Physics Insight

The Collapse That Couldn't Be Stopped

By: Lucy O'Shea - Researcher

There are objects in space that we can never directly observe. We might reach them, but we’d never come back. And alarmingly, our entire solar system is being pulled towards one at a speed of over 200 kilometres per second. 

The object is Sagittarius A*, just one of any number of supermassive black holes that populate our universe. But because of their absurd nature, physicists fought over their existence for nearly half a century. When a star dies, it’s violent. Scientists had known this for decades. But it wasn’t until they realised the implications of general relativity when they knew just how violent their death could be. 

Einstein’s theory of general relativity kicked off the study of modern black holes. By unifying previously unrelated fields of physics concerned with gravity, light, and the fabric of the universe, he led physicists to the realisation that the largest stars in our universe are destined to collapse behind an event horizon, creating an object so dense that not even light can escape. However, this prediction concerned physicists at the time. 

Black holes challenge our entire understanding of reality. At the heart of their study is general relativity, which tells us the speed of light is constant. But for this to be the case, time has to be subjective. For an observer outside a black hole, they can never see an object cross the event horizon - only get closer and closer until it seems to freeze on the edge of the black hole. For the person unfortunate enough to fall in, they might have a few hours before they reach the end of time itself. 

Black holes also break the most fundamental rule of the universe. When a black hole absorbs something, it swallows information about that object forever. This information can never be recovered. The only thing we can see is Hawking radiation, which can only tell us a tiny amount about what went in. Information - like energy - should not be destroyed. This is true everywhere except on the edge of a supermassive black hole. 

This is why many prominent physicists rejected black holes - they seemed impossible. Einstein himself said “Black holes are where God divided by zero”, and claimed that they did “not exist in the real world”. 

Karl Schwarzchild, a German astrophysicist, agreed with him, and created an equation that could determine the event horizon - the point of no return - for a black hole of any mass. If you compressed the earth enough, it would form a black hole when it got to a radius of under a centimetre.. For a star, it was about 3 kilometres. Rather than taking this as theoretical proof, Schwarzchild described this equation to be the maximum radius an object could be compressed to. Anything lower than that was supposedly impossible, though it wasn’t exactly clear why. 

His apparent answer came 9 years later when the Royal Astronomical Society published R. H. Fowler’s paper, On Dense Matter.

This article outlines how closely packed atoms can avoid gravitational collapse because of Pauli’s Exclusion Principle. This law tells us how there’s a finite number of energy levels that fermions - a family of particles including electrons - can have. Electrons always fill the lowest energy level available. But when they’re tightly compressed, all the lower levels get filled up and they’re forced to reach increasingly high levels of energy, vibrating so fast they can support a star. This is why if you touch something, your hand doesn’t go straight through - the electrons create a strong pressure that stops them from occupying the same space. This is called electron degeneracy pressure. Luckily for Fowler, this works off the energy used to compress the electrons, so the process can continue forever. A star could never collapse. Or so he thought. 

In 1930, Subrahmanmyan Chandrasekhar calculated the limits of electron degeneracy pressure. Fowler believed that they would create the same amount of energy as the energy used to compress them. However, higher energy electrons move faster. A lot faster. Chandrasekhar reasoned that electrons could not move higher than the speed of light. The Chandrasekhar limit is the maximum mass of an object that can be supported by electron degeneracy pressure - around 1.44 solar masses. All white dwarves are below that. Once you go higher, they turn into something else. 

It was two years later when Fritz Zwicky and Lev Landau proved that this “something else” wasn’t a black hole. At incredibly high pressures, a process called electron capture can happen, where a proton and electron can turn into a neutron and an ultralight particle called a neutrino. In much the same way as electrons, the resulting neutrons can create neutron degeneracy pressure. As they were heavier, they would be able to stop a larger mass from gravitational collapse. These “neutron stars” would be incredibly stable and live long lives. These were more than just theoretical - the first was discovered by Jocelyn Bell in 1967. 

After Chandrasekhar’s discovery, scientists were sceptical about the limits of neutron degeneracy pressure. However, nobody fully understood the behaviour of large particles such as neutrons at the time, especially as nuclear physics was still a young field. Many physicists theorised that neutron stars would eject matter as they contracted, fizzling out before they could fully collapse. We still believed that nature would prevent the creation of a black hole. 

This ended when J Robert Oppenheimer and his student, Hartland Snyder, published their paper On Continued Gravitational Contraction, where they proved that the largest stars were destined to collapse to a singularity. Building on their previous work establishing the Tolman-Oppenheimer-Volkoff Limit, they showed that the maximum mass a neutron star could be was around 3 times the mass of the sun, largely ending the black hole debate. 

Through this often heated dispute, the world discovered some of the best minds in science. Einstein became famous. Schwarzchild helped us navigate a relativistic universe. Chandrasekhar won the Nobel Prize. Landau, Zwicky and Fowler were pioneers in the field of degenerate matter. And Oppenheimer made the atomic bomb. 

Despite all of this research, there’s still so much that we don’t know. With discoveries relating to quantum mechanics, relativity and nuclear physics, black holes have helped to shape our understanding of the universe and revealed how all of these fields are connected. Nearly all of what we know about the life cycle of a star came out of this debate. With new and exciting discoveries, such as the detection of gravitational waves in 2015, black holes are here to stay, and will continue to push our understanding of the universe.