I recently receive the following e-mail question:
Got a question for you: do you consider the Search for Extraterrestrial Intelligence to be science or pseudoscience? I recently got into an online debate and found myself in the minority because I maintained that the central thesis — that if intelligent life exists somewhere out there in the greater universe, we would be able to recognize it based upon patterns in radio waves — is not falsifiable.
It would seem to me that the only way to truly falsify SETI, we’d need to map quite literally every body in the universe and rule them out one by one and say that they don’t have anything there in terms of extraterrestrial intelligence. Unlike other complex hypotheses that are limited by available technologies, I’m not convinced that the task of mapping the universe is even possible, even with a sufficiently advanced technology.
I have received some version of this question many times over the years, always by people who are trying to be skeptical and apply what they have learned about the differences between science and pseudoscience. It therefore seems like an excellent opportunity to explore this important issue.
SETI is the search for extraterrestrial intelligence and refers to a number of programs over the years that have listened for intelligent radio signals from space. NASA for a time had a SETI program, but this was canceled in 1993. The SETI Institute now carries on this endeavor with private funding.
Whether or not you think SETI is a good idea, is it real science? The issue here is how do we define science. One major criterion for science is that a scientific hypothesis must be falsifiable. This, however, is not strictly true and is an oversimplification.
A hypothesis does not need to be falsifiable in the sense that it is possible to be proven 100% wrong. All that is necessary is that the hypothesis is testable – there is some observation or experiment that you can perform that will make the hypothesis more or less likely to be true.
Sometimes a hypothesis can be stated in such a way that a single counter-example will disprove it. The now classic example is that all swans are white. A single non-white swan will falsify this hypothesis. How thoroughly do you have to search, however, before we can conclude that all swans are white? Would you have to simultaneously survey every swan in the world? If it takes 10 years to conduct a thorough survey can you be sure that a black swan was not born in the last 10 years?
The problem here is in thinking in absolutes. Scientific theories, rather, often deal with probabilities and are not necessarily wrong when exceptions are found. In the case of swans, the more thoroughly we look for non-white swans without finding them the greater our confidence is that all swans are white, and we can certainly conclude that most swans are white and that any exceptions are rare.
Of course this is the classic example because black swans were discovered in Australia.
With regard to SETI the hypothesis is this – life arose spontaneously on Earth, there is nothing special about the Earth and therefore it is possible for life to arise elsewhere in the universe. It is possible that some of that life evolved intelligence, and some of that intelligence developed technology. One method for a technological civilization to communicate across stellar distances is through radio signals. Therefore, perhaps the Earth is being bathed at this moment with intelligent radio signals from other worlds.
Every link in that logical change is perfectly reasonable. The best way to test that hypothesis is to simply look. Looking is part of science. It is a valid way to test many hypotheses. It is not necessary to be able to prove that there are no intelligent radio sources anywhere in the universe in order for this endeavor to be properly scientific.
Like the search for non-white swans, a single example is all that is necessay, in this case to prove that the hypothesis is valid. The more we search without success the more information we will have about the density of radio-transmitting civilizations in the universe. This survey will never be complete, but that is irrelevant.
The broader issue here is the importance of understanding that science is not one method but a collection of various methods. It is important to a proper understanding of science not to have an artificially narrow view of what counts as science. As long as there are hypotheses that are testable with empirical evidence, you are doing science (whether or not you are doing rigorous high quality science is a separate issue).
Frequently the opponents of science try to limit what counts as science in order to deny legitimate science (it is a major tactic of denialism). To be clear, the e-mailer is not doing that here, and he states later in his e-mail that he supports SETI as an endeavor.
It is, however, a common ploy of creationists. They try to deny the legitimacy of all historical sciences because what has happened in the past was either not directly observed or cannot be run as an experiment in the lab. Historical sciences, however, can still make observations and generate hypotheses that can be tested with further observations. There is even a field of experimental archaeology that conducts experiments to test hypotheses about how things were done in the past.
So, yes, SETI is legitimate science. It is searching for evidence that directly tests a very interesting hypothesis. The fact that it can never prove a negative version of that hypothesis (there are no intelligence radio sources in the universe) is irrelevant.