These days it's actually more like "58 Years of Science" but I am going to continue to stick with the original theme anyhow. This is the ninth post in the series. You can find an index to all of the posts in the series at http://sigma5.blogspot.com/2017/04/50-years-of-science-links.html. I update that post every time I add a new entry to the series.
Anyhow, I take the Isaac Asimov book "The Intelligent Man's Guide to the Physical Sciences" as my baseline for the state of science as it was when he wrote the book (1959 - 60). In these posts I am reviewing what he reported and examining what's changed since. For this post I am starting with the chapter Asimov titled "Magnets" and then moving on to "Meteors". These sections are from his section entitled "The Atmosphere".
Asimov starts with where the word "Magnet" came from (a town called "Magnesia") and notes that "lodestone" is another name for a naturally occurring magnet. Magnets were only a curiosity until people figured how to impart magnetism to materials like steel. This made it relatively easy to float a magnetized needle in a bowl of water or mount it on a pivot. Either way it could now rotate freely. And one end generally pointed toward the north. It could be used as a compass when landmarks were invisible or not available. In 1600 Gilbert figured out that the earth itself had a magnetic field.
The earth's magnetic field made one end of the compass point toward the north. But a local source of magnetism could deflect the needle away from north. That's why I had to say "generally" above. But the method was pretty reliable as there were few competing magnetic fields about. It was certainly better on average than other methods available at the time. A critical observation that led him to this conclusion was that if a magnetized needle was allowed to revolve through three dimensions it did not stay level. Instead it pointed down into the earth.
Initially there was no satisfactory explanation for why earth had a magnetic field. Asimov attacks this problem by moving on to Faraday's observation that if you jiggle small fibers of iron on a piece of paper over a magnet they will line up. This makes the pattern of the magnetic field visible. From there imagining that a similar but much larger field was somehow created by the earth was a small step.
Faraday also intuited that if an electric field could make a magnetic field as Oersted had observed then it might be possible to reverse the process. He succeeded in showing that his intuition was correct. The result was the first demonstration of how to use a moving magnet to create electricity. This led directly to the electrical transformer and many other devices we now depend on.
This in turn led to the invention of the dynamo, a device for turning other kinds of power into electric power. From there it was a short step to the invention of the electric motor, a device for turning electric power into other kinds of power. This led directly to the "unification" of electricity and magnetism into what we now call electromagnetism.
A Scottish scientist named James Clerk Maxwell codified how electromagnetism worked into a set of laws he published in 1861 and 1862. We now speak of Maxwell's Equations, the mathematical embodiment of his laws. These laws have stood the test of time. Neither relativity nor quantum mechanics has required their revision or abandonment. And nothing that's happened since Asimov's book was published has changed this.
Returning to earth's magnetic field, by 1960 some early ideas were being developed as to what was going on. Elsasser, for instance, speculated about how slow eddies in molten Iron might produce the field. His and other ideas have been enhanced and elaborated on since. But there are still a lot of questions and not much in the way of solid answers as of now.
Understanding of solar magnetism, sun spots, and the like was just in its infancy. This is currently a field of very active investigation. Giant whorls of solar material periodically erupt from the surface of the sun. Magnetism is involved. There is a "solar wind" of charged particles that is launched from the sun and hits the earth. A strong enough solar wind can damage satellites, damage or knock out electric power grids, and by a number of methods generally wreck havoc on the earth and the lives of its inhabitants. And all of it is tied up with solar magnetism. In the interest of brevity I am going to leave it at that.
Asimov observes that the magnetic and rotational poles of the earth do not align. Besides confusing scientists this means that magnetic compasses don't necessarily point where we want them to. This discrepancy is called "compass deviation" and led to the development of the gyrocompass in 1911. Gyrocompasses depend on high tech tops spinning at very high speed to maintain a sense of direction.
Since 1911 gyrocompasses have gotten a lot better. They quickly became more accurate than magnetic compasses but they were also much more expensive. We now have even better alternatives. One, creating an even more expensive but also more accurate gyrocompass-like device by sending light through long coiled up fiber optic cables, was popular with the military and on commercial airplanes for a while. But mostly we now rely on GPS.
The idea of being able to detect and suitably process weak radio signals using small and relatively inexpensive hand held devices was unthinkable in 1960. It is now commonplace. And the other marvel necessary to make GPS work, the ability to easily measure time to the level of accuracy required to make GPS work in such a device, was also unimaginable in 1960. And yet your run of the mill smart phone now does it so effortlessly we no longer notice.
Asimov mentions the then controversial idea that the magnetic poles of the earth could flip completely. We now know that the poles have completely reversed their polarity a number of times over millions of years. The history proving this is written in the rocks that make up, among other things, the bottom of the Atlantic Ocean.
The orientation of the magnetic field can be measured by studying small particles of Iron in lava that has long since solidified. These particles measure the direction of the earth's magnetic field at the time the lava solidified. We have other ways of knowing when the lava solidified. These bands showing magnetism switching back and forth are written on the floor of the Atlantic. This discovery was one of the most important lines of evidence that led to the general acceptance of the idea of Plate Tectonics. In 1960 Plate Tectonics was inside knowledge known only to the few who were directly involved in the research. But the word spread quickly and the Plate Tectonics revolution was well under way by 1970, only a decade after the publication of this book.
Scientists were just starting to investigate solar weather, the causes and impact of the Aurora Borealis (northern lights) and the similar Aurora Australis (southern lights), sun spots (what they were and what caused them), and many "solar" subjects in 1960. A lot has happened since then. Several satellites have been launched specifically to study the sun. We now know that sun oscillates (ring like a bell) in various complicated ways that tell us a lot about what's happening under the surface. But the state of the art when it came to studying the sun previous to the publication of the book had made enough progress to produce a discovery about something much closer to home. A prediction was made and validated that something called the "ionosphere" existed and was part of earth's upper atmosphere.
Just before the book was published the "Van Allen" radiation belts that circle the earth and make space travel dangerous for humans had just been discovered. Space is "empty vacuum", right? Well, actually not. There are lots of different things in space at extremely low concentrations. Investigating this sort of thing was just starting in 1960. We have now mapped out a lot of this in some detail. Low concentrations of electrically charged particles are flying around in complex patterns. A lot of these particles come from the sun but some of them come from other sources. Moving electric particles mean moving magnetic fields. The earth swims in a complex set of continuously changing magnetic fields. These fields literally keep us humans alive by diverting lots of very nasty stuff away from the surface of the earth.
Before leaving this chapter let me make the following observations. Scientists have known for some time that earth's atmosphere has layers such as the ionosphere. By the time of Asimov's book they knew that sun's "atmosphere" also had it's own set of layers including the corona. We now know a lot more about these subjects than we did in 1960 but we are still just scratching the surface. But what about the post-atmosphere, the stuff above the regular atmosphere?
In the case of the earth I am talking about the Van Allen radiation belts, what we now call the "magnetosphere" (the structure of magnetic fields that surround the earth), etc. Since then we have gone from having just discovered the existence of one component to having a more complete inventory of components and knowing something about each component.
And we now know there is also a post-atmosphere around the sun. It consists of things like the solar wind. And in a manner analogous to the discovery of the Van Allen belts just before Asimov's book was published, we think we have just recently detected the edge of the solar system. A couple of years ago the Voyager I space probe got far enough from the sun to detect changes in the solar post-atmosphere. This seems to demark the boundary between the solar system and interstellar space.
Interstellar literally means "between suns". So the interstellar space we are talking about here is in actuality the "atmosphere" of our local galaxy, the Milky Way. It is possible there is a boundary that demarks the end of the Milky Way and the beginning of intergalactic space. We don't know and presently have no way of ever finding out the details or even if such a boundary exists. If it exists it is just too far away to study. But the Milky Way is part of a galactic cluster boringly called the "Local Cluster". This too might have a boundary. As might the local galactic super-cluster (which may or may not be called the Virgo super-cluster - I've seen it called a number of things) that our Local Cluster is a part of.
On to meteors. The Greeks gave them the nickname of "shooting stars" because they behaved like stars that shot across the sky. Stars are not supposed to do that, at least not according to the Greek cosmology of the time. (And this cosmology remained unchanged until the advent of the scientific revolution.) Instead, they are supposed to stay in one fixed place in the heavenly firmament for eternity.
The visible lifetime of a meteor is short, perhaps a minute. But Aristotle observed that the number of stars visible in the heavens seemed to remain the same even after a "shower" of meteors. The name meteor means "things of the air" because that's where Aristotle concluded they hung out. (This conveniently meant that the "unchanging heavens" part of cosmology could be retained as the air was not part of the heavens.)
We now know that meteors actually are things of the air. What we see is them burning up as the move through the air at incredible speed. And most of them burn up completely before they hit the ground. Confusingly, the odd meteor, or more likely meteor fragment, that makes it all the way to the ground is called a meteorite. I always found that confusing.
Very occasionally the ancients were able to connect debris on the ground (what we now call meteorites) with a particular meteor. This seemed like a rare and profound and important event, an omen if you will. So on a number of occasions an object of meteoric origin of significant size took on religious significance. So the Kabala, the most sacred object in Islam is a meteorite. A meteorite is mentioned in the Iliad. This also happened at other times and in other places.
The beginnings of science, at least in the western world, occurred in a place and at a time when the influence of the Catholic Church was great. Meteors and especially meteorites did not fit into Catholic cosmology. That spilled over into the behavior of western scientists in that they initially denied the cosmic origins of meteorites. This slowly changed in the early 1800's. A giant meteor shower (an event where a large number of meteors streak across the sky in a short period of time) in the US in 1833 was one of several events that caused scientists to decide to take a fresh look.
Careful study elicited the information that some meteor showers seemed to reoccur on a fixed schedule and the meteors associated with a specific shower seemed to be coming from roughly the same place in the sky. (A number of popular meteor showers that are easily seen with the unaided eye are now named for the constellation they appear to originate from.) This allowed astronomers to treat these events like observations of a heavenly body and see if they could calculate an orbit. Schiaparelli was the first to notice that these orbits looked like cometary orbits. We now know that in a number of instances a comet lost all its gas and ice and that the cluster of rocks and gravel that remained became a cloud. When the earth passes through that cloud a meteor shower results.
Once scientists believed that meteorites were a real thing and deserved careful study they initially concluded that most meteorites were made of Iron. This turned out to be an error caused by sampling bias. It's just that a chunk of meteor iron is pretty distinctive so it is easy to identify correctly so it is correctly categorized a high percentage of the time. A more careful study determined that most meteorites actually are "stony". They contain non-metallic elements that are "refractory", they have a very high melting point. This class of meteorites is easy to misclassify as something other than a meteorite.
Meteorites are rare. As of 1960 (I don't think this has changed since) no one was known to have been killed by one. But big ones have made a splash. Asimov lists a few like the one that landed in Siberia in 1908. Around the publication date what we now call Meteor Crater in Arizona was thought by only a few scientists to be of meteoric origin. Most scientists thought it was of volcanic or other non-meteorite origin. We now know for sure it was caused by a meteor impact. Interestingly enough, we now know that the opposite situation pertained to Crater Lake in Oregon. Many people used to think it was a meteor crater. We now know it was created when a volcano called Mount Mazama exploded about 7,700 years ago..
And we now know that the Dinosaurs were wiped out 67 million years ago by a large meteorite that hit the Yucatan peninsula in Mexico. It and numerous other large but old meteorite craters have been discovered since 1960 with the help of satellites and other modern tools. Many of them are invisible to the naked eye.
The study of lunar craters was just beginning at the time of the book's publication. The Russian "Luna" space probe had just released pictures of the far side of the moon in 1958. And, as I discussed in a previous post in this series, telescopes (i.e. Hubble Space Telescope) and telescopic techniques (i.e. adaptive optics) that would deliver really sharp pictures of the surface of the moon (and whose coverage extended to the far side) did not exist in 1960. Nor at that time had the Apollo missions made it to the moon, collected samples from its surface, and returned them to earth for study.
We now have sophisticated methods for estimating the age of lunar craters. So we now know that most of the craters of the moon happened early in the life of the solar system. And we have been able to study other parts of the solar system in some detail. Pretty much everything has craters. Even Venus, which has a surface temperature of about 800 degrees and a fierce atmosphere, has a few (very few) craters. More informative are the craters on Mercury, Mars, and some of the moons of the gas giant planets. In the case of some of these moons, for instance, we know something is going on. If the surface has too few craters it is young. A number of these moons have areas almost completely devoid of craters so we know that fresh surface is being created right now. That's exciting.
At the time little was known about the origin of meteors and meteorites. There was the business about meteor showers I noted above. And, as noted by Asimov, the Asteroid Belt between Mars and Jupiter was known of and seemed like a likely source. There has been a lot of progress since then. We now know that some meteorites actually do originate in the Asteroid Belt. But most come from somewhere else. For instance, some meteoroids have been positively identified as coming from the moon and others from Mars. This has proved more useful in learning about the history and composition of the moon and Mars than it has been about meteors and meteorites. Scientists have also learned some tricks like prospecting in parts of Antarctica for meteorites. So they now have a lot more meteorites to study now than they did back then.
And the science of celestial mechanics has advanced quite a bit. So we know a lot about processes that are likely to sent things our way. This ties into something Asimov talks about. There are rocks passing close to earth all the time. Certainly the big ones are dangerous. That was recognized back then. But only the largest rocks like Eros were big enough to be identified, let alone studied. Eros presents no threat. But even the meteorite that wiped out the dinosaurs would have been hard to detect with 1960's technology. And if detected there was nothing at the time that could have been done.
We (in the form of NASA) now takes the business of looking for anything that is big enough to be dangerous and is also heading our way very seriously. And we have some capability that would allow us to deal with a dangerous one if we are given enough lead time. We are now confident we have spotted or will soon spot all of the very large (but still way smaller than Eros) rocks. And if it is small enough it will burn up in the atmosphere harmlessly. But there is still a group that is too big to burn up but too small to detect. Tools and techniques for handling these potential bad boys are getting better but they are not at 100% yet. They are, however, working well enough that every couple of years the news ponderously announces that some rock missed us by a million or so miles. This is truly a case of "a miss is as good as a mile".
Asimov notes the same thing I did above. Meteorites allow the study of things that would otherwise be out of reach. Some are thought to be "pristine", little changed since the early days of the formation of the solar system. Studying one of these is like studying the early solar system without needing a combination time machine and space ship. He notes that by using radioactive dating techniques age can be measured. One such measurement Asimov reports pegs the age of the solar system at 5 billion years. This is very close to the modern figure of 4.6 billion years.
Using various techniques we now know a lot about the sequence and timing of various events in the formation of the solar system. There are dating techniques that tell us approximately when it formed. But other techniques allow us to know with surprising precision how long various processes took. So we know that the sun formed and by a very short time later (perhaps a hundred million years or less) pretty much all of the planets, moons, etc. had also formed and evolved to pretty much their current form. It now looks, however, like the distance from the sun at the time of formation of the gas giant planets, for instance, was quite different than it is now. There is a lot that is not understood about where in the solar system various items were formed and how they got to where they are now. But that's a subject for another time.
Interesting enough, as Asimov informs us, there is a lot of stuff bombarding the earth on a daily basis. 4.6 billion years is a long time so it wouldn't take a very high rate of bombardment for the total to add up to a significant percentage of the current mass of the earth by now. But calculations done then (circa 1960) and calculations done now indicate that the amount of additional material being added to the mass of the earth by this process is insignificant. Most of what hits earth on a daily basis is truly tiny. 20,000 meteorites weighing at least a gram hit the earth each day. And at least 200,000 smaller meteors hit. But the fact that the total amount of mass is so small doesn't mean they pass unnoticed. There is a phenomena called "Zodiacal light". Zodiacal light is not very bright. But it is still bright enough that it can be used to make the above estimates with considerable confidence.
Next time: The Origin of Air.
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