Shells Around Suns May Have Been
June 18, 1960
page 389, Astronomy
Intelligent beings in another solar
system could have hidden their sun by knocking their planets
apart and using the pieces to build a hollow ball around their
Dr. Freeman J. Dyson of the
Institute for Advanced Study,
Princeton, N. J., says that other civilizations may be millions
of years ahead of the earth. They may have rearranged their
solar systems to meet the needs of their exploding populations.
A hollow ball built around the sun
would solve the space and energy problems. It would also cut off
the sun's light. To detect such an advanced civilization,
earthlings would have to detect the invisible heat radiation
from the hollow ball.
A search for such infrared radiation
should be coordinated with,
Project Ozma, a program now
underway for detecting artificial radio waves from nearby stars,
Dr. Dyson reports in Science, Vol. 131, 1960, page 1667
Using our own solar system as an
example, Dr. Dyson calculates that it would take about 3,000
years for population and technology to expand one trillion times
at the rate of one percent a year. Pressures of population and
energy needs could be met only by trapping all of the sun’s
To trap the energy, earthlings could
knock apart the planet Jupiter and rearrange it as a hollow ball
about 10 feet thick with a diameter twice the size of earth's
orbit. This would take all the energy given off by the sun in
800 years. Such a sphere would be “comfortably habitable.”
Dr. Dyson states he is not
suggesting that this is what will happen in the solar system,
but only proposes what may have happened in other stellar
Search for Artificial Stellar
Sources of Infrared Radiation
by Freeman John Dyson
June 3, 1960
If extraterrestrial intelligent
beings exist and have reached a high level of technical
development, one by-product of their energy metabolism is likely
to be the large-scale conversion of starlight into far-infrared
It is proposed that a search for
sources of infrared radiation should accompany the recently
initiated search for interstellar radio communications.
Cocconi and Morrison  have called
attention to the importance and feasibility of listening for
radio signals transmitted by extraterrestrial intelligent
beings. They propose that listening aerials be directed toward
nearby stars which might be accompanied by planets carrying such
beings. Their proposal is now being implemented .
The purpose of this report is to
point out other possibilities which ought to be considered in
planning any serious search for evidence of extraterrestrial
We start from the notion that the
time scale for industrial and technical development of these
beings is likely to be very short in comparison with the time
scale of stellar evolution.
It is therefore overwhelmingly
probable that any such beings observed by us will have been in
existence for millions of years, and will have already reached a
technological level surpassing ours by many orders of magnitude.
It is then a reasonable working hypothesis that their habitat
will have been expanded to the limits set by Malthusian
We have no direct knowledge of the
material conditions which these beings would encounter in their
search for lebensraum. We therefore consider what would be the
likely course of events if these beings had originated in a
solar system identical with ours.
Taking our own solar system as the
model, we shall reach at least a possible picture of what may be
expected to happen elsewhere. I do not argue that this is what
will happen in our system; I only say that this is what
may have happened in other systems.
The material factors which
ultimately limit the expansion of a technically advanced species
are the supply of matter and the supply of energy. At present
the material resources being exploited by the human species are
roughly limited to the biosphere of the earth, a mass of the
order of 5 x 1019 grams.
Our present energy supply may be
generously estimated at 1020 ergs per second.
The quantities of matter and energy
which might conceivably become accessible to us within the solar
system are 2 x 1030 grams (the mass of Jupiter) and 4
x 1033 ergs per second (the total energy output of
The reader may well ask in what
sense can anyone speak of the mass of Jupiter or the total
radiation from the sun as being accessible to exploitation. The
following argument is intended to show that an exploitation of
this magnitude is not absurd.
First of all, the time required for
an expansion of population and industry by a factor of 1012
is quite short, say 3000 years if an average growth rate of 1
percent per year is maintained.
Second, the energy required to
disassemble and rearrange a planet the size of Jupiter is about
1044 ergs, equal to the energy radiated by the sun in
Third, the mass of Jupiter, if
distributed in a spherical shell revolving around the sun at
twice the Earth's distance from it, would have a thickness such
that the mass is 200 grams per square centimeter of surface area
(2 to 3 meters, depending on the density). A shell of this
thickness could be made comfortably habitable, and could contain
all the machinery required for exploiting the solar radiation
falling onto it from the inside.
It is remarkable that the time scale
of industrial expansion, the mass of Jupiter, the energy output
of the sun, and the thickness of a habitable biosphere all have
consistent orders of magnitude.
It seems, then a reasonable
expectation that, barring accidents, Malthusian pressures will
ultimately drive an intelligent species to adopt some such
efficient exploitation of its available resources. One should
expect that, within a few thousand years of its entering the
stage of industrial development, any intelligent species should
be found occupying an artificial biosphere which completely
surrounds its parent star.
If the foregoing argument is
accepted, then the search for extraterrestrial intelligent
beings should not be confined to the neighborhood of visible
stars. The most likely habitat for such beings would be a dark
object, having a size comparable with the Earth's orbit, and a
surface temperature of 200 deg. to 300 deg. K.
Such a dark object would be
radiating as copiously as the star which is hidden inside it,
but the radiation would be in the far infrared, around 10
It happens that the earth's
atmosphere is transparent to radiation within the wavelength in
the range from 8 to 12 microns. It is therefore feasible to
search for "infrared stars" in this range of wavelengths, using
existing telescopes on the earth's surface.
Radiation in this range from Mars
and Venus has not only been detected but has been
spectroscopically analyzed in some detail .
I propose then that a search for
point sources of infrared radiation be attempted, either
independently or in conjunction with the search for artificial
radio emissions. A scan of the entire sky for objects down to
the 5th or 6th magnitude would be desirable, but is probably
beyond the capability of existing techniques of detection.
If an undirected scan is impossible,
it would be worthwhile as a preliminary measure to look for
anomalously intense radiation in the 10-micron range associated
with visible stars. Such radiation might be seen in the
neighborhood of a visible star under either of two conditions.
A race of intelligent beings might
be unable to exploit fully the energy radiated by their star
because an insufficiency of accessible matter, or they might
live in an artificial biosphere surrounding one star of a
multiple system in which one or more component stars are
unsuitable for exploitation and would still be visible to us. It
is impossible to guess the probability that either of these
circumstances would arise for a particular race of
extraterrestrial intelligent beings.
But it is reasonable to begin the
search for infrared radiation of artificial origin by looking in
the direction of nearby visible stars, and especially in the
direction of stars which are known to be binaries with visible
G. Cocconi and P.
Nature, Vol. 184, 1959, pp. 844-846.
131, April 29, 1960, page 1303.
Astrophysics Journal, Vol. 31, 1960, pp. 459, 470.
Search for Artificial
Stellar Sources of Infrared Radiation
Science Vol. 132, July
22, 1960, pp. 250-253.
It is unfortunate that Dyson's
suggestion [Vol. 131, 1960, page 1667] as to how intelligent
beings might survive after reaching "the limits set by
Malthusian principles" does not do justice to the intelligence
of these beings by explaining how they would overcome some of
the obstacles which, at first sight, would seem to militate
against their curious way of life.
Dyson's report describes a uniformly
thick shell of fluid with a thickness of a meter or two and a
radius twice the earth's distance from the sun.
The shell is said to revolve around
the central star, which implies that the material revolves as a
whole. Presumably the material of the shell must be enclosed on
both surfaces by transparent plastic sheaths of similar
constructions, for self-gravitation cannot be expected to make
the material cohere.
However it is not conceivable that
it would be possible to quarry from the material of a planet
like Jupiter sufficient structural steel to keep the shell rigid
against the shear forces and those that would tend to move
material towards the equatorial plane.
Therefore it must be assumed that
radiation pressure must play a part in supporting the shell, so
that its form will be that of an oblate spheroid rather than a
For example, material at the poles
of revolution of the shell would be supported entirely by
radiation pressure, so that the polar radius of the shell would
necessarily be less than the equatorial radius.
However, a cursory calculation will
show that this would be possible only at a distance from the
central star comparable to but less than the radius of the sun.
Beings of lesser intelligence, not
having discovered the appropriate laws of physics, might
therefore seek some other distribution of their dismantled
Jupiter that would have more intrinsic stability - for example,
a torus lying in a plane perpendicular to the axis of its own
The mass of Jupiter distributed in
this way would yield a torus whose cross-sectional area was
comparable with that of the moon, but unfortunately the flux of
stellar radiation would be reduced by a factor of 109.
With conventional laws of physics,
however, as Laplace was the first to show, even this arrangement
would not be stable, and it is to be expected that the material
of the torus would coalesce into one or more planetary objects.
This suggests that the present state
of intelligence, the dispersal of Jupiter into a thin shell
about the sun would not be an effective means of escaping the
consequences of continued population growth but that it might be
an experiment with important bearing on various theories of
origin of the solar system.
It would, for example, be
interesting to see whether the outcome of the experiment was the
recreation of Jupiter or the creation of a number of asteroids.
Another point is that a search for
infrared stars would be valuable even in conventional science
for the light it might throw on the evolution of stars which are
very young or very small as compared with the sun.
Freeman Dyson's report suggesting
that intelligent life elsewhere in the universe may be detected
by looking for sources of infrared radiation was delightful.
However, as an old science-fiction
hand, I feel obliged to sound a cautionary note to the
scientists. Or am I merely to dense to recognize a satire?
The basis of Dyson's argument is
that an industrial culture may eventually occupy an artificial
biosphere completely surrounding its sun, thus maximizing the
territory and energy available for population expansion,
"to the limits set by Malthusian
principles. The mass of Jupiter could be converted into a
"spherical shell revolving around the sun at twice the
Earth's distance from it," utilizing incident solar
radiation which would be reradiated into space in the
Offhand, I should think rotational
and gravitational stresses alone would rule out such a structure
of such dimensions.
But since it is admittedly dangerous
to assert that anything is impossible, I shall confine myself to
the questions of economics. Even Dyson intimates that the
project would take several thousand years to complete; he
calculates the energy required as equal to the sun's total
output for eight centuries, and one does have to eat meanwhile.
And meanwhile, too, the population
growth necessitating this project will presumably continue.
As Hauser remarks in the same issue
Vol. 131, 1960, page 1642], at our present-day rate of increase
we would reach "a population of one person per square foot of
the land surface of the earth in less than 800 years.
Thus, the economic surplus needed
for the biosphere project would be consumed long before the
latter got well started.
If we assume a ratio of population
increase to industrial expansion low enough so that this
contretemps does not occur, we must ask ourselves how any
intelligent species could be induced to patiently to continue
this enormous task, millennium after millennium. True, our human
history contains epochs of grandiose and 'useless'
construction, such as
the pyramid building of Egypt,
but they never lasted very long.
Any revolutionist who promised
relief from the crushing burden of the biosphere project would
be well received! He could doubtless get support for some or
other population-control program; those who demurred would be
martyred by exasperated taxpayers, or the equivalent thereof.
Of course, the entire species
might by advanced psychological techniques, be conditioned
into such an antlike state that its government could never be
overthrown, or break down from internal stresses, or evolve into
But given subjects as meek as this,
and nor reason to breed vast armies (for only a well-established
world government could seriously entertain these ideas in the
first place), the masters could regulate birth and death by
fiat. Thus, the population would have stabilized at some
rational figure and projects such as Dyson's would never be
In short, uncontrolled population
growth will make the construction of artificial biospheres
impossible, and control will make them unnecessary. So
astronomical discovery of infrared sources won't prove anything
about the inhabitants of other planets.
3 Las Palomas Road
The suggestion by Freeman J. Dyson
for investigating solar far-infrared radiation as one way to
detect extraterrestrial intelligence sounds quite practical and
This leads me to suspect that if
Dyson's assumption is correct - that intelligent beings exist of
a far higher order technological achievement than our own - it
would be well - nigh impossible for such beings not to
have detected us.
Eugene A. Sloane
In reply to Maddox, Anderson and
Sloane, I would only like to add the following points, which
were omitted from my earlier communication.
A solid shell or ring
surrounding a star is mechanically impossible. The form
of "biosphere" which I envisaged consists of a loose
collection or swarm of objects traveling on independent
orbits around the star. The size and shape of the
individual objects would be chosen to suit the
inhabitants. I did not indulge in speculations
concerning the constructional details of the biosphere,
since the expected emission of infrared radiation is
independent of such details.
It is a question of taste
whether one believes that a stabilization of population
and industry is more likely to occur close to the
Malthusian limit or far below that limit. My personal
belief is that only a rigid "police state" would likely
to stabilize itself far below the Malthusian limit. I
consider that an open society would be likely to expand
by proliferation of the "city-states" each pursuing an
independent orbit in space. Such an expansion need not
be planned or dictatorially imposed; unless it were
forcibly stopped it would result in the gradual
emergence of an artificial biosphere of the kind I have
suggested. This argument is admittedly anthropomorphic,
and I resent it in full knowledge that the concepts of
"police state" and "open society" are probably
meaningless outside our own species.
The discovery of an intense
point source of infrared radiation would not by itself
imply that extraterrestrial intelligence has been found.
On the contrary, one of the strongest reasons for
conducting a search for such sources is that many new
types of natural astronomical objects might be
Freeman J. Dyson
Institute for Advanced Study
Princeton, New Jersey