Gravitational waves detected yet again | Science News for Students

Gravitational waves detected yet again

These spacetime vibrations were kicked up by a merger of black holes 3 billion light-years away
Jun 14, 2017 — 7:10 am EST
gravity waves

A pair of black holes, illustrated above, merge to become one in a powerful collision some 3 billion light-years from Earth. That smashup churned up newly detected ripples in spacetime.

Aurore Simonnet/Sonoma State, MIT, Caltech, LIGO

Long ago, some 3 billion light-years from Earth, two black holes orbited one another. In time, their gravitational tugs had them spiraling inward toward one another. Eventually, they embraced in a cataclysmic merger. The event fused them into one jumbo black hole. Its mass was about 49 times that of our sun. Moving across the universe at the speed of light, the wiggles this smashup imparted to spacetime finally reached Earth this past January 4. And a pair of incredibly sensitive twin instruments detected the teensy stretch and squeeze of space and time that they caused.

The pair of detectors are known as LIGO, for the Advanced Laser Interferometer Gravitational-Wave Observatory. Scientists described this gravity-wave discovery by LIGO on June 1 in Physical Review Letters. At a news conference one day earlier, the scientists heralded this, the third confirmation of gravity waves’ existence.

Gravity waves “are the most powerful astronomical events witnessed by human beings,” argues Michael Landry. He heads the LIGO observatory in Hanford, Wash.

In announcing this latest spotting of the elusive waves, Landry noted that the black-hole merger that spawned them had converted about two suns’ worth of mass into energy. That energy then radiated out across the cosmos in all directions as gravity waves. 

A black hole likely starts as a collapsed star. Its mass all condenses into a space so densely stuffed with matter that its gravitational field also becomes enormous. Indeed, the gravity associated with a black hole is so strong that nothing — even light — can escape it. So a black hole isn't actually a hole. It just appears to be empty space because no light can illuminate it.

gravity waves
Based on when the gravity waves reached each of LIGO’s two detectors, scientists were able to determine what part of the sky they arrived from. LIGO’s three detections are shown (plus a fourth possible detection that was not strong enough to confirm). Outermost curves indicate 90 percent probability that this is where the signals came from; inner curves mark where the probability falls to 10 percent.
Leo Singer/LIGO, Caltech, MIT; Axel Mellinger (Milky Way image)

LIGO’s two detectors, in Hanford and Livingston, La., each consist of a pair of 4-kilometer-long arms. They act as outrageously oversized rulers to measure the stretching of spacetime caused when gravity waves ripple by.

According to Einstein’s theory of gravity — the general theory of relativity — massive objects will bend the fabric of space. This will create ripples when they accelerate. That can happen, for instance, when two objects orbit one another.

These ripples are tiny: LIGO is tuned to detect waves that stretch and squeeze the arms by a thousandth of the diameter of a proton. Black hole collisions are one of the few events in the universe that are catastrophic enough to produce spacetime ripples big enough for people to detect from a great distance.

The black holes that spawned the latest waves were particularly hefty. One had a mass about 31 times that of our sun. The other had about 19 times the sun’s mass. LIGO’s first detection of gravity waves, announced 16 months ago, came from an even bigger duo: colliding black holes 36 and 29 times the mass of the sun.  LIGO’s second detection featured two smaller black holes, 14 and eight times the mass of the sun.

Ligo sightings

LIGO’s three sightings of gravity waves, thus far, have all been spawned by merging black holes. But those mergers varied in mass, distance and the energy released as gravitational waves.

First detection
Date:September 14, 2015
Mass of first black hole:36.2 solar masses
Mass of second black hole:29.1 solar masses
Merged mass:62.3 solar masses
Energy radiated as gravitational waves:3 solar masses
Distance from Earth:1.4 billion light-years
Second detection
Date:December 26, 2015
Mass of first black hole:14.2 solar masses
Mass of second black hole:7.5 solar masses
Merged mass:20.8 solar masses
Energy radiated as gravitational waves:1 solar mass
Distance from Earth:1.4 billion light-years
Third detection
Date:January 4, 2017
Mass of first black hole:31.2 solar masses
Mass of second black hole:19.4 solar masses
Merged mass:48.7 solar masses
Energy radiated as gravitational waves:2 solar masses
Distance from Earth:2.9 billion light-years

Weighty black holes are difficult to explain. The reason: The stars that collapsed to form them must have been even more massive. Typically, stellar winds steadily blow away mass as a star ages. This can lead to a small black hole. But under certain conditions, those winds might be weak. For example, the stars might contain few elements heavier than helium or have intense magnetic fields. The large masses of LIGO’s black holes suggest that they formed in such environments.

Clearly, astrophysicists don’t fully understand how such big black holes form. But now, “it seems that these are not so uncommon,” says physicist Clifford Will. “So clearly there’s a way to produce these massive black holes.” Will works at the University of Florida in Gainesville.

Scientists also disagree about how black holes partner up. One theory is that two neighboring stars each explode and produce two black holes, which then spiral inward. Another is that black holes find one another within a dense cluster of stars, as massive black holes sink to the center of the clump.

The new detection of gravity waves provides some support for the star-cluster theory. The wave pattern that LIGO observed hints that one of the black holes might be spinning in the opposite direction of its orbit. Like a cosmic dance, each black hole in a pair twirls on its own axis as it spirals inward. Black holes that pair up as stars are likely to have their spins aligned with their orbits. But if the black holes instead find one another in the chaos of a star cluster, they could spin any which way. That’s what LIGO appears to have seen. But that interpretation is “not definite,” says Avi Loeb. An astrophysicist, he works at Harvard University in Cambridge, Mass.

Scientists will need more data to sort out how the black hole pairs form, agrees physicist Emanuele Berti. He works at the University of Mississippi in Oxford. It’s likely, he says, that various processes contribute to these pairings.

Having seen evidence of three black hole mergers, scientists are looking forward to a future in which gravity-wave detections become routine. The more waves detected, the better scientists can test theories of what creates them and how. “There are already surprises that make people stop and revisit some old ideas,” Will says. “To me that’s very exciting.”

NOTE: This story has been corrected for a transciption error in the "Third detection" table in the original version. The solar masses for merged mass and energy radiated as gravity waves had been inadvertantly swapped.

Power Words

(more about Power Words)

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.

axis     The line about which something rotates. On a wheel, the axis would go straight through the center and stick out on either side.

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

condense     To become thicker and more dense.

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

diameter     The length of a straight line that runs through the center of a circle or spherical object, starting at the edge on one side and ending at the edge on the far side.

element     (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

environment     The sum of all of the things that exist around some organism, event or system.

field     (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).

gravity     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.

helium     An inert gas that is the lightest member of the noble gas series. Helium can become a solid at -272 degrees Celsius (-458 degrees Fahrenheit).

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.

Laser Interferometer Gravitational wave Observatory    (or LIGO) 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.

matter     Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."

observatory     (in astronomy) The building or structure (such as a satellite) that houses one or more telescopes.

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

physical     (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).

physicist     A scientist who studies the nature and properties of matter and energy.

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.

relativity     (in physics) A theory developed by physicist Albert Einstein showing that neither space nor time are constant, but instead affected by one’s velocity and the mass of things in your vicinity.

speed of light     A constant often used in physics, corresponding to 1.08 billion kilometers (671 million miles) per hour.

star     (adj. stellar) 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.

sun     The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

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).

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


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The LIGO Scientific Collaboration and the Virgo Collaboration. GW170104: Observation of a 50-solar-mass binary black hole coalescence at redshift 0.2. Physical Review Letters. Vol. 118, June 1, 2017, p. 221101. doi: 10.1103/PhysRevLett.118.221101.