12 AUSTRALIAN SKY & TELESCOPE November | December 2017
DISCOVERIES by David EllyardAn invisible ‘rain’ from space
ROBERT MILLIKAN WAS a leading
United States physicist of the early 20th
century, who was awarded the Nobel
Prize for Physics in 1923 for his famous
‘oil drop’ experiment by which he was
able to measure the electric charge on a
single electron.
He had also done important
work on the ‘photoelectric effect’,
whereby electrons are released from
a metal surface when light falls on it.
Millikan’s interest in the effect had
been sparked by the explanation put
forward by Einstein (and for which he
was awarded the Nobel Prize in 1921).
Einstein had argued that a beam of light
consists of tiny packets of energy called
photons. Millikan was a conservative
19th-century type of physicist, and
rejected such radical ‘quantum’ ideas.
But after years of careful and laborious
experiment he had to admit that
Einstein’s explanation was the best one.
So it was with the announcement he
made in November, 1925. He was again
trying to prove someone else wrong.
This time the issue was atmospheric
electricity. Air is normally an insulator
unless the voltages are extremely high,
as in a lightning strike. But researchers
had found that radioactivity has the
power to ‘ionise’ air, that is, to give its
atoms an electric charge so that the
air will conduct electricity. This could
be made to happen in the laboratory,
but it also occurs in nature. Sensitive
‘electrometers’ were used to measure
the level of ionisation at various
locations, and so to chart the level of
the radiation that caused it.By 1909 it was known that the rate
of air ionisation is higher at the top of
the Eiffel Tower than on the ground. In
1912, German physicist Victor Hess had
flown with three electrometers to an
altitude of 5000 metres in a balloon, and
found that the rate of air ionisation, and
by implication the level of background
radiation, increased with altitude.
He had concluded “the results of my
observations are best explained by the
assumption that radiation of very great
penetrating power enters our atmosphere
from above”. His work secured him the
Nobel Prize for Physics in 1936.
Millikan would have none of it, arguing
that ionising radiation came from all
the radioactive elements in the rocks.
He had repeated Hess’ balloon-borne
measurements and achieved very different
results, much less than Hess had. Being
the methodical experimenter that he
was, Millikan set out to study the matter
further. He sank his sealed electrometers
to various depths in high-altitude lakes,
measuring among other things the power
of water to absorb the radiation.
Speaking to the US National
Academy of Science on November 9,
1925, he reported observations in two
lakes at elevations of 1500 metres and
3600 metres. The rates of ionisation at
various depths in the lower lake were
the same as in the upper lake but at
depths 2 metres less. To Millikan, this
was evidence that the ionising radiation
did indeed come from above, from
space, with 2100 metres of air absorbing
as much radiation as 2 metres of water.
It looked like Hess was right after all,and later observations agreed.
Having switched sides as to the
source of radiation, Millikan now made
his mark by devising the term ‘cosmic
radiation’ for the phenomenon. The new
term entered the public consciousness
when newspapers published reports of his
announcement two days later. He went
on to claim the radiation was a form
of electromagnetic radiation, akin to
gamma rays from radioactive elements,
though with twenty times the penetrating
power. He argued that the photons
were the ‘birth cries’ of new atoms
being constantly created to prevent the
running-down of the universe.
Millikan lost that one. It later became
clear that cosmic rays were fragments
of matter (mostly protons) rather than
packets of energy. The matter fragments
carry electric charge and so follow
paths bent by the Earth’s magnetic field,
accounting for the difference between
Hess’ balloon measurements in Europe
and Millikan’s in Texas.
It’s now clear that ionising radiation
fills space, raining down on Earth and
the other planets. Some comes from the
Sun, but the more powerful stuff arrives
from much further out. Such radiation
will pose a definite risk to astronauts
who want to trek long distances in
space, such as to Mars. As to the source
of the highest energy ‘primary’ cosmic
rays, such as in the ‘gamma ray bursts,’
the argument goes on.■ DAVID ELLYARD presented SkyWatch
on ABC TV. His StarWatch StarWheel
has sold over 100,000 copies. ESO.Scientific rigour led one researcher to abandon his ideas and accept a new reality.