Physicists use supercomputers and AI to create the most accurate model yet of black hole mergers

January 24, 2019 by Whitney Clavin
Physicists use supercomputers and AI to create the most accurate model yet of black hole mergers
A simulated picture of two merging black holes, each about 30 solar masses. This is approximately what a human would see if they could travel in spaceship to take a closer look at merging black holes. Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project (

One of the most cataclysmic events to occur in the cosmos involves the collision of two black holes. Formed from the deathly collapse of massive stars, black holes are incredibly compact—a person standing near a stellar-mass black hole would feel gravity about a trillion times more strongly than they would on Earth. When two objects of this extreme density spiral together and merge, a fairly common occurrence in space, they radiate more power than all the stars in the universe.

"Imagine taking 30 suns and packing them into a region the size of Hawaii. Then take two such objects and accelerate them to half the speed of light and make them collide. This is one of the most violent events in nature," says Vijay Varma, a graduate student at Caltech.

In a new study in the January 11 issue of the journal Physical Review Letters, Varma and his colleagues report the most accurate computer model yet of the end stage of black hole mergers, a period when a new, more massive black hole has formed. The model, which was aided by supercomputers and machine-learning, or artificial intelligence (AI) tools, will ultimately help physicists perform more precise tests of Einstein's general theory of relativity.

"We can predict what's left after a black hole merger—properties of the final black hole such as its spin and mass—with an accuracy 10 to 100 times better than what was possible before," says co-author Davide Gerosa, an Einstein Postdoctoral Fellow in Theoretical Astrophysics at Caltech. "This is important because tests of general relativity depend on how well we can predict the end states of black hole mergers."

The research is related to a larger effort to study with LIGO, the Laser Interferometer Gravitational-wave Observatory, which made history in 2015 by making the first direct detection of gravitational waves emitted by a black hole merger. Since then, LIGO has detected nine additional black hole mergers. Gravitational waves are ripples in space and time, first predicted by Einstein more than 100 years ago. Gravity itself, according to general relativity, is a warping of the fabric of spacetime. When massive objects like black holes accelerate through spacetime, they generate gravitational waves.

One of the goals of LIGO and the thousands of scientists analyzing its data is to better understand the physics of black hole collisions—and to use these data, in turn, to assess whether Einstein's still holds true under these extreme conditions. A breakdown of the theory might open the door to new types of physics not yet imagined.

But creating models of colossal events like black hole collisions has proved to be a daunting task. As the colliding black holes become very close to one another, just seconds before the final merger, their gravitational fields and velocities become extreme and the math becomes far too complex for standard analytical approaches.

"When it comes to modeling these sources, one can use the pen-and-paper approach to solve Einstein's equations during the early stages of the merger when the black holes are spiraling toward each other," says Varma. "However, these schemes break down near the merger. Simulations using the equations of general relativity are the only means to predict the outcome of the merger process accurately."

That is where supercomputers help out. The team took advantage of nearly 900 black hole merger simulations previously run by the Simulating eXtreme Spacetimes (SXS) group using the Wheeler supercomputer at Caltech (supported by the Sherman Fairchild Foundation) and the Blue Waters supercomputer at the National Center for Supercomputing Applications (NCSA) at the University of Illinois at Urbana-Champaign. The simulations took 20,000 hours of computing time. The Caltech scientists' new machine-learning program, or algorithm, learned from the simulations and helped create the final model.

"Now that we have built the new model, you don't need to take months," says Varma. "The new model can give you answers about the end state of mergers in milliseconds."

The researchers say that their model will be of particular importance in a few years, as LIGO and other next-generation gravitational-wave detectors become more and more precise in their measurements. "Within the next few years or so, gravitational-wave detectors will have less noise," says Gerosa. "The current models of the final black hole properties won't be precise enough at that stage, and that's where our new can really help out."

The Physical Review Letters study is titled "High-accuracy mass, spin, and recoil predictions of generic black-hole remnants."

Explore further: Scientists detect biggest known black-hole collision

More information: Vijay Varma et al. High-Accuracy Mass, Spin, and Recoil Predictions of Generic Black-Hole Merger Remnants, Physical Review Letters (2019). DOI: 10.1103/physrevlett.122.011101 ,

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not rated yet Jan 24, 2019
I don't understand how we can see black holes merging. Light from the point at which their event horizons connect will take an infinite amount of time to reach an external observer so they will only see what was there before ie space outside either event horizon. Similarly how can we see an object falling into a BH ie crossing its event horizon. As it gets closer to the event horizon the light takes longer to reach an external observer till at the point it crosses it takes an infinite time so only what was there before, ie the gap between the object and the event horizon, is ever visible. Any answers?
3.4 / 5 (5) Jan 24, 2019
Any answers?

Gravitational waves.
1 / 5 (1) Jan 24, 2019
In search of black holes and dark matter astrophysicists are relying on indirect observations. It would seem that the measurement of the event horizon of a black hole directly would be a direct evidence. However, by the nature of a horizon, any real measurement of the event horizon will be indirect. The Event Horizon Telescope will get picture of the silhouette of the Sgr A* which is due to optical effects of spacetime outside of the event horizon. The result will be determined by the simple quality of the resulting image that does not depend on the properties of the spacetime within the image. So, it will be also indirect and an existence of BH is a hypothesis.
5 / 5 (2) Jan 25, 2019
I don't understand how we can see black holes merging.

Replace "see" with "detect". In xray or radio astronomy we also don't 'see' anything. In this case LIGO is looking for time-dependent changes in spacetime curvature (as gravity has no problem escaping a black hole).

Event horizons are also not some material thing, but just a mathematical threshold. What we can measure is the curvature of spacetime around black holes (by looking at stars behind them and also the gravitational effect of the black hole on nearby objects). The definition is sometimes a bit iffy, but a BH is the collapsed mass that is strong enough to form an event horizon. So technically the event horizon isn't the black hole even though the two notions are often used interchangeably.
(Note that there exist solutions for 'naked singularities' that do not form event horizons)

5 / 5 (1) Jan 25, 2019
I studied physics and came top of my class in GR, not that we studied anything so esoteric as Black Holes (in the 1990s). Gravitational waves can't provide any more information that EM waves. I get that in the Space-time metric a BH can coalesce but can that information be transmitted to an external observer?
1 / 5 (1) Jan 26, 2019
jd, v_p & a_p are all correct in how scientists "observe" all the fun puns of singular events at extreme astrophysics.

As a non-scientist? My answer to your question "How do we see BH, etc?"
Aside from gravitational waves, what is being observed is all the crap being dragged around by the high-gravity wells.
All that crap crashing together, smushed into spectacular spectrum of radiation & the debris expelled from the wreckage.

That in turn, is the cause for the pretty petty, weakass EM insignificant sparklees that appeal to our monkey brains.

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