The Outline of Science, Vol. 1 (of 4)

J >> J. Arthur Thomson >> The Outline of Science, Vol. 1 (of 4)

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Measuring the Speed of Light

But this is not all; soon a new use was found for the spectroscope. We
found that we could measure with it the most difficult of all speeds
to measure, speed in the line of sight. Movement at right angles to the
direction in which one is looking is, if there is sufficient of it, easy
to detect, and, if the distance of the moving body is known, easy to
measure. But movement in the line of vision is both difficult to detect
and difficult to measure. Yet, even at the enormous distances with which
astronomers have to deal, the spectroscope can detect such movement and
furnish data for its measurement. If a luminous body containing, say,
sodium is moving rapidly towards the spectroscope, it will be found that
the sodium lines in the spectrum have moved slightly from their usual
definite positions towards the violet end of the spectrum, the amount of
the change of position increasing with the speed of the luminous body.
If the body is moving away from the spectroscope the shifting of the
spectral lines will be in the opposite direction, towards the red end of
the spectrum. In this way we have discovered and measured movements that
otherwise would probably not have revealed themselves unmistakably to us
for thousands of years. In the same way we have watched, and measured
the speed of, tremendous movements on the sun, and so gained proof that
the vast disturbances we should expect there actually do occur.

[Illustration: THE SPECTROSCOPE IS AN INSTRUMENT FOR ANALYSING LIGHT; IT
PROVIDES THE MEANS FOR IDENTIFYING DIFFERENT SUBSTANCES

This pictorial diagram illustrates the principal of Spectrum Analysis,
showing how sunlight is decomposed into its primary colours. What we
call white light is composed of seven different colours. The diagram is
relieved of all detail which would unduly obscure the simple process by
which a ray of light is broken up by a prism into different
wave-lengths. The spectrum rays have been greatly magnified.]

IS THE SUN DYING?

Sec. 3

Now let us return to our consideration of the sun.

To us on the earth the most patent and most astonishing fact about the
sun is its tremendous energy. Heat and light in amazing quantities pour
from it without ceasing.

Where does this energy come from? Enormous jets of red glowing gases can
be seen shooting outwards from the sun, like flames from a fire, for
thousands of miles. Does this argue fire, as we know fire on the earth?
On this point the scientist is sure. The sun is not burning, and
combustion is not the source of its heat. Combustion is a chemical
reaction between atoms. The conditions that make it possible are known
and the results are predictable and measurable. But no chemical reaction
of the nature of combustion as we know it will explain the sun's energy,
nor indeed will any ordinary chemical reaction of any kind. If the sun
were composed of combustible material throughout and the conditions of
combustion as we understand them were always present, the sun would burn
itself out in some thousands of years, with marked changes in its heat
and light production as the process advanced. There is no evidence of
such changes. There is, instead, strong evidence that the sun has been
emitting light and heat in prodigious quantities, not for thousands, but
for millions of years. Every addition to our knowledge that throws light
on the sun's age seems to make for increase rather than decrease of its
years. This makes the wonder of its energy greater.

And we cannot avoid the issue of the source of the energy by saying
merely that the sun is gradually radiating away an energy that
originated in some unknown manner, away back at the beginning of things.
Reliable calculations show that the years required for the mere cooling
of a globe like the sun could not possibly run to millions. In other
words, the sun's energy must be subject to continuous and more or less
steady renewal. However it may have acquired its enormous energy in the
past, it must have some source of energy in the present.

The best explanation that we have to-day of this continuous accretion of
energy is that it is due to shrinkage of the sun's bulk under the force
of gravity. Gravity is one of the most mysterious forces of nature, but
it is an obvious fact that bodies behave as if they attracted one
another, and Newton worked out the law of this attraction. We may say,
without trying to go too deeply into things, that every particle of
matter attracts every other throughout the universe. If the diameter of
the sun were to shrink by one mile all round, this would mean that all
the millions of tons in the outer one-mile thickness would have a
straight drop of one mile towards the centre. And that is not all,
because obviously the layers below this outer mile would also drop
inwards, each to a less degree than the one above it. What a tremendous
movement of matter, however slowly it might take place! And what a
tremendous energy would be involved! Astronomers calculate that the
above shrinkage of one mile all round would require fifty years for its
completion, assuming, reasonably, that there is close and continuous
relationship between loss of heat by radiation and shrinkage. Even if
this were true we need not feel over-anxious on this theory; before the
sun became too cold to support life many millions of years would be
required.

It was suggested at one time that falls of meteoric matter into the sun
would account for the sun's heat. This position is hardly tenable now.
The mere bulk of the meteoric matter required by the hypothesis, apart
from other reasons, is against it. There is undoubtedly an enormous
amount of meteoric matter moving about within the bounds of the solar
system, but most of it seems to be following definite routes round the
sun like the planets. The stray erratic quantities destined to meet
their doom by collision with the sun can hardly be sufficient to account
for the sun's heat.

Recent study of radio-active bodies has suggested another factor that
may be working powerfully along with the force of gravitation to
maintain the sun's store of heat. In radio-active bodies certain atoms
seem to be undergoing disintegration. These atoms appear to be splitting
up into very minute and primitive constituents. But since matter may be
split up into such constituents, may it not be built up from them?

The question is whether these "radio-active" elements are undergoing
disintegration, or formation, in the sun. If they are undergoing
disintegration--and the sun itself is undoubtedly radio-active--then we
have another source of heat for the sun that will last indefinitely.

THE PLANETS

LIFE IN OTHER WORLDS?

Sec. 1

It is quite clear that there cannot be life on the stars. Nothing solid
or even liquid can exist in such furnaces as they are. Life exists only
on planets, and even on these its possibilities are limited. Whether all
the stars, or how many of them, have planetary families like our sun, we
cannot positively say. If they have, such planets would be too faint and
small to be visible tens of trillions of miles away. Some astronomers
think that our sun may be exceptional in having planets, but their
reasons are speculative and unconvincing. Probably a large proportion at
least of the stars have planets, and we may therefore survey the globes
of our own solar system and in a general way extend the results to the
rest of the universe.

In considering the possibility of life as we know it we may at once rule
out the most distant planets from the sun, Uranus and Neptune. They are
probably intrinsically too hot. We may also pass over the nearest planet
to the sun, Mercury. We have reason to believe that it turns on its axis
in the same period as it revolves round the sun, and it must therefore
always present the same side to the sun. This means that the heat on the
sunlit side of Mercury is above boiling-point, while the cold on the
other side must be between two and three hundred degrees below
freezing-point.

The Planet Venus

The planet Venus, the bright globe which is known to all as the morning
and evening "star," seems at first sight more promising as regards the
possibility of life. It is of nearly the same size as the earth, and it
has a good atmosphere, but there are many astronomers who believe that,
like Mercury, it always presents the same face to the sun, and it would
therefore have the same disadvantage--a broiling heat on the sunny side
and the cold of space on the opposite side. We are not sure. The
surface of Venus is so bright--the light of the sun is reflected to us
by such dense masses of cloud and dust--that it is difficult to trace
any permanent markings on it, and thus ascertain how long it takes to
rotate on its axis. Many astronomers believe that they have succeeded,
and that the planet always turns the same face to the sun. If it does,
we can hardly conceive of life on its surface, in spite of the
cloud-screen.

[Illustration: FIG. 14.--THE MOON

Showing a great plain and some typical craters. There are thousands of
these craters, and some theories of their origin are explained on page
34.]

[Illustration: FIG. 15.--MARS

1} Drawings by Prof. Lowell to accompany actual photographs of Mars
showing many of the
2} canals. Taken in 1907 by Mr. E. C. Slipher of the Lowell Observatory.
3 Drawing by Prof. Lowell made January 6, 1914.
4 Drawing by Prof. Lowell made January 21, 1914.

Nos. 1 and 2 show the effect of the planet's rotation. Nos. 3 and 4
depict quite different sections. Note the change in the polar snow-caps
in the last two.]

[Illustration: FIG. 16.--THE MOON, AT NINE AND THREE-QUARTER DAYS

Note the mysterious "rays" diverging from the almost perfectly circular
craters indicated by the arrows (Tycho, upper; Copernicus, lower), and
also the mountains to the right with the lunar dawn breaking on them.]

We turn to Mars; and we must first make it clear why there is so much
speculation about life on Mars, and why it is supposed that, if there
_is_ life on Mars, it must be more advanced than life on the earth.

Is there Life on Mars?

The basis of this belief is that if, as we saw, all the globes in our
solar system are masses of metal that are cooling down, the smaller will
have cooled down before the larger, and will be further ahead in their
development. Now Mars is very much smaller than the earth, and must have
cooled at its surface millions of years before the earth did. Hence, if
a story of life began on Mars at all, it began long before the story of
life on the earth. We cannot guess what sort of life-forms would be
evolved in a different world, but we can confidently say that they would
tend toward increasing intelligence; and thus we are disposed to look
for highly intelligent beings on Mars.

But this argument supposes that the conditions of life, namely air and
water, are found on Mars, and it is disputed whether they are found
there in sufficient quantity. The late Professor Percival Lowell, who
made a lifelong study of Mars, maintained that there are hundreds of
straight lines drawn across the surface of the planet, and he claimed
that they are beds of vegetation marking the sites of great channels or
pipes by means of which the "Martians" draw water from their polar
ocean. Professor W. H. Pickering, another high authority, thinks that
the lines are long, narrow marshes fed by moist winds from the poles.
There are certainly white polar caps on Mars. They seem to melt in the
spring, and the dark fringe round them grows broader.

Other astronomers, however, say that they find no trace of water-vapour
in the atmosphere of Mars, and they think that the polar caps may be
simply thin sheets of hoar-frost or frozen gas. They point out that, as
the atmosphere of Mars is certainly scanty, and the distance from the
sun is so great, it may be too cold for the fluid water to exist on the
planet.

If one asks why our wonderful instruments cannot settle these points,
one must be reminded that Mars is never nearer than 34,000,000 miles
from the earth, and only approaches to this distance once in fifteen or
seventeen years. The image of Mars on the photographic negative taken in
a big telescope is very small. Astronomers rely to a great extent on the
eye, which is more sensitive than the photographic plate. But it is easy
to have differences of opinion as to what the eye sees, and so there is
a good deal of controversy.

In August, 1924, the planet will again be well placed for observation,
and we may learn more about it. Already a few of the much-disputed
lines, which people wrongly call "canals," have been traced on
photographs. Astronomers who are sceptical about life on Mars are often
not fully aware of the extraordinary adaptability of life. There was a
time when the climate of the whole earth, from pole to pole, was
semi-tropical for millions of years. No animal could then endure the
least cold, yet now we have plenty of Arctic plants and animals. If the
cold came slowly on Mars, as we have reason to suppose, the population
could be gradually adapted to it. On the whole, it is possible that
there is advanced life on Mars, and it is not impossible, in spite of
the very great difficulties of a code of communication, that our "elder
brothers" may yet flash across space the solution of many of our
problems.

Sec. 2

Jupiter and Saturn

Next to Mars, going outward from the sun, is Jupiter. Between Mars and
Jupiter, however, there are more than three hundred million miles of
space, and the older astronomers wondered why this was not occupied by a
planet. We now know that it contains about nine hundred "planetoids," or
small globes of from five to five hundred miles in diameter. It was at
one time thought that a planet might have burst into these fragments (a
theory which is not mathematically satisfactory), or it may be that the
material which is scattered in them was prevented by the nearness of the
great bulk of Jupiter from uniting into one globe.

For Jupiter is a giant planet, and its gravitational influence must
extend far over space. It is 1,300 times as large as the earth, and has
nine moons, four of which are large, in attendance on it. It is
interesting to note that the outermost moons of Jupiter and Saturn
revolve round these planets in a direction contrary to the usual
direction taken by moons round planets, and by planets round the sun.
But there is no life on Jupiter.

The surface which we see in photographs (Fig. 12) is a mass of cloud or
steam which always envelops the body of the planet. It is apparently
red-hot. A red tinge is seen sometimes at the edges of its cloud-belts,
and a large red region (the "red spot"), 23,000 miles in length, has
been visible on it for half a century. There may be a liquid or solid
core to the planet, but as a whole it is a mass of seething vapours
whirling round on its axis once in every ten hours. As in the case of
the sun, however, different latitudes appear to rotate at different
rates. The interior of Jupiter is very hot, but the planet is not
self-luminous. The planets Venus and Jupiter shine very brightly, but
they have no light of their own; they reflect the sunlight.

Saturn is in the same interesting condition. The surface in the
photograph (Fig. 13) is steam, and Saturn is so far away from the sun
that the vaporisation of its oceans must necessarily be due to its own
internal heat. It is too hot for water to settle on its surface. Like
Jupiter, the great globe turns on its axis once in ten hours--a
prodigious speed--and must be a swirling, seething mass of metallic
vapours and gases. It is instructive to compare Jupiter and Saturn in
this respect with the sun. They are smaller globes and have cooled down
more than the central fire.

Saturn is a beautiful object in the telescope because it has ten moons
(to include one which is disputed) and a wonderful system of "rings"
round it. The so-called rings are a mighty swarm of meteorites--pieces
of iron and stone of all sorts and sizes, which reflect the light of the
sun to us. This ocean of matter is some miles deep, and stretches from a
few thousand miles from the surface of the planet to 172,000 miles out
in space. Some astronomers think that this is volcanic material which
has been shot out of the planet. Others regard it as stuff which would
have combined to form an eleventh moon but was prevented by the nearness
of Saturn itself. There is no evidence of life on Saturn.

THE MOON

Mars and Venus are therefore the only planets, besides the earth, on
which we may look for life; and in the case of Venus, the possibility is
very faint. But what about the moons which attend the planets? They
range in size from the little ten-miles-wide moons of Mars, to Titan, a
moon of Saturn, and Ganymede, a satellite of Jupiter, which are about
3,000 miles in diameter. May there not be life on some of the larger of
these moons? We will take our own moon as a type of the class.

A Dead World

The moon is so very much nearer to us than any other heavenly body that
we have a remarkable knowledge of it. In Fig. 14 you have a photograph,
taken in one of our largest telescopes, of part of its surface. In a
sense such a telescope brings the moon to within about fifty miles of
us. We should see a city like London as a dark, sprawling blotch on the
globe. We could just detect a Zeppelin or a Diplodocus as a moving speck
against the surface. But we find none of these things. It is true that a
few astronomers believe that they see signs of some sort of feeble life
or movement on the moon. Professor Pickering thinks that he can trace
some volcanic activity. He believes that there are areas of vegetation,
probably of a low order, and that the soil of the moon may retain a
certain amount of water in it. He speaks of a very thin atmosphere, and
of occasional light falls of snow. He has succeeded in persuading some
careful observers that there probably are slight changes of some kind
taking place on the moon.

[Illustration: FIG. 17.--A MAP OF THE CHIEF PLAINS AND CRATERS OF THE
MOON

The plains were originally supposed to be seas: hence the name "Mare."]

[Illustration: FIG. 18.--A DIAGRAM OF A STREAM OF METEORS SHOWING THE
EARTH PASSING THROUGH THEM] [Illustration: _Photo: Royal Observatory,
Greenwich._

FIG. 19.--COMET, September 29, 1908

Notice the tendency to form a number of tails. (See photograph below.)]

[Illustration: _Photo: Royal Observatory, Greenwich._

FIG. 20.--COMET, October 3, 1908

The process has gone further and a number of distinct tails can now be
counted.]

But there are many things that point to absence of air on the moon. Even
the photographs we reproduce tell the same story. The edges of the
shadows are all hard and black. If there had been an appreciable
atmosphere it would have scattered the sun's light on to the edges and
produced a gradual shading off such as we see on the earth. This
relative absence of air must give rise to some surprising effects. There
will be no sounds on the moon, because sounds are merely air waves. Even
a meteor shattering itself to a violent end against the surface of the
moon would make no noise. Nor would it herald its coming by glowing into
a "shooting star," as it would on entering the earth's atmosphere. There
will be no floating dust, no scent, no twilight, no blue sky, no
twinkling of the stars. The sky will be always black and the stars will
be clearly visible by day as by night. The sun's wonderful corona, which
no man on earth, even by seizing every opportunity during eclipses, can
hope to see for more than two hours in all in a long lifetime, will be
visible all day. So will the great red flames of the sun. Of course,
there will be no life, and no landscape effects and scenery effects due
to vegetation.

The moon takes approximately twenty-seven of our days to turn once on
its axis. So for fourteen days there is continuous night, when the
temperature must sink away down towards the absolute cold of space. This
will be followed without an instant of twilight by full daylight. For
another fourteen days the sun's rays will bear straight down, with no
diffusion or absorption of their heat, or light, on the way. It does not
follow, however, that the temperature of the moon's surface must rise
enormously. It may not even rise to the temperature of melting ice.
Seeing there is no air there can be no check on radiation. The heat that
the moon gets will radiate away immediately. We know that amongst the
coldest places on the earth are the tops of very high mountains, the
points that have reared themselves nearest to the sun but farthest out
of the sheltering blanket of the earth's atmosphere. The actual
temperature of the moon's surface by day is a moot point. It may be
below the freezing-point or above the boiling-point of water.

The Mountains of the Moon

The lack of air is considered by many astronomers to furnish the
explanation of the enormous number of "craters" which pit the moon's
surface. There are about a hundred thousand of these strange rings, and
it is now believed by many that they are spots where very large
meteorites, or even planetoids, splashed into the moon when its surface
was still soft. Other astronomers think that they are the remains of
gigantic bubbles which were raised in the moon's "skin," when the globe
was still molten, by volcanic gases from below. A few astronomers think
that they are, as is popularly supposed, the craters of extinct
volcanoes. Our craters, on the earth, are generally deep cups, whereas
these ring-formations on the moon are more like very shallow and broad
saucers. Clavius, the largest of them, is 123 miles across the interior,
yet its encircling rampart is not a mile high.

The mountains on the moon (Fig. 16) rise to a great height, and are
extraordinarily gaunt and rugged. They are like fountains of lava,
rising in places to 26,000 and 27,000 feet. The lunar Apennines have
three thousand steep and weird peaks. Our terrestrial mountains are
continually worn down by frost acting on moisture and by ice and water,
but there are none of these agencies operating on the moon. Its
mountains are comparatively "everlasting hills."

The moon is interesting to us precisely because it is a dead world. It
seems to show how the earth, or any cooling metal globe, will evolve in
the remote future. We do not know if there was ever life on the moon,
but in any case it cannot have proceeded far in development. At the most
we can imagine some strange lowly forms of vegetation lingering here and
there in pools of heavy gas, expanding during the blaze of the sun's
long day, and frozen rigid during the long night.

METEORS AND COMETS

We may conclude our survey of the solar system with a word about
"shooting stars," or meteors, and comets. There are few now who do not
know that the streak of fire which suddenly lights the sky overhead at
night means that a piece of stone or iron has entered our atmosphere
from outer space, and has been burned up by friction. It was travelling
at, perhaps, twenty or thirty miles a second. At seventy or eighty miles
above our heads it began to glow, as at that height the air is thick
enough to offer serious friction and raise it to a white heat. By the
time the meteor reached about twenty miles or so from the earth's
surface it was entirely dissipated, as a rule in fiery vapour.

Millions of Meteorites

It is estimated that between ten and a hundred million meteorites enter
our atmosphere and are cremated, every day. Most of them weigh only an
ounce or two, and are invisible. Some of them weigh a ton or more, but
even against these large masses the air acts as a kind of "torpedo-net."
They generally burst into fragments and fall without doing damage.

It is clear that "empty space" is, at least within the limits of our
solar system, full of these things. They swarm like fishes in the seas.
Like the fishes, moreover, they may be either solitary or gregarious.
The solitary bit of cosmic rubbish is the meteorite, which we have just
examined. A "social" group of meteorites is the essential part of a
comet. The nucleus, or bright central part, of the head of a comet (Fig.
19) consists of a swarm, sometimes thousands of miles wide, of these
pieces of iron or stone. This swarm has come under the sun's
gravitational influence, and is forced to travel round it. From some
dark region of space it has moved slowly into our system. It is not then
a comet, for it has no tail. But as the crowded meteors approach the
sun, the speed increases. They give off fine vapour-like matter and the
fierce flood of light from the sun sweeps this vapour out in an
ever-lengthening tail. Whatever way the comet is travelling, the tail
always points away from the sun.

A Great Comet

The vapoury tail often grows to an enormous length as the comet
approaches the sun. The great comet of 1843 had a tail two hundred
million miles long. It is, however, composed of the thinnest vapours
imaginable. Twice during the nineteenth century the earth passed through
the tail of a comet, and nothing was felt. The vapours of the tail are,
in fact, so attenuated that we can hardly imagine them to be white-hot.
They may be lit by some electrical force. However that may be, the comet
dashes round the sun, often at three or four hundred miles a second,
then may pass gradually out of our system once more. It may be a
thousand years, or it may be fifty years, before the monarch of the
system will summon it again to make its fiery journey round his throne.

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