The instrumentation for this endeavor, though, is vastly different from that of an ordinary telescope. There are no lenses to spy on the universe. Instead, long, wide tubes are set at right angles to each other. The LIGO tubes, for example, extend out into both the Washington and Louisiana countryside for two and a half miles (four kilometers), each forming a giant L in the landscape. Resembling oil pipelines, the tubes are as empty of air as the vacuum of space. And at each end mirrors have been suspended, with a laser beam continuously bouncing back and forth between them.
Gravity-wave observatories are set up in this way because of the unique effect a gravity wave has as it travels through and disturbs space-time. The wave compresses space in one direction—say, north and south—while simultaneously expanding it in the perpendicular direction—east and west. So, a gravity wave coming straight down on the L-shaped observatory would squeeze one of the arms, causing the mirrors at each end to draw closer together, while spreading the mirrors in the other arm farther apart. A millisecond later, as the gravity wave continues on its way, this effect would reverse, with the compressed arm expanding and the expanding arm contracting. The laser beams, continually measuring the distance between the mirrors, would take note of this cyclic change.
This is far trickier than it sounds. The gravity waves emitted by two black holes colliding are very powerful. Space is shaken, and shaken hard. Such a colossal collision would send out a spacequake that surges through the cosmos at the speed of light. But they wouldn’t propagate in the manner a light wave travels through space. Rather, they would be an agitation of space itself. The waves would alternately compress and stretch the fabric of space-time as they travel along. Such waves would be deadly near the crash site. They could stretch a six-foot man to twelve feet and within a millisecond squeeze him down to three, before stretching him out once again. Any planets in the vicinity would be torn asunder. But those waves would get weaker and weaker as they travel outward, not unlike the ripples that die out after a rock is thrown into a pond. By the time those waves reach Earth, the expansion and contractions they create in space-time will be far smaller than the width of a proton particle.
To measure such a minuscule movement, gravity-wave astronomers have taken great pains to eliminate within their observatories as many local disturbances as possible, so that a passing truck or seismic tremor could be ruled out. And as the data streams in, it is continually compared to various “templates,” theoretical predictions of what a gravity-wave signal might look like from differing events. Simultaneous detections of a signal by separate observatories, located half a continent or more apart, would be the best confirmation.
Neutron-star collisions may be their bread-and-butter sources. The observatories are set up to register the final minutes of a neutron-star binary, as the two city-sized balls of dense matter spiral into one another. LIGO will be most receptive to signal frequencies from 100 to 3,000 cycles per second, which is coincidentally the same frequencies our ears pick up as sound. So once the wave is electronically recorded, you could actually listen to it. Gravity-wave observatories will be adding sound to our cosmic senses. The neutron-star clash would start off as a whine and then rapidly rise in pitch, like the sound of a swiftly approaching ambulance siren.
The biggest prize of all, though, will be two black holes colliding. As the twirling holes are about to meet, spiraling inward faster and faster at speeds close to that of light, it’s predicted the whine will turn into a quick chirp, a birdlike trill that races up the scale in a matter of seconds. A cymbal-like crash, a mere millisecond in length, would herald the final collision and merger. The two black holes become one, followed by a ring-down, akin to the diminishing tone of a struck gong, as the new and bigger hole wobbles a bit and then settles down.
There are ways to detect gravity waves indirectly. Teams of radio astronomers operating very sensitive detectors at the South Pole are on the lookout for the distinct imprint of gravity waves upon the Big Bang’s now-weak afterglow—a wash of radio waves known as the cosmic microwave background. Gravity waves can affect the microwaves in an unmistakable way, imposing a faint spiraling pattern into the polarization of the signal. The gravity waves were born as quantum fluctuations in the newborn force of gravity itself, whizzing through the tiny speck of a universe. These waves were fueled and blown up in size as the cosmos underwent a brief, accelerated spurt of expansion called inflation in the first trillionth of a trillionth of a trillionth of a second of our cosmic beginnings, when afterward the cosmos settled down into a slower expansion. By stretching and squeezing space-time, the primordial gravity waves can imprint a slight swirling pattern on the remnant radiation that had become “polarized” (the electric fields of the light waves oscillating back and forth in one preferred direction) as the light began to freely traverse the cosmos. As they rippled space-time, the waves gives the light a little kick that causes its orientation to curl. Dust in our galaxy can lead to the same effect, so any signal must be closely examined to confirm that the primordial universe was the source.
If such a signal is verified, it would be the first detection of Hawking radiation, although not in the context of black-hole event horizons. The observable universe was so tiny at first that it also had a “horizon,” which emits radiation just as hypothesized for a black hole. In this case, the radiation is in the form of gravitons, those quantized particles of gravity that grow into the gravity waves that stretched and squeezed the primordial radiative soup. If the signature of Hawking radiation is assuredly found within the Big Bang, it makes it highly likely that such radiation is emitted by black holes as well. This would open up an entirely new arena for astronomy and cosmology, one that began years earlier when Jacob Bekenstein and Stephen Hawking began thinking outside the box.
There’s more definitive proof of gravity waves’ effects closer in. Two neutron stars in our galaxy, located some twenty-one thousand light-years distant, are rapidly orbiting each other and also drawing closer and closer together. The rate of their orbital decay—the orbit shrinking by about 11.5 feet (3.5 meters ) each year—is just the change physicists expect if this binary pair is losing orbital energy in the form of gravitational waves. The amount of energy the waves carry off matches, with exquisite precision, just what general relativity predicts. Astronomers Joseph Taylor and Russell Hulse won the Nobel Prize in 1993 for this discovery. The gravity waves being emitted by this system are currently too weak to be recorded by the earthbound observatories, but the gravitational ripples will be far more powerful once the two stars finally merge about three hundred million years from now.
But there are lots of other gravity-wave sources that are currently detectable, including supernova explosions, black-hole mergers, and neutron-star collisions that regularly occur throughout the cosmos. Once the observatories are fully up and running, sensitive enough to pick up waves originating from up to billions of light-years away, scientists hope to see some kind of event daily. Even more sources could be recorded if the technology is taken up to space, far from ground-based disruptions, endeavors that are on the drawing board.
Firm detection of a gravity wave is such a high priority in relativistic astrophysics that scientists are not relying on one method alone. There’s another clever scheme based on well-studied astronomical objects—pulsars, the most exquisite timepieces in the universe. By closely monitoring the pulses arriving from an array of particularly fast pulsars, situated around the sky, astronomers are on the lookout for slight changes in the pulsing due to a gravity wave passing between the pulsar and the earthbound detector. No matter which way the gravity wave from a black hole is detected, such sightings would provide the final, undeniable proof that black holes are real, which would be a historic moment for astronomers, who for so long denied their existence.
A rare polar express swept over the Dallas area in December 2013, heavily icing both the airport and the roads. It was a freezing start to the 27th Texas Symposium on Relativistic Astrophysics, it
s fiftieth anniversary. Held nearly every two years since 1963, the conference has been located in cities around the globe, from Munich and Melbourne to Jerusalem and Vancouver. Yet, wherever it is held, the meeting still retains its Texas name to honor its origin.
The first symposium was primarily focused on quasars, with the phrase “relativistic astrophysics” newly introduced. Over the next fifty years the list of topics discussed at the symposium expanded like a raging wildfire; there are now sessions on the inflationary universe, gravitational waves, searches for dark matter, gamma ray bursts, and the cosmic microwave background. Some of these subjects weren’t even imagined half a century ago, when pulsars hadn’t yet been discovered. “We didn’t know that neutron stars would come equipped with a handle and a bell,” as one wit quipped. Now more than 2,300 pulsars have been cataloged within our galaxy.
As for black holes, no longer are eyebrows raised at the sound of their name. In fact, the 2013 Texas Symposium served up a succulent banquet of talks on the subject; researchers reported on the origin of supermassive black holes, gamma-ray bursts from newly formed holes, black-hole mergers, magnetized black holes, jets shooting out of black holes, and new searches for these collapsed objects. They are as readily discussed at current astronomy conferences as a galaxy, nebula, or star. The stellar-sized holes are just another possible endpoint (albeit rarer) in the lifetime of a star. It’s estimated about one star in a thousand ends its life hidden behind an event horizon, with one hundred million of them residing in the Milky Way alone. A new one is being born somewhere in the cosmos with each tick of the clock. And the supermassive ones, grandly residing in the hearts of most galaxies, are now standard equipment in a galaxy’s very structure.
John Wheeler once remarked that he never read science fiction. “All the science fiction I need is right out there in front of us,” he said. He was absolutely right. Black holes, long considered so fanciful, have now transformed into some of the most wondrous and necessary denizens of the cosmos. Once scorned but now accepted, the black hole is commencing a brand new chapter of its life.
Timeline
1687
Sir Isaac Newton publishes his revolutionary law of gravity in the Principia.
1758
A comet predicted by Edmond Halley to return in 1758 arrives on schedule, a victory for Newton’s law of gravity.
1783
John Michell in Great Britain introduces the Newtonian version of a black hole. He calculates the mass at which a star would be so heavy that light cannot escape from it, rendering it invisible.
1796
Following similar reasoning as Michell, Pierre-Simon de Laplace in France independently suggests the existence of corps obscurs, or hidden stellar bodies, in the heavens.
1862
Alvan Graham Clark in Massachusetts discovers that the bright star Sirius has a faint companion. But astronomers were later puzzled how it could be so dim and yet still weigh as much as the Sun.
1905
Albert Einstein publishes what came to be known as his special theory of relativity, which abolishes Newtonian notions of absolute space and time.
1907
Mathematician Hermann Minkowski demonstrates that Einstein, with special relativity, has turned time into just another dimension, leading to the single, absolute entity of space-time.
1915
Introducing his general theory of relativity, Einstein successfully broadens relativity to handle other types of motion, specifically gravity. Gravity is now seen as arising from masses indenting the flexible mat of space-time, with objects moving along the curvatures.
1916
The German astronomer Karl Schwarzschild publishes the first full solution to the equations of general relativity. The result leads to the appearance of the Schwarzschild sphere, which surrounds a mass concentrated at a point in its center. Space and time appear to stop at the sphere’s surface. It is a version of what we today call the black hole, this one uncharged and nonspinning. Some assume it is an artifact of the coordinates being used; others are sure stars could never shrink to such a state.
Estonian Ernst Öpik and later Britisher Arthur Eddington calculate that the solar-mass companion of Sirius must be little larger than the Earth, which explained its faintness. Such stars came to be called “white dwarfs.”
1916
British solar eclipse expeditions to West Africa and Brazil verify that starlight indeed bends its path as it passes close to the Sun, following the indentation the Sun makes in space-time. General relativity is triumphant.
1926
British theorist Ralph Fowler uses the newly established rules of quantum mechanics to explain how the mass of the Sun, crushed to an Earth-sized space, can remain stable as a white dwarf star.
1930
On a voyage from India to Great Britain, Subrahmanyan Chandrasekhar discovers that there is a maximum limit to the mass of a white dwarf star. He doesn’t know what happens to the star should it pass that threshold.
1931
The Soviet theorist Lev Landau calculates a star could collapse to a point if heavy enough, but deeming such a result “ridiculous,” he suggests that the stellar core instead forms “one gigantic nucleus.”
1932
James Chadwick in Great Britain discovers the neutron.
Bell Telephone physicist Karl Jansky discovers radio waves emanating from the center of the Milky Way galaxy. Radio astronomy begins.
1933
At a meeting of the American Physical Society, Fritz Zwicky and Walter Baade suggest that a tiny neutron star forms in the midst of a stellar explosion, a supernova. Astronomers consider the idea far-fetched.
1935
At a meeting of the Royal Astronomical Society, Arthur Eddington infamously challenges Chandrasekhar’s conclusion that a white dwarf star would suddenly shrink if its density passed a maximum limit.
1939
J. Robert Oppenheimer and George Volkoff are the first to examine the physics of a neutron star and discover that neutron stars, just like white dwarfs, have a maximum limit to their mass.
Oppenheimer and Hartland Snyder publish the first modern description of a black hole. They call it “continued gravitational contraction.” Oppenheimer then abandons this line of work. Interest in general relativity continues to plummet within the physics community.
Einstein publishes his “worst scientific paper,” an attempt to prove that stars could never totally collapse to a point (or singularity).
1948
American financier Roger Babson founds the Gravity Research Foundation to renew interest in gravitational studies (so that antigravity devices might be developed one day). Its funding spurs new interest in general relativity.
1952–53
Princeton physicist John Archibald Wheeler teaches the first courses on special and general relativity ever offered by his physics department. He hopes to find a physical reason that prevents a star from collapsing to a singularity, as Oppenheimer and Snyder had suggested.
1955
Einstein dies, believing his colleagues regarded him as an “old fool” and his greatest work—general relativity—in the shadows of physics research.
1957
A new institute for gravitational studies at the University of North Carolina holds a conference at Chapel Hill on the role of gravitation in physics. The meeting proves to be a landmark in reenergizing the field.
At an international conference John Wheeler and two student collaborators attempt to demonstrate how an imploding star could save itself from collapse to a singularity. Oppenheimer, in the audience, politely disagrees.
1958
David Finkelstein develops a new reference frame for general relativity that makes it easier to comprehend the physics of a black hole. It allows physicists to picture how a collapsing star appears like a “frozen star” from afar, yet still fully implodes from the standpoint of the hole. Martin Kruskal did this earlier, but his work wasn’t published unti
l 1960.
∼ 1960
At a colloquium at the Institute for Advanced Study, Princeton physicist Robert Dicke jokingly compares the complete collapse of a star, where gravity is so strong that nothing can escape, to the “Black Hole of Calcutta.” Physicist Hong-Yee Chiu is in the audience.
1962
Using the new mathematical tools developed by Finkelstein and Kruskal, Princeton undergraduate David Beckedorff, working with Charles Misner, arrives at a more detailed description of the space outside a black hole’s event horizon. It was the first depiction of a black hole as a real object.
The field of X-ray astronomy is initiated when a rocketborne X-ray detector discovers the first cosmic X-ray source, Sco X-1, later revealed to be a neutron star in a binary star system.
Early 1960s
Computer simulations carried out at the Livermore National Laboratory in California demonstrate that a star of sufficient mass at the end of its life will collapse to a black hole. Similar results are obtained by Soviet physicists. Convinced by these findings, as well as Beckedorff’s results, Wheeler completely reverses his opinion and comes to champion black holes. The Soviets hardly doubted their existence.
1963
A radio star known as 3C 273 is revealed to be the superluminous nucleus of a galaxy some two billion light-years distant. Such objects are soon dubbed “quasars.”
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