PBS Space Time

Aliens.

We did a prior episode about whether it makes sense to believe they exist.

But for today, let's assume that they do exist and that we want to meet some.

How would we let them know we're here?

[THEME MUSIC] Let's leave aside for the moment the fact that signaling aliens might not be a good idea.

If we wanted to do it, how we go about it?

Today, I'm going to summarize three different signaling methods that have been discussed in the literature along with their pros and cons.

I make no claim that this list is comprehensive.

If I've missed something major, you can let me know in the comments.

But if you're going to bring up probes, which you can, you should know that I'm deliberately ignoring that option.

It's too slow, it's too hard to do in bulk, and it's too relying on aliens being able to decipher the content of the probe.

I want to discuss signaling without the added complications of messaging.

Let's start with the radio.

Contrary to popular misconception, it could be very difficult for aliens to pick up the radio and TV transmissions that we've been leaking into space since around World War II.

The growing consensus is that, even if nearby aliens have space-based radio receivers with the total area equivalent to a large city pointed right at Earth full time, our leakage would be hard to detect, let alone decipher.

You can see the links in the description for papers that have analyzed this.

So what would it take to have a radio beacon that operated fairly continuously and could be detected?

Well, a 2010 paper linked below by Benford, Benford, and Benford-- it's three physicists all related-- analyze the physics and the economics of this question.

Now, I'm simplifying a bit here.

But they suggested that the optimal solution would be a phased array of many radio telescopes sending narrow pulses of high-powered microwaves at frequencies of 10 gigahertz or higher, sweeping the whole sky about once a year or so.

Now, depending on the available and affordable transition power-- and we're talking gigawatts here minimum-- an array like this could be detected from a few thousand light years away.

But the cost would be enormous.

100 billion US dollars just to get an omnidirectional transmitter built or only about 10 billion if you aimed it in the galactic plane alone, where, for reasons I can't get into here, many people think life is more likely.

Now, ongoing cost for operation, personnel, and empower would add billions more dollars.

So while large distances with microwave transmission are possible, it would require a massive ongoing investment of humanities resources to go loud enough and last long enough to go beyond the mere cosmic blade.

So let's move on to method two, lasers.

Nowadays, lasers can produce high-power ultra-short pulses that can outshine the sun for less than a nanosecond.

Nothing natural pulses with stellar brightness for a nanosecond at a time, so laser pulses would look distinctly artificial to our Vulcan friends.

They're also detectable.

Even we can notice pulses that short with photomultiplier tubes, although you need repeated flashes to know that you don't have a false positive from a cosmic ray or something.

Attenuation from interstellar dust makes the range for optical lasers less than that for microwave beacons.

You only get about 1,000 light years or so before it becomes too dim.

But at those distances, a laser's spot size would only grow to around the size of Jupiter's orbit.

So you could target the pulse to individual star systems and avoid wasteful transmission into empty space.

With a lot of lasers, you could even flash several star systems per second, re-aim, and repeat over and over.

Now, we could probably implement this in a few decades.

But as with radio, the continuous energy usage would be very expensive.

And that is the major overhead of the options we've discussed so far.

You have to keep transmitting for a very long time to increase the odds that someone will see the signal or there's not much point.

But there's another signaling option that wouldn't require us to transmit anything at all.

In a recent video, we discussed the transit method for finding exoplanets.

Using something like NASA's Kepler satellite, astronomers stare at 150,000 or so stars at once continually for a few years, looking for the tiny dips in starlight that occur when a planet transits in front of its host star along our line of sight.

This is actually how the majority of exoplanets have been identified.

Now, if Kerbals are doing astronomy-- and that's our target audience-- they're probably also looking for planets.

And they're probably using, among other things, a device like Kepler to do it.

Why?

Because even though there are other ways to detect exoplanets, the transit method is the only one that scales well.

With a few Kerbal Keplers, the Kerbal astronomers could look at hundreds of thousands or millions of stars at once.

All right, here's where it gets interesting.

What if, instead of a planet, the object that's transiting in front of its star looks like this?

Or like this?

Before Kepler was even launched, French astronomer Luke Arnold wrote a paper-- link below-- showing that the transits of objects like these could be distinguished from those of round planets.

And a sequence of louvered slits would look especially distinct and artificial, all without any transmissions or energy requirements.

Now, these objects wouldn't need to be the same mass as a planet.

But they would need to be the same size.

So I'm picturing an ultra-thin, ultra-lightweight, opaque material that we could launch into solar orbit and then unroll like a sail.

That's not doable today.

But in a few hundred years, maybe.

Now, we would need to tweak the orbit occasionally due to solar wind and radiation pressure.

But if that could be automated or if space maintenance missions inside the solar system become easier for us in the future, then this setup could serve as a really long-lasting billboard.

Now, like the lasers, billboard transits could only be seen within a few light years of earth or maybe a couple thousand if the Kerbal Kepler instrument is really sensitive.

So to signal aliens very far away near the galactic center, you'd still need high-powered microwaves.

But all in all, a billboard seems like a distinct, cheap, ongoing signal that aliens could notice just by doing exoplanets astronomy, even if they aren't looking for other aliens.

I want to finish with a comment.

Suppose we conclude that billboards are the most efficient, most noticeable way to signal aliens, or at least one of them, that aliens might apply Princess Bride poison logic.

They might think that we'd think that they'd think that billboards are a good signal so that we would go looking for them.

In which case, in order to catch our attention, maybe alien have put up billboards.

To my knowledge, no one has actually analyzed exoplanet transit data to look for these signals.

But the Kepler data set is public.

We put a link in the description.

Everyone has access, including SETI.

So besides looking for radio signals and laser pulses, which SETI does now, should SETI also be looking for geometric alien billboards orbiting nearby stars.