Cosmology and our View of the World
Life in the Universe
Lead: Alex DeJong & Brent Tardiff
Summary by Austin Purves
Search for Life and Intelligent Beings in the Universe
Currently the only universally accepted instance of life in the universe is life on Earth. The planet Earth is about 4.5 billion years old, and life therein is about 3.7 billion years old. Life on Earth (and thus, all known and universally accepted life) is carbon based, uses DNA to encode a blueprint for its construction, and needs water. Certain properties of Earth make it unique in its ability to support the earth-based life. Earth has a nearly circular orbit in the “habitable” region around the Sun. Jupiter in its orbit and with its large mass is deflecting asteroids and comets that might otherwise collide with Earth. The Moon maintains appropriate tilt of the Earth’s axis preventing seasonal extremes. The active geological state promotes chemical processes necessary for life, such as the carbon cycle. It is possible that other planets with similar life-promoting properties might be found in other solar systems.
Present understanding of life has all been derived from observation of life on Earth. But the biogenic elements that are found on Earth are not necessarily unique to Earth. They are synthesized in stars and it seems reasonable to expect that they can be found on other planets. Furthermore, alternative biochemistries have been suggested. If these proposed alternative biochemistries could give rise to life, it is possible that a planet with a chemical composition unlike that of Earth could nevertheless support life.
So it seems there may be other planets with life-promoting characteristics and chemistries. The next consideration is the likelihood of life arising, given appropriate background conditions. Some say spontaneous appearance of life is impossible (without an intelligent designer or other initiator) while others believe it to be inevitable. Of course, there is a range of opinions in between.
If there is life on other planets, is it likely that it is intelligent? Or communicating? Stephen J Gould put forth the idea that the shaping of human intelligence is unique to the “shape” of Earth. Therefore it is not likely that there would exist anything similar to human intelligence except on a planet nearly identical to Earth. This suggests that if we do find intelligent communicating life, it is likely to differ from that on Earth in behavior, communication, interaction and appearance.
Simon Conway claimed that evolution will inevitably fill certain niches. Does human-like intelligence fill one of these niches? If so, that niche should be filled in any other instances of evolving life. It is clear that there are niches for woodpeckers, herbivores, and carnivores (to name some examples) but it is not clear that there is a niche for intelligence that will inevitably be filled by evolution.
The fact that life appeared relatively quickly after Earth was formed (~1 billion years) can be used as evidence that life is highly likely or inevitable given the right circumstances. Of course, life on Earth may be an unusual case. There is no way to conclude because there is nothing to which to compare.
The Drake equation gives us a way to estimate the number of intelligent communicating societies that would be likely to exist in a galaxy (such as the Milky Way). However, the equation contains quite a few free parameters whose values can only be roughly guessed. This is primarily because life on Earth is the only instance of life that can be studied, and it is therefore difficult to estimate how life might behave more generally. The factors in the Drake equation are as follows: aggregate number of stars to be considered, probability of any given star having a planet system, number of potentially life-supporting planets expected in each planet system, probability of life arising on an potentially life-supporting planet, probability that said life will become intelligent, probability that said intelligent life will be able to communicate as far as the other stars, length of time said communicating intelligent life will remain as such.
Currently active searches for life are underway. These include investigations of potential life in our own solar system, (for example on Europa, a moon of Jupiter), and searches for earthlike planets orbiting other appropriately sized stars.
There are also attempts to send messages that could be interpreted by other intelligent life. The Pioneer Plaques on Pioneer 10 and 11 show the location of Earth relative to several pulsars and also show the size of a human relative to the dish on the pioneer spacecraft. The Voyager Golden Records on the Voyager spacecraft contained audio recordings of human languages and music. The Arecibo message was transmitted containing numbers one through ten, atomic numbers of carbon and other elements, a graphic of the solar system, man, DNA, and a satellite dish.
There are also passive searches that involve “listening” with radio telescopes in hopes of receiving an alien transmission. Searches like these have been carried out by The Very Large Array and the Allen Telescope Array.
There are inherent difficulties in contacting extraterrestrial intelligent life. Large distances (Alpha-Centauri is 4.3 ly away) and language barriers are most obvious.
Possible indications of alien life have been claimed. A meteorite found in Antarctica from mars appeared to have fossilized bacteria. A strange red rain fell in India that contained what appeared to be extraterrestrial organic materials. There have also been claims of UFO sightings.
At the end of the presentation several questions were put forward:
Equation Calculator” can be used to experiment with the drake equation:
Plugging in the following numbers:
yields number of communicating civilizations N=0.01.
These numbers apparently yield a small likelihood (~1%) that there would be another communicating civilization in the Milky Way. These numbers however are very rough guesses. Of course, with the knowledge that is available about life in the universe, it is not possible to uniquely determine an appropriate value for each of these numbers. However, it may be possible to come up with values that are better justified...
One model for planet formation around a lone star (not a star system) is put forward. As a star is formed from the collapse of a large, rotating, non-rigid cloud of gas, it is necessary that planets are also formed from the same cloud of gas in order to conserve angular momentum. Without the formation of planets, all angular momentum would wind up in the star, which would then be spinning impossibly fast. In our solar system 99% of the mass is in the sun, while 98% of the angular momentum is in planets. Of course, this assumes that the dust cloud has some angular momentum to begin with, but the rotation of the galaxy implies that a minimum angular momentum that is enough to require planet formation. This implies that all stars formed in relative isolation will have orbiting planets. With this information, leaving the number of stars with orbiting planets at 50% seems reasonable.
Potentially life-supporting planets have been observed. But we can only see those ones that are easier to find. This makes it difficult to accurately estimate the number of potentially life-supporting planets per star. Supposing that each star with a planet system has one life-supporting planet is probably an overestimation and 0.5 life supporting planets per star with planet system may be a better estimate. This yields N=0.005.
The only known instance of life formation on a potentially life-supporting planet is of course life formation on Earth. In this instance, life formed very quickly once the appropriate background conditions were satisfied. This has been used as evidence that the formation of life is almost certain given appropriate background conditions. But this argument is somewhat sloppy as we cannot be sure exactly what was present/necessary at life’s conception. There could have been other factors present that are not obvious to us while being necessary for the conception of life. This is reason for expecting that life should not develop on all planets that we would call “potentially life-supporting.”
On the other hand, life has survived a series of climate changes on Earth. When life formed, the sun was 30% less luminous than it is now. And Earth’s environment has gone through severe changes since then, such as ice ages. But life has nevertheless proliferated, implying that life may be more robust and flexible than is initially evident. Furthermore, Earth’s life-supporting background conditions may be necessary for intelligent life such as human life to form, but in harsher conditions, simpler life forms such as bacteria may be able to thrive. This extends the flexibility of life even more. In any case, 50% chance of life formation on an earthlike planet is a nice middle-of-the road choice.
A definition of intelligence is necessary to estimate the likelihood of for life developing any intelligent life form. A reasonable delineation might be linguistic communication. But it is still difficult to estimate how widespread linguistic communication might be once life exists. Life on Earth developed at least one instance of intelligence; however this took quite some time. In principle, there could have been other instances of intelligent life which were exterminated by the dominant intelligence. With little justification, 10% seems like a reasonable choice.
The galaxy is generally homogeneous in composition. This means that wherever intelligent life develops in the galaxy, it is likely to have access to necessary resources, such as metals, to develop technology and long-distance communication. Increasing the probability of developing technology to 10% seems justified.
Based on looking at the proliferation of self-destructive pollution, war, overpopulation, etc. on Earth, it might appear that intelligent communicating life or Earth will destroy itself rather quickly after developing communication. A 1000 year lifetime seems reasonable to expect from intelligent communicating civilizations.
This yields N=25 communicating civilizations.
On the other hand, suppose intelligent communicating life on Earth does not become a victim of its own self-destructive tendencies, what will limit humans’ lifetime then? As the sun ages it gradually becomes more luminous, eventually bringing the surface temperature of the Earth too high for human life (after about 900 million years, according to Wikipedia). Drastic climate change could occur for other reasons, too, such as an asteroid collision with Earth, which may have been what caped the dominion of the dinosaurs at 160 million years. It appears that a lifetime of at least 100 million years is reasonable if humans are not to self destruct. This yields N=2,500,000.
Clearly it is difficult to estimate the lifetime of intelligent communicating life or Earth, much less a “typical” instance of intelligent communicating life in the galaxy.
A back-of-the-envelope calculation can tell us that with N=2,500,000 intelligent communicating civilizations in the whole of the Milky Way galaxy, there would be a distance between neighboring civilizations on the order of hundreds of light-years. It is possible to communicate over such distances using electromagnetic radiation, and it is far easier to conduct this sort of passive search than an active search for life on nearby planets. At the same time expecting to achieve communication with a nearby civilization by simply waiting and firing off random communication signals seems very optimistic and is based on a number of hopeful assumptions (the ones assumed in obtaining the number N=2,500,000). It may be more realistic to search for signs of life on nearby planets.
The signals used for communication across hundreds of light-years would have
to be much stronger than typical television or radio signals in order to stand
out above the cosmic microwave background radiation. And in order for them to
be distinguishable from random signal bursts due to natural causes, they would
have to contain a recognizable complex pattern. The Arecibo message is an example
of such a signal.
An important thing to consider is how much extra terrestrial life may or may not resemble “life as we know it.” Alternative biochemistries have been suggested, involving silicon instead of carbon for example. Silicon may not be quite as flexible as carbon. It has the same number of valance electrons, and it is sufficiently abundant, but it is not as likely as carbon to form water soluble compounds.
There is no obvious reason why the method of coding used in DNA is the only one that would work. It may be reasonable to expect that life found on another planet would have a very different way of encoding its “blueprint.”
Non water based life could perhaps be based on methane or ammonia instead. In order to remain liquid, methane or ammonia must be kept much cooler than water. This means that chemical reactions in methane based life may be very slow. Alternatively, the pressure would have to be kept much higher to keep the methane in liquid form. This could happen on a larger planet with a stronger gravitational field. Another potential problem with methane is that, unlike water, it is a non-polar molecule, making it not so good a medium for life. There would probably need to be something analogous to pH, which is an essential component in the biochemistry found on Earth. Another nice thing about water is when it ionizes, it yields protons. This may not be the case with methane and ammonia, they may yield something else. The most important properties of water are that it is a polar molecule and it is liquid at a very “comfortable” temperature.
If there are other forms of life on planets orbiting stars other than the sun, and two forms of life orbiting neighboring stars would like to meet each other, then there remains the substantial problem of interstellar travel. This would need to be addressed in order to have direct contact between the two forms of life. Sending a significant amount of mass at or near the speed of light is not practical due to energy requirements. Wormholes in principle could be used to aide interstellar travel, but actually carrying this out is far from practical at present. Another principle is traveling through higher dimensions; bending space to bring two distant regions closer together. Imagine an example of a piece of paper (two-dimensional space) being bent so that opposite corners are near to each other, then a hypothetical traveler could jump from one corner to the distant opposite corner by traveling a short distance through a third dimension. The problem with this is that bending space requires mass and the same prohibitive energy costs that are encountered in near-light-speed travel arise in this scheme as well.