# Can we “Communicate” with Tau Ceti?

It seems like four earth-like planets were found near Tau Ceti as per this article

A new study by an international team of astronomers reveals that four Earth-sized planets orbit the nearest sun-like star, tau Ceti, which is about 12 light years away and visible to the naked eye.

Assuming that the planets have a civilization as advanced as ours, let's say an exact analogue (not implying anything, purely hypothetical), could we make our presence known to them?

Or is this simply to great a distance, no matter how much energy we spent on the communication?

If not, how would it be done?

• Voyager 1 is about 30 light minutes away from Earth, but the 12 light years to Tau Ceti is a factor of 210,000. The DSN network antennas use a 20 kW transmitter for transmission to Voyager, for Tau Ceti about 884 TW terawatt would be needed. – Uwe Aug 11 '17 at 17:40
• @Uwe I think it's safe to assume a DSN or Arecibo- class receiving antenna at the other end, rather than the dinky little dish Voyager has. Voyager has a transmitter power of only 22.4 watts, yet the DSN is still able to receive telemetry from it. Scale that up and maybe sending a signal to Tau Ceti isn't that far-fetched. – Jim Lewis Aug 11 '17 at 18:30
• @JimLewis By my calculations, if Tau Ceti is 210,000x the distance that Voyager is, to receive a signal at the same strength as Voyager's (22.4 W) would still require the power of the transmitter at Tau Ceti to be almost one terawatt (9.8e+11 W)! – Mike Harris Aug 14 '17 at 12:14
• I took a stab some time ago at answering a very similar question on Worldbuilding, How far away would an alien civilization need to be for us to not notice them? -- While my answer does have a number of assumptions embedded in its calculations, I at least tried to be clear about what those assumptions were, use at least semi-reasonable values for those quantities, and show the math so that a reader can substitute their own assumptions for mine. I make no claims that the answer is perfect, but you still might be interested in it. – user Aug 14 '17 at 13:52
• @MichaelKjörling Am interested in any answers regarding this. – VSO Aug 14 '17 at 14:14

In my Astronomy SE question What (actually) does “peculiar periodic spectral modulations” refer to in this preprint? I link to the October 2016 ArXiv preprint Discovery of peculiar periodic spectral modulations in a small fraction of solar type stars. The abstract reads:

A Fourier transform analysis of 2.5 million spectra in the Sloan Digital Sky Survey was carried out to detect periodic spectral modulations. Signals having the same period were found in only 234 stars overwhelmingly in the F2 to K1 spectral range. The signals cannot be caused by instrumental or data analysis effects because they are present in only a very small fraction of stars within a narrow spectral range and because signal to noise ratio considerations predict that the signal should mostly be detected in the brightest objects, while this is not the case. We consider several possibilities, such as rotational transitions in molecules, rapid pulsations, Fourier transform of spectral lines and signals generated by Extraterrestrial Intelligence (ETI). They cannot be generated by molecules or rapid pulsations. It is highly unlikely that they come from the Fourier transform of spectral lines because too many strong lines located at nearly periodic frequencies are needed. Finally we consider the possibility, predicted in a previous published paper, that the signals are caused by light pulses generated by Extraterrestrial Intelligence to makes us aware of their existence. We find that the detected signals have exactly the shape of an ETI signal predicted in the previous publication and are therefore in agreement with this hypothesis. The fact that they are only found in a very small fraction of stars within a narrow spectral range centered near the spectral type of the sun is also in agreement with the ETI hypothesis. However, at this stage, this hypothesis needs to be confirmed with further work. Although unlikely, there is also a possibility that the signals are due to highly peculiar chemical compositions in a small fraction of galactic halo stars.

The idea here would be that you could modulate sunlight somehow (their method is a little complicated) in time (imagine giant vibrating mirrors or optical shutters) and this modulation would cause a spectral feature in wavelength as well time. You imprint your simple information as a tiny wiggle in the spectrum of the sunlight somehow reaching another star. That means whenever they look with their telescope and spectrograph, they will see that the blackbody spectrum is wiggly. They don't need a time base or to look at the right moment, a conventional spectral survey of stars might now it up.

Phys.org's Existing laser technology could be fashioned into Earth's 'porch light' to attract alien astronomers links to the recent paper in the Astrophysical Journal Optical Detection of Lasers with Near-term Technology at Interstellar Distances but a quick search for terms like "interstellar", "SETI" and "laser" will turn up many papers.

Phys.org says:

Clark started looking into the possibility of a planetary beacon as part of a final project for 16.343 (Spacecraft, and Aircraft Sensors and Instrumentation), a course taught by Clark's advisor, Associate Professor Kerri Cahoy.

"I wanted to see if I could take the kinds of telescopes and lasers that we're building today, and make a detectable beacon out of them," Clark says.

He started with a simple conceptual design involving a large infrared laser and a telescope through which to further focus the laser's intensity. His aim was to produce an infrared signal that was at least 10 times greater than the sun's natural variation of infrared emissions. Such an intense signal, he reasoned, would be enough to stand out against the sun's own infrared signal, in any "cursory survey by an extraterrestrial intelligence."

He analyzed combinations of lasers and telescopes of various wattage and size, and found that a 2-megawatt laser, pointed through a 30-meter telescope, could produce a signal strong enough to be easily detectable by astronomers in Proxima Centauri b, a planet that orbits our closest star, 4 light-years away. Similarly, a 1-megawatt laser, directed through a 45-meter telescope, would generate a clear signal in any survey conducted by astronomers within the TRAPPIST-1 planetary system, about 40 light-years away. Either setup, he estimated, could produce a generally detectable signal from up to 20,000 light-years away.

Both scenarios would require laser and telescope technology that has either already been developed, or is within practical reach. For instance, Clark calculated that the required laser power of 1 to 2 megawatts is equivalent to that of the U.S. Air Force's Airborne Laser, a now-defunct megawatt laser that was meant to fly aboard a military jet for the purpose of shooting ballistic missiles out of the sky. He also found that while a 30-meter telescope considerably dwarfs any existing observatory on Earth today, there are plans to build such massive telescopes in the near future, including the 24-meter Giant Magellan Telescope and the 39-meter European Extremely Large Telescope, both of which are currently under construction in Chile.

Clark envisions that, like these massive observatories, a laser beacon should be built atop a mountain, to minimize the amount of atmosphere the laser would have to penetrate before beaming out into space.

Here the use of a laser is twofold

1. A huge amount of power in a very small etendue or phase space. The expanded beam will diverge slowly.
2. A huge amount of power in a very narrow spectral feature. Unlike sunlight, the laser will have a very narrow spread in wavelength, and so it could show up in a spectrograph as a little blip.

Yes, such communication is unequivocally possible given our current technology!

Michael Busch (from the SETI institute) has calculated that an Arecibo-class receiver could detect TV carrier wave leakage at a range of about 16 light years, slightly further than the distance to Tau Ceti, assuming favorable geometry for the receiving antenna.

That's for leakage -- not an intentional directed signal, but RF energy that happens to go over the horizon from routine television broadcasts. The situation would be much more favorable for a deliberately beamed signal using the full power of an Arecibo-class transmitter.

Keep in mind that you don't have to come close to matching the total energy output of the Sun to still be detectable. The Sun's energy, being close to a blackbody spectrum, is spread over an enormous range of wavelengths from radio, through infrared and visible, to X-ray and beyond, while the energy of a directed radio or optical transmission would be extremely tightly concentrated around a single frequency.

Reference: Quora

Could we make our presence known to them? We have definitely tried our hardest. In the 1950s and 60s, we broadcast TV and radio directly into space in the hope that one day the signals will reach a civilization capable of receiving them. You describe such a situation, but alas it is more complicated than that.

The main problem with any civilization receiving signals is that those signals will be incredibly weak. Think about it this way: our sun is an incredibly powerful signal emitter, and there are millions of other stars just as bright and far, far more luminous. But they are also far, far away, just like Tau Ceti. Take Sirius A, the brightest star in the night sky. It is only 8.6 lyr away and a far more luminous star than our sun, and yet Jupiter outshines it by about double. So you can imagine that the signals that have reached Tau Ceti are incredibly weak. Given our current technology, it is hard to say whether they would have detected our broadcasts or no.

If we knew to broadcast in that specific direction, it is conceivable* that communication would be possible. But the roundtrip time of 24 years is not exactly productive for conversation.

*Just some numbers: Lets say we broadcast a megawatt (gonna fry birds) of radio waves from a dish 100 meters in diameter. This gives a signal strength at Tau Ceti of around $10^{-23}\mathrm{W/m^2}$. I would venture to guess that even our most sensitive equipment would struggle to pick up this signal.

• Radio waves are also a stream of photons like visible light. If there is only one photon per square meter and year at Tau Ceti, detection would require a lot of patience. Data transmission is nearly impossible with a data rate of some bits per year. – Uwe Aug 11 '17 at 18:06
• Thats a good point, I never thought about the quantized nature of light limiting the information flow rate, but indeed that is what will happen unless we what-if.xkcd.com/13 Tau Ceti – Quietghost Aug 11 '17 at 18:19
• At a frequency of 10 GHz, the signal strength of 1E-23 W per square meter is 1.5 photons per second or 90 photons per minute. Detection would be very difficult and data transmission very slow. But a photon per year was too pessimistic. – Uwe Aug 11 '17 at 21:09

It's too great a distance.

Look at it this way: the sun to that system barely emits enough energy to make that planet visible to our technology.

So, if that planet wanted to send us data, they would have to send as much energy as gets reflected by that planet in our direction.

Since you said "as advanced as ours": We're not capable of producing that much power. Not even remotely. So they're not capable of doing that, either. A star is amazingly powerful.

Let's look at this way. The estimated power of the sun is something like 1026 W.

Humanity's electrical production is something like 1012 W.

So, one sun is roughly 100 trillion earth electricity productions.

Heck, we don't freaking matter on a stellar scale.

• "Look at it this way: the sun to that system barely emits enough energy to make that planet visible to our technology." Pretty intuitive way to describe it! – VSO Aug 11 '17 at 17:41
• A laser beam diverges over great distances just like electromagnetic waves from a large antenna dish of the DSN, only the angle may be smaller. The very powerful radiation of our Sun would hide the laser beam to a observer near Tau Ceti. Again we are not capable to produce a laser beam with enough power. – Uwe Aug 11 '17 at 17:57
• @VSO see my calculation, even a laser diffuses according to the inverse square law. Our most powerful military lasers barely make a blip on the far end – Quietghost Aug 11 '17 at 17:58
• One of the ways we detect exoplanets is by the change in brightness of the star when they pass in front. How does that difference in brightness compare with the strength of a signal we could send? – Barmar Aug 11 '17 at 19:58
• @Bramar I would ask that as a question on its own. I think its a good question, and provides a better place for an answer than the comment section of another answer – Quietghost Aug 11 '17 at 21:03

Let's use Voyager 1 as a reference. We can receive from it at about 1400 bits per second, it is about 20 light hours away and has a 3.7m dish and a 22W transmitter. So 12 ly is a bit less than 5300 times further away. Inverse square law applies, so we have factor of 27 milllion to overcome. Stepping up to a 100m dish buys us a factor of about 1000 (100/3.7 squared). Stepping up to a 20kW transmitter buys us another factor 1000, so very naively, we would cut our bandwidth by a factor of 27 to 50 or so bits per second.

Realistically, better transmitter technology will help this a bit, but the noise from Tau Ceti nearby will hinder it. On the other hand, for the prize of communicating with another civilization, we could build arrays of 300m radio telescopes pretty quickly.

In fact, come to think of it, we are in the process of building the square kilometer array, which has, as the name suggest a square kilometer of collecting surface (spread over several thousand kilometers). That buys us another factor of 100 or so compared to a 100m dish. With one of those at each end of the link, we could get up to about half a megabit per second.