In February of 2016, scientists made the historic announcement that they had observed the universe in an entirely new way with the first ever detection of gravitational waves—ripples in space produced by the catastrophic collision of two merging black holes.
What is LIGO?
Since then, the same telescope known as LIGO (short for Laser Interferometer Gravitational wave Observatory) has detected three additional black hole merger events which have confirmed major predictions from Einstein’s theory of general relativity. This detection was so important to our understanding of the universe that LIGO scientists Barry Barish, Kip Thorne, and Rainer Weiss earned the 2017 Nobel Prize in Physics earlier this month.
This week the scientists at LIGO, joined by astronomers from 70 other telescopes all around the world, are back again with big news. They have detected for the first time gravitational waves produced by the final collision of two neutron stars and observed the event across the electromagnetic spectrum kicking off the era of not just multi-wavelength but multi-messenger astronomy.
This news is so big that astronomers have been rumbling about it for months ever since the detection was made in August. It turns out, astronomers are terrible at keeping secrets. A few scientists leaked the information on Twitter, but, even more obvious was the fact that suddenly almost all of the world’s largest and most technologically advanced telescopes stared simultaneously at an otherwise normal-seeming galaxy about 130 million lightyears away. Many large observatories like Hubble, Green Bank, and Fermi post publicly where they are currently observing. There was so much off-the-record information flying around that the journal Nature published an article summarizing the rumors all the way back in August.
Here are six reasons astronomers are so excited about this new discovery and why you should be too.
1. The merger of two neutron stars has never before been observed.
On August 17th, 2017, the LIGO telescope detected a clear signal but one that was significantly longer—on the order of a minute—than the signals from merging black holes that last only a few seconds. LIGO scientists are constantly scanning their incoming signals and looking for matches in a library of hundreds of thousands of templates. In other words, the match-making software looks to see whether the detected signal matches what simulations predict we should observe for any of a variety of different kinds of merger events. Unlike all of LIGO’s previous detections which have all been signals from distant mergers of black holes, the August 17th signal the first ever detection of the final collision between two neutron stars with masses of 1.1 and 1.6 times the mass of our Sun at a distance of 130 million lightyears away off in the southern hemisphere.
Neutron stars are incredibly dense, dead stars produced by the supernova explosions that occur during stellar death. They are made up purely of neutrons and are three times denser than atomic nuclei. Neutron stars typically pack between one to two times the mass of our Sun into a space that’s 20-25 kilometers in diameter, or only about the length of Manhattan.
Models suggest the first neutron star was living a relatively quiet existence until the second neutron star arrived to form a binary system which then sent the pair careening throughout their host galaxy. As neutron stars orbit each other, general relativity predicts that the system will lose energy over time by radiating gravitational waves. This signal is exactly what LIGO detected.
The inevitable collision is then among the most violent and powerful events in the universe. But more on that later…
2. The event almost came too soon to find the host galaxy.
Even with its two detectors (in Louisiana and Washington in the US), LIGO can only pinpoint the source of one of its detections to within an area of 1,000 square degrees on the sky—a region far too large to be able to effectively narrow down the source of the signal.
However, just in the nick of time, LIGO’s sister observatory Virgo located in Cascina, Italy had turned back on earlier that month after a long break for an upgrade. When working together, LIGO and Virgo can localize mutual detections to within 30 square degrees.
Scientists were able to narrow down the source of the detection even further when they noticed a Fermi alert had been triggered at almost the exact same time as the LIGO and Virgo detections. A realization that led to the “Wake up!” message that then triggered a period of swift action and intense discovery. Julie McEnery, the Fermi Project Scientist called that first morning “the most exciting morning of the 9-year Fermi mission.”
The Fermi Gamma-Ray Space Telescope scans the sky for gamma rays, the highest energy form of light and triggers an alert to let scientists know when it has found something. INTEGRAL, the European gamma ray observatory, later also confirmed a detection of what is known as a short duration Gamma Ray Burst. Using the LIGO, Virgo, and Fermi observations together, scientists were able to narrow down the source of the neutron star collision to roughly 50 potential host galaxies.
3. Gravitational waves and electromagnetic waves were detected for the first time from the same object.
The detection of a gamma ray burst that coincided with the neutron star merger event that produced the gravitational waves did more than help localize the detection. They marked the first time electromagnetic radiation (the kind of light we are used to talking about from X-rays and ultra-violet to optical and infrared wavelengths) and gravitational waves could both be used to observe an astronomical object. Thus describing astronomical pursuits as “multi-wavelength” is no longer sufficient and we have now entered the era of "multi-messenger” astronomy.
But astronomers did not stop at gamma rays. Communities across the globe worked fast to attempt to detect the quickly fading afterglow from this catastrophic event, an effort that Marcelle Soares-Santos from the Fermi National Accelerator Laboratory and Brandeis University described as trying to find a needle in a haystack except that your needle is fading away and the haystack is moving.
The Swope Supernova Search team was up to the challenge, and within 12 hours of the LIGO detection, they noticed that there was a bright optical emission coming from one of the galaxies in the area of interest identified by LIGO, Virgo, and Fermi that hadn’t been there before. The kilonova event they found in the galaxy called NGC 4993 was the first optical look at the explosion inspired by two merging neutron stars. Within an hour, there were six independent discoveries of the flaring optical counterpart to the gravitational wave event.
Over the next few days and weeks, 70 telescopes across the world, including seven space-based observatories and a telescope on every continent (even Antarctica!) were able to detect the source at optical, X-ray, infrared, and radio wavelengths.
David Reitze, the Executive Director for LIGO, described the multiple detections as moving “from the era of silent movies to the era of talking movies.” Laura Cardonati, Deputy Spokesperson for LIGO described the addition of the multi-wavelength and now multi-messenger observations as going from “looking at a black and white picture of a volcano to sitting in a 3D movie of an eruption of Mount Vesuvius.” Vicky Kalogera, an astrophysicist with the LIGO collaboration summarized that for the first time “we hear the death spiral of two neutron stars and see the fireworks that came from the final merger.”
For the first time “we hear the death spiral of two neutron stars and see the fireworks that came from the final merger.”
4. The gravitational wave discovery helps us further test Einstein’s theories.
Einstein not only predicted the existence of gravitational waves but also that they should travel at the same speed as electromagnetic radiation; that is, at the speed of light. The gamma ray burst observed by Fermi and INTEGRAL was detected less than two seconds after the gravitational wave detection from LIGO and Virgo. Since both types of radiation took similar lengths of time to reach our telescopes, this coincidence offers strong evidence that they are in fact traveling at the same speed. The longer signals coming from merging neutron stars relative to those detections of black hole mergers, may also provide more precise tests of general relativity.
5. The neutron star merger solves a long-standing mystery in the creation of heavy metals.
While all elements may appear equal on the periodic table, those elements heavier than iron actually harbor a secret past. The formation of light elements like oxygen or carbon from even lighter elements produces energy, but forming heavier elements like gold, platinum, or uranium instead requires energy. Since processes that require significant amounts of energy can’t happen in the universe without help, a long standing question has been how those elements form.
Astronomers have predicted that powerful collisions like in supernovae explosions or in the mergers of neutron stars could smash atoms together hard enough to produce these heavier elements. However, direct signatures of the production of heavy elements had never been seen in any astronomical event until the LIGO/Virgo neutron star merger. In fact, Edo Berger, a Professor of Astronomy at Harvard, noted that the mass of all of the heavy elements produced in this event totals roughly 16,000 times the mass of the Earth, including ten times the mass of the Earth in gold and platinum alone. That’s a lot of jewelry.
6. If the neutron star merger had happened nine days later, we could have missed it.
The two neutron stars in NGC 4993 spent 11 billion years in their decaying orbit before finally merging and creating a thrilling journey of discovery for the astronomical community on a distant planet called Earth. The detection was made on August 17th and just nine days later LIGO began a year-long shut down for an upgrade. After 11 billion years of orbiting each other, if those neutron stars had taken another nine days to finally coalesce, LIGO would have missed it entirely.
As is with most exciting new scientific discoveries, the LIGO news raises even more questions. What is left behind after a merging of two neutron stars? Is it a black hole? Another neutron star? How many more of these events can we hope to detect?
As France Cordova, the Director of the National Science Foundation, noted, this multinational collaboration which has been decades in the making represents the best of our “courage to push beyond the limits of our knowledge.” Reitze noted that the National Science Foundation “swang for the fences scientifically [when it funded LIGO] back in the ’80s and now we’ve hit a home run.”
Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.
Image courtesy of ligo.caltech.edu and NASA/Swift/Dana Berry
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