Experiment #22 · Scientific experiment

Eddington's Eclipse Expedition

Stars shift; general relativity is confirmed

Arthur Eddington and Frank Dyson · 1919 · General relativity

First published: F. W. Dyson, A. S. Eddington, & C. Davidson, "A Determination of the Deflection of Light by the Sun's Gravitational Field, from Observations made at the Total Eclipse of May 29, 1919", *Phil. Trans. Roy. Soc.* A 220 (1920): 291–333.

During a total solar eclipse, the positions of stars near the Sun shift by 1.75 arc seconds — exactly as Einstein predicted, and twice what Newton allowed.

Einstein's 1915 theory predicted that starlight grazing the Sun would be deflected by 1.75 arc seconds — twice the value Newtonian theory could be made to give. The May 1919 total eclipse offered the first opportunity to photograph stars near the solar limb. Eddington led one expedition to Príncipe and Dyson dispatched another to Sobral. The reported deflections matched Einstein to within experimental error and were announced at the joint Royal Society / Royal Astronomical Society meeting in November 1919. The result made Einstein a global celebrity overnight and is the first decisive observational confirmation of general relativity. Historians have since debated the precision and selectivity of the original data; subsequent expeditions and modern VLBI measurements have abundantly confirmed the prediction.

Formulation

Photograph star field during totality with the Sun present; compare to the same field photographed months later without the Sun. Measure systematic radial displacements of stars closest to the solar limb. Predicted (GR): 1.75″. Predicted (Newton, photon-as-particle): 0.87″. Observed: 1.61″ ± 0.30″ (Príncipe), 1.98″ ± 0.16″ (Sobral, four-lens telescope).

Compare this experiment

Dimensions Engaged

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Space

Confirms Space · Curvature: matter curves spacetime, and light follows geodesics in the curved geometry. Newtonian flat space is empirically ruled out at solar-system scales.

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Time

Indirect support for Time · Ontological Status as part of a unified spacetime: gravitational deflection of light is a geometric phenomenon, not a force acting in time.

Responses — How Schools Engage

Affirms / takes the bait 5

Relationalism

A vindication: gravity is the geometry of spacetime, not a force in absolute space. The Leibniz-Mach critique of Newtonian substantivalism is empirically redeemed.

Naturalism

A model case of hypothetico-deductive confirmation: a quantitative prediction from a radical theory, tested at one of its few accessible regimes, and observed. Scientific naturalism's preferred kind of success.

Realism

Scientific realism: GR really describes the spacetime geometry of the actual world. The light-bending is genuine, not a calculational artifact.

Structuralism

What survives is structural content (the metric, geodesic equations) rather than stuff (aether, absolute space). The expedition is a structural confirmation.

Logical Positivism

The kind of crisp empirical test the positivists liked: an unambiguous quantitative prediction, an unambiguous measurement, an unambiguous theory choice.

Reframes the question 1

Kantian Transcendental Idealism

A challenge to Kant's Euclidean *a priori*: physics forces non-Euclidean geometry as the form of outer intuition. Reichenbach and the neo-Kantians spent decades absorbing the consequences.