Black Holes: The Most Mysterious Objects in the Universe?

Often black holes are portrayed as totally mysterious objects. Black holes are certainly bizarre and astonishing. Even Einstein doubted whether such outlandish objects could really exist. But in this video I argue that rather than being totally mysterious, black holes are actually the most well-understood objects in the universe. The video ends with a wonderful simulation from LIGO of the gravitational waves generated by a binary black hole system leading up to a black hole merger event.

00:00 Introduction
00:32 A Journey Through the Solar System
01:48 The Kerr Solution
03:40 Black Holes Have No Hair!
06:10 LIGO, Black Hole Mergers and Gravitational Waves
07:31 Back to Earth

In 1963 Roy Kerr physicist found a remarkable solution to Einstein’s equation that describes the shape of spacetime outside a spinning spherical body. Its most important application is to describe the spacetime in and around a black hole. What is truly incredible is that, according to general relativity, the Kerr solution describes a black hole exactly. The great Indian astrophysicist Chandrasekhar found this overwhelming. He recorded the impact it had on him:

‘In my entire scientific life, extending over 45 years, the most shattering experience has been the realization that an exact solution of Einstein’s equations of general relativity, discovered by the New Zealand mathematician Roy Kerr, provides the absolute exact representation of untold numbers of massive black holes that populate the universe.’

NASA’s orbiting Chandra X-ray telescope was named after Chandrasekhar. The Chandra Deep Field South image was produced by pointing the X-ray telescope at a small patch of sky continuously for six weeks. In this small patch, Chandra gathered data from the blazing-hot accretion discs of 5000 supermassive black holes in distant galaxies.

All these black holes are described exactly by the Kerr solution. Remarkably, the Kerr solution depends on just two parameters – the black hole’s mass and the rate at which its spins. So given these two quantities we know everything there is to know about a black hole. John Archibald Wheeler summed this up with the statement:

Black holes have no hair.

Furthermore, the structure of a black hole does not evolve. It only changes due to outside influences, such as when material falls into the black hole increasing its mass and altering its rate of spin. So the Kerr solution describes a black hole for all time. This means that a black hole shows no scars of its earlier history. We cannot tell by looking at a black hole whether it formed from a collapsing star, from the collision of two neutron stars or some other esoteric process.

Can we be sure that the Kerr solution accurately describes the real world. Plenty of simple mathematical results are used to model physical phenomena without representing them exactly. So how do we know that black holes really are described by the Kerr solution? In 2015, the LIGO gravitational wave observatories in the United States detected the first gravitational wave signals. Hundreds of such signals have now been received.

These signals allow us to test general relativity in ultra-strong gravitational fields. Indeed, computer models based on general relativity are required to interpret this data. The good new is that general relativity has passed every test and it has enabled physicists to extract a great deal of information from these signals – such as the masses of the colliding black holes and the distance to the merger event. So it seems that general relativity is accurate even in the most extreme gravitational scenarios, which means that the Kerr solution really does give a precise picture of a black hole.

Physicists have generated a library of computer simulations of the gravitational waves produced in a range of astrophysical scenarios. When LIGO receives a signal it is matched to a simulation in the library and this reveals the data about the objects that have produced the gravitational wave signal.

Many thanks to John Eastwood.

The page of text showing the Kerr metric is from The Physical World by Nicholas Manton and Nicholas Mee (OUP, 2017):

The LIGO gravitational wave and black hole merger simulation was created by the Simulating eXtreme Spacetimes (SXS) Project:


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