SCIENCE IN THE IMAGINATION

Imagine it
first.

Some experiments need no lab at all.

Hands resting on an open blank notebook beside a pencil and a cup of tea on a desk by a window — AI-generated image, not a photograph of a real person or place.

Set up a scene
Follow it through
Look for the clash
Ideas tested by imagination

A thought experiment is a scene you run in your head to test an idea. Galileo used one to overturn Aristotle's account of falling, and Einstein used one to predict that gravity bends light.

Updated 16 October 2026

THE SHORT ANSWER

A scene you run
in your head.

Thought experiments are devices of the imagination.
The best ones expose a hidden contradiction.

THE IDEA

Devices of the imagination

The Stanford Encyclopedia of Philosophy calls thought experiments devices of the imagination, used in physics, biology, economics, mathematics and philosophy.

Try one

FALLING

Tie two balls together

If heavy things fall faster, a cannonball tied to a musket ball should fall both faster and slower. Galileo's way out: they all fall at the same speed.

How it was tested

LIGHT

A beam in a lift

In a sealed, accelerating lift a light beam would seem to curve down. Einstein's elevator argues gravity must do the same: gravity bends light.

The 1919 test

FOUR FAMOUS SCENES

What did nature
say later?

Pick a famous thought experiment to see what it set out to show. The bar shows how directly a later observation settled it.

Summaries from the Stanford Encyclopedia of Philosophy, NASA and the Royal Society's Notes and Records.

Thought check

Choose a thought experimentGalileo's falling ballsEinstein's elevatorNewton's bucketSchrödinger's cat

Galileo's falling ballsShown on the Moon

An astronaut dropped a hammer and a feather together on the Moon. With no air resistance, they hit the ground together, as Galileo had concluded.

Einstein's elevatorTested in 1919

British eclipse expeditions measured starlight passing the Sun. Two of three data sets matched the prediction of general relativity.

Newton's bucketArgued over

Newton offered a spinning bucket of water as evidence for absolute space. Ernst Mach answered with a counter thought experiment, so the debate stayed in the head.

Schrödinger's catNever meant to be run

A cat that is both alive and dead was Schrödinger's way of showing that one reading of quantum theory clashes with common sense about everyday objects.

Sources: Stanford Encyclopedia of Philosophy, Thought Experiments (2023 revision); NASA, Apollo 15 hammer-feather drop; Gilmore and Tausch-Pebody (2020), Notes and Records.

TWO NUMBERS WORTH KNOWING

Imagined first,
measured later.

One number from the test of Einstein's lift,
one from the history of the word itself.

1.75 arcseconds

Predicted bending of starlight at the Sun's edge

Expeditions to Príncipe and Sobral photographed stars during totality on 29 May 1919 to test whether gravity bends light by this tiny amount.

THE 1919 ECLIPSE

1811

Earliest known use of the term

Ernst Mach is often credited with the word, but the Danish 'Tankeexperiment' was used by Hans Christian Ørsted as early as 1811.

THE WORD

Sources: British Astronomical Association review of No Shadow of a Doubt (Kennefick, 2019); Stanford Encyclopedia of Philosophy, Thought Experiments.

A beam of torchlight crossing the inside of an empty, plain steel lift — AI-generated image, not a photograph of a real person or place.

RUN YOUR OWN

Galileo's argument
in four steps.

You can follow the most famous thought experiment of all at your kitchen table, without dropping anything.

Take the old rule
Assume, as Aristotle's account did, that heavy bodies fall faster than light ones.
Tie two balls together
Imagine a heavy cannonball tied to a light musket ball. Together they are heavier, so they should fall faster.
Spot the drag
But the light ball falls slower, so it should hold the heavy one back. The pair falls both faster and slower: a contradiction.
Find the way out
The contradiction disappears only if all bodies fall at the same speed, which is what Galileo concluded.

WHEN NATURE WAS ASKED

The lift met
the eclipse.

Einstein's 1915 theory predicted how much starlight should bend passing the Sun. The results of two British expeditions were announced at a meeting in London on 6 November 1919, and Einstein became world-famous the next day.

Three data sets were obtained: two matched the prediction and one was discarded as defective. A 1980 study questioned that choice; a 2020 reanalysis in the Royal Society's Notes and Records identified an error in that critique.

A heavy iron cannonball and a small lead ball tied together with cord on an old oak table — AI-generated image, not a photograph of a real person or place.

Questions

Questions people ask

What is a thought experiment?

An imagined scenario used to test or explore an idea, usually told as a short story, often with a diagram. The Stanford Encyclopedia of Philosophy calls them devices of the imagination.

Can you learn something new just by thinking?

That is the central question philosophers ask about thought experiments. Some argue Galileo's falling-bodies argument gives knowledge of nature without new data; others, like John Norton, treat thought experiments as arguments in disguise.

Was Galileo's idea ever tested?

Yes. On the Apollo 15 mission an astronaut dropped a hammer and a feather together on the Moon, where there is no air resistance, and they landed at the same time.

Is Schrödinger's cat a real experiment?

No. Schrödinger described it in 1935 to show that one interpretation of quantum theory conflicts with common sense about large, everyday objects.

Who invented the term?

Ernst Mach is often credited, but the Danish term was used by Hans Christian Ørsted as early as 1811, and Georg Lichtenberg wrote of experiments with thoughts in the 1700s.

Imagine it
first.