Will We Ever Contact Aliens? A Physicist’s Analysis (Part II)

space_laserIn my previous post, I calculated how much power it would take to send out a signal that would be detectable at our nearest neighboring star, Proxima Centauri. It was equivalent to the output of a large power plant. I then pointed out that by the time this signal reaches any stars that are farther away, it will have dissipated to an undetectable level. I concluded by promising to examine other possibilities in the next post. Here they are.

Rather than sending out a signal in all directions, a better strategy would be to use something like a laser, focusing the signal into a beam that doesn’t spread out much over large distances. Then the signal would still be fairly strong when it reached some distant planet.

Whoa! Brilliant idea. Problem solved, right?

Unfortunately, no. The problem with this is that you can only point a laser at one star at a time (or a relatively small group of stars). No one is sure what the chances are that a given star has a life-supporting planet orbiting it, but one thing people agree on is that it’s a pretty tiny probability. And so we run into the problem that if we focus our laser on a single star, chances are almost zero that it’s a star with life orbiting it. In other words, our signal is almost certain to go undetected.

Calculations based on the popular Drake equation suggest that there’s most likely quite a bit of intelligent alien life out there in the universe. That sounds pretty exciting. But it leads naturally to a very obvious question, which Enrico Fermi is famous for asking: “If that’s true, then where is everybody?”

The contradiction between the conclusion that the universe is almost certainly teeming with life and the fact that we haven’t seen any evidence of extraterrestrial life has become known as Fermi’s paradox. Is it just the case that there isn’t life out there, or is there some other resolution to this paradox? People have been asking this question for some time.

Well, our simple calculation suggests that the answer to Fermi’s question is rather simple. “Everybody” else out there is in the same situation we’re in: floating on a rock so isolated by its distance from the rest of the stars that it’s impossible to travel to or communicate with even the nearest neighbor. And so it may turn out that even if the galaxies are positively teeming with life, we might as well be alone for all practical purposes.

But wait. There’s at least one thing we haven’t considered. Using spectroscopy, it may be possible to identify planets that are likely to have life on them. We can do this by analyzing light from distant planets to determine what kinds of chemical compounds are there (that’s what spectroscopy is). Assuming that extraterrestrial life is based on familiar chemical processes, the detection of organic molecules on a planet would indicate a good chance for the presence of life. Then we can aim a laser at it and try to say hello.

There are still two problems here. First, even among planets that have organic molecules on them, only a tiny percentage could be expected to have intelligent life forms that are advanced enough to detect and respond to such signals. And second, most of the candidates are much farther away than our nearest neighbor, which is already over four light-years away. That means that it would take years for any aliens to receive our signal, and then just as many years for us to receive a response.

Even within our own galaxy, the majority of stars are not just a few light-years away, but thousands of light-years away. Thus, if someone does detect and respond to a signal that we send now, the response probably wouldn’t arrive in time for our grandchildren to receive it. In fact, by the time the response arrives, it’s likely that no one on earth would even remember that we sent a signal to begin with.

And so my conclusion about interstellar communication is, sadly, the same as my conclusion about interstellar travel: Barring some truly revolutionary discoveries in physics, it will remain nearly impossible. That is probably why we have never heard from anyone, even if there are countless alien civilizations out there.

Nevertheless, I will still examine in my next post what might happen if aliens ever do happen upon Earth.

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