Death By Black Hole & Other Cosmic Quandaries

by Neil DeGrasse Tyson

  So fertile was this concept of universality that it was successfully applied in reverse. Further analysis of the Sun’s spectrum revealed the signature of an element that had no known counterpart on Earth. Being of the Sun, the new substance was given a name derived from the Greek word helios (the Sun). Only later was it discovered in the lab. Thus, “helium” became the first and only element in the chemist’s periodic table to be discovered someplace other than Earth.

  OKAY, THE LAWS of physics work in the solar system, but do they work across the galaxy? Across the universe? Across time itself? Step by step, the laws were tested. The nearby stars also revealed familiar chemicals. Distant binary stars, bound in mutual orbit, seem to know all about Newton’s laws of gravity. For the same reason, so do binary galaxies.

  And, like the geologist’s stratified sediments, the farther away we look, the further back in time we see. Spectra from the most distant objects in the universe show the same chemical signatures that we see everywhere else in the universe. True, heavy elements were less abundant back then—they are manufactured primarily in subsequent generations of exploding stars—but the laws describing the atomic and molecular process that created these spectral signatures remain intact.

  Of course, not all things and phenomena in the cosmos have counterparts on Earth. You’ve probably never walked through a cloud of glowing million-degree plasma, and you’ve probably never stumbled upon a black hole on the street. What matters is the universality of the laws of physics that describe them. When spectral analysis was first turned to the light emitted by interstellar nebulae, an element appeared that, once again, had no counterpart on Earth. But the periodic table of elements had no missing boxes; when helium was discovered there were several. So astrophysicists invented the name “nebulium” as a placeholder, until they could figure out what was going on. Turned out that in space, gaseous nebulae are so rarefied that atoms go long stretches without colliding with each other. Under these conditions, electrons can do things within atoms that had never before been seen in Earth labs. Nebulium was simply the signature of ordinary oxygen doing extraordinary things.

  This universality of physical laws tells us that if we land on another planet with a thriving alien civilization, they will be running on the same laws that we have discovered and tested here on Earth—even if the aliens harbor different social and political beliefs. Furthermore, if you wanted to talk to the aliens, you can bet they don’t speak English or French or even Mandarin Chinese. You don’t even know whether shaking their hands—if indeed they have hands to shake—would be considered an act of war or of peace. Your best hope is to find a way to communicate using the language of science.

  Such an attempt was made in the 1970s with the Pioneer 10 and 11 and Voyager 1 and 2 spacecraft, the only ones given a great enough speed to escape the solar system’s gravitational pull. Pioneer donned a golden etched plaque that showed, in pictograms, the layout of our solar system, our location in the Milky Way galaxy, and the structure of the hydrogen atom. Voyager went further and included diverse sounds from mother Earth including the human heartbeat, whale “songs,” and musical selections ranging from the works of Beethoven to Chuck Berry. While this humanized the message, it’s not clear whether alien ears would have a clue what they were listening to—assuming they have ears in the first place. My favorite parody of this gesture was a skit on Saturday Night Live, appearing shortly after the Voyager launch. NASA receives a reply from the aliens who recovered the spacecraft. The note simply requests, “Send more Chuck Berry.”

  AS WE WILL see in great detail in Section 3, science thrives not only on the universality of physical laws but also on the existence and persistence of physical constants. The constant of gravitation, known by most scientists as “big G,” supplies Newton’s equation of gravity with the measure of how strong the force will be, and has been implicitly tested for variation over eons. If you do the math, you can determine that a star’s luminosity is steeply dependent on big G. In other words, if big G had been even slightly different in the past, then the energy output of the Sun would have been far more variable than anything that the biological, climatological, or geological records indicate. In fact, no time-dependent or location-dependent fundamental constants are known—they appear to be truly constant.

  Such are the ways of our universe.

  Among all constants, the speed of light is surely the most famous. No matter how fast you go, you will never overtake a beam of light. Why not? No experiment ever conducted has ever revealed an object of any form reaching the speed of light. Well-tested laws of physics predict and account for this. These statements sound closed-minded. True, some of the most embarrassing science-based proclamations in the past have underestimated the ingenuity of inventors and engineers: “We will never fly.” “Flying will never be commercially feasible.” “We will never fly faster than sound.” “We will never split the atom.” “We will never go to the Moon.” You’ve heard them. What they have in common is that no established law of physics stood in their way.

  The claim “We will never outrun a beam of light” is a qualitatively different prediction. It flows from basic, time-tested physical principles. No doubt about it. Highway signs for interstellar travelers of the future will surely read:

  * * *

  The Speed of Light:

  It’s Not Just a Good Idea

  It’s the Law.

  * * *

  The good thing about the laws of physics is that they require no law enforcement agencies to maintain them, although I once owned a nerdy T-shirt that loudly proclaimed, “OBEY GRAVITY.”

  Many natural phenomena reflect the interplay of multiple physical laws operating at once. This fact often complicates the analysis and, in most cases, requires supercomputers to calculate things and to keep track of important parameters. When comet Shoemaker-Levy 9 plunged into and then exploded within Jupiter’s gas-rich atmosphere in 1994, the most accurate computer model of what was to happen combined the laws of fluid mechanics, thermodynamics, kinematics, and gravitation. Climate and weather represent other leading examples of complicated (and difficult-to-predict) phenomena. But the basic laws governing them are still at work. Jupiter’s Great Red Spot, a raging anticyclone that has been going strong for at least 350 years, is driven by the identical physical processes that generate storms on Earth and elsewhere in the solar system.

  THE CONSERVATION LAWS, where the amount of some measured quantity remains unchanged no matter what are another class of universal truths. The three most important are the conservation of mass and energy, the conservation of linear and angular momentum, and the conservation of electric charge. These laws are in evidence on Earth and everywhere we have thought to look in the universe—from the domain of particle physics to the large-scale structure of the universe.

  In spite of all this boasting, all is not perfect in paradise. As already noted, we cannot see, touch, or taste the source of 85 percent of the gravity of the universe. This mysterious dark matter, which remains undetected except for its gravitational pull on matter we see, may be composed of exotic particles that we have yet to discover or identify. A tiny subset of astrophysicists, however, remain unconvinced and have suggested that dark matter does not exist—you simply need to modify Newton’s law of gravity. Just add a few components to the equations and all will be well.

  Perhaps one day we will learn that Newton’s gravity indeed requires adjustment. That’ll be okay. It has happened once before. In 1916, Albert Einstein published his general theory of relativity, which reformulated the principles of gravity in a way that applied to objects of extremely high mass, a realm unknown to Newton, and where his law of gravity breaks down. The lesson? Our confidence flows through the range of conditions over which a law has been tested and verified. The broader this range, the more powerful the law becomes in describing the cosmos. For ordinary household gravity, Newton’s law works just fine. For black holes and the large-scale structure of the universe, we need
general relativity. They each work flawlessly in their own domain, wherever that domain may be in the universe.

  TO THE SCIENTIST, the universality of physical laws makes the cosmos a marvelously simple place. By comparison, human nature—the psychologist’s domain—is infinitely more daunting. In America, school boards vote on the subjects to be taught in the classroom, and in some cases these votes are cast according to the whims of social and political tides or religious philosophies. Around the world, varying belief systems lead to political differences that are not always resolved peacefully. And some people talk to bus stop stanchions. The remarkable feature of physical laws is that they apply everywhere, whether or not you choose to believe in them. After the laws of physics, everything else is opinion.

  Not that scientists don’t argue. We do. A lot. When we do, however, we are usually expressing opinions about the interpretation of ratty data on the frontier of our knowledge. Wherever and whenever a physical law can be invoked in the discussion, the debate is guaranteed to be brief: No, your idea for a perpetual motion machine will never work—it violates laws of thermodynamics. No, you can’t build a time machine that will enable you to go back and kill your mother before you were born—it violates causality laws. And without violating momentum laws, you cannot spontaneously levitate and hover above the ground, whether or not you are seated in the lotus position. Although, in principle, you could perform this stunt if you managed to let loose a powerful and sustained exhaust of flatulence.

  Knowledge of physical laws can, in some cases, give you the confidence to confront surly people. A few years ago I was having a hot-cocoa nightcap at a dessert shop in Pasadena, California. I had ordered it with whipped cream, of course. When it arrived at the table, I saw no trace of the stuff. After I told the waiter that my cocoa was plain, he asserted I couldn’t see the whipped cream because it sank to the bottom. Since whipped cream has a very low density and floats on all liquids that humans consume, I offered the waiter two possible explanations: either somebody forgot to add the whipped cream to my hot cocoa or the universal laws of physics were different in his restaurant. Unconvinced, he brought over a dollop of whipped cream to test for himself. After bobbing once or twice in my cup, the whipped cream sat up straight and afloat.

  What better proof do you need of the universality of physical law?

  THREE

  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?

  Nope.

  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.

  Nope again.

  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)