What they look for
Almost every kind of body in the universe transmits energy (except black holes, which absorb energy and refuse to allow it out again. But more on that later in this book). The energy is transmitted in waves and the wavelength determines how people on earth “see” the energy and what name they give it. The light waves picked up by optical telescopes and the light waves picked up by radio telescopes are on the same spectrum but at different places – it is the length of the wave that determines where the source sits on the spectrum.
A possible confusion that should have been removed by that last paragraph is between light and sound. The waves picked up by a radio telescope are not – despite the word “radio” in radio telescope – the same as sound. Radio telescopes receive a form of light wave, and not sound waves. Light can travel through a vacuum, and sound cannot (which is why, when you scream in space, no one hears). There are also differences in the shape of the wave; light waves are transverse and sound waves are longitudinal.
The fact that everything is transmitting energy, and transmitting it in every possible direction, means that the universe is a very noisy place and it is very difficult to pick out the specific kind of signal that SETI is looking for, which is signals that raise at least the possibility that they were generated by intelligence. The radio frequencies used on earth lie between 20 kHz and 300 GHz, but the whole radio spectrum is a lot bigger than that – it runs from 3 Hz at one end of the scale to 3 THz (3000 GHz) at the other. Signals being generated by heavenly bodies simply because they are heavenly bodies take up the whole of that radio spectrum. SETI’s scientists think it is extremely unlikely that any intelligent being would attempt to signal over such a wide range, and what they are looking for is primarily narrow-band signals (signals covering only a small part of the radio spectrum).
They also look for brief flashes of light – and by brief they mean flashes that last nanoseconds.
Signals meeting both of these criteria – narrow-band radio spectrum signals and flashes of light lasting nanoseconds – have been discovered. What happens then is that they are considered possible candidates for intelligent signals and examined in greater depth.
Scientists are also aware that picking up a radio signal would not necessarily indicate that it had been intended for us. Some signals, if strong enough, may simply have left the planet’s atmosphere and reached us although no intention existed that that should happen. The same thing may be true in reverse; somewhere out there in space may be a civilisation that is trying to extract intelligent messages from chat shows and reality TV originating on earth.
And that raises one of the problems with the SETI Project: if they succeed in identifying a signal as definitely originating from an intelligent life form, how likely is it that they will be able to decode the message? As well as looking for communications or signals beamed into space, and those seemingly intended for Earth, SETI also looks at signals passing between two worlds where the line of sight extends to this planet. Such signals may be messages between a planet and another planet or a satellite, and may continue past the intended receiver and eventually reach here. It’s possible to reason that an intelligent life form capable of sending signals into space is also capable of reducing them to the barest minimum in order to make them comprehensible to a different civilisation. That won’t be the case if the signal represents a message between two places sharing a common language.
What stage has SETI reached?
Here are some interesting points from current and recent searches.
FRB121102 is, as the letters FRB in its name indicates, a fast radio burst. FRBs last for perhaps a millisecond, but the energy emitted in that millisecond can be as much as the sun has given out since the first wheat and barley were cultivated some 10,000 years ago in what was then Mesopotamia and is now Iraq. Scientists argue furiously over what causes an FRB and the fact is that no-one knows. Probably, they have a variety of causes. Possibly, one of those causes is the desire of an intelligent life form to transmit a signal.
A number of FRB’s have been found; 121102 was first seen in 2012 at the Arecibo Observatory in Puerto Rico. It was seen to recur a number of times, and a team at Cornell University led by Dr Shami Chatterjee made arrangements to watch in case it returned. That’s one of the risks in this kind of work, because it might have been a century before 121102 came back to life; in fact, however, there were nine flashes in six months (and there may have been more, because only 83 hours were devoted to observing the location of the pulses during those six months). Doctor Chatterjee was previously a Janskey Fellow at the Harvard-Smithsonian Centre for astrophysics and it was at the extremely powerful Carl G Janskey radio telescope array that the pulses were observed. Doctor Chatterjee’s team was able to locate 121102 in a dwarf galaxy more than three billion light years distant from Earth.
Does this mean that the dwarf galaxy is home to intelligent life? It does not mean that at all (and nor does it mean the opposite, which would be that there is no intelligent life in that galaxy, or that these flashes were not sent by intelligent life. There is simply not enough information to say one way or the other). It is, though, helpful in ruling out some of the other possibilities that have been considered.
One widely held view of FRBs was that they were formed as a result of some cataclysmic event – the collapse of a neutron star into a black hole, for example, or perhaps a star exploding into a supernova. And it is possible that either of those events could produce the kind of burst seen from 121102 – but they could not do it repetitively.
It was also considered that FRBs were likely to be coming from within our own galaxy or, if not within, then very close by. And that may still be the case – for other FRBs – but at a distance of more than 3 billion light years from Earth, it is clearly not the case here. (Our galaxy, which is known as the Milky Way Galaxy, has a diameter of between 100,000 and 180,000 light years).
It is possible that 121102 is a magnetar, a kind of neutron star that has a very powerful magnetic field. Neutron stars are small, very dense, and created by the collapse of a star with insufficient mass to produce a black hole.
But, when all of those caveats have been stated, it is also possible that some form of intelligence there is attempting to send a signal to intelligence elsewhere.
*Excerpt from 'Where are They?' available here on Amazon