What a measurement is

The order, and why it is this one

Join anywhere. Each stop carries the route with it, so the foot of every question here offers the next one and the way back to this page.

1. 01What actually happens when we look?The most precisely tested theory in science comes with a set of mutually exclusive stories about what it describes, and no experiment has yet decided between them.Abyssdepth, meaning: Fundamental questions where our understanding is extremely limited.Unknown. Humanity does not currently have a satisfactory answer.Read
2. 02Can two things be connected across any distance?Measurements on entangled particles are correlated more strongly than any theory in which each particle carried its answer with it could produce. In 2015 the experiment was finally done with the detection and locality loopholes closed at once, and the correlations held.Deepdepth, meaning: Major unanswered questions.Known. Strongly supported by evidence and broadly established.Read
3. 03Is anything truly random?Classical randomness is ignorance about details. Quantum randomness appears to be different in kind, and a 2018 experiment generated bits that no theory forbidding faster-than-light signals could have predicted, provided the measurement choices were made independently of the devices. The physics inside the devices is not assumed; the timing electronics and computers around them are trusted.Deepdepth, meaning: Major unanswered questions.Uncertain. Meaningful scientific, historical or philosophical disagreement remains.Read
4. 04How big can something be and still be quantum?Interference experiments have been pushed from electrons to metal clusters of thousands of atoms without encountering a boundary. Whether the limit is merely technical, or whether something physically destroys superpositions above some mass, is a question being decided by pushing further.Deepdepth, meaning: Major unanswered questions.Uncertain. Meaningful scientific, historical or philosophical disagreement remains.Read
5. 05What does quantum mechanics mean?Thirty-three physicists, philosophers and mathematicians at a foundations conference were polled on what their own theory is about, and no interpretation took a majority. A survey of more than a thousand researchers in 2025 found the same. Some candidate theories have been ruled out by measurement, which makes this a scientific dispute rather than only a philosophical one. What is left includes accounts built to make identical predictions, which no experiment can tell apart.Abyssdepth, meaning: Fundamental questions where our understanding is extremely limited.Unknown. Humanity does not currently have a satisfactory answer.Read