We've all wondered "Are we alone in the universe?" and perhaps we are. I am inclined to think that we are not. When you look out into space, with or without a telescope, thinking that we humans are the ONLY ("intelligent") life out there in the trillions and trillions of miles of the inky vastness of space? Feels a little bit selfish to me, but that's one man's opinion. But to know whether or not we humans are alone means we have to do two things: listen and "speak".
The first thing we humans started to do in earnest, was listen. Once the technology was readily available, humans created SETI or the Search for Extra-Terrestrial Intelligence. What eventually became SETI began operations on February 1, 1985, even though NASA had been previously funded a smaller search for extraterrestrial intelligence. With the discussion between the Chief of Life Sciences at NASA Ames Research Center at the time, and others who were involved, a plan was put in place and the creation of the 501(c)3 Non-Profit Corporation happened on November 20, 1984 and thus SETI was born and the search began.
But now they had to listen, and according to SETI's website, their scientists "... use space-based NASA telescopes such as Kepler and TESS to investigate possibly Earth-like planets orbiting other stars, and searching for technology that could be evidence of extraterrestrial intelligence. We carry out the last at our facility, the Allen Telescope Array, and at other radio telescopes such as the Very Large Array in New Mexico. LaserSETI searches for brief laser flashes originating from far beyond our solar system. In addition to these efforts to find so-called "technosignatures,” our study of the semantic content of whale songs can give insight into how we might judge whether any signals come from another intelligent species."
And listen they would. Even before SETI was a thing, humans were listening and detecting signals! The first signal found of any real significance was discovered by Jocelyn Bell Burnell on November 28, 1967! The power and regularity of a received signal was briefly thought to resemble an extraterrestrial beacon! This lead the source to be nicknamed "LGM", later LGM-1 (for "little green men"). What Dr. Burnell had inadvertently discovered was something almost as strange as little green men: a pulsar.
It was the very first discovered radio pulsar (a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles). It was a repeating signal but alas, not a message from intelligent, space-faring aliens.
10 years would have to pass before the next signal would be received that got researchers excited and wow, were they excited; eventually and informally dubbed the "Wow! signal". The Wow! signal was a strong narrowband radio signal detected on August 15, 1977, by Ohio State University's Big Ear radio telescope, then being used to support the search for extraterrestrial intelligence.
Appearing to come from the constellation Sagittarius this signal had all the traits searchers could have hoped for in an extraterrestrial signal! Astronomer Jerry R. Ehman discovered the anomaly a few days after it was received while going through the data. He was so impressed by the intensity of the signal that he circled it and wrote the now-famous "Wow!" moniker for this mysterious signal. Lasting an incredible 72 seconds, this signal has never been heard again, despite multiple attempts to find it. Many hypotheses have been put forward to help solve this conundrum, but so far, none have adequately explained the source of this one-time signal. So, once again a radio signal but, no aliens...
That doesn't mean humans have stopped listening, far from it. We humans have huge banks of radio telescopes like the Green Bank Telescope in West Virginia or the Five-hundred-meter Aperture Spherical Telescope (FAST) in China or the MeerKAT Telescope in South Africa. But like I said before, we humans have to listen for ETs to contact us. However, that is a passive approach to learning whether we're alone and more effort would have to be put forth in humanity's search for aliens. 1974 brought the first real attempt at interstellar communication when SETI decided to actively reach out to whoever's out there.
Using the now-defunct Arecibo radio telescope in Puerto Rico. The Arecibo message was created and sent carrying basic information about humanity and Earth; the message was sent to globular star cluster M13 in November of 1974. By reaching out with this radio message it was hoped it would demonstrate humans achieving a level of technological advancement, rather than hoping to engage in extraterrestrial conversation. Some scientists suppose that because the resolution of the message will degrade, any civilization that receives the message will be unable to understand it. But how do you communicate across thousands of light years (one light year = 5.88 trillion miles)? How do you speak without using any language heard here on Earth? Also, what do you say!? Good thing the really smart people thought about that before clicking "send" on a message that was sent about 25,000 light years from Earth!
The simple answer is "math". Math is what allowed the Arecibo message (at right) to be sent in to space towards M13, specifically binary code: 1s and 0s. The content of the Arecibo message was designed by Frank Drake, creator of the Drake equation, who wrote the message with help from Carl Sagan and others. Math is and remains the hope that intelligent life outside of our solar system will be able to understand. (I want you to read this so you'll fully understand what these colors mean.) The binary transmission sent carried no color information just the binary code, the colors have been added to help you understand what was sent back in 1974.
But that was not the only time humans have sent messages into space! There are twelve recognized interstellar radio messages (IRMs) that have been sent off into the cosmos, although some are considered more serious than others. Wikipedia has thankfully compiled that list so please take a look if you're curious: List of interstellar radio messages. But what about these IRMs that might last to reach their intended target? There's discussion among astronomers that after a few light years, even Earth's strongest radio and TV signals will simply blend into the background "noise" that permeates space... So any alien listening in our direction might not hear anything but static. Plus, we must consider that the message we sent to M13 arrived 22 YEARS after it was sent! This is the same when it comes to any IRM humans will send or already have. Because IRMs ONLY travel at the speed of light (670,616,629 mph), to travel the vast distance means that even the latest message that was sent, Sónar Calling GJ273b in 2018 to Luyten b (a potentially habitable exo-planet located 12.4 light years from Earth) is still 8.4 years or 49,380,454,546,486 miles before the 10-second message even shows up!
We humans are an intrepid and curious species, always seeking to explore what's beyond the horizon or over that hill. Space is no different and because we cannot easily go there ourselves, we have a small fleet of space-faring probes doing the job we cannot fathom doing ourselves. There are currently five space probes on an escape trajectory which will carry them out of the solar system! Voyager 1 and Voyager 2, Pioneer 10 and Pioneer 11 as well as New Horizons. All five of these probes will continue on their journeys out of the solar system and eventually, like Voyager 1 & 2 already have done, no longer feel the pull of the sun and become true interstellar travelers! Fortunately Voyager 1 & 2 and Pioneer 10 & 11 carry messages with them. Both sets of probes have plaques attached to them, created by astronomers to carry a message from Earth.
On the left is an illustration of the Pioneer plaque that (hopefully) remains firmly attached to the probes and one that caused a great deal of uproar because its depiction of the nude male and female form. The depictions of humans, nude or not, was important enough to astronomers who thought that if the Pioneer probes were ever intercepted, that any space-travelers would understand what the beings looked like that sent the probe(s). On the right is an image of the Voyager record affixed to both probes as they hurtle through the cosmos. Notice this time, the human form was omitted from the outside of the record. However the record itself contains music, pictures and everything that might represent the human species as an entirety. The designs that you see on the front, have instructions, written in binary code to help triangulate where the probe came from (the star-shaped object) as well as an illustration on how to play the record. The famous astronomer Carl Sagan called the launching of the Voyager probes with their golden record, in the hopes that they’d be intercepted by an intelligent species, “hopelessly quixotic” and it might be... Think about the vast emptiness and distance between even the planets, let alone the stars, it's almost impossible they'll be intercepted and studied. But like Sagan said the Voyager record has really two audiences, one is the extraterrestrial and the other is right here on Earth.
While we have had no contact with alien civilizations (yet), that doesn't meant it's been for a lack of trying. Modern humans have only been around for roughly 300,000 years, using radio for 127 years, using radio telescopes for 85 years and actively exploring space for a mere 65 years (when Sputnik 1 was launched by the Soviet Union). If, in ANY TIME before 1937, when the first purpose-built radio telescope was constructed (by Grote Reber), a signal was received, we'd have no idea. Extra-terrestrial intelligences may have already tried to contact us but we could have missed their call because our technology wasn't ready and waiting to hear the message. But it's very possible that these four probes and their messages will outlast humanity and even our sun!! Silently carrying their interstellar "message in a bottle" from the humans that made and sent them on their trek through the stars.
But we keep searching, keep reaching out, keep looking and scanning the sky for someone, anyone out there, to tell us we're not alone. And who knows, maybe some species is doing the same thing we are, around some distant sun? Looking, listening towards a small yellow star with eight planets circling it, wondering if they're alone? Who's to say?
But we continue to look because you never know what or who is out there.