[BLANK_AUDIO] If there are intelligent civilizations out there, obviously we want to try and communicate with them. And there have been past attempts to send messages to alien civilizations. Let's look at a few of these efforts. The earliest message to be sent on a spacecraft were plaques sent on the Pioneer Ten and 11 space craft in 1972 and 1973. They had a very simple plaque on them that showed an image of a man and a woman, and also the location of our star system. So that if the spacecraft were ever found by alien intelligences, they would have some idea of what we looked like and where we had originated the spacecraft. In 1977, the two Voyager spacecraft, Voyager One and Two that are now leaving the solar system, also had records placed on them. These records contained images and music of planet Earth, pictures of different locations in different countries on the Earth. And music from different cultures from around the world to give aliens some idea of culture, civilization on our home planet. Carl Sagan, who was one of the people instrumental in developing these records and determining what their content might be. Said of these records, the spacecraft were being counted and the record played only if there are advanced space-faring civilizations in interstellar space. But the launching of this bottle into the cosmic ocean says something very hopeful about life on this planet. Other attempts have also been made, so we sent messages on spacecraft, we've also sent radio messages into space using our radio telescopes. In 1974, a pixelated message was sent from the Arecibo radio telescope in Puerto Rico, towards the N13 star cluster. This was a binary message that when it's interpreted in the right way, gives aliens information about our planet. The message contained the numbers from one to ten, the atomic numbers of hydrogen, carbon, nitrogen, oxygen and phosphorus, very important elements for life. It also gave the formulas for sugars and base pairs of nucleotides, of the information storage system DNA. It gave the number of nucleotides in DNA and showed a picture of the double helix of DNA as well. And within that picture was also a figure of a human, a number of the population of the earth. When that message was sent, a diagram of our solar system, and also an image of the Arecibo telescope. So, a diversity of information about the civilization that sent that message, and also about the biology of the organisms that thought up that message in the first place. There have been some other recent attempts as well. In 2001, a Russian group put together a message called the Teenage Message, which was a message sent by school children constructed of ideas that they had put together. There were greetings in Russian and English and also musical compositions for the aliens, if they ever find them. In 1999 and 2003 there was also a message called Cosmic Call which was a digital message containing basic maps to complex physical concepts. A bilingual image glossary, a Rosetta Stone of symbols and images submitted by people all over the world. Well, there are problems with these messages, and these plaques sent on spacecraft. They're very unlikely to be found by aliens. The messages that we've been sent by radio telescopes are very short messages, and it's very unlikely that over such a short period of transmission they would be picked up by alien civilizations. And of course those spacecraft are very small. It's very unlikely that they will be picked up by aliens travelling through the space given their small size, but these are early attempts to communicate with alien civilizations if they are out there. A somewhat more problematic question that was first raised by Physicist Enrico Fermi is called the Fermi paradox, and this is the paradox. If there are intelligent civilizations out there, if there are billions of stars with planets around them that might have developed intelligent civilizations, then we would expect that perhaps by now we would have been visited by alien intelligences. If the Universe is 13 billion years old then the question is, why have we not found any extraterrestrial intelligences? Why have they not visited our planet? And in a later lecture we'll look at some of the reasons why there might be a Fermi paradox. Some of the explanations for why there might not be aliens visiting Earth. So what have we learned? Well, we've learned that we live in the hope that other civilizations are looking for us, like we are looking for them. We've sent out numerous messages into space, like bottles in the ocean. We've sent plaques on spacecraft, and we've transmitted messages by radio telescopes. We still have no real evidence for extraterrestrial civilizations. But the hunt for them and the attempt to communicate with them continues.
