Have astronomers observed any phenomena that could be produced by extraterrestrial civilizations?
I was just wondering if there are any unexplained or weakly theorized high-power activities (like Gamma Ray Bursts) that could possibly be caused by activity of other advanced civilizations in our universe? Forgive me, I know that the question is pretty "out there", it's just that I was wondering if anyone was looking for signs of extra-terrestrial life in this manner.
There's no way we can rule it out completely, of course. However, I think there are several reasons why this is probably not the case.
First, all the astronomical phenomena we see tend to be "broadband," which basically means that they emit light containing a lot of different colors. By contrast, many of the signals that humans produce that would be detectable from outer space are narrowband, meaning that the light contains primarily one specific color. This is especially true of signals used for communication -- for example, when you listen to a radio station you have to tune your radio to a very specific frequency (i.e. "color" of the radio wave) in order to receive the signal that is being sent, and if you change it a bit you will no longer detect that station's broadcast.
Narrowband signals are the best way to concentrate as much of the signal power as possible into a detectable regime, which saves energy and makes it easier to discern the signal from background noise. So it is likely that extraterrestrial civilizations would also use narrowband signals to communicate, and none of the astronomical phenomena we have seen fall into this category (the SETI@home project, in fact, only searches for narrowband signals, and that's one of the ways they would be able to distinguish any "real" extraterrestrial signal from a naturally occurring phenomenon).
On the other hand, it's possible that extraterrestrial civilizations have receivers that work differently than ours do (so that their definition of "narrowband" would be different from ours), and it's certainly possible that they might produce signals not intended for communication that we would be able to detect.
Second, the astronomical phenomena we observe tend to be extremely energetic events, and it is difficult to imagine what an extraterrestrial civilization might be doing to use that much energy that quickly. Gamma Ray Bursts (GRBs) in particular are among the most energetic events in the universe, typically releasing around 1051 ergs of energy in less than one minute -- this is equivalent to taking a planet with the mass of Jupiter and completely converting its mass into energy (as in a nuclear bomb) in an extremely short period of time! Why extraterrestrial civilizations would want to make Jupiter-sized planets completely disappear and send the energy beaming off into the universe is a mystery to me (although I certainly can't claim to have any idea how they think).
Third, the astronomical phenomena that we see tend to fall into distinct categories, and within each category there are many different objects whose properties are the same, except for statistical fluctuations. For example, there have been thousands of GRBs discovered, and if you look at a histogram of their durations (how long each event lasts), you see that they cluster around a particular value with some statistical variation around that value (actually, in the case of GRBs there are two values with statistical variation around each, but that's because what we call "GRBs" are actually now thought to be two different types of objects).
This may be speculation, but I don't think advanced civilizations are so predictable that they would all evolve towards the point where they decided to do the same, extreme event (such as obliterate one of the planets in their solar system). I think we see evidence of this on Earth, where human beings who were initially isolated from each other developed such different cultures all around the world. To me, the fact that the different astronomical events we see are so similar all over the universe is evidence that they have a simple physical explanation, rather than being due to the activity of living creatures and their vastly more complex chemistry and behavioral patterns.
Interestingly enough, when the first radio pulsar was discovered in 1967, the idea was briefly considered that the periodic signal might have been from an alien civilization. As described by Jocelyn Bell (the graduate student who made the initial discovery), at first she was jokingly upset about the possibility ("Here was I trying to get a Ph.D. out of a new technique, and some silly lot of little green men had to choose my aerial and my frequency to communicate with us"), but once she discovered a second pulsar, she realized immediately that it was very unlikely to be the work of an extraterrestrial civilization. In her words: "I left the recording on Tony's desk and went off, much happier, for Christmas. It was very unlikely that two lots of little green men would both choose the same, improbable frequency, and the same time, to try signalling to the same planet Earth."
We now know that pulsars can be well-explained as rapidly rotating neutron stars, and since the initial discovery astronomers have found over a thousand more of these objects. I think it is very likely that any new categories of suspicious objects which are discovered in the future will follow a similar history as pulsars did.
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