As far as mainstream science is concerned, our world is deterministic. A leads to B which leads to C. Despite the best efforts of revisionist politicians, the past is immutable. Just between you, me, and this handsome implementation of WordPress, Lisbon University’s Vitor Cardoso and UC Berkeley’s Peter Hintz reckon they’ve modelled a class of extra-special black holes that can erase your past.
To better understand what Hintz is hinting at, you must understand black holes. Black holes are the corpses of stars where the entire mass is packed into an infinitesimal area. In that space gravity approaches infinity, becoming so strong not even light can escape. This is an event horizon, a black hole’s point-of-no-return. Everything outside this barrier, even by a whisker, has some hope of escape. As a hypothetical astronaut falls towards the event horizon, time slows down in accordance with Einstein’s theory of general relativity. At first, centuries will pass in minutes. Closer, millennia would pass in the span of a heartbeat. Deeper in the black hole, aeons will fly by in the course of an eyeblink ’til time as we know it comes to a screaming halt. At that point, everything outside the black hole’s event horizon appears to happen all at once, should a black hole explorer be looking in that direction.
The journey doesn’t stop there, since there’s another, deeper barrier called the Cauchy horizon. A Cauchy horizon is the spot where determinism breaks down. A place where the past no longer determines the future. If someone doesn’t know a star’s initial state, then no one can know the future of that object. In hopes of heading off the confusion of indeterminism at the pass, Roger Penrose hypothesized ‘strong cosmic censorship‘ where general relativity prevents anything and everything from passing through a Cauchy horizon. Instead every captured photon, all of a black hole’s absorbed energy, and everything stuck in cosmic amber is annihilated, or censored, simultaneously against the Cauchy horizon.
Pretty neat, except general relativity says we exist in an ever-accelerating and expanding universe.
In fact, the energy available to fall into the black hole is only that contained within the observable horizon: the volume of the universe that the black hole can expect to see over the course of its existence. For us, for example, the observable horizon is bigger than the 13.8 billion light years we can see into the past, because it includes everything that we will see forever into the future. The accelerating expansion of our universe will prevent us from seeing beyond a horizon of about 46.5 billion light years.
In that scenario, the expansion of the universe counteracts the amplification caused by time dilation inside the black hole, and for certain situations, cancels it entirely. In those cases – specifically, smooth, non-rotating black holes with a large electrical charge, so-called Reissner-Nordström-de Sitter black holes – an observer could survive passing through the Cauchy horizon and into a non-deterministic world.
Anything passing through a Cauchy horizon no longer has a past. All those cringey moments from calling your teacher “Mommy” to accidentally sexting Grandma, will be lost in spacetime like tears in rain. My question is, has the past been erased or are you in an parallel universe? A It’s A Wonderful Life-esque dimension where you were never born? No birth means no past, and a survivor who crossed a Cauchy horizon may find themselves in a pickle without Clarence (angel, second-class) for guidance. It’s just one of the philosophical questions physicists like to shelve for a later date. The probability of something surviving a black hole and passing through a Cauchy horizon looks good on paper. Especially to those capable of reading esoterica like Quasinormal Modes and Strong Cosmic Censorship, yet cosmologists can only say the math works.
One caveat says these special Reissner-Nordström-de Sitter black holes eventually lose their charge. Without a charge, the Cauchy horizon is an impassible dead zone for a black hole explorer. All isn’t lost, as Hintz points out another species called Kerr-Newman-de Sitter black holes, which are smooth and rotating, could also have the perfect conditions for Cauchy horizons without needing an electric charge. And Cardoso and Hintz have a clue how to detect these objects.
[Peter Hintz] discovered these types of black holes by teaming up with Cardoso and his colleagues, who calculated how a black hole rings when struck by gravitational waves, and which of its tones and overtones lasted the longest. In some cases, even the longest surviving frequency decayed fast enough to prevent the amplification from turning the Cauchy horizon into a dead zone.
With gravitational observatories popping up like mushrooms in hopes of detecting new physics, there’s a strong possibility humanity will eventually observe these modulated gravitational waves. If we’re clever enough, we could replicate the phenomenon here on Earth and kickstart a booming industry to clean up messy backgrounds or give folks a new leash on life.
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