It is easy to be led astray by the mystery of black holes. As a young person, listening to Stephen Hawking describe what we didn’t understand about them was what convinced me that I needed to become a theoretical physicist. They are so strange. They have this thing called an event horizon and when you cross it, the properties of space and time seem to reverse. Time might lose all meaning. Space becomes one way, in a sense, since the event horizon is a hard boundary that you can only cross once.
Black holes are unlike anything else in the known universe: points of no return embedded into the very structure of space-time. Plus, they are so massive, but also quite compact. A black hole that has the same mass as the sun would stretch only about 3 kilometres across. There are high streets that would longer than a stellar-mass black hole.
However, even discussing a black hole in terms of mass raises questions because it isn’t really a material object in the same way the sun is. Rather, it is a shape-shifted piece of space-time that has intense gravitational power, pulling on things as if it were an object made of matter.
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A black hole is a region, a place in space-time. It can possibly be produced through the presence of a high density of matter, which would be packed together so tightly that space-time folds in on itself, creating a phenomenon that isn’t actually a hole, but isn’t really a material object either.
On some level, I am speaking in terms of conjecture. No human has ever been inside a black hole. And had someone been inside, there is a decent chance they wouldn’t be back again. That said, some things seem able to emerge from one. Quantum mechanics tells us that perhaps information can leak out of black holes. This idea, known as Hawking radiation, is part of what made him a big name in physics before becoming a global celebrity.
So black holes present us with an interesting conundrum: we can draw conclusions about them using calculations, but they are actually quite hard to investigate.
Hawking radiation, for example, is a phenomenon where particle pairs that are created as a result of random fluctuations get separated by the event horizon. Usually these pairs of particles, one of matter and one of antimatter, would quickly annihilate each other, but the event horizon can act as a barrier that permanently separates them.
The problem? Hawking radiation won’t happen at rates high enough for us to detect the particles directly and distinguish them from the other busy stuff that is happening in the universe, such as hot gases orbiting the exterior of a black hole.
Those gases themselves create all kinds of interesting effects. Not only do they radiate powerfully across the electromagnetic spectrum, but we also see interesting structures like jets of particles moving close to the speed of light that seem to shoot out from either side of a black hole. These jets are themselves a mystery because we don’t fully understand what causes them to start and why only some galaxies with highly active black holes at their cores have them. They radiate strongly in radio waves and also X-rays, which gives us some indication of their inner workings, but the mysteries still abound.
For example, the galaxy M87, with a supermassive black hole at its heart, not only has jets, but the jets have a unique structure, with bright spots known as knots distributed across them. A team led by Rameshan Thimmappa, a research fellow in Joey Neilsen’s group at Villanova University in Pennsylvania, has been exploring the nature of these knots through careful observations using the Chandra X-ray Observatory. This space telescope, which is sadly being defunded by the US government, allows teams like Thimmappa’s to look at 20 years of data on the knots, providing insights that help us understand their fundamental nature.
In a recent that has been accepted by The Astrophysical Journal, his team found that variations in brightness of one of the knots corresponded to its distance from the core of the galaxy. This piece of information allowed the team to deduce exactly how fast the particles in the jet appear to be travelling, and make more informed guesses about what the internal shape of the jet might be.
Black holes, in the end, are more than the parts that we can’t see – at least to us scientists. Of course, as a theoretical physicist, I also love the mystery of black holes from a mathematical perspective. What is really happening in the spot that we call a singularity at the centre of a black hole? Our equations break down here. But part of doing physics is about connecting with the cosmos as we witness it. And it turns out that even though we can’t see black holes, they offer up a quite glorious vision.
Chanda’s week
What I’m reading
Arguing for a Better World: How philosophy can help us fight for social justice by Arianne Shahvisi. It is so smart and thoughtful.
What I’m watching
I recently saw the film Farewell My Concubine for the first time, and wow!
What I’m working on
Trying to understand quantum mechanics. Again.
Chanda Prescod-Weinstein is an associate professor of physics and astronomy, and a core faculty member in women’s studies at the University of New Hampshire. Her most recent book is The Disordered Cosmos: A journey into dark matter, spacetime, and dreams deferred
