Rick Robinson

Aug 6th 2021

Your Black Hole Tour Guide: Beyond This Horizon


A journey through the event horizon into a black hole could be the ultimate cosmic vacation experience — and we do mean ultimate. As in, the journey is strictly one-way. There are no round-trip tickets, and you can’t even send back a postcard or post a selfie on your social media accounts.

With that disclaimer, we can also report that your adventure won’t necessarily be as quick and abrupt as some popular accounts make it sound. It all depends on what sort of black hole you choose as your destination.

A Peculiar Corner of Space

First of all, what exactly are black holes? For a class of celestial objects that we cannot actually see (at most, we can only see their shadows), black holes loom large in the popular culture. In a nutshell, a black hole is an object dense enough and massive enough that its escape velocity is greater than the speed of light.

Thus, no message sent from inside can ever get to the outside. And at the very center of the black hole is a singularity — a single point in space into which the former star (or galaxy, or whatever it might previously have been) collapses. What happens at the singularity itself is unknown, as our understanding of physics doesn’t yet provide any explanation. Perhaps, as the University of California at Riverside (UCR) suggests, the theory of quantum gravity will eventually help us puzzle this out, but not yet.

This tour will offer only the relatively simple physics of a journey into a pure “Schwarzschild” black hole, one that has no rotation and no magnetic charge. Out in the real universe, most black holes probably have both, limiting your selection. But in a Schwarzschild, we can at least say something about the region between the event horizon and the singularity, and what a tourist will experience while passing through that region.

Avoid Spaghettification

To enjoy your tourist experience inside a black hole, choose a massive one, the kind found at the center of many galaxies. The ordinary kind formed by collapsing stars are very poor tourist destinations. The problem with these black holes is that they not only have intense gravity, but they also squeeze all that gravity into a small volume.

This is a problem, because differences in gravity between nearby points are what produce tidal forces. For example, the difference between the moon’s gravity acting on the side of Earth facing the moon and on the opposite side is enough to produce ocean tides.

A compact black hole produces much more powerful tidal forces. Objects caught in these tides are pulled in some directions and squeezed in others, long before reaching the event horizon — a process that has rather vividly been dubbed “spaghettification.”

But as UC Berkeley explains, galaxy-mass black holes are larger, so their gravitational fields produce less tidal stretching, and you can pass through the event horizon without getting pulled out of shape. Ultimately, you’ll still get spaghettified as you approach the central singularity, but at least you’ll have a chance to look around the interior of the event horizon before it happens.

Black hole sizes can vary from relatively tiny to mindbogglingly huge. Stellar-mass black holes, while still immensely massive in the technical sense, are only a few miles across. The black hole at the center of the Milky Way galaxy is about 8 million miles across, while a supermassive black hole in the galaxy M87 is 11 billion miles across — as large as the solar system out to the Kuiper Belt, well past the orbit of Pluto. This will make your tourist journey a bit more leisurely.

The Games Time Plays

There’s a popular rumor that you’d never actually fall into the singularity because the effects of relativity make time slow down, extending your travel time to eternity. But according to UCR, this is a misunderstanding. A friend watching you from a distance as you approach the event horizon will see your clock seem to run slower, but to you, it will still run at normal speed. (And once you pass the event horizon, your friend won’t see either you or your clock at all.)

All this applies mainly to classical Schwarzschild black holes, not to rotating or magnetically charged ones. Black holes in the real universe are likely to have more complex properties, for which the term “weird” is an understatement.

Postdoctoral fellow Peter Hintz tells UC Berkeley that some black holes may produce environments in which past and future become indeterminate, and determinism itself breaks down. In principle at least, a tourist who enters this type of black hole might be able to survive the trip into indeterminacy — whatever exactly that means.

Hintz plays the spoilsport, saying that no physicist is ever actually going to make the trip. However, he adds, “This is a question one can really only study mathematically, but it has physical, almost philosophical implications, which makes it very cool.”

On the other hand, with the risks of spaghettification and sliding into indeterminacy, maybe it’s best that this trip remains purely mathematical.