Sorry, Stephen Hawking, But Every Black Hole Is Still Growing, Not Decaying
“It remains true that every black hole that exists in the Universe should emit Hawking radiation, and that if you wait long enough, all of these black holes will eventually decay. But in our Universe so far, based on the black holes that actually exist, not a single black hole has even begun to decay in a meaningful way. The amount and energy of the radiation that’s out there, from starlight and left over from the Big Bang, ensures that black holes will absorb it and grow much more quickly than they lose energy from radiating it away.
Even though it’s been more than 45 years since Hawking first figured out that black holes do emit radiation, as well as what that radiation must look like, it’s far too faint and sparse for us to have ever detected it. Unless there’s a surprisingly low-mass black hole or we’re willing to wait an enormous, cosmic time for the Universe to cool, we’ll never be able to see it. Black holes are growing, not decays, and astrophysics teaches us exactly why.”
Yes, black holes emit radiation of exactly the type described by Stephen Hawking: a low-energy spectrum of photons and other particles arising from the physics of quantum field theory in curved spacetime. But, for much longer, we’ve known that black holes must also be absorbing energy over time, from matter, from starlight, and from the cosmic microwave background radiation.
The results are in, and every realistic black hole in the Universe is growing, not decaying, and it isn’t even close.
Ask Ethan: Can Black Holes Ever Spit Anything Back Out?
“Do black holes ever spit things out at any time? And if they do, do they ever spit out light?”
Sometimes, it’s the simplest-sounding questions that are the most profound. If you consider the name “black hole,” you’ll likely think that it doesn’t emit any type of light, and hence, it must be black. Only, if you look at a realistic black hole, it turns out it isn’t black at all. Black holes with material around them emit both jets and radiation; black holes in the presence of light will develop photon spheres and display that famous donut shape like the Event Horizon Telescope revealed earlier this year; even black holes in isolation will still emit Hawking radiation!
But can any of these types of radiation, varied and real though they may be, actually constitute something being spit back out of a black hole? Find out today!
Ask Ethan: When Do Black Holes Become Unstable?
“Is there a critical size for black hole stability? [A] 1012 kg [black hole] is already stable for a couple of billion years. However, a [black hole] in the range of 105 kg, could explode in a second, thus, definitely not stable… I guess there is a critical mass for a [black hole] where the flow of gained matter will equal to the Hawking evaporation?”
Wherever you have a black hole in the Universe, you have two competing processes. On the one hand, anything that crosses the event horizon, whether it’s normal matter, dark matter, or even pure energy, can never escape. If you fall in, you just add to the overall mass of the black hole, and grow it in size, too. But on the other hand, all black holes radiate away energy in the form of Hawking radiation, and that subtracts mass over time, shrinking your black hole. For all realistic-mass black holes, the rate-of-growth far outstrips the rate of mass loss, meaning they’ll grow for a very long time before they start to shrink.
But eventually, they will shrink. And although we think they don’t exist, a low-enough mass black hole would start shrinking today. Find out when black holes become unstable today!
Ask Ethan: How Do Black Holes Actually Evaporate?
“[J]ust what is Hawking radiation? The science press articles keep referring to the electron-positron virtual pair production at the event horizon, which makes a lay person think that the Hawking radiation consists of electrons and positrons moving away from the black hole.”
Halloween may be over now, so you are free to return to your regularly scheduled existential crises instead of being scared by ghouls and goblins. To help you with that, let’s think about the fact that everything in the Universe, given enough time, will eventually die and decay away. The longest-lived entities, as far as we know, are the supermassive black holes at the centers of galaxies. While stars will burn out after billions or trillions of years, and white dwarfs will cool down after quadrillions, and galaxies will gravitationally dissociate after perhaps 10^24 years, black holes will stick around for far longer: up to 10^100 years. But even they don’t live forever. Hawking radiation ensures that they will decay away, eventually, too.
But if you learned about Hawking radiation from Hawking’s explanation itself, you were lied to. Come find out how black holes actually evaporate today!
The Black Hole Information Paradox, Stephen Hawking’s Greatest Puzzle, Is Still Unsolved
“Despite our best efforts, we still don’t understand whether information leaks out of a black hole when it radiates energy (and mass) away. If it does leak information away, it’s unclear how that information is leaked out, and when or where Hawking’s original calculations break down. Hawking himself, despite conceding the argument more than a decade ago, continued to actively publish on the topic, often declaring that he had finally solved the paradox. But the paradox remains unresolved, without a clear solution. Perhaps that’s the greatest legacy one can hope to achieve in science: to uncover a new problem so complex that it will take multiple generations to arrive at the solution. In this particular case, most everyone agrees on what the solution ought to look like, but nobody knows how to get there. Until we do, it will remain just another part of Hawking’s incomparable, enigmatic gifts that he shared with the world.”
When anything falls into a black hole, it adds to the black hole’s mass, electric charge, and angular momentum, which is what General Relativity predicts. But there’s also quantum information encoded in what falls in, and that information can’t be destroyed. There’s a neat solution for that: information can be encoded on the event horizon of a black hole, getting “smeared out” from the perspective of an outside observer. But then, what happens to this information when the black hole evaporates via Hawking radiation? Hawking himself predicted that information was lost, which is now thought to be wrong. But the question of exactly how that information gets encoded onto the outgoing radiation is still a matter of massive uncertainty. Despite declarations by many (including Hawking) that the paradox has been resolved, the fact is that the black hole information paradox is still an open area of study.
Come find out what the greatest problem in black hole physics, the one that plagued Hawking all his life (and continues to plague him even posthumously) is all about!
How Stephen Hawking’s Greatest Discovery Revolutionized Black Holes
“Increasing entropy, over time, should be okay, but decreasing it should be forbidden. The only way to ensure that would be by forcing an increase in the black hole’s mass to cause entropy to go up by at least the largest amount you can imagine.
The way that people working on that problem – including Hawking – assigned an answer was to make entropy proportional to the surface area of a black hole. The more quantum bits of information you can fit on a black hole, the greater its entropy was. But that brought up a new problem: if you have entropy, then that means you have a temperature. And if you have a temperature, you have to radiate energy away. Originally called “black” because nothing, not even light, can escape, now it became clear it had to emit something after all. All of a sudden, a black hole isn’t a static system anymore; it’s one that changes over time.”
Stephen Hawking may be most famous, today, for his popular accounts of astrophysics and cosmology, for his inspirational personal story, and for his overcoming of adversity and disability to achieve all that he has. But he was truly a tremendous physicist, and his contributions to our understand of black holes was truly transformative. His greatest discovery was that they weren’t merely static objects in space, but that they were active and evolving. They didn’t just absorb mass, but had an entropy, a temperature, and radiated energy away. How this occurred was far from intuitive, and it took a decade of research for Hawking to arrive at his incredible 1974 result, where he derived the existence of Hawking radiation. All of a sudden, black holes had a finite lifetime, and would eventually evaporate away entirely.
Here’s the story of Stephen Hawking’s greatest scientific find, and his greatest contribution to the canon of scientific knowledge. If you want to understand what he did and why it’s so important, you won’t want to miss this.
Ask Ethan: How Do Hawking Radiation And Relativistic Jets Escape From A Black Hole?
“Everything you read about a black indicates that “nothing, not even light, can escape them”. Then you read that there is Hawking radiation, which “is blackbody radiation that is predicted to be released by black holes”. Then there are relativistic jets that “shoot out of black holes at close to the speed of light”. Obviously, something does come out of black holes, right?”
When it comes to black holes, the cardinal rule is that there exists an event horizon: a region from which nothing inside can ever escape. Once you cross over, you can never get out. No matter how fast you move, how quickly or what direction you accelerate in, or even if you travel at the speed of light, your inevitable destiny lies at the central singularity. So how, then, are things like relativistic jets and Hawking radiation emitted from black holes? The key to understanding them lies in examining the conditions that occur outside the event horizon, in the region near (but not exactly at) the black hole itself. This is the critical environment where spacetime is curved, matter achieves relativistic speeds, and the quantum fields themselves are affected by relativity.
Hawking radiation and relativistic jets may be real, but they don’t break the laws of physics to exist! Find out how they really do escape on this edition of Ask Ethan.
Star Trek: Discovery Is Smart-Sounding Scientific Nonsense, Season 1, Episode 4 Recap
“Hawking radiation is the radiation emitted by the very slow decay of black holes due to quantum effects just outside the event horizon. There will not be a black hole that emits enough Hawking radiation to ionize a single atom in the Universe for another 10^60 years or so, and even when they do, it will be because the black hole is on the verge of evaporating entirely. A firewall is the idea that either at or just outside a black hole’s event horizon, high-energy radiation surrounds it, frying whatever falls in before sucking it into a singularity. Combining them into “Hawking radiation firewall” and having that murder the crew while leaving the ship intact is what someone whose science education comes from memes on the internet might write into a television show.“
Science is full of great ideas and brilliant discoveries, and some of those more recent ones have made their way into the popular consciousness. TED talks, popular blogs and online magazines, and Facebook pages and internet memes have helped disseminate bits of knowledge to millions. But how much of what’s come through is actually worth knowing, versus how much is simply science-sounding buzzwords that’s content-free? As we dive deeper into the world of Star Trek: Discovery, that’s what I fear we’re looking at: the IFLS of a Star Trek series. Invented terms an misinterpreted legitimate science is the norm now, as though no one could be bothered to speak with a science consultant. It’s like the filming/script-writing crew is suffering from the same myopia as the crew of the Discovery: unable to look beyond of their own, bull-headed path.
The terms may sound smart, but this is jumping from science fiction into science fantasy, and leaving morality behind. Catch the latest recap, and hope it gets better from here!
Ask Ethan: Do Black Holes Grow Faster Than They Evaporate?
“Wondering why black holes wouldn’t be growing faster than they can evaporate due to [Hawking] radiation. If particle pairs are erupting everywhere in space, including inside [black hole] event horizons, and not all of them are annihilating one another shortly thereafter, why doesn’t a [black hole] slowly swell due to surviving particles that don’t get annihilated?”
So, you’ve got a black hole in the Universe, and you want to know what happens next. The space around it is curved due to the presence of the central mass, with greater curvature occurring closer to the center. There’s an event horizon, a location from which light cannot escape. And there’s the quantum nature of the Universe, which means that the zero-point-energy of empty space has a positive value: it’s greater than zero. Put them together, and you get some interesting consequences. One of these is Hawking radiation, where radiation is created and moves away from the black hole’s center. It occurs at a specific rate that’s dependent on the black hole’s mass. But another is black hole growth from the mass and energy that falls through the event horizon, causing that black hole to grow. At the present time, realistic black holes are all growing faster than they’re decaying, but that won’t be the case for always.
Eventually, all black holes will decay away. Come find out the story on when evaporation will win out on this week’s Ask Ethan!
Ask Ethan: What Happens When A Black Hole’s Singularity Evaporates?
“What happens when a black hole has lost enough energy due to hawking radiation that its energy density no longer supports a singularity with an event horizon? Put another way, what happens when a black hole ceases to be a black hole due to hawking radiation?”
One of the most puzzling things about Black Holes is that if you wait around long enough, they’ll evaporate completely. The curved spacetime outside of the event horizon still undergoes quantum effects, and when you combine General Relativity and quantum field theory in exactly that fashion, you get a blackbody spectrum of thermal radiation out. Given enough time, a black hole will decay away completely. But what will that entail? Will an event horizon cease to exist, exposing a former black hole’s core? Will it persist right until the final moment, indicative of a true singularity? And how hot and energetic will that final evaporative state be?
Incredibly, even without a quantum theory of gravity, we can predict the answers! Find out on this week’s Ask Ethan.