"I never thought that one could grow so attached to science."
These words are from a letter
young Russian physicist Pyotr Lebedev wrote to his
mother from Strassburg, where
he had gone to complete his education.
Lebedev was born in Moscow on March 8, 1866, he was the son of a
wealthy merchant. His father wanted
him to follow a business
career and sent him to the
German Commercial School in Moscow. In those days
many of the schools were run by foreigners, and it was
the fashion among merchants to educate their children
in life, however, Lebedev was attracted by science. Gradually
he became convinced that his calling
was research. Training for
research work could only be had at the universities, but
to enter one of those it was essential to know Latin and
Greek, which Lebedev had
never studied. He was therefore compelled to go abroad for a scientific education.
In Strassburg Lebedev made a thorough study of
physics and became very good at experiments. In
received his doctor's degree, after completing a
brilliant dissertation on the dielectric permeability of
gases. These studies
convinced him that an electric field exerts a strong
influence on the properties of molecules,
a subject that continued to hold his interest.
The study of this problem led Lebedev to an important
discovery, which concerned the tails of comets. Down the agespeople
have watched the movement of comets with wonder.
Gradually comets came to be
accepted as a part of the Universe, just like the stars
and planets. On the basis of physical laws, scientists
determined the orbits in
which comets move in space and even learned to predict
the dates of their periodic appearance. It was
established that a comet consists of a small "head", made up of extremely dense matter, and a "tail", a huge amount
of rarified matter.
However, scientists could not explain one exceedingly
interesting circumstance. As it approaches the
Sun, a comet changes its shape in a very definite
manner. Its tail lengthens out at a rate of a million
kilometres a day until it reaches a maximum length of
50—100 million kilometres at the point nearest to the
Sun. Then, as it withdraws from the Sun, its tail
shortens in a similar manner. The most remarkable
thing, however, is that the tail always points away
from the Sun. This fact appeared to contradict the law
of gravity, according to which the tail of a comet, like
any other material body, should be attracted to the Sun.
opposite seems to be true: the tail appears
to be repelled by the Sun.
early as 1619 Johannes Kepler had suggested
that this behaviour of a comet's tail was due to the
repulsive action of the Sun's rays; but nobody could provide a
plausible explanation of the mechanism of this repulsion. In
1891 Lebedev wrote a paper entitled "On the Repulsive Force of
Radiating Bodies", in which
he offered a theory explaining the changes in the shape of a comet's tail. In his
paper he demonstrated that there are forces of
repulsion in space, which are
brought into being by
the pressure of light.
Moreover, for very small
cosmic particles the force of
repulsion due to the pressure of
light is considerably greater
than the force of
attraction due to the law of
tails, Lebedev argued, consist of
gases, which are actually a vast quantity of
molecules. As the comet draws nearer to the Sun, each of these
molecules experiences an
increasing repulsive force of the Sun's rays.
It is the light rays that
drive the particles of the
tail away from the Sun.
Lebedev next set out to prove in laboratory conditions
that light exerts pressure. Some years earlier the
English physicist James
Clerk Maxwell had formulated an electromagnetic theory of light, in which it was assumed that light does
exert pressure. But Maxwell's
theory was purely mathematical. There was no experimental
confirmation of it, and many very prominent scientists were opposed to
If the pressure of light could be established
clearly prove Maxwell's theory correct. But
to detect this pressure was a very difficult matter: a mosquito alighting on a mirror
will press on it with a greater force than a ray of
by means of an extremely
clever experiment, Lebedev
succeeded in actually
measuring the pressure of light. Lebedev first
reported his experiment at
meeting of a Swiss Scientific Society in 1899. The
following year he reported the results of his work to
the World Congress of Physicists in Paris.
The discovery made a profound
Russian scientist's success was applauded by all the
leading physicists of the world. The great Lord Kelvin, on
meeting Kliment Timiryazev, said to him: "Perhaps you know that
all my life I have been
fighting Maxwell, refusing
to recognize his light pressure, and now your Lebedev
has forced me to surrender in
the face of his experiments!"