The Physics of Star Trek (16 page)

On to the Earth. What is it about our fair green-blue planet that makes it special? In the
first place, it is in the inner part of the solar system. This is important, because the
outer planets have a much higher percentage of hydrogen and heliummuch closer to that of
the Sun. Most of the heavy elements in the disk of gas and dust surrounding the Sun at its
birth appear to have remained in the inner part of the system. Thus, one might expect
potential sites for life to be located at distances smaller than, say, the distance of
Mars from a 1-solar-mass star.

Next, as Goldilocks might have said, the Earth is just rightnot too big or too small, too
cold or too hot. Since the inner planets probably had no atmospheres when they formed,
these had to be generated by gases produced by volcanoes. The water on the Earth's surface
was also produced in this fashion. A smaller planet might well have radiated heat from its
surface rapidly enough to prevent a large amount of volcanism. Presumably this is the case
with Mercury and the Moon. Mars is a borderline case, while Earth and Venus have
successfully developed an atmosphere. Recent measurements of radioactive gas isotopes in
the terrestrial rocks suggest that after an initial period of bombardment, in which the
Earth was created by the accretion of infailing material over a period of 100 million to
150 million years about 4.5 billion years ago, volcanism produced about 85 percent of the
atmosphere within a few million years. So, again, it is not surprising that organic life
formed on Earth rather than on other planets in the solar system, and one might expect
similar proclivities elsewhere in the galaxyon Class M planets, as they are called in the
Star Trek universe.

The next question is how quickly life, followed by intelligent life, might take to evolve,
based on our experience with the Earth. The answer to the first part of the question is:
Remarkably quickly. Fossil relics of blue-green algae about 3.5 billion years old have
been discovered, and various researchers have argued that life was already flourishing as
long as 3.8 billion years ago. Within a few 100 million years of the earliest possible
time that life could have evolved on Earth, it did. This is very encouraging.

Of course, from the time life first began on Earth until complex multicellular structures,
and later intelligent life, evolved, almost 3 billion years may have elapsed. There is
every reason to believe that this time was governed more by physics than biology. In the
first place, the Earth's original atmosphere contained no oxygen. Carbon dioxide,
nitrogen, and trace amounts of methane, ammonia, sulfur dioxide, and hydrochloric acid
were all present, but not oxygen. Not only is oxygen essential for the advanced organic
life-forms on Earth, it plays another important role. Only when there is sufficient oxygen
in the atmosphere can ozone form. Ozone, as we are becoming more and more aware, is
essential to life on Earth because it screens out ultraviolet radiation, which is harmful
to most life-forms. It is therefore not surprising that the rapid explosion of life on
Earth began only after oxygen was abundant.

Recent measurements indicate that oxygen began building up in the atmosphere about 2
billion years ago, and reached current levels within 600 million years after that. While
oxygen had been produced earlier, by photosynthesis in the blue-green algae of the
primordial oceans, it could not at first build up in the atmosphere. Oxygen reacts with so
many substances, such as iron, that whatever was photosyn-thetically produced combined
with other elements before it could reach the atmosphere. Eventually, enough materials in
the ocean were oxidized so that free oxygen could accumulate in the atmosphere. (This
process never took place on Venus because the temperature was too high there for oceans to
form, and thus the life-forming and life-saving blue- green algae never arose there.)

So, after conditions were really ripe for complex life-forms, it took about a billion
years for them to evolve. Of course, it is not clear at all that this is a characteristic
timescale. Accidents such as evolutionary wrong turns, climate changes, and cataclysmic
events that caused extinctions affected both the biological timescale and the end results.

Nevertheless, these results indicate that intelligent life can evolve in a rather short
interval on the cosmic timescalea billion years or so. The extent of this timeframe has to
do with purely physical factors, such as heat production and chemical reaction rates. Our
terrestrial experience suggests that even if we limit our expectations of intelligent life
to the organic and aerobicsurely a very conservative assumption, and one that the Star
Trek writers were willing to abandon (the silicon-based Horta is one of my
favorites)planets surrounding several- billion-year-old stars of about 1 solar mass are
good candidates.

Granting that the formation of organic life is a robust and relatively rapid process, what
evidence do we have that its fundamental ingredientsnamely, organic molecules, and other
planetsexist elsewhere in the universe? Here, again, recent results lead to substantial
optimism. Organic molecules have been observed in asteroids, comets, meteorites, and
interstellar space. Some of these are complex molecules, including amino acids, the
building blocks of life. Microwave measurements of interstellar gas and dust grains have
led to the identification of dozens of organic compounds, some of which are presumed to be
complex hydrocarbons. There is little doubt that organic matter is probably spread
throughout the galaxy.

Finally, what about planets? In spite of the fact that to date only one direct observation
of a planetary system other than our own has been made, it has long been believed that
most stars have planets around them. Certainly a fair fraction of observed stars have
another stellar companion, in so-called binary systems. Moreover, many young stars are
observed to have circumstellar disks of dust and gas, which are presumably the progenitors
of planets. Various numerical models for predicting the distribution of planetary masses
and orbits in such disks suggest (and I emphasize here the word “suggest”) that they will
produce on average at least one Earthlike planet at an Earth-like distance from its star.
Most recently, another planetary system was finally directly detected, 1400 light-years
from Earth. Somewhat surprisingly, the system observed is one of the least hospitable
places one might imagine for planets: three planets all orbiting a pulsarthe collapsed
core of a supernovaat a distance closer than Venus is to our Sun. These planets could
easily have formed after rather than before the supernova, but either way, this discovery
indicates that planetary formation is probably not rare.

I do not want to lose the forest for the trees here. It is almost miraculous that the
normal laws of physics and chemistry, combined with an expanding universe more than some
10 billion years old, lead to the evolution of conscious minds that can study the universe
out of which they were born. Nevertheless, while the circumstances that led to life on
Earth are special, they appear to be by no means peculiar to Earth. The arguments above
suggest that there could easily be over a billion possible sites for organic life in our
galaxy. And since our galaxy is merely one out of 100 billion galaxies in the observable
universe, I find it hard to believe that we are alone. Moreover, as I noted earlier, most
Population 1 stars were formed earlier than our Sun wasup to 5 billion years earlier.
Given the time frame discussed above, it is likely that intelligent life evolved on many
sites billions of years before our Sun was even born. In fact, it might be expected that
most intelligent life in the galaxy existed before ours. Thus, depending upon how long
intelligent civilizations persist, the galaxy could be full of civilizations that have
been around literally billions of years longer than we have. On the other hand, given our
own history, such civilizations may well have faced the perils of war and famine, and many
may not have made it past a few thousand years; in this case, most of the intelligent life
in the universe would be long gone. As one researcher cogently put the issue over 20 years
ago, “The question of whether there is intelligent life out there depends, in the last
analysis, upon how intelligent that life is.”
3

So, how will we ever know? Will we first send out starships to explore strange new worlds
and go where no one

has gone before? Or will we instead be discovered by our galactic neighbors, who have
tuned in to the various Star Trek series as these signals move at the speed of light
throughout the galaxy? I think neither will be the case, and I am in good company.

In the first place, we have clearly seen how daunting interstellar space travel would be.
Energy expenditures beyond our current wildest dreams would be neededwarp drive or no warp
drive. Recall that to power a rocket by propulsion using matter-antimatter engines at
something like 3/4 the speed of light for a 10-year round-trip voyage to just the nearest
star would require an energy release that could fulfill the entire current power needs in
the United States for more than 100,000 years! This is dwarfed by the power that would be
required to actually warp space. Moreover, to have a fair chance of finding life, one
would probably want to be able to sample at least several thousand stars. I'm afraid that
even at the speed of light this couldn't be done anytime in the next millennium.

That's the bad news. The good news, I suppose, is that by the same token we probably don't
have to worry too much about being abducted by aliens. They, too, have probably figured
out the energy budget and will have discovered that it is easier to learn about us from
afar.

So, do we then devote our energies to broadcasting our existence? It would certainly be
much cheaper. We could send to the nearest star system a 10-word message, which could be
received by radio antennae of reasonable size, for much less than a dollar's worth of
electricity. Howeverand here again I borrow an argument from the Nobel laureate Edward
Purcellif we broadcast rather than listen, we will miss most of the intelligent
life-forms. Obviously, those civilizations far ahead of us can do a much better job of
transmitting powerful signals than we can. And since we have been in the
radio-transmission business for only 80 years or so, there are very few societies less
advanced than we are that could still have the technology to receive our signals. So, as
my mother used to say, we should listen before we speak. Although as I write this, I
suddenly hope that all those more advanced societies aren't thinking exactly the same
thing.

But what do we listen to? If we have no idea which channel to turn to in advance, the
situation seems hopeless. Here we can be guided by Star Trek. In the
Next Generation
episode “Galaxy's Child,” the
Enterprise
stumbles upon an alien life-form that lives in empty space, feeding on energy.
Particularly tasty is radiation with a very specific frequency1420 million cycles per
second, having a wavelength of 21 cm.

In the spirit of Pythagoras, if there were a Music of the Spheres, surely this would be
its opening tone. Fourteen hundred and twenty megahertz is the natural frequency of
precession of the spin of an electron as it encircles the atomic nucleus of hydrogen, the
dominant material in the universe. It is, by a factor of at least 1000, the most prominent
radio frequency in the galaxy. Moreover, it falls precisely in the window of frequencies
that, like visible light, can be transmitted and received through an atmosphere capable of
supporting organic life. And there is very little background noise at this frequency.
Radioastronomers have used this frequency to map out the location of hydrogen in the
galaxywhich is, of course, synonymous with the location of matterand have thus determined
the galactic shape. Any species intelligent enough to know about radio waves and about the
universe will know about this frequency. It is the universal homing beacon. Thirty-six
years ago, the astrophysicists Giuseppe Cocconi and Philip Morrison proposed that this is
the natural frequency to transmit at or listen to, and no one has argued with this
conclusion since.

Hollywood not only guessed the right frequency to listen to but helped put up the money to
do the listening. While small-scale listening projects have been carried out for more than
30 years, the first large-scale comprehensive program came on line in the autumn of 1985,
when Steven Spielberg threw a big copper switch that formally initiated Project META,
which stands for Megachannel Extra Terrestrial Array. The brainchild of electronics wizard
Paul Horowitz at Harvard University, META is located at the Harvard/Smithsonian 26-meter
radiotŽlescope in Harvard, Massachusetts, and funded privately by the Planetary Society,
including a $100,000 contribution from Mr. ET himself. META uses an array of 128 parallel
processors to scan simultaneously 8,388,608 frequency channels in the range of 1420
megahertz and its so-called second harmonic, 2840 megahertz. More than 5 years of data
have been taken, and META has covered the sky three times looking for an extraterrestrial
signal.

Of course, you have to be clever when listening. First, you have to recognize that even if
a signal is sent out at 1420 megahertz, it may not be received at this frequency. This is
because of the infamous Doppler effecta train whistle sounds higher when it is approaching
and lower when it is receding. The same is true for all radiation

emitted by a moving source. Since most of the stars in the galaxy are moving at velocities
of several hundreds of kilometers per second relative to us, you cannot ignore the Doppler
shift. (The Star Trek writers haven't ignored it; they added “Doppler compensators” to the
transporter to account for the relative motion of the starship and the transporter
target.) Reasoning that the transmitters of any signal would have recognized this fact,
the META people have looked at the 1420 megahertz signal as it might appear if shifted
from one of three reference frames: (a) one moving along with our local set of stars, (b)
one moving along with the center of the galaxy, and (c) one moving along with the frame
defined by the cosmic microwave background radiation left over from the big bang. Note
that this makes it easy to distinguish such signals from terrestrial signals, because
terrestrial signals are all emitted in a frame fixed on the Earth's surface, which is not
the same as any of these frames. Thus terrestrial signals have a characteristic “chirp”
when present in the META data.