What a Black Hole Actually Is
The simplest definition Ole Eggers Bjælde offers is this: a black hole is a spherical volume in space where gravity wins every battle. Regardless of what enters this volume, it cannot escape. The boundary of this volume is called the event horizon. Everything within it is the black hole.
What sits at the centre is the part physics struggles to fully account for. If you believe the theory, all of the mass is concentrated into a point with no volume at all. The density at that point is mathematically infinite. "If we believe in the theory, it is so compressed that everything will actually be situated in something that has no volume in a point in the center of this black hole." Most of the black hole's interior, by volume, is simply empty space.
Black holes typically form when a massive star reaches the end of its life and collapses. The collapse is extreme enough that for the largest stars, nothing stops it. A star like our Sun, if it were to collapse into a black hole, would produce an object roughly the size of a small city. The mass would be the same. The volume would be almost nothing.
The Event Horizon and the Point of No Return
The event horizon is not a physical surface. There is nothing there. It is the precise boundary at which the escape velocity required to leave exceeds the speed of light. Cross it moving inward, and you cannot get out. Cross it from the outside moving inward, and what you cross is essentially invisible.
An object falling toward a black hole does not experience anything unusual at the event horizon itself. For the falling astronaut, nothing dramatic happens at that moment. "He will not recognize what's going on. He will not see the difference." The strangeness is entirely visible from outside. What the falling astronaut does eventually encounter is the singularity at the centre, and the journey is not slow. "It sounds a little bit slow when you're talking about it in this way, but it's actually very fast."
The event horizon also scales with mass. Stellar black holes, formed from a single star's collapse, have an event horizon measured in kilometres. The black hole at the centre of our Milky Way has an event horizon roughly the size of the Solar System.
Time Near a Black Hole
One consequence of general relativity is that time does not pass at the same rate everywhere. Gravity bends spacetime, and clocks run slower where gravity is stronger. On Earth, this effect is measurable but tiny: a clock on a mountaintop runs very slightly faster than one at sea level, because the mountaintop is marginally further from Earth's centre and experiences weaker gravity. Near a black hole, the effect becomes extreme.
Bjælde illustrates this with a thought experiment. An astronaut falls toward a black hole, firing a flashlight every ten seconds. For the astronaut, nothing changes. The flashes come every ten seconds as measured by their own experience. For an observer watching from a safe distance, the apparent interval between flashes grows. Ten seconds, then twelve, then fifteen. Time for the falling astronaut appears to slow from the outside, approaching zero at the event horizon. "We will see this happen. And it looks like the astronaut is indeed stopping."
Simultaneously, the light from the astronaut is red-shifted. The wavelength stretches as it climbs out of the gravitational well. Eventually the wavelength shifts beyond the range of human vision, and the image fades. The astronaut does not vanish suddenly. They slow, fade, and disappear.
White Holes, Wormholes, and What Theory Permits
Physics permits two related objects that have not been directly observed. A white hole is the inverse of a black hole: the same extreme density, the same event horizon, but the direction of crossing is reversed. You can exit a white hole but cannot enter it from outside. "For the white hole, it's the opposite. You can only cross it from inside out."
A wormhole connects a black hole and a white hole, creating a theoretical bridge between two points in spacetime, possibly between separate universes. Bjælde is careful to note that theoretical permission is not the same as physical existence. "Just because something is possible from a theoretical point of view, it doesn't mean that it exists in reality. And this is probably an example of that."
White holes have no known formation mechanism. Wormholes, even if they existed, would collapse when anything tried to pass through them. They remain among the most intellectually compelling objects in physics, and for now, objects of theory only.
How Black Holes Die
Stephen Hawking predicted a process by which black holes would very slowly lose mass over time, through a quantum mechanical effect near the event horizon. In the quantum vacuum, pairs of particles can briefly come into existence and then annihilate each other. Near the event horizon, one of these particles can be captured by the black hole while the other escapes. The black hole loses a tiny amount of energy with each such event. Over time, it evaporates.
The timescales involved are difficult to grasp. For a black hole with the mass of the Sun, Hawking radiation would take approximately 10 to the power of 66 years to complete the process. The current age of the universe is roughly 10 to the power of 10 years. "That is a very long time." No one has ever observed Hawking radiation. It remains theoretical.
Millions of Black Holes We Cannot See
Of the hundreds of billions of stars in the Milky Way, perhaps one in a hundred or one in a thousand are massive enough to eventually become black holes. That produces millions of black holes within our own galaxy. "There are literally millions of black holes in our Milky Way galaxy. And we have maybe observed a little more than a handful of these."
The reason is simple. A galaxy is mostly vacuum, and a black hole is dark against a dark sky. You can detect a black hole when it is actively pulling in gas, which heats to millions of degrees and emits x-rays. You can detect it when its gravity measurably perturbs a nearby star. But most black holes are simply floating, inert, in the vast empty spaces between stars. "We don't know where they are."
In 2019, the Event Horizon Telescope collaboration released the first direct image of a black hole, produced by combining observations from radio telescopes distributed across the Earth. The result was an instrument effectively the size of the planet. Even so, the image captured was of a black hole 55 million light years away, not one in our own galaxy. The black hole at the centre of the Milky Way has not yet been directly imaged.
What We Still Do Not Know
"If someone came and told me that these objects exist and I didn't have the background and I didn't know anything about it, I would probably not believe them. I would probably say, okay, in Sci-fi, sure. But not in reality."
That statement captures something important. The objects Bjælde describes are not speculative. They are well-established, mathematically consistent, and observationally supported. But the depth of the strangeness, the infinite density, the time dilation, the millions of invisible objects sharing our galaxy, sits entirely outside normal human intuition. The physics is solid. The intuition has nowhere to go.
What remains open is what happens at the singularity. The theory predicts infinite density, and in physics, infinity usually signals that the theory is incomplete. A future understanding of quantum gravity may replace that mathematical infinity with something that has a physical meaning. That understanding does not yet exist.
What exists instead is a field of observation still catching up with theoretical predictions made decades ago, using instruments that were unimaginable when those predictions were first written down. The first image of a black hole was taken using a telescope the size of the Earth. The next discoveries will require methods that do not yet exist. That, as Bjælde describes it, is precisely what makes the field worth following.