For
decades the main assumption about extraterrestrial intelligences is
that our galaxy contains many civilizations much older than
ours, perhaps millions of years older. The reason for this assumption
is the fact that the universe is thousands of times older than the
human race. Humanity is estimated to be only a few hundred thousand
years old, but the universe is some 13 billion years old. Apparently
intelligent life could have arisen on other planets any time during
the past three or four billion years. A period of about three billion
years is about 10,000 times longer than a period of only a few
hundred thousand years. So it would seem that if intelligence arose
in our galaxy at random times, it would have arisen mainly during the
first 99% of this three-billion-year period, which would mean most
extraterrestrial civilizations would have arisen millions of years
ago. Under such a scenario, our species is a very inferior species,
and there are extraterrestrial minds as superior to our minds as our
minds are superior to mice or insects.
Such
reasoning seems pretty solid, but there is one big problem with it:
the fact that we do not see any evidence of other extraterrestrial
civilizations (with the possible exception of UFO's, a matter of
controversy). If many civilizations arose on other planets millions
of years ago, we might expect that such civilizations would have left
signs of themselves which we would have detected. But no such sign
has been indisputably found. Our searches for extraterrestrial radio
signals have not been successful; we have seen no evidence of
extraterrestrials in deep space; and we see no artifacts from
extraterrestrials anywhere in the solar system.
This
discrepancy is known as Fermi's Paradox, the paradox that asks: where
is everybody? I discussed various possible solutions to Fermi's
Paradox in this earlier blog post. I would now like to suggest
another possible solution. I will call this possible answer the SAGE
hypothesis. SAGE is an acronym standing for Simultaneous Appearance
of Galactic Extraterrestrials.
The
idea behind the SAGE hypothesis is that all intelligent life that has
appeared in the galaxy has appeared only in the past 300,000 years .
Rather than asserting our galaxy has many civilizations millions or
many thousands of years older than man, the SAGE hypothesis asserts
that while our galaxy may have many civilizations, none of them are
much older than mankind.
The
answer that the SAGE hypothesis gives to the “Where is everybody?”
question of Fermi's Paradox is: they exist, but we have not detected
them because they appeared not very long ago; they are about as old
as we are.
If
this hypothesis is correct, it can adequately explain Fermi's
Paradox. We would not expect that we would have received radio
signals from extraterrestrial civilizations that are only about as
old as our civilization, since our civilization has made almost no
attempts yet to send radio signals to other civilizations. We also
would not expect to see any signs of extraterrestrial civilizations
in deep space if they are only about as old as we are, nor would we
expect that they would have reached our planet with spacecraft from
their planets.
The
graph below illustrates the difference between conventional thinking
about the origin date of extraterrestrial civilizations and the
assumption of the SAGE hypothesis. Each dot represents the appearance of an extraterrestrial civilization at a point in time and space. The pattern of red dots
illustrates the pattern we might expect under conventional
assumptions, with extraterrestrial civilizations appearing at random
intervals in the past billion years. The pattern of blue dots
illustrates the pattern that might occur under the SAGE hypothesis,
with all the civilizations appearing relatively recently.
Two
Contexts in Which the SAGE Hypothesis Would Be Credible
Despite
its success in answering Fermi's Paradox, one might argue that the
SAGE hypothesis is not credible, because it seems to require too much
of a coincidence for all intelligent life in the galaxy to have
appeared only during the past small sliver of cosmic history (a
period less than a thousandth of the total length of cosmic history).
But there are two contexts in which the SAGE hypothesis would be
entirely credible.
The
first context in which the SAGE hypothesis would be credible is a
theistic context. Let us imagine for a moment the possibility that
the universe was specifically created billions of years ago by a
cosmic designer, a possibility that cannot be casually dismissed in
the light of all we know about the anthropic principle, apparent
cosmic fine-tuning, and remarkable coincidences required for our
existence. Under such a possibility it is plausible enough that the
universe might be either programmed or controlled so that there is a
widespread simultaneous appearance of intelligent life, all in the
relatively recent past, rather than at random intervals over a span
of billions of years.
We
can imagine why a cosmic designer or controller might want the
earth-like planets of the galaxy to produce intelligent life at
roughly the same time – perhaps as a sign of that being's control
over things, or perhaps to prevent one civilization from being able
to take over the galaxy before other civilizations appeared. By
arranging for civilizations to appear simultaneously throughout the
galaxy, such a cosmic designer or controller might be guaranteeing a
more even-handed distribution of things, so that each civilized
planet gets a fairly equal share of the galactic pie.
The
second context in which the SAGE hypothesis would be credible is a
context in which the universe has some kind of information
capabilities beyond any that we currently understand. Let us imagine
that the universe has some strange capability in which the following
astonishing thing happens: once a highly unlikely event occurs in one
place in the galaxy, it then becomes radically more likely to start
occurring in other places in the galaxy. This might happen if the
galaxy had some type of information field or computational layer, a
field perhaps allowing the universe to in some sense “learn” from
great successes of the past . We can imagine some context under
which the chance of intelligent life appearing on a planet is, say, 1
in a billion – until the time that it first appears, and then the
probability changes to be vastly higher (perhaps only 1 in 10 or 1 in
100). It could be that for some information-related reasons, once
some great but highly improbable event occurs, it is then almost as
if the universe “learns” how to accomplish this thing; and once
that happens it could then be relatively easy for the event to occur
elsewhere.
Under
this idea (which does not require any assumption of a divine creator
or designer or controller, but which does require assuming some
unusual computation-related feature of the universe), we have a
second context in which this SAGE hypothesis could plausibly be true. If
the probability of intelligent life appearing on a particular planet
were somehow to be radically improved once it had occurred one time,
there might be a significant chance of intelligent life appearing
more or less simultaneously on many planets in the galaxy.
The
Predictions of the SAGE Hypothesis, and How It Could Be Falsified
To
many philosophers of science, a scientific idea should ideally be
falsifiable, and it should make specific predictions. In this
regard, the SAGE hypothesis is in good shape, because it can be
falsified, and does make specific predictions.
The
SAGE hypothesis would be falsified if we were to look for and receive
radio signals or television signals from a very old civilization
vastly older than ours. We might then learn that the civilization was
far older than ours. That would instantly falsify the SAGE
hypothesis, which maintains that no civilizations in our galaxy are
very much older than ours. The hypothesis would also be falsified if
our planet was to receive a spaceship from another planet, and those
beings told us their civilization was very much older than ours.
Below
are specific predictions that follow from the SAGE hypothesis:
- We will find no evidence of Dyson Spheres, or any other gigantic galactic engineering projects that would have required many thousands or millions of years to complete.
- If we receive extraterrestrial radio signals or television signals, they will not show us pictures of some vastly superior mega-civilization with godlike technology, but will merely show us a civilization not very much more advanced than our own.
- If we ever receive an extraterrestrial spaceship, it will not be from some civilization vastly older than ours, but will at most be from some civilization that only has started to explore the galaxy fairly recently.
We
can imagine how the SAGE hypothesis could be pretty well verified in
the next century. Looking in one direction of the sky, we might find
radio or television signals from an extraterrestrial civilization only slightly more
advanced than ours. Then looking in some opposite direction of the
sky, to a completely different part of the galaxy, we might then find
the same thing – signals from another civilization about the same
age as ours. Once the same thing happened three or four times, we
would have a choice between believing in a one in a billion
coincidence, or believing in something like the SAGE hypothesis,
which would pretty well clinch the hypothesis.
Do
I personally think that the SAGE hypothesis is correct, and that
civilizations have only recently appeared in our galaxy? No, I think
it is somewhat unlikely that the SAGE hypothesis is correct. I still
tend to prefer the idea that there are some civilizations in our
galaxy much older than our civilization. However, I think that the
SAGE hypothesis is a respectable hypothesis that might well be true,
a hypothesis that deserves a mention in a discussion of Fermi's
Paradox. I would say the SAGE hypothesis is rather unlikely to be
true, but perhaps not very unlikely to be true. I also think that the SAGE
hypothesis has some good aspects, particularly the fact that it is
falsifiable and the fact that it makes specific predictions that we
might well be able to verify in a reasonable time frame.
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