In a recent column, I wrote about black holes. A curious reader sent in a letter asking some interesting follow-up questions about topics that I didn’t really have time to delve into, more specifically about the relationship between matter and a black hole, as well as what happens at the centre of it all.
The easy answer to the second part is that we have no idea. End of column! More seriously, part of what makes black holes such interesting phenomena to study is that perspective really matters. We know that they are demarcated by a boundary known as the event horizon. Outside of it, we can calculate trajectories for photons (particles of light) and massive matter like spaceships where they escape the black hole’s gravity. By contrast, inside the event horizon, all trajectories lean towards the centre of the black hole. There is no escaping. Even light, which travels at the fastest speed in the cosmos, can’t go fast enough to get out. In the simplest scenario, this is actually what defines the event horizon: it is the location in space-time where escape velocity is the speed of light.
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From the perspective of someone or something that crosses the event horizon, there is no sign saying “No Return Beyond This Point”. Space-time continues like normal. But the gravitational forces at work here are so intense that, in practice, it is unlikely a person would be intact by the time they reached the event horizon.
The perspective of someone watching an object cross the event horizon is completely different. The reason why has to do with the way gravity affects the passing of time. Time moves more slowly in a strong gravitational field than in a weak one. As a result, time passes faster on Mount Kilimanjaro than it does in New York, since New York is closer to the majority of Earth’s mass. The primary gravitational force any clock experiences on Earth is from the matter that makes up our beautiful home planet. This effect is called gravitational time dilation.
Now imagine how much this effect is amped up by a black hole, which will typically be much more massive than Earth: an observer at a safe distance will measure time such that an object near the black hole is moving very slowly. In fact, it will look like the object never actually crosses the event horizon, since the closer it gets, the stronger the gravitational time dilation.
In other words, what happens at the edge of a black hole is a matter of perspective. The other thing to keep in mind is that a black hole behaves like it is a massive object, but it isn’t necessarily made of matter in the same sense that, say, a baby is. The simplest black hole solution to the equation that governs gravity in Albert Einstein’s general relativity is actually what we call a vacuum solution. That is, there is no matter present. Just a black hole acting like matter.
In practice, the black holes that we observe astrophysically tend to have a lot of matter around them. But this isn’t a property unique to black holes. Many massive objects tend to accrete – that is, attract – more matter into their orbit. The sun, for example, has a solar system that probably formed from an accretion disc around it. Earth has a satellite, the moon, that pulls our planet and causes its oceans to have tides.
What’s more, a black hole doesn’t need to have matter around it to be a black hole. The properties of space-time that define a black hole can exist in that state with no matter present. There is something of a caveat to this, however. Einstein’s relativity doesn’t take quantum mechanics into account. Fifty years ago, Stephen Hawking showed that when quantum features are taken into account, black holes radiate particles. Given enough time, a black hole can even evaporate because of this. Unfortunately for the experimentally curious, that timescale for any observable black hole is longer than the age of the universe, so it is unlikely we will ever witness it.
This might make you think that there is no point to researching quantum black holes, because we can’t see their effects. But special relativity came about as a theory because Einstein used a series of thought experiments that turned out to be quite insightful. Similarly, research about the quantum nature of black hole event horizons may provide insight into what happens at the centre of a black hole.
You may recall that the black hole equations as we understand them suggest there is a singularity there. What this really means is our equations break down. Applying quantum theory to black holes and studying associated quantum gravity ideas might help us figure out how to make a functional mathematical description of singularities. Understanding the black hole event horizon may give us insight into the black hole singularity in the middle, and whether we can ever see it or not, this would be pretty exciting to know!
Chanda’s week
What I’m reading
The plot of Octavia Butler’s Parable of the Sower starts in July 2024, so, like many others, I am reading it.
What I’m watching
For better or for worse, a lot of the Dutch soap opera Goede Tijden, Slechte Tijden.
What I’m working on
Developing a research programme for a grant proposal.
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
