The Science Book

88

THE DIMINUTION

OF THE VELOCITY OF

LIGHT

JOHN MICHELL (1724–1793)

I

n a 1783 letter to Henry

Cavendish at the Royal Society,

British polymath John Michell

set out his thoughts on the effect of

gravity. The letter was rediscovered

in the 1970s and found to contain

a remarkable description of black

holes. Newton’s law of gravity states

that an object’s gravitational pull

increases with its mass. Michell

considered what might happen to

light if it is affected by gravity. He

wrote: “If the semidiameter of a

sphere of the same density with the

sun were to exceed the sun in the

proportion of 500 to 1, a body falling

from an infinite height toward it

would have acquired at its surface

a greater velocity than that of light,

& consequently, supposing light

to be attracted by the same force...

all light emitted from such a body

would be made to return towards

it.” In 1796, French mathematician

Pierre-Simon Laplace came up with

a similar idea in his Exposition du

Système du Monde.

However, the idea of a black

hole would lie dormant until Albert

Einstein’s 1915 paper on general

IN CONTEXT

BRANCH

Cosmology

BEFORE

1686 Isaac Newton formulates

his law of universal gravitation,

in which the strength of

the gravitational attraction

between objects is

proportional to their masses.

AFTER

1796 Pierre-Simon Laplace

independently theorizes about

the possibility of black holes.

1915 Albert Einstein shows

that gravity is a warping of the

space-time continuum, which

is why massless light photons

are affected by gravity.

1916 Karl Schwarzschild

proposes the event horizon,

beyond which no data can be

received about a black hole.

1974 Stephen Hawking

predicts that quantum effects

at the event horizon will emit

infrared radiation.

Newton shows that

the gravitational

attraction of an object

is proportional

to its mass.

If light is affected

by gravity, a massive

enough object will have

such a strong gravitational

field that no light will be

able to escape it.

Einstein explains

gravity as a distortion

of space-time, meaning

that massless light is

affected by gravity.

The velocity

of light will appear

to diminish.