Tuesday, February 9, 2010

Part 7: Book Review - Prospects for Interstellar Travel

In this continuing book review series we look into Chapter 11 where the author reviews the search for other intelligent beings and explores the related issues this has with interstellar travel. Whether life can begin on other planets is still an open question as so far there is no strong evidence to suggest so. The author points out that despite all the research papers published on this subject, conferences held etc, "The work has been informed speculation due to the lack of experimental and observational results". However given what we do know about life here so far, a number of interesting educated guesses can be made on the possibility of life elsewhere.

The Search for Extra-Terrestrial Intelligence (SETI) is a vast subject incorporating many disciplines including: "astronomy (which stars might have planets), planetary studies (which planets might have conditions for life), biology (what sorts of biospheres can evolve on planets), a mix of biology, sociology, and philosophy (what kinds of intelligent beings can arise and how would they act), technology (what means would "they" use to send and receive messages, or themselves, across many lightyears), mathematics (expected to be a common language), and other fields (as we define them). SETI includes Communication with Extra-terrestrial Intelligence (CETI) and can include sending messages as well as receiving them."

Interest on the possibility of other beings has been around for a long time looking back in our history, authors going back from ancient Greece to 16th Century Giordano Bruno for eg. [CI: my favourite quotes from Christiaan Huygens and part of Edward Young's Night Thoughts followed below]

Several terms over the years have been used to refer to extra-terrestrials, such as "ETs" (from the movie), "humanoid" (somewhat assuming human like two legs, two arms and a head) and "aliens" (blood thirsty bug eyed monsters). The author uses the term intelligent beings or simply "beings", implying: "creatures who have evolved to the level of recognizing their own existence and therefore likely to study their place in the Universe ... We assume intelligence also includes rational thinking (which some humans practice some of the time), problem solving, artistic creation, development of science, technical competence (tool use expanded to building, flying, computing, transmitting messages, etc.), complex societies with evolving cultures, and more." The study of life as it might arise elsewhere in different conditions and its evolution not necessarily leading upto intelligent life, is known as exobiology. So far we are only familiar with carbon-based lifeforms as on Earth. Carbon based molecules are very versatile for life however there is no basic proof showing that life cannot use other chemistry .

Mention is made of the Drake equation (shown above) which makes an attempt to predict the number of civilisations in our galaxy at any given time which have achieved technology for radio communications by multiplying a series of probabilities together which include the number of new stars per year, those with habitable planets, the probability of life on those planets, the probability of that life achieving intelligence, the probability that those beings develop the ability to send and receive interstellar messages and the average lifetime of a technological civilisation. As pointed out, most of these factors are very difficult to estimate and the result of the equation may be no better than a wild guess however the number of stars formed per year in our galaxy is given to be between 1 and 10 per year. Some estimates given on the number of civilisations in our galaxy range from 10 for short lived ones and millions for long lived ones giving a geometric mean of 10,000 civilisations [CI: see also this interesting paper].

[CI: we haven't been able to leave the Solar System to explore possible habitable exoplanets to find closeup evidence to make more informed guesses. We would need to send out starships, interstellar probes or large space telescopes could provide some indirect evidence, however given the number of galaxies in our observable Universe and the number of stars per galaxy (in the hundreds of billions, upper guess 400 billion for the Milky Way alone) and given that astronomers are finding more and more planets around stars (ie planetary formation may be a common phenomenon around certain stars) and given that some of the base molecules required for life as we know it have been confirmed in the interstellar medium, my point of view on this is despite the fermi paradox (explained later), it is highly unlikely that the only lifeforms (intelligent or otherwise) reside only on Earth.]

An average density of civilisations in our galaxy can also be guessed and notes that they are less likely to be near the galactic core because of the shortage of heavier elements and the possible violent black hole at the center: "For galactic volume about 10^13 CLy (Cubic Lightyears), excluding core, each civilisation is surrounded by 10^9 CLy, and the average spacing is thus about 1000 Ly, or about 1/6 the thickness of our galaxy." which would be problematic for communications and any plans to meeting them in person.

[CI: Photo: A panoramic view of our galaxy with closeup towards the center. Anyone out there? (ESO)]

The author then moves on to discuss the Fermi Paradox or "the great silence" and explains the key link between other beings and interstellar travel: "Life has been possible in our galaxy for so long that some other civilisations should have been around a long time and engaging in widespread travel, if only by probe. Yet they are not known to have visited us." and hopes that this paradox is an apparent one and an explanation will eventually remove this paradox. Many reasonable and wild explanations have been put forth as to why no contact has been made so far and the author outlines many of them. Some have suggested that "absence of evidence is evidence of absence", others that habitable planets may be rare or life may not start easily on habitable planets or intelligent life might be a rare step in evolution, technical civilisations even rarer or some civilisations may not want to or are unable to venture out to other star systems or we are in a region of the galaxy where no beings have ventured or sent messages or we already have been detected and unknowing to us a fleet of starships are on their way to Earth or because of a "Codex Galactica", Earth has been quarantined by "them" and no contact allowed, or etc. etc.

Searches for evidence of other beings (SETI) are ongoing and have been mostly in the radio spectrum using large radio dishes and very sensitive receivers searching for any meaningful signals buried in the galactic noise. Some of the technology outlined was already discussed by the author in Chapter 7 and as pointed out SETI searches are difficult: "The million directions in space (or million stars) at which to point a receiving dish, the billions of one-cycle channels to be examined, the short time available to "listen", and the rapid decrease of power with distance are some of the factors making search very difficult. Power is not the main problem. Presently we have a receiver (using the Arecibo 300m dish) which could detect messages sent with a similar dish from 15,000 Ly away, and further if they use more power than Arecibo engineers have." and extensive searches of several hundread stars have been conducted at and between 1420 Mhz, 21 cm wavelength (hydrogen) and 1666 Mhz, 18cm wavelength (hydroxyl molecules) the band being referred  to as "the water hole", the logic being the two combine to form water which is a necessity for life, other beings might decide to search or transmit radio signals in this band which is also relatively quiet.

It is pointed out that several intelligent signals may be reaching us right now however we may not be looking at the right directions or frequencies and furthermore: "SETI has been thought of as a cheap alternative to interstellar travel (still requiring billions of dollars and much donated effort for a semi-thorough search) [a-Finney 1985]. But SETI would be useful in finding destinations for travel, and could be a program parallel to a starship program."

What if we receive an interstellar message? According to the author this would profoundly affect the world. Principles and protocols have already been prepared for initial action on confirmation of the message. Basically scientists have agreed to cooperate in the confirmation then announce the evidence to the public fully and openly "presumably this includes all data on the content of the message even though deciphering it early is unlikely." [CI: for an interesting movie on all this see Contact]. If it comes from a star nearby, it is pointed out that a serious study of traveling there would begin and many subsequent messages planned in preparation.

So far our main SETI work has has been listening however one message has been sent intentionally from Earth in 1974 using Arecibo. The message was transmitted to the Hercules Globular Cluster M13 25,000 Ly away at an effective power of 3 TW [CI: M13 shown below with the depicted message and a relevant quote from James Jeans, see also this 35th anniversary message sent last year from Arecibo.]

Interesting point made by the author on p268:  "Considering the time and cost of a full search in the radio spectrum, not to mention other possible channels and sending our own messages, interstellar travel is not necessarily slower or more costly in the long run." Further considerations are also given to search in other parts of the spectrum such as laser signals (10 KJ pulses would outshine stars). Further studies are also looking into the possibility of detecting other possible signatures left by large and advanced galactic civilisations such as excess heat for a given class of star due to a hypothetical Dyson sphere partly surrounding the star, such gradual change over time may also indicate their space engineers have been busy working on the structure.
What would happen if visitors actually came to Earth? Apart from the expected mass histeria, scientists would get real busy with almost every field of study deeply affected: "visits with the visitors would need to be leisurely to treat them kindly and respectfully, and new complex programs and procedures would be needed to allow many people fair and full access to the visitors and their knowledge." hopefully they won't be hostile beings with sinister purposes in mind and keen to exchange knowledge with us.

The author then moves on to discuss habitable planets which are defined: "(somewhat anthropocentric) as planets in the pleasantly warm biozone around a star with adequate water, breathable (to us) air, stable orbit, low eccentricity, reasonable rotation and axis tilt, absence of very violent weather and high radiation (including UV), and other factors." The interstellar medium must be relatively stable with very little instellar dust clouds passing by (which would block the precious sunlight). Also mentioned are the dangers of closeby supernovae which can increase radiation levels by 1000 to 10,000 if within 30 Ly and would be harmful for developing life. One study suggests that a supernova is expected in any spiral arm galaxy every 100 million years or so. Long term stable stars (with life spans in the billions of years) are also required which are stars of

   [CI: Diagram: Theoretical habitable zones for various stellar masses.]

types F, G, K and M spectral classes which make up 90% of all stars. A, B and O type stars are more massive than our Sun and are said to change luminosity too rapidly with fast rotations and unstable habits not very good for a stable biosphere for life which requires some stability in the system over billions of years. The past ice ages here alone are said to be due to variations in the Earth's orbital distance and orientation from the Sun by only a few percent. Current theories also suggest that planetary formation from collapsing nebulae can only form around a single star however some research has suggested otherwise: "A system with two or more stars would not permit stable orbits for planets unless the stars are far apart, at least four times the distance to the best place for habitable planets."

[CI: Photo: The Orion Nebula with closeups of planetary
systems in formation around their host stars. (APOD)]

Other interesting points mentioned: "Low-mass planets do not have the gravity to hold the lightest gaseous elements hydrogen, helium, nitrogen, and oxygen, especially if they are warm. A small difference in gravity or temperature has a large effect on the time over which gas molecules can escape [recall molecular speed in Appendix B for 1.2]." and many other considerations are given as to the possiblity of habitable planetary formation. Taking account these various factors and considering planetary formation models, the author gets back to the Drake equation and mentions that the number of stars with planets could be 1/2 but the number of habitable planets per such star (not including M stars) could be less than 1/10 and some have suggested a factor of 0.001.

The origin and evolution of life is also discussed, how life begins on a planet is very important to whether intelligent life eventually develops or not. We also don't fully understand life as is on Earth and have only one biosphere to study however: "The self-regulation of the biosphere would be an important explanation of how a planet can remain habitable over long times and extend the chances for life and intelligence. Indeed, the dominant form of life on Earth, by mass and by global effect, is micro-organisms, mainly bacteria. Bacteria are a large source of oxygen. Plankton in the ocean surface remove much CO2 and release dimethyl sulfide that helps nucleate clouds (which helps reflect excess solar input)."

How life began on Earth is still uncertain in detail however it is mentioned that organic molecules may have originated from comet collisions and points out the Miller-Urey experiment which produced amino acids and other biochemicals with UV radiation in a simulated primordial Earth ocean and atmosphere [CI: some more recent studies have also indicated that early genetic code was based on a smaller number of amino acids (only those available in prebiotic nature), see this interesting paper and the Last Universal Ancestor.] Spectral analysis of the atmosphere of exoplanets using sophisticated telescopes may give an indication of life on those planets. Some molecules to look for would include oxygen, methane, ammonia, nitrous oxide, dimethyl sulfide (from Planckton as explained above), terpene vapours, chlorofluorocarbons, and other gases emitted by lifeforms would be an atmosphere not in equilibrium. NASA's exobiology program is studying the above and many other factors including the possiblity of life on Mars, Titan and other places.

The author also points out that several studies have also looked into the possiblity of whether life can develop in environments other than luke warm water: "A few other liquids besides water (e.g. ammonia, oils, silicates) might be good solvents for the complex chemistry needed for life at other temperatures from cold (-50°C) to very hot (1000°C)." however notes that no experiments have been succesful in making interesting chemistry with other base multi-bonding elements such as silicon which could provide the basis for complex "organic" structures at higher or lower temperatures. Several moons in our Solar System do have interesting compounds including liquid methane, ammonia, nitrogen, sulfur etc however no evidence of intelligent beings have been detected on them.

Even if life turns out to be spontaneous on planets with the right conditions, according to some evolutionary biologists, the development of intelligent life (the next Drake equation factor) with humanoid bodies would be very unlikely however other scientisits suggest intelligence would readily appear within a few billion years however: "More generously, at present no one can say how likely a repeat evolution of humanoid form and/or abilities is under similar conditions. Biologists are familiar with the nearly endless number of variations and branchings in evolution and the lack of ultimate purpose or design (teleology) in biological nature. Thus they are inclined to a view of uniqueness for any life form. And almost any form may appear as time goes on and the environment changes." The chance of intelligent life developing communications technology is given for the next Drake equation factor between 1/100 to 1/10 and opinions are speculative.

Several speculative theories on the spread of civilisations throughout the galaxy are also outlined and some interesting numbers for thought are given: "If a high estimate of 10,000 civilizations are actively spreading in our galaxy in an epoch of 10,000 years and typically advance 100 Ly (or 0.01 Ly/year), then the chance of encountering one in a given region is only about 0.001, rather low." A scale known as the Kardashev scale lists three types of civilisations based on energy usage: I Earth scale, II stellar scale, and III galactic scale, we are currently below Type I. The author also mentions the interesting cosmological princliple: "the Universe must be physically (therefore biologically) about the same everywhere, that nothing special occurs in one place that cannot occur elsewhere. This principle is a modern version of the Copernican view in astronomy, that Earth is not the center of everything."

Several possible case scenarios of humans visiting a habitable planet without life, with primitive life and/or with intelligent life are then outlined. If the civilisation is capable of radio or otherwise communication, contact should be made first via this avenue to agree on the timing, method and purpose of the visit instead of just dropping by. If no communication is possible for them to space then a cautious landing may be needed for first contact however as discussed in a previous chapter by the author, landing has severe technical problems attached to it notably the problem of getting back to space. One interesting point raised on p284: "Just by their presence humans would begin altering local life." If a planet is found which lacks habitability, making a suitable biosphere suited for humans may be possible over a very long time. Just as was done on Earth, bacteria could be used to make an oxygen atmosphere, remove methane and CO2 and even aid in warming or cooling it, this is known as "ecopoiesis".
 [CI: Image: Terraforming of Mars one day a possibility?]

Many ethical issues would need to be considered if there are already lifeforms (intelligent or otherwise) on the planet. The chapter concludes with an interesting discussion on how humans might coexist with possible beings on their planet (hopefully peacefully) and discusses various issues including possible competition for resources, developing a human outpost on their planet while not interfering with them and even possibly continue to explore the galaxy with those beings to seek out other new life and new civilisations together.

In the next part of this book review series we'll look into the last chapter of the book where the author outlines other more speculative energy and propulsion ideas related to interstellar travel and some conclusions on the various topics discussed in the book.

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