Newton’s Bucket Experiment 

The Newton’s bucket experiment is a classic thought experiment described by Isaac Newton in 1689. By observing the shape of the rotating water surface, Newton argued for the existence of absolute space. This experiment was later reinterpreted by Ernst Mach and Albert Einstein, becoming a key case in the debate between classical and relativistic spacetime. Its principle is also applied in the construction of liquid-mirror telescopes.

The experiment proceeds as follows:

A bucket half-filled with water is suspended by a soft rope. When the bucket and water are stationary, the surface is flat.

When the bucket begins to rotate but the water has not yet moved, the surface remains flat.

As friction between the bucket wall and the water gradually sets the liquid into rotation, the angular velocity of the water layers becomes uniform, and the surface forms a concave paraboloid.

Newton interpreted the concavity as proof that the water possesses absolute motion, not merely relative motion, thereby supporting his concept of absolute space.

Mach argued that the concavity arises from the relative motion between water and the distant stars, whose gravitational interaction causes the surface deformation.

Einstein’s general relativity introduced the frame-dragging effect. In 1966, Brill and Cohen showed that a rotating hollow spherical shell can induce sufficient centripetal force to produce the concave surface, providing a relativistic explanation.

To test these theories, Gravity Probe A (1976) and Gravity Probe B (2004) were launched. The observed precession of gyroscope axes indirectly supported Mach’s view.