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.
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 determinism 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 from 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 rely on two related, but logically distinct conjectures, both originally developed by the physicist Roger Penrose nearly 50 years ago: 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).
The weak cosmic censorship hypothesis, on the other hand, suggests that naked singularities don’t exist in the universe, apart from the Big Bang. Today, Penrose’s weak cosmic censorship hypothesis is widely held to be a necessary condition of the universe by physicists, although its validity is still an open question.
The strong cosmic hypothesis is much more contentious, and the new research published this week offers the strongest refutation of its validity yet. UC Berkeley postdoc Peter Hintz and his colleagues’ paper suggests that there are some types of black holes in the universe that would allow an observer access to the indeterministic universe on the other side of a black hole’s Cauchy horizon.
BLACK HOLES, SON
For the last century, Einstein’s theory of relativity has managed to predict the results of every test thrown at it. Perhaps its strongest validation occurred in 2016, when physicists at the Laser Interferometer Gravitational-Wave Observatory managed to measure gravitational waves produced by two colliding black holes for the first time, exactly as Einstein’s theory predicted. Yet general relativity’s ability to describe gravity falters on the threshold of singularities, where the curvature of spacetime becomes infinite.
Let us imagine that we are space explorers again and that we are approaching the type of theoretical black hole studied by Hintz and his colleagues: A non-rotating black hole with an electrical charge known as a Reissner-Nordström-de Sitter black hole. According to the general theory, as we approach the black hole, time begins to slow down due to the increasing strength of the gravitational field. As we fall into the black hole, we would also see all the light and matter falling in as well. Eventually we would reach the Cauchy horizon, an object within the event horizon found in these types of black holes.
The Cauchy horizon can be thought of as the barrier between the deterministic and non-deterministic universe. After an observer crosses this threshold, the past no longer determines the future. An observer crossing this threshold would, as a result, actually see all the energy the black hole will ever encounter over the entire existence of the universe hitting its Cauchy horizon at the same time. This is why the strong cosmic censorship hypothesis states that it is impossible for an observer to pass over the Cauchy horizon—they would be totally obliterated by all that energy.
Yet Hintz and his colleagues realized that this wasn’t necessarily the case, since the universe is also expanding at an accelerating rate. This means that while spacetime is condensing to an infinite point in a black hole, it is also being pulled apart or stretched by the expansion of the universe. So rather than all the energy in the universe hitting the Cauchy horizon at the same time, only a relatively small portion of the energy in the universe makes it to the black hole because that energy can’t travel from the farthest corners of the universe to the black hole faster than the speed of light.
As detailed by Hintz and his colleagues, the amount of energy that will fall into the black hole is only the amount of energy contained within the observable horizon from the black hole’s perspective. This observable horizon is ‘smaller’ than the whole universe because the universe is expanding at an accelerating rate.
To see why this is the case, consider our perspective on Earth. Although we can see 13.8 billion years in the past, our observable horizon is actually around 46 billion light years since it includes everything we will see in the future. We will never be able to see ‘further’ than this because the universe is expanding at a speed faster than the speed of light, so the light from objects beyond this cosmological horizon will never reach us and objects on the ‘brink’ of this horizon will eventually fade and disappear from our perspective.
The same is true for the theoretical Reissner-Nordström-de Sitter black hole we are visiting. The accelerating expansion of the universe essentially ‘cancels’ the time dilation experienced while falling into the black hole under certain conditions. This would, in theory, allow an observer to pass through the Cauchy horizon and exist in a non-deterministic world where their past no longer determines their future. For all intents and purposes, crossing this threshold obliterates the observer’s past by opening up an infinite number of possible futures.
“There are some exact solutions of Einstein’s equations that are perfectly smooth, with no kinks, no tidal forces going to infinity, where everything is perfectly well behaved up to this Cauchy horizon and beyond,” Hintz said. “After that, all bets are off; in some cases, one can avoid the central singularity altogether and live forever in a universe unknown.”
This is all theoretical, of course. Hintz and his colleagues aren’t suggesting that a physicist ever will travel to the inside of one of these types of black holes. In fact, Hintz said, these charged black holes used in the model might not even exist. The reason is that a charged black holes would attract oppositely charged matter and eventually become neutral. Still the mathematical model is useful as a way of studying rotating black holes, which Hintz said are probably the norm.
“No physicist is going to travel into a black hole and measure it,” Hintz said. “This is a question one can really only study mathematically, but it has physical, almost philosophical implications. From that point of view, this makes Einstein’s equations mathematically more interesting.”