But these wavelengths are not missing from the spectrum of a simple candle flame. Aha: the sun isn't quite what it seems to be.
It wasn't for a few more decades that a puzzle piece clicked into place when Robert Bunsen (of “the burner” fame) and Gustav Kirchhoff (of “who?” fame) figured out that when specific elements were tossed into a flame, bright lines would pop out of the flame's spectrum. It's as if every element has a fingerprint—a pattern of lines in an otherwise featureless spectrum that is unique to that element.
Perhaps—work with me here, guys—when an element adds its light, we get bright lines appearing, but when an element blocks a background light, the same lines appear, but dark. Like a cosmic crime scene, the spectral fingerprints point the way to the suspect elements.
And now we can figure out what the sun is made of. And the stars. And nebulae. And planet atmospheres. And…everything. Granted, at this time nobody understood why the elements produced these strange lines (and even the concept of “element” was still gaining ground in chemistry circles), but what mattered was that they did, and we could test that in a laboratory in the back of the office.
That's kind of a big deal. This technique, known as spectroscopy (because, again, “rainbowscopy” doesn't sound sciencey enough), is the ultimate key that unlocks the farthest reaches of the universe. We can taste the surface of the sun without having to visit it, simply by comparing sunlight to various heated gases. We can discover brand-new elements, as Jules Janssen and Joseph Lockyer did in 1868 when they identified in sunlight a never-before-understood series of lines as helium, which they named in honor of our own sun.
It wasn't all roses and unicorns, though. Every new discovery in science leads to a thousand more questions. Hey, there's oxygen in that nebula way over there, awesome! Wait—how did oxygen get into that nebula? Way over there?
It gets even better/worse. Once folks started to come to terms with the concept of light as waves (waves of what would have to wait until 1865, when Maxwell realized that they're waves of electricity and magnetism), there was another trick that spectroscopy could play. If you've ever heard something loud passing by, you're familiar with the Doppler effect: on the approach, sound waves get squished, pushing them into higher pitches. On the way out, sound waves get stretched, pulling them to lower tones.
It happens with sound waves, and it happens with light waves. It's a very subtle effect, though, since most things don't move very fast compared to the speed of light itself, so it's not like we see entire colors shifting redder or bluer. But the fingerprint pattern of spectral lines can shift. It's a wonderful tool: perhaps you recognize a particular arrangement in the spectrum of a star—oh, there's some iron!—but the whole pattern is shifted to the left or right by a few wavelengths. Well, if the spectrum from that star is shifted toward the blue end compared to something stationary, like a light source in the room you're sitting in, not only can you conclude that the star is moving, but you can measure very precisely its speed.
Not its entire speed—sideways movements won't change the spectrum from our perspective—but the in-out speed is fair game for measurement. And measuring the speed of star after star revealed that we don't live in a fixed cosmos. We live in a beehive.
There's one other piece of technology that opened a window of confusion to our universe: photography. Where the telescope acts as a super-eye that creates a bigger bucket to collect light and magnifies separations to make them more distinct, adding a photographic plate to the back end of that device amps it up to a hundred. No matter how good your telescope is, if you only look with your eyes, you'll be fundamentally limited by what you can see.
But a photographic plate can collect, collect, collect. Restless and unblinking, it continually absorbs light, adding it to the pile, revealing fainter and dimmer objects. And it records! No longer do you have to alternate between staring and sketching to record what you're seeing—the photograph does it all in one handy-dandy, convenient device.
Astrophotography is the ultimate extension of the human sense of sight into the cosmos. It's everything a human eye does, just way better. Combined with spectroscopy—the study of spectra—it opened up the heavens like never before. Information poured in from observatories across the globe. Expeditions were launched; telescopes were fashioned by professionals and enthusiasts alike. Never before had so much interest been focused onto the night sky. The number and variety of phenomena in the universe around us were almost overwhelming. Breathlessly, astronomers recorded and published their findings in between sessions of staring dumbstruck at their celestial revelations.
Over the course of the eighteenth and nineteenth centuries, we discovered and cataloged new kinds of nebulae. We confirmed that the distant stars were like our sun but also different. Some were smaller, some larger. Some hotter, some cooler. Comets came and went on repeatable cycles. Our solar system was belted with a ring of asteroidal debris. Dozens of moons danced around the outer worlds of our solar system. Dust glinted in the pale sunlight and was flung out between the stars themselves. The universe itself was beginning to open up before us.
I'm always hesitant to pull random quotes out of history just to mock them, because it's kind of challenging to predict the future, but this one is too juicy to pass up. In 1835, the philosopher Auguste Comte wrote, “I regard any notion concerning the true mean temperature of the various stars as forever denied to us” due to their extreme distance.10 He wrote many other things that turned out to be useful and respected, but in this one instance, the scientific community, after decades of labor, analysis, and careful study, responded with a resounding “Bite me.”
But nature has a habit of biting back, and I'm sure Heinrich Olbers thought he was making a really great point in 1823. Turns out he was far from the first person to have this thought, but he was pretty much the last, so his name got stuck to an apparent paradox.11 The paradox goes back to the old sun-centered versus Earth-centered arguments of generations past. In the solipsistic view, where we here on Earth are the literal center of the universe, the fixed stars are simply that: fixed. There are a few thousand of them, and they're all attached to the outermost celestial sphere, wheeling away through cosmic time. No big deal.
But in a sun-centered universe, you have to grapple with the distinct possibility that the fixed stars are really distant suns. And as soon you point your telescope into the deep void between them, you find other, fainter—and possibly more distant—fireballs.
So how far back in space does it go? How deep could we possibly perceive? What lies in between the in-between? Do stars stop? Is there a limit to their light? If that's the case, does that kind of universe even make sense? To have a cosmos filled with void, except a little cluster of lights in one corner?
Isaac Newton provided one answer. If the universe were finite in space, then eventually gravitational interactions would, over the course of uncountable eons, cause all the stars to accumulate into a single pitiful lump.
The easier thought to swallow is that the universe is infinite. It simply goes and goes, with countless stars backed by ever-larger multitudes. Thus any spot in space is perfectly gravitationally balanced by the infinity on either side.
But how far back in time does it go? Various religious traditions teach about the (re-)creation of the physical universe at distinct points in the past, but if there's one thing the Copernican revolution taught us, it's that maybe we should give the scientific method a chance at answering some of those questions.
The Earth isn't going anywhere soon, and neither is the sun. And neither are the stars, or comets, or nebulae. They're just there. Sure, a new star will occasionally appear, or a comet will break apart upon encountering the inner solar system, or the moon might be slowly circling away from us, but for the most part, the universe today looks like the universe yesterday. And the day before, and the day before. Maybe the universe is simply infinite both in space and time. Maybe there is no beginning, no primordial ooze, no “Let the
re be.” The universe is.
But that doesn't work, and that's where Olbers’ paradox comes in. In a universe with infinite extent in both time and space, there shouldn't be any dark. If you look in any random direction in the sky in an infinite universe, then you must be looking in the direction of a star, somewhere, at some distance. “But maybe the light hasn't reached me yet,” you retort. Good point, except that in a universe that has existed for eternity, there's been way more than enough time for that light to reach out.
So night shouldn't be night at all; instead it should be aglow with the fire of literally an infinite number of suns. But it's pretty dang dark, which means the universe isn't infinite in time and/or space. But all the lines of thinking and evidence point toward infinity. What gives?
It will take me a lot of words to fully deconstruct the apparent paradox in detail—and don't worry, I certainly will in later chapters—but the short version is that the universe is definitely not infinite in time (at least, into the past) and most likely not infinite in space. But our dear nineteenth-century friends didn't know that, so they had to grapple with the central conundrum.
They attempted to tackle it one step at a time, and the first step is getting a distance to a star: any star at all will do. Just give us one hook into the extrasolar system, and we can start putting together a map of the cosmos and figure out the flaw in Olbers's reasoning.
I'm never one to call Newton naïve, but he did advocate a naïve method for measuring distances. If you assume that all the stars are the same true brightness—in essence, that they're all identical copies of the sun—then if you can measure their perceived brightness with incredible precision, you can do some math and figure out a distance. Astronomers over the following decades confirmed that most stars are totally unlike our sun in color and temperature and elemental composition, so that's a nonstarter. The method isn't totally without merit, and later generations will use the same principles to great effect, but that's a story for a later chapter.
Instead they had to give parallax a try. Parallax is the simple geometric measurement where you pick a star, record its absolute position in the sky, then wait six months until the Earth is on the opposite side of the solar system. Repeat your measurement, and if you're very good and even luckier, you'll have recorded a small shift in its position. That gives you an angle, and since you (hopefully) know the distance to the sun, you can construct a long skinny triangle toward the star, do some basic trigonometry, and compute a distance.
Simple, but not easy. We first encountered Tycho Brahe himself attempting a parallax measurement to put the nail in the coffin of all this sun-centered universe nonsense. He succeeded: according to the very best measurements the world had ever produced (ahem, his own), there was no observed parallax, and hence if the sun were the center of the cosmos with the Earth flinging itself about it, that would mean the celestial sphere had to be…let's see here…seven hundred times farther away than the planet Saturn. Preposterous that the universe should be so large! Earth-centered it is, chaps.12
Even though Kepler and then Newton won the sun-centered day based on other arguments, the problem stuck. Surely somebody would eventually measure a reliable parallax, get a fix on a star, and start to put the nomy in astronomy. But decades, and even centuries, churned by without a measurement. Telescopes got bigger and better. Catalogs of the heavens grew thick with entries. Innovative techniques were developed and deployed. The heavyweights I've already introduced in this chapter all took a crack at it.
Nothing. Not a single distance. With every failed attempt, we had to stretch the yardstick of space out farther. With every false report, the universe grew larger: the greater the distance to the stars, the smaller the seasonal wobble, and the better our instrumentation had to be. It was getting kind of scary, honestly.
Finally, after centuries of previous attempts and years of his own hard labor, Friedrich Bessel nailed it: 61 Cygni, a star in the constellation Cygnus that's unremarkable except that it's close to Earth. Bessel didn't know it was close, but he guessed based on its larger proper motion over the decades and centuries. (“Proper” here is a bit of astronomical jargon to mean motion that belongs to the star itself, not due to any “fake” motion that we might observe from the rotating vantage point on the surface of the Earth.)
It was already realized that stars move of their own accord, even before later spectroscopic measurements would confirm it. Since the stars are very far away (as has been established), it takes time for their motion to be noticed, but noticed and measured they can be, and it was (correctly) argued that if a star is closer to the Earth, it should have a bigger proper motion, because that's how geometry works: cars moving across the intersection right in front of you will have greater proper motion than ones a few blocks away, even if the cars are all moving at the same speeds.
61 Cygni is one of the fastest stars, so Bessel figured it would give him a shot of winning the big prize, and he was right. In 1838, after a few years of observations, he came up with a distance of ninety-six trillion kilometers. That's right, “trillion” with a terrible t.
Remember, just a couple of centuries earlier, Tycho Brahe was nauseated by the thought of the stars sitting seven hundred times farther from the sun than Saturn. Bessel's measurement, which is only 10 percent off from the current best measurement, placed 61 Cygni about sixty thousand times farther than Saturn.
In one clean measurement, Bessel (who, I feel compelled to note, received no higher formal education and also managed to develop suites of mathematical functions that bear his name today) finally put to rest the ultimate question of the heliocentric debate. It was already on firm theoretical grounds thanks to Kepler, Newton, and others, but this was a key piece of data that had been missing from the arguments.
Other parallax measurements quickly followed (Bessel wasn't the only one interested in the problem). The universe was getting larger and more complex with every new telescope and every new catalog. It was a heyday for the experimentalist and a nightmare for the theorist. None of it made any sense: just how big is our home, and what is it made of? These stupidly simple questions were getting frustratingly hard to answer.
At about this same time, astronomy was finally splitting off from astrology. For millennia, the two had been intertwined, and the words were roughly synonyms. Measurements of the stars went hand in hand with their forecast effects on our daily lives. In a complex, chaotic world where nothing made sense and everything was changing all the time, it was no wonder our ancestors looked to the steady, regular patterns overhead and drew solace from them. The clockwork regularity of the stars and planets must hold the keys to the underlying order of life on Earth.
But by the close of the nineteenth century, we knew that the universe at large was as frightful and complex as anything here on Earth—and even more so. There were forces and motions at play too great to comprehend. 61 Cygni itself was computed to have a proper motion of hundreds of thousands of miles per hour. How does a star, a massive burning ball of gas, achieve such incredible feats? How can new stars appear and familiar ones go silent? How can Newton's laws account for all this?
The telescope, the spectrometer, and the photograph opened up the cosmos before us, but it was a cosmic Pandora's box. We struggled and grasped to connect the physics we were learning on the Earth—electricity and magnetism, heat and energy, chemistry and the element, and other hot topics of the day—to the scales of the heavens, and we failed, terribly.
It was becoming painfully obvious that the cosmos was not connected to us, did not care about us, and did not care for us. We were an ant climbing on a branch of a vast tree that was incomprehensibly larger and more complex than we ever thought. We were reaching out with our enhanced senses and the powerful tool of the scientific method, and we were not liking what we were seeing.
While astrologers clung—as some still do today—to the notion that the motions of the planets govern and predict our lives, astronomers were left in a
much more befuddled state. They could record, measure, and study, but they could not understand.
By the early twentieth century, astronomers were especially concerned by the nature of the spiral nebulae—just one branch of that fuzzy family tree, but one that seemed to be different from the others. How far away were they? Were they part of our universe—an unbroken field of stars stretching from one end of the cosmos to the other—or somehow isolated from us in their own “island universe”? Data and argument swung either way depending on who was more persuasive and whose data you believed to be more reliable.
Our understanding of the universe was at a breaking point. A hurricane of raw data was slamming into previously held notions. We couldn't crack the code; we couldn't navigate the storm of conflicting ideas and theories.
Our perception of the cosmos was ready, if you will, for a phase transition.
I want you to imagine visiting a hotel, one familiar but with some odd properties that you usually don't encounter. Feel free to call it Hotel Dirac if you want. If you don't, you'll get the joke later.
Here's how the hotel works. There are multiple floors and rooms on each floor, as usual. But there can only be one person assigned to any room at any one time, and the rooms are filled starting with the first room on the first floor, then the second room, and so on, until the lowest floor is filled; then rooms get assigned on the second floor. There's technically an infinite number of floors, but that's not really relevant.
Sometimes a guest will feel like moving up in the world and bump themself up to a room on a higher floor. But they only get to visit that penthouse suite for a little bit of time—as soon as one of the hotel staff notices (and they're very diligent about these sorts of things), the guest gets scolded and pushed back down to a lower room.
Your Place in the Universe Page 6