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LIGO detects gravitational waves for third time

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Staff Writer |
LIGO
Space science   More observations with LIGO are needed

The Laser Interferometer Gravitational-wave Observatory (LIGO) has made a third detection of gravitational waves, ripples in space and time, demonstrating that a new window in astronomy has been firmly opened.

As was the case with the first two detections, the waves were generated when two black holes collided to form a larger black hole.

The newfound black hole, formed by the merger, has a mass about 49 times that of our sun. This fills in a gap between the masses of the two merged black holes detected previously by LIGO, with solar masses of 62 (first detection) and 21 (second detection).

"We have further confirmation of the existence of stellar-mass black holes that are larger than 20 solar masses - these are objects we didn't know existed before LIGO detected them," says MIT's David Shoemaker, the newly elected spokesperson for the LIGO Scientific Collaboration (LSC), a body of more than 1,000 international scientists who perform LIGO research together with the European-based Virgo Collaboration.

"It is remarkable that humans can put together a story, and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us. The entire LIGO and Virgo scientific collaborations worked to put all these pieces together."

The new detection occurred during LIGO's current observing run, which began November 30, 2016, and will continue through the summer. LIGO is an international collaboration with members around the globe. Its observations are carried out by twin detectors - one in Hanford, Washington, and the other in Livingston, Louisiana - operated by Caltech and MIT with funding from the National Science Foundation (NSF).

LIGO made the first-ever direct observation of gravitational waves in September 2015 during its first observing run since undergoing major upgrades in a program called Advanced LIGO.

The second detection was made in December 2015. The third detection, called GW170104 and made on January 4, 2017, is described in a new paper accepted for publication in the journal Physical Review Letters.

In all three cases, each of the twin detectors of LIGO detected gravitational waves from the tremendously energetic mergers of black hole pairs.

These are collisions that produce more power than is radiated as light by all the stars and galaxies in the universe at any given time.

The recent detection appears to be the farthest yet, with the black holes located about 3 billion light-years away. The black holes in the first and second detections are located 1.3 and 1.4 billion light-years away, respectively.

The newest observation also provides clues about the directions in which the black holes are spinning.

As pairs of black holes spiral around each other, they also spin on their own axes - like a pair of ice skaters spinning individually while also circling around each other.

Sometimes black holes spin in the same overall orbital direction as the pair is moving - what astronomers refer to as aligned spins - and sometimes they spin in the opposite direction of the orbital motion.

What's more, black holes can also be tilted away from the orbital plane. Essentially, black holes can spin in any direction.

The new LIGO data cannot determine if the recently observed black holes were tilted but they imply that at least one of the black holes may have been non-aligned compared to the overall orbital motion.

More observations with LIGO are needed to say anything definitive about the spins of binary black holes, but these early data offer clues about how these pairs may form.


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