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Collisions felt across the universe

We have all heard the apocryphal story of Newton’s discovery of gravity. He really wasn’t sitting under an apple tree but there was one visible from his window. And he really didn’t discover gravity.
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We have all heard the apocryphal story of Newton’s discovery of gravity. He really wasn’t sitting under an apple tree but there was one visible from his window. And he really didn’t discover gravity. Instead, he worked out the law of gravity, which explained the relationship between the mass of two objects, their distance apart and the force of attraction.

The law of gravity tells us how gravity works but not why. We still don’t have a good understanding of why but the best explanation we have is provided by Einstein’s work. In essence, gravity is a consequence of the curvature of spacetime.

An often used analogy is a sheet of rubber with a heavy object placed in the middle. The object stretches the rubber to make a parabolic indentation and if you introduce, say, a marble it will roll down the surface of the rubber sheet until it meets the heavy object. From a two dimensional perspective, it is not a bad analogy. It gets a bit stretched when you apply both a third dimension and time but in essence gravity is a consequence of the underlying curvature.

One of the consequences of Einstein’s equations is the existence of gravity waves – ripples in spacetime. In our stretch rubber sheet analogy, hitting the sheet produces a rippling effect which propagates outward. If you push on the heavy object and let go, it will oscillate up and down resulting in oscillations in the surrounding rubber. Or maybe a better image is the rippling effect which surrounds Neo when he gains control of the Matrix. Gravity waves are a ripple in both space and time – for a brief moment neither time nor space are smooth.

Some physicists became convinced that with the right apparatus and a lot of careful measurements, they might be able to see gravity waves. Not from ordinary objects like you and me as the waves we produce are very tiny (an ant walking across our rubber sheet wouldn’t really be noticed) but from collisions of gravitationally massive objects such as black holes and neutron stars.

When LIGO or the Advanced Laser Interferometer Gravitational-Wave Observatory was turned on in 2015, scientists were able to detect gravity waves for the first time. With LIGO, we now have a brand new way of looking at the universe and observing phenomena we couldn’t detect by any other means.

When the Italian equivalent of LIGO (Advanced Virgo) started up, even better measurements could be made and when a new Japanese version comes online in the next year, measuring gravity waves will become even more refined.

Over the past five years, LIGO and Advanced Virgo have detected dozens of collisions between super-massive objects. The signal they received on May 21, 2019, however, was different. Not only was it the most powerful and distant ever seen but the black holes involved should not have existed according to our present understanding. The mass of each black hole falls into a mass gap in which theorists believe it is impossible to make black holes by the standard route of collapsing a star.

The stellar class of black holes arise when a large star (bigger than our sun) runs out of nuclear fuel. It stops exploding outwards and gravity forces the mass inwards. The bigger the star, the more massive the catastrophic event. But for truly massive stars (more than 65 solar masses) their cores have so much energy they start converting photons into particle/antiparticle pairs through a process called pair instability. This triggers an explosion of oxygen nuclei which totally destroys the star. The stars disappear rather than creating a black hole.

This means theoretically there should be a cutoff of about 65 solar masses for the size of a first generation black hole. Some theorists even argue the cutoff should be only 45 solar masses. The event detected on May 21 involved two black holes of 66 and 85 solar masses and at least one of the black holes is beyond the expected theoretical limit. Further, the black hole created by the merger is 151 solar masses, which is unique. No other black hole has been detected in this range.

We do know massive black holes with 1,000 to 1,000,000 solar masses, like the one imaged at the heart of M87, exist but how these massive holes form has been something of enigma. In detecting a collision which happened 7 billion years ago to generate this 151 black hole, astronomers may have uncovered a vital clue. The monsters at the hearts of galaxy may have formed from multiple collisions of smaller black holes and slowly accumulated over time. This collision provides the first indirect evidence such a path is possible.

A long time ago in a galaxy far, far away, two black holes danced towards each other and we are just now seeing the results of that collision.