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A trip across the event horizon into a black hole could be the ultimate cosmic vacation experience – and we mean ultimate. As in, the trip is strictly one way. There are no round-trip tickets, and you can’t even resend a postcard or post a selfie to your social media accounts.
With this disclaimer, we can also point out that your adventure won’t necessarily be as quick and abrupt as some popular accounts suggest. It all depends on what type of black hole you choose as your destination.
A special corner of space
First of all, what exactly are black holes? For a class of celestial objects that we can’t actually see (at most, we can only see their shadows), black holes are a big part of popular culture. In short, a black hole is an object dense and massive enough that its exit speed is greater than the speed of light.
Thus, no message sent from the inside can ever reach the outside. And at the very center of the black hole is a singularity – a single point in space into which the old star (or galaxy, or whatever it may have been before) is collapsing. What is happening to the singularity itself is unknown, as our understanding of physics does not yet provide an explanation. Perhaps, as the University of California at Riverside (UCR) suggests, quantum gravity theory will eventually help us solve this problem, but not yet.
This tour will only offer the relatively simple physics of a trip to a pure “Schwarzschild” black hole, without rotation or magnetic charge. In the real universe, most black holes probably have both, which limits your selection. But in a Schwarzschild one can at least say something about the region between the event horizon and the singularity, and what a tourist will experience while passing through this region.
To enjoy your sightseeing experience inside a black hole, choose a massive one, the one 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 also compress all of that gravity into a small volume.
This is a problem, because the differences in gravity between neighboring points is what produces the tidal forces. For example, the difference between the gravity of the moon acting on the side of the Earth facing the moon and 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 they reach the event horizon – a process that has been quite vividly dubbed “spaghettification”.
But as UC Berkeley explains, galactic mass black holes are bigger, so their gravitational fields produce less tidal stretch, and you can cross the event horizon without warping. Ultimately, you’ll still be spaghetti-sized as you approach the central singularity, but at least you’ll have the opportunity to look inside the event horizon before that happens.
The size of black holes can range from relatively tiny to incredibly huge. Stellar-mass black holes, while still immensely massive in a technical sense, are only a few kilometers in diameter. The black hole in the center of the Milky Way galaxy is about 8 million kilometers in diameter, while a supermassive black hole in the M87 galaxy is 11 billion kilometers in diameter – as large as the solar system until the Kuiper belt, well beyond the orbit of Pluto. This will make your sightseeing trip a little quieter.
Play time is playing
There is a popular rumor that you would never fall into the Singularity because the effects of relativity slow down time, extending your travel time to eternity. But according to UCR, this is a misunderstanding. A friend who watches you from a distance as you approach the event horizon will see your clock appear to be slowing down, but for you it will still run at normal speed. (And once you’re past the event horizon, your friend won’t see you or your clock.)
All of this applies mainly to classic Schwarzschild black holes, not to rotating or magnetically charged holes. Black holes in the real universe are likely to have more complex properties, for which the term “bizarre” is an understatement.
Postdoctoral fellow Peter Hintz told UC Berkeley that certain black holes can produce environments in which the past and future become indeterminate and determinism itself crumbles. In principle at least, a tourist who enters this type of black hole might be able to survive the indeterminate journey – no matter what exactly that means.
Hintz plays spoiler, saying no physicist will ever make the trip. However, he adds, “this is a question that you can only really study mathematically, but it has physical, almost philosophical implications, which makes it very cool.”
On the other hand, with the risks of spaghettification and slipping into indeterminacy, it might be better that this trip remain purely mathematical.