How Do Black Holes Actually Work?

1. From dying star to black hole

2. The event horizon and the point of no return

note

Event horizon: the boundary of no return

The event horizon is the surface where escape becomes impossible, even for light.

Key properties

• It is not a physical wall.
• Nothing special has to happen locally at the horizon for a large black hole.
• For a distant observer, infalling clocks appear to slow down.
• For the falling observer, crossing the horizon can feel ordinary if the black hole is large enough.

Schwarzschild radius

For a non-rotating black hole:

r_s = 2GM / c^2

That is the radius of the event horizon.

Concrete numbers

• Sun mass black hole: about 3 kilometers
• Earth mass black hole: about 9 millimeters

Why the horizon matters

The horizon is not where gravity becomes infinite. The real singularity, in the classical theory, is deeper inside. The horizon is where the escape route disappears.

diagram

equation

$$r_s = \frac{2GM}{c^2}$$

illustration

3. Spaghettification, tidal forces, and time dilation

4. Hawking radiation and evaporation

5. The information paradox and what we still do not know

Transcript

Welcome to Slate. Today we're looking at How Do Black Holes Actually Work?. We'll cover How a black hole forms from a dying star, What the event horizon is and why nothing escapes it, Spaghettification, time dilation, and what falling in would feel like, and Hawking radiation and the information paradox. Let's get into it.

A black hole starts with gravity winning a fight against pressure. In a massive star, fusion pushes outward for millions of years. When the fuel runs low, that support fades. If the star’s core is heavy enough, the collapse can outrun the force that normally stops it. For a core above about 2 to 3 times the mass of the Sun, even neutron pressure can fail. Then the core can keep shrinking until it becomes a black hole. The key idea is simple: gravity is not a force pulling from far away like a rope. In Einstein’s general relativity, mass and energy curve spacetime, and the star’s own collapse makes that curve steeper and steeper. A useful picture is a heavy ball placed on a stretched rubber sheet. The sheet sags. Now imagine the sag getting so deep that the slope never lets anything climb back out. That is the direction a black hole takes. The diagram on screen shows the path: massive star, supernova, collapsing core, then a compact object. Not every supernova makes a black hole. Some leave behind neutron stars. The outcome depends on the core mass, how much material falls back, and how the star lost mass before it died. Observations of stellar-mass black holes in X-ray binaries, and of black holes formed in mergers detected by LIGO in 2015, show that nature makes them in more than one way.

The event horizon is the boundary that makes a black hole a black hole. It is not a solid surface. It is the place where escape speed reaches the speed of light. Once you cross it, every future path through spacetime points deeper inward. Think of it like a waterfall edge in a river. Upstream, a swimmer can fight the current. Near the lip, the current is too fast. Over the edge, every route leads down. The same idea is true here, except the “current” is spacetime itself. For a non-rotating black hole, the horizon sits at the Schwarzschild radius. For the Sun, if it were compressed into a black hole, that radius would be about 3 kilometers. For Earth, it would be about 9 millimeters. The horizon is tiny compared with the mass it contains. That is why black holes can be extremely compact and extremely dense. But density is not the only useful idea. The geometry matters more. From far away, an infalling clock appears to slow down because light it sends out loses energy climbing out of the gravity well. That is gravitational redshift. In the simplest picture, an outside observer never sees the object quite finish crossing. The falling object, though, crosses in finite time. Both descriptions are correct in their own frames. The visual here separates those two viewpoints, because that distinction is where many black hole myths begin.

What would falling in feel like? The answer depends on the black hole’s mass. Near a small black hole, tidal forces can be brutal well before the horizon. Your feet feel a stronger pull than your head. That stretching and squeezing is what people call spaghettification. It is the same reason Earth raises tides on the oceans, but scaled to an extreme. The Moon pulls more on the near side of Earth than the far side, so the ocean bulges. Near a black hole, the difference in gravity across your body can become enormous. For a stellar-mass black hole, that difference can be lethal near or even outside the horizon. For a supermassive black hole, like the one in the center of our galaxy, Sagittarius A* with about 4 million solar masses, the horizon is so large that the tidal forces at the horizon are much weaker. You could cross it without feeling a sharp jolt at that instant. The danger comes later, deeper inside. Time dilation adds another layer. Clocks deeper in gravity run slower relative to distant clocks. Near the horizon, that effect becomes extreme. In the math of general relativity, the falling observer and the faraway observer disagree about the timing, but not about the physics. The diagram shows both the stretching force and the clock-rate difference, because those are the two ideas that make the experience vivid and the equations less mysterious.

Classical general relativity says nothing escapes once it is inside the horizon. But quantum theory adds a twist. In 1974, Stephen Hawking showed that black holes should emit radiation because quantum fields near the horizon are not empty in the simple sense. The result is Hawking radiation. A useful analogy is a shoreline at night. The water looks still, but tiny waves are always forming and disappearing. Near the horizon, quantum fluctuations can let one member of a particle pair fall in while the other escapes. To a distant observer, the black hole slowly loses mass. The temperature is incredibly low for big black holes. A black hole with the mass of the Sun would have a Hawking temperature of about 60 nanokelvin, far colder than the cosmic microwave background at 2.725 kelvin. That means stellar black holes today absorb more background heat than they emit. Only very tiny black holes would evaporate quickly. A black hole with the mass of Mount Everest would be hot enough to shine intensely and vanish in a fraction of a second. None have been observed. Hawking radiation is one of the deepest clues that black holes are not just simple sinks. They have thermodynamics. They have entropy. And they may force us to unify quantum theory with gravity. The image and chart here help show why the effect is real in theory but hard to detect in practice.

The information paradox is the sharpest unresolved question in black hole physics. Quantum mechanics says information is not destroyed. If you know the full state of a system, the future should preserve that information in some form. But if a black hole forms from a detailed arrangement of matter and then evaporates completely through Hawking radiation, where did the information go? Hawking originally argued that the radiation looked purely thermal, which would erase the details. That clashes with quantum theory. The paradox became a major research problem after the 1990s, and it still shapes modern work on gravity, holography, and quantum entanglement. One important clue is the Bekenstein-Hawking entropy formula, written in the 1970s, which says black hole entropy scales with surface area, not volume. That is strange. It is as if the black hole stores its bookkeeping on the horizon, like a library whose catalog is written on the cover rather than inside the shelves. We do not yet have a complete experimentally tested theory that resolves everything. Candidates include holographic ideas, black hole complementarity, and quantum gravity approaches such as string theory and loop quantum gravity. What we know is solid: black holes exist, horizons behave as relativity predicts, and Hawking radiation follows from quantum field theory in curved spacetime. What we do not yet know is how all the pieces fit together in the deepest, final theory.