Death by Black Hole: And Other Cosmic Quandaries - Neil deGrasse Tyson (2014)
SECTION 1. THE NATURE OF KNOWLEDGE
Chapter 3. SEEING ISN’T BELIEVING
So much of the universe appears to be one way but is really another that I wonder, at times, whether there’s an ongoing conspiracy designed to embarrass astrophysicists. Examples of such cosmic tomfoolery abound.
In modern times we take for granted that we live on a spherical planet. But the evidence for a flat Earth seemed clear enough for thousands of years of thinkers. Just look around. Without satellite imagery, it’s hard to convince yourself that the Earth is anything but flat, even when you look out of an airplane window. What’s true on Earth is true on all smooth surfaces in non-Euclidean geometry: a sufficiently small region of any curved surface is indistinguishable from a flat plane. Long ago, when people did not travel far from their birthplace, a flat Earth supported the ego-stroking view that your hometown occupied the exact center of Earth’s surface and that all points along the horizon (the edge of your world) were equally distant from you. As one might expect, nearly every map of a flat Earth depicts the map-drawing civilization at its center.
Now look up. Without a telescope, you can’t tell how far away the stars are. They keep their places, rising and setting as if they were glued to the inside surface of a dark, upside-down cereal bowl. So why not assume all stars to be the same distance from Earth, whatever that distance might be?
But they’re not all equally far away. And of course there is no bowl. Let’s grant that the stars are scattered through space, hither and yon. But how hither, and how yon? To the unaided eye the brightest stars are more than a hundred times brighter than the dimmest. So the dim ones are obviously a hundred times farther away from Earth, aren’t they?
That simple argument boldly assumes that all stars are intrinsically equally luminous, automatically making the near ones brighter than the far ones. Stars, however, come in a staggering range of luminosities, spanning ten orders of magnitude—ten powers of 10. So the brightest stars are not necessarily the ones closest to Earth. In fact, most of the stars you see in the night sky are of the highly luminous variety, and they lie extraordinarily far away.
If most of the stars we see are highly luminous, then surely those stars are common throughout the galaxy.
High-luminosity stars are the rarest of them all. In any given volume of space, they’re outnumbered by the low-luminosity stars a thousand to one. The prodigious energy output of high-luminosity stars is what enables you to see them across such large volumes of space.
Suppose two stars emit light at the same rate (meaning that they have the same luminosity), but one is a hundred times farther from us than the other. We might expect it to be a hundredth as bright. No. That would be too easy. Fact is, the intensity of light dims in proportion to the square of the distance. So in this case, the faraway star looks ten thousand (1002) times dimmer than the one nearby. The effect of this “inverse-square law” is purely geometric. When starlight spreads in all directions, it dilutes from the growing spherical shell of space through which it moves. The surface area of this sphere increases in proportion to the square of its radius (you may remember the formula: Area = 4πr2), forcing the light’s intensity to diminish by the same proportion.
ALL RIGHT: the stars don’t all lie the same distance from us; they aren’t all equally luminous; the ones we see are highly unrepresentative. But surely they are stationary in space. For millennia, people understandably thought of stars as “fixed,” a concept evident in such influential sources as the Bible (“And God set them in the firmament of the heaven,” Genesis 1:17) and Claudius Ptolemy’s Almagest, published circa A.D. 150, wherein he argues strongly and persuasively for no motion.
To sum up, if you allow the heavenly bodies to move individually, then their distances, measured from Earth upward, must vary. This will force the sizes, brightnesses, and relative separations among the stars to vary too from year to year. But no such variation is apparent. Why? You just didn’t wait long enough. Edmond Halley (of comet fame) was the first to figure out that stars moved. In 1718 he compared “modern” star positions with the ones mapped by the second-century B.C. Greek astronomer Hipparchus. Halley trusted the accuracy of Hipparchus’s maps, but he also benefited from a baseline of more than eighteen centuries from which to compare the ancient and modern star positions. He promptly noticed that the star Arcturus was not where it once was. The star had indeed moved, but not enough within a single human lifetime to be noticed without the aid of a telescope.
Among all objects in the sky, seven made no pretense of being fixed; they appeared to wander against the starry sky and so were called planetes, or “wanderers,” by the Greeks. You know all seven (our names for the days of the week can be traced to them): Mercury, Venus, Mars, Jupiter, Saturn, the Sun, and the Moon. Since ancient times, these wanderers were correctly thought to be closer to Earth than were the stars, but each revolving around Earth in the center of it all.
Aristarchus of Samos first proposed a Sun-centered universe in the third century B.C. But back then it was obvious to anybody who paid attention that irrespective of the planets’ complicated motions, they and all the background stars revolved around Earth. If Earth moved we would surely feel it. Common arguments of the day included:
· If Earth rotated on an axis or moved through space, wouldn’t clouds in the sky and birds in flight get left far behind? (They aren’t.)
· If you jumped vertically, wouldn’t you land in a very different spot as Earth traveled swiftly beneath your feet? (You don’t.)
· And if Earth moved around the Sun, wouldn’t the angle at which we view the stars change continuously, creating a visible shift in the stars’ positions on the sky? (It doesn’t. At least not visibly.)
The naysayers’ evidence was compelling. For the first two cases, the work of Galileo Galilei would later demonstrate that while you are airborne, you, the atmosphere, and everything else around you get carried forward with the rotating, orbiting Earth. For the same reason, if you stand in the aisle of a cruising airplane and jump, you do not catapult backward past the rear seats and get pinned against the lavatory doors. In the third case, there’s nothing wrong with the reasoning—except that the stars are so far away you need a powerful telescope to see the seasonal shifts. That effect would not be measured until 1838, by the German astronomer Friedrich Wilhelm Bessel.
The geocentric universe became a pillar of Ptolemy’s Almagest, and the idea preoccupied scientific, cultural, and religious consciousness until the 1543 publication of De Revolutionibus, when Nicolaus Copernicus placed the Sun instead of Earth at the center of the known universe. Fearful that this heretical work would freak out the establishment, Andreas Osiander, a Protestant theologian who oversaw the late stages of the printing, supplied an unauthorized and unsigned preface to the work, in which he pleads:
I have no doubt that certain learned men, now that the novelty of the hypothesis in this work has been widely reported—for it establishes that the Earth moves and indeed that the Sun is motionless in the middle of the universe—are extremely shocked…. [But it is not]necessary that these hypotheses should be true, nor even probable, but it is sufficient if they merely produce calculations which agree with the observations. (1999, p. 22)
Copernicus himself was not unmindful of the trouble he was about to cause. In the book’s dedication, addressed to Pope Paul III, Copernicus notes:
I can well appreciate, Holy Father, that as soon as certain people realize that in these books which I have written about the Revolutions of the spheres of the universe I attribute certain motions to the globe of the Earth, they will at once clamor for me to be hooted off the stage with such an opinion. (1999, p. 23)
But soon after the Dutch spectacle maker Hans Lippershey had invented the telescope in 1608, Galileo, using a telescope of his own manufacture, saw Venus going through phases, and four moons that orbited Jupiter and not Earth. These and other observations were nails in the geocentric coffin, making Copernicus’s heliocentric universe an increasingly persuasive concept. Once Earth no longer occupied a unique place in the cosmos, the Copernican revolution, based on the principle that we are not special, had officially begun.
NOW THAT EARTH was in solar orbit, just like its planetary brethren, where did that put the Sun? At the center of the universe? No way. Nobody was going to fall for that one again; it would violate the freshly minted Copernican principle. But let’s investigate to make sure.
If the solar system were in the center of the universe, then no matter where we looked on the sky we would see approximately the same number of stars. But if the solar system were off to the side somewhere, we would presumably see a great concentration of stars in one direction—the direction of the center of the universe.
By 1785, having tallied stars everywhere on the sky and crudely estimated their distances, the English astronomer Sir William Herschel concluded that the solar system did indeed lie at the center of the cosmos. Slightly more than a century later, the Dutch astronomer Jacobus Cornelius Kapteyn—using the best available methods for calculating distance—sought to verify once and for all the location of the solar system in the galaxy. When seen through a telescope, the band of light called the Milky Way resolves into dense concentrations of stars. Careful tallies of their positions and distances yield similar numbers of stars in every direction along the band itself. Above and below it, the concentration of stars drops symmetrically. No matter which way you look on the sky, the numbers come out about the same as they do in the opposite direction, 180 degrees away. Kapteyn devoted some 20 years to preparing his sky map, which, sure enough, showed the solar system lying within the central 1 percent of the universe. We weren’t in the exact center, but we were close enough to reclaim our rightful place in space.
But the cosmic cruelty continued.
Little did anybody know at the time, especially not Kapteyn, that most sight lines to the Milky Way do not pass all the way through to the end of the universe. The Milky Way is rich in large clouds of gas and dust that absorb the light emitted by objects behind them. When we look in the direction of the Milky Way, more than 99 percent of all stars that should be visible to us are blocked from view by gas clouds within the Milky Way itself. To presume that Earth was near the center of the Milky Way (the then-known universe) was like walking into a large, dense forest and, after a few dozen steps, asserting that you’ve reached the center simply because you see the same number of trees in every direction.
By 1920—but before the light-absorption problem was well understood—Harlow Shapley, who was to become director of the Harvard College Observatory, studied the spatial layout of globular clusters in the Milky Way. Globular clusters are tight concentrations of as many as a million stars and are seen easily in regions above and below the Milky Way, where the least amount of light is absorbed. Shapley reasoned that these titanic clusters should enable him to pinpoint the center of the universe—a spot that, after all, would surely have the highest concentration of mass and the strongest gravity. Shapley’s data showed that the solar system is nowhere close to the center of the globular clusters’ distribution, and so is nowhere close to the center of the known universe. Where was this special place he found? Sixty thousand light-years away, in roughly the same direction as—but far beyond—the stars that trace the constellation Sagittarius.
Shapley’s distances were too large by more than a factor of 2, but he was right about the center of the system of globular clusters. It coincides with what was later found to be the most powerful source of radio waves in the night sky (radio waves are unattenuated by intervening gas and dust). Astrophysicists eventually identified the site of peak radio emissions as the exact center of the Milky Way, but not until one or two more episodes of seeing-isn’t-believing had taken place.
Once again the Copernican principle had triumphed. The solar system was not in the center of the known universe but far out in the suburbs. For sensitive egos, that could still be okay. Surely the vast system of stars and nebulae to which we belong comprised the entire universe. Surely we were where the action was.
Most of the nebulae in the night sky are like island universes, as presciently proposed in the eighteenth century by several people, including the Swedish philosopher Emanuel Swedenborg, the English astronomer Thomas Wright, and the German philosopher Immanuel Kant. In An Original Theory of the Universe (1750), for instance, Wright speculates on the infinity of space, filled with stellar systems akin to our own Milky Way:
We may conclude…that as the visible Creation is supposed to be full of sidereal Systems and planetary Worlds,…the endless Immensity is an unlimited Plenum of Creations not unlike the known Universe…. That this in all Probability may be the real Case, is in some Degree made evident by the many cloudy Spots, just perceivable by us, as far without our starry Regions, in which tho’ visibly luminous Spaces, no one Star or particular constituent Body can possibly be distinguished; those in all likelyhood may be external Creation, bordering upon the known one, too remote for even our Telescopes to reach. (p. 177)
Wright’s “cloudy Spots” are in fact collections of hundreds of billions of stars, situated far away in space and visible primarily above and below the Milky Way. The rest of the nebulae turn out to be relatively small, nearby clouds of gas, found mostly within the Milky Way band.
That the Milky Way is just one of multitudes of galaxies that comprise the universe was among the most important discoveries in the history of science, even if it made us feel small again. The offending astronomer was Edwin Hubble, after whom the Hubble Space Telescope is named. The offending evidence came in the form of a photographic plate taken on the night of October 5, 1923. The offending instrument was the Mount Wilson Observatory’s 100-inch telescope, at the time the most powerful in the world. The offending cosmic object was the Andromeda nebula, one of the largest on the night sky.
Hubble discovered a highly luminous kind of star within Andromeda that was already familiar to astronomers from surveys of stars much closer to home. The distances to the nearby stars were known, and their brightness varies only with their distance. By applying the inverse-square law for the brightness of starlight, Hubble derived a distance to the star in Andromeda, placing the nebula far beyond any known star within our own stellar system. Andromeda was actually an entire galaxy, whose fuzz could be resolved into billions of stars, all situated more than 2 million light-years away. Not only were we not in the center of things, but overnight our entire Milky Way galaxy, the last measure of our self-worth, shrank to an insignificant smudge in a multibillion-smudge universe that was vastly larger than anyone had previously imagined.
ALTHOUGH THE MILKY WAY turned out to be only one of countless galaxies, couldn’t we still be at the center of the universe? Just six years after Hubble demoted us, he pooled all the available data on the motions of galaxies. Turns out that nearly all of them recede from the Milky Way, at velocities directly proportional to their distances from us.
Finally we were in the middle of something big: the universe was expanding, and we were its center.
No, we weren’t going to be fooled again. Just because it looks as if we’re in the center of the cosmos doesn’t mean we are. As a matter of fact, a theory of the universe had been waiting in the wings since 1916, when Albert Einstein published his paper on general relativity—the modern theory of gravity. In Einstein’s universe, the fabric of space and time warps in the presence of mass. This warping, and the movement of objects in response to it, is what we interpret as the force of gravity. When applied to the cosmos, general relativity allows the space of the universe to expand, carrying its constituent galaxies along for the ride.
A remarkable consequence of this new reality is that the universe looks to all observers in every galaxy as though it expands around them. It’s the ultimate illusion of self-importance, where nature fools not only sentient human beings on Earth, but all life-forms that have ever lived in all of space and time.
But surely there is only one cosmos—the one where we live in happy delusion. At the moment, cosmologists have no evidence for more than one universe. But if you extend several well-tested laws of physics to their extremes (or beyond), you can describe the small, dense, hot birth of the universe as a seething foam of tangled space-time that is prone to quantum fluctuations, any one of which could spawn an entire universe of its own. In this gnarly cosmos we might occupy just one universe in a “multiverse” that encompasses countless other universes popping in and out of existence. The idea relegates us to an embarrassingly smaller part of the whole than we ever imagined. What would Pope Paul III think?
OUR PLIGHT PERSISTS, but on ever larger scales. Hubble summarized the issues in his 1936 work Realm of the Nebulae, but these words could apply at all stages of our endarkenment:
Thus the explorations of space end on a note of uncertainty…. We know our immediate neighborhood rather intimately. With increasing distance our knowledge fades, and fades rapidly. Eventually, we reach the dim boundary—the utmost limits of our telescopes. There, we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial. (p. 201)
What are the lessons to be learned from this journey of the mind? That humans are emotionally fragile, perennially gullible, hopelessly ignorant masters of an insignificantly small speck in the cosmos.
Have a nice day.