1900 · Planck and the Quantum Hypothesis
On December 14, 1900, a 42-year-old German professor named Max Planck stood up at a meeting of the Berlin Physical Society and presented a formula that fit the experimental data perfectly — and made no sense at all.
The problem was called black-body radiation: heat a lump of iron and it glows, first red, then orange, then white-hot. Physicists had measured exactly which colours come out at each temperature. But every equation of classical physics predicted the wrong answer — at high frequencies, the formulas blew up to infinite energy. Something was deeply broken.
Planck found a formula that matched the measurements beautifully. But to derive it, he had to assume something absurd: that energy is not a smooth, continuous flow, but comes in tiny indivisible packets — quanta — each carrying energy \(E = h\nu\), where \(h\) was a brand-new constant of nature. He called it a mathematical trick. He spent the next years trying to get rid of his own idea and derive the formula without it. He never could. The universe really is granular.
For this discovery of energy quanta, Planck received the 1918 Nobel Prize in Physics (announced in 1919, delayed by the First World War) — "in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta."
Common myth: "Planck was trying to overthrow physics." Not at all — he was a conservative who wanted to save classical physics. The quantum revolution began as an accident, by a man who didn't want it.
Go deeper — the math & the rigor
Planck's radiation law gives the energy emitted per unit frequency by a perfect absorber (a "black body") at temperature \(T\):
\[B(\nu, T) = \frac{2h\nu^3}{c^2}\,\frac{1}{e^{h\nu/kT} - 1}\]
The key move was the counting: to derive this, Planck assumed the oscillators in the cavity walls could only hold energy in whole multiples of \(h\nu\). The constant he introduced, Planck's constant \(h \approx 6.626 \times 10^{-34}\,\text{J·s}\), turned out to be one of the fundamental constants of nature — it sets the scale at which quantum effects appear.
Classically, energy could be divided without limit, so high-frequency modes each carried their "fair share" of thermal energy — and there are infinitely many of them, hence the infinite prediction (the so-called ultraviolet catastrophe). Quantization cuts this off: a mode of frequency \(\nu\) needs at least one whole quantum \(h\nu\) to be excited at all, so high-frequency modes simply freeze out. Infinity disappears.
Why this prize matters for quantum computing: the qubit is a quantized energy system. Every qubit is a physical system — an atom, a superconducting circuit — whose energy comes in discrete levels, exactly as Planck discovered. When we say a qubit is \(|0\rangle\) or \(|1\rangle\), we mean it sits in one of two quantized energy states, and quantum gates drive transitions between them. Without Planck's quanta, there is no two-level system, and no qubit.
Key takeaways
- In 1900 Planck explained black-body radiation by assuming energy comes in discrete packets, E = hν.
- He introduced Planck's constant h, which sets the scale of all quantum phenomena.
- He called it a mathematical trick and tried for years to remove the assumption — the data wouldn't allow it.
- He received the 1918 Nobel Prize (announced 1919) for the discovery of energy quanta.
- Quantized energy levels are the physical basis of the qubit: |0⟩ and |1⟩ are two discrete energy states.
Check your understanding
Q1.What radical assumption did Planck make to derive his radiation law?
Planck's law only follows if the cavity oscillators exchange energy in whole quanta hν — continuous energy gives the wrong (infinite) answer.
Q2.For which discovery did Planck receive the 1918 Nobel Prize in Physics?
The official motivation was 'in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta.'
Q3.Why does Planck's discovery matter for quantum computing?
A qubit is |0⟩ or |1⟩ precisely because real systems have quantized energy levels — no Planck, no discrete levels, no qubits.
Tip: press ← / → to turn pages.