SETI

The search for extra-terrestrial intelligence

Radio Telescope
Credit: Dave Evans

With the cancellation of funding for an alien-hunting telescope in California, we look at the science behind the search for extra-terrestrial intelligence.

Where is everybody?
This was the question reportedly asked by nuclear physicist Enrico Fermi during a lunch at Los Alamos laboratory in New Mexico back in 1950.

It’s now known as the Fermi Paradox. It’s intended to highlight the apparent contradiction between high estimates of the probability that there are alien civilizations in our galaxy, and the lack of any evidence that they really do exist.

There are a number of attempted explanations of why this should be the case, such as alien civilizations being too far away, eventually being destroyed by themselves or by natural events – or because they aren’t there to find.

A decade after Fermi’s question, astrophysicist Frank Drake, the pioneer of the search for extraterrestrial intelligence, created what is now referred to as the Drake Equation to try to estimate the number of alien civilizations in our galaxy.

The equation is based on the star formation rate, the fraction of stars that have planets and the probabilities of, for example, the planets developing life and that life evolving intelligence.The problem is that we don’t accurately know the values of most of the variables, and Drake himself has said that his equation is just a way of “organising our ignorance” on the matter.

Most approaches aimed at detecting aliens involve listening out for signals and hoping we find something.

Listening out
Efforts to detect signs of extraterrestrial intelligence usually use radio telescopes, typically utilising a portion of their time that is not devoted to traditional radio astronomy.

Assuming that alien signals are much like our own, the data analysts are on the lookout for repeating signals with a narrow bandwidth.

But because there is little time dedicated to SETI activities, and resources are scarce, there have only been studies at a handful of frequencies from a few thousand star systems – out of more than 100 bn stars in total.

Arguably the best candidate for the detection of an artificial transmission so far is the “Wow!” signal, picked up in 1977 by the Big Ear radio telescope at Ohio State University.

It lasted for the entirety of the maximum 72 seconds for which it could be observed, suggesting a constant signal, and was at a frequency similar to that of hydrogen resonance, which has been suggested as one at which strong interstellar signals might be transmitted.

Repeated attempts to relocate the signal have not found anything, however.

Because using radio telescopes can only possibly detect civilizations that have reached a certain level of technology, some have suggested finding planets around other stars and monitoring their atmospheres.

This could first determine whether they’re capable of supporting life like that which is found on Earth, and then detect any changes in composition that would result from an industrial revolution.

Announcing our presence
Signals from Earth have been leaking into space, unwittingly announcing our presence to any extraterrestrial civilizations, since the first FM radio and TV transmissions.

Unlike some other frequencies, which bounce off the atmosphere, FM signals can penetrate it and be carried through space.

But any aliens wanting to catch 24-hour rolling news coverage would have to be nearby.

Until recently, TV transmissions have been omnidirectional – they spread out roughly equally in every direction through space. So their power decreases with the square of the distance that the signal has travelled through space, meaning that broadcasts would be extremely difficult to pick up at a range of more than a few tens of lightyears.

It will be even harder in the future because of both satellite television and the digital switchover.

Whereas the old type of broadcasts sent radio waves in every direction, the requirements of satellite TV mean sending them in tighter beams up to satellites and then back down to Earth again, so less of the signal leaks into space.

The UK is also due to switch over to digital rather than analogue broadcasting, and digital signals only need about a quarter of the power, making any transmission even weaker still by the time it reaches neighbouring stars.

But even if accidentally leaked messages are weak, we can still try to make contact deliberately by beaming messages towards a particular location in space.

This has been attempted several times, the best known of which was transmitted by the Arecibo radio observatory in 1974. The message was a sequence of binary digits, which, when decoded shows pictorial and mathematical representations of a human being, our solar system and DNA.

It was aimed at the M13 globular cluster of around 30 000 stars 21 000 light years away, which will no longer be there by the time the message arrives – it was intended more as a demonstration of technology and to be thought-provoking.

Although the power of deliberately transmitted beams doesn’t drop as rapidly as with accidental broadcasts, they are still only likely to be strong enough to be detected within perhaps a few hundred to a few thousand lightyears. The effective range of a transmission depends on several factors, including the frequency, bandwidth and transmission power, as this range-calculator shows.

Even for messages at high power, any extraterrestrials waiting to hear from us could need a radio dish several kilometres across. They may not have the budget for it.



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