The mind-bending reality of quantum mechanics - with Jim Al Khalili
What this covers
Physicist Jim Al-Khalili charts the full history and implications of quantum mechanics in this hour-long discourse at the Royal Institution, tracing the framework from its pioneers in the 1920s through to the technologies now being built on its foundations. He argues that quantum mechanics, despite remaining philosophically unresolved and deeply counterintuitive, stands as the most experimentally validated framework in science. The talk moves chronologically from the early breakthroughs of Planck, Bohr, and Heisenberg through the mid-century refinements, then pivots to explain what makes quantum mechanics so strange—superposition, tunneling, and entanglement—before turning to its practical applications, past and coming.
Al-Khalili gives sustained attention to entanglement and the historical puzzle it posed. He walks through the EPR paradox, Einstein's famous objection to "spooky action at a distance," and how Bell's theorem provided a testable prediction. He describes the experiments by Clauser, Aspect, and Zeilinger that proved Einstein wrong, confirming that entanglement is not merely incomplete knowledge but a real feature of nature. From there, the scope widens considerably. He traces how the first quantum revolution produced lasers, semiconductors, computers, GPS, and MRI machines—essentially all of modern electronics. The second quantum revolution, now underway, encompasses quantum clocks of extraordinary precision, quantum sensing that can detect single neurons firing, ghost imaging that sees objects indirectly, and quantum computing, though he tempers expectations there, noting that practical machines remain one to two decades away. Throughout, he emphasizes that entanglement may be so fundamental that space and time themselves could emerge from it.
Al-Khalili argues that quantum mechanics, though deeply counterintuitive and still philosophically unresolved (especially regarding entanglement), is the most powerful and experimentally validated framework in science, and that its 'weird' features—entanglement, superposition, tunneling—are now being harnessed in a second quantum revolution of real technologies.
- Bell's theorem and subsequent experiments (Clauser, Aspect, Zeilinger) empirically proved Einstein wrong and entanglement real
- The first quantum revolution built modern electronics; the second is producing quantum clocks, sensors, imaging, and computing
- Entanglement may be so fundamental that space and time themselves emerge from it
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The sun shines because of quantum tunneling: hydrogen nuclei (protons) can quantum tunnel together as the first step toward nuclear fusion into helium, enabling thermonuclear fusion that produces the sun's energy.
“the sun shines because of quantum tunneling. Hydrogen uh nuclei protons can quantum tunnel together in the first step towards nuclear fusion to make helium”
In quantum mechanics, what you observe depends on what you measure: if you probe a photon's wavelength it behaves as a wave, and if you measure its position it behaves as a particle, and until measured it is neither but a mixture of both.
“in quantum mechanics, the lesson is that what you get depends on what you measure. If you want to see it behaving like a wave, sure enough, it'll oblige and behave like a wave.”
The EPR paradox argues that since measuring one of two entangled photons instantly determines a property of the distant photon without touching it, that second photon must have possessed those properties from the start, implying quantum mechanics is incomplete rather than that spooky instantaneous communication occurs.
“they they could know or could have known both the position and momentum of photon 2 without interacting with it. How could it possibly have that? It must have had those properties from the start”
Quantum entanglement means two photons are described by a single shared quantum state, so neither has decided whether it is a particle or wave; forcing one photon to make up its mind instantly affects what the other is doing regardless of distance.
“Quantum mechanics says photon one and photon 2, neither of them have decided whether they're a particle or a wave... they are combined together. We say they're quantum entangled. They're described by the same quantum state.”
Experimental tests of Bell's inequality by Clauser (early 1970s), Aspect (early 1980s) and Zeilinger consistently yielded correlation values greater than two, proving Einstein wrong and confirming that quantum entanglement is real; the three won the 2022 Nobel Prize in Physics for this work.
“all three of those men had to wait a long time 2022 they won the Nobel Prize for tests of Bell's inequality what do they find quantum mechanics is right you get a number more than two Einstein couldn't be right”
The first quantum revolution produced essentially all of modern technology—lasers, diodes, LEDs, integrated circuits, computers, GPS, the internet, smartphones, MRI machines, and electron microscopes—because understanding the quantum world gave us semiconductors and chips.
“the first quantum revolution involved developing many devices that relied on these ideas in quantum mechanics. The laser is is a famous one. But of course we have the diode, LEDs, integrated circuits, the computer, GPS, internet... smartphones.”
Bell's theorem (1964), formalized by the CHSH inequality, provided a testable formula: if nature is local (as Einstein believed), a correlation quantity between entangled particles cannot exceed two; a measured value greater than two proves non-local instantaneous entanglement.
“if uh if if Einstein's right and nature is local then there's a formula that says there's only so much that... those two particles can have in common... This quantity here cannot be greater than two.”
The Copenhagen interpretation holds that the atomic world cannot be pictured or visualized; quantum mechanics only gives predictions about the results of measurements, and one cannot speak about what an atom is doing when not being observed.
“the atomic world wasn't something that we could picture we couldn't talk about an atom as a miniature solar system... What quantum mechanics could give you, they argued, was predictions about the results of measurements”
Quantum entanglement is so prevalent that it is probably the most important feature of physical reality, and some current fundamental theories suggest entanglement may be the source code of reality, with even time and space themselves possibly emerging from it.
“current theories in very the fundamentals of physics are even suggesting that quantum entanglement may be the the the source code base of reality. Even time and space themselves may emerge from quantum entanglement”
Optical lattice clocks use lasers to pump energy into atoms that emit at optical (higher) frequencies rather than microwave, yielding far more accurate timekeeping—so accurate they would lose less than a second over the entire age of the universe.
“these atomic clocks are so accurate they lose something like less than a second over the age of the entire universe.”
Ghost imaging exploits quantum entanglement so that one can obtain an image of an object using entangled photons where the photon that interacts with the object is not the one that reaches the camera—allowing you to see something while not directly looking at it.
“ghost imaging is this wonderful idea where you can look at something while you're actually not looking at it. You send light to probe it and then light in another direction goes to a camera and in the camera is the image of the thing that you weren't looking at.”
Quantum sensing using atom interferometry with entangled atoms can measure Earth's gravity extremely accurately, including at sea or underground, and a brain-scanner version worn like a cycling helmet is sensitive enough to detect the magnetic field of a single firing neuron—offering an alternative to confining MRI scanners.
“a brain scanner... using lasers and atoms and quantum entanglement that is so sensitive they can pick up the weakest of magnetic fields even the magnetic fields created by the firing of a single neuron in the brain”
GPS works because navigation satellites carry atomic clocks that measure time extremely accurately, which is necessary to measure distance from the satellites precisely enough to triangulate a phone's location.
“those satellites that are sending radio signals to your phone have atomic clocks on board. And those atomic clocks are needed to measure time very very accurately in order for us to measure the distance from those satellites very accurately because that's the only way you can triangulate where you are.”
Quantum electrodynamics (QED), developed by Feynman, Schwinger and Tomonaga in the 1940s, applies quantum mechanics to all matter and electromagnetic radiation and is often regarded as the most accurate theory in all of science, describing most phenomena except gravity.
“Quantum electronamics is often regarded as as as the most accurate theory in all of science. Very very powerful.”
Quantum mechanics is not a theory but a mathematical framework for understanding the microcosm, analogous to how classical Newtonian mechanics is a framework for the everyday world rather than a theory.
“Quantum mechanics isn't a theory. Quantum mechanics is a framework for un a mathematical framework for understanding the workings of the microcosm”
Quantum tunneling allows a particle given insufficient energy to surmount an energy barrier to nonetheless disappear and reappear on the other side with a certain probability, behaving like a ghost passing through a solid wall, because particles are waves and waves can leak through the barrier.
“There's a certain probability that it gets halfway up, disappears, reappears on the other, and rolls down like a ghost phantom walking through a solid wall.”
Quantum mechanics explains the structure of the periodic table by governing the rules for how electrons arrange themselves around atomic nuclei, which in turn determine the physical and chemical properties of the elements.
“Quantum mechanics explains the rules for how electrons arrange themselves around atomic nuclei and give us the properties of the elements. Without quantum mechanics, you wouldn't understand why and how the periodic table looks the way it does.”
Decoherence is the process by which the everyday world disturbs and destroys delicate quantum entanglement; for example, detecting whether one photon is a particle or wave destroys the entanglement, and this concept (developed largely by Zeh and Zurek) resolves paradoxes like Schrödinger's cat.
“decoherence is when our everyday world messes with disturbs quantum entangle quantum entanglement... if you measure it... you destroy the entanglement”
Entanglement-based imaging cameras are being used to detect tiny details in breast cancer tumor biopsies, picking up far more detail than infrared cameras alone because the visible-light photon provides much sharper images while the entangled infrared photon probes the object.
“This is now being used to uh pick up um tiny details in in biopsies of tumors in breast cancer. They can pick up far more detail than you could just with infrared cameras for example because visible light cameras are much more precise, much sharper images.”
Despite corporate claims from Google, IBM and others that quantum computers are imminent, realistically practical quantum computers are one to two decades away because of challenges in scaling up qubits, error correction against decoherence, uncertainty over the physical platform, and the scarcity of quantum algorithms.
“you'll hear the big um corporations working at Google and IBM and others saying you know we've we've done it. We've cracked it... Realistically we're talking about one or two decades away before quantum computers actually arrive.”
Quantum computing relies on superposition so that a quantum bit (qubit) can be both 0 and 1 at the same time rather than a binary 0 or 1, and could eventually aid drug discovery, battery and solar panel design, finance, logistics, and simulation of subatomic systems.
“it relies on this idea that uh of superposition that rather than having a logic uh gate that's 0 or one binary digits, now we have zero and one at the same time.”
There are multiple competing physical platforms for building quantum computers—superconducting qubits, photonic qubits, neutral-atom qubits cooled and manipulated by lasers, and trapped-ion qubits held by electromagnetic fields—and it is not yet known which will prove the most efficient and powerful.
“We don't know which of these quantum uh computers is going to be the one that ends up being the most efficient, the most powerful, the one that's going to give us quantum computing in in the years to come.”
Gravity is the last remaining major phenomenon in nature that has not been brought under the framework of quantum mechanics, and reconciling it is the central challenge of fundamental physics.
“the big challenge in in fundamental physics is how do you bring gravity in? That's the last remaining big phenomenon in in nature that hasn't come under the spell of quantum mechanics.”
Einstein did not actually write the EPR paper—Boris Podolsky wrote it and leaked it to the New York Times, which displeased Einstein not because he disagreed with its content but because of the publicity and that it was not really his work.
“Einstein didn't write that paper. It was Boris Podsolski who'd written the paper. And Einstein was unhappy to say the least that Boris Podolski then leaks it to the New York Times as well.”
Max Planck initiated quantum theory in 1900 by proposing that radiation from warm bodies is not continuous but discrete, coming in lumps or quanta like separate drips rather than a continuous stream.
“Plank suggested that radiation given off by warm bodies isn't continuous like a the stream of water from a tap but if you look carefully enough it's discreet. It comes in lumps like separate drips.”
Superconductivity, where materials cooled to near absolute zero lose their electrical resistance so current can flow without hindrance, is one of the quantum phenomena underpinning superconducting qubit quantum computers.
“materials cooled down to near absolute zero lose their chem their um electrical resistance in their wiring and so there's no reason so they so the current can flow through without any hindrance”
Heisenberg developed the mathematical framework now called quantum mechanics in the summer of 1925 while recuperating from hay fever on the island of Helgoland.
“A young German scientist, Vera Heisenberg, was famously recovering from a bout of hay fever on the treeless island of Helgoland in the North Sea... he uh came up with the mathematical framework that today we call quantum mechanics.”
Marie Curie was the only woman in the 1927 Solvay conference photograph and is distinguished by having won two Nobel Prizes, which none of the other attendees managed.
“Mari Cury, the only woman in the whole photo photograph. However, she has something over the rest of them. She won two Nobel prizes. None of them managed that.”
Niels Bohr said that if you are not astonished by quantum mechanics then you clearly haven't understood it.
“the famous quote by the great uh Danish physicist Neils Boore who says if you're not astonished by quantum mechanics then clearly you haven't understood it.”
The clip from his 2013 Royal Institution lecture on the two-slit experiment, where he challenged anyone with a common-sense explanation to contact him for a Nobel Prize recommendation, has been viewed several million times and still generates one or two emails a week from people claiming to have solved it.
“That clip has been viewed several million times. I to this day 12 years later I get one or two emails a week from people saying I have solved the two experiment.”