Heinrich Hertz was a German physicists who propelled the human understanding of electromagnetic radiation forward in leaps and bounds. Using a spark gap to essentially transmit electromagnetic radiation (EMR) energy, and receiving the signal across the room with an antenna, Hertz produced the first human radio transmission. Of course, the signal carried no intelligence. In the 1890's the Italian radio pioneer Marchese Guglielmo Marconi would combine Hertz's spark gap with the telegraph key and Samuel F.B. Morse's code to create the first wireless transmission of intelligible information. By the turn of the century, wireless telegraph stations on either side of the Atlantic were sending messages back and forth at the speed of light. It was the precursor of much longer lines of EMR communication to come.
In 1909, Charles David Herrold kind of combined Alexander Graham Bells microphone and speaker inventions and imposed or modulated the carrier frequency of a spark gap to produce the first ever voice transmission with radio waves, broadcasting the words San Jose Calling. It was pretty crude, but it worked, setting the stage for a communications revolution that was just about to happen. English inventor Ambrose Fleming added the plate element to Edison's light bulb, creating a diode or full wave rectifier tube, but it was the addition of the screen grid by Lee de Forest in 1906, creating a vacuum tube that could amplify a radio signal, which greatly enhanced radio receiver performance and gave birth to the commercial radio industry. It was an absolutely brilliant invention that would take radio transmission and reception into a whole new dimension of possibilities. By 1916, it was all coming together, Hertz's spark gap, Herrold's modulated carrier, Bells telephone equipment, and de Forests amplifier. Over the next decade, radio broadcast stations would begin to spring up all over the place.
But the radio broadcasts that became the epitome of communications in the 20's and 30's were trapped here on earth, due to an effect caused by the ionosphere commonly referred to as skip or bounce, which causes frequencies bellow 30 Mhz to be reflected back to the earth. Voice communications require only a narrow bandwidth, about 10 kilohertz, so lower frequency carriers can effectively handle all the channels you might want. When the invention of television came along in the 1930's, low frequency carriers could not handle the bandwidth required for both audio and video components of the Television signal. Besides that, most of the frequencies below 30 MHz had already been assigned for commercial, governmental, and military use. The solution was to use Very High Frequency (VHF), between 30 to 300 MHz, and Ultra High Frequency (UHF) 300 to 1000 MHz, for television broadcasting. But these frequencies of EMR are unaffected by the ionosphere and radiate out into space. I the 1930's and 40's therefore, human television broadcasts became the first forms of interstellar messages emanating from Earth.
The nineteen thirties were actually quite a prolific period of human discovery and invention, albeit out of the view of the general populous. In 1926 the American Physicist Robert Goddard invented the liquid fueled rocket engines which 15 years later would propel the NAZI V-2 rocket, and ultimately, would take humans beyond the confines of Earth to the Moon. During the same period, Astronomers like Edwin Hubble were realizing the vastness of the universe from whole new perspectives of it and whole new orders of magnitude. At the same time, scientists like Linus Pauling and Erwin Schrodinger were unraveling the secret of life and immutable proof of Darwin's theory of variation and natural selection.
In the second half of the 19th century, human space travel and alien visitation had become successful themes for novelists to cash in on, but we must remember that in that time not a whole lot of people were literate enough to read them. But when H.G. Wells novel War Of The Worlds was turned into a radio theater presentation in 1938, no one needed to be able to read to experience the sensationalism of an extraterrestrial visitation. Thereafter, notions of UFO's and visitations on Earth by alien species became a common occurrence, but one alien visitation event that occurred in 1948 in Roswell, New Mexico, would have greater effect, and still does, than any other. Are there really other beings out there somewhere in space who have developed the technology to traverse the immense void of space that separates star systems?
By 1950, all of the afore mentioned threads wove a tapestry of intrigue in the minds of scientist and laymen alike, the notion that we are not alone in the universe, that out there somewhere, around other Sun-like stars encircled by Earth-like planets, the odds were pretty good that life had evolved much the same way that it has on Earth. Therefore, there are likely other species out there transmitting radio signals we can detect. In fact, there are conceivably thousands of such alien species spread across the Milky Way galaxy alone, and their presence should be obvious and robust, thought Physicist Enrico Fermi in 1950. And so began the search for extraterrestrial intelligence, and an interstellar message saying we are here. After all, given that planet Earth had only just arrived on the stage of interstellar EMR transmission, we were far more likely to receive a signal from some alien species preceding us, before they would receive any signal from us. But never the less, the zeal to announce our arrival could not be dampened in the minds of astronomers and rocket scientist alike.
In 1972, the United States launched the Pioneer 10 and 11 space craft which would be the first to traverse the solar system and exit into interstellar space. The Pioneer craft were a kind of advanced scouting party for two more spacecraft called Voyager 1 and 2, which would ultimately overtake Pioneer 10 and 11 to become the first spacecraft that might come in contact with an extraterrestrial intelligence. On the side of each of the Voyagers, a golden phonograph record was attached containing a message for any extraterrestrial species who might come across the space craft. Imprinted on the back of the disk were presumably easy to understand pictograms describing how to play the records. The content of the records were a menagerie of sounds representative of life on Earth. Ironically, it is more probable than not, that human life on Earth will have long since vanished before any extraterrestrial intelligence will come across the little spacecraft and there interstellar message.
The motion of an Interstellar space craft would be almost imperceptible in deep space and inconsequential in comparison to the speed of light, and warp drive is the stuff of science fiction, not a real possibility, at leased within the foreseeable future. A much more efficient and expeditious means of letting alien species know we are here is to send them a message via radio waves which travel through space at very close to the speed of light. Such effort has been attempted once, back in 1974. It was the most powerful broadcast ever made into space an the target was a globular star cluster 21,000 light years away. The Arecibo 305 meter parabolic antenna was used to focus a megawatt of power from the telescopes transmitter into a 20 trillion watt beam of EMR. Our very distant descendents can be expecting a return call from ET about 42,000 earth years from now.
If you think about it, 42 thousand years ago, human beings on Earth had not even begun to leave messages on cave walls for their own future descendants to marvel at. In this context, the concept of interstellar messages and communications takes on mind numbing proportions, and the arrogant perspectives we humans so often evince, give way to a more humble sense of our inadequacy to comprehend ourselves, and the immensity of the universe we find ourselves in.