Why are they confident a radio wave will reach Earth from hundreds of light years away?
My car can only listen to a radio station within a few hundred miles.
The short answer is, lots of digital signal processing.
The signals are received using a huge radio telescope; your car probably doesn’t have a 1000-foot dish on it. Once the data is recorded, they set computers looking through it for signals with particular characteristics, eg narrow bandwidth, signs of modulation like pulses, and Doppler shifting. Of course, when you look at the raw data, it is buried in lots of noise. It’s not like they expect to tune in and hear I Love Lucy.
As for how many light years away one can expect to distinguish a good signal, that also depends on the strength at the point of origin and its directionality. People have actually beamed messages into space in megawatt strength, using basic modulation techniques that are easy to decipher.
The Voyager probes are 20 billion and 17 billion kilometres from Earth and are still sending back messages with the power of a light bulb.
There is still a difference between receiving a signal from 1 light year out, versus 100 ly. In the latter case, ideally someone is using a multi-gigawatt transmitter instead of a mere megawatt or so.
There is a very small probability that there is an extant civilization capable of sending or receiving radio signals within the few hundred light years of our solar system that such a signal could be transmitted even if intelligent life is “common”, e.g. arises in potentially habitable systems in our relatively sparse region of the galaxy between two major arms. Even if we assume that the approximately 15,000 stars within a 100 light year radius of Earth could support intelligent life, the odds that they would host a civilization within the last hundred years that we’ve been able to receive signals over the many hundreds of milllions (or more) of years of development of such life its virtually infinitesimal. The basic conceit of the Search for Extra-Terrestrial Intelligence is that they will continue to use the same basic communication technologies that we’ve been using for only our recent ascension to understanding electrodynamics despite the prohibitive restrictions of such methods. In fact, an advanced civilization would likely use gravitational waves (which are not attenuated by interference from the interstellar medium) or some other more advanced means to communicate, and may well consider communication via electromagnetic radiation again to ants communicating by pheromones. As for receiving our signals, our most powerful transmissions have been due to ballistic missile early warning radars that could only be detected above the random noise level of the interstellar medium at distances of about 150 light-years even using an array the size of our solar system; actually receiving, say, high frequency television transmissions (VHF and UHF bands) could not be done beyond the limits of our solar system.
The Voyager probes transmit to the NASA Deep Space Network (DSN), a trio of dedicated facilities with substantial capacity to amplify the weak signals from known sources, and these narrow band transmitters are pointed directly back at Earth. It is not realistic that they or anything like them could be received even that the distance of the nearest star systems.
Unless technological civilizations are incredibly prevalent and persistent in our galaxy, SETI is unlikely to find them. It is more likely that life (the organized and self-propagating moderation of thermal and informational gradients) is common, to the point of existing in nearly any environment which can host stable chemistry, but technological civilizations, even if they are reasonably prevalent, are so vastly spread out that detection or communication requires an understanding of physics and capabilities for interstellar measurement or exploration that are vastly beyond what we can do. This isn’t an argument against SETI per se, but we should not have a high expectation of discovering alien civilizations with our primitive state of technology, which in the scheme of things is barely beyond sending smoke signals. We barely even understand electrodynamics at the fundamental (quantum) level enough to actually make use of it, much less the deeper implications of cosmology and quantum field theory. And the thought of contacting a civilization that does have command of these basic mechanics should worry us very much, as we’d be as helpless against a more advance civilization than the American natives against Cortez and Pizarro.
Stranger
What makes you think they’re confident?
Forget about using gravitational radiation for communications. Gravity is the weakest force we know, which is why it’s taken us 100 years to see the first direct evidence of gravity waves.
Planets tend to rotate and orbit…both of which doppler shifts the frequency/wavelength of the EM transmission. Is technology available to adjust for this shift?
I like this idea a lot, and I might have to steal it. However the basic premise of SETI is that friendly advanced civilisations would try to communicate with other civilisations, including civilisations like ourselves which are not yet as advanced as themselves. It is quite possible that such communications will occur using relatively simple means, including radio, or (perhaps more likely) laser-beam communications.
It is not impossible that all, or most of, the civilisations in the galaxy are communicating with each other in this relatively simple and inclusive way, and that an ‘interstellar internet’ exists that we may be able to detect and join.
More about the Interstellar internet concept here
https://www.centauri-dreams.org/?p=34223
Yes; it’s called multiplication!
The old SETI screensaver would (among other operations) just guess thousands of different values for what the frequency shift rate might be, loop through and for each value dechirp (I presume by multiplying with an artificial reference signal) the samples by the guessed rate, perform a bunch of windowed FFTs, and look for interesting spikes and other features.
Interstellar internet? Your lag time would be simply horrendous…
One can see it in the cut of their jib.
Any signal sent by aliens would be direct, line-of sight from them to us. The signals received by your car radio have to contend with the curvature of the Earth. Some frequencies don’t bend or bounce well.
To illustrate my earlier point, I did a little back-of-the-envelope calculation.
In spite of the vast distance of Voyager 1, we can pick up that puny 20 Watt radio signal with enough signal-to-noise ratio to communicate at a modest data rate. Compare that to the recent detection of gravitational radiation from the merger of black holes. By my back-of-the-envelope calculation, the power density of that gravity wave signal at Earth was brighter than the light of the moon, and something like ten trillion times more powerful than Voyager’s signal. Even with a square kilometer detector, we could barely see it after decades of research and development.
Space is extremely stiff and gravity waves just don’t couple well to matter. I am not contesting your larger point, but electromagnetic waves are far superior to gravity waves for communication at any distance.
To the extent that one can ascribe an area to the LIGO detectors, it’d be more accurate to call it 32 square kilometers. Which only amplifies your point, of course.
On the other hand, that’s for natural gravitational wave sources. With sufficiently-advanced technology, one might be able to envision an artificial source with both a greater intensity at the target, and a higher (and precisely-known) frequency.
Well, Seth Shostak is on record as expecting to find extraterrestrial civilizations within a few decades, and is apparently buffered by the fact that many nearby stars have planetary systems. (Why this is a surprise to anyone I don’t know; planetary scientists entirely expected that we would find planets around other star systems.)
Gravity is “weak” in comparison to electromagnetic interactions but the range it can travel without distortion or absorption is essentially infinite. It has taken us so long to detect gravitational waves after they were first hypothesized because of how primitive our detection systems (interferometry using lasers) are and how physically large they have to be to react with gravitational waves with wavelengths on the order of kilometers or larger, but an advanced technological civilization will almost certainly have better methods for detecting fluctuations in the background gravitational field. The real challenge is generating sufficiently powerful waves, which would essentially require the manipulation of compact masses on the scale of large stars. We don’t really have any concept of how to do that in a fashion that would facilitate communication, as manufacturing and juggling black holes to send messages seems unlikely, but a more advanced civilization may have the means to directly create and manipulate singularities such as cosmic strings as readily as we use a microphone and amplifier.
I know for many it seems improbable that an alien civilization would have capabilities beyond our ability to even conceive of them, but consider that as of two hundred years ago electricity was a novel phenomenon useful only for parlor tricks that only a few cranks with a lot of spare time had any interest in, and now we can store and generate thousands of times of the equivalent energy produced by a work animal with the flip of a switch. In another few hundred years we may be able to directly control intranuclear forces, or artificially generate gravitational waves, or any of another of abilities which are pure speculation today.
The problem with this, again, is the relatively short distances that electromagnetic signals, and particularly radio, can be transmitted compared to interstellar scales. You’d literally need a transmitter with a power throughput on the scale of an O-type star to communicate across thousands of light years, and a civilization with those kinds of energy resources and technological capability probably has as little interest in communicating via radio as you do in using tin cans and a string to make an international call.
There is an even more facile conceit in SETI, thought: that an extraterrestrial civilization would be willing and able to communicate with us, and that some sort of confederation of interstellar civilizations exists and talks with each other. Even if we assume that technological intelligence is relatively common in high metallicity star systems, their distribution both in space and over the potential time such civilizations could develop that the likelihood of two civilizations that are technologically comparable in reasonable proximity is most probably quite low. Imagine if an alien civilization contacted us tens of thousands of years ago, or we contacted a civilization in a comparable state today. We’d have very little to exchange even if the mismatch in technologies could be overcome. And the odds are that any given technological civilization will have evolved on a different timeline on the order of millions of years; discovering alien life of intellectual capacity comparable to apes would be interesting, but we wouldn’t be discussing mathematics or philosophy with them.
And even two civilizations with compariable technological advancement may be very limited in the ability to communicate. Our ways of communication are very much structured around how our brains are structured and how we perceive the world, which would likely be vastly different for an intelligent alien species which evolved in a completely separate fashion in a very different environment. We can’t even communicate effectively with most land mammals, much less cetaceans or cephalopods, and we share a world and evolutionary history with them. The mental image of sitting in Draco’s Tavern swapping star tales with alien traders and smugglers has an aesthetic appeal but is about as far from reality as dragons and hobbits.
Stranger
That range is a direct result of the fact that it does not interact with matter in any significant way, which is why it is so difficult to detect, unlike electromagnetism. Electromagnetic radiation also has essentially infinite range. We can see all the way back to 300,000 years after the big bang at microwave frequencies. We can’t see the center of our galaxy with visible light, but we can with infrared and radio.
I beg to differ. Gravitational radiation may be described as weakly interacting with mass, but that is exactly the property that makes it desirable for long distance communication. Electromagnetic radiation is absorbed and scattered by interstellar dust, and the universe is awash with radiation from a vast number of sources at essentially all frequencies. On the other hand, an artificial source of high frequency gravitational radiation is likely to be readily distinguishable from natural sources. That we cannot detect such signals with conventional technologies does not make it unviable.
In any case, unless intelligent technological civilizations are extremely common (and it took nearly four billion years for life on Earth to develop such a capability) it is unlikely that we’ll find one within the range of detection of our radio telescopes even if they are transmitting radio signals. The optimism that we’ll find signs of an intelligent civilization within a few decades is nothing more than unbridled optimism.
Stranger
Actually, your AM car radio can detect terrestrial stations thousands of miles away if at nighttime due to ionospheric propagation.
The very reason for certain radio stations having “clear channel” status is because those signals cannot be prevented from being received thousands of miles away: Clear-channel station - Wikipedia
Re what is the maximum distance an incidental (not intentional) alien radio signal could be detected, according According to Dr. H. Paul Shuch who is an expert on this area, our current technology could detect earth-like TV and radio leakage signals from an extra-solar planet 1,000 light years away:
If the alien signal was intentional, IOW directional and restricted to stars with rocky planets in a habitable zone, the detection distance would be far greater.
“I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”