‹ Shelves ■ Quantum physics made simple — slits, waves and tunnellingNo formulas on the card: why light comes in packets, what the double-slit experiment shows when you send a single electron, what ‘observing’ actually means, how a particle passes through a wall (and why the Sun shines because of it), entanglement and what quantum physics does NOT say. Every lesson has a quiz and an audioguide.
18 lessons · 46 min
The reading track 18 lessons All With quiz · 18 All authors Richard P. Feynman · 3 Royal Swedish Academy of Sciences · 3 Richard P. Feynman, Robert B. Leighton, Matthew Sands · 2
01 In 1900 Max Planck found that the energy of light given off by a hot body does not flow as if down a ramp but comes in small steps: packets called quanta. The bluer the light, the taller the step. Scrisoare către Robert W. Wood (1931), despre ipoteza cuantelor din 1900 · Max Planck · 1931 ✓ Quiz · 3 questions › 02 Light is not only a wave: it arrives in packets called photons. That is why, to knock an electron out of a metal, what matters is the colour of the light, not how bright it is. Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt · Albert Einstein · 1905 ✓ Quiz · 3 questions › 03 If light, which we thought was a wave, also comes as particles, perhaps particles such as the electron also behave as waves. Louis de Broglie proposed this in 1924, and in 1927 electrons did in the lab exactly what waves do. The wave nature of the electron (prelegerea Nobel, 12 decembrie 1929) · Louis de Broglie · 1929 ✓ Quiz · 3 questions › 04 Throw balls through a board with two slits and the wall shows two stripes; send waves and it shows many bands, bright and dark, because waves add up and cancel out. Light makes bands, so it behaves like a wave. The Bakerian Lecture: Experiments and Calculations Relative to Physical Optics · Thomas Young · 1804 ✓ Quiz · 3 questions › 05 When you send electrons through two slits one at a time, each hits the screen at a single point, like a ball. But after thousands of electrons, the points line up in bands, like a wave. Each electron, alone, behaves as if it had gone through both slits. The Feynman Lectures on Physics, Vol. III: Quantum Mechanics · Richard P. Feynman, Robert B. Leighton, Matthew Sands · 1965 ✓ Quiz · 3 questions › 06 If you place a detector that finds out which slit each electron goes through, the bands vanish and two stripes remain, as with balls. What matters is not a person looking but the fact that the information about the path exists somewhere in the world. The Feynman Lectures on Physics, Vol. III: Quantum Mechanics · Richard P. Feynman, Robert B. Leighton, Matthew Sands · 1965 ✓ Quiz · 3 questions › 07 In quantum physics, ‘observing’ means any interaction that leaves a lasting trace in the world: a detector, an air molecule, heat. No person or consciousness is needed. The more things an object touches around it, the faster it loses its bands; this is called decoherence. Law Without Law · John Archibald Wheeler · 1983 ✓ Quiz · 3 questions › 08 It is not only electrons that make bands. In 1999, football-shaped molecules of 60 carbon atoms made interference bands, and in 2019 molecules with almost 2,000 atoms did. There is no known border where quantum physics stops; there are only objects ever harder to keep untouched. Wave–particle duality of C60 molecules · Markus Arndt, Olaf Nairz, Julian Vos-Andreae, Claudia Keller, Gerbrand van der Zouw, Anton Zeilinger · 1999 ✓ Quiz · 3 questions › 09 Quantum physics does not say where a particle will land, only how likely each place is. Every possible path carries an ‘arrow’; the arrows add up and can cancel, and the chance is the length of the final arrow, squared. QED: The Strange Theory of Light and Matter · Richard P. Feynman · 1985 ✓ Quiz · 3 questions › 10 Superposition does not mean the particle is secretly in one state and we do not know which, nor that it is ‘in two places’ like a duplicated object. It means its state is a combination with arrows of its own, which can give results impossible for any mixture of ordinary options. Die gegenwärtige Situation in der Quantenmechanik (The Present Situation in Quantum Mechanics) · Erwin Schrödinger · 1935 ✓ Quiz · 3 questions › 11 You cannot know at the same time, however good your instruments, exactly where a particle is and exactly how fast it moves. It is not a clumsy instrument but a property of every wave, and it is precisely what keeps atoms from collapsing. Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik · Werner Heisenberg · 1927 ✓ Quiz · 3 questions › 12 A ball without enough energy never gets over the hill. A quantum particle, because it also behaves like a wave, has a small chance of turning up beyond a wall it could not jump: that is tunnelling, and the chance falls very fast with the wall's thickness, its height and the particle's mass. Press release: The 1973 Nobel Prize in Physics · Royal Swedish Academy of Sciences · 1973 ✓ Quiz · 3 questions › 13 The protons at the centre of the Sun repel each other electrically and, at 15 million degrees, lack the energy to touch. They touch anyway, rarely, by tunnelling: without it the Sun would not burn, and its rarity is why the Sun burns slowly enough for life to have appeared. The Internal Constitution of the Stars · Arthur Stanley Eddington · 1926 ✓ Quiz · 3 questions › 14 Tunnelling works for you every day: Esaki's diode, the microscope that sees atoms and the flash memory in your phone all live on it. It is also a wall for ever-smaller transistors, through which electrons start to leak. Press release: The 1986 Nobel Prize in Physics · Royal Swedish Academy of Sciences · 1986 ✓ Quiz · 3 questions › 15 Tunnelling is not only for tiny particles. In 1984–1985 John Clarke, Michel Devoret and John Martinis showed that an electric circuit on a one-centimetre chip, with billions of electron pairs, tunnels like a single particle and takes in energy only in steps. For that they received the 2025 Nobel Prize in Physics. Press release: The Nobel Prize in Physics 2025 · Royal Swedish Academy of Sciences · 2025 ✓ Quiz · 3 questions › 16 Two entangled particles give linked results however far apart they are, and experiments show the link is stronger than any ‘instructions written in advance’ could allow, as with a pair of gloves; yet you cannot send any message through it. On the Einstein Podolsky Rosen Paradox · John S. Bell · 1964 ✓ Quiz · 3 questions › 17 A quantum computer does not ‘try every answer at once’: it arranges probability waves so that wrong answers cancel and the right one is reinforced, like the bands in the double-slit experiment — which works only for a few kinds of problems. Simulating Physics with Computers · Richard P. Feynman · 1982 ✓ Quiz · 3 questions › 18 Quantum physics is strange, but it is not magic: observing needs no consciousness, entanglement is not telepathy, and delicate quantum effects fade fast in large, warm objects — what is genuinely open is what is real, not what works. The Character of Physical Law · Richard P. Feynman · 1965 ✓ Quiz · 3 questions ›