This is the fifth in a series. The first one can be found at http://sigma5.blogspot.com/2012/07/50-years-of-science-part-1.html. Part 2 can be found in the August 2012 section of this blog. Parts 3 and 4 can be found in the September 2012 section. Taking 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 - 1960), more than 50 years have now passed but I am going to stick with the original title anyway. In these posts I am reviewing what he reported and examining what has changed since. For this post I am starting with the chapter Asimov titled "The Windows of the Universe". This is the last chapter in his "The Universe" section.
Asimov starts the chapter with yet another reference to the 200" Hale telescope situated on Mt. Palomar in California. That gives me a chance to digress into some telescope basics. The telescope was invented about 1609 and popularized by Galileo. Before that astronomical observations were made by eye. About 3,500 stars are visible in the northern hemisphere with the naked eye. We now know that this is literally a drop in the ocean compared to the actual number of stars in the sky. Even in this period it turns out that what was most important to astronomers was the ability to measure angles as accurately as possible.
To address this problem people devised instruments. These included the quadrant, octant, and later the sextant commonly used by sailors for hundreds of years. This process culminated with the efforts of Tycho Brahe. He developed the most precise instruments ever devised to make human eye angle measurements. He died in 1601 just before the telescope was invented. His super-precise (for the time) measurements uncovered problems with the Ptolemaic astronomical system that had been in use for centuries. But before people could digest this and decide on an appropriate response the observations of Galileo started becoming known and they really threw a monkey wrench into the works. This diverted attention from what Brahe had found but the Brahe measurements ultimately bolstered the anti-Ptolemy side of the argument. Back to telescopes.
So what's the point? What does the telescope bring to the table? The first and most obvious thing a telescope does is magnify things. This instantly makes it possible to measure angles far more accurately than was possible with the naked eye. It also makes it possible to view things that are too small to see with just the naked eye. Galileo discovered the "mountains of the moon". He observed shadows cast by what appeared to be mountains when observing the edge between the bright and dark parts of the face of the moon. These features are too small to be seen reliably with the naked eye. He also was able to make out moons around Jupiter. Again, these are too small to see with the naked eye. Galileo saw a lot more things that upset the traditional authorities but that's enough for my purposes.
But being able to see the moons of Jupiter leads to another characteristic of telescopes. They can make dim things brighter. It is hard to see the moons of Jupiter not only because they are small but because they are dim. Galileo was able to easily make them out because his telescope made them brighter. This is generally referred to as "light gathering power". The previous property (making things bigger) is generally referred to as "magnification". And these are the two primary characteristics that the telescope brings to the table.
Now let's take a quick look at telescope design. The telescope we are most familiar with is the sailor's telescope prominently on display in swashbuckler movies. It consists of a tube with mirrors at each end. It does both of the things associated with telescopes. The lens at the front is bigger than the human eye and gathers more light making the image brighter. The two lenses work together to magnify the image. This makes it easy to see the other guy's ship in detail when it is many miles away on the horizon. And this is the design Galileo used. But it has a problem.
Lenses work because the material they are made of has a different index of refraction. All you need to know is that it is a property of things and that it is easily measured. A vacuum has an index of refraction. Air has a slightly different index of refraction. Water has a still different index of refraction as does the glass telescope lenses are made of. As light passes from material with one index of refraction to material with a different index it bends. Clever design allows this idea to be turned into a telescope. So what's the problem?
It turns out that the index of refraction depends on frequency. So lens glass bends red light through a different angle than it bends blue light. This isn't much of a problem with a sailor's telescope but it soon became a big problem with the large very precise telescopes that people built to observe the heavens with. This problem is called "chromatic aberration". It affects any optical device that passes light through lenses and processes more than one frequency of light. But there is a solution. And the man that found it was Isaac Newton. Talk about genius.
By now the general idea should be obvious. Don't pass light through lenses. But how could a telescope be made without lenses? Mirrors also change the direction light travels. Curved mirrors can bend light in just the way necessary to make a telescope. And that's what Newton (yes -- same guy) did. He put a big for the time (6") mirror at the bottom of a tube. Then he put a small (compared to the big mirror) flat mirror near the top of the tube. This mirror was angled at 45 degrees so that the light could come out a hole in the side of the telescope. That's where he put his eye. The eye contains a lens but the amount of chromatic aberration is still much smaller than with the old design (called a refracting telescope). This general class of designs is called a reflecting telescope as light reflects off the mirror. Pretty much all professional astronomical telescopes now use this reflecting design but with a modification.
A "cassegrain" telescope uses a perpendicular mirror at the front instead of a 45 degree one. The idea is to bounce the light back down the tube and through a relatively small hole in the center of the primary mirror. The astronomer's eye (then -- now sophisticated instruments) sits slightly behind the main mirror. The Hubble Space Telescope, for instance, is a cassegrain telescope. And for the first 400 years telescopes depended on the naked eye to make observations. But starting in the nineteenth century then more and more often as the twentieth century advanced photographic plates, often a foot on a side, were substituted. Since about 1970 the CCD or charged-couple device has been taking over from photographic plates. CCDs are the electronic equivalent of the photographic plate and are what the Hubble uses.
Now let me circle back to the Hale telescope. It was the largest in the world for about thirty years. Why? Isn't bigger always better. Maybe not. The point of a telescope is to make things larger and brighter. Lets take each in turn starting with magnification. For a while there was a telescope race as people came up with new designs to increase magnification. But the race didn't last long. Remember the "twinkle, twinkle, little star" nursery rhyme? Stars do actually twinkle.
We now know that this is caused by small irregularities in the air. Some air is slightly thicker than average and some air is slightly thinner than average. Winds move these thicker and thinner chunks around continuously. And it turns out that thicker air has a very slightly different index of refraction than thinner air. This causes these chunks of air to act like lenses and change the path of light. This means that sometimes the light from a star is being directed into your eye and sometimes it isn't. The star twinkles.
It 's kind of pretty when you are gazing fondly at the evening sky. It is really annoying when you are trying to observe something with a telescope. The greater the magnification the greater the twinkle effect. Telescopes were quickly built that were all twinkle and no observation. There was a limit to the amount of useful magnification a telescope could employ.
But none of this messes with the idea of increasing brightness, right? You have to work harder but it makes problems here too. We want to focus all the light of a small star onto a very small point so we get a sharp image of it. But the twinkle effect moves the location of the star around. And that results in blur. And if the star is dim enough you never end up with enough light in any one place to be able to see it at all. So this means there is a limit to the amount of light amplification that is useful. The 200" telescope superseded an earlier 100" model. Doubling the diameter theoretically gave the mirror four times the light gathering ability but in actuality it didn't work four times better. And there was another problem, gravity.
The mirror needs to have an extremely specific shape. If not then the light from a star won't all land at exactly the same place. It turns out polishing the mirror to the extreme level of precision necessary to give it the right shape was relatively easy. The problem was to make the mirror keep its shape. To do this it was made extremely stiff. And that was achieved by using a large, thick piece of glass. And that made it very heavy.
And as you use the telescope you are moving it around. And that means gravity is coming at it from one angle now and another angle later. It worked. But all the calculations that went into the design said it wouldn't work if you made the mirror a lot bigger, say 400". And all this weight made the telescope hard to operate. You needed very big motors to move it around. And these motors had to position the mirror very accurately or everything was a big waste of time. So for a long time it looked like 200" was as big as it was practical to go.
But modern telescopes are much bigger. Astronomers use the metric system so astronomers call the Hale not a 200" telescope but a 5 meter one (200" is almost exactly 5 meters). There are now lots of telescopes with larger mirrors. The Keck telescopes in Hawaii have 10 meter mirrors. The Europeans are currently building a 40 meter telescope and designs have been proposed for even larger ones. The James Web Space Telescope will house a mirror larger than that of the Hale but in space. Something obviously changed but what?
The easiest to understand change is that of going from one single main mirror to a main mirror made up of several segments. Each individual mirror segment is much smaller than the mirror as a whole. This involved solving the math problem of determining the specific shape each mirror segment needed to be. Then the harder problem of making mirror segments in these odd shapes needed to be solved but it was. So what's the advantage of segmenting the mirror? It means that the stiffness problem is much easier to solve. One edge of the 200" Hale mirror has to be maintained in a very precise relationship with the other edge. But if the edges are now say 50" apart this becomes much easier to do. So the mirror glass can be much thinner and still be stiff enough. And that saves a lot of weight.
The other design change was to dial way back on the whole "stiff" thing. If instead of depending on the mirror's built in strength to maintain its shape what if we take very precise measurements, determine what corrections need to be made, and then bend the glass until it is in the proper shape. Computer power and lasers made it possible to perform the measurements and calculate the corrections to the necessary accuracy. Then it was a simple process to put a gadget (actually several gadgets) on the back of each mirror segment to bend it the right amount to get the shape right.
It now became important to make the glass thin enough that it could be bent appropriately. This in turn took a lot more weight out and allowed everything to be lighter and cheaper. So this new "bend on demand" approach made it possible to have a mirror whose effective size was much larger than before but which still had the right shape. But what about the twinkle problem?
It turns out that this "bend on demand" capability came to the rescue here too. With even more computing power (now cheap) it was possible to calculate exactly how much the irregularities in the atmosphere were messing things up. This made it possible to calculate how to bend the mirror out of what would normally be the "correct" shape just enough to undo the twinkling the atmosphere was causing. The corrections would have to be calculated and applied frequently (about 100 times per second) but it meant that it was possible to take a much sharper picture of the sky from the bottom of the atmosphere.
This process is called "adaptive optics" and all the big telescopes now have adaptive optics systems. The last thing you need to make it work is a "guide star". Measuring it allows the distortions and corrections to be calculated. The guide star can also be used to verify that the right correction was applied. If there is a bright star handy close to the portion of the sky you want to point your telescope at then it can be used. Otherwise, synthetic guide stars can be created using lasers and other tricks. And with that, let me return to Asimov's book.
Remember that chromatic aberration I was talking about. And remember the old saw about "turning a problem into an opportunity". That's the first subject Asimov gets into. If you introduce a piece of glass into the path of the light from your telescope it will bend the light. And it will bend the light by different amounts that depend on the frequency of the light. The piece of glass used for this purpose is called a "prism". A well designed prism will accentuate this phenomenon. Why do it? Because this allows us to study each frequency of say the light from a specific star, separately. And that allows us to learn a lot about the star. Studying the various frequencies is called spectroscopy and a device for spreading those frequencies out so that each frequency can be examined individually is called a spectroscope and the resulting pattern a spectrograph. (And this whole business of studying the spectrum of light also goes back to Isaac Newton.) So what can we learn from the spectrum of a star?
"White" light will contain all frequencies and the intensity of each frequency will follow a specific pattern. But real light always has bands that are either brighter or darker than they are supposed to be. Franhoffer first reported this in 1814. The dark lanes are "absorption" lines where something has absorbed a particular frequency as the light passes through it. The brighter lines are "emission" lines. Something, say a candle, has emitted extra light in particular frequencies. It didn't take long for scientists to speculate that specific elements caused specific lines. It turned out that they were right.
Early work identified the new at the time elements of Cesium and Rubidium. Then in 1868 Helium was identified in the spectrum of sunlight. Cesium and Rubidium were rare but could be found on earth if you looked hard. At that time no one had found any Helium anywhere. (It was later discovered to be a trace component of natural gas and can also be found in even smaller quantities elsewhere.) The use of spectroscopy to discover solar Helium was a big deal and really put the technique on the map.
These early spectroscopy studies were done "by eye". The first major step in moving away from this was the invention of the daguerreotype, an early photographic method, in 1839. As the century progressed photographic techniques improved and photographs became more common in astronomy. And photography made it possible to use telescopes in a different way. The standard way is to peer very closely at a small part of the sky. But this means you can't do a broad "survey" of a larger portion of the sky. Then in the 1930's Schmidt came up with a telescope design that could do surveys. But you could not do it with your eye. You had to use photography. This was an early example of moving on beyond what the naked eye could show us.
But spectroscopy turned out have very down to earth uses. Around 1800 Herschel moved a thermometer beyond the visible part of the spectrum. He detected a warming. He had discovered infrared light, light with a frequency lower than that of red. Additional investigation by others led to the discovery of ultraviolet, light with a frequency higher than violet. The "electromagnetic" spectrum has since been expanded to include (from highest to lowest) gamma rays and x-rays (usually subdivided into hard (higher frequency) and soft (lower frequency)) on the high end and various forms of radio waves (from highest to lowest: microwave, shortwave, and long wave) on the low end.
It turns out gamma rays, x-rays, and infrared waves are all very effectively blocked by our atmosphere. But radio waves are not. In 1933 Jansky detected radio waves coming from the sky. This was the start of what is now a booming field of endeavor, radio astronomy. Radio astronomy was in its infancy when Asimov wrote his book. Big dish antennas were just coming into being. And the now famous giant dish at Arecibo in Puerto Rico was not completed until 1963. Nor had Lasers been invented yet. But the predecessor to the Laser, the first Maser had been built. The idea is the same as that behind the Laser but a Maser uses radio waves and a Laser uses light waves. It was easier to pull the necessary engineering off with radio waves so the Maser came first by about ten years. And, although radio astronomers invented the Maser and had done so before the book was written, they were still figuring out how to make the best use of Masers so Asimov does not even mention them.
Another technique that is now in common use is radio interferometry. This is the process of combining signals from two or more widely separated radio antennas to create a "synthetic aperture" that is as large as the distance between the antennas. A problem Asimov does address is that fact that some radio wavelengths are very long. This means you need very large equipment to do anything with them. The synthetic aperture scheme got around this problem and allowed radio astronomers to make effective use of low frequency (long wavelength) radio waves. But this came well after Asimov's book. And this synthetic aperture scheme has now been adapted for use with light telescopes. There are actually two Keck telescopes in Hawaii. They can be operated together in such a way as to behave like they are one single telescope with a mirror diameter of 85 meters (almost 300 feet).
One issue Asimov does get into is mapping the Milky Way. As Asimov put it, "[i]n a sense, the galaxy hardest for us to see is our own." Radio astronomy has been a big help. Light is blocked by clouds of dust like the "coal sack" in the southern hemisphere. But radio waves can penetrate dust easily. This led to the mapping of the Orion, Perseus, and Sagittarius arms of the Milky Way. But these arms are only partially mapped and there has not been a lot of progress since Asimov's book. And the existence of a giant black hole (it weighs millions of times as much as our Sun) in the center of our galaxy was totally unsuspected at that time. As was the Cosmic Microwave Background, the single biggest discovery in the field of radio astronomy. It was discovered less than a decade after the book was written.
Asimov does list a number of achievements that had been racked up by radio astronomy. These include a number of bright radio sources in the sky, the fact that sunspots emit radio waves, the fact that the atmospheres of both Venus and Jupiter are turbulent enough to emit radio waves (as does the cloud surrounding the Crab Nebula), the fact that galaxies collide, and others. And then there is that workhorse of spectroscopy that was first made use of by radio astronomers, the Doppler effect. As I have indicated elsewhere, it can be used to measure the speed with which stars (and any other object bright enough to allow a detailed spectrograph to be taken) are moving toward or away from the Earth. Asimov reports the results of early Doppler work.
At first it might seem like this is a story of scientific results being overturned but that is not so. In fact, what is on display is a progress toward more and better information. This does result in some scientific theories being overturned. But that's why scientific theories are theories. The possibility always exists that new data will come along that will show them to be wrong in some important way. But before scientists move on to a new theory they make sure it accounts for all the old observations.
And scientists are pretty good at figuring out how solid their theories are. Scientists see it as part of their job to theorize before all the data is in. But they admit it. They even have a name for this sort of thing. It's called a WAG, a Wild-Assed Guess. As more data comes in this might be replaced by a SWAG, a Scientific Wild-Assed Guess. Neither rises to the level of a "theory", which must have more support. And some theories are "tentative" while others are "pretty solid". From there it can move on to being a "well tested" or "foundational" theory or not. It depends on the data.
And mostly what we see are new discoveries made possible by new and better tools. The new information does not overturn the old. It supplements it or "opens new vistas". Large though it is, there were no tools in 1960 that could have detected the giant black hole at the center of our galaxy. Scientists knew roughly where the center of our galaxy was ("somewhere in the Sagittarius Constellation") but were the first to admit they knew little to nothing about what was there.
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