Extra-terrestrials have been imagined for at least 2300 years, if not longer. Epicurus wrote to Herodotus in 300BCE proposing there could be "infinite worlds both like and unlike this world of ours", even ones inhabited by "living creatures and plants and other things we see in this world"1.
“When he redesigned the cosmos, Descartes diminished the significance of human beings by opening up the possibility of life elsewhere. The concept of multiple universes was strange and new, but by the middle of the eighteenth century, many natural philosophers believed not only that they existed, but also that they provided homes for intelligent beings.”
It is really hard to guess as to how many places in the Universe intelligent life has developed. You need to know things like how many habitable planets there are, what the chances are of life developing and what the chances are of life surviving long enough to develop as we have (and beyond). Seven such factors were formulated into a single construct known as the Drake Equation in 1961, by astronomer Francis Drake3. The result of this is as estimate of how many times advanced civilisation has appeared in the Universe. But so many of the necessary values were unknown, that previous generations of scientists have concluded that the error margins are too high to draw any conclusion4. However, advanced new telescopes and the slowly dawning space age have allowed many more accurate measurements to be made and current calculations are that life may have arisen on one hundred billion Earthlike planets throughout the Universe if using some dire statistics on how likely life is to appear on any given planet (one in a billion).
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An Unidentified Flying Object (UFO) describes any object in the sky until it can be given a different name5. The most exciting result possible is that some are intelligent aliens visiting us from another planet. But a great number of rational investigations have been conducted by dedicated and well-funded space programmes across the world, by military researchers, government bodies, hundreds of university departments, and many well-funded independent space-related organisations as well as a horde of scientifically-minded independent analysts. Where conclusions can be made, UFO always turn out to be ordinary phenomenon and hoaxes6.
Common causes of UFOs are the planet Venus, blurred insects and birds close to a camera, Chinese lanterns, sweeping lighthouse beams, balloons and launched scientific instruments7, planes, objects such as plastic bags whipped up by the wind and presumed to be further away than they really are, and a variety of unusual but perfectly terrestrial weather phenomenon.
Conspiracy-minded people think that the many people involved in detecting space and near-orbit events are altogether involved in a mass lifelong pact, for rather unclear reasons, to hide the evidence. They must be constantly coercing tens of thousands of scientists, operators, data analysts, institution staff, government workers and the entire IT security departments at all those places, silently making them all comply, without nosy national governments ever noticing and with no defectors. Needless to say, skeptics routinely bang their heads in confusion as to what kinds of psychology is in operation when people think that they've personally discovered the truth, when flocks of dedicated professionals cannot.
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Let us presume that we have detected intelligent aliens, 100 light-years away. This is not a long way but it still covers a thousand stars and their planets. If we want to communicate with someone, we must first attract their attention. Likewise, someone could be trying to contact us but so far we could have missed the signal by unfortunately not facing a suitable receiver in the correct direction at the correct time.
If we sent a message to them it would take 100 years to get there. They would have to detect it and then send one back, which would take another 100 years. This means that a reply would take 200 years from the time our first message was sent. This is the fastest possible two-way communication allowed by the laws of physics. And this assuming they are close by. We would have to keep scanning from about 190 yrs after our first message up to 250 years after our transmission to see if they have replied to our one message. We would have to keep re-sending the signal for the 100 years in case they have not detected it yet! Only when we receive a response could we stop resending the original signal. Even if we did discover intelligent alien life in our lifetimes we certainly would not be alive to learn the outcome. If Human life was extended to beyond 200 years on average then perhaps it would seem more exciting - the prospect of still being alive when we are expecting an answer.
How long would it take us to establish a dictionary of mathematical terms with which to communicate? Depending on the length of the transmissions (they would have to be highly repetitive in order to be easily recognizable and to prevent data corruption) it could take well over a thousand years to learn anything about them, or them about us, apart from the fact that we both have well-financed radio astronomers and reasonable mathematicians.
If we receive transmissions from further away, they could have been sent thousands or even millions of years ago. We have to consider the very likely possibility that by the time signals have bounced around the Universe for long enough to accidentally stumble upon the antenna of an intelligent species, the original sender, their planet, and their sun, are all extinguished, and such a frustrating event is one that sorely worries astronomers and scientists8.
On the other hand, luckily, our transmissions into space actually got a head start on our present intention to find life in the Universe:
“Humans have been sending radio waves into space at the speed of light for nearly one hundred years now. Although our goal has seldom been interstellar communication, radio is especially appropriate to that task because electromagnetic waves generally require no travel media, and because radio waves penetrate very efficiently through gas and dust clouds. To date, our "radio bubble," as Tyson calls it, encompasses every star around which a planet has thus far been found, including Alpha Centauri, at 4.3 light-years away; Sirius, our sky's brightest star at ten light-years out; and about a thousand others. So, if aliens are listening, they might have an opportunity to partake in any message sent at frequencies greater than twenty megahertz, including FM radio and television.”
The mechanics of listening to such weakened signals requires massive effort, massive receivers pointing in the right direction, "and, if the yearning stargazer actually wanted to decode our message's content, it would have to first adjust for Doppler's shifts caused by Earth's rotation and revolution, and then build a dish about twenty miles wide, or four hundred times Arecibo. That's a lot of work - and for what? A weekly dose of Desperate Housewives and reruns of Welcome Back, Kotter"6.
So although we might despair at the vastness of space, the remoteness of life on Earth and the incredible chances against radio waves happening to travel from one inhabited planet to another, we might also get lucky. I explore the potential results of discovering alien intelligence in: "Discovering Alien Intelligence: The Politics of War and Fear" by Vexen Crabtree (1999).
Searching for aliens via their radio emissions may not prove to be that useful. Satellites and probes which we have sent into space transmit their radio signals in a directional manner (which saves energy) back towards the Earth, meaning, that often our transmission don't just spread across the Galaxy, but head straight for Earth. It would take an incredible stroke of luck for such intelligently directed signals to be intercepted or spotted by anyone that they were not directed at. Likewise we are unlikely to accidentally stumble across alien communication channels - the alien planet's network of satellites and spaceships, needless to say, will do most of their communicating (just like ours) by transmitting directly to their home world(s).
The same problem occurs with our own "radio bubble", expanding outwards from Earth. Unlike Terran omnidirectional antennas, our transmissions that escape into space are not omnidirectional. Our radio bubble is not like a detectable shockwave, but rather, like a very thinly spread net. Signals in space do not spread out the further they travel, like they do within a planet's atmosphere. With no media such as air to travel through, radio signals travel in long, but very thin, straight lines. You have to be within the direct path of this lightspeed needle in order to detect it. "When you consider that entire galaxies can pass through one another without any stars colliding, you can get some idea of how empty the universe is. [...] Any signal from any planet in the universe broadcast in any direction is unlikely to be in the path of another inhabited planet" says Robert Todd Carroll11, "if you were a magical massless being who hopped aboard a photon shot from Earth in any direction, it is highly likely that you would pass through the entire universe without hitting anything larger"8.
Now, in addition to all that, Human use of radio transmissions is dwindling. Things like the Internet have replaced radio as a good default method of communication, but, do not use radio waves that might travel into space. Paul Davies, physicist at Arizona State University, states that the mass use of radio may not persist, and already we have largely moved on to use things like fibre instead. 'Moreover, many modern radio devices (such as mobile phones) rely on a technique called 'spread spectrum' encoding. It uses signals that look like background noise, except to a receiver equipped with the right unscrambling code. Us Humans figured this out within a century of inventing radio technology, so aliens might have done the same. Radio signals that are clearly artificial in origin may, then, be only a transient sign of civilisation.'12 In other words, planets harbouring intelligent life probably only emit radio waves (just like us) for a short period in their development. To be in the right spot, in the right direction, and have receivers pointing in the right direction, at exactly the right point in time, requires a bit more luck than many scientists hold out for.
One possibility considered by Paul Davies, a physicist at Arizona State University, stems from a problem that our species will face in the future: the death of our star, the Sun. Star manipulation, in order to make it last longer, or to shield it, or obtain more energy from it, could be a tell-tale sign of an ancient civilisation trying to protect itself from the star's demise.
“Red giants are created when a star exhausts its supply of hydrogen at its core, with the result that the inner layer contracts and the outer layers expand, forming a redder and much larger star. If a star's outer layers could be mixed into the core, that would slow the process of inflation down. [...] Such a star would look odd, though. It would be bluer than it should be and would be of a type known to astronomers as a 'blue straggler'. [...] Perhaps, then, it will be a sign like this - of a technological civilisation millions of years old - that is seen, rather than some upstart that has not even got its radio waves under control.”
The Economist (2010)13
So it might be that although SETI using radio waves is not likely to bear much fruit, it is possible to use some similar techniques but shifting the emphasis to stellar spectrography.