Death by Black Hole: And Other Cosmic Quandaries - Neil deGrasse Tyson (2014)
SECTION 3. WAYS AND MEANS OF NATURE
Chapter 16. 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 space probes, which we can rightly call robots, became (and still are) the standard tool for space exploration. Robots in space 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 cumbersome pressure suit; and they are not alive in any traditional sense of the word, so they cannot be killed in a space 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.
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 classes 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 assembled 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 noble 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 eyeballs 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 assemble 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:
While some are deadly:
CO (carbon monoxide)
HCN (hydrogen cyanide)
Some remind you of the hospital:
C2H5OH (ethyl alcohol)
And some don’t remind you of anything:
N2H+ (dinitrogen monohydride ion)
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 Nobel Prize–winning 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.
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 passes mostly unhindered through glass and air. Ultraviolet, however, is summarily absorbed by ordinary glass, so glass 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 analysis 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 Nobel Prize ever awarded in physics went to the German physicist Wilhelm C. Röntgen 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 space-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 pass 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 space and time, have never been detected from any source. Physicists at the California Institute of Technology are developing a specialized gravity-wave detector that consists of an L-shaped evacuated pipe with 2.5-mile-long arms housing laser beams. If a gravitational wave passes 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.