Maybe too cold for water-based life, but methane-based life???
What do you mean by "Intelligent"?
Most of the extreme variations on life we have seen thus far are
all quite simple: bacteria and tube worms. But what about complex
and intelligent life.
When most people talk about "intelligent life", this is shorthand for
In other words, life like us...
- Possessing a highly advanced technological civilization.
- Capable of communicating across interstellar distances.
- Capable of interstellar travel by spacecraft.
- Interested in finding and communicating with other intelligences.
Do we qualify as "Intelligent"?
By the previous definition, Homo sapiens just barely qualifies:
- Only had radio communication technology for ~100 years.
- Only had limited (short-duration) manned spaceflight for ~40 years.
- Only sent robotic spacecraft to the edges of our solar system in the
last 10 years.
- May or may not yet have sufficiently sensitive radio reception technology.
Sheer Weight of Numbers
- Why do I think that intelligent life elsewhere is highly likely?
- The sheer numbers of possible planets:
To understand this, let's do a brief cosmic census:
This gives about 2x1022 or 20 billion trillion stars in the
- ~200 Billion galaxies in the visible universe
- ~100 Billion stars per galaxy
Even if we assess the probability of life at one chance in a trillion,
that gives 20 billion intelligent species somewhere in the Universe.
Planetary Requirements for Life
Some astrophysical factors that influence which kinds of stars could have
Long-lived, stable star
- Good stars: F, G, and some K stars: last >3 Gyr, long enough to allow
life time to evolve intelligence.
- Bad Stars: O, B & A stars: These are all short-lived and hot,
and so have high UV radiation output (damaging to organic molecules)
- M stars: small and dim, often have powerful flaring (bad radiation
A Stable orbital environment
- Exclude most binary star systems from consideration
Metals (heavy elements)
- Need metals to make rocky planets
- Need carbon for complex molecules
- Need a metal-rich star in a chemically evolved environment
On the latter point, there is some observational indication that those
stars around which we have detected planets so far are all relatively
metal rich - slightly more so than the local average.
The Drake Equation
One way of assessing the odds of finding intelligent life in our
Galaxy was proposed by radio astronomer Frank Drake in 1961:
The terms in the Drake equation are:
- N = number of advanced civilizations in the Galaxy
- R* = rate at which suitable stars are formed
- fp = fraction of stars with planetary systems
- ne = number of Earth-like planets per system
- fl = fraction of Earth-like planets that have life
- fi = fraction where intelligent life has evolved
- fc = fraction with communication technology
- L = lifetime of an advanced civilization
If we could assign numbers to each of these, we could estimate how many
intelligent civilizations (by our criteria) we would expect in our Galaxy.
Measurement & Conjecture
Only the first three terms of the Drake Equation can be measured by
All of the other terms in the Drake Equation are purely conjectural (and
highly controversial) at the present time, and at least for the
- R* is about 1 suitable (F, G, and K-type) star
per year in the Milky Way galaxy.
- fp is at least 0.1-0.2 from recent searches for
planetary systems, but still rather uncertain.
- ne may be measurable in the next few years (various
ground-based and space-borne projects proposed)
Let's take our situation here on Earth at face value and make the
following very optimistic assumptions:
Putting these all together gives:
- If a solar system forms, an earth like planet forms 1 in 10 times
- If such a planet forms, life inevitably arises (fl=1)
- Given enough time (~4.5 Gyr), intelligent life will inevitably emerge
- All intelligent beings will eventually develop radio technology
and curiosity about other intelligences in the Universe
- We did it in at least 100 years and haven't blown ourselves up, yet,
Or about 2 intelligent civilizations in the Galaxy. Good thing it is
at least one if you count us.
The only way we can increase the count is to increase the number of
earth-like planets expected, increase the number of solar systems, or
increase the lifetime of advanced civilizations (I've already maxed-out
most of the conjectural terms, which would probably excite controversy
among my biologist friends). These increases are plausible, but
only the first two are observables.
The bottom line is, if you take the Drake Equation at face value, the
answer may be "life is rare in our galaxy".
Extraterrestrial Visitations? No.
Has the earth been visited by extraterrestrials, either recently or in
the past? I believe the answer is a firm "No".
My guiding principle in evaluating this question is the maxim
"Extraordinary claims require extraordinary proof."
None of the "proofs" offered to date qualify as "extraordinary" or
- Fuzzy photographs
- Anecdotal accounts of visits and abductions
- Claims of government conspiracies (Roswell, Area 51, etc.)
Are there unexplained sightings? Yes. But admitting they are
"unexplained" does not mean we are justified in leaping to truly wild
explanations. On careful examination, most "unexplained" phenomena
eventually have rather mundane explanations. No claim of
extraterrestrial visitation has ever withstood even modest scrutiny.
The whole UFO business is, in my opinion, nothing more than a singular
failure of imagination. The claimed visitations all have an
unmistakable ring of the familiar about them (especially given the
intensive saturation of our popular culture by Hollywood depictions of
extraterrestrials and their vessels). If a truly "alien" civilization
did visit us, it would probably be totally different than our
expectations (if you had never seen or had described to you an elephant,
could you have imagined one?).
The claim that our ancestors may have been visited (and even instructed)
by extraterrestrial visitors is the reprehensible "Ancient Astronauts"
nonsense that borders on a form of racism. I completely dismiss this
offensive notion on the grounds that it shamelessly belittles our
ancestors and the considerable achievements of their cultures. In the
words of astronomer Frank Shu, it "represents a form of grave robbing".
Where are they?
A plausible explanation for the lack of visits is that interstellar
travel is extremely difficult.
The distances between stars are enormous:
The fastest human spacecraft: Voyager 1 and 2
- Need very large amounts of time, or
- Extremely large amounts of energy
- Outward bound at ~15 km/sec (0.005% c)
- Would take ~80,000 years to reach the nearest stars.
The "fast route" to interstellar travel is to accelerate a starship to
The energy costs are simply enormous:
- Need to reach at least 0.1c to reach the nearest stars in 50 years
- The amount of fuel required increases exponentially with the time
spent accelerating (you have to burn more fuel at the start, and
then to slow down again)
- The best fuel, matter/antimatter annihilation, is only ~50% efficient
at best (if you use anti-protons, there are other annihilation
channels than photons that are poorly-suited for propulsion)
- The production efficiency of such fuel is extremely low (it
takes us a lot of energy to make a small number of anti-protons in
a particle collider).
It would require a maximum committment of time and resources on the part
of any sufficiently advanced civilization to attempt even local
interstellar travel. If the estimates of the relative paucity of such
civilizations (the Drake Equation above) are correct, then the mean
distance between such civilizations in the Galaxy would be far larger
than the typical distances between the stars, making matters worse.
Even so, a sufficiently advanced civilization might decide they can
pull it off. If so, the trick appears to be that you need to be patient
and travel light.
My guess is that if our Solar System is ever visited by an
extraterrestrial starship, it will be a surprisingly small robotic craft
on a one-way trip (saves enormously on life-support, food, and crew
Note: This is where we will pick up the
topic on Friday, March 10
Talk is cheap!
If you really want to bridge interstellar distances, use light
(electromagnetic radiation) to send messages.
- Messages travel at the speed of light.
- Very low energy cost per message.
What wavelengths to use?
- Microwaves at 1000-10,000 MHz frequency. This is a region of
relatively low cosmic background signal (also known as "The Water Hole").
- Lasers at visible or infrared wavelengths. There are very few
naturally occuring lasers in the sky, so they would stand out.
Earth is already on-the-air...
We have been inadvertently beaming radio signals into space for the last
- Regular AM radio broadcasting started in the 1920s
- Television broadcasts started in the 1950s.
It is a somewhat sobering thought that episodes of "I Love Lucy" will
already have reached most of the stars in the solar neighborhood (stars
within ~50 light years).
Curiously, the earth's "radio brightness" has been decreasing:
- Introduction and spread of "closed channel" cable TV transmission.
- Increased use of "directed" communications (optical fibers, low-power
distributed radio, beamed high-frequency satellite up/downlinks) instead
It may well be that sufficiently advanced civilizations emit less
"waste radio" and so become "radio quiet" as they advance.
If a civilization wants to be found, it may have to purposely broadcast
its presence with that end in mind.
This brings us to SETI, the "Search for Extra-Terrestrial Intelligence".
SETI is a relatively inexpensive strategy to search for radio signals
from extraterrestrial civilizations.
Much of the funding for these efforts is private.
What are we looking for?
One persistent problem is to ask what kind of signals will clearly be from
an intelligent source, rather than naturally occuring.
Some properties of radio signals that might mean they are "artificial":
- Narrow "bandwidth" (<300 Hz, the narrowest naturally
occuring maser sources)
- Pulsed signals (a common way to encode information)
- High polarization (another way of encoding information)
- Very little frequency "drift"
So far, there have been no detections, but the searches continue and
will likely expand over the next few years.
What if we detect something?
Interesting question. What do you think?
I am again beholden to Prof. Barbara Ryden, whose own lecture on this
topic for her Astronomy 162 class was a direct source of inspiration and
whose approach I adopted.
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Updated: 2006 March 5
Copyright © Richard W. Pogge, All Rights Reserved.