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The Real-Life Search for Aliens

Absorption spectroscopy, from Coel Hellier's blog post

Diagram explaining absorption spectroscopy, from Coel Hellier’s blog post

In a previous series of posts, I presented a simple analysis of our chances of ever contacting aliens. The foundation of my argument was that any planet that does harbor intelligent life will almost certainly be too far away, not just for us to reach by spaceship, but even for us to contact with any type of signal. Thus, although the universe may be teeming with life, we’re never going to see any of it (except what’s right here on Earth).

Today, I ran across this blog post by Coel Hellier, an actual scientist who searches for the signatures of extraterrestrial life (though I think that’s not the only goal of his work). In his blog, he does a great job of explaining in layman’s terms how real-life searches are actually carried out, from identifying stars that have planets orbiting them to analyzing the chemical composition of those planets’ atmospheres. I encourage you to read his original article, but I’ll give a brief summary of it below in case you’re too lazy to click a mouse.

  • We can identify a star that has a planet orbiting it by the periodic dimming of the star as the planet passes between us and the star, blocking part of the star’s light.
  • The planet’s size is determined from how much of the star’s light it blocks.
  • Its mass is determined by measuring the redshift of the star’s light when the planet is behind the star (pulling the star away from us) or the blueshift of the star’s light when the planet is in front of the star (pulling it toward us).
  • From size and mass, we get density, and from density, we get a rough idea of what chemical elements are most abundant on the planet.
  • We can even determine what molecules are present in large quantities in a planet’s atmosphere by looking at the fringe of light that passes through the planet’s atmosphere as the planet passes in front of the star. Different molecules will absorb light from different parts of the spectrum of the star’s light, so all we have to do is compare unimpeded light from the star to light that passed through the planet’s atmosphere to determine what types of molecules are in the planet’s atmosphere. This is called spectroscopy.

Coel writes that, using spectroscopy,

we are beginning to be able to detect the atmospheres of extra-solar planets, despite them being hundreds of light-years away. If we can detect molecules in the atmospheres of exoplanets then, in principle, we might detect “biomarker” molecules that indicate organic activity (such as free oxygen). Thus it is realistic that, within a couple of decades, we will have found other Earth-like planets that we know to bear life.

The beauty of this approach is that with an array of sensitive telescopes that can record light from stars all over the sky, we will be able to use automated analysis software that will process the data from thousands or millions of stars in a relatively short period of time. Thus, it might actually be possible to verify the presence of life on other planets in a relatively short time frame. Exciting!

Having said all of that, it is still the case that even if we do find planets that harbor life, they will almost certainly be hundreds if not thousands of light-years away from us. We can try sending them a signal, but even on the off-chance that the life there is advanced enough to detect and decode our signal, it would be hundreds or thousands of years before their reply ever reached us.

Thus, I maintain for now my pessimistic conclusion that we’ll never contact aliens.

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.

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

OLYMPUS DIGITAL CAMERAIn a previous post, I calculated how much energy it would take to travel to Proxima Centauri, the nearest star outside of our own solar system, within a reasonable amount of time. The results were rather discouraging; barring any monumental revolutions in physics, energy considerations alone suggest that interstellar travel might be downright impossible.

So let’s set aside thoughts of space travel and consider a far more modest project: merely broadcasting a signal to the stars (in hopes of contacting intelligent alien life, assuming there is any). Surely, simply sending a signal would be much easier than transporting a massive spaceship over such a long distance. So let’s see what it would take.

First, we need to define the problem.

Suppose our goal is just to send out a signal that is detectable on Proxima Centauri, our nearest neighbor. And suppose that we have at our disposal a transmitter that sends a uniformly intense signal out in all directions. The question we wish to answer is how much energy it would take to generate such a signal.

To complete the setup of the problem, there are two things that we need to specify:

  1. What exactly does it mean for a signal to be “detectable”?
  2. What is the nature of this signal? (Visible light? Radio waves? X-rays?)

Let’s tackle the first question first, and let’s be optimistic about aliens’ signal-detection capabilities. We’ll assume that aliens can detect our signal if it consists of at least one photon (a particle of light) per second flowing through one square meter of area when it reaches the aliens’ location. This is actually an über-weak signal, but as I said, we’re going to be optimistic here.

The second question is important because the amount of energy carried by each photon depends on the nature of the signal. If it’s a radio wave, which is low-frequency, then the amount of energy required is relatively low. Microwaves, visible light, and X-rays have higher frequencies and would require more energy. To keep the requirements low, let’s assume our signal consists of radio waves; and to keep the numbers simple, let’s suppose these waves have a wavelength of one meter.

We now have enough information to solve the problem.

Since the signal is being sent out uniformly in all directions, it is essentially a sphere that’s expanding at the speed of light, with the Earth at its center. Assuming the photons are uniformly distributed over this sphere, and keeping in mind that we want there to be one photon per square meter, we simply need to calculate the surface area (in square meters) of this expanding sphere when it arrives at the destination, Proxima Centauri.

Well, Proxima Centauri is 4.24 light-years away from us, so when the signal arrives there, the radius of the sphere is 4.24 light-years. Calling this distance R, the formula for the surface area of a sphere tells us that a total of 4*pi*R^2 square meters must be covered. And if we want one photon to pass through each of those square meters per second, that’s also the number of photons per second that our transmitter must send out. (Note that we have to convert R to meters.)

That’s 2×10^34 photons.

The amount of energy per photon is E = hc/L, where h is Planck’s constant and L is the wavelength of the radio waves (and c, of course, is the speed of light). So the total amount of energy (per second) is just hc/L times the number of photons, which works out to be 4 billion joules. Since that’s the amount of energy per second, the amount of power is 4 billion Watts.

All right. So what?

Well, that’s about the output of a large power plant, which is a lot of power to put into one signal. And that signal will thin out to just one tiny photon per square meter per second by the time it reaches our nearest neighbor in the universe. In reality, it would have virtually no chance of being detected, even if someone were looking for it with highly advanced technology. Farther away, the signal would be even weaker.

So the sad truth is that even if we devoted a huge amount of energy into attempts to contact extraterrestrial life forms, our signals would dissipate to undetectable levels long before they reached any of the distant planets that might harbor life. We can conclude from this analysis that a radio transmitter that sends a signal in all directions just won’t cut it.

There are other possibilities, though.

To be continued …