SALT LAKE CITY, Utah — When black holes collide, they create gravitational waves. These gravity waves aren’t exactly like waves of water or light. They are instead tiny ripples in the fabric of space. Astronomers expect to record these ripples as a kind of “chirp,” similar to the sound a bird might make. But not all colliding black holes chirp, scientists now say. Some may prefer to sing.
Those that sing will be the ones that spin extremely fast. The spinning black hole featured in the 2014 film Interstellar is a perfect example. It is called Gargantua, and rightly so. It is huge. It would have had a mass 100 million times as great as that of our sun. And it whirled about its axis at breakneck speeds. That’s according to the calculations of Kip Thorne. He is a physicist at Caltech who served as scientific consultant for Interstellar.
Such mass and speed would explain the extreme slowdown of time — what scientists call a time dilation. But it would be seen only on the world where the film’s planet hunters had landed. In one hour there, seven hours would have elapsed on Earth. This phenomenon is predicted by Albert Einstein’s theory of general relativity. That theory is a set of mathematical expressions that define gravity as the warping of space and time. It also helped astronomers capture the “chirp” of gravitational waves from two colliding black holes. The discovery was announced in February 2016. This was the first detection of gravity waves.
A “chirp” is the standard signal of merging black holes. It gets its name from the changes that happen to gravity waves created as the black holes spiral inward. These changes are seen in the frequency and amplitude of the waves. Frequency is the number of waves that occur over a particular period of time. Amplitude describes the height of those waves. Both increase as the black holes get closer. And both can be represented as sound waves. In sound waves, the frequency determines a note’s pitch, and the amplitude determines its volume. When scientists convert gravity waves to sound waves, they get what sounds like a bird’s chirp.
But if a rapidly spinning black hole merges with a companion, it would produce a “completely different gravitational wave signature,” said Niels Warburton. He is a physicist at the Massachusetts Institute of Technology, or MIT, in Cambridge. He spoke April 18 at a meeting, here, of the American Physical Society. He also coauthored a related paper posted online at arXiv.org on March 3.
Warburton and his colleagues wanted to know what the gravity-wave signature from a merger with a black hole spinning at nearly full tilt would be. Their calculations show that instead of a chirp, the waves would maintain a constant pitch, but slowly fade away. This would sound like a singer holding a note.
“It was certainly very unexpected to see something that didn’t chirp,” says Jolyon Bloomfield. He is a physicist and a lecturer at MIT. He was not involved with the research but says it is interesting work. “It shows that the chirp actually goes away — something else is happening here.”
Waiting to see Gargantua-size singers
In his talk, Warburton said that gravity wave detectors might be able to observe the unique signal from collisions of of spinning black holes. The Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, made the first detection of gravity waves this past February. LIGO can’t observe the mergers of black holes as massive as Gargantua, Warburton said. But smaller spinning black holes would produce a similar effect, which could be observed.
Next-generation gravity-wave observatories, however, may be able to detect the songs of extremely large, spinning black holes. One of these upcoming observatories is called the Evolved Laser Interferometer Space Antenna, or eLISA. Plans call for eLISA to measure gravitational waves from space beginning in 2034. Singing gargantuan-black-hole collisions should be “definitely detectable with eLISA,” Warburton said.
These kinds of detections would be exciting for exploring extreme physics. Spinning black holes are “really interesting from a fundamental physics point of view,” says Samuel Gralla. He is a physicist at the University of Arizona in Tucson. He also was a coauthor of the new paper.
Black holes can spin faster and faster as they suck in matter, he explains. But there’s a limit to how fast they can go. That’s because at the center of a black hole is a singularity, or region of infinite density. It is hidden by an event horizon — the surface beyond which nothing can escape a black hole’s greedy pull. If the black hole twirls too fast, then the singularity is exposed. This is known as a “naked singularity.” Scientists think it is impossible to reach because the known laws of physics would break down.
The calculations made by Gralla, Warburton and their team showed that black-hole mergers sing when the larger black hole is spinning at just below its limit. That would be when it spins at 99.99 percent of its maximum speed. This makes the detection of singing black holes an enticing prospect for understanding physics at its extremes.
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amplitude A measure of the height of a recurring wave in some signal, water or beam of radiation. In sound, wave amplitude corresponds with intensity — loudness or softness.
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
black hole A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.
consultant Someone who performs work as an outside expert, usually for a company or industry. “Independent” consultants often work alone, as individuals who sign a contract to share their expert advice or analytical skills for a short time with a company or other organization.
density The measure how condensed an object is, found by dividing the mass by the volume.
event horizon An imaginary sphere that surrounds a black hole. The more massive the black hole, the bigger the sphere. Anything that happens inside the event horizon is invisible, because gravity is so strong that under normal circumstances even light can’t escape. But according to some theories of physics, in certain situations small amounts of radiation can escape.
frequency The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.
general relativity A set of mathematical expressions that define gravity and space over time (also known as spacetime). It was first published by Albert Einstein in November 1915. The field of research that focuses on this is described as relativistic.
gravitational waves (also known as gravity waves) Ripples in the fabric of space that are produced when masses undergo sudden acceleration. Some are believed to have been unleashed during the Big Bang, when the universe got its explosive start.
gravity Schools tend to teach that gravity is the force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity. But Einstein’s general theory of relativity redefined it, showing that gravity is not an ordinary force, but instead a property of space-time geometry. Gravity essentially can be viewed as a curve in spacetime, because as a body moves through space, it follows a curved path owing to the far greater mass of one or more objects in its vicinity.
infinite Endless, or without limit. The term can be applied to the spatial extent of the universe or something that is similarly impossible to measure or calculate.
LIGO (short for Laser Interferometer Gravitational wave Observatory) A system of two detectors, separated at a great geographical distance, that are used to register the presence of passing gravitational waves.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
pitch (in acoustics) The word musicians use for sound frequency. It describes how high or low a sound is, which will be determined by the vibrations that created that sound.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study which emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in that field is known as a physicist.
singularity A region of infinite density at the center of a black hole.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
theory (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.
time dilation A difference of elapsed timebetween two events as measured by observers who are either moving relative to each other or are differently situated from a gravitational mass (or masses), such as a black hole.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.