Hello, is this Bloomfield? Or, how we will communicate with other intelligent life in the universe.

But if you ask a scientist a little smarter than an ordinary scientist if he believes in aliens - there is a very reasonable chance that he will answer yes, that is - he believes in the possibility of the existence of aliens, but he does not believe that they have visited us yet

If an ordinary scientist is asked whether he believes in aliens, the answer is likely to be negative. The scientific establishment denies, and probably rightly so, the possibility that beings from outer space are visiting the earth and this for a very wide variety of reasons. One of the reasons, for example, is that the distances between the stars of the Sabbath in space are enormous, and even light itself takes tens and hundreds of years to cross them.

But if you ask a scientist a little smarter than an ordinary scientist if he believes in aliens - there is a very reasonable chance that he will answer yes, that is - he believes in the possibility of the existence of aliens, but he does not believe that they have visited us yet. This distinction, although at first reading it seems rather artificial, is actually quite important. The universe is vast and vast above and beyond anything we humans can perceive. There are so many stars in space, that it is hard to believe that any event - such as the formation of life - that occurred in the universe at a certain time, occurred only once.

One of the most famous mathematical equations in this field, of the search for intelligent life in space, is the Drake equation, named after Prof. Frank Drake, the astronomer who devised it. I won't go into the technical details of the equation here, but I want to treat it for a moment like a meat grinder. On one side of the equation, you put in all kinds of data about our galaxy: for example, how many new stars are formed in the galaxy each year, and how many of those stars have planets orbiting them, like the Earth orbits the Sun. Additional data is put into this grinder, such as the percentage of these planets where there are suitable conditions for the development of any life at one point or another, and how long an advanced civilization - one that can even communicate with other intelligent beings - may exist before it destroys itself or is destroyed by an external disaster .

From the other side of the machine, the meatball comes out. The meatball, in our case, is the number of advanced civilizations that exist at any given moment in our galaxy. Sounds simple, doesn't it? You put numbers into the equation, you get a number. The reality is a bit more complex, unfortunately. As with a real meat grinder, you have to be very careful when you start putting your fingers in and touching all these ingredients, otherwise you can't really be sure that what comes out the other side is fit for human consumption if you're not in the fast food industry. From astronomical observations scientists are quite convinced that six new stars are formed each year in the galaxy, but this is about where the agreement ends. How many of these Saturn stars have planets around them? No one can say for sure. What is the percentage of planets where life will eventually develop? Any number would be just a guess, since we only know of one planet that has had life on it. All these data in the Drake equation are so fluid and not anchored in observations, that the numbers that come out of it range from ten advanced civilizations in the galaxy if you are a particularly optimistic scientist, to something very close to zero if you are a scientist who tends to see everything in black. Depending on the choice of data in the Drake equation, we may live in a universe that is as bustling with life as an alien bar in a Star Wars movie, or a universe where we can keep talking to the lamp because no one will ever hear us.

For the purposes of this episode, let's assume that there are other intelligent beings in the universe - otherwise I have nothing to talk about for another thirty or so minutes.

So if there are aliens around us who are smart enough to be able to communicate with them - how exactly will we do it? We may not be aware of it in our day-to-day lives, but our communication with other people relies on a multitude of agreed signs, cues, body movements and facial expressions without which it is difficult, very difficult to understand each other. Me, personally, as an electronics engineer in a company whose business spans the world, I find myself trying to communicate with engineers from other countries - and it's really, really difficult, especially over the phone where you can't even guess the facial expressions of the other party in the conversation.

This question has troubled the scientists who have been searching for intelligent life in the universe since time immemorial. They realized that it's not enough to find someone to talk to in outer space - you also need to know how to talk to them, otherwise all we get will be awkward silences.

Mathematics has always been the main and primary solution to the problem of communication. There is a lot of sense in the idea that mathematics is the most basic common language between us and every other intelligent alien race in the universe.

There is an absolute consensus among the scientists of the planet that mathematical knowledge is a necessary condition for any technological development in any branch and in any field of science, here or on another planet. 'Mathematics is the language of nature', says the famous cliché, and rightly so. The only way to describe phenomena in nature, both the simplest and the most complex, is through mathematics. You can take, for example, a basketball and throw it towards the ring - and if you're black, you can even score - but there's no way to accurately describe the path the ball takes from your hand to the basket without going into formulas and equations. It just doesn't work any other way.

Mathematics has, of course, its own language. Intervals, powers, parentheses, roots - you must know the accepted notations in mathematics otherwise you will not be able to follow the descriptions. Even the numbers are written differently from language to language, although if there's one thing I learned at the Technion very early on it's that if there are numbers on the board it's not a math lesson, it's a physics lesson. In real mathematics there are only letters, and real mathematicians use only Greek letters even when you can just write X and Y.

But even though the notations change from language to language, the basic mathematical principles behind them never change, and this immutability can, perhaps, be exploited to communicate with other intelligent beings.

In the nineteenth century, for example, the most common astronomical tool was the telescope, and scientists naturally assumed that if there were intelligent beings on the Moon or Mars, they too were studying the Earth with telescopes. To signal to the people of the moon that there is an advanced civilization on Earth, someone suggested drawing the Pythagorean theorem on the vast and desolate steppes of Siberia, and drawing it on a huge scale, across many miles, so that it could be seen from space.

The Pythagorean Theorem, let's recall, is one of the most basic theorems in geometry and we all learned it in middle school: in a right-angled triangle (that is, one of whose angles is equal to ninety degrees), the length of one side squared plus the length of the other side squared is equal to the length of the remainder of the triangle squared (the remainder is the longest side in the triangle, the one opposite the right angle). As a mathematical formula the sentence is written as X²+Y²=Z². The Pythagorean theorem is one of the most basic insights into the way the universe behaves, and it is important not only for geometry but also for other areas of mathematics, such as trigonometric calculus, irrational numbers and all kinds of other such curses. It is quite clear that any alien who dares to call himself a civilized alien, must know the Pythagorean theorem.

But writing the Pythagorean theorem just like that in the steppes of Siberia, it won't help anything. If we write X²+Y²=Z² in big letters, not only the aliens will not understand what we mean, even Chinese scholars will not understand us, because they have no X and no Y.

The solution, then, is to outline the basic principle behind the sentence, rather than the sentence itself. In this case, what can be done is to draw a huge right-angled triangle - say, by planting thousands of trees in the appropriate way, and then draw a square on each side of the triangle, so that the side of the triangle is one of the sides of the square. We will get three squares on the three sides of the triangle - for those who have trouble visualizing the whole thing in their head, I will try to put a link on the website to draw an example. The principle behind the drawing is that according to the Pythagorean theorem, the sum of the areas of the two small squares in the drawing will always be equal to the area of ​​the large square, and this is always true in any language: you just see it. Whoever sees these shapes from space, will immediately understand that they are not accidental - but that there is an attempt to explain the Pythagorean theorem without using letters and words.

This idea, of drawing the contiguous triangles and squares, by the way, is attributed to the mathematician Carl Friedrich Gauss, one of the greatest mathematicians of all time, although I know of no evidence for this. This idea has also not been realized in practice in Siberia or anywhere else in the world, as far as I know. A similar idea that was brought up at about the same time, was to dig long and wide trenches in the Sahara desert, fill them with a flammable liquid and set them on fire - thus creating similar letters or drawings that would be visible even at night. This idea, like the previous one, did not come to fruition either.

As the telescopes were perfected, it was clear that the moon (or at least its visible side) has no settlements of advanced civilizations, and is probably devoid of life. The attention shifted, naturally, to the planets closest to us - Venus and Mars. Prolonged observations of Venus revealed the fact that it is always covered by an impenetrable layer of clouds. As such, it was unlikely that intelligent beings there, if any, had developed advanced astronomical capability.

Mars, however, showed promising potential. It is considered to be an older planet than Earth, so the chance that intelligent life had time to develop on it was higher. The long canals that were discovered on it at the end of the 19th century also planted the feeling that it should be focused on as part of the attempts to search for life outside the earth and communicate with them. The French inventor Charles Cru, who was also known as a talented poet, heard in 1869 that mysterious points of light had been observed on the Red Planet. He became convinced that these spots, which were actually reflections of sunlight from high clouds in the Martian atmosphere, were actually signs of some advanced civilization. He tried to convince the French government to build a huge mirror that would allow him to focus the sun's rays and direct them to Mars, where he would burn large marks on the sand. It is unclear whether he actually tried to plan this appearance, or what the effect of an Earth-side 'death ray' attack might be on diplomatic relations with the surprised Martians, but the French government did not express too much interest in the idea.

Therefore, the question of communication between the stars continued to disturb the rest of the scientists, who were looking for more effective ways to realize it. The French Academy of Sciences announced, in 1900, a prize of one hundred thousand francs (a considerable sum) to whoever would find, within ten years, an effective method of transmitting messages to another planet and receiving messages back. It is interesting to note that the award, which was named 'Guzman Award' after the rich widow who donated it, did not apply to communication with beings from Mars - the popular opinion at the time was that it was too easy. Ten years passed, but no one was able to meet the conditions of the competition and win the prize.

But difficult problems are, happily, only a catalyst for interesting solutions and research in unconventional directions. In 1960, Dr. Hans Friedenthal proposed an artificial language called 'Lincus' (combination of the Latin words 'Lingua Cosmic', cosmic language) designed to allow two civilizations that have no No common background, understand each other. The ideas he put forward are based on the research of another mathematician from the early fifties, and he developed them into something more complete.

As Dr. Friedenthal understood, language is a very general concept that may contain much more than the words, letters and pictures that we humans use. Bees, for example, use smells to explain to other bees where the good flowers are, and chameleons change colors to convey to other chameleons... what interests chameleons, whatever it is.

The language invented by Friedenthal is very similar in principle to other languages ​​invented by humans to communicate with another alien entity that does not have much in common with us - the computer. Linux is in many ways reminiscent of a programming language, and is based on binary coding of information (that is, zeros and ones instead of letters) and the transfer of ideas through mathematical operations such as addition and subtraction, and with the help of basic logic. The use of binary coding is done because it is the most basic coding by which mathematical concepts or concepts in general can be described. It's like saying there is and there isn't or black and white in different combinations to describe a certain letter or some sound, because we can't do that if we have only black or only white. And as in the case of the Pythagorean theorem, once again we find that mathematics is the most basic common language.

At the moment, Linux is only a theoretical exercise, and has not been used in reality because several equally successful ideas have been put forward over the years. However, Linkos grew in its wake some interesting research and ideas that developed and refined it, as well as an interesting and not so well known field of research called 'anti-cryptography'. If cryptography aims to develop methods to hide the content that passes through a stream of information, 'anti-cryptography' seeks to do exactly the opposite: to create the simplest and clearest presentation of information, so that everyone (be it a computer or an alien) can easily understand the content .

The most intriguing idea, in my opinion, which has its roots in Linux, is algorithmic communication. The idea of ​​algorithmic communication is quite simple, despite the impressive name: to send a computer program to the aliens. The software itself will be sent via radio waves, of course, using the principles of the Linux language, but once the people on the other end of the line understand that it is a computer program (ie, an algorithm), everything becomes simpler. They don't need to understand the software, but just need to build a simple computer that knows how to run the software, and see what happens. The software we pass on to them could be, for example, some kind of artificial intelligence that can learn the language of the aliens by itself and then explain to them - in their own language - about the Earth, about humans and how wonderful, smart, etc. we are. An example of such software is the CosmicOS, a computer program that creates a simple but effective virtual reality environment where the aliens can experiment with interacting with simulated humans and learn from their reactions.

Algorithmic communication assumes, of course, that the aliens will be sophisticated enough to build a computer that will be able to successfully run software written here on Earth. This sounds like a very optimistic assumption, especially in light of the fact that the operating system here, on my computer, is not always capable of running software written on Earth. Nevertheless, we have reasonable grounds to assume that if the aliens were able to receive the transmitter from Earth in the first place, then they understand the technology of radio waves and astronomy, and from here to computer technology the road is not very long.

But when I want to describe to someone what the trip in Nepal was like, for example, I won't start pouring hours of stories on him: I'll show him a photo album. Sometimes, this is the simplest and easiest way to convey the message - especially when you don't have much time for long plans.

This was also the situation faced by the scientist Carl Sagan, in the early seventies. Dr. Carl Sagan, who passed away about ten years ago, was probably the most famous astronomer in the United States at that time. He frequently lectured in front of audiences and wrote highly successful books about astronomy and science in particular, and was an influential factor in the American space agency.

In 1972, the spacecraft Pioneer 10, and its sister Pioneer 11, were about to take off from Earth. Their initial purpose was to study the giant planets beyond the asteroid belt, namely Jupiter and Saturn. They are also slated to be the first man-made objects to leave the confines of the solar system and enter interstellar space.

Pioneer 10 and 11 are far from being a fast interstellar messenger service: even in the most optimistic estimates, it will be hundreds of thousands of years, perhaps millions of years, before the spacecraft pass near any star, and who knows what will be left of them by then.

Nevertheless, when two of Dr. Sagan's colleagues approached him and suggested that he attach some kind of message from Earth to the spacecraft, he was immediately enthusiastic about the idea. In the end, there is nothing to lose from attaching a 'greeting card', if you will, to spaceships. At the very least, if intelligent beings in space find the spacecraft one day they will, at least, know that they are not alone.

The problem was that NASA agreed to give him only three weeks to prepare the interstellar message, so that it would be ready in time for the launch of the first spacecraft, Pioneer 10.

Sagan, together with Dr. Frank Drake - this is from the Drake equation that I mentioned earlier - decided to create a visual message, an image and symbols, which would be engraved on a plate made of aluminum and gold. The two formulated the ideas and Sagan's wife, Linda Sagan, actually sketched them out.

The message that Sagan and Drake wanted to convey was quite simple: where did the spaceship leave for its journey, how long ago, and what do the creatures that sent it on its journey look like.

To convey concepts such as distance and time, you must first define meaningful units. Even here, on Earth, we first define what a 'meter' is and then count distances in units of meters, kilometers or centimeters. The meter, in this case, is defined as the distance a light beam travels in a vacuum in such and such a fraction of a second.

Sagan and Drake found an interesting way to define basic units of time and distance for the aliens. The element hydrogen is the most common element in the universe, it seems. The electron that surrounds the nucleus in the hydrogen atom has a property called 'spin': it doesn't matter for our purposes what exactly 'spin' is, and no, it has nothing to do with the electron's image and public relations. What is important is that spin can be in the up or down direction, and in the hydrogen atom the spin of the electron changes 1420 million times per second - or in the accepted technical language, at a frequency of 1420 MHz. Now that we have frequency, we also have a unit of time: the hertz, one cycle per second.

What's even more beautiful about Sagan and Drake's solution is that you can kill two birds with one stone here. If we have a frequency, we have a wavelength. Wavelength and frequency are complementary quantities: the wavelength is one of the parts of the frequency. In this case, the wavelength is twenty-one centimeters. Length, centimeter: here we got a unit of distance.

A schematic drawing of the hydrogen atom conveys this figure, on the reasonable assumption that the intelligent beings know hydrogen, understand what spin is and what frequency is.

Now that the units of time and distance have been defined, how can it be explained from where and when the spacecraft left its point of origin? Luckily for us, scattered in the universe are 'natural beacons', and they are called 'pulsars'. Pulsars are small, compact stars that spin around their axis at high speed, usually emitting very narrow radio beams. With each rotation, the radio rays wash over us for a short time, so that we receive radio pulses - hence the name pulsar.

The rotation rate of most pulsars is quite stable, and changes only over millions of years and even then at a known and constant rate. The drawing on the aluminum and gold plate shows, in the units of time and distance we defined earlier, the position of fourteen pulsars relative to the Earth, and the rate of their pulses. The creatures that will place their tentacles on the spacecraft will be able to understand from this, if they know at least some of the fourteen pulsars, where the Earth is according to the relative distance of each of the pulsars from the Earth and when Pioneer 10 and Pioneer 11 were launched into space according to the rate of their pulses, After all, only at one point in time did a combination of 14 different frequencies of the pulsars occur.

The presentation of the figures of the people of the Earth is, apparently, the simplest problem: just draw a man and a woman, and that's more or less. But precisely here the scientists ran into unexpected problems.

At first the painting showed a man and a woman holding hands, to emphasize the close connection between these two forms of life. But then Sagan realized that the aliens might not understand the message correctly, and think that we are four-legged creatures, with two heads and a long, narrow central part. That is why the man and the woman were separated in the drawing.

The man's right hand is raised in what I can describe as a 'hello man' pose, a completely universal movement assuming the universe only contains the earth. Obviously, aliens wouldn't understand that the raised hand is a greeting of peace, but Sagan explained that this way they would be able to understand that our limbs have a certain flexibility.

Then came the most serious problem of all: the genitals. As silly as the question of how the genitals should be drawn on the message board sounds, and it is indeed silly when you realize that there is a fairly low chance that an alien in a million years will find himself sexually aroused by a picture of a naked couple - this topic created a bit of a stir in the conservative and puritanical United States. Some people loudly criticized the decision to send 'gross pictures' into space, as if the scientists had attached an issue of Playboy to the Pioneer scientific instrument. Sagan himself was not sure about the level of conservatism in the US space agency, and to be safe he omitted the small vulva slit from the drawing of the female genital organ. In retrospect, NASA was liberal enough, and smart enough, to completely ignore the whole question of genitalia in the paintings.

There were also other criticisms of the message sent to space with the spacecraft. There were those who were concerned that revealing the exact location of the Earth might invite a brutal 'War of the Worlds' style attack on Earth. There were scientists who claimed that the message was too complex and that the aliens would not understand it at all. Either way, the two spacecraft carry the message to the stars with them, almost forty years after their launch.

For NASA's next mission outside the solar system, there was already enough time to prepare something a little more sophisticated. The Voyager 1 and Voyager 2 spacecraft left Earth in 1977 carrying yet another message in a bottle into the mighty ocean of space, if you will.

This time it was a golden record, which contained pictures, music and audio recordings of human conversations. The cover of the record contains drawings that explain to the recipient how to play it using a phonograph-like device, and also the time and location of the appearance on the Pioneer spacecraft.
The content that was burned on the disc was chosen by a NASA committee, again headed by Carl Sagan, and it includes natural sounds such as wind, thunder, etc., animal sounds and greetings of peace in fifty-five different languages, including Hebrew. The pictures are also representative, but this time - following the public criticism from the Pioneer days - without explicit pictures of naked people.

Representative musical pieces were also added to the record, from classical pieces by Bach, through African tribal rhythms to Chuck Barry and Louis Armstrong. Carl Sagan asked to add the song 'Here comes the Sun', by the Beatles, to the album - but even though the band members willingly agreed, their record company refused the idea and it was shelved. You can understand them: if the aliens start passing illegal copies of the song between them, go sue them after a million years, with the statute of limitations and all that.

The president of the United States at the time, Jimmy Carter, also added some of his own sentences to the record - a message that I personally find particularly moving. He said: "This is a gift from a small and distant world. A little of our voices, our science, our pictures, our music, our thoughts and feelings. We are trying to survive this period, so that we can also reach your times."

Sending aluminum plates and gold records into space is a favorite pastime for scientists, a beautiful theoretical exercise. But there are not many chances that these messages will result in real communication with other cultures, and in my opinion there is a good chance that the metal thieves are already waiting for the pioneer to leave Saturn in order to disassemble it into its components and sell it to the Chinese.

The only way to create any kind of communication between the stars is through radio waves, which travel at the speed of light. Even at this speed, many years still pass before a message reaches its destination, but twenty or thirty years is better than half a million.

I already mentioned earlier messages that can be transmitted via radio waves, mainly mathematical or algorithmic messages. It is also possible to send the visual messages, such as those engraved on Pioneer's plates, through the radio: this is a kind of compromise designed to exhaust the best of all worlds.
Quite a few such messages have been sent over the years, probably because it is easier to point an antenna into space than to launch a spacecraft. However, even this form of communication is not without limitations. Just as Tel Aviv Radio can be received in the central area and not far beyond, the radio signals transmitted from Earth to space fade with distance. To overcome the problem, it is necessary to transmit with a tremendous power, far beyond the transmission power required by a normal radio station. The problem can be minimized by transmitting the message in a very narrow beam of radio waves, similar to a laser beam, so that the signals are not scattered in all directions. This way you can reduce the required transmission power, but then you already have to direct the beam precisely towards the star you want to transmit to.

One of the most famous examples of a visual message sent via radio waves is the so-called 'Arecibo Message' in 1974. This message was sent from the Great Arecibo Radio Telescope in Puerto Rico, as part of a ceremony held there to mark an important upgrade of this important telescope. It is difficult to say that the Arecibo message is a real attempt to create interstellar communication, since it was mainly ceremonial. The message was sent only once, and its destination - the star cluster M13 which is twenty-five thousand light years from Earth - was chosen simply because it was in a convenient position in the sky on the day and time of the ceremony. Nevertheless, a lot of thought was put into the message being sent: here, too, you can find a symbolic representation of numbers, important chemical elements, even the chemical formulas of DNA and, of course, a drawing of a human figure. The broadcast received widespread attention in the media, and it was followed by several more similar broadcasts, some of which were rebroadcasts of the Arecibo message with additional messages - visual or otherwise.

What perhaps distinguishes the Arecibo message from other messages sent into space, is that in this case we even received an answer. The answer came in the form of a crop circle. A crop circle, for those who haven't heard the name, is a name for a shape that is created when agricultural crops such as wheat, soybeans or corn are spread over many meters and create a figure or painting that can usually only be recognized from the air. Astronomers attribute these circles, which usually appear mysteriously overnight, to attempts by aliens to contact us. The scientists, who are usually a bit more cynical, attribute crop circles to particularly bored and creative teenagers, and in fact it's pretty clear that this is the real explanation for crop circles.

Nevertheless, it is difficult to convince those who do not want to be convinced, and the crop circle that appeared in an agricultural field in the Hampshire region of Great Britain was a real celebration for all kinds of paleontologists. The message that appeared in the field was an almost perfect copy of the Arecibo message, but with some significant changes. Instead of the double helix, the same twisted helix that comes to indicate the shape of DNA we know, the answer to Arecibo contains a triple helix, and what is even more interesting: the human figure in the drawing is replaced by the familiar and iconic figure of the standard alien, the short one with the big head and the strange eyes .

Throughout this episode I focused on the messages that we, the people of Earth, are trying to send to the extraterrestrials wherever they are. But since I mentioned the crop circles as a possible attempt at an answer on the part of the aliens, albeit an obviously stupid and somewhat unclear attempt on the part of a culture that crossed light years just to disturb the corn growing quietly, I am obliged to mention, if only slightly, a slightly more serious attempt to receive a real message from the stars .

SETI, English acronym for 'Search for extraterrestrial intelligence', is the collective name of several projects whose goal is to try to receive radio transmissions originating from extraterrestrial civilizations. The motivation for these attempts is clear: if we can transmit messages to them, there is no reason why they should not try to transmit to us. Quite a few countries and many scientific organizations have invested considerable budgets in building radio telescopes whose purpose is to scan the sky, and try to find out from the constant noise that reaches us from space, real messages from extraterrestrial beings.

Here too, the technical challenges facing the researchers are not simple. Not only do you have to scan the sky piece by piece over a long period of time, you also need to target exactly the frequency on which the message was broadcast: even if you examined the part of the sky from which the message comes at exactly the right time, but you were not tuned to the appropriate frequency - you will not receive the message. Moreover, interference from television and radio broadcasts here on Earth does not make life easy for scientists trying to distinguish between a historical message from another culture and episode 4387 of the brave and the beautiful.

One of the accepted hypotheses is that there is a very reasonable chance that any message received from the aliens will arrive at a frequency of 1420 MHz, the frequency we have already mentioned as the signal emanating from the hydrogen atom. The explanation is that since this noise, of the hydrogen atoms, appears everywhere in the universe and comes from all directions at the same time with equal intensity, it will be relatively easy to distinguish between a real signal, a real transmitter, and just another random noise.

So far, no SETI project has been able to pick up a real message from space. The most intriguing signal in this respect was picked up in 1977 at Ohio University in the United States. The university's radio telescope, which was scanning the sky at a frequency of 1420 MHz, suddenly picked up a strong radio signal, more than thirty times the known noise level.

The scientist who examined the sheet of paper with the measurements on it, noticed the unusual letter, circled the relevant area and scribbled 'Wow!' with an exclamation mark. Hence this signal, which received much publicity, got its name - 'signal wow'. Unfortunately, the telescope that received the signal only scanned this area of ​​the sky for only seventy-two seconds, and when the scientists returned to scan the source of the signal - they did not discover anything there. Just the constant and familiar background noise. A real message from the stars, or maybe just a random bug in the space telescope? We may never know.

We on Earth will continue to listen and hope. Open your ears and listen carefully. Maybe the aliens are trying to contact us. Maybe even at the end of this chapter...

מקור

This article is taken from the show's script.Making history!', a bi-weekly podcast about the history of science and technology.

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