Explainer
What Is the Uncertainty Principle?
The uncertainty principle turns 100 next year, and it is probably the most misquoted idea in physics. Werner Heisenberg never called it a principle. The exact inequality in every textbook…
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Explainer
Evergreen physics: the concepts the other two rely on.
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Explainer
What Is Schrödinger’s Cat?
Schrödinger's cat is the most famous image in quantum physics, and it is almost always told backwards. In 1935 Erwin Schrödinger did not claim a cat could be alive and…
Explainer
What Is Bell’s Theorem?
John Bell worked on accelerator design at CERN. Quantum foundations was the hobby, and in 1964 it produced four pages in a journal that folded soon afterwards. The strange thing…
Explainer
What Is 5 Sigma? Probability and the Discovery Rule
Five sigma corresponds to a one-sided p-value of about one in 3.5 million. Learn how that probability is calculated, why particle physics uses it, and why it cannot tell you…
Explainer
What Is the Schrödinger Equation?
In late 1925 Peter Debye told Erwin Schrödinger that talk of electron waves was childish without a wave equation. Within weeks Schrödinger had one, and a century later it still…
Explainer
What Is Dark Energy?
Dark energy is a name for an observation, not an explanation of it. The observation is that cosmic expansion is speeding up, and the whole question is whether the thing…
Explainer
How Do We Know How Bright a Supernova Was?
Type Ia supernovae are the rulers cosmology uses to measure the expanding universe, and they are usually called standard candles, as if they all shone equally brightly. They do not.…
Explainer
What Is the Double-Slit Experiment?
Fire electrons one at a time at two slits and they land as single dots, yet the dots build up into the stripes of a wave. Try to find out…
Explainer
What Does a Dark Matter Detector Actually Measure?
On 26 August 2026, two kilometres under Ontario, SuperCDMS began collecting its first scientific data. No detector can see dark matter. What it measures is heat and charge, two numbers…
Explainer
What Is the Hubble Tension?
Measure how fast the universe is expanding using the afterglow of the Big Bang, and you get one number. Measure it using nearby stars and exploding suns, and you get…
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