Death By Black Hole Part 6

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...o...b..t, 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 s.p.a.ce 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...o...b..t and force us on a relentless spiral inward toward thermonuclear oblivion.

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

Yes, interstellar s.p.a.ce is empty. But like the remarkable visibility of rarefied comet tails in interplanetary s.p.a.ce, 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 a.n.a.lyzed 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 s.p.a.ce.

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 s.p.a.ce, 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 s.p.a.ce 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 s.p.a.ce all the more rarefied.

What lies beyond?

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

FIFTEEN.

OVER THE RAINBOW.

Whenever cartoonists draw biologists, chemists, or engineers, the characters typically wear protective white lab coats that have a.s.sorted 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 s.p.a.ce. 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 pa.s.sed 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 pa.s.sed 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 (17871826), who devoted his professional career to the quant.i.tative a.n.a.lysis 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 pa.s.sed 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. 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 (17871826), who devoted his professional career to the quant.i.tative a.n.a.lysis 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 pa.s.sed 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 cla.s.s) were making a cottage industry of pa.s.sing 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 before it was discovered on Earth. The element's name bears this history with its prefix derived from it was discovered on Earth. The element's name bears this history with its prefix derived from Helios 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. 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 Cours de la Philosophie Positive by Auguste Comte (17981857): by Auguste Comte (17981857): 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.) (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. 4654) (Schwippell 1992, pp. 4654) 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 1800 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 Astrophysical Journal, was founded in 1895, and, until 1962, bore the subt.i.tle: An International Review of Spectroscopy and Astronomical Physics An International Review of Spectroscopy and Astronomical Physics. Even today, nearly every paper reporting observations of the universe gives either an a.n.a.lysis 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 a.n.a.lyze 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 a.n.a.lysis of spectra is removed by several levels of inference from the things being studied. a.n.a.logies and metaphors help, by linking 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.

In the design of exhibits for a natural history museum, or for any museum where real things matter, what you typically seek are artifacts for display cases-rocks, bones, tools, fossils, memorabilia, and so forth. All these are "level 0" specimens and require little or no cognitive investment before you give the explanation of what the object is. For astrophysics displays, however, any attempt to place stars or quasars on display would vaporize the museum.

Most astrophysics exhibits are therefore conceived in level 1, leading princ.i.p.ally to displays of pictures, some quite striking and beautiful. The most famous telescope in modern times, the Hubble s.p.a.ce Telescope Hubble s.p.a.ce Telescope, is known to the public primarily through the beautiful, full-color, high-resolution images it has acquired of objects in the universe. The problem here is that after you view such exhibits, you leave waxing poetic about the beauty of the universe yet you are no closer than before to understanding how it all works. To really know the universe requires forays into levels 3, 4, and 5. While much good science has come from the Hubble Hubble telescope, you would never know it from media accounts that the foundation of our cosmic knowledge continues to flow primarily from the a.n.a.lysis of spectra and not from looking at pretty pictures. I want people to be struck, not only from exposure to levels 0 and 1, but also from exposure to level 5, which admittedly requires a greater intellectual investment on the part of the student, but also (and perhaps especially) on the part of the educator. telescope, you would never know it from media accounts that the foundation of our cosmic knowledge continues to flow primarily from the a.n.a.lysis of spectra and not from looking at pretty pictures. I want people to be struck, not only from exposure to levels 0 and 1, but also from exposure to level 5, which admittedly requires a greater intellectual investment on the part of the student, but also (and perhaps especially) on the part of the educator.

IT'S ONE THING to see a beautiful color picture, taken in visible light, of a nebula in our own Milky Way galaxy. But it's another thing to know from its radio-wave spectrum that it also harbors newly formed stars of very high ma.s.s within its cloud layers. This gas cloud is a stellar nursery, regenerating the light of the universe. to see a beautiful color picture, taken in visible light, of a nebula in our own Milky Way galaxy. But it's another thing to know from its radio-wave spectrum that it also harbors newly formed stars of very high ma.s.s within its cloud layers. This gas cloud is a stellar nursery, regenerating the light of the universe.

It's one thing to know that every now and again, high-ma.s.s stars explode. Photographs can show you this. But x-ray and visible-light spectra of these dying stars reveal a cache of heavy elements that enrich the galaxy and are directly traceable to the const.i.tuent elements of life on Earth. Not only do we live among the stars, the stars live within us.

It's one thing to look at a poster of a pretty spiral galaxy. But it's another thing to know from Doppler shifts in its spectral features that the galaxy is rotating at 200 kilometers per second, from which we infer the presence of 100 billion stars using Newton's laws of gravity. And by the way, the galaxy is receding from us at one-tenth the speed of light as part of the expansion of the universe.

It's one thing to look at nearby stars that resemble the Sun in luminosity and temperature. But it's another thing to use hypersensitive Doppler measurements of the star's motion to infer the existence of planets in orbit around them. At press time, our catalog is rising through 200 such planets outside the familiar ones in our own solar system.

It's one thing to observe the light from a quasar at the edge of the universe. But its another thing entirely to a.n.a.lyze the quasar's spectrum and deduce the structure of the invisible universe, laid along the quasar's path of light as gas clouds and other obstructions take their bite out of the quasar spectra.

Fortunately, for all the magnetohydrodynamicists among us, atomic structure changes slightly under the influence of a magnetic field. This change manifests itself in the slightly altered spectral pattern caused by these magnetically afflicted atoms.

And armed with Einstein's relativistic version of the Doppler formula, we deduce the expansion rate of the entire universe from the spectra of countless galaxies near and far, and thus deduce the age and fate of the universe.

One could make a compelling argument that we know more about the universe than the marine biologist knows about the bottom of the ocean or the geologist knows about the center of Earth. Far from an existence as powerless stargazers, modern astrophysicists are armed to the teeth with the tools and techniques of spectroscopy, enabling us all to stay firmly planted on Earth, yet finally touch the stars (without burning our fingers) and claim to know them as never before.

SIXTEEN.

COSMIC WINDOWS.

As noted in Section 1, the human eye is often advertised to be among the most impressive of the body's organs. Its ability to focus near and far, to adjust to a broad range of light levels, and to distinguish colors are at the top of most peoples' list of eye-opening features. But when you take note of the many bands of light that are invisible to us, then you would be forced to declare humans to be practically blind. How impressive is our hearing? Bats would clearly fly circles around us with a sensitivity to pitch that extends beyond our own by an order of magnitude. And if the human sense of smell were as good as that of dogs, then Fred rather than Fido might be the one who sniffs out contraband from airport customs searches.

The history of human discovery is characterized by the boundless desire to extend the senses beyond our inborn limits. It is through this desire that we open new windows to the universe. For example, beginning in the 1960s with the early Soviet and NASA probes to the Moon and planets, computer-controlled s.p.a.ce probes, which we can rightly call robots, became (and still are) the standard tool for s.p.a.ce exploration. Robots in s.p.a.ce have several clear advantages over astronauts: they are cheaper to launch; they can be designed to perform experiments of very high precision without the interference of a c.u.mbersome pressure suit; and they are not alive in any traditional sense of the word, so they cannot be killed in a s.p.a.ce accident. But until computers can simulate human curiosity and human sparks of insight, and until computers can synthesize information and recognize a serendipitous discovery when it stares them in the face (and perhaps even when it doesn't), robots will remain tools designed to discover what we already expect to find.

Unfortunately, profound questions about nature can lurk among those that have yet to be asked.

The most significant improvement of our feeble senses is the extension of our sight into the invisible bands of what is collectively known as the electromagnetic spectrum. In the late nineteenth century the German physicist Heinrich Hertz performed experiments that helped to unify conceptually what were previously considered to be unrelated forms of radiation. Radio waves, infrared, visible light, and ultraviolet were all revealed to be cousins in a family of light that simply differed in energy. The full spectrum, including all parts discovered after Hertz's work, extends from the low-energy part that we call radio waves, and continues in order of increasing energy to microwaves, infrared, visible (comprising the "rainbow seven": red, orange, yellow, green, blue, indigo, and violet), ultraviolet, x-rays, and gamma rays.

Superman, with his x-ray vision, has no special advantage over modern-day scientists. Yes, he is a bit stronger than your average astrophysicist, but astrophysicists can now "see" into every major part of the electromagnetic spectrum. In the absence of this extended vision we are not only blind but ignorant-the existence of many astrophysical phenomena reveal themselves only through some windows and not others.

WHAT FOLLOWS IS a selective peek through each window to the universe, beginning with radio waves, which require very different detectors from those you will find in the human retina. a selective peek through each window to the universe, beginning with radio waves, which require very different detectors from those you will find in the human retina.

In 1932 Karl Jansky, in the employ of Bell Telephone Laboratories and armed with a radio antenna, first "saw" radio signals that emanated from somewhere other than Earth; he had discovered the center of the Milky Way galaxy. Its radio signal was intense enough that if the human eye were sensitive to only radio waves, then the galactic center would be among the brightest sources in the sky.

With some cleverly designed electronics, you can transmit specially encoded radio waves that can then be transformed into sound. This ingenious apparatus has come to be known as a "radio." So by virtue of extending our sense of sight, we have also, in effect, managed to extend our sense of hearing. But any source of radio waves, or practically any source of energy at all, can be channeled to vibrate the cone of a speaker, although journalists occasionally misunderstand this simple fact. For example, when radio emission was discovered from Saturn, it was simple enough for astronomers to hook up a radio receiver that was equipped with a speaker. The radio-wave signal was then converted to audible sound waves whereupon one journalist reported that "sounds" were coming from Saturn and that life on Saturn was trying to tell us something.

With much more sensitive and sophisticated radio detectors than were available to Karl Jansky, we now explore not just the Milky Way but the entire universe. As a testament to our initial seeing-is-believing bias, early detections of radio sources in the universe were often considered untrustworthy until they were confirmed by observations with a conventional telescope. Fortunately, most cla.s.ses of radio-emitting objects also emit some level of visible light, so blind faith was not always required. Eventually, radio-wave telescopes produced a rich parade of discoveries that includes the still-mysterious quasars (a loosely a.s.sembled acronym of "quasi-stellar radio source"), which are among the most distant objects in the known universe.

Gas-rich galaxies emit radio waves from the abundant hydrogen atoms that are present (over 90 percent of all atoms in the universe are hydrogen). With large arrays of electronically connected radio telescopes we can generate very high resolution images of a galaxy's gas content that reveal intricate features in the hydrogen gas such as twists, blobs, holes, and filaments. In many ways the task of mapping galaxies is no different from that facing the fifteenth-and sixteenth-century cartographers, whose renditions of continents-distorted though they were-represented a n.o.ble human attempt to describe worlds beyond one's physical reach.

IF THE HUMAN EYE were sensitive to microwaves, then this window of the spectrum would enable you to see the radar emitted by the radar gun from the highway patrol officer who hides in the bushes. And microwave-emitting telephone relay station towers would be ablaze with light. Note, however, that the inside of your microwave oven would look no different because the mesh embedded in the door reflects microwaves back into the cavity to prevent their escape. The vitreous humor of your peering eyeb.a.l.l.s is thus protected from getting cooked along with your food. were sensitive to microwaves, then this window of the spectrum would enable you to see the radar emitted by the radar gun from the highway patrol officer who hides in the bushes. And microwave-emitting telephone relay station towers would be ablaze with light. Note, however, that the inside of your microwave oven would look no different because the mesh embedded in the door reflects microwaves back into the cavity to prevent their escape. The vitreous humor of your peering eyeb.a.l.l.s is thus protected from getting cooked along with your food.

Microwave telescopes were not actively used to study the universe until the late 1960s. They allow us to peer into cool, dense clouds of interstellar gas that ultimately collapse to form stars and planets. The heavy elements in these clouds readily a.s.semble into complex molecules whose signature in the microwave part of the spectrum is unmistakable because of their match with identical molecules that exist on Earth.

Some cosmic molecules are familiar to the household: NH3 (ammonia) (ammonia)H2O (water) While some are deadly: CO (carbon monoxide)HCN (hydrogen cyanide) Some remind you of the hospital: H2CO (formaldehyde)C2H5OH (ethyl alcohol) And some don't remind you of anything: N2H+ (dinitrogen monohydride ion)CHC3CN (cyanodiacetylene) Nearly 130 molecules are known, including glycine, which is an amino acid that is a building block for protein and thus for life as we know it.

Without a doubt, a microwave telescope made the most important single discovery in astrophysics. The leftover heat from the big bang origin of the universe has now cooled to a temperature of about three degrees on the absolute temperature scale. (As fully detailed later in this section, the absolute temperature scale quite reasonably sets the coldest possible temperature to zero degrees, so there are no negative temperatures. Absolute zero corresponds to about-460 degrees Fahrenheit, while 310 degrees absolute corresponds to room temperature.) In 1965, this big bang remnant was serendipitously measured in a n.o.bel Prizewinning observation conducted at Bell Telephone Laboratories by the physicists Arno Penzias and Robert Wilson. The remnant manifests itself as an omnipresent and omnidirectional ocean of light that is dominated by microwaves.

This discovery was, perhaps, serendipity at its finest. Penzias and Wilson humbly set out to find terrestrial sources that interfered with microwave communications, but what they found was compelling evidence for the big bang theory of the origin of the universe, which must be like fishing for a minnow and catching a blue whale.

MOVING FURTHER ALONG the electromagnetic spectrum we get to infrared light. Also invisible to humans, it is most familiar to fast-food fanatics whose French fries are kept warm with infrared lamps for hours before purchase. These lamps also emit visible light, but their active ingredient is an abundance of invisible infrared photons that the food readily absorbs. If the human retina were sensitive to infrared, then an ordinary household scene at night, with all lights out, would reveal all objects that sustain a temperature in excess of room temperature, such as the household iron (provided it was turned on), the metal that surrounds the pilot lights of a gas stove, the hot water pipes, and the exposed skin of any humans who stepped into the scene. Clearly this picture is not more enlightening than what you would see with visible light, but you could imagine one or two creative uses of such vision, such as looking at your home in the winter to spot where heat leaks from the windowpanes or roof. the electromagnetic spectrum we get to infrared light. Also invisible to humans, it is most familiar to fast-food fanatics whose French fries are kept warm with infrared lamps for hours before purchase. These lamps also emit visible light, but their active ingredient is an abundance of invisible infrared photons that the food readily absorbs. If the human retina were sensitive to infrared, then an ordinary household scene at night, with all lights out, would reveal all objects that sustain a temperature in excess of room temperature, such as the household iron (provided it was turned on), the metal that surrounds the pilot lights of a gas stove, the hot water pipes, and the exposed skin of any humans who stepped into the scene. Clearly this picture is not more enlightening than what you would see with visible light, but you could imagine one or two creative uses of such vision, such as looking at your home in the winter to spot where heat leaks from the windowpanes or roof.

As a child, I knew that at night, with the lights out, infrared vision would discover monsters hiding in the bedroom closet only if they were warm-blooded. But everybody knows that your average bedroom monster is reptilian and cold-blooded. Infrared vision would thus miss a bedroom monster completely because it would simply blend in with the walls and the door.

In the universe, the infrared window is most useful as a probe of dense clouds that contain stellar nurseries. Newly formed stars are often enshrouded by leftover gas and dust. These clouds absorb most of the visible light from their embedded stars and reradiate it in the infrared, rendering our visible light window quite useless. While visible light gets heavily absorbed by interstellar dust clouds, infrared moves through with only minimal attenuation, which is especially valuable for studies in the plane of our own Milky Way galaxy because this is where the obscuration of visible light from the Milky Way's stars is at its greatest. Back home, infrared satellite photographs of Earth's surface reveal, among other things, the paths of warm oceanic currents such as the North Atlantic Drift current that swirls 'round the British Isles (which are farther north than the entire state of Maine) and keeps them from becoming a major ski resort.

The energy emitted by the Sun, whose surface temperature is about 6,000 degrees absolute, includes plenty of infrared, but peaks in the visible part of the spectrum, as does the sensitivity of the human retina, which, if you have never thought about it, is why our sight is so useful in the daytime. If this spectrum match were not so, then we could rightly complain that some of our retinal sensitivity was wasted. We don't normally think of visible light as penetrating, but light pa.s.ses mostly unhindered through gla.s.s and air. Ultraviolet, however, is summarily absorbed by ordinary gla.s.s, so gla.s.s windows would not be much different from brick windows if our eyes were sensitive to only ultraviolet.

Stars that are over three or four times hotter than the Sun are prodigious producers of ultraviolet light. Fortunately, these stars are also bright in the visible part of the spectrum so discovering them has not depended on access to ultraviolet telescopes. The ozone layer in our atmosphere absorbs most of the ultraviolet, x-rays, and gamma rays that impinge upon it, so a detailed a.n.a.lysis of these hottest stars can best be obtained from Earth orbit or beyond. These high-energy windows in the spectrum thus represent relatively young subdisciplines of astrophysics.

AS IF TO herald a new century of extended vision, the first n.o.bel Prize ever awarded in physics went to the German physicist Wilhelm C. Rontgen in 1901 for his discovery of x-rays. Both ultraviolet and x-rays in the universe can reveal the presence of one of the most exotic objects in the universe: black holes. Black holes emit no light-their gravity is too strong for even light to escape-so their existence must be inferred from the energy emitted by matter that might spiral onto its surface from a companion star. The scene resembles greatly what water looks like as it spirals down a toilet bowl. With temperatures over twenty times that of the Sun's surface, ultraviolet and x-rays are the predominant form of energy released by material just before it descends into the black hole. herald a new century of extended vision, the first n.o.bel Prize ever awarded in physics went to the German physicist Wilhelm C. Rontgen in 1901 for his discovery of x-rays. Both ultraviolet and x-rays in the universe can reveal the presence of one of the most exotic objects in the universe: black holes. Black holes emit no light-their gravity is too strong for even light to escape-so their existence must be inferred from the energy emitted by matter that might spiral onto its surface from a companion star. The scene resembles greatly what water looks like as it spirals down a toilet bowl. With temperatures over twenty times that of the Sun's surface, ultraviolet and x-rays are the predominant form of energy released by material just before it descends into the black hole.

The act of discovery does not require that you understand either in advance, or after the fact, what you have discovered. This happened with the microwave background radiation and it is happening now with gamma ray bursts. As we will see in Section 6, the gamma-ray window has revealed mysterious bursts of high-energy gamma rays that are scattered across the sky. Their discovery was made possible through the use of s.p.a.ce-borne gamma-ray telescopes, yet their origin and cause remain unknown.

If we broaden the concept of vision to include the detection of subatomic particles then we get to use neutrinos. As we saw in Section 2, the elusive neutrino is a subatomic particle that forms every time a proton transforms into an ordinary neutron and positron, which is the antimatter partner to an electron. As obscure as the process sounds, it happens in the Sun's core about a hundred billion billion billion billion (1038) times each second. Neutrinos then pa.s.s directly out of the Sun as if it weren't there at all. A neutrino "telescope" would allow a direct view of the Sun's core and its ongoing thermonuclear fusion, which no band from the electromagnetic spectrum can reveal. But neutrinos are extraordinarily difficult to capture because they hardly ever interact with matter, so an efficient and effective neutrino telescope is a distant dream, if not an impossibility.

The detection of gravity waves, another elusive window on the universe, would reveal catastrophic cosmic events. But as of this writing, gravity waves, predicted in Einstein's general theory of relativity of 1916 as ripples in the fabric of s.p.a.ce and time, have never been detected from any source. Physicists at the California Inst.i.tute of Technology are developing a specialized gravity-wave detector that consists of an L L-shaped evacuated pipe with 2.5-mile-long arms housing laser beams. If a gravitational wave pa.s.ses by, the light path in one arm will temporarily differ in length from that of the other arm by a tiny amount. The experiment is known as LIGO, the Laser Interferometer Gravitational-wave Observatory, and it will be sensitive enough to detect gravitational waves from colliding stars over 100 million light-years away. One can imagine a time in the future where gravitational events in the universe-collisions, explosions, and collapsed stars-are observed routinely this way. Indeed, we may one day open this window wide enough to see beyond the opaque wall of microwave background radiation to the beginning of time itself.

SEVENTEEN.

COLORS OF THE COSMOS.

Only a few objects in Earth's nighttime sky are bright enough to trigger our retina's color-sensitive cones. The red planet Mars can do it. As does the blue supergiant star Rigel (Orion's right kneecap) and the red supergiant Betelgeuse (Orion's left armpit). But aside from these standouts, the pickings are slim. To the unaided eye, s.p.a.ce is a dark and colorless place.

Not until you aim large telescopes does the universe show its true colors. Glowing objects, like stars, come in three basic colors: red, white, and blue-a cosmic fact that would have pleased the founding fathers. Interstellar gas clouds can take on practically any color at all, depending on which chemical elements are present, and depending on how you photograph them, whereas a star's color follows directly from its surface temperature: Cool stars are red. Tepid stars are white. Hot stars are blue. Very hot stars are still blue. How about very, very hot places, like the 15-million-degree center of the Sun? Blue. To an astrophysicist, red-hot foods and red-hot lovers both leave room for improvement. It's just that simple.

Or is it?

A conspiracy of astrophysical law and human physiology bars the existence of green stars. How about yellow stars? Some astronomy textbooks, many science-fiction stories, and nearly every person on the street, comprise the Sun-Is-Yellow movement. Professional photographers, however, would swear the Sun is blue; "daylight" film is color-balanced on the expectation that the light source (presumably the Sun) is strong in the blue. The old blue-dot flash cubes were just one example of the attempt to simulate the Sun's blue light for indoor shots when using daylight film. Loft artists would argue, however, that the Sun is pure white, offering them the most accurate view of their selected paint pigments.

No doubt the Sun acquires a yellow-orange patina near the dusty horizon during sunrise and sunset. But at high noon, when atmospheric scattering is at a minimum, the color yellow does not spring to mind. Indeed, light sources that are truly yellow make white things look yellow. So if the Sun were pure yellow, then snow would look yellow-whether or not it fell near fire hydrants.

TO AN ASTROPHYSICIST, "cool" objects have surface temperatures between 1,000 and 4,000 degrees Kelvin and are generally described as red. Yet the filament of a high-wattage incandescent lightbulb rarely exceeds 3,000 degrees Kelvin (tungsten melts at 3,680 degrees) and looks very white. Below about 1,000 degrees, objects become dramatically less luminous in the visible part of the spectrum. Cosmic orbs with these temperatures are failed stars. We call them brown dwarfs even though they are not brown and emit hardly any visible light at all.

While we are on the subject, black holes aren't really black. They actually evaporate, very slowly, by emitting small quant.i.ties of light from the edge of their event horizon in a process first described by the physicist Stephen Hawking. Depending on a black hole's ma.s.s, it can emit any form of light. The smaller black holes are, the faster they evaporate, ending their lives in a runaway flash of energy rich in gamma rays, as well as visible light.

MODERN SCIENTIFIC IMAGES shown on television, in magazines, and in books often use a false color palette. TV weather forecasters have gone all the way, denoting things like heavy rainfall with one color and lighter rainfall with another. When astrophysicists create images of cosmic objects, they typically a.s.sign an arbitrary sequence of colors to an image's range of brightness. The brightest part might be red and the dimmest parts blue. So the colors you see bear no relation at all to the actual colors of the object. As in meteorology, some of these images have color sequences that relate to other attributes, such as the object's chemical composition or temperature. And it's not uncommon to see an image of a spiral galaxy that has been color-coded for its rotation: the parts coming toward you are shades of blue while the parts moving away are shades of red. In this case, the a.s.signed colors evoke the widely recognized blue and red Doppler shifts that reveal an object's motion. shown on television, in magazines, and in books often use a false color palette. TV weather forecasters have gone all the way, denoting things like heavy rainfall with one color and lighter rainfall with another. When astrophysicists create images of cosmic objects, they typically a.s.sign an arbitrary sequence of colors to an image's range of brightness. The brightest part might be red and the dimmest parts blue. So the colors you see bear no relation at all to the actual colors of the object. As in meteorology, some of these images have color sequences that relate to other attributes, such as the object's chemical composition or temperature. And it's not uncommon to see an image of a spiral galaxy that has been color-coded for its rotation: the parts coming toward you are shades of blue while the parts moving away are shades of red. In this case, the a.s.signed colors evoke the widely recognized blue and red Doppler shifts that reveal an object's motion.