As a field, the Search for Extraterrestrial Intelligence suffers from some rather significant constraints. Aside from the uncertainty involved (e.g., is there life beyond Earth we can actually communicate with?), there are the limitations imposed by technology and the very nature of space and time. For instance, scientists are forced to contend with the possibility that by the time a message is received by an intelligent species, the civilization that sent it will be long dead.
Harvard astronomers Amir Siraj and Abraham Loeb tackle this very question in a new study that recently appeared online. Taking their cue from the Copernican Principle, which states that humanity and Earth are representative of the norm (and not an outlier), they calculated that if any transmissions from Earth were heard by an extraterrestrial technological civilization (ETC), it would take about 3000 years to get a reply.
Their stud, titled “Intelligent Responses to Our Technological Signals Will Not Arrive In Fewer Than Three Millennia,” recently appeared online and is being considered for publication. Whereas Siraj is a concurrent undergraduate and graduate student of astrophysics at Harvard, Prof. Loeb is the Frank B. Baird Jr. Professor of Science, the Director of Harvard’s Institute for Theory and Computation (ITC), the Chair of the Breakthrough Starshot Advisory Committee, a bestselling author, and Siraj’s academic advisor.
The globally distributed dishes of the European VLBI Network are linked with each other and the 305-m William E. Gordon Telescope at the Arecibo Observatory in Puerto Rico. Credit: Danielle FutselaarLoeb is also renowned for theorizing that the interstellar object ‘Oumuamua, which flew past Earth in 2017, could have been extraterrestrial lightsail. This theory was originally put forth in a 2018 paper he co-wrote with postdoctoral researcher Shmuel Bialy (of the ITC). The arguments presented therein have since been expanded upon in Loeb’s most recent book, Extraterrestrial: The First Sign of Intelligent Life Beyond Earth.
Prof. Loeb recently partnered with Dr. Frank Laukien and other colleagues to launch the Galileo Project, a multinational non-profit dedicated to the study of Unidentified Aerial Phenomena (UAPs). Siraj serves as the Director of Interstellar Object Studies for this project, and he and Loeb have published extensively on subjects ranging from black holes and meteors to panspermia and interstellar objects (many of which were on the subject of ‘Oumuamua).
For the sake of this study, Siraj and Loeb focused on a particular aspect of SETI, which they dubbed the Search for Extraterrestrial Responding Intelligence (SETRI). By this, they mean ETIs that would be motivated to message Earth in response to the detection of technological activity on our planet (aka. “technosignatures”). This addresses a question of growing importance to the SETI community.
In short, does humanity have a chance of ever hearing from an ETC before our civilization collapses or is wiped out by a natural disaster? As Siraj told Universe Today via email:
“It is important to estimate the response time from extraterrestrial responding intelligences (ETRIs*) since such an estimate informs the nature of effective SETI searches — as well the implications of a confirmed signal if we ever receive one. The question we try to answer in our paper is: when might we expect our first cosmic conversation to take place?”
This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. Credit: ESO/M. KornmesserThis establishes the first parameter of their study, which was the amount of time that humanity has been emitting detectable signatures. Of all potential technosignatures that have been considered to date, the most likely and most widely investigated by SETI researchers are still radio transmissions. In keeping with the Copernican Principle, we can assume that ETIs are also engaged in the search for signs of intelligence other than their own.
“The Copernican principle asserts that we are unlikely to live at a privileged time and so the likelihood of another habitable planet like Earth going right now through an analog of our first century of radio communication, given a few billion years of its history, is below one part in ten million,” said Loeb. “Therefore, a response is expected only within a large enough volume, containing more than ten million stars.”
It can also be safely assumed that an ETI would see radio signals as a possible technosignature and would be actively listening for them. The first long-range radio broadcast took place in 1901, when Italian inventor Guglielmo Marconi sent the first transatlantic broadcast from Cornwall, England, to St. John’s, Newfoundland. Since then, humans have been sending radio transmissions to space without thinking about the consequences.
This means that if there is a civilization within a hundred light-years of Earth with sensitive radio telescopes, they may have already heard from us. In short, we may have already “started a conversation” with an intelligent species and are just waiting for a response. Beyond this, said Siraj, they went with a number of parameters that were consistent with the Copernican Principle and the conditions under which life is known to flourish:
“[W]e considered ETIs able to communicate via electromagnetic radiation, located on Earth-like planets orbiting Sun-like stars (aka, “life as we know it”). Furthermore, we considered radio signals (which at the speed of light) as well as physical probes, which would travel slower. We used the Copernican principle, which is inherently optimistic about the prevalence of life in the Universe, to establish a lower limit on the expected response time from ETRIs.
In this illustration, NASA’s Hubble Space Telescope is looking along the paths of NASA’s Voyager 1 and 2 spacecraft as they journey through the solar system and into interstellar space. Credit: NASA/ESA/Z. Levy (STScI).Transmission technologies can extend beyond radio waves to include other types of electromagnetic (EM) radiation, such as microwave lasers, X-rays, gamma-rays, and more. Since the only constraint is the speed of light – 299,792,458 m/s (1079 million km/h; 670.6 million mph) – it remains the fastest available option. It also means humans would only need to wait until the 22nd century for a transmission from a civilization located a hundred light-years away.
That being said, it is also possible that an ETC would choose to explore our planet more closely rather than send a transmitted reply. In this respect, Siraj and Loeb considered possibilities like the Voyager 1 and 2 missions, New Horizons, and the Pioneer 10 and 11 spacecraft. All of these robotic missions have or will enter interstellar space (or will in the near future) and could someday be intercepted by an ETC.
It was for this reason that the Pioneer Plaques and the Golden Records were created. However, it will take millions of years before any of these missions reach even the closest star systems to Earth. This means that if a civilization sent a probe to investigate Earth in response to radio signals from a hundred years ago, it wouldn’t arrive for hundreds of thousands of years. As Loeb explained:
“Although the latter response method results in physical contact with alien objects, it requires millions of years for the journey across a hundred light-years. This means that we still have a waiting time as long as the time that has elapsed since humans first appeared on Earth before we will witness chemically-propelled crafts in response to our radio broadcasts.”
This graphic shows the relative positions of NASA’s most distant spacecraft in early 2011, looking at the solar system from the side. Credit: NASA/JPL-CaltechOther possible concepts, like directed-energy propulsion (a la Breakthrough Starshot), could make the transit in much less time – at 20% the speed of light, it would reach Alpha Centauri in just 20 years. However, such concepts are effective for reaching the nearest star systems, but not stars 1000 light-years away within a reasonable timeline. As a final parameter, they considered just how many planets out there are likely to host an ETC.
“The Copernican principle asserts that we are unlikely to live at a privileged time and so the likelihood of another habitable planet like Earth going right now through an analog of our first century of radio communication, given a few billion years of its history, is below one part in ten million,” said Loeb. Working from this, they determined that a response could only be expected within a large enough volume, containing more than ten million stars.
Assuming that our galaxy is relatively homogenous in terms of the distribution of stars in its disk, this results in a volume of 1 billion cubic light-years (ly3) or one thousand light-years in any direction. This, in turn, entails a two-way travel time of more than two thousand years. This essentially means that if an ETC is aware of us and wants to talk, we would not be hearing from them until 4000 CE at the earliest. Or as Siraj summarized:
“We found that the fact that we have only existed as a technological civilization for about a hundred years means that, right now, we should not expect to hear back from an extraterrestrial civilization in response to our own signals. In other words, it’s extraordinarily unlikely that we could start a cosmic conversation.”
An artist’s illustration of a light-sail powered by a radio beam (red) generated on the surface of a planet. Credit: M. Weiss/CfAThis conclusion is supported by previous research (conducted with the help of Dr. Frank Drake himself!) that indicated that within various parameters, a call-and-answer scenario would take longer than the average civilization’s lifespan. In other words, any signals we receive from an ETC (whether they are a response or an attempt to “start a conversation”) are likely to have been sent by a species that has since become extinct.
This, according to Siraj, is the most significant aspect of their study, which is that civilizations have a life expectancy (which they have a measure of control over). In essence, it underscores the importance of ensuring that humanity doesn’t succumb to self-destruction or a cataclysmic fate. “The big takeaway here is that we’d better get our act together and figure out how to survive long-term if we ever want to participate in a cosmic conversation!”
Further Reading: arXiv
