```Class events: week 15
This week’s goals:
- What is the Great Silence?
- Possible resolutions to the Great Silence
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An important take-home message
If nothing else…
…I hope you leave today understanding why you should never again utter
the phrase, “There may be aliens in the galaxy, but I think they just haven’t
gotten here yet.”
This terribly flawed perspective reveals a basic lack of understanding of
our galaxy’s size, age, and history.
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Timescales of space exploration
How long will it take for a spacefaring civilization to explore the galaxy?
Consider a colonization model where the exploring species travels to a
new planet, resides there long enough to resupply, and then continues on
its way.
This move-rest-move approach results in an overall average speed of
exploration that would be the same as if the explorers travelled the same
distance, at a slower velocity.
Move-rest-move:
Continuous motion at a slower speed:
Obviously, an expanding civilization would not explore the galaxy with
only a single spaceship—it would create more as it went. This is
called the coral model of exploration.
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Timescales of space exploration
Consider two colonization models.
In the slow model, spaceships move slowly (0.01c), perhaps as generational
arks, which then reside for a long time (5000 years) before moving on.
In the fast model, the spaceships are very speedy (0.1 c), and only reside for
150 years before moving on.
Slow
Travel speed:
0.01 c
Travel time:
500 y
Rest on a planet: 5000 y
Fast
0.1 c
50 y
150 y
Resulting velocities and timescales
Vexplore:
0.0009 c
 Texplore:
9.2×107 y
0.025 c
3.3×106 y
(Stars are about 5 LY apart.)
While these times may seem very long, compared to the age of the galaxy
(1010 years) they are exceedingly short:
Texplore / Agegalaxy: 1/110
1/3000
4
Galactic differential rotation
Our galaxy rotates differentially, so stars slide past each other with
time. In effect, even the inhabitants of a stationary solar system
explore different star systems, as its neighbors change over time.
Because of galactic differential rotation, it will take a sub-light civilization only about
0.5-50 million years to completely explore the galaxy, instead of 3.3-92 million years.
The new time ratio to explore the galaxy is as small as 1/20,000.
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Timescales of space exploration
Von Neumann Probes
Self replicating machines that would have simple programming:
Find new planets, find raw materials, replicate new machines.
Once the probes are ready, they move on. And they do this fast…
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Exploration by von Neumann Probes
Exploration by von Neumann probes is hugely effective:
– No need for bulky life-support → smaller crafts;
– Faster rebuilds, less time between generations;
– Can make far more crafts with limited resources;
– Easier to accelerate to high speed.
– No need for delicate accelerations, when 30g would do!
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Exploration by von Neumann Probes
Recall our biological exploration wave through the galaxy:
Robot probes, by any calculation, would explore the galaxy about ten times faster,
exceeding even the rate of a fast biological exploration, 0.5 million years—or about
1/20,000th of the galaxy’s age.
Robot exploration, even if it started much later than biological exploration, would
expand in a wave that would quickly overtake the biological exploration wave.
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“They are out there, but they haven’t gotten here yet”
Recall that star formation has been occurring in our galaxy for about 10 billion
years, making stars (and presumably planets) just like ours for all that time.
If there is just one other civilization in our galaxy, it might have formed at any time
during those 10 billion years. Indeed, it would be preposterous to expect that its age
is within 0.5 million years, 5 million years, or even 50 million years of our own.
0.5 million years (fast robot probes, fastest biological exploration)
5 million years (slow robot probes)
50 million years (slow biological exploration)
10 billion years
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“They are out there, but they haven’t gotten here yet”
0.5 million years (fast robot probes, fastest biological exploration)
5 million years (slow robot probes)
50 million years (slow biological exploration)
10 billion years
Instead, it would be vastly more likely that if even one other civilization EVER
occurred in our galaxy, it would have had billions of years to explore the entire
galaxy, before we even gelled from the primordial soup.
Saying they haven’t gotten here yet, in the case of a 0.5 million year exploration
time, would have the same likelihood as spending one day per year in a rental
cabin, just to find that someone else—the only other renter all year—arrives within
If there is more than one other civilization in the galaxy, the situation is even less
likely that “they haven’t gotten here yet.”
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Two observations in conflict
1) If aliens exist in our galaxy then they should have reached the Earth long, long ago.
2) Daily public experience tells us that we are not surrounded by a constant barrage of
aliens.
WHERE ARE THEY??
WHY CAN’T WE HEAR THEM??
This is the Great Silence.
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Systems in equilibrium
In general, science proceeds on the assumption of near-equilibrium. This
assumption is valid because changes to systems usually transpire slowly…
- stars evolve, but slowly;
- mountains grow or erode, but slowly;
- trees grow larger over time, but very slowly.
Of course, many systems can move rapidly from one state to a dramatically
different state. But such systems spend very little time in the actual transition
process.
In other words, dramatic things CAN happen, but the time between dramatic events
is large…
- stars explode, but very rarely;
- earthquakes happen, but between long spaces of time;
- trees fall over, but rarely.
The lesson from this is that it is almost always very safe to assume that a system is
in a slowly-changing stage, and that it is not in a stage of rapid transition.
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Equilibrium and the Copernican Principle
Recall the Copernican Principle…
We cannot assume we are at a special place in our Universe.
We are not at…
the center of the Earth’s surface (for flat-Earthers);
the center of the solar system;
the center of our galaxy;
the center of the Universe.
Nor can we assume that we are in a special moment, such as a transitional time.
Such transitional times are rare and short-lived.
Reflect: this is really a restatement of the Copernican Principle.
Avoiding the assumption of a special time or place is called the Copernican
Principle. (In cosmology: The Cosmological Principle.)
It is foolish to assume we are at a special time in our galaxy’s history.
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Equilibrium and galactic evolution
Assuming we survive into the future, we (or our von
Neumann probes) will explore the galaxy during the next
0.5-50 million years.
If we are alone in the galaxy, we are about to transform it
from an unexplored one, to a fully explored one.
This would be a huge, dramatic transition for our galaxy,
implying that the galaxy is in a vast state of disequilibrium.
This is a huge violation of the Copernican Principle.
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In case you lost focus for a moment…
The Milky Way Galaxy is ancient (10 billion years), but in contrast, the time needed to
explore it is but a few million years. Other life forms in the galaxy should have
explored it by now.
But we don’t see them, nor have we detected them.
(Statement A)
(Thinking that they evolved simultaneously as us would require an incredibly
improbable coincidence.)
If other life forms simply don’t exist, we are in the unique position of being the first
galactic explorers.
In that case, we are about to change the galaxy forever, but the Copernican Principle
says that almost certainly isn’t likely to happen.
Therefore, something is just about to descend upon us,
and stop us in our tracks!
(Statement B)
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We now seek possible resolutions to this quandary. How do we circumvent
either…
– Statement A—we don’t see or detect aliens that should be swarming
throughout the galaxy!
– Statement B—something is going to stop us from exploring the galaxy.
There are many possible explanations. Let us look at some…
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1:4 Other space-faring civilizations do not exist
Rare Earth Hypothesis
Other civilizations never formed in the first place!
Perhaps developing civilizations need:
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Water – to allow life to form;
Dry land – to force technology;
Plate tectonics – to regulate CO2 and avoid a catastrophic greenhouse effect;
A magnetic field – to prevent atmospheric stripping;
A large satellite – to stabilize the planet’s axis;
An absence of hot Jupiters – inward spiraling giants are devastating;
A cold Jupiter – to remove comets from the inner solar system.
BUT…
Is this argument simply rooted in carbon chauvinism?
This hypothesis violates the Cosmological Principle by saying we are unique, and
does not address the fact that our galaxy is about to change character because of
our own exploration.
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1:4 Other space-faring civilizations do not exist
Self Destruction Hypothesis
All civilizations destroy themselves before reaching space:
– Nuclear disaster?
– Ecological disaster?
– Technological singularity?
BUT…
Could these mechanisms really have a 100% kill rate?
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2:4 Critical barriers to communication exist
Unattractive Earth Hypothesis
Aliens are widespread, but either the Earth is difficult to reach, or it might not fit
the notion of a habitability for alien species.
– Are nearby black holes necessary for space travel by spectacular
technologies?
– The Sun is leaving a spiral arm—is this inconvenient for space travel?
– Are all other alien civilizations associated with K-M dwarf stars?
BUT….
Von Neumann probes could reach “inconvenient” planets with little delay.
It is non-Copernican to expect our civilization is unique in being the only one
orbiting a G star.
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2:4 Critical barriers to communication exist
Overwhelming Technology Hypothesis
Communication with aliens would be easy, if we knew how to look.
– Exotic communication (neutrino modulation), etc.
– Are we misinterpreting obvious beacons such as pulsars, etc?
– Communication is out there, but it is mostly carefully masked to
be energy-efficient.
While islanders in a Southeast Asian stone-age culture might
communicate with each other via drum beats, they would be unaware
BUT…
Wouldn’t aliens be able to dumb down their communications to talk to
us?
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2:4 Critical barriers to communication exist
Government Conspiracy Hypothesis
Aliens are, or have been, in communication with us, but our government
is not letting the truth out. The hypothesis includes…
– In 1947, aliens crashed in Roswell, NM. (“Operation Mogul” is a
cover-up.) President Truman started the “Majestic 12” secret
committee of scientists. The FBIT denies this.
– Area 51 is a high-security Air Force installation in Nevada (130 km
NW of Las Vegas). Its contents are secret, but may contain alien
aircraft and bodies.
– In the 1970s, cattle were found in 15 states, surgically and
mysteriously mutilated. Even an investigation by the FBI came up with
questions about the cause. Aliens might need to experiment on cattle.
– The cover-up is international in scope.
BUT…
Conspiracy theories are, in general, largely impractical because of the huge
financial and celebrity gains associated with revealing them.
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3:4 Aliens choose to observe us quietly
Sneaky UFO Hypothesis
UFO-proponents argue that aliens have
gone through all their efforts and energy
expenditures to visit us secretly—and
then accidentally reveal themselves!?
Rice #1: If aliens wanted to sneak around,
why don’t they just turn off their lights?
A: those are their engines, not just their
in-flight lights!
Rice #2: If those are their engines lights, then why not use a craft that
is a little more sneaky? After all, they knew they were coming here to
sneak around!
A: They are so powerful, they don’t need to sneak around, after
all—we can’t threaten them.
Rice #3: Then why sneak, and in particular, sneak so poorly?
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3:4 Aliens choose to observe us quietly
Even though UFO reports are generally lame, we cannot discount
the possibility that we are being observed.
Immaturity Hypothesis
We must reach maturity before the aliens make themselves known.
– Prime Directive—Roddenberry, 1966-1969;
– Codex Galactica—Newman & Sagan, 1981.
Zoo Hypothesis
We are oddities worth watching for obscure reasons.
Quarantine Hypothesis
Something is wrong with us?
Is it our aggressive nature?
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3:4 Aliens choose to observe us quietly
Have astronomers spotted mystery spacecraft?
We have filled our sky with countless bits of space debris. Nearly all the pieces of
debris are in orbit around the Earth.
The military is interested in bombing
humans, not in creating space probes
to explore the solar system. We can
expect that secret military debris
would only be orbiting the Earth, and
not the Earth-Moon system.
There is no supportable reason to
envision that the military would
create weaponry that would escape
the Earth-Moon gravity, to enter
orbit around the Sun.
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3:4 Aliens choose to observe us quietly
There are a few threads of evidence consistent with our being watched.
J002E3 has a strange, temporary geocentric orbit. It is a discarded
Saturn V stage from Apollo 12.
6Q0B44E orbits the Earth-Moon system every 80 days. From its brightness, it is a
few meters across. Presumably artificial junk from Earth research.
1991VG has a heliocentric, near-Earth orbit, passing us every 16 years. It is ~9-19 m
across. In 1991, it approached within 450000 km of the Earth.
Its unstable orbit argues it is a recently captured (lunar?) meteoroid. Glinting
brightness variations suggest it is metallic, but its orbit does not match with any space
missions. Steel (1995) notes that:
Pnatural + Pspace junk + Palien =100%
If both Pnatural and Pspace junk are very small, then Palien is close to 100%!
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3:4 Aliens choose to observe us quietly
The problem with all these hypotheses (of aliens watching us quietly) is that
galactic shear would move us to new stellar systems in a few million years;
enforcement of a “DO-NOT-CONTACT-EARTHLINGS” rule would require
cooperation at a galactic scale.
In other words, if you think we are being quietly observed by aliens…
…the fact that we have not seen them suggests there must be a galactic federation
of allied planetary civilizations!
It starts to sound like, “the fact that we
have never seen leprechauns, proves that
they exist and are magical
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Suppose there were bad von Neumann probes in the
galaxy…
– Created by a xenophobic race?
– Created to terraform planets, but disregarding the natives?
– Arising from imperfect probe replication, resulting in the evolution
of a hostile strain?
– Arising from an unanticipated technological singularity?
The new programming…
To explore strange new worlds;
To seek out new life and new civilizations;
To boldly go where no probe has gone before…
…then replicate massive attack forces, and sterilize it all.
In response, civilizations would tend to be quiet (hide) and cautious
about making contact with other civilizations.
Contact beacons would be hidden or nonexistent.
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The Deadly Probes scenario explains all the observations we have made so far.
– The galaxy is in equilibrium—it has been fully explored by previous aliens, and
– We are certainly not the first race to explore the galaxy; in compliance with the
Copernican Principle, it has already been explored by aliens—including the
– The galaxy is filled with aliens—but they are not communicating with us because
they are hiding from the Deadly Probes. Hence the Fermi Paradox and Great
Silence.
– Problems with other hypotheses (such as unlikely 100% efficiencies for social
changes) do not plague this scenario—it only had to happen once.
The Deadly Probes scenario makes a prediction!
have contacted about 2500 star systems within 50 LY! These star systems
need not be inhabited…they only need to have Deadly Probes, which
are about to descend upon us, and stop us in our tracks!
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A semester in review
Astrobiology
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The building blocks of life occur throughout the galaxy.
Life evolved with great speed on Earth.
Primitive life may occur elsewhere in our solar system.
Primitive life seems likely at many sites in our galaxy.
It is quite possible that we are not alone.
It is quite possible that we are the only life in the galaxy.
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Might we become members of a Galactic Union?
Are we destined to be explorers of the ruins of prior civilizations?
Are we fated, by probability, to destroy ourselves?
Against all odds, are we the first technological species to develop?
Are, even now, alien von Neumann probes such as 6Q0B44E and 1991VG
preparing to overwhelm us with superior numbers and technology?
Any of those possibilities is profound.
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