# Physicists Demonstrate What Occurs When Black Holes Collide In Simple Equation

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Being the result of black holes colliding and merging, gravitational waves are made, well, waves when they were discovered.

Scientists typically require the use of supercomputers to study wave patterns and create simulations, but two from the University of Barcelona have come up with a simpler approach to demonstrate two black holes merging — granting one of the black holes is drastically smaller than the other.

In a study published in the journal Classical and Quantum Gravity, Roberto Emparan and Marina Martinez explained the equation they arrived at using knowledge every physicist has, like the equivalence principle and what defines an event horizon.

"Surprisingly, the ideas and techniques used in our work are elemental and allow us to thoroughly study the properties of the horizon at the moment both black holes join together to form a new one," said Emparan.

An event horizon is characterized by the boundary a black hole has, where events within cannot affect those on the outside but an outsider can affect what transpires within. When black holes merge, their event horizons become one.

The theoretical physicists likened the smaller black hole to an astronaut on the International Space Station. The astronaut is observed to be floating in space but this is not due to a lack of gravity. In fact, the ISS has about 90 percent of the Earth's surface gravity. Rather, the noticeable weightlessness is caused by the space station freely moving with the gravitational field of Earth.

According to Albert Einstein's theory of gravity, an observer or an object can't tell the difference between floating in deep space and falling in a gravitational field.

In the case of the two black holes, the smaller black hole is unable to tell apart situations where it is floating on its own in space or falling into the larger black hole. This is what allowed Emparan and Martinez to simplify the equation for merging black holes.

To describe the event horizon resulting from the two black holes, the physicists used geometric elements, plotting out null geodesic lines on the Schwarzschild metric, which is the solution to Einstein's field equations for black hole gravitational fields.

Using their equation then, Emparan and Martinez have made it possible to more easily identify geometric properties present in an event horizon as two black holes merge. More importantly, they noted that the results of their work point to a universal behavior that occurs when two black holes come together, regardless of wherever they are in different parts of the universe.

Recently, two studies published in The Astrophysical Journal and The Astrophysical Journal Letters also shed light on black hole behavior, determining that they leave light echoes or flares after eating stars that get too close.

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