Ask Ethan: Does A Time-Stopping Paradox Prevent Black Holes From Growing?
“[F]or any object falling into a black hole, time slows down upon approach and comes to a standstill as the object reaches the event horizon. Reaching and passing that border would take an infinite amount of time measured by a distant observer… if ‘eating’ matter would take infinite time… how could supermassive black holes come into existence?”
From the perspective of an infalling particle, when you pass the event horizon of a black hole, you simply go straight through and head inevitably towards the central singularity. That should increase the black hole’s mass and cause the black hole and its event horizon to grow. On the other hand, from an external observer’s perspective, any particle that falls in will never be seen to cross the event horizon, creating an apparent paradox by where black holes would be disallowed from growing.
So, who’s right, and how do we reconcile these two points of view? Find out, and here’s the spoiler: black holes really do grow!
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: Did We Just Find The Universe’s Missing Black Holes?
“As interesting as this new black hole is, and it really is most likely a black hole, it cannot tell us whether there’s a mass gap, a mass dip, or a straightforward distribution of masses arising from supernova events. About 50% of all the stars ever discovered exist as part of a multi-star system, with approximately 15% in bound systems containing 3-to-6 stars. Since the multi-star systems we see often have stellar masses similar to one another, there’s nothing ruling out that this newfound black hole didn’t have its origin from a long-ago kilonova event of its own.
So the object itself? It’s almost certainly a black hole, and it very likely has a mass that puts it squarely in a range where at most one other black hole is known to exist. But is the mass gap a real gap, or just a range where our data is deficient? That will take more data, more systems, and more black holes (and neutron stars) of all masses before we can give a meaningful answer.”
Last week, an incredible new story came out: scientists discovered a massive object some 10,000 light-years away that emits no light of its own. From the giant star in orbit around it, we were able to infer its mass to a well-constrained range, with the mean value hovering right at 3.3 solar masses.The lack of X-rays from it, based on the field strength associated with neutron stars and the orbit of the giant star itself, very strongly indicates that this object is not a neutron star, but a black hole.
Does this mean we’ve discovered a black hole in the so-called “mass gap” range? Yes! But does it disprove the existence of a mass gap overall? Not so much. Come get the full story on this edition of Ask Ethan!
Ask Ethan: How Dense Is A Black Hole?
“I have read that stellar-mass black holes are enormously dense, if you consider the volume of the black hole to be that space which is delineated by the event horizon, but that super-massive black holes are actually much less dense than even our own oceans. I understand that a black hole represents the greatest amount of entropy that can be squeezed into [any] region of space expressed… [so what happens to the density and entropy of two black holes when they merge]?”
The entropy of a black hole, if you simply applied the laws of General Relativity (and nothing else), would simply turn out to be zero. By giving it a quantum description, however, we can get a meaningful formula for entropy: the Bekenstein-Hawking equation. When two black holes merge, the entropy is greater than even the pre-existing entropies combined.
If you think that’s weird, you might suspect that your instinct for density would also be incorrect. Sure, density is just mass divided by volume, but which volume do we use for a black hole? The volume of the event horizon? The volume of a (volume-less) singularity? Something else?
The question of how dense a black hole is has a lot of potential pitfalls, but if we follow the physics closely, we can answer it. Here’s how it’s done.
Is The Universe Filled With Black Holes That Shouldn’t Exist?
“What about at the high end of the stellar mass range of black holes? It’s true that pair instability supernovae are real and are indeed a limiting factor, as they don’t produce black holes. However, there’s an entirely separate way to produce black holes that is not particularly well understood at this time: direct collapse.
Whenever you have a large enough collection of mass, whether it’s in the form of a cloud of gas or a star or anywhere in between, there’s a chance that it can form a black hole directly: collapse due to insufficient pressure to hold it up against gravitation. For many years, simulations predicted that black holes should spontaneously arise through this process, but observations failed to see a confirmation. Then, a few years ago, one came in an unlikely place, as the Hubble Space Telescope saw a 25 solar mass star simply “disappear” without a supernova or other cataclysm. The only explanation? Direct collapse.”
As far as our best theories are concerned, the Universe isn’t filled with black holes of all different masses. Instead, the black holes that the Universe forms are inextricably linked to the processes by which the Universe makes the objects that then become black holes. From stars, there’s a theoretical lower limit of about 5 solar masses, and yet we saw a black hole of about 3 solar masses get created. There should be an enormous drop in black hole frequency above about 50 solar masses, but LIGO may be about to challenge that. And even at the highest end, there should be an upper limit to the masses of supermassive black holes, but a few of the ones we’ve found challenge that limit, too.
Does this mean the Universe is filled with black holes that shouldn’t exist? Or does it simply mean that we need superior models? Get the full story today.
Astronomers Find A ‘Cloaked’ Black Hole 500 Million Years Before Any Other
“The first stars should lead to modest black holes: hundreds or thousands of solar masses. But when we see the Universe’s first black holes, they’re already ~1 billion solar masses. The leading idea is black holes form and merge, and then rapidly accrete matter at maximal rates. But those rapidly growing black holes should be invisible, obscured by the dense gas clouds they feed upon. They were, until now. New observations have revealed the earliest “cloaked” black hole ever.”
How do black holes get so big so quickly in this Universe? It’s one of the great mysteries in astrophysics, but the hope has been that better observations of the first billion years of the Universe will help solve this puzzle. If the seeds of black holes can gather enough gas around them to feed on, they just might get there. But the difficultly then comes in locating and finding these obscured, “cloaked” black holes. While they’ve been found relatively nearby, telling us that such a phenomenon does occur, they’ve never been found at very early times: within the first billion years of the Universe.
Well, with new Chandra X-ray observations, all of that has changed. We found a cloaked black hole just 850 million years after the Big Bang. It might be the key to solving this cosmic puzzle at long last.
This Is Why Black Holes Must Spin At Almost The Speed Of Light
“Realistically, we can’t measure the frame-dragging of space itself. But we can measure the frame-dragging effects on matter that exist within that space, and for black holes, that means looking at the accretion disks and accretion flows around these black holes. Perhaps paradoxically, the smallest mass black holes, which have the smallest event horizons, actually have the largest amounts of spatial curvature near their horizons.
You might think, therefore, that they’d make the best laboratories for testing these frame dragging effects. But nature surprised us on that front: a supermassive black hole at the center of galaxy NGC 1365 has had the radiation emitted from the volume outside of it detected and measured, revealing its speed. Even at these large distances, the material spins at 84% the speed of light. If you insist that angular momentum be conserved, it couldn’t have turned out any other way.”
Have you ever wondered how black holes, ranging from a few times our Sun’s mass up to billions of times as massive, can spin so rapidly? Most black holes, as far as we can tell, are spinning very close to the speed of light: the ultimate speed limit of the Universe. Yet most stars, like our Sun, rotate extremely slowly: just once over a period of many days (or even longer).
So how does a slowly-rotating star, which goes supernova and forms a black hole, give rise to an object spinning near the cosmic speed limit? Find out today.
General Relativity Rules: Einstein Victorious In Unprecedented Gravitational Redshift Test
“The most interesting part of this result is that it clearly demonstrates the purely General Relativistic effect of gravitational redshift. The observations of S0-2 showcase an exact agreement with Einstein’s predictions, within the measurement uncertainties. When Einstein first conceived of General Relativity, he did so conceptually: with the idea that acceleration and gravitation were indistinguishable to an observer.
With the validation of Einstein’s predictions for the orbit of this star around the galactic center’s black hole, scientists have affirmed the equivalence principle, thereby ruling out or constraining alternative theories of gravity that violate this cornerstone of Einsteinian gravity. Gravitational redshifts have never been measured in environments where gravity is this strong, marking another first and another victory for Einstein. Even in the strongest environment ever probed, the predictions of General Relativity have yet to lead us astray.”
If you want to test Einstein’s General Relativity, you’ll want to look for an effect that it predicts that’s unique, and you’ll want to look for it in the strongest-field regime possible. Well, there’s a black hole at the center of our galaxy with 4 million times the mass of the Sun, and there’s a star (S0-2) that passes closer to it, during closest approach, than any other. In May of 2018, it made this closest approach, coming within 18 billion km (about twice the diameter of Neptune’s orbit) of the black hole, and zipping around at 2.7% the speed of light.
Did Einstein’s predictions for gravitational redshift come out right? You bet they did: 5-sigma, baby! Come get the full, amazing story here!
Ask Ethan: What’s It Like When You Fall Into A Black Hole?
“[W]hat is it like to be/fall inside a rotating black hole? This is not observable, but calculable… I have talked with various people who have done these calculations, but I am getting old and keep forgetting things.”
I get a lot of questions that people submit for Ask Ethan, but only rarely do they come to me from other scientists who tower above me in the field. This week’s question, from Event Horizon Telescope scientist extraordinaire Heino Falcke, asks me to help him visualize what it would look like if you fell into a black hole. Not just any black hole, mind you, but a realistic, rotating black hole. There’s really only one person on Earth who understands this well enough: Andrew Hamilton, who has devoted the last 15 years of his life to figuring out what it looks like and what it means when this actually happens.
So what did I do? I went and met Andrew, interviewed him, read his papers, and used his simulations to give everyone the best answer I could. I hope you love it, and I hope (even moreso) that I got it right!
Ask Ethan: How Does The Event Horizon Telescope Act Like One Giant Mirror?
“I’m having difficulty understanding why the EHT array is considered as ONE telescope (which has the diameter of the earth).
When you consider the EHT as ONE radio telescope, I do understand that the angular resolution is very high due to the wavelength of the incoming signal and earth’s diameter. I also understand that time syncing is critical.
But it would help very much to explain why the diameter of the EHT is considered as ONE telescope, considering there are about 10 individual telescopes in the array.”
Humanity has imaged a black hole’s event horizon! It’s been less than a month since the news was announced, and it’s still hard to get over what a phenomenal achievement it was. It’s very difficult to conceive of how, though, we can treat 8 different telescopes and telescope arrays, all stitched together, as acting like a single giant mirror. But that’s exactly what the Event Horizon Telescope did. In fact, that’s what it needed to do, or it would never have been able to achieve the resolutions necessary to construct the first image of a black hole’s event horizon.
But we have it! We achieved it! And here’s the conceptual way you can understand it, even if you barely understand the way a single telescope works.