Imagine, for a moment, that our species has perfected interstellar space travel and we can visit anywhere we want in the universe. There will be a lot of interesting places to check out and astrophysical phenomena to investigate, but a trip to a black hole will surely be at the top of the itinerary. Why would anyone want to visit something from which nothing, not even light, can escape? Mostly because physicists have debated for decades what will happen if someone were to enter one.

A caveat here: Most physicists harbor little doubt that you would be ripped to shreds long before you came anywhere near smaller black holes (the technical term is ‘spaghettified,’ where intense gravitational forces stretch you into a string of atoms). But—* but*—new research from an international team of mathematicians suggests that there may be certain black holes that are theoretically accessible to an observer, albeit with bizarre consequences.

As detailed in a report published last week in * Physical Review Letters*, observers entering certain kinds of theoretical black holes wouldn’t necessarily be obliterated—or at least not in the way you’re probably imagining. Instead, an observer’s entrance into these black holes would destroy their past and potentially open up an infinite number of futures. They’d never emerge from the black hole to tell their tale, but that doesn’t really matter—they’d have no one from their past to return to anyway.

## MUCH ADO ABOUT BLACK HOLES

There’s a lot to unpack here, so let’s start with some background. You may have heard of this guy named Albert Einstein who, among other things, fundamentally changed the way we thought about space and time when he published his general theory of relativity about a century ago.

Einstein’s general theory of relativity describes gravity as a property of spacetime, a four-dimensional scaffolding that is ubiquitous in the universe. More to the point, the theory described the curvature of spacetime as a function of matter’s mass, energy, and motion. This curvature of spacetime by objects in motion is felt as gravity.

One of the phenomena predicted by the general theory is the existence of spacetime singularities in black holes, a mass that is so dense that nothing can escape its gravitational effects—not even light. For our purposes, a black hole might be imagined as a funnel whose spout tapers to a point of infinite density known as a singularity.

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The structure of these singularities is a subject of contention among physicists. We can’t see them because a black hole’s event horizon effectively acts as a barrier between these infinite densities and the rest of the universe. This is a good thing because if we could see the singularities at the heart of black hole—what is called a ‘naked’ singularity—this would destroy the causality that is fundamental to physics.

The reason that physics can be used to predict things in nature is because the universe is deterministic. What this means is that if you knew the exact starting conditions of the universe, you could theoretically predict exactly how the universe would develop over time frsingulom those initial conditions. This would also include your thoughts and actions since, as cognitive scientists like Dan Dennett have argued, consciousness is determined by material interactions among neurons. The important thing here is that determinism means that the past determines exactly one future.

So physicists are presented with a problem: Singularities must exist as a consequence to the theory of general relativity, but observing these singularities seems to be impossible. To account for this discrepancy, physicists created two related, but logically distinct conjectures, both developed by the physicist Roger Penrose: the strong and weak cosmic censorship hypotheses.

The strong cosmic censorship hypothesis states that there is a boundary within the event horizon of black holes known as the Cauchy horizon that is a limit to the applications of the theory of general relativity. Beyond the Cauchy horizon, the deterministic physical world breaks down into indeterminacy. A consequence of this is that it is impossible for an observer to transcend the Cauchy horizon without being destroyed (more on this later).