Are We All Alone?(An Inquiry by an Amateur Scientist)

By: John Winders

The image on the cover was provided by NASA. Seeing the earth suspended in empty space always gives me a lonely feeling, despite the fact that 7 billion souls share our tiny planet.

The question, “Are we alone?” has been asked by humans ever since we attained self consciousnessand realized that we are different from all the other animals. This is perhaps the most important question ever asked, because the answer to it – yes or no – determines how each of us views the cosmos and what our role in it is. Humans have surrounded themselves with spirits, gods, goddesses, angels, demons, and extraterrestrial beings in part to explain the occurrences of natural phenomena, but also to reassure ourselves that there are other intelligent entities who are guiding and caring for us. This urge to populate the cosmos is not hard to understand, because being alone is terrifying to most people. What I intend to do in this essay is to explore what scientific evidence there is on this question and try to reach a tentative answer.

As far as spirits, gods, etc. are concerned, there is very little evidence that there are any supernaturalevents taking place in our daily lives. At one time, it was believed that everything had a supernatural cause.1 Gradually, scientific knowledge began to replace supernatural causes as explanations for weather, diseases, and how the universe works in general. This is the “God in the gaps” argument2 and even scientists had a hard time abandoning those gaps. Isaac Newton was unable to find stable solutions to orbital mechanical problems involving three or more bodies. Since the solar system consists of many planets, Newton postulated that when a planet started to veer into an unstable trajectory, God nudged it back into its proper orbit. Pierre-Simon Laplace later found that the planetary orbits are in fact quasi-stable, and there is no need for divine intervention to prevent the planets from colliding or being ejected from the solar system.

With the recent explosion of scientific knowledge beginning in the late 19th century, it seems that the gaps are getting ever smaller. This has led many of today's scientists to deny the existence of God altogether, in some cases quite forcefully. Naturally, this antagonizes people of religious persuasions, who see science as the enemy of their religion. Although I agree there is scant evidence that supernatural personalities actually exist, I think it's a mistake for members of the scientific community to weigh in on religious matters. It's one thing to reject the notion that God creates hurricanes to punish gays, because there's a perfectly natural scientific explanation for hurricanes that has nothing to do with breaking sexual taboos. But it's quite another matter to categorically state that the universe sprang into being without any cause and that intelligent life is simply the end product of random collisions between molecules. This is just as intellectually dishonest as stating that God created the universe in six days based solely on writings compiled by Jewish scribes during their Babylonian captivity.3

So let us consider the hypothesis that we are not alone; the world is under the direct control of an almighty God, who cares for us and watches over us. What can we say – scientifically – concerningthis hypothesis? The scientific method draws upon empirical evidence and controlled experiments to show whether a conjecture or hypothesis is correct. The hypothesis of an almighty, supremely good, and caring God who personally directs events here on earth immediately runs into a major

1 There are people today who still believe that, such as Bill O'Reilly of Fox News, who says that only a supernatural cause can explain why “Tides go in and tides go out.”

2 Neil deGrasse Tyson is quoted as saying, “Does it mean, if you don’t understand something, and the community of physicists don’t understand it, that means God did it? Is that how you want to play this game? Because if it is, here’s a list of things in the past that the physicists at the time didn’t understand [and now we do understand] [...]. If that’s how you want to invoke your evidence for God, then God is an ever-receding pocket of scientific ignorancethat’s getting smaller and smaller and smaller as time moves on – so just be ready for that to happen, if that’s how you want to come at the problem.”

3 The creation story in Genesis is remarkably similar to other creation myths that were circulating in the Babylon region around 600 BCE. Some Biblical scholars believe two of these creation stories were woven together to produce Genesis 1-2.

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empirical problem – the problem of evil. If God is omnipotent and truly abhors evil, then why is evil allowed to exist? The customary answer is that God permits humans to commit evil acts because He values human free will even more than goodness. Why do bad things happen to good people? There are no good people because everyone falls short of the infinite goodness of God. Okay then, why do bad things happen to people who have faith and good things happen to people without faith? God is testing their faith, or alternatively, God's ways are mysterious. None of these answers appear to support the hypothesis of a good and loving God who tends to our needs. In fact,looking at empirical evidence leads directly to one of the following depressing conclusions.

1. There an almighty God who is supremely good, but He doesn't care about us.

2. There is a supremely good God who cares about us, but He is powerless to prevent evil.

3. God is evil.

4. There is no God.

These conclusions do relate directly to the question, “Are we alone?” Both Conclusions 1 and 2 imply that we are not alone, but we may as well be. Conclusion 3 implies that we are not alone, butwe would be much better off if we were. And Conclusion 4 implies that we are indeed all alone, insofar as there is no supreme Being to take care of our problems and concerns.

There is a way out of the dilemma caused by the existence of evil: God set the universe into motionwith all the necessary physical, laws and then simply allowed it to unfold entirely on its own without any further direct intervention – but with a catch. In addition to the physical laws, Got put in place a moral law that causes appropriate consequences in the long term as a result of committinggood and evil acts. The “long term” could mean an afterlife, in the Christian sense, or it could meana series of physical lives, according to the doctrine of karma and rebirth embraced by Eastern religions.4 This is a partially-satisfying answer to the “Are we alone?” question, because at least there is a Creator who cared enough about his creation to set up some moral laws to guide us; however, having a moral law isn't nearly as good as being able to talk things over with someone.

If a moral law involving good and bad consequences is all our Creator left us, there must be an immortal entity – a soul – for the law to work on. Okay, is there any scientific evidence that there issuch a thing? Well, there is some. A number of people have claimed to “recall” previous physical lives and have even provided some details about these lives that appear to have been independently corroborated. Then there are the near death experiences (NDEs) that have been reported and verified by medical workers. The NDEs seem to be quite similar in the major features: leaving the body and making accurate observation about its surroundings while outside the body, traveling toward a bright light, being accompanied by angelic beings, feeling a sense of well-being, being metby God or a god-like being, being given a life review, and being instructed as to the purpose of life. When a person returns to the physical body following an NDE, he generally has the sense of immortality and that “death” is just a passage from one form of life to another. The details of the NDE seem to depend on the cultural background of the person experiencing it, which is rather reassuring because there doesn't appear to be a “right” or “wrong” way to have the NDE, and so different religious views appear to be equally valid. Beyond occasional recollections of previous lives, NDEs and other “out of body experiences” there is little empirical evidence to go on concerning the afterlife or even the existence of a higher moral law. Religions speak volumes aboutsuch things, and prescribe long lists of “dos” and “don't” in minute detail. But much of what

4 I think karma is often misconstrued as a cosmic tally sheet of good and evil acts committed by a person while livingon earth; a person's soul then rewarded by an easy life or punished by a miserable life the next time around. I think a more accurate way to view karma is being a tendency to make good or bad choices. Good choices lead us down one path in life and bad choices lead us down another path. The cumulative effect of making good and bad choices is finding ourselves in situations and circumstances of our own making that we think of as “good” or “bad” karma. These effects may carry over into an afterlife or future physical lives – I think the jury is still out on that.

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religions teach on these matters is self-contradictory and almost all of it is pure conjecture.

Putting the supernatural aside, what about the existence of other physical, intelligent entities who are equal or superior to humans? It certainly would be reassuring to know that there are advanced beings – watchers – who will swoop down and rescue humankind when it is on the verge of destroying itself. Throughout the ages, there have been stories about fairies, elves, genies, etc., whocan materialize and intervene in human matters. Since around 1947 these entities have been replaced by extraterrestrials who travel in UFOs. Today, believing in supernatural beings is considered ridiculous and uncool, whereas space aliens from other solar systems are considered to be at least scientifically plausible. But are they? We'll test that hypothesis in a little while.

The idea of civilizations living on other planets is not all that new. Giordano Bruno suggested the sun is just a star and that there are numerous, perhaps infinite, worlds inhabited by intelligent beingsaround other suns. Unfortunately, Bruno lived in the 16th century when believing such things could get you into some really serious trouble with the Establishment. In Bruno's case, the Roman Inquisition found him guilty of heresy and burned him at the stake.5

We know that intelligent life exists on earth6, but we don't have much of a theory about how it cameabout. My essay Order, Chaos, and the End of Reductionism delves into this question. Since we don't know how life came about here, we don't know if it's easy or hard it is for life to get started in other places. Not only that, but there doesn't seem to be any necessity for intelligent life to evolve at all. For example, according to the evolutionary model, chimpanzees and humans branched off from a common ancestor. But whereas humans learned how to build the Large Hadron Collider andsend rockets to other planets, chimps are content with fashioning twigs to fish for termites. In other words, we really don't have a clue as to why intelligent life arose on earth in the first place, much less whether it could arise here again, or if it has evolved in other places in the universe. Of course creationists have a ready answer to that: Around the sixth day, God created Adam from the dust andthen He created Eve from Adam.7 Thus, God certainly could have created as many intelligent beings on as many different planets as He saw fit; but He probably didn't create any people besides Adam and Eve, because the Bible doesn't mention them. (I'm not saying the Bible story is wrong; I'm just saying there is zero corroborating evidence to support it.)

Enrico Fermi provided his own answer to this puzzle in the form of another question: “Where is everybody?” In other words, if evolving intelligent life is as easy as falling off a log, we should be in daily contact with extraterrestrials. Well, not so fast, Enrico. First of all, what does it mean to “coexist” with aliens, when the nearest alien planet may be thousands of light years away? Any exploratory mission from one civilized star system to another would effectively be a one-way trip. Here's why: The Milky Way is approximately 100,000 light years in diameter. If an exploratory mission from the opposite side of our galaxy traveled to earth at nearly light speed, the journey backand forth could be done within the lifetime of an alien crew member, thanks to time dilation. But his home planet would have gotten 200,000 years older in the meantime. The human race has only existed for about 200,000 years; the alien's race could go extinct or evolve into a whole new speciesin 200,000 years, so the trip home would be like traveling to another alien planet. There's simply nogetting around special relativity.

So even if the Milky Way were teeming with intelligent life, meaningful communication or physicalcontact with aliens might simply be an impossibility. As far as making physical contact with aliens from Andromeda (at least 2.5 million light years away): fuggedaboutit. Unless …

What if aliens discovered a way to achieve inter-dimensional travel? Could travelers slip through a

5 Actually, what really pissed off the Church was Bruno's belief in pantheism, which holds that God = Creation.6 Despite much contrary evidence, as illustrated in the television series “Here Comes Honey Boo Boo.” 7 This is according to the oft-cited second version of creation found in Genesis. The first version of creation in

Genesis merely says that Adam and Eve – male and female – were created at the same time.

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fourth dimension or a “wormhole” and make a 100,000 year journey in a matter of seconds? Well, think about the ramifications of that. Getting around the light-speed barrier would make it possible to transmit information faster than light, and that is essentially equivalent to backward time travel. This is a huge no-no in our universe because it violates all sorts of causality laws. So unless you're willing to royally screw up causality, thereby disrupting the entire history of the universe, I'd strongly recommend traveling at a more sedate speed, say less than 300,000 km/sec.

What about all those reports of UFO sightings, close encounters of the third kind, abductions, etc.? Well, I don't doubt that some of the people making these claims sincerely believe they experienced those things, but where's the logic behind it? Any physical trans-galactic journey would require stupendous amounts of energy and very advanced technology. If the aliens' purpose were to study humans, what could they possibly learn by observing primitive beings like us? It would be like us traveling to Alpha Centauri to study ants. Furthermore, as stated earlier, any physical journey across the galaxy is essentially one-way trip, so how would their home planet be able to glean any information about us? If aliens are here to help us, why haven't they made their presence known to the whole world? After all, they must know about cable news by now. UFO enthusiasts have a ready answer to this: There's a vast government conspiracy to hide the truth from a panic-prone public, and the aliens are part of that conspiracy. But any conspiracy such as this would involve thousands or tens of thousands of people in all levels of government from every country on earth. That degree of global cooperation has never existed before and it probably never will. Maybe the aliens have a much more sinister agenda than gathering information or helping us along the evolutionary path. What if their real purpose is to invade and take over the earth? Okay, but if that were the case, why are all of the world governments cooperating with them?

Maybe I'm just too stupid to grasp the concept physical contact with aliens, but I've pretty much ruled out that possibility in my mind due to the limitations of special relativity and because of the sheer amount of effort and resources that would be involved. If aliens can't physically travel aroundthe galaxy, that wouldn't rule out contact via radio transmissions, however. The SETI Project is looking really hard for alien radio signals, but so far it hasn't found any. So maybe Fermi was right.If the galaxy is truly teeming with intelligent life, then alien versions of the “I Love Lucy” show should be streaming in from all directions, even if they were sent thousands or even hundreds of thousands of years earlier. Since they aren't, shouldn't we therefore conclude that no advanced civilizations exist, at least in our vicinity? Well perhaps, unless those advanced alien civilizations communicate by some means other than radio waves. Or unless their signals are encoded in such a way that they simply appear as noise to us.8

So lacking any empirical evidence that advanced alien civilizations exist, what are we to conclude? Well, a gentleman by the name of Frank Drake proposed an equation that would actually calculate the number of advanced civilizations in the Milky Way that are presently communicating. This equation is known as … the Drake Equation (duh). Here it is:

N = R* × fp × ne × fl × fi × fc × L

Here is what these factors represent.

N = the number of civilizations in the galaxy where communication is possible

R* = the average rate of star formation in the galaxy

fp = fraction of stars that have planets

ne = the number of planets per star that can support life

fl = the fraction of planets that can support life that actually do

8 In my essay Order, Chaos, and the End of Reductionism, I discuss the fact that signals encoded using pseudorandom number sequences are indistinguishable from random noise.

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fi = the fraction of planets with life that have intelligent life

fc = the fraction of planets with intelligent life that have civilizations that communicate

L = the average life span of a civilization that communicates

The last factor is rather ominous, because it implies that most civilizations stop communicating for one reason or another. One reason they stop communicating is because they self destruct. Some of Drake's factors can be determined with some precision. R* can be estimated by simply observing star formation. Lately, some exciting data are starting to come in from telescopes designed to detectextra-solar planets. These telescopes are both earth-bound and mounted on spacecraft. It seems that some stars really do have planets, so at least we know fp is greater than zero. Some planets have been detected in the “habitable zones” around those stars, enabling scientists to estimate ne. Unfortunately, the rest of the factors in the Drake Equation are simply SWAGs.9

Based on these SWAGs, calculations of N range from a pessimistic 10-19, which is essentially zero, to an exuberant 36 million. The latter number would place the nearest communicating civilization right around the corner. But as Fermi would ask, “Where is everybody?” Well, there's some additional information that needs to be considered before reaching any conclusions.

First off, the most common star in our galaxy is the red dwarf. These stars live for a really long time: hundreds of billions of years.10 Unfortunately, the vast majority of them have been around fornearly as long as the universe has existed, and they are comprised almost entirely of hydrogen and helium. This means that any planets orbiting around them are also comprised almost entirely of hydrogen and helium. Such planets don't support life, so ne = 0 for red dwarf stars.

In order to support life, you need chemistry involving hydrogen, oxygen, nitrogen, carbon, and water with a dash of metals. Thus, second- and third-generation stars, born out of the debris of supernovae, are needed for life to exist. Among these latter-day stars, the majority of them form double or triple star systems. The Sirius system, which includes three stars, is very typical. Stable planetary orbits are nearly impossible to attain in these systems; thus, fp is essentially zero for them.

So we've ruled out the vast majority of stars that exist in our galaxy as candidates for supporting life. There are, however, still a great number of single sun-like second- or third-generation stars that have mean lifetimes in the range of billions of years. And the good news is that data from telescopes reveal that some of these stars have planets orbiting in their “habitable zones.” The problem is that not any old planet in the habitable zone will do. Many of these planets are Jupiter-sized monsters. At the risk of sounding like a chauvinist, I believe the most likely places forlife to form are medium-sized rocky planets like earth. The chemistry just isn't right on the gas giants. Furthermore, even if floating balloon-like creatures did evolve on Jupiter-style planets, I don't think you'd find too many radio inventors like Guglielmo Marconi among them.

But fortunately, telescopes have detected several earth-like planets in habitable zones near sun-like stars. Yippee! But before we fire up our radio transmitters and point them at those planets, let's consider earth's special circumstance, which is somewhat unusual among planets in the Milky Way.

Asteroids and comets are an ever-present danger for the inner planets of any star system. A single hit by an average-sized comet could render the earth uninhabitable for a long time. One of the things that triggers comets raining down on the inner planets are close encounters with other stars and large inter-stellar clouds of gas and dust. The gravitational force from a large passing object disturbs the sun's halo of comets and wreaks havoc. Fortunately for us, the sun is pretty much a “loner” in the galaxy. You see, we live in one of the empty spaces between two spiral arms of the Milky Way, making it less probable that we would encounter extra-solar objects in our travels

9 For the benefit of my non-engineer readers: SWAG stands for Silly (or Scientific) Wild-Ass Guess.10 Or as Carl Sagan used to say, “Billions and billions of years.”

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around the galaxy. Things would be a lot more tense if we lived among the stars in the spiral arms (where most stars happen to be). Furthermore, we happen to have a couple of gas giants (Jupiter and Saturn) that act as bodyguards for the inner planets. Many of the comets and asteroids that would cause problems for us are deflected or intercepted by our friendly giants. What fraction of planetary systems are configured the same as ours? Probably not a very large one.

There's another reason why you don't want to live too close to a spiral arm. A lot of novae and supernovae occur in those regions. When a star dies, it usually undergoes a colossal explosion that hollows out the entire region and bathes it with deadly radiation. If the sun were near a supernova, life on earth would end; no more evolution and no more intelligent life. Oh, and you don't want to live too close to the center of the galaxy either, because there's a colossal black hole lurking there. At present, the Milky Way's central black hole is fairly quiet. But based on observations of other galaxies, central black holes erupt from time to time when a lot of stars or large amounts of gas and dust fall into the hole's accretion disk. When that happens, huge jets of protons, electrons, and othersubatomic particles are ejected at close to the speed of light from the opposite poles of the black hole. Any planets that happen to be in the paths of those jets are vaporized.

Gamma-ray bursts fill out the list of galactic hazards. They are the most violent explosions in the universe, and are not well understood. What is understood, however, is that all life on a planet is extinguished if that planet happens to be in the path of one of those gamma-ray bursts. It seems thatthe earth is very lucky to be located in a region of the Milky Way where such catastrophes are rare.

Then there is our moon. Our satellite is huge compared with the size of our planet. The moon was probably formed from a collision with a Mars-sized object billions of years ago. This accidental encounter was fortunate for us, because our large moon stabilizes the earth's rotation, keeping the north-south axis tilted about 23.5° from the vertical. Also, the earth's orbit is nearly circular so the distance to the sun is about the same year-round. This gives us seasons that aren't too hot or too cold. Without the moon, the earth's tilt would be erratic, resulting in huge seasonal temperature variations and violent weather if the tilt were too extreme. It's probably not an exaggeration to say that advanced life on earth would be difficult if not impossible without the moon. How many earth-like planets have large satellites to stabilize their axes of rotation? Probably very few.

The bottom line is that earth enjoys an unusually favorable set of conditions. It's just the right distance from just the right sun, which happens to be in just the right place in the galaxy, and it's just the right size with just the right moon, and just the right chemistry, and so on. The alignment ofall these favorable “just rights” is pretty unlikely, but it's probably not that strange that we live in such a place, thanks to the anthropic principle. The strong version of the anthropic principle postulates that there are conditions favorable for intelligent life because the universe is compelled tocreate intelligent life. I don't like the strong version of the anthropic principle because it sounds a lot like creationism or intelligent design – only with God missing from the picture. The weak form of the anthropic principle is somewhat a rhetorical tautology: Intelligent life evolves only where intelligent life can evolve. I don't like the weak version either because cosmologists try to use it to explain why the fundamental constants of the universe are favorable for life.11 But the weak anthropic principle does explain why intelligent life is more likely here on earth than most other places in the galaxy, and it also suggests where we should look for extraterrestrial life.

So what's the end result according to Frank Drake? His early estimates (from the 1960s) were:

R* = 1 per year

fp = 1

11 The anthropic principle involves a common fallacy in logic. Suppose the result R is observed and R can be caused by either A or B. Even if R is more likely if A occurs than if B occurs, then it does not logically follow that the existence of R implies that A occurred.

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ne = 2

fl = 1

fi = 1

fc = 0.5

L = 1,000 – 100,000 years

Plugging these numbers into the Drake Formula results in a value of N between 1,000 and 100,000 civilizations that are actively communicating. Based on the hazards to life mentioned previously, my answer would be much smaller: I would say it's more like around 10, which would answer Fermi's question. With so few communicating civilizations spread around a galactic disk 100,000 light years in diameter, the mean distance between neighboring civilizations is huge. Our “I Love Lucy” signals would be so severely attenuated by the time they reached our nearest neighbor that there is almost no chance that an alien will ever hear Desi shout, “LUCY, I'M HOME!”

There's one final thing to consider. Even with all the conditions lining up so favorably, life on earth could have been extinguished any number of times in our history. There was the “snowball earth” event that occurred about 650 million years ago, when the entire earth literally froze over. There were numerous asteroid and comet collisions, volcanic eruptions on an unimaginable scale, and so on. Then there is this: The human race almost went extinct – twice in fact. This information was inferred through genetic evidence. It so happens that mitochondrial DNA is passed from mothers totheir daughters and is unaffected by DNA supplied by the fathers. Thus, tracing mutations in mitochondrial DNA over many generations is fairly straightforward, and this allows geneticists to make fairly accurate estimates of how many individuals were in the human population over time.

It turns out that there's strong evidence that the entire human population “crashed” about 150,000 years ago. This was roughly 50,000 years after the species Homo sapiens emerged.12 It is believed that some drastic climate change was responsible for that first crash and that only 2,000 individuals survived. An even worse human population crash occurred 70,000 years ago when the Toba supervolcano on the island of Sumatra erupted. Toba dumped 3,000 times as much solid material into the atmosphere as Mt. Saint Helens did in 1980, drastically altering the global climate. It is believed that no more than 1,000 reproducing individual humans survived that calamity, and some estimates place that number as low as 40 individuals.13 I don't know about you, but I think that's pretty humbling. If the human race had been completely eradicated 70,000 years ago, would there ever be intelligent life on earth again? Would our chimpanzee cousins go on to invent mathematics and discover nuclear physics, or would they just continue to act like chimps? Would dolphins climb back on land and create an advanced civilization, or would they just be content to cavort in the waves and chase down schools of herring and mackerel?

Thinking about this makes me realize how fragile life is, and yet how awesomely robust life can be at the same time. Imagine being one of those few survivors 70,000 years ago. Imagine how frightened they were, with full human self consciousness and awareness of their own fragility and mortality. Did they have gods and did they feel abandoned by them? They must have felt so utterlyalone in the universe. They only had each other. That's the way I think of us, clinging to our tiny lifeboat planet as we flounder aimlessly through the vast Milky Way. There is no safe harbor anywhere else, and we are completely alone. There is no one to look out for us besides each other.

12 Modern humans evolved right around the time of “Mitochondrial Eve.” Eve was an early Homo sapiens individual who lived about 200,000 years ago. Only Eve's mitochondrial DNA has survived down the ages, and every one of the seven billion humans living on the planet today carry her DNA. Homo sapiens is a remarkably homogeneous species – we are all virtually brothers and sisters.

13 Of course those who take the Bible literally will say that Noah and his family represented an even smaller number of survivors after the Flood. In any case, the fact is that the survival of the human race has dangled by a thread at least once.

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Appendix A – Andromeda to SETI: Look Over Here

SETI (Search for Extraterrestrial Intelligence) began in the 1960s. The movie Contact, starring Jodie Foster who played Dr. Ellie Arroway, was based on a story by Carl Sagan. Sagan was a believer in extraterrestrial life and big fan of SETI. If you recall, Ellie was in charge of this big array of radio telescopes pointed at the sky, listening for signals that stood out from all the random background radio noise. Well, it didn't seem that Dr. Arroway had much of a strategy, but she got really lucky and found an intelligent signal almost right away. In fact, she hit the jackpot. She not only got a signal, but it turns out that the signal was actually an encoded set of blueprints of an interstellar space-time portal that she later used to visit a planet near the center of the galaxy.

The real SETI doesn't seem to have much of a strategy either, and unfortunately they haven't been as lucky as Ellie. They have heard nothing ... nada, zilch.14 Well here's the thing: There are two types of signals that an advanced civilization could be transmitting. Type I signals are an ordinary broadcast communication signals, like the “I Love Lucy Show” and police radio calls. That kind of signal is sent out omnidirectionally (in every direction). Type I signals are definitely not aimed at us, so they are very attenuated (weak). Furthermore, over the long haul most Type I signals are not going to be simple Morse code, AM or FM signals. Those use obsolete 20th century analog technology. No, they're going to be digitally encoded, using pseudorandom number sequences that make them appear just like random noise unless you have a matched key to decode them. So most Type I signals are going to be very weak noise-like signals that are impossible to detect.

Type II signals are the kind of signals aliens would send through space to notify other civilizations of their presence. Those would probably be simple on/off Morse code type signals that are easy to distinguish from background noise. To be most effective, Type II signals would be beamed at a specific area instead of sent out in all directions. Now if I were going to send out Type II signals, I would probably aim them where I thought alien civilizations would be when the signal crosses their paths. Where would that be? Well, if my target audience were somewhere in the Milky Way, I wouldn't have a clue. They could be anywhere, and as I indicated earlier, there may only be around 10 or so civilizations scattered around the galaxy who would have the technology to receive Type II signals. So instead of beaming the signal at where I knew an alien civilization would be, I'd have tosend the signal out in all directions and hope for the best – not a very good strategy.15

On the other hand, the entire Andromeda galaxy – spiral arms and all – subtends an angle of 3° as seen from Earth. If Andromeda is anything like the Milky Way, then it should potentially have 10 or more advanced civilizations that our signal might intersect. If we beamed a signal at Andromeda,all of those civilizations would be contained within that small 3° angle. Admittedly, the signal would still be greatly attenuated by the time it reached Andromeda because of the inverse-square law. But that signal would be focused 24/7 on a spot only 3° wide, so it might have a much better chance of being picked up than signals sent out randomly all over the Milky Way.

Now the Andromedans should have figured this out too if they are as smart as I am, which they probably are. So if they sent out Type II signals, they should have beamed them to where we are now. All we have to do is point our antennas in their direction. So here's my advice to SETI: Forget about listening for signals coming from inside the Milky Way. Point your antenna arrays at Andromeda instead. Granted, Andromeda is a long way off, and there is zero chance that we'll be

14 Actually, back in 1967 there was a pulsating signal that stood out. For a while, it was a candidate for a signal coming from extraterrestrial intelligent life, and the scientists labeled it LGM-1 (the LGM stood for Little Green Men). Alas, it turned out to have a natural origin: an ordinary pulsar.

15 Assuming the receiving civilization is using a directional antenna, à la SETI, it would make no sense to use directional antennas to send Type II signals unless you were absolutely sure where to aim them. No signals could be received unless both antennas were pointed at precisely the right directions, which is highly unlikely.

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able to visit anybody living there, and they wouldn't even receive a return signal from us for at least another 2½ million years. But wouldn't it be cool to know that we're not quite alone after all?

The figure below shows how a focused signal sent to Andromeda compares with a signal sent out inall directions.

The red circle with a diameter D subtends an angle of 3° on a sphere of radius R, representing Andromeda as seen from Earth. D = α R, where α is the subtended angle in radians. An angle of 3°is equal to π/60 radians, so R = 60 D/π.

The area of the sphere is As = 4 π R2. Substituting 60 D/π for R gives the area of the sphere in termsof D:

As = 14,400 D2/π

The area of the red circle is Ac = π D2/4. Let Γ = As /Ac .

Γ = 57,600 / π2 = 5,836

A signal that spreads out evenly over the entire sphere will be weaker than a signal focused on the red circle. Γ gives the ratio of the signal strengths of the two signals having the same total power. In other words, Γ is the gain we get by using a directional antenna, both in sending and receiving.

Suppose Signal A is a Type II signal sent throughout the Milky Way. Its sphere will spread to a radius of about 50,000 light-years to encompass the galaxy.16 Suppose Signal B is a Type II signal sent to Andromeda. Its sphere will spread to a radius of 2.5 million light years when it reaches Andromeda. According to the inverse square law, a signal 2.5 million light years away will be attenuated a lot more than a signal 50,000 light years away. The ratio of Signal B to Signal A is:

σ = (50,000/2,500,000)2 = 0.0004

But concentrating the entire signal on Andromeda increases the gain by a factor of Γ = 5,836. So Signal B arriving at Andromeda is σ Γ = 0.0004 × 5,836 = 2.3 times stronger than Signal A at the outer fringes of our own galaxy, assuming both transmitters have the same total power output. This 2.3 signal-strength ratio also holds for signals sent from Andromeda directed at us compared to signals sent omnidirectionally from the fringes of the Milky Way. This is why SETI should be aiming their receiving antennas at Andromeda (assuming the Andromedans also figured this out andaimed their transmitters at us). Of course, if everybody is only receiving but not transmitting – like SETI – then the universe will still turn out to be a very lonely place.

16 This is an approximation because the Earth's location in the Milky Way is off-center. A bubble 50,000 light-years inradius wouldn't reach the far side of the galaxy, but it would encompass most of it.

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Appendix B – A Short Tutorial on Radio Communication

Communication using radio-frequency waves began around 1887 with experiments conducted by Heinrich Hertz. His transmitting apparatus consisted of a DC source of high voltage connected to spark gap in series with a coil that is magnetically coupled to a secondary coil with two metal platesat both ends. The secondary coil and its plates form a tuned circuit. The receiving apparatus consisted of a wire loop antenna with a spark gap in the middle. The DC source supplies energy to an electric arc having negative dynamic resistance that regeneratively amplifies currents circulating in the tuned circuit. The metal plates connected to the tuned circuit excite electromagnetic waves that radiate into space. These waves induce voltages in the remote receiving loop that can be strongenough to produce sparks, making a crude communication device.

Around 1900, Guglielmo Marconi perfected the spark-gap transmitter and also designed receivers that were much more sensitive than Hertz's wire loop, making ship-to-shore communication possible. Information using Morse code was applied to the radio waves by simply switching the arcon and off with a telegraph key. This method is called continuous wave (CW) modulation, althoughthe waves – being switched on and off – are not really continuous. In 1914 Edwin Armstrong invented the regenerative circuit using the vacuum tube, which is a much more consistent source of radio frequencies than electric arcs. Vacuum tubes ushered in the modern electronic era.

Amplitude modulation (AM) and frequency modulation (FM) make it possible for radio signals to carry audio (and later video) information. It turns out that all the information in an AM or FM signal is contained in two “sidebands” that occupy frequencies below and above the “carrier wave” (the wave whose amplitude or frequency is being modulated). In fact, the AM carrier wave can be eliminated altogether, concentrating all of the transmitter's power on only the sidebands. Amateur radio operators use single sideband modulation (SSB) that uses only one of the AM sidebands. Thismakes very efficient use of available bandwidths; unfortunately, reducing a signal's bandwidth has amajor drawback – higher susceptibility to noise and jamming.

Claude Shannon studied ways of transmitting information through a noisy channel with as few errors as possible at the receiving end. He came up with the famous Shannon-Hartley theorem, which states that the channel capacity, C, in bits per second is given by a very elegant formula:

C = B log 2 (1 + SNR)

, where B is the channel's bandwidth in Hz and SNR is the linear signal-to-noise power ratio. Shannon proved it is theoretically possible to receive signals with zero errors as long as the information rate is kept below C. The key is to make B is large as practicable, spreading the signal over a very wide bandwidth instead of squeezing it into a narrow range as in SSB. Modern spread-spectrum modulation does just that, taking information at thousands of bits per second, encoding it using sophisticated error-correcting digital codes, and then spreading the signal over millions of Hz. It is now possible to approach Shannon's capacity limit and establish highly reliablecommunication even at low signal-to-noise ratios. NASA uses spread-spectrum modulation to communicate with its deep-space probes. Most earthbound communication systems also utilize digital encoding techniques with or without spread spectrum. The days of analog TV broadcasts arefinally over; television was fully converted to digital format in the early 21st century.

The problem is that modern encoding techniques, designed to make communication more secure and less prone to interference and jamming, make it virtually impossible to decode the signal without a filter that is digitally matched to the encoder. In fact, Type I signals from aliens who employ spread-spectrum modulation might be buried far below the ambient noise so that listeners like SETI, who lack the necessary matched filters, couldn't even tell those signals are there.

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