Chapter 5D
A NEW VISION OF THE UNIVERSE
When Galileo turned his telescope to the heavens a new sense was born. Galileo's telescope gave us the moons of Jupiter, the phases of Venus, and thousands of new stars. But this new sense was no mere addition, for it helped usher in a view that contradicted direct sensory experience. As Galileo himself said, "…there is no limit to my astonishment, when I reflect that Aristarchus and Copernicus were able to make reason so to conquer sense that, in defiance of the latter, the former became mistress of their belief."[1] But in the telescope he found a "superior and better sense than natural and common sense".[2] This new sense, furthermore, was not just a refinement of sight, but rather an alternative that agreed with the Copernican view, unlike plain sight. Indeed, whereas to plain sight the magnitude of Mars changed little--a troublesome fact for the Copernican view according to which the distance between Mars and the Earth varied considerably--in looking through the telescope the magnitude behaved as if God had been a Copernican. Likewise, the phases of Venus explained why the magnitude of Venus remained constant: when Venus is closest we only see a small portion of its disk lighted, but as it moves away from us we see more and more of its disk lighted, until when it is furthest from Earth we see it fully lighted.
In this way a new technology came to the rescue of an idea that was to transform our view of nature most profoundly. Not that the telescope was free from reasonable question, for on the contrary, given what was known about optics and perception at the time, its reliability was a lucky assumption made plausible more by Galileo's enthusiasm than by his argument. Among other things, we should keep in mind that the brain does not merely experience the pattern of light that strikes the retina: it interprets that pattern on the basis of past experience, expectation, and environmental clues. It also imposes constancies of shape, color, and size upon the visual image. When peering into the skies, however, many of those clues are absent and thus the untrained eye can experience many illusions. And indeed many people looking through Galileo's telescope did not see what he saw.[3] Moreover, it was generally thought that perception cannot be trusted when the natural medium through which the information travels has been tampered with (e.g., a haze or a drunken brain). Galileo's telescope was theoretically suspect, then, because it clearly altered the natural medium through which the light from distant objects was transmitted to the eye.
But the telescope did open up many avenues of observation and investigation that would not have been there otherwise. It was a promise from the heavens better kept in the course of the new science than perhaps Galileo had a right to imagine. For him it was too striking a coincidence that the telescope would so match the new astronomy of Copernicus. For others, whose fundamental views were at stake, it was a case of a distorting instrument of observation presuming to support a refuted and obnoxious view of the cosmos.
To most of us now it seems fortunate that, as Galileo put it, Copernicus "with reason [theory] as his guide...resolutely continued to affirm what sensible experience seemed to contradict."[4] Many of those who were in a position to choose decided in favor of the most exciting of the alternatives (in accordance with a principle that NASA scientist Brian Toon fondly calls "Sagan's Razor"). Whether they had other, more compelling reasons I shall not discuss here. Suffice it to say that the invention by Newton of the reflecting telescope and the refinement of both kinds of telescopes permitted not only the discovery of many new objects in the universe, but also a shift in perspective about its nature. And having embarked on a different approach to nature, the new scientists also had the motivation and opportunity to develop the auxiliary sciences (e.g., optics, electromagnetism) that eventually established the reliability of the telescope as well.
Such reliability is rather limited, as we have learned. Apart from obvious problems like background lights and bad weather, optical telescopes until recently had to contend with the effects of columns of air within them and of gravitational distortions of their very large mirrors. Worse still, even though a large telescope such as the 5-meter telescope on Mount Palomar could in principle separate the images of objects in the sky as close to each other as 0.02 arc second (this is called angular resolution), the motion of air molecules blur the path of light through the atmosphere to the point that the best angular resolution we can achieve is about 50 times worse than that. Such was the situation at the point when space technology could begin to place optical telescopes in orbit. Today those space telescopes work in concert with new generations of Earth-bound telescopes in which computers compensate for some of the distortions of the atmosphere and in which combinations with other such telescopes (in interferometers) allows the collection of much greater amounts of light.
Moreover we know now things that Galileo could not have the tools to imagine. Light is a form of electromagnetic radiation and the Earth's atmosphere blocks many other forms of that radiation: More specifically, it absorbs the X-ray and gamma frequencies, as well as most of the ultraviolet and several bands of the infrared. The envelope of gases that made life possible in the beginning, and has protected it ever since, has given us only a small window into the universe at large. Through that window we have seen and we have dreamt, and through that window we have also adjusted our views of nature. Galileo's new sense became better and better, and when it seemed that it was reaching its limitations, in the middle of the 20th century, we discovered radio astronomy, and then infrared astronomy.
By then we had found that the sun was not the center of the universe either, that it was one among billions of stars in the outskirts of a rather common galaxy, and that the galaxies were receding from each other, that is, that the universe was expanding. This last discovery prompted Einstein to recognize what he called his greatest mistake. The idea that the universe might change in size had seemed so preposterous at the time he proposed his general theory of relativity that he introduced a constant into the theory to ensure that the universe would appear static in it.
So much resulted from the evolution of Galileo's instrument. But then, with the discovery of radio and infrared astronomy, new kinds of objects and new kinds of activity in objects already known gave us a glimpse of what a look at the universe in the full electromagnetic spectrum may do. Quasars and pulsars, and the radiation left over from the big bang began to tell us about a universe far stranger than we had imagined. With the advent of space exploration we can go above the atmosphere to examine the universe in the full electromagnetic spectrum. We can also do far better astronomy in the more traditional wavelengths (radio, optical, and infrared).
For example, in the visible range, NASA's Hubble Space Telescope, unencumbered by most of the handicaps of its terrestrial counterparts, swept a volume 350 times larger and produced images ten times finer than most traditional telescopes could at that time. Even more sophisticated future space telescopes may allow us to detect planets the size of Earth around stars within 10 light years from us. Radio astronomy, which allows us to study regions obscured by dust, as well as some very cold or very hot objects, is not hampered much by the atmosphere, but it is limited by the size of the Earth. Even though the angular resolution of the largest radio telescopes is very poor (about 10 arc seconds), they can be placed together in large arrays (with the telescopes at large distances from each other) that are equivalent to gigantic telescopes. The largest such array (an interferometer called the Very Long Baseline Array) has a baseline almost as long as the diameter of the Earth and attains a resolution of 0.001 arc seconds. To attain a finer resolution we need a larger baseline, and the only way to secure that is to place some radio telescopes in space (with the first such extensions the resolution of the Very Long Baseline Array will improve by a factor of 100).
Infrared astronomy permits us to study the incredible energy released in the center of remote galaxies and other events taking place at the edge of the universe. For the objects in question are receding so fast from us that their light has been shifted into the infrared spectrum. But infrared astronomy is also of importance closer to home, as we will see in the last section of this chapter.
Let us see now some of the ways in which this developments help us understand how space physics and space astronomy are fundamental sciences.
Posts
No comments:
Post a Comment