What Happens to Time Inside a Black Hole, and Why Physicists Still Disagree

The video begins with the claim that inside a black hole, time either ends, loops, becomes a place in space, or becomes wildly unpredictable. The speaker says physicists cannot agree on which of these actually happens.

The question is presented as open rather than settled, and the rest of the video is framed as an attempt to explain both what the mathematics says and why the answer remains disputed.

The speaker says that most people taught about black holes learn a very oversimplified version: that you fall in and are crushed into an infinitely small point called a singularity, which does weird things to time on the way. The speaker calls this the mathematics of a version of black holes that does not exist in the real universe.

Real black holes spin, and the speaker says spin changes everything, including the nature of the singularity, which becomes a donut shape. The speaker describes it as an infinitely thin donut that does even weirder things to time, so weird that it could make time travel possible. The same idea is said to be the source of the notion of alternate universes inside black holes, along with white holes and a breakdown of time and determinism as we know them.

The speaker says the video will first get viewers on the same page about what a black hole is, then examine what happens to time inside a static black hole, then inside a realistic spinning black hole, and finally why physicists cannot agree on what is going on.

The speaker also says the hope is that, if physicists had a theory of quantum gravity, they might be able to solve the problem.

The speaker defines a black hole as a place in space where an object such as a dead star has collapsed under its own gravity. Nothing pushes outward anymore to counteract gravity crushing inward, so the object becomes infinitely dense, and escaping it would require traveling faster than the speed of light.

The speaker attributes the ban on faster-than-light travel to Einstein's theory of special relativity. As a result, everything from matter to light to astronauts would be trapped inside what is called the event horizon, a rough sphere that marks the edge of the black hole and the region from which no light or information reaches us.

A playful exchange follows: the speaker's partner notes that the boundary is a sphere, and the speaker replies that of course it is, since the speaker had perhaps expected just a hole.

Because no light or information comes from inside the event horizon, the speaker says there are no observations of what is going on there and no real-universe evidence or data for the interior. The only thing available to understand what happens inside is the mathematics of the best theory of gravity.

That theory is Einstein's general relativity, published in 1915. The speaker says it has been tested for over a century and has held up as the best explanation for observations, with predictions that match what is seen in the real universe. Its predictions follow from the idea that massive objects curve spacetime, and the heavier the object, the more spacetime is curved. As an object collapses under gravity, spacetime is curved to an extreme, so much that all the matter ends up in an infinitely dense region that the equations can no longer describe. This is what the speaker calls a singularity.

The speaker says the word singularity is borrowed from mathematics, where it means that the maths breaks down. One of the most common ways this happens is when one tries to divide by zero.

In a black hole, the speaker explains, the matter is crushed into an infinitely small volume, so when one works out the density, one is dividing by zero. This is why a singularity is described as an infinitely small point at the center of the event horizon. Apart from that point, the speaker says general relativity provides equations that describe what happens to space, time, and moving objects both outside and inside the event horizon. These are known as the Einstein field equations.

The speaker says the Einstein field equations show that as you get closer to heavy objects with stronger gravity, such as black holes, time ticks more slowly for you relative to someone not next to that object. You would still feel time passing normally, but if you went back out away from the black hole, you would find that much more time had passed.

The speaker notes that this was a major plot point in the film Interstellar, and says the speaker was shouting at the screen for the characters to leave the planet. When the speaker's partner says the speaker knew about relativity, the speaker agrees that this is what is called time dilation, and calls it very weird. The speaker adds that things get even weirder once you cross the event horizon.

In the simplest case of a non-spinning static black hole, the speaker says that once you cross the event horizon, you no longer have control over how you move through space, because everything is drawn toward the singularity. The speaker describes this as inevitable.

The speaker calls this a space-like singularity: it cannot be defined in space, so it becomes a moment in time.

The speaker compares this to ordinary life, where we control how we move through space in three directions but have no control over how we move through time. Next Monday will arrive whether we want it or not.

Inside a black hole, the speaker says, space and time swap. You can no longer control where you end up in space, only in time. The singularity will arrive whether you want it to or not, and you have only a tiny bit of control over when.

The speaker says everything just described applies only to a static black hole, the kind everyone is taught about and the simplest case in the mathematics. The speaker says this is not considered realistic for the actual universe.

The reason given is that black holes form in supernovae, when a star dies and its core collapses. A star spins during its life and retains that spin as it collapses, so the speaker thinks all black holes in the universe will be spinning at some rate. The speaker adds that even supermassive black holes at the centers of galaxies have measured spins. Spinning changes the predictions of Einstein's field equations a great deal: there is no longer a pointlike singularity at the center, and a second horizon appears inside the event horizon. The speaker says this change is what matters most for time.

The speaker asks what happens to time inside a real spinning black hole and says viewers need to set aside what they knew about black holes. In a spinning black hole there is no point singularity; there is a ring singularity, an infinitely thin donut.

The speaker explains that spin throws matter outward, which is analogous to the feeling of being pushed outward when spinning while holding a friend's hands. The speaker says Earth is an oblate spheroid for this reason, slightly squashed and bulging at the equator, and that Saturn, which spins faster, is even more squashed. Taken to the extreme, this produces a ring singularity.

The speaker says the ring has an infinitely thin height and width, but a set radius from the center, which depends on how fast the black hole spins and how heavy it is. Because the ring is a definable place in space, it is called a timelike singularity, meaning it can exist across any moment in time. The speaker says that an astronaut trapped beyond the event horizon of a spinning black hole could, technically, fly around in a spacecraft and avoid the ring.

The speaker says that if you look at the equations describing the ring, at its very center all the forces cancel out, so you would not be pulled in any direction toward the ring. This is what the equations give when r equals zero.

The speaker adds that the equations behave perfectly normally when r is less than zero. This is where the idea of alternate universes on the other side of a black hole comes from: a universe that is an inverse of our own, reachable only through the strange center of the ring singularity. The speaker admits this is a tangent but says it is fun.

Returning to time, the speaker says that at the equator of the ring, spacetime is bent to the extreme. Because the ring is a defined place in space, one is forced to travel in a specific direction in space, while in ordinary life one is forced to travel forward in time with no control. If one is forced in a specific direction in space instead, the speaker says one can travel in any direction one wants in time.

The speaker says the ring singularity sets up what is known as time loops. One could travel back to another moment in time and meet oneself, or travel back to a moment before one fell into the event horizon.

The speaker says the mathematics also allows negative time, where time can be less than zero. This has led people to speculate about white holes in other universes on the far side of a black hole, the exact opposite of a black hole, where the singularity is a place in one's past rather than future.

The speaker clarifies that these time loops do not occur anywhere inside a spinning black hole's event horizon. They are confined to the negative-r region and very close to the ring singularity.

The speaker says that another weird thing happens to time inside a spinning black hole: beyond what is known as the Cauchy horizon, everything breaks down. The Cauchy horizon is described as a second, inner event horizon, lying between the outer event horizon, the sphere of no return, and the singularity.

Between the outer event horizon and the Cauchy horizon, general relativity can be used to say that because something happened in the past, something will happen in the future. The speaker illustrates this with a kicked football, whose landing spot can be predicted from its force, angle, and Earth's gravity, and with a spacecraft whose future position can be predicted from its direction and speed. The speaker says this kind of mathematics is called deterministic.

The speaker says that inside the Cauchy horizon, the mathematics technically still works, but it is no longer deterministic. The equations can no longer predict that because something happened in the past, something will happen in the future.

The speaker says that beyond the Cauchy horizon there are infinitely many completely different yet equally valid mathematical futures, and that the Einstein field equations cannot say which one is correct. When a companion says this means infinite timelines and possibilities, the speaker says it is total chaos, and explains that the mass allows multiple incompatible futures to exist at the same time.

The speaker says that it is not even known whether the idea of the Cauchy horizon survives in real black holes that have actually been observed, and this is why physicists cannot agree on what actually happens.

Because of this indeterminism, the speaker says the Cauchy horizon acts like a kind of time sink, where stuff gets stuck on its way to the singularity. This is called mass inflation, which results in a complete breakdown of the equations and predictions. Some people argue this would not actually happen in the real universe, a position traced to Roger Penrose, who raised the beginnings of the idea at the end of the 1960s and called it cosmic censorship, meaning nature would ensure mass inflation does not occur. Others argue it would really happen and cannot be ignored, and the debate then turns to how it should be handled. The speaker concludes that all of these discussions boil down to needing a theory of quantum gravity.

The speaker says general relativity, the best theory of gravity, and quantum mechanics, the theory of how things behave at the smallest scales, are currently two completely separate theories. General relativity deals with the very big and makes concrete predictions, whereas quantum mechanics deals with the very small, where things are governed by probabilities, even for the location of a specific particle. What is missing is a unified theory combining the two. The speaker says this is not for lack of trying, and that people have tried for decades.

The speaker says a unified theory would help with the black hole question because in theories of quantum gravity, matter, space, and time cannot be infinitely compressed. The universe would then have a finite pixel scale, giving the ring singularity a very small but finite edge and giving spacetime curvature that is extreme but finite. The hope is that in such a theory, one is no longer in an infinite curvature state, so closed time loops do not form, mass inflation does not happen, and everything becomes deterministic again, though probabilistic as in quantum mechanics for small particles. The speaker stresses that this theory does not yet exist.

The speaker notes that a theory of quantum gravity has long felt like a pipe dream and will not arrive overnight. It took about 230 years to get from Newtonian gravity to Einstein's general relativity, so the speaker suggests it may take another 100 years before a theory explaining what happens to time inside a black hole is finally available. The speaker closes by saying perhaps only time will tell.

Partway through the video, the speaker pauses the science to promote UPDF, a PDF tool, saying it helps with reading research papers, highlighting key points, adding sticky notes, redacting unpublished drafts, and syncing across devices with one account. The speaker says the tool allows PDFs to be edited, can use OCR on scanned documents, and costs around a sixth of Adobe Acrobat, and includes a link in the description for a limited-time offer. This segment does not add to the physics of time inside a black hole.