quantum is 3 discoveries, 2 easy and 1 hard." - Carlo Rovelli, quantum physicist

From Weirdness to Understanding: The One-Question Rule

The podcast then tests a radical method: a single guiding question, aimed at forcing clarity rather than decoration. The host and guests navigate the uncomfortable parts of quantum theory, such as how observation collapses uncertainty and how a particle’s properties can depend on whether or what is being measured. The rule is designed to avoid the usual tidy, well-worn explanations and instead sit with the actual stubbornness of quantum ideas, asking what it would mean to frame reality under a different kind of scrutiny.

"To be somewhere is to answer the question where you are." - Carlo Rovelli, quantum physicist

Entanglement and the Metaphysical Shift: Elise Krall

Elise Krall, associate professor of philosophy at the City University of New York, shares a philosophical lens. She argues that quantum theory rests on a radical departure from classical objects: no system can be fully described in isolation, and when two systems interact they become entangled, losing the sense of independent states. Her metaphor—a world that cannot be neatly carved into Lego blocks but behaves more like a smeared, interdependent fabric—highlights how quantum mechanics compels a rethink of what counts as individual reality.

"Not an easy thing to answer. I take quantum theory to be our best and most fundamental physical description of the world, and at its core is this idea that no system can be described as an object all on its own." - Elise Krall, associate professor of philosophy

From Theory to Practice: Quantum Computing with Michelle Simmons

The discussion moves from interpretation to application with Silicon Quantum Computing founder Michelle Simmons. She reframes quantum as a computational paradigm, where a qubit can exist in superpositions between 0 and 1, and where entanglement allows two qubits to share information in parallel. Simmons emphasizes how these ideas translate into real computing potential, explaining how quantum states between 0 and 1 enable new forms of information processing and how entanglement creates powerful, simultaneous operations across components.

"In the quantum world, we use those states in between. One quantum bit called a qubit has two classical bits of information to describe it." - Michelle Simmons, CEO Silicon Quantum Computing

Not What Quantum Is Not: Dockside Reflections with Philip Ball

Science writer Philip Ball offers a practical corrective to popular tropes. He argues that quantum mechanics does not claim particles physically reside in two places at once; rather, meaningful statements about a particle’s properties depend on measurement. This reframing invites readers to rethink which questions are valid in the quantum domain and to appreciate how the theory reshapes our understanding of what can be described as real.

"Quantum is not the many things that we often hear the quantum actually is." - Philip Ball, science writer

Conclusion: A Quantum World Reframed

Across these voices, the documentary suggests quantum mechanics is as much a philosophical journey as a scientific one. Helgeland serves as a backdrop for a broader truth: asking one precise question can rearrange the way we understand reality, observation, and knowledge. The host finishes with a candid admission of how hard it remains to picture atoms, while acknowledging the value of the ongoing pursuit to understand this perplexing but foundational domain of science.

Quotes, metaphors, and practical insights braid together to show that quantum theory does not merely explain the micro world; it invites a shift in how we think about measurement, reality, and possibility, a shift that might just shape the future of computing, communication, and epistemology.