The Test
Testing of atomic explosives is usually carried out in beautiful
surroundings. There is a good reason for this: the radioactive fallout.
Because of the fallout, the test site must be isolated. The presence of
human population does not improve nature (with exceptions which are
quite rare and the more notable). Also, to keep the site clean, tests
must be carried out in the absence of rain. Therefore, at the site one
usually finds sunshine and solitude.
For the participants the beauty of nature forms the back-drop to
preparations of experiments which are difficult and exciting to everyone
involved. At the end, the atomic explosion is always dwarfed by its
setting. But the work that culminates in the detonation is rewarded by
something quite different from a flash and a bang.
The really important results of a test consist in marks on photographic
plates. Most of the apparatus that produced the plates has been
destroyed in the explosion. But enough is saved so that one can conclude
what has happened in the short fractions of a second that pass between
the pressing of the button and the knowledge in the observer: this was
it. In those fractions of a second another stone was added to the
structure which we may call astrophysical engineering. What happens and
what is observed in nuclear explosions are closely related to the
behavior of matter in the interiors of the stars.
The details of the nuclear explosion cannot be described here for three
reasons. First, the details are secret. Second, the size of this book
and the forbearance of the reader set limitations. And third, we
understand only a small part of the process. Within these limitations,
this is what happens:
The actual nuclear reaction takes only a fraction of a microsecond (one
microsecond = one millionth of a second). All the energy of the bomb is
released in this short period. At the end of this period, the main body
of the nuclear material is moving apart at a rapid rate and by this
motion further nuclear reactions are stopped. In addition to the more or
less orderly outward motion, considerable portions of the energy are
found in the disorderly temperature motion, which has stripped most of
the electrons off the nuclei and has transformed the atoms into a freely
and chaotically moving assembly of charged particles. By this time many
of the original nuclei have been transformed into nuclei of radioactive
species, partly by the fission process and partly by the capture of
neutrons in all sorts of atoms which had been originally present in the
bomb materials.
Still another portion of the energy is present as electromagnetic
radiation. This radiation closely resembles light except that it is of
shorter wave length and is therefore not actually visible; but it can be
absorbed and re-emitted by all sorts of materials, and is in a violent
exchange of energy with the exploded bomb fragments.
All this perturbation spreads outward from the region where the nuclear
reaction has taken place into the surrounding components of the bomb.
During the outward spread, more atoms and more space get engulfed. The
agitation and the radiation become somewhat less hot.
This hot region tends to be limited by a sharply defined boundary which
is called a shock front and which is moving outward at a speed of
several hundred miles per second. This front finally reaches the limits
of the more or less dense material in which the whole bomb structure was
originally encased. It then breaks through into the surrounding air. The
air heats up in the immediate vicinity, and this is the beginning of the
fireball.
From this point on, the energy spreads due to the push of the
high-temperature air. A sharp shock front forms and keeps moving outward
at a speed greatly surpassing ordinary sound speed. The radioactive
material is contained within this hot and expanding sphere.
As the fireball expands and the temperature falls, more and more visible
radiation is emitted. Actually, the surface is growing less brilliant as
the structure expands and cools, but its greater size and the longer
time that is available for the emission of radiation overcome this
disadvantage. Finally, at a radius of perhaps a few hundred feet for a
small bomb and a mile for a big one, the fireball expansion halts. This
happens because the shock front is no longer strong enough to make the
air luminous. The luminosity not only stops advancing but is actually
partly dimmed by absorbing substances formed by the badly mistreated air
molecules.
The time which has elapsed to reach this stage of the explosion depends
on the bomb energy. If two explosions are compared, and the bigger one
has a thousand times the explosive power of the smaller one, then the
time needed to reach the extreme expansion of the fireball will be
approximately ten times greater for the more violent event. In any case,
a reasonably close observer has to use strongly absorbing glasses during
this time if he is not to be blinded. For small bombs, the expansion of
the fireball is too short to register. For the really big ones, you can
see the expansion developing and you wonder when it will stop. To the
unprotected eye the small bombs are almost as dangerous as the big ones,
because there is not enough time to blink.
In the meantime, the shock wave, now separated from the fireball,
travels through the air and carries with it a considerable fraction of
the original explosive power. An important part of the damage which a
bomb can cause is due to this invisible pressure wave which spreads with
a speed close to that of sound, over a distance of miles, before it
settles down into harmless rumbling.
The rest of the energy is still sitting in the fireball near the point
where the explosion occurred and the hot air now commences to ascend,
breaking up into a turbulent mushroom as it goes. The hot interior
portions get occasionally exposed and the object gives the appearance of
an enormous flaming mass, at least when seen in a motion picture which
slows down the action and reduces the size. The radiant tongues are too
big and too fast for any ordinary flames.
During this stage the display gradually pales sufficiently so that it
can be viewed with the naked eye. The originally hot masses have now
emitted enough energy in the form of light and mixed with a sufficiently
great mass of cool air that they no longer glow violently. This mass of
central and rising gas contains practically all the radioactivity, not
only that originally formed in the explosion but also some produced by
neutrons which leaked out of the bomb and got captured by a variety of
nuclei in the air, water, or ground within the neighborhood.
And now the aftermath of the explosion is turning into a display growing
rapidly and yet in a measured manner so that not only the eye of the
observer but his mind and his feelings can follow the events. The
mushroom which has been formed by the first updraft develops into a
column with more and more agitated boiling masses added on the top and
with slanting skirts of a snowy appearance descending toward the sides.
What is this white mass that looks just like a cloud of peculiar shape
and that has grown up to the high heavens (or as the meteorologists call
it: the stratosphere) in a few minutes before our eyes?
It is actually a cloud: a collection of droplets of water too small to
turn into rain but big enough to reflect the white light of the sun. And
it is formed in a similar way to the cumulus clouds of a thunderstorm.
Indeed it is a beautiful example of a many-storied castle of cumulus
upon cumulus. But strangely enough what makes this cloud is not the heat
of the bomb. It is the cooling of the air masses that have been sucked
in as the remnants of the fireball rush upward like a giant balloon.
Under this balloon air is drawn upward. As this air rises, it cools and
water vapor contained in it condenses into droplets: precisely the same
mechanism which gives rise to thunderheads on a hot summer day.
The white skirts (which are not always present) do not consist of any
material that is falling out of the cloud. On the contrary, a moist
layer of air is sucked up into the cloud from the side and the droplets
which form in this layer give rise to a cloud-sheet with the appearance
of a skirt.
In big bombs near the top a particularly smooth and white cap is seen.
This is again condensation, not into droplets but into fine crystals of
ice. In some explosions more than one of these caps are present.
Finally the cloud has gained its full height. Depending on the size of
the bomb it may have grown to 20,000 feet, to 100,000 feet or more. Then
the wind blowing at various levels in various directions tears the
structure apart sweeping some of it to the east, some to the west. The
radioactive debris in the cloud has started on its travel.
What this radioactivity will do, how it can affect living beings, how
dangerous it actually is, we shall discuss in succeeding chapters. But
one thing is clear and remains present in the minds of all participants
in an atomic test: The danger of the test is nothing compared to the
catastrophe that may occur if great numbers of these weapons should be
used in an unrestricted nuclear war.
It has been frequently asserted that our present atomic explosives can
wipe out the cities and industries of the greatest countries. Why
continue with further development and testing?
The answer is simple: The main purpose of a war is not to destroy the
enemy’s civilian centers but rather to defeat his armed forces, and for
this purpose we need flexible refined weapons of all kinds and sizes. We
also need weapons with which to defend our own cities. We need weapons
with which to defend our allies and in particular we need weapons which
will do their job against an aggressor and will do the least possible
damage to the innocent bystander.
In this last respect, in particular, notable progress has been made. We
are developing clean weapons which are effective by their blast and
their heat, but which produce little radioactivity. Of course, blast and
heat will do damage only near the point of detonation. Radioactivity may
be carried by the winds and escape the control of man to a considerable
extent.
It is clear that war is and always has been terrible. We refuse to
believe that wars will always be with us but we cannot disregard the
danger of war as long as the world is half free and half slave.
An atomic war, limited or even unlimited, need not be connected with
more suffering than past wars. However, such a war would probably be
more violent and it would be shorter.
The story is told that a war which turned out to be perhaps the most
dreadful in the history of mankind was started with this message: “Thou
hast chosen war. That will happen which will happen and what is to be we
know not. God alone knows.” Perhaps the only possible path for a free
people is to be well prepared for war but never to choose war as long as
the choice is free. But what will happen God alone knows.