Explainer: What are gravitational waves?

Energy from large, dramatic events in space create tiny waves that can ripple past Earth
Feb 11, 2016 — 10:34 am EST
gravity waves

This is an artist’s rendering of the gravity waves emanating from the movement of massive celestial bodies, such as two black holes.

Caltech/MIT/LIGO Lab

Gravitational waves are ripples in the fabric of space. Throw a rock into a pond and it will create ripples — waves in the water — that appear to stretch and squeeze back again. Similarly, accelerating masses should send gravity waves into space. These ripples would cause space to stretch and squeeze back again.

On February 11, 2016, after decades of trying to directly detect such waves, scientists announced that they appear to have found them. The waves came from another galaxy far, far away. How far? Try between 750 million and 1.86 billion light-years away! There, two black holes collided, shaking the fabric of space and time, or spacetime. Here on Earth, two giant detectors in different parts of the United States quivered as gravity waves washed over them.

In his theory of general relativityAlbert Einstein predicted that ripples in spacetime should radiate energy away from enormously violent events, such as colliding stars. Such events are powerful. Still, the ripples they trigger are subtle. By the time they reach Earth, some compress spacetime by as little as the width of a proton. (A proton is one of the particles that makes up an atom.)

DO THE WAVE After decades trying to directly detect the waves, the recently upgraded Laser Interferometer Gravitational-Wave Observatory, now known as Advanced LIGO, appears to have succeeded, ushering in a new era of astronomy. Science News

The newfound waves were picked up by the recently upgraded Laser Interferometer Gravitational-wave Observatory. It is now known as "advanced" LIGO. To spot a signal, LIGO uses a special mirror to split a beam of laser light. The mirror sends each beam down one of two 4-kilometer-long tubes. These tubes sit at a 90-degree angle to each other. Light ricochets back and forth 400 times down each tunnel in the detector. This turns each beam’s journey into a 1,600 kilometer (990 mile) roundtrip. Then the light recombines near its source.

The experiment was designed so that under normal conditions the light waves will cancel one another out when they recombine. When that happens, no signal moves on to a nearby detector.

But a gravity wave will stretch one tube while squeezing the other. That alters the distance the two beams travel relative to one another. Make no mistake: The difference is tiny. But it's enough that when the beams recombine, their waves no longer align perfectly align. Because they no longer cancel each other out, the detector  will pick up a faint glow. This signals a passing gravity wave.

To ensure the signal is not triggered by some local phenomenon (and to help scientists triangulate its source), LIGO has two detectors. One is in Louisiana and another is in Washington State. Any signal appearing at only one detector — meaning it’s local — will be ignored.

Scientists initially found gravity waves coming from the collision of two black holes. But those are not the only sources they think they will be able to detect. By working with computer simulations, also known as computer models, scientists can figure out what type of signals to expect from other sources.

A neutron star is the core left behind after a massive star explodes. A spinning neutron star should whip up spacetime at frequencies similar to those produced by colliding black holes.

Powerful explosions known as supernovas are triggered when a massive star dies. They can shake up space and blast the cosmos with a burst of high-frequency gravity waves.

Pairs of gargantuan black holes, each more than 1 million times as massive as the sun — and larger than the ones that Advanced LIGO detected — radiate long, undulating waves. Advanced LIGO can’t detect waves at this frequency. But scientists might spot them by looking for subtle variations in the steady beats of pulsars. Pulsars are spinning, ultra-dense neutron stars.

The Big Bang might have triggered universe-sized gravitational waves 13.8 billion years ago. These waves would have left an imprint on the first light released into the cosmos 380,000 years later. Scientists now are looking for these waves today in the cosmic microwave background. That’s the radiation left behind from the Big Bang.

Power Words

(for more about Power Words, click here)

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

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

black hole     A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.

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.

compression   Pressing on one or more sides of something in order to reduce its volume.

computer model   A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

cosmic    An adjective that refers to the cosmos — the universe and everything within it.

cosmic microwave backgroundradiation     The heat left over from the Big Bang and that should exist throughout the universe. It is estimated to be about 2.725 degrees above absolute zero.

cosmos    (adj. cosmic) A term that refers to the universe and everything within it.

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.

galaxy     (adj. galactic) A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

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.

laser   A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.

light-year     The distance light travels in one year, about 9.48 trillion kilometers (almost 6  trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.

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.

neutron star  The very dense corpse of what had once been a star with a mass four to eight times that of our sun. As the star died in a supernova explosion, its outer layers shot out into space. Its core then collapsed under its intense gravity, causing protons and electrons in its atoms to fuse into neutrons (hence the star’s name). Astronomers believe neutron stars form when large stars undergo a supernova but aren’t big massive enough to form a black hole. A single teaspoonful of a neutron star, on Earth, would weigh a billion tons.

particle   A minute amount of something.

phenomenon   Something that is surprising or unusual.

proton   A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.

pulsar   The name for a spinning, ultra-dense neutron star. A single teaspoonful, on Earth, would weigh a billion tons. It represents the end of life for stars that had started out four to eight times the mass of our sun. As the star died in a supernova explosion, its outer layers shot out into space. Its core then collapsed under its intense gravity, causing protons and electrons in the atoms that had made it up to fuse into neutrons (hence the star’s name). When these stars rotate, they emits short, regular pulses of radio waves or X-rays (and occasionally both at alternate intervals).

radiation   (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

simulate   To deceive in some way by imitating the form or function of something. A simulated dietary fat, for instance, may deceive the mouth that it has tasted a real fat because it has the same feel on the tongue — without having any calories. A simulated sense of touch may fool the brain into thinking a finger has touched something even though a hand may no longer exists and has been replaced by a synthetic limb. (in computing) To try and imitate the conditions, functions or appearance of something. Computer programs that do this are referred to as simulations.

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    The basic 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.

subtle   Some feature that may be important, but can be hard to see or describe. For instance, the first cellular changes that signal the start of a cancer may be visible but subtle — small and hard to distinguish from nearby healthy tissues.

supernova     (plural: supernovae or supernovas) A massive star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass.

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.

triangulate   To figure out where something is by analyzing the timing of signals arriving at different receivers.

undulate   To rise and fall in a predictable, wavelike way. This pattern can refer to motion, sound or shapes. Ocean waves are one example of undulations. So is the wavelike motion of a snake.

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).

wave    A disturbance or variation that travels through space and matter in a regular, oscillating fashion.


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B. P. Abbott et al. Observation of gravitational waves from a binary black hole merger. Physical Review Letters. Published online February 11, 2016. doi: 10.1103/PhysRevLett.116.061102. 

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