Wave after wave, each mightier than the last, Till last, a ninth one, gathering half the deep And full of voices, slowly rose and plunged Roaring, and all the wave was in a flame.
—ALFRED, LORD TENNYSON, THE COMING OF ARTHUR
Admittedly, Rottnest Island was not the main reason I had come to the Perth area. My official destination was the Gravity Discovery Centre in Gingin, an hour north. Unfortunately, I was going to have to miss out on visiting a radio telescope array that looks more like a swarm of mechanized spiders than an astronomical observatory. The 4,096-unit spider brigade, the official name of which is the Murchison Widefield Array, is a few hours’ drive beyond Geraldton, which is an hour’s flight from Perth, which is already at the ends of the Earth.
In a place as sparse as Western Australia, it’s a wonder that any two things ever come together. In a universe that’s even bigger and more sparsely populated—and getting bigger and more sparsely populated by the second—it seems an impossibility.
But on a sunny day off, smiling quokkas (“The world’s happiest animals!” assured the tourism videos) beckoned. Twenty kilometers off the coast of Western Australia, Rottnest Island gets the full brunt of thrashing Indian Ocean waves. To get there requires a 45-minute fast ferry ride that slides up and down the swells. On this particular autumn day, the swells were rather pronounced because of a disturbance off the coast of Africa a few days earlier.
Yes, Africa. There is nothing but the vast Indian Ocean separating that mighty continent from Perth, fully 8,000 kilometers to its east. Nasty weather near Nairobi can bring seasickness to Australian ferry riders within a week, energy from a distant storm driving the swells an ocean away.
The ferry hummed rhythmically over the swells, and as I gazed out at the undulating sea, with small ripples superimposed on the rolling hills of water, my thoughts naturally turned to binary stellar corpses, decaying orbits, and the invisible tapestry in which we live. As one’s mind often does . . .
In 1916 Albert Einstein published his general theory of relativity. In it, he revealed our universe to be an interwoven fabric of matter, space, and time. It took less than a year for him to further calculate that when matter moves, the rest of the fabric—space and time—ripples in response. Those resulting ripples are known as “gravitational waves,” and like any wave, they transfer energy from one place to another. Rolling waves near Rottnest can signal the energy dumped by a storm system near Africa, just as a buzzing sound lets you know a fly is frantically beating its wings nearby.
Einstein himself had little optimism that any consequence of gravitational radiation would ever be observable. Indeed, he occasionally doubted that such a thing even existed, going so far as to author an unpublished paper entitled “Do Gravitational Waves Exist?” At that time, two decades after he first formalized his general theory of relativity, he had become convinced that the answer was no, and he said as much. Unsurprisingly, attempting to describe how the imperceptible fabric of spacetime is affected by accelerating masses requires novel mathematical tools whose results are frequently tough to interpret physically. His mathematics were correct. His interpretation, however, needed some help.
Once it was largely accepted that, yes, gravitational waves do exist, the question remained whether they—or anything resulting from them—could ever be measured. Gravitational radiation, unlike electromagnetic radiation, has no choice but to be subtle. Light waves arise from the interplay between electricity and magnetism, forces that are 100 trillion trillion trillion times stronger than gravity, making them much easier to observe. Those waves bounce around the universe, potentially agitating any charged particles they encounter, including in our retinas, in radio telescopes, or in orbiting X-ray observatories. As it turns out, charged particles, like two young siblings, really enjoy bothering each other, even from incredible distances.
But conversations between masses are much more subdued, their voices carrying as fast and as far as light waves, but at a much, much, much lower volume. This “volume” is not a literal loudness. These are not sound waves. Instead, interacting masses create minute compressions and elongations in the tapestry fibers, a tapestry that is stubbornly resistant to carrying any messages the masses want to send.
In this picture, spacetime is a thing. A substance. And it can be manipulated like one. Every substance is theoretically compressible, assuming you can apply enough pressure. Steel, for instance, is about 20 times harder to compress than wood, which is about 100 times harder to compress than rubber. The strongest, least compressible material humans have ever created is something called carbyne, which puts up an enormous fight to keep its dimensions. This substance is about 18 times more resistant to compression and elongation than steel, a figure that seems impressive until you consider spacetime.
We live on a planet that seems to glide effortlessly through something that is about 10,000,000,000,000,000,000 (1019) times stiffer than carbyne. All the analogies in the world cannot begin to convey the intransigence of the cosmic scaffolding. It requires the most monumental pressure to deform spacetime, but this deformation has one very obvious consequence in our lives: gravity.
It was the well-understood behavior of masses subject to gravity that allowed Einstein in 1915 to formulate the equations that encompassed the properties of spacetime. For those who don’t speak mathematics, Charles Misner, Kip Thorne, and John Wheeler neatly summarized the situation in their 1973 book, Gravitation: “Matter tells space-time how to curve; space-time tells matter how to move.”
We had seen plenty of moving matter through the centuries. Now, thanks to relativity, we could understand the behind-the-scenes curvature responsible for those motions.
The most common way to visualize the concept of a curvable spacetime is to think of a trampoline flexing and warping with each bounce. Gleeful children never have to worry about hitting the ground below because their masses are sufficiently small, their jumps sufficiently tame, and the trampoline sufficiently taut. A child who doesn’t want to make a steep dip in the trampoline need only lie down. Spreading out her mass over a greater area exerts less pressure on the fabric. The steepness and depth of the dips depends on how much mass there is and how concentrated it is. And when a child lying down on a trampoline exerts less pressure on the fabric than the same child standing upright, the curvature is even lower.
Earth, in this analogy, is a child lying down. She creates a gentle slope to the fabric of the trampoline, such that if you were to put a marble near the child, it would roll casually down and rest next to her. Because its mass and size are much greater than Earth’s, the Sun’s imprint would be much wider but also deeper. Concentrate the Sun’s mass into a tiny package like a white dwarf, and the dent becomes narrower and steeper. Further squish it into the city-sized package of a neutron star, and the dent becomes so narrow and so steep that it’s almost vertical, the result of an elephant on a pogo stick. It’s not hard to imagine the ever-faster circles that a marble rolled near such a pit would trace out as it spiraled toward the bottom.
In this formulation of gravity, orbits are nothing more than the natural motion along a curved surface, no force required. There is, however, a payment to be exacted. If, for instance, two objects whirl around and around and around their balance point, they are perpetually creating undulations in the universal tapestry. Those undulations sap energy from the pair and then send off that energy at the speed of light to the farthest reaches of the tapestry in the form of gravitational waves.
Puny Earth with its year-long orbit around the Sun creates shallow ripples with such long wavelengths that only one of them washes over our cosmic neighbors each year, each wave a light-year from peak to peak. The combination of masses, separation, and orbital period means that the Earth-Sun system is losing a measly 200 watts annually through gravitational radiation. If this were the only process at play, this energy loss would gradually force Earth’s orbit to shrink by about a billionth of a millimeter per year, hardly a cause for concern given that we are 150 trillion millimeters from the Sun. But our solar system is a dynamic mess with far more significant factors at play, the sum of which is actually causing Earth’s orbit to grow.
Heftier and faster, the pair of stars that makes up the not-soon-to-be-red-nova KIC 9832227 creates steeper ripples with a wavelength of 11 light-hours, about the diameter of Pluto’s orbit. Their greater masses and smaller separation mean that more energy is being carried away by gravitational waves. But like the Earth-Sun system, the stars of KIC 9832227 have 99 energy-altering problems, and gravitational radiation isn’t one of them.
To really notice the effects of gravitational radiation, what astronomers need is a clean binary system, preferably living in very close quarters and preferably with intensely concentrated masses so that other effects are minimized.
Thankfully, the universal warehouse has something for everyone.
In the early 1970s, pulsars were the name of the astronomical game. Radio telescopes like the Dish and Arecibo were racking up pulsar discoveries left and right, but not as mere collector’s items. Pulsars were the best chance of seeing extreme physics play out, a stage upon which the usual rules of Newton’s physics didn’t apply. “To lovers of general relativity theory like me,” physicist Kip Thorne gushed, “this is a very exciting state of affairs, because relativistic modifications of Newtonian theory should be important in all neutron stars!”
So what does the extreme curvature of spacetime around neutron stars do? First, there is the potential for measurable gravitational radiation. Perhaps astronomers can’t directly measure the gravitational waves themselves, but they can divine how much energy a pair of neutron stars loses over time if at least one of the pair presents itself as a pulsar. Second, if astronomers are extremely lucky, they might get a chance to see an eclipsing binary pulsar, one pulsar passing in front of the other as they orbit. Because space and time are intertwined in general relativity, the stretching of spacetime by masses means that time ticks more slowly closer to a gravitating object. If one pulsar passes in front of the other, it appears to Earthlings that someone lengthened the background pulsar’s beat. And finally, if astronomers hit the line-of-sight jackpot, they can even measure how the pulse beam itself follows the deep pit in spacetime caused by its foreground partner.
But let’s not get too greedy. Before any of this could be done, astronomers needed to find an actual pulsar in a binary system.
Of course, it didn’t take all that long. Binary stars are the norm, and while massive stars aren’t the most abundant stars in the universe, there are still plenty in a galaxy of several hundred billion stars.
In 1974, less than seven years after the discovery of the first pulsar, Princeton astronomers Russell Hulse and Joseph Taylor announced an exciting new find. It wasn’t a binary pulsar, but it was a pulsar with a fairly massive unseen partner. The two (objects, not scientists) danced about each other in mere hours, a dance that caused the pulsar’s heartbeat first to appear faster, then to appear slower. It was the Doppler shift, but instead of the “eeeeeeeeeyoooooooooo” of sound waves or the blueshift-redshift of light waves, astronomers measured the pulse-to-pulse timing change. Instead of picking up a reliable clock ticking every 59 milliseconds, Arecibo detected slightly more frequent pulses when the pulsar moved toward us and slightly less frequent pulses as it receded.
The universe had given scientists the perfect laboratory to test one of the most audacious claims in science. If Einstein was right, the orbiting neutron stars should radiate away energy, not as light but as gravitational waves. The very fabric of the universe would be rippling, and all researchers needed to do was watch.
And wait.
And wait some more.
There is, it turns out, a considerable amount of waiting involved in trying to measure any changes in the orbit of a pair of neutron stars that are separated by, on average, 2 million kilometers. The neutron stars are engaged in a complicated and elongated dance, whipping quickly around each other when they are near, and more casually sweeping across the dance floor when they’re far. Over the course of seven years, astronomers made thousands of observations, looking for evidence that gravitational waves were siphoning over 7 trillion trillion watts of power from this system.
Along the way, they made a number of other discoveries, including that the mass of each neutron star in the system is 1.4 times that of the Sun. This in and of itself was a pretty spectacular find, but authors Joseph Taylor and Joel Weisberg seemed almost blasé about it. “It is interesting to note, in passing, that PSR 1913 + 16 is the only radio frequency pulsar whose mass has been measured.”
This apparent indifference can be excused, though. What they then went on to show is that the time between each close encounter between the neutron stars—the point in the dance where they whip quickly around each other—is gradually decreasing. The dance partners are moving ever closer together, each pass taking infinitesimally less time than the previous one. With one of the most stunning graphs in the history of astronomy, Taylor and Weisberg’s 1982 paper shows how their measurements are precisely what you would expect if the energy thief were none other than gravitational radiation.
Once again, a Nobel Prize would be awarded to scientists who used stellar corpses as natural laboratories. This one went to Hulse and Taylor in 1993. By then, astronomers had confirmed year after year that the pair of neutron stars was, in fact, behaving precisely as Einstein’s theory predicted. (Scientists are still confirming this behavior.) “The good agreement between the observed value and the theoretically calculated value of the orbital path can be seen as an indirect proof of the existence of gravitational waves,” read the press release for the 1993 Nobel Prize.
Astronomers will have to wait much longer to see the inevitable finale of this shrinking dance. It will take 300 million years for gravitational waves to carry off all the orbital energy of the two neutron stars, at which time they will collide.
At that point, the Nobel Prize press release promises, things could get very interesting. “Perhaps the violent perturbations of matter that take place when the two astronomical bodies in a binary star (or a binary pulsar) approach each other so closely that they fall into each other may give rise to gravitational waves that could be observed here.”
Perhaps.
But that’s not all that will be created.