Zombie stars: A source of gravitational waves?

Scientists have witnessed motions that would be predicted by Einstein’s general relativity


A pair of pulsars emitting beams of radio waves form a system locked in tight orbits. An artist has illustrated them here. These pulsars offer an ideal test for measuring gravitational waves and other effects of general relativity.

© MPIfR, M. Kramer

GENEVA, Switzerland —  Pulsars are the dense cores of dead stars. But these zombies still communicate. They emit intense beams of radio waves with the regularity of a nearly perfect clock. A dancing pair of these cosmic radio beacons has just provided scientists with the best gauge that gravitational waves exist.

Gravitational waves are ripples in the fabric of space. If you throw a rock into a pond, it creates ripples — waves in the water — that stretch and squeeze back again. Similarly, accelerating masses should send gravitational waves into space, ripples that cause space to stretch and squeeze back again. For instance, the universe should have unleashed gravitational waves right after its explosive start in the Big Bang.

In recent years, scientists have been looking for such waves. In March 2014, one group reported finding them. Follow-up work, reported in early 2015, concluded that the earlier claim had been a mistake. Now other scientists think they have very strong evidence of those elusive waves. They didn’t see the space ripples. They just saw changes in pulsar orbits that should be due to the production of such waves.

Michael Kramer is an astrophysicist. He works at the Max Planck Institute for Radio Astronomy in Bonn, Germany. He and his colleagues precisely tracked the pulsars’ orbits over time. And they found a slight shrinking in those orbits. This indicates they have lost energy. And that energy likely went into generating gravitational waves. Kramer’s team reported its discovery, here, on December 16 at the Texas Symposium on Relativistic Astrophysics. (Despite its name, the meeting was not held in Texas, or even in the United States.)

The rate of the pulsars’ orbital slowing — and shrinkage — matches precisely what scientists would predict based on general relativity. This theory, first proposed by Albert Einstein a century ago, redefined the rules of gravity and how they must play out across the universe.

The new analysis comes from studying the only known pair of pulsars that are bound together by gravity. This double pulsar system was discovered in 2003.

In many ways, these pulsars are a physicist’s dream. By analyzing the pulsars’ radio beams, physicists can probe the wild things that happen when small — but massive — celestial objects circle each other at roughly a million kilometers (more than 600,000 miles) per hour. Under the rules of general relativity, the pulsars should plow through space and generate gravitational waves. The result: Those pulsars should gradually fall toward one another.

Kramer and his team studied observations made from several telescopes over more than a decade. From these data, they calculated that the pulsars are approaching each other by 7.152 millimeters (0.28 inch) a day, give or take a micrometer (0.00004 inch). And that’s exactly what general relativity would predict.

Though no instrument on Earth has yet detected gravitational waves, Kramer’s work adds to growing evidence that such waves do exist.

Power Words   

(for more about Power Words, click here)

acceleration A change in the speed or direction of some object.

astronomy    The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.

astrophysics   An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.

Big Bang  The rapid expansion of dense matter that, according to current theory, marked the origin of the universe. It is supported by physics’ current understanding of the composition and structure of the universe.

celestial object  Any naturally formed objects of substantial size in space. Examples include comets, asteroids, planets, moons, stars and galaxies.

equation  In mathematics, the statement that two quantities are equal. In geometry, equations are often used to determine the shape of a curve or surface.

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.

gravitational waves   (also known as gravitational 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.

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. 

orbit   The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

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.

pulsar    The name for a spinning, ultra-dense neutron star. When these stars rotate, they emits short, regular pulses of radio waves or X-rays (and occasionally both at alternate intervals).

radio waves  Waves in a part of the electromagnetic spectrum; they are a type that people now use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals; it is also used in radar.

spacetime   A term made essential by Einstein’s theory of relativity, it describes a designation for some spot given in terms of its three-dimensional coordinates in space, along with a fourth coordinate corresponding to time.

star  Thebasic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

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.

universe The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).

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