Main differences between SETI and Extended SETI (E-SETI) by SETI

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Main differences between SETI and Extended SETI (E-SETI)

By SETI

(This article was written on the basis of the book, ‘E-SETI: Communicating with Alien Civilisations, published by Cambridge University Press)

In the past, we have seen many of our efforts to communicate with extraterrestrial intelligence (ETI) fall by the wayside.

First, it was when we thought that ETI was an extraterrestrial race that was coming to get us and destroy the Earth (see the films A.I. Artificial Intelligence, etc.). However, the second problem was more important: communication was not going to work, because our technology was not up to the task.

We could use powerful transmitters like those of the Voyager space probes, but the chances of us being able to detect a signal, decipher any of it, and communicate with it are very small. The reason is that we have an advanced culture on Earth that has developed technology and social institutions over a long period of time. When a civilisation similar to ours comes into existence, this usually takes a long time. A civilisation might reach this stage of development in tens or hundreds of thousands of years, much longer than the estimated arrival time of any space-faring civilisation.

Even if a civilisation reaches this stage, it may be completely oblivious to us because it is so different to the society we find ourselves in.

Let us look at the technology of today and see how well we can expect to communicate with an extraterrestrial civilisation, even if it has been developing for millions of years.

The problem today

The way we communicate today is, in general, a form of one-to-one communication. That is, the message is sent from sender to receiver in a very specific way.

To do this in a distant universe, it is necessary to broadcast a message with a high signal-to-noise ratio.

Signal-to-noise ratio

For a signal to be noticeable to us, it must be at least twice as strong as the background noise. This means that if the radio waves that we use today to transmit our signal from Earth to outer space are a hundred millionth of the intensity of the background noise, the message will be detectable, even if the message is only one one millionth of a second long. That would be far too brief to be useful.

One solution would be to get bigger transmitters. We have an enormous array of transmitters that could be used for interstellar communications. With the largest transmitters we could hope to produce a signal with one million times greater power than the noise level. But even so, this would still not be adequate. For example, we could only transmit at a single frequency, such as the FM radio band. If we were able to communicate with a far away civilisation using FM, we would only have a chance of detecting a message sent to us at exactly the right frequency. For a distant signal, it is only by receiving a constant waveform that we can be sure it is the signal that we want to receive.

The radio wave is not constant. The radio waves that travel through space can vary in intensity as well as frequency. This variation is known as the carrier wave. To receive a signal from a distant star or galaxy, it is necessary to do much more than keep the receiver tuned to the specific frequency of the signal you want to receive. A much better idea is to keep receiving a small sample of the many frequencies that make up the radio wave. This sample is much more constant and regular than the regular radio wave. We can sample this regularity using a device known as a digital storage oscilloscope. With a digital storage oscilloscope, we can keep track of the many frequencies making up the radio wave. This can then be translated into a number that represents the total energy in radio waves coming from a distant object. We can then calculate the energy in a particular frequency band, such as the FM radio band, and find the total amount of energy in that band over time.

The regularity of the radio wave can be used to tell if the signal is something coming from our own solar system or something from the distant future.

In the past it would have been easy to make measurements to tell us if the FM wave was travelling in our direction or from the past. A digital storage oscilloscope could measure the relative strength of a radio wave coming from the future.

Nowadays, the regularity of the radio wave can be measured by the regularity of the electrical current making up a radio wave. Modern receivers can measure these currents and use the information to tell if they are a regular signal or an impulse signal. They can even measure the signal in time, like the regularity of a clock.

These measurements allow the distant civilisation to receive the very same measurements that we receive. We can receive a regular series of electrical current pulses or we can receive a highly sporadic signal, and then we can use the regularity of the incoming signal to tell which is which.

We have not been given any evidence that a distant civilisation has been able to receive anything other than a regular series of signals, and we have therefore assumed that their technology is just the same as ours. But, the technology used to detect radio waves does have some capability to work with sporadic signals.

Sporadic signals would have a different way of being received. For example, if a civilisation has a range finder on a spacecraft, it could try to measure the distance of an object and then transmit that information to Earth.

To get a response back from Earth, the receiving civilisation would need to be on the receiving side of the same spacecraft. They might use a different frequency to send the data. If that frequency had a different level of regularity than the transmitter, then that civilisation would detect a different frequency. This could be a clue as to whether they have a regular radio signal or a highly sporadic one.

An important consideration with regard to radio waves is that some radio frequencies are absorbed or reflected by the atmosphere. If this is the case for the signal you are receiving, then it might be able to show that the signal is a signal and not just random noise.

If you have a radio and listen to what you get, it will still sound random. But, if you have the radio in a special place that is shielded from the elements and has the right tuning, you may get random noise but there are also regular signals buried in the static.

We use radio to communicate with far away devices. If a civilisation is using radio as an inter-spacecraft communication tool, they could use radio frequencies that have different frequencies to ours. If they were using an antenna, you could detect that and then be able to get a clearer picture as to whether it was transmitting.

If the signal has a long wavelength then it could be absorbed or reflected by the atmosphere, but if the signal is short wavelength it will travel through the atmosphere. If that civilisation can detect our radio signal and it has a large antenna, that could be the only way they can receive our signals.

If we detect radio waves, and our radio signal is long wavelength, it may be possible that the Earth’s magnetic field is the reason why we have not received that signal.

If our signal is long wavelength, and we detect that signal and we don’t receive any transmissions, this could be because the radio signal is being absorbed, or reflected in the atmosphere.

If our signal is transmitted in a specific frequency that has a certain wavelength, and if our signal is large enough to travel through the atmosphere, then we may be able to receive that signal.

If we do receive a signal, it may just be random noise, or it may be a message from a far away civilisation.

We know our signal is reaching out to them because they are picking up our signal. If you look at the Doppler effect in the sky you may see something moving away from us. If we received a signal that it is moving away from us, then we could confirm that the signal is from another civilisation.

If our signal is very low-power, and if our signal is very short-wavelength, then that may not be able to travel through the atmosphere.

How our signal reaches a far away civilisation, and how long that signal has to travel may also depend on the distance between our planet and another world. If you measure the mass of another planet and its distance to us, you may be able to calculate the speed that the signal would travel at.

The distances between us and other worlds can be calculated using the Kepler space telescope. If you know the distances, then you can calculate how long it will take for that signal to reach that world.

If a civilisation wants to communicate with us, they must be able to transmit a signal. If they have a large and powerful transmitter, then that could be why our signals have not yet been received.

If our signal is long wavelength, then our signal may be absorbed in the atmosphere.

If the civilisation is very far away from us, and if they receive our signal in time to do anything with it before our signal is lost, then the signal can also be received on the Earth.

If the other civilisation is very far away from us, and if the message is much bigger in proportion to their distance, then the message may not fit in our radio-sonde.

We know that the size of a signal could be different from planet to planet. When it is possible to send a message to a distant planet, then the sizes of the messages are usually not equal. For example, the messages sent by earthlings are usually much bigger than the messages sent to other planets.

The way that interstellar communication is done

The space is not empty. There are other stars, and there are objects that are moving in space around the stars.

There are stars that are our neighbours, and there are planets around them. There are many things that happen in space. The stars and planets have their own lives.

The stars and planets are usually not stationary. They move along their orbits and sometimes they revolve around their centre. The stars rotate about their axis. The planets rotate around their sun.

Planets and stars are very important objects, because they have much life around them. They have living things on them. They have planets and moons, and they also have planets.