Some Black Holes Erase Your Past and Could Send You Back in Time


Why would anyone want to visit a black hole, 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.


For a long time physicists have insisted that black holes are impenetrable ciphers. Whatever goes in is lost, impossible to study or meaningfully understand.


However, in a paper published Jan. 17 in the journal Physical Review Letters, Berkeley postdoctoral fellow Peter Hintz and colleagues showed that certain kinds of theoretical black holes could exist that would allow theoretical observers to pass through their outer borders without being instantly destroyed. Instead, they would destroy their past and potentially open up an infinite number of futures.


"If someone were to venture into one of these relatively benign black holes, they could survive, but their past would be obliterated and they could have an infinite number of possible futures," he claimed in an article in the journal Physical Review Letters.



Physicists are constantly debating the nature of these collapsed stars. Physics is deterministic, meaning that knowing exactly what happened in the past, such as the origins of the universe, determines just one future.


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. A black hole might be imagined as a funnel whose spout tapers to a point of infinite density known as a singularity.


The only issue is that singularities are invisible to us. Most argue that there is no possible way we could ever see beyond the event horizon and into the naked singularity deep within a black hole, because that would basically upset physics as we know it — this would destroy the determinism that is fundamental to physics.


The notion that black holes must be walled off, that their interiors are necessarily impossible to observe, is called the cosmic censorship hypothesis. First proposed by mathematician Roger Penrose in 1969 and later debated by the likes of Stephen Hawking and Kip Thorne, it's been modified over the decades and has never been formally stated as a theory.


Sir Roger Penrose introduced his twistor theory of space-time in the Sixties (SCIENCE PHOTO LIBRARY).


But the new paper implies that in the border regions of these special black holes, cosmic censorship breaks down. An observer could travel beyond the zone of what physics can predict and watch what happens there. And if that's true, it would mean the world of physics that makes sense is starting to leak into the zone of the incomprehensible.



The weak cosmic censorship hypothesis insists that there are no naked singularities in the universe except the one that arose in the wake of the Big Bang, and that the event horizons of black holes must obscure their singularities. 


The strong cosmic censorship hypothesis limits how far a universe governed by the determinism of physics extends. After you pass a strange and intangible boundary within the event horizon of a (smooth, non-rotating, highly electrically charged) black hole, known as the Cauchy horizon, bizarre things start to happen. 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. 


(Serge Droz, Physics World magazine)


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. The only problem is that no one will know what you saw, because you’ll never return.


Scientists who argue against this theory believe that gravity would slow down time as you approached the Cauchy horizon, and all the energy that the black hole has ever seen since the universe began will fall in with you and obliterate you right there.


Yet Hintz and his colleagues showed that, since the universe is also expanding at an accelerating rate, the wall of death around black holes could break down. 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. After that, all bets are off.”


Hintz said it's important to understand that his and his colleague's model of the universe is "far-fetched." In fact, he 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. However, this kind of abstract research can pierce widely accepted notions of reality and open up areas of inquiry in ways experimental science cannot.


“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.”



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