Einstein, Eddington and the 1919 Eclipse

Expeditions

Peter Coles(Cardiff University)

http://telescoper.wordpress.com

“..the Supreme God is a Being eternal, infinite, absolutely perfect…He endures forever and is everywhere present; and by existing always and everywhere, he constitutes duration and space..”

Newton, Principia (General Scholium).

Absolute Space, Absolute Time

General Relativity

• Published in 1915

• The “Principle of Equivalence”

• Acceleration and Gravity

• The curving of space

• ..and the bending of light!

is the angular deflection at grazing incidence

=4GM/Rc2

Light Bending..a comedy of errors.

• Newton “Do not Bodies act upon Light at a distance, and by their action bend its Rays?”

• 1801 Johann Georg von Soldner, calculates bending of light by the Sun: = 0.87 seconds of arc: the “Newtonian” Prediction

• 1907 Einstein thinks about light bending, but then shelves the idea.

• 1911 Einstein tries again using “E=mc2”; gets Soldner’s answer: = 0.87 seconds of arc.

• 1915 Einstein tries again, and finds a mistake, a factor of two. The new value is = 1.74 seconds of arc.

The Factor Two

ds2=c2dt2−dr 2−r2 dΩ2

ds2=(1− 2GMrc 2 )c2dt 2−(1− 2GM

rc2 )−1

dr 2−r 2d Ω 2

Flat Space:

Schwarzschild:

Energy Momentum

Eddington and the Expeditions

• In 1912 Eddington had been involved in an Eclipse expedition to Argentina. It rained.

• 1916 de Sitter tells him about Einstein’s prediction and suggests the idea of light bending measurements during an eclipse.

• 1917, Frank Watson Dyson, the Astronomer Royal realises the eclipse of 29 May 1919 would be perfect.

The Eclipse of 1919

• Date: 29 May 1919• Path of Totality is across the South Atlantic

from Sobral to Principe• Duration is long…7 minutes or so at

Principe (max for a total eclipse is ~ 9 mins)• Behind the Sun during totality was a cluster

of stars, The Hyades.

War and Peace

• BUT Eddington was a Quaker, and therefore a pacifist.

• The First World War had started in 1914, but conscription was not introduced in the British Army until 1917.

• Eddington refused to be drafted…• He was saved by a deal by Dyson, which

protected him on condition he agreed to lead an expedition in 1919 if the war was over.

The Equipment

• Funding: £100 for equipment, £1000 for travel and labour costs

• Two “astrographic” object glasses, one to Principe (Oxford), Sobral (Greenwich), both stopped down to 8 inches.

• A 4 inch telescope taken to Sobral as a backup

• All were equipped with coelostats• The two astrographic object glasses were

mounted in stainless steel tubes

The Measurements

• Eddington went to Principe (off the coast of Spanish Guinea)

• Crommelin went to Sobral (Northern Brazil).• Eddington was nearly rained out

“THROUGH CLOUD. HOPEFUL”• Crommelin was luckier “ECLIPSE

SPLENDID”• The results were presented as supporting

Einstein• But it wasn’t quite as simple as that..

The Paper

• Dyson, Eddington and 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

• Philosophical Transactions of the Royal Society of London. Series A, Containing Papers of a Mathematical or Physical Character, Volume 220, pp. 291-333

• 81 citations on ADS!

Some Data Analysis…

ax+ by+ c+ αE x=Dxdx+ ey+ f + αE y=Dy

Coordinates of stars Measured deflections

Gravitational deflections at star positions

The Controversy

• Principe astrographic: 2 “poor” plates. (=1.62 ± 0.45)

• Sobral astrographic: 18 “poor” plates (= 0.86 ± 0.48)• Sobral 4”: 8 “good” plates: (=1.98 ± 0.18)• Eddington included the Principe results, despite not

really getting enough measurements for an astrometric solution

• The Sobral astrographic suffered from “serious optical problems” but remeasurement in the 1970s gave results consistent with the Einstein value.

The Aftermath

• This made Einstein more famous than any scientist before or since.

• Reconciliation of Britain and Germany

• What might have been…the two expeditions of 1912 and 1914 failed to take measurements when the prediction was wrong!

• Much better measurements were made in 1922, and later using radio observations.