Death By Black Hole & Other Cosmic Quandaries

by Neil DeGrasse Tyson

  Earth’s atmosphere ends where it blends indistinguishably with the very low density gas of interplanetary space. Normally, this blend lies several thousand miles above Earth’s surface. Note that the space shuttle, the Hubble telescope, and other satellites that orbit within only a few hundred miles of Earth’s surface would eventually fall out of orbit from the residual atmospheric air resistance if they did not receive periodic boosts. During peak solar activity, however (every 11 years) Earth’s upper atmosphere receives a higher dose of solar radiation, forcing it to heat and expand. During this period the atmosphere can extend an extra thousand miles into space, thus decaying satellite orbits faster than usual.

  BEFORE LABORATORY VACUUMS, air was the closest thing to nothing that anyone could imagine. Along with earth, fire, and water, air was one of the original four Aristotelian elements that composed the known world. Actually, there was a fifth element known as the “quint”-essence. Otherworldly, yet lighter than air and more ethereal than fire, the rarefied quintessence was presumed to comprise the heavens. How quaint.

  We needn’t look as far as the heavens to find rarefied environments. Our upper atmosphere will suffice. Beginning at sea level, air weighs about 15 pounds per square inch. So if you cookie cut a square inch of atmosphere from thousands of miles up all the way down to sea level and you put it on a scale, it would weigh 15 pounds. For comparison, a square-inch column of water requires a mere 33 feet to weigh 15 pounds. On mountaintops and high up in airplanes, the cookie-cut column of air above you is shorter and therefore weighs less. At the 14,000-foot summit of Mauna Kea, Hawaii, home to some of the world’s most powerful telescopes, the atmospheric pressure drops to about 10 pounds per square inch. While observing on site, astrophysicists will intermittently breathe from oxygen tanks to retain their intellectual acuity.

  Above 100 miles, where there are no known astrophysicists, the air is so rarefied that gas molecules move for a relatively long time before colliding with one another. If, between collisions, the molecules are slammed by an incoming particle, they become temporarily excited and then emit a unique spectrum of colors before their next collision. When the incoming particles are the constituents of the solar wind, such as protons and electrons, the emissions are curtains of undulating light that we commonly call aurora. When the spectrum of auroral light was first measured, it had no counterpart in the laboratory. The identity of the glowing molecules remained unknown until we learned that excited, but otherwise ordinary, molecules of nitrogen and oxygen were to blame. At sea level, their rapid collisions with each other absorb this excess energy long before they have had a chance to emit their own light.

  Earth’s upper atmosphere is not alone in producing mysterious lights. Spectral features in the Sun’s corona long puzzled astrophysicists. An extremely rarefied place, the corona is that beautiful, fiery-looking outer region of the Sun that’s rendered visible during a total solar eclipse. The new feature was assigned to an unknown element dubbed “coronium.” Not until we learned that the solar corona is heated to millions of degrees did we figure out that the mystery element was highly ionized iron, a previously unfamiliar state where most of its outer electrons are stripped away and floating free in the gas.

  The term “rarefied” is normally reserved for gases, but I will take the liberty to apply it to the solar system’s famed asteroid belt. From movies and other descriptions, you would think it was a hazardous place, wrought with the constant threat of head-on collisions with house-sized boulders. The actual recipe for the asteroid belt? Take a mere 2.5 percent of the Moon’s mass (itself, just 1/81 the mass of Earth), crush it into thousands of assorted pieces, but make sure that three-quarters of the mass is contained in just four asteroids. Then spread them all across a 100-million-mile-wide belt that tracks along a 1.5-billion-mile path around the Sun.

  COMET TAILS, as tenuous and rarefied as they are, represent an increase in density by a factor of 1,000 over the ambient conditions in interplanetary space. By reflecting sunlight and re-emitting energy absorbed from the Sun, a comet tail possesses remarkable visibility given its nothingness. Fred Whipple, of the Harvard-Smithsonian Center for Astrophysics, is generally considered to be a parent of our modern understanding of comets. He has succinctly described a comet’s tail as the most that has ever been made of the least. Indeed, if the entire volume of a 50-million-mile-long comet tail were compressed to the density of ordinary air, all the tail’s gas would fill a half-mile cube. When the astronomically common yet deadly gas cyanogen (CN) was first discovered in comets, and when it was later announced that Earth would pass through the tail of Halley’s comet during its 1910 visit to the inner solar system, gullible people were sold anticomet pills by pharmaceutical charlatans.

  The core of the sun, where all its thermonuclear energy is generated, is not a place to find low-density material. But the core comprises a mere 1 percent of the Sun’s volume. The average density of the entire Sun is only one-fourth that of Earth, and only 40 percent higher than ordinary water. In other words, a spoonful of Sun would sink in your bathtub, but it wouldn’t sink fast. Yet in 5 billion years the Sun’s core will have fused nearly all its hydrogen into helium and will shortly thereafter begin to fuse helium into carbon. Meanwhile, the luminosity of the Sun will increase a thousandfold while its surface temperature drops to half of what it is today. We know from the laws of physics that the only way an object can increase its luminosity while simultaneously getting cooler is for it to get bigger. As will be detailed in Section 5, the Sun will ultimately expand to a bulbous ball of rarefied gas that will completely fill and extend beyond the volume of Earth’s orbit, while the Sun’s average density falls to less than one ten-billionth of its current value. Of course Earth’s oceans and atmosphere will have evaporated into space and all life will have vaporized, but that needn’t concern us here. The Sun’s outer atmosphere, rarefied though it will be, would nonetheless impede the motion of Earth in its orbit and force us on a relentless spiral inward toward thermonuclear oblivion.

  BEYOND OUR SOLAR SYSTEM we venture into interstellar space. Humans have sent four spacecraft with enough speed to journey there: Pioneer 10 and 11, and Voyager 1 and 2. The fastest among them, Voyager 2, will reach the distance of the nearest star to the Sun in about 25,000 years.

  Yes, interstellar space is empty. But like the remarkable visibility of rarefied comet tails in interplanetary space, gas clouds out there, with a hundred to a thousand times the ambient density, can readily reveal themselves in the presence of nearby luminous stars. Once again, when the light from these colorful nebulosities was first analyzed their spectra revealed unfamiliar patterns. The hypothetical element “nebulium” was proposed as a placeholder for our ignorance. In the late 1800s, there was clearly no spot on the periodic table of elements that could possibly be identified with nebulium. As laboratory vacuum techniques improved, and as unfamiliar spectral features became routinely identified with familiar elements, suspicions grew—and were later confirmed—that nebulium was ordinary oxygen in an extraordinary state. What state was that? The atoms were each stripped of two electrons and they lived in the near-perfect vacuum of interstellar space.

  When you leave the galaxy, you leave behind nearly all gas and dust and stars and planets and debris. You enter an unimaginable cosmic void. Let’s talk empty: A cube of intergalactic space, 200,000 kilometers on a side, contains about the same number of atoms as the air that fills the usable volume of your refrigerator. Out there, the cosmos not only loves a vacuum, it’s carved from it.

  Alas, an absolute, perfect vacuum may be impossible to attain or find. As we saw in Section 2, one of the many bizarre predictions of quantum mechanics holds that the real vacuum of space contains a sea of “virtual” particles that continually pop in and out of existence along with their antimatter counterparts. Their virtuality comes from having lifetimes that are so short that their direct existence cannot ever be measured. More commonly known as the “vacuum energy,” it can act as
antigravity pressure that will ultimately trigger the universe to expand exponentially faster and faster—making intergalactic space all the more rarefied.

  What lies beyond?

  Among those who dabble in metaphysics, some hypothesize that outside the universe, where there is no space, there is no nothing. We might call this hypothetical, zero-density place, nothing-nothing, except that we are certain to find multitudes of unretrieved rabbits.

  FIFTEEN

  OVER THE RAINBOW

  Whenever cartoonists draw biologists, chemists, or engineers, the characters typically wear protective white lab coats that have assorted pens and pencils poking out of the breast pocket. Astrophysicists use plenty of pens and pencils, but we never wear lab coats unless we are building something to launch into space. Our primary laboratory is the cosmos, and unless you have bad luck and get hit by a meteorite, you are not at risk of getting your clothes singed or otherwise sullied by caustic liquids spilling from the sky. Therein lies the challenge. How do you study something that cannot possibly get your clothes dirty? How do astrophysicists know anything about either the universe or its contents if all the objects to be studied are light-years away?

  Fortunately, the light emanating from a star reveals much more to us than its position in the sky or how bright it is. The atoms of objects that glow lead busy lives. Their little electrons continually absorb and emit light. And if the environment is hot enough, energetic collisions between atoms can jar loose some or all of their electrons, allowing them to scatter light to and fro. All told, atoms leave their fingerprint on the light being studied, which uniquely implicates which chemical elements or molecules are responsible.

  As early as 1666, Isaac Newton passed white light through a prism to produce the now-familiar spectrum of seven colors: red, orange, yellow, green, blue, indigo, and violet, which he personally named. (Feel free to call them Roy G. Biv.) Others had played with prisms before. What Newton did next, however, had no precedent. He passed the emergent spectrum of colors back through a second prism and recovered the pure white he started with, demonstrating a remarkable property of light that has no counterpart on the artist’s palette; these same colors of paint, when mixed, would leave you with a color resembling that of sludge. Newton also tried to disperse the colors themselves but found them to be pure. And in spite of the seven names spectral colors change smoothly and continuously from one to the next. The human eye has no capacity to do what prisms do—another window to the universe lay undiscovered before us.

  A CAREFUL INSPECTION of the Sun’s spectrum, using precision optics and techniques unavailable in Newton’s day, reveals not only Roy G. Biv, but narrow segments within the spectrum where the colors are absent. These “lines” through the light were discovered in 1802 by the English medical chemist William Hyde Wollaston, who naively (though sensibly) suggested that they were naturally occurring boundaries between the colors. A more complete discussion and interpretation followed with the efforts of the German physicist and optician Joseph von Fraunhofer (1787–1826), who devoted his professional career to the quantitative analysis of spectra and to the construction of optical devices that generate them. Fraunhofer is often referred to as the father of modern spectroscopy, but I might further make the claim that he was the father of astrophysics. Between 1814 and 1817, he passed the light of certain flames through a prism and discovered that the pattern of lines resembled what he found in the Sun’s spectrum, which further resembled lines found in the spectra of many stars, including Capella, one of the brightest in the nighttime sky.

  By the mid-1800s the chemists Gustav Kirchhoff and Robert Bunsen (of Bunsen-burner fame from your chemistry class) were making a cottage industry of passing the light of burning substances through a prism. They mapped the patterns made by known elements and discovered a host of new elements, including rubidium and caesium. Each element left its own pattern of lines—its own calling card—in the spectrum being studied. So fertile was this enterprise that the second most abundant element in the universe, helium, was discovered in the spectrum of the Sun before it was discovered on Earth. The element’s name bears this history with its prefix derived from Helios, “the Sun.”

  A DETAILED AND accurate explanation of how atoms and their electrons form spectral lines would not emerge until the era of quantum physics a half-century later, but the conceptual leap had already been made: Just as Newton’s equations of gravity connected the realm of laboratory physics to the solar system, Fraunhofer connected the realm of laboratory chemistry to the cosmos. The stage was set to identify, for the first time, what chemical elements filled the universe, and under what conditions of temperature and pressure their patterns revealed themselves to the spectroscopist.

  Among the more bone-headed statements made by armchair philosophers, we find the following 1835 proclamation in Cours de la Philosophie Positive by Auguste Comte (1798–1857):

  On the subject of stars, all investigations which are not ultimately reducible to simple visual observations are…necessarily denied to us…. We shall never be able by any means to study their chemical composition…. I regard any notion concerning the true mean temperature of the various stars as forever denied to us. (p. 16, author’s trans.)

  Quotes like that can make you afraid to say anything in print.

  Just seven years later, in 1842, the Austrian physicist Christian Doppler proposed what became known as the Doppler effect, which is the change in frequency of a wave being emitted by an object in motion. One can think of the moving object as stretching the waves behind it (reducing their frequency) and compressing the waves in front of it (increasing their frequency). The faster the object moves, the more the light is both compressed in front of it and stretched behind it. This simple relationship between speed and frequency has profound implications. If you know what frequency was emitted, but you measure it to have a different value, the difference between the two is a direct indication of the object’s speed toward or away from you. In an 1842 paper, Doppler makes the prescient statement:

  It is almost to be accepted with certainty that this [Doppler effect] will in the not too distant future offer astronomers a welcome means to determine the movements…of such stars which…until this moment hardly presented the hope of such measurements and determinations. (Schwippell 1992, pp. 46–54)

  The idea works for sound waves, for light waves, and in fact, waves of any origin. (I’d bet Doppler would be surprised to learn that his discovery would one day be used in microwave-based “radar guns” wielded by police officers to extract money from people who drive automobiles above a speed limit set by law.) By 1845, Doppler was conducting experiments with musicians playing tunes on flatbed railway trains, while people with perfect pitch wrote down the changing notes they heard as the train approached and then receded.

  DURING THE LATE 1800S, with the widespread use of spectrographs in astronomy, coupled with the new science of photography, the field of astronomy was reborn as the discipline of astrophysics. One of the pre-eminent research publications in my field, the Astrophysical Journal, was founded in 1895, and, until 1962, bore the subtitle: An International Review of Spectroscopy and Astronomical Physics. Even today, nearly every paper reporting observations of the universe gives either an analysis of spectra or is heavily influenced by spectroscopic data obtained by others.

  To generate a spectrum of an object requires much more light than to take a snapshot, so the biggest telescopes in the world, such as the 10-meter Keck telescopes in Hawaii, are tasked primarily with getting spectra. In short, were it not for our ability to analyze spectra, we would know next to nothing about what goes on in the universe.

  Astrophysics educators face a pedagogical challenge of the highest rank. Astrophysics researchers deduce nearly all knowledge about the structure, formation, and evolution of things in the universe from the study of spectra. But the analysis of spectra is removed by several levels of inference from the things being studied. Analogies and metaphors help, by linki
ng a complex, somewhat abstract idea to a simpler, more tangible one. The biologist might describe the shape of the DNA molecule as two coils, connected to each other the way rungs on a ladder connect its sides. I can picture a coil. I can picture two coils. I can picture rungs on a ladder. I can therefore picture the molecule’s shape. Each part of the description sits only one level of inference removed from the molecule itself. And they come together nicely to make a tangible image in the mind. No matter how easy or hard the subject may be, one can now talk about the science of the molecule.

  But to explain how we know the speed of a receding star requires five nested levels of abstraction:

  Level 0:

  Star

  Level 1:

  Picture of a star

  Level 2:

  Light from the picture of a star

  Level 3:

  Spectrum from the light from the picture of a star

  Level 4:

  Patterns of lines lacing the spectrum from the light from the picture of a star

  Level 5:

  Shifts in the patterns of lines in the spectrum from the light from the picture of the star

  Going from level 0 to level 1 is a trivial step that we take every time we snap a photo with a camera. But by the time your explanation reaches level 5, the audience is either befuddled or just fast asleep. That is why the public hardly ever hears about the role of spectra in cosmic discovery—it’s just too far removed from the objects themselves to explain efficiently or with ease.