E.T. Phone Home

In Astronomy by Brian Koberlein49 Comments

In the movie Contact, astronomers receive a radio signal from the star Vega. Buried within the signal is a broadcast of Hitler’s speech for the opening of the 1936 Olympic games. The television signal had made the 25 light year journey to Vega, which let the aliens know we’re here. The idea that our television and radio signals are gradually reaching ever more distant stars is a popular one, but in reality things aren’t so simple.

The reach of TV signals. Credit: Abstruse Goose

The reach of TV signals. Credit: Abstruse Goose

The opening ceremony of the 1936 Olympics was the first major television signal at a frequency high enough to penetrate Earth’s ionosphere. From there you could calculate that any star within about 80 light years of Earth could detect our presence. There’s even a website that shows which TV shows might be reaching potentially habitable worlds. But the problem with this idea is that it isn’t good enough for the signal to reach a distant star, it also needs to be powerful and clear enough to be detectable.

For example, the most distant human-made object is Voyager I, which has a transmission power of about 23 Watts, and is still detectable by radio telescopes 125 AU away. Proxima Centauri, the closest star to the Sun, is about 2,200 times more distant. Since the strength of a light signal decreases with distance following the inverse square relation, one would need a transmission power of more than 110 million Watts to transmit a signal to Proxima Centauri with the strength of Voyager to Earth. Current TV broadcasts (at least in the States) is limited to around 5 million Watts for UHF stations, and many stations aren’t nearly that powerful.

One might argue that an advanced alien civilization would surely have more advanced detectors than we currently have, so a weaker signal isn’t a huge problem. However the television signals we transmit aren’t targeted at space. Some of the signal does leak out into space, but they aren’t specifically aimed at a stellar target the way Voyager I’s signal is aimed at Earth. They also lack a clear mechanism for how transform the signal to an image. On Earth this works by implementing a specific standard, which any alien civilization would need to reverse engineer to really watch TV. On top of that, there is the problem of scattering and absorption of the signal by interstellar gas and dust. This can diminish the power and distort the signal. Even if aliens could detect our signals, they might still confuse it with background noise.

A color-coded version of the Arecibo message.

A color-coded version of the Arecibo message.

That doesn’t mean it’s impossible to communicate between stars. It just means that communication would require an intentional effort on both sides. If you really want to communicate with aliens, you need to make sure your signal is both clear and readable. To make it stand out among all the electromagnetic noise in the universe, you’d want to choose a wavelength were things are relatively quiet. One good region is known as the water hole, which spans a range from 18 to 21 cm. Hydrogen (H) emits at about 21 cm, and hydroxyl (HO) has a strong emission at about 18 cm. Together they can form water, hence the name for the quite gap in between. You also need to make your signal easy to recognize as an artificial signal. In Contact the aliens did this by transmitting a series of prime numbers.

In 1974 humanity made its most famous effort to send a signal to the stars. It was a radio transmission sent from the Arecibo observatory, and consisted of 1,679 binary digits, lasting three minutes. Since 1,679 is the product of the primes 23 and 73, the bits can be arranged into an image of those dimensions. There have been other efforts to send messages to the stars, but they haven’t been as powerful or as simple.

Beyond a few light years, our leaky TV broadcasts are likely undetectable. As we’ve switched to digital television and lower transmission powers they’ve become even harder to detect.  Any aliens looking for us will have to rely on other bits of evidence, such as the indication of water in our atmosphere or chlorophyl on Earth’s surface, just as we will strive to detect such things on distant worlds. Either way, the first message received won’t be a complex text of information. It will simply be a recognition of life on another world.


    1. Author

      SETI is privately funded, so it’s hard to criticize as a waste of money. Also, if a civilization made deliberate efforts to make their presence known, SETI could detect them.

  1. Brian, how about our communications with spacecraft like Voyager and New Horizons?

    1. Author

      Communications with various satellites are targeted, but they are aimed at the satellites that move relative to the direction of particular stars, so it wouldn’t be much better.

      1. What would a civilization need to send a deliberate signal with a chance of being received? Thank you for your reply.

        1. Author

          The best chance would be a strong, long-lasting, directed signal at a wavelength with little background noise, such as the “water hole” region. Preferably it would be something with a clear non-natural pattern to it, such as prime numbers or some other mathematical relation. Even then their message would be based upon what they think another civilization would look for, which is really just a guess.

  2. “On Earth this works by implementing a specific standard,” The television signal used in the United States from about 1953 through 2009, NTSC, was never standardized.

    The proposed standard RS-170A pending, was withdrawn in the first decade of this century. I chaired the Working Group of CEA that choose to withdraw the proposed standard, which was never approved.

    1. Any alien engineers advanced enough to pick up the signal would not have any trouble decoding NTSC. This is not in any way like decrypting a secret message. It really is trivial to figure out once they have the signal.

    2. Whether or not a standard was approved, there was a method for broadcasting TV signals that receiving TVs understood. Between the broadcaster and the receiving TV was essentially a “standard”.

  3. Suppose an advanced civilization has been scanning the sky for (say) hundreds of years; it would have a pretty good idea of the kinds of signals coming from our corner of the sky. The signals leaking from our planet could well represent an anomaly in the expected pattern and be flagged for analysis. (Point being that it’s one thing to separate signal from noise in the cross-section, which is what you imply in your post, and quite another to do so in a time series or panel).

  4. Whilst I agree that aliens would not be able to detect our television transmissions due to the weak signal strength, the calculation that it would take 110MW to communicate with the nearest star does not take into account the fact that we could use a much higher gain antenna than that used by Voyager One, which would send a much narrower but more concentrated signal.

    If, as I understand, the gain of an antenna is proportional to the square of the diameter divided by the square of the wavelength, then using a dish only 37m diameter, which is ten times that of Voyager One would reduce the power required to a quite manageable 1.1MW.

    In addition, if we were to reduce the data rate by a factor of, say 10, to allow the receiving planet to integrate a weak signal over a period of time, we could increase the range by a factor of 3.16 which could be enough to reach the nearest habitable planet. Although at this rate it might take a week or so to send a high definition photo of life on our planet, this is not unreasonable when you consider that it would take 24 years before our potential alien friends are able to reply with one of theirs. The signal could be interspersed with (say) horizontal and vertical sync codes to make it easier to detect.

    It therefore seems to me that communicating with extra-terrestrials is well within the bounds of current technology, and the real problem is that if we are unwilling to transmit information to them for fear of alien invasion, then why should we expect them to send information to us?

    What do you think?

    1. We’re very unlikely to find a civilization within communication range. There are less than 100 starts within 20 light years of earth.


      Most of these stars do not have the characteristics necessary to support life on planets, even those in the theoretically ideal distance from the star.

      It’s taken 4.5 billion years for intelligent life to evolve on Earth. Assuming this is a good representative time needed for intelligence to evolve, you’d need a star that’s at least 4.5 billion years old. Starts larger than the Sun end their life sooner than that. So there are perhaps ten stars capable of supporting life within 20 light years of earth.

      It’s questionable how long our civilization can last given our impact of the planet. If the expected life of an advanced civiliztioin is 100,000 years, and that’s highly optimistic figure, the probabililty at least one of those ten stars has a planet with an advanced civilization in existence at the same time as ours is extremely small if not close to zero.

      So it’s almost certain that many intelligent civilizations exist among the 300 billion solar systems in our galaxy, the chances that any of them are within twenty light years, and thus within communication distance is highly unlikely. If such a civilization exists and it wanted to communicate with us, they’ve had a chance to do that. Since we haven’t heard from anyone we can pretty much give up the effort.

      1. As of today, they are now estimating about 150 stars within 20 light years (http://www.solstation.com/stars/s20ly.htm), and 56 possible within 32.6 light years. Perhaps a generational craft en route to one exoplanet, e.g. Proxima Centauri b, subsequently detected radio signals from earth.

  5. Very interesting article. Let me ask this hypothetical question. Assuming there are humans just like us with the exact same technology living on a planet orbiting Proxima Centauri, is it not possible that we wouldn’t know about each other?

    According to your article, detection and communication would require high-watt radio transmissions targeted directly at the other planet. It would also require the other planet to be listening at the right time.

      1. And vice versa, as not only our earth’s transmissions fading, they are intermittent and their source location constantly moving with both the rotation of the planet and the planet’s orbiting of our sun.

  6. Although I believe that it’s worth making the effort to announce our existence to the rest of the galaxy, in the hope of receiving a signal back, we should be prepared for either a very long or eternal wait. I agree with Gabe’s overall theme i.e. low likelihood of ‘nearby’ civilisations overlapping. It annoys me when I read assumptions that not only life, but intelligent life is almost guaranteed to become manifest on any planet or moon with a Goldilocks temperature. There are probably dozens of factors crucial to life even starting that we are unaware of or grossly underestimate, e.g. it may be crucial to have both a strong stable magnetic field to retain an atmosphere. It may be equally important to have the kind of tidal forces produced by our moon early in it’s life to wash salts into the oceans, this in turn required a large moon to be created via a very fortunate planetary collision. It would therefore suggest that signals are sent by apparatus designed to reach 10000 light years rather than 20, (see what I mean by a long wait!). By the way, don’t bother with signals based on prime numbers. They will only be prime in base 10, which ETs may not use! Simple pulses that are definitely unnatural sound like a good idea, maybe with some parameter added in that is relevant to the target star so ET knows it was intentional.

  7. Whoops, ignore comment on ‘only be prime in base 10’, don’t think that’s true!

    1. Good self-catch, Jim. Prime numbers are prime in any base. More exactly, a set contains a prime number of elements if it cannot be divided into any set of smaller sets, where each of the smaller sets contains more than one element and exactly as many elements as every other smaller set. Note that you don’t have to represent the number of elements in the original set in any particular base. In fact, you don’t need a numeric representation of that number at all. Whatever that number is, it’s prime if it meets the criteria I’ve outlined.

      What I’ve never understood is why they used two different numbers. Yes, you can only reconstruct the diagram if you lay out the dots in a 23-by-73 grid, but it only works if you use 73 rows of 23 dots. If you lay out the dots in 23 rows of 73 dots, you get a horizontal phase shift and the picture is nonsense. That wouldn’t have been an issue if they had squared a single prime number and made the grid, say, 41-by-41. The message would have been almost the same size (1,681 bits instead of 1,679 bits), with no way to get it wrong at the receiving end.

  8. I seriously hope that any aliens cannot pick up our TV and radio broadcasts. Imagine Trump or a artificially induced reality program became the first thing they see from us. They may regard it as a hostile act and decide to wipe us out for the good of the universe.

  9. Truth, to get effective radio power one needs use a beam dish antenna. Such antennas look at on a very tiny part of the sky. If somebody was attempting to radio signal, they would first guess we were here. Then aim the dish. What is the likely possibility of that? Almost zero. Life may indeed exist in some intelligent form in the Universe. The nearest Galaxy Andromeda is 2.6 million light years distant. A signal from there would be from an extinct civilization. Humans are not more than 2.5 million years away from the Ape.

  10. It isn’t as simple as just the power. Space probes use error detecting and correcting codes like Reed Muller codes so that the signal can be reconstructed when it is affected by interference like Solar wind. TV signals that were analogue like the 1936 Olympic broadcast would degrade badly i would have thought.

  11. Question 1: How many “earht-like” planet has received our broadcats by now?
    Question 2: What is the “possibility of coinciding-any distanced alien broadcast” by listening just for a few decades (meant; SETI)?
    Question 3: One of the reason we couldnt received anythings, may be that: “they ended radio comunication by some reasons, such as achieiving quantum cominication?
    Question 4: How many years does “civilisation A” need minimum and maximum, between “inventing radio” and ” blockading cominications between civilİisation B and civilisation C”?
    Question 5: Actually; at which point of technological eveluation is the radio?

    Sorry for imperfect English. Thanks.

  12. So the answer to my question, “how close to the Earth would aliens need to be todetect our radio transmission?”would be, We have yet to broadcast a radio signal strong enough for anyone outside the solar system to detect.

    Would this be the correct answer?

  13. Interesting conversation (albeit somewhat disappointing).

    Other than seeking direct evidence of alien life, another way to approach this would be to get a better sense of the variety of conceivable lifeforms given planetary conditions vastly different from our own. Does anyone know where I might find scientifically informed articles and/or discussion of this?

  14. Maybe we should build a Dyson Sphere; or another type of structure around our own star; the Sun, and send signals by purposely blocking the starlight from the Sun in a way that would be completely unnatural. And, Or pick a code that proves intelligence.

  15. Or, maybe we should focus and redirect the starlight from the Sun. Focus a portion of it into a signal beam. And, or reflect it off another celestial object. Reflectors, laser beams, etc.

  16. The sun gives off a lot more energy than then our radio stations. Would that make it harder for aliens to hear us? Imagine that I am standing right next to a jack hammer. I am one mile from you. I whisper something in your direction. To hear me, you would have to be able to detect my voice and discern it from the jack hammer. Does the sun make noise that would confound our radio signals? Or is the sun quiet enough in the “water hole”? Thanks.

  17. I for one I wouldn’t be surprised if some intelligent civilization has detected a signal coming from our blue planet. However if a civilization is advanced enough to detect and reconstruct our weak signal this means they would have the ability to send something back. But would they? They would only have a suspicion that we are here. Assuming that they would have the same relative issues we have as a civilization, would they bother themselves with trying to communicate with a species that they could learn nothing from. I’m sure an advanced civilization would have the same curiosities we have. Technology only brings more curiosity, with every advancement there are more possibilities. Any communication from a distant planet would inherently change our civilizations path. What if they are advanced enough to respect all life. Sort of like Star Treks prime directive they wouldn’t make contact until we could handle it. We have to populate our star system before anything will be heard. We may have to begin traveling to alpha centauri before they know that we are advanced enough to handle a first contact. We still make alien invasion movies which by all means shows how unintelligent we are. A civilization needs resources no matter the advancement of any civilization. Unless your a child just to see what happens do you go out of your way to step on an anthill? Do you actually go out to buy a bullet to shoot at the anthill?
    This is only an analogy of circumstances put on everyone by life itself, just because a civilization is advanced doesn’t mean they would waste resources unless they could gain from it. There would be nothing to gain for the would be conquerors when the universe is populated by resources.

  18. What about listening to von Neumann probes? It would be a lot easier (if they exist).

  19. I believe that SETI is just looking for *any* communication that isn’t natural. Since our radio communication is meant primarily for our own consumption, our signals are not strong enough to be sent much past a few light years. I’m not sure that’s even enough to reach Alpha Centauri, the nearest star other than our own sun. Such would probably be the case for alien civilizations. The window that a civilization even uses high power broadcast radio may be short. Our own communication standard is rapidly becoming the Internet, which relies on fiberoptic cable for the most part to achieve high bandwidths. There could come a time that we don’t use radio at all, except for short distance wireless like WI-FI. Earth would become radio silent. For SETI to succeed, they would need to locate an alien civilization at the peak of their own “radio window”. It would be a great discovery indeed. But, communication would probably be impossible. Even if we could somehow understand them and “speak” their language, in order for them to hear us, they would have to be listening to us as well at the exact moment we sent communications to them, and, we would have to be ready, some number of years later to receive their reply, if any. It all seems very far-fetched, and I tend to be optimistic.

  20. Send them a year of “Married with children” and they’ll realize that we have the same problems here that they have there.

  21. Ok. An off the wall question I suppose. Do you remember the tv show from the late 70s called Battlestar Galactica? Towards the end of the series they received the transmission of the Apollo 11 landing. How close to Earth would they have had to have been to get it?

  22. I write science fiction. Would it be credible for a human character’s cell phone to have any data the alien ship’s computer could download, such as Internet sites? (I expect the answer will be no.)

  23. The old TV signals were basically radio waves SO what about the first radio signals? When did those happen and the aliens migh be sending someone to rescue the Titanic. How far out are those signals?

  24. The only way I can think of to make a signal that could be detected by aliens far far away would be to detonate many many nuclear bombs. Literally, it would take thousands to just have a chance. Send them out into solar orbit somewhere between here and Mars. Line them up and set the detonation timing to create an unnatural timing pattern. These would give off a wide spectrum of energy so almost any detector would pick it up. Also, it would radiate in all directions.
    Then this should be repeated every year or so for as long as possible.

  25. Do you think they’ll ever find intelligent life on earth? (-_-)

    1. I doubt it. I think the aliens will find organisms that like to over-reproduce to the point of destroying their environment. The question is, what group will the aliens classify them under via their taxonomy? A virus or a Cancer?

  26. I wonder how many alien civilizations we humans have annoyed with our noise pollution…for all we know our radio and television broadcasts might interfere with some poor aliens thought processes or hearing….go stand on any sidewalk next to a busy road and try talking to someone while endless cars and trucks speed past belching noise from exhaust,tires on road,loud radios,horns…lets hope we don’t get a noise violation warning on our first contact.

    1. I was having similar thoughts. there you have an actual case for extermination of noisy neighbours.

  27. I’ve never understood the use of TV signals as a first time introduction from us to any alien out there. Wouldn’t Morse code signals and radio be first and a stronger wave than TV signals. If suddenly picked up by another very distant receiver it would still be an anomaly.

  28. It would seem logical to launch a satellite or spacecraft of some sort in the southern Hemisphere (where Alpha Centauri is) towards Alpha Centauri with a 110 million watt television broadcast transmitter…is this possible or has it been done?

    1. Author

      Laser instead of TV transmitter, and Proxima Centauri instead of Alpha, but that’s basically the plan of Breakthrough Starshot.

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