PART 4: EXTRATERRESTRIAL INTELLIGENCE,
INTERSTELLAR
TRAVEL, FERMI PARADOX
Read book chapters: 12 and 13.
What are the odds? The Drake
"equation"
Number of civilizations =
(number of habitable planets in
galaxy) times
(fraction of habitable planets
that have life) times
(fraction of habitable planets
with life on which intelligent
civilization capable
of interstellar communication
has developed at
some point in the planet's history) times
(fraction of planets that have
intelligent civilizations now)
Example.
- Suppose that every one
in ten stars has a planet that is habitable,
- that one in 100 of those developes life on it,
- that one in 1000 of those has at some point developed
intelligent life,
- and that one in 10000 of those has intelligent life
form present now.
If our Galaxy has
100,000,000,000 stars with habitable zones, then:
- 10,000,000,000 will have habitable planets,
- 100,000,000 will have planet(s) with life on it,
- 100,000 will have at some point have had
intelligent life forms on it,
- and 10 would have a present day civilization
capable of communicating
with us.
Obviously, it is all in the numbers! We don't know the odds of any of
the terms in the equation yet, although we are making good progress
with the first couple of items on the list.
The BIG questions:
- odds for finding
planets with life elsewhere?
- odds for developing intelligent life?
- how long will an intelligent civilization last?
Important aspect: idea of "convergent
evolution". This is related to the question of what are the odds
of developing advanced life if life is present in more primitive forms.
The idea of convergent evolution is that even if life evolves through
different paths for different species, many share characteristics
anyway, even if they followed different evolutionary (in a biological
sense) paths. Under this scenario, the odds of developing advanced life
forms may be large. See the graph in the book on brain mass versus body
mass. Can you think of arguments in favor of this notion and against
this notion?
The reality of the Drake equation:
with the example of
only one planet with advanced life forms known, it cannot give us any
reliable answer as to the number of civilizations that might be out
there.
So, let us move on to a more
active initiative:
How to find evidence of
extra-terrestrial intelligence?
Past claims that we have been "visited" are highly spurious at best,
and outright lies as worst. There is a lot of money to be made selling
books about pseudo science!
We are currently left with
one option:
Search for signals from outer space
that cannot be explained by natural
phenomena (remember Ockham's razor!). This is done by project
SETI, the
Search for Extraterrestrial Intelligence.
Considerations:
We have already been broadcasting our existence to the outside world;
they may be watching our soap operas as we speak. There are about 2500
stars within 50 light years from Earth. And we have been "leaking" t.v.
and radio waves into space for longer than that...
Think about this: if aliens were listening for our radio signals, only
those within about 100 lightyears would even know we exist. This region
spans a tiny, tiny section of the entire MW, so most alien
civilizations, if they exist, can not know about us from listening
in....
Questions:
- should we be concerned about announcing our
existence into space?
- what is the strongest signal we have been sending
into space?
PROJECT SETI
and here is how you can help: SETI
at home
What to search for?
Radio waves makes most
sense: lowest energy electromagnetic waves, cheap to produce, very good
for communication since they are not blocked by atmosphere,
interstellar dust, etc. Radio waves are part of the electromagnetic
spectrum, just as visible light, and travel at the speed of light in
vacuum.
Challenge: the radio
spectrum is vast. What frequency ("color") of waves to search for, and
what type of signal? The signal has to be artificial, e.g. restricted
to very narrow band width (but not a known spectral line), and
likely periodic or distinctly repetitive. Maybe the signal would be
close to a well-known frequency, such as that of the element hydrogen,
which has a distinct spectral line at 21-cm wavelength in the radio
spectrum. There is also some prominent OH lines lines around 18 cm, so
some astronomers talk about looking in the "water hole" of the
spectrum, between wavelengths of 18 and 21 cm.
If the signal is not sent on purpose, but e.g. just leakage of radio
waves into space, like our radio, t.v., and radar signals, then we
might be able to detect a periodic variation in the signal of such a
planet due to its rotation and the location of its major cities/centers.
False alarm: pulsars
Where to search?
Well, it makes sense to start with local stars, since the signals will
quickly get fainter with distance (as the inverse of distance squared).
Also, pick stars like Sun or somewhat later in spectral type. There are
plenty to chose from. Recent results on extra-solar planet detections
might help to focus the search on particularly interesting stars.
Challenge: there are many many stars to chose from, and it is quite
possible the nearby ones won't have anything interesting: if they did,
why aren't "they" here?
How close would they be? If there are 20,000 civilizations in the Milky
Way, randomly distributed through stars in space, the closest one, on average, would be 1000
light years away. Signals get pretty faint from that far away!
How to search? See above section on radio waves.
Challenge: establishing contact is by no means trivial. Remember, we
need to decode their signal, then send a response back. If they are 50
lightyears away from us, and it takes us 5 years to figure out the
message and send a response, it will take another 50 years for our
signal to get to them. The larger the distance to the star, the harder
"communication" will be!
How might signal be decoded? Hopefully they use "binary notation" like
we do in all our computers and messages in the "digital age".
E.g. number 12,345 = 1x104 + 2x103 + 3x102
+
4x101 + 5x100
Likewise, we can write any number in binary notation, pick a simple
one: 45 which equals 101101 in binary notation:
101101 = 1x25 + 0x24 + 1x23 + 1x22
+ 0x21 + 1x20
(=
32 + 0 +
8 + 4
+ 0 + 1)
Every number can be written using the binary system, as can letters,
even pictures (see book Figure 11.6; the squares should be only be
black or white!):
Arecibo message
INTERSTELLAR TRAVEL: Dreams and
realities...
Challenges for manned space flight we discussed in class:
- extra development cost
- safety
- additional weight of missions
- need to return
- time limit to space travel, while unmanned missions can keep going as
long as there is no failure or loss of power.
Why is space travel expensive, even "local" (close to Earth and in
solar system)?
- Every object launched from Earth needs to get a speed that is of
order the escape speed from Earth. That figure is 11 km/s. To put
something in close Earth orbit (like the spac shuttle, which orbits at
about 200-300 miles above Earth) requires a speed of about 7 km/s. In
class, we compared these speeds with typical every day speeds:
walking - 1.5 m/s
car - 35 m/s
airplane - 220 m/s
The amount of energy required to accelerate something from zero
velocity to a velocity v scales
with the velocity squared and with the
mass of the object. This is called "energy of motion" or "kinetic
energy".
So, per unit mass, it takes (11,000/220) squared = 2500 times more
energy to launch something in orbit than have it go up in a commercial
airplane. This does not include any considerations of forces acting to
slow down the object once it is being accelerated (e.g. friction by air
and the pull of gravity). This energy requirement explains why in any
launch from Earth, most of the space craft is taken up by rockets and
fuel, not scientific or human payload.
If we want to leave the solar system or travel from Earth to other
planets, the gravitational pull from the Sun becomes important too. To
leave the solar system from Earth's location requires a speed of 42
km/s. Since we are already going at 30 km/s around the Sun, we need to
gain additional speed to make it to 42 km/s.
So, if we launch a space craft from Earth in a direction that takes it
out of the solar system and it attains just the velocity of escape from
the solar system by the time most of the fuel is gone, what will happen?
First, the craft experiences
no friction in space, so it is not being
slowed down by friction. It is being slowed down by the gravitational
pull of the Sun. If it moves at just escape speed, it will
continue to
slow down and never go fast by the time it reaches large distances from
the Sun. So, instead we accelerate it further on its way out of the
solar system, using the gravitational
"assist" from other planets. By
cleverly using the knowledge of orbits and dynamics in the solar
system, we can actually increase the speed of space craft greatly by
having them "swing by" other planets. Thus, we can increase the speed
from that required to leave Earth to higher speeds and send the craft
on its way.
Still, there are enormous hurdles to get far at the typical speeds that
can be reached. For example,
going at 100 km/s if we could reach that,
it would still take about 12,000 years to reach the closest star to us.
We would not need to power the space craft much with rockets, but it
would require energy to send signals back to us. And those signals
would take longer and longer to reach us, and get fainter and fainter
as the space craft gets further away. Once far away from the Sun, the
craft cannot be powered by solar energy, so it must have its own
internal power source; most likely, with current technology, this would
be a nuclear reactor, using radioactive elements to generate energy.
A manned mission out
of the Solar System is clearly completely beyond
our technological capabilities. Will it always be?
The space craft has to be
self-sufficient, it has to provide food,
medical care, energy, life support systems etc to a large crew; if it
were to travel for generations in terms of human life times, it would
have to be very large and take along a lot of people. With conventional
fuels and thinking we will not get very far. Several exotic ideas have
been proposed in the past, e.g. using nuclear bombs behind the space
craft to accelerate it to high speed. Other ideas include using the
hydrogen in space as a source of fuel (nuclear fusion!) by having the
space ship collect the hydrogen gas as it moves at high speed through
the space between stars. The density of the hydrogen is very low and
you would need to move fast and have a very large funnel to collect the
hydrogen to make this work...
How fast could we travel?
Einstein's theory of special relativity sets
a hard limit on the highest possible speed; nothing with mass can reach
the speed of light. If we keep accelerating an object by giving it more
energy it will still not exceed the speed of light, instead it will act
as if its mass is increasing and since the energy of motion goes as
product of mass and velocity squared, the gain in velocity will be less
and less as we approach the speed of light. The relativistic effects
become most noticeable only at speeds close to the speed of light, so
in principle we could travel at e.g. 25% speed of light without too
much non-Newtonian effects.
There is one important
advantage however if you could travel at close
to the speed of light; for the travellers, the time will slow down
compared to an observer at rest, and the distances will be
foreshortened. This peculiar effect, called time dilation and length
contraction, are the result of Einsteins postulate that the speed
of
light is the same for every observer in any frame of reference moving
at constant speed or at rest.
The implications of
Einstein's postulates are:
- clocks slow down inside fast moving space craft
compared to Earth (this
has been measured). At the extreme limit of the speed of light, no time
passes. In other words, for a light beam traveling through space there
is no change in time if a clock could come along (which it can't, since
it has mass!).
- astronauts could in fact reach the nearest stars
in their life time if
we could reach speeds close to the speed of light. Many generations
might have passed on Earth but not in the space craft. The
challenges would be enormous to manage such "expeditions", you would
not find your relatives if you could ever come back on Earth.
We discussed: twin paradox.
More on the twin paradox than you ever wanted to know.
Are there better ways to
travel? Can we beat Einstein's limit to speed?
There is nothing known within physics today that contradicts Einstein's
results. There is speculation of being able to travel by short cuts
through "worm holes"
associated with black holes, or other exotic ways.
If the Universe is as we understand it today, 4-dimensional (3 space
and 1 time coordinate) such short cuts are unlikely. However, there is
much we don't know about the Universe. It may be possible that there
are "hidden" dimensions that allow for short cuts and make travel
possible. If we have ever been visited by aliens, they more than likely
would have used such shortcuts.
Here is an excellent primer on black holes and the effects that happen
there. Note that gravity slows down the light speed too and hence time
goes slower near a black hole. Actually, you can think of the "slowing
down of light" more like a distortion in space-time so that the light
has to travel larger distances near the massive object than if the
object were not there. The space time is also distorted, such that
there is a gravitational Doppler shift to the wavelength of the light.
One way to see the effect of gravity on light is through gravitational
lenses, which we discussed before:
Hubble
picture of lensing galaxy.
Explore here: Black
holes, gravity's relentless pull
Make sure that you explore several topics in the black hole web page,
including:
- travel to a black hole
- seeing what happense to a clock that is dropped
into a black hole
- seeing how the orbits are different around a black
hole compared to Kepler's elliptical orbits
- how black holes form
- how we know black holes exist
LAST SECTIONS
THE FERMI PARADOX AND CONTACT
Read book chapters: rest of chapter 13
The Fermi Paradox.
Who was Enrico
Fermi? Famous 20th century physicist, working on
elementary particle physics, discoverer of the "Fermi-exclusion
principle", involved with the Los Alamos Manhattan project in the 2nd
world war.
The Fermi paradox: Where is everyone?
If life is common and present on many other planets
in the Milky Way, then surely some civilization would be far more
advanced than we are today, and they should have been able to colonize
our Galaxy, even if interstellar travel proceeded at slow pace. They
could have hopped from star to star and colonized large sections of the
Milky Way in millions of years, even travelling well below the speed of
light (see book).
Even if the civilizations themselves could not travel, they would have
been able to design self-replicating robots that could have traveled
across the Milky Way. Where are they?
Motives for colonization:
- if our history on Earth is any indication...
- avoid extinction: send out groups of an advanced species to settle
new colonies. Not everyone has to go!
- threats of star ending its life or "impending" impacts.
Non-motives for colonization:
- Cannot solve population growth problem on Earth.
- Conquest?
- Bring natural resources back to Earth. Seems too expensive.
Possible explanations for the
Fermi paradox:
1. We are in fact the most advanced
civilization. In this case, SETI
will not find a signal for a long time...
Seems
unlikely: our most rapid advance has happened in a relatively
short period (see "cosmic calendar argument" in the book) and our solar
system was formed 8 billion years after the Milky Way began to form so
there are many stars and presumably planets out there that preceded us
by many eons. While the very early MW might have lacked heavier
elements, successive generations of stars would have created them and
so there would be ample opportunity for life to develop elsewhere.
Also: everything we
have learned since Copernicus has shown us that
Earth is not the special place it was once thought to be. We are not
the center of the solar system, there are billions of stars like the
Sun, we are not near the center of the Milky Way, and there are
billions of galaxies like ours.
Proponents of this idea:
"Rare Earth Hypothesis". Many conditions may
have conspired to make Earth more special than we think.
2. Civilizations are not
interested or able to colonize other
solar systems.
a. Technological challenges?
b. sociological considerations?
c. self-destruction
3. They exist but don't reveal
themselves to us.
Why
would we be
interesting to them? The "Zoo
hypothesis" (or "wildlife refuge").
The "sentinel" hypothesis.
They are monitoring our progress and are
waiting for us to advance more.
What is the most popular explanation in your eyes?
Or have we been visited?
Roswell
"Alien abductions"
The true answer :)
SETI fiction: They are made out of meat.
BACK TO EARTH: LET'S PLANT OUR FEET IN SOLID SOIL AND MAKE A BALANCE OF
WHAT WE DO AND DON'T KNOW AND WHAT SITUATION IS FROM SCIENTIFIC
PERSPECTIVE
Scientific results thus far:
THE GOOD:
Billions and billions:
- Sun is a normal star of which there are many in
the Milky Way.
- Stars like the sun can shine for billions of years
due to the nuclear fusion in their centers. Our solar system is 4.5
billion years old, less than the age of the universe and the age of the
Milky Way (12-14 billion years).
- The Milky Way is a normal spiral galaxy of which
there are hundreds of billions in the Universe.
- The Earth is one of 4 terrestrial planets in the
solar system. The other planets are like us in size and composition,
but lack water (either liquid or all together) and have very different
atmospheres.
- Planets around other stars are being found in
large numbers. We cannot yet find planets of Earth mass easily but so
far this is a technological limitation. There is no reason to claim
that planets like Earth don't exist elsewhere. In fact, there are
probably (you guessed it) billions and billions of them...
- Our solar system is not located in any special
place in the Milky Way.
- Our Milky Way Galaxy is not located in any special
place in the Universe.
- The Earth and its inhabitants are made out of
atoms that are equally common elsewhere in the universe.
- Living beings are made out of a subset of those
atoms.
- There is strong evidence that early life on Earth
was primitive and that complexity of species increased over time due to
evolution. Biological evolution is the result of mutations in genetic
material upon reproduction, with natural selection as the "guiding
mechanism" to push evolution in particular directions depending on the
local and global conditions.
- There is overwhelming evidence for biological
evolution
occurring on earth, including evolution of species, mass extinction of
species, increases in intelligence among some species with time,
advances in use of tools and technological discoveries.
All of this taken together, in my mind, makes the
existence of life elsewhere a likely prospect.
THE
(as of now still) BAD:
The uncertain or unknown:
- Science argues for the spontaneous generation of
life in some primitive form on Earth, but we have not been able to
replicate this in the laboratory.
- We have not yet found evidence of life on other
places in the solar system. The likeliest places to find such life may
be on Mars and on some of the major moons near the giant planets. Our
guiding principle for establishing likely presence of life is the need
for liquid water to exist. This establishes the "habitable zone" around
other stars and planets.
- The Earth's moon seems rather special and its
formation a fluke of nature. How essential has it been for development
of advanced life on Earth?
THE UGLY....?
The perhaps impossible:
- The physical laws that we have discovered place
severe limits on our ability to travel to other stars in reasonable
amounts of time from the perspective of earthlings. The
speed of light in vacuum is faster than any speed
an object that has mass can attain. If
we could travel at close to the speed of light, time dilation would
allow the astronauts to reach the other stars, but we have no way of
generating the amounts of energy required to do this in a spaceship. We
have no scientific evidence that
these same limitations would not apply to other civilizations.
- The jump from current technology in space travel
to traveling through the Milky Way to other stars is an enormous one.
It will require completely different technology than "conventional
rocket" engines. It will require self-sustaining space ships in areas
far from direct energy sources. It is a jump that is very much larger
than the one from cars to airplanes, or airplanes to current space
flight capabilities.
- We can in principle communicate with other
civilizations
through electro-magnetic waves which travel at the speed of light. It
seems plausible that radio waves are the most likely wavelength range
of electromagnetic waves to use for such communication. We are
searching for signals of artificial nature that may be coming to us
from other civilizations. We have not found any such signals but have
only started looking recently and over a limited range in wavelength
and search space. Given the weakening of signals with distance traveled
and the enormous distances between stars, absence of signals will
perhaps never imply evidence of absence of alien civilizations.
We don't have scientific
evidence to support:
- Past or present visits by aliens
- Descent from aliens
- Monitoring by aliens
- Advice from aliens
- Materials from aliens
- Contacts from aliens
While absence of evidence is not evidence of absence, we
should be guided by scientific principles to make conclusions, and with
no evidence or theoretical supporting arguments, at this point absence
of evidence at least implies no scientific case for support of the
mysterious. Sorry, Captain Kirk.
All in all though, the universe is a
big and miraculous place. Predictions about what we can and cannot do
in the future have always been wrong. We should therefore not be guided
by pessimism, but by the search for the unknown. It is only in this way
that increases in our knowledge and technological improvements have
come about. Enjoy the journey.