In the search for extraterrestrial civilizations, it's difficult to know what to look for. During the pursuit, we have tended to search for detectable signals and signs that we ourselves would emit (on purpose, or by accident) on the assumption that aliens will use similar technology, given that they have access to the same physics. 

It's perfectly logical to do so, but it's not altogether ideal. As we've seen over the last few hundred years on our own planet, intelligent civilizations can quickly abandon old detectable technology as their understanding of the universe increases. For obvious example, we have quickly moved from using analog signals to digital for communication. Though analog signals in the range we used for communication would be a fairly ineffective way of communicating with alien planets, it's possible that alien civilizations could go "radio quiet" in a cosmological half-blink of 100 or so years, just like we have, making detection all the more difficult.

As such, scientists have speculated about what kind of signal a more advanced civilization might send, and what technology level would be needed to send them. 

While of course speculative, we do have some ideas of what kind of signal would make sense, as well as what the message should contain to make it clear it comes from an intelligent being. 

“In the 1960s, the idea was to focus on a region around a well-known frequency where neutral hydrogen emits radiation in interstellar space, 1.42 GHz," astronomy graduate student at UC Berkeley, Bryan Brzycki, explained to Universe Today. "Since this natural emission is prevalent throughout the galaxy, the idea is that any intelligent civilization would know about it, and potentially target this frequency for transmission to maximize the chance of detection. Since then, especially as technology has rapidly advanced, radio SETI has expanded along all axes of measurement."

Sending signals across the galaxy or universe, particularly continuous signals that would offer our best chance of detection, requires a lot of energy, far more than humans are capable of. In 1963, Soviet astronomer Nikolai Kardashev attempted to calculate the kind of energy involved in sending such information-carrying signals, and the levels of advancement that civilizations would have to be at in order to send them.

Kardashev divided these hypothetical civilizations into three types, based on how much energy they are able to harness from their surroundings.

Type I civilizations are civilizations which are able to harness all the energy available to them on their planet (given at around 4 x 10¹⁹ erg per second) and use it for their own purposes. Type II civilizations are able to harness the energy of their star, e.g. by constructing a Dyson Sphere, theoretical megastructures constructed around stars built for this purpose. Type III civilizations, meanwhile, are alien civilizations capable of harnessing the energy of their whole galaxy. 

Though the energy productions of Type II and III civilizations are extremely high, based on an increase in energy production on Earth of 1 percent per year Kardashev calculated that humanity would reach those milestones in 3,200 and then 5,800 years. An extended scale, proposed in 2020, suggests a Type IV civilization which is able to harness the energy of their observable universe. According to this team, looking at our energy consumption, humans are currently a Type 0.72 civilization.

Detecting Type I civilizations, with their puny output (though far in excess of our own) would be very unlikely, according to Kardashev. But signals sent by Type II and Type III civilizations could be detected (though not replied to) by a Type I civilization with conventional radio telescopes not too far in advance of our own. The assumption in the work is that alien civilizations would be broadcasting scientific information far in advance of our own, with the intent of being heard by less advanced civilizations, which may not be a great idea for any civilization with the goal of staying alive. 

However, the Kardashev scale gives us an idea of what kind of civilizations are capable of sending signals we may soon be able to detect. If such advanced civilizations exist (and given how vast the universe is, and how long it has dragged on for, it would be a reasonable assumption) it would also give us other things to look for, such as giant megastructures used for harnessing energy.

Though we have a pretty good idea of what we are capable of, and might soon be capable of, the universe has been going for far longer timescales. Looking at what an advanced alien species could be capable of may also tell us about our own potential futures. If we search the skies and find no signs of Type III civilizations able to harness energy on galactic scales – which so far we haven't – it may tell us that something prevents intelligent species from reaching that level, and a Great Filter looms ahead of us. 

An earlier version of this article was published in May 2024.