- Brian Cox explains the double slit experiment by starting with modern theory, not historical confusion.
- Quantum probability is fundamental to nature, not a gap in human knowledge.
- The double slit experiment captures all quantum behavior in one setup.
Brian Cox sat in front of a Big Think camera and explained the double slit experiment the way universities now actually teach it: starting not from history, but from the theory itself.
The University of Manchester particle physicist has spent decades making quantum mechanics accessible to general audiences. In this 22-minute video, he dropped the historical approach that most textbooks still follow and began where modern physics actually begins.
The Confusion That Taught for Decades
When Cox learned quantum mechanics in the 1990s, universities taught the subject historically. Students started with the photoelectric effect, worked through Niels Bohr's atomic orbitals, and absorbed decades of physicists' bewilderment before arriving at the modern framework.
That approach, he argued, passes the confusion of early 20th-century pioneers directly on to students who never needed it.
The alternative is to begin with what a qubit actually is. Cox described it as a coin that can be both heads and tails at once, existing in a superposition where the outcome remains genuinely undetermined until measurement.
What is superposition?
In quantum mechanics, superposition means a particle exists in a combination of states simultaneously. A quantum coin can be 30% heads and 70% tails until someone observes it. These probabilities are not due to ignorance. They are built into the fabric of nature itself.
Classical probability reflects incomplete knowledge, like forecasting rain without enough weather data. Quantum probability is different. It is fundamental to the description of nature, not a gap in our understanding.
The Double Slit Experiment, Explained One Electron at a Time
Cox built his explanation around the experiment that captures all of quantum mechanics in a single setup.
The double slit experiment: A single particle passes through both slits at once, causing wave-like interference. (Science Reader)
Fire electrons one at a time through two slits in a barrier. On the detector screen behind it, a striped interference pattern emerges, matching what waves produce, not what particles should.
The implication is that each electron somehow explores both paths simultaneously. Richard Feynman's formulation, which Cox recommended via the freely available Feynman Lectures Volume Three, makes the calculation straightforward: assign a complex number to every possible route, add them up, and the sum predicts exactly what the experiment shows.
Whether this means the electron literally travels every conceivable route remains a question of interpretation. Cox noted that many physicists now accept it as a correct description of reality, not merely a mathematical convenience.
The problem with quantum mechanics, I suppose, is when you try to interpret what the calculation means for the nature of reality.
Brian Cox, University of Manchester
From Philosophical Puzzle to Engineering Problem
Cox's sharpest argument came at the intersection of theory and technology.
Two entangled qubits can exist in what physicists call a Bell state, where measuring one instantly determines the other regardless of distance. Einstein found this deeply troubling in the 1930s. Experiments confirming the behavior earned Alain Aspect, John Clauser, and Anton Zeilinger the 2022 Nobel Prize in Physics.
Key figure
500
Qubits whose possible configurations exceed the number of atoms in the observable universe
Scale the system to 100 entangled qubits, and the number of possible configurations exceeds anything representable by a classical computer.
At 500 qubits, the combinations surpass the number of atoms in the observable universe.
More Quantum Physics
Quantum Tunneling Won the 2025 Physics Nobel Prize
Three physicists proved quantum tunneling works at macroscopic scale, laying the foundation for modern quantum computers.
→That exponential scaling is what quantum computers attempt to harness. Companies like Google, Microsoft, and IBM are investing heavily.
In 2026, Microsoft and Atom Computing delivered the first error-corrected quantum machines, and a recent paper in Science described the field as reaching its "transistor moment."
Cox was candid about limitations. The engineering remains enormously difficult, and current machines cannot yet exploit this power reliably.
But quantum mechanics is no longer an abstract philosophical puzzle. It is becoming an engineering discipline, and understanding it, Cox argued, is no longer optional.
Sources
- Primary Source: Physicist Brian Cox explains quantum physics in 22 minutes (Big Think)
- Additional Context:
- Neutral Atom Quantum Computing: 2026's Big Leap (IEEE Spectrum)
- Scientists say quantum tech has reached its transistor moment (ScienceDaily)
Fact Check: Claim-by-Claim Verification Verified
The article accurately summarizes Brian Cox's Big Think interview on quantum mechanics, with all scientific claims, explanations, and recent developments matching reliable sources.
Commentary
- Article appropriately hedges on interpretations like "many paths" as per Cox: question of interpretation, not fact.
- Quantum computing claims note engineering challenges, avoiding overhyping current capabilities.
Sources used for verification
Academic/Peer-reviewed:
- Physicist Brian Cox explains quantum physics in 22 minutes - bigthink.com
- The Feynman Lectures on Physics Vol. III Ch. 1: Quantum Behavior - caltech.edu
- Microsoft advances quantum error correction with Atom Computing - microsoft.com
Other reliable sources:
- Scientists say quantum tech has reached its transistor moment - sciencedaily.com
- Nobel Prize Physics 2022 Aspect Clauser Zeilinger - nobelprize.org
- Time, space, and the quantum code - Brian Cox transcript - bigthinkmedia.substack.com
- Neutral Atom Quantum Computing advances - spectrum.ieee.org
Fact-checked by Perplexity Sonar Pro on 2026-03-11
