When it comes to the Universe, there’s one class of object that achieves more extreme conditions than any other: black holes. While other objects may be dense — the centers of an iron-rich planet, a white dwarf star, a uranium nucleus, or a neutron star — only a black hole packs so much mass into such a small volume that it actually creates an event horizon. An event horizon represents a boundary between our observable Universe, where we reside, and a black hole’s interior, where all infalling quanta inevitable head towards a central singularity. Once you cross over that horizon, from the outside to the inside, you can never escape no matter what you are, no matter how hard you fire a rocket or how fast you go: even if you travel at the speed of light.

But the black holes that we routinely observe actually emit many things that we can see directly: light of all different wavelengths, particles, spectacular jets, and event gravitational waves. If once something falls in, it can’t get out, how do we see all of these things? That’s what Patreon supporter Dominic Turpin wants to know, writing in to ask:

“There is something about Black holes that I have never understood. [They’re] so powerful that nothing crossing over the even horizon can escape, [but] they produce jets and winds, where we just found winds from Sagittarius A*. How can both things be true?”

It’s a confusing and counterintuitive puzzle, but it turns out that both things actually are true at once. Nothing can get out once it falls in, and we see all sorts of signals escaping from them. Here’s how to make sense of it.

The supermassive black hole at the center of our galaxy, Sagittarius A*, emits X-rays due to various physical processes. The flares we see in the X-ray indicate that matter flows unevenly and non-continuously into the black hole, leading to the flares we observe over time. In X-rays, no event horizon is visible at these resolutions; the “light” is purely disk-like.

Rather than start with a black hole and talk about the astrophysical conditions that occur around it, something that most of us aren’t particularly familiar with, I want to start with something that we’re a little more experienced with: a kitchen.

Imagine that you have a large amount of water: a giant pot, cauldron, or jug full of it, for example. Imagine, also, that you have a drain: a place that can easily accept all of that water, no matter how much of it you have. Even if the drain opening itself is small in terms of its physical size, as long as there’s enough room on the inside — room so that there’s a place for that water to “go,” so to speak — then every last drop of that water can fit through the drain, no matter how big your cauldron is or how small the drain opening is.

What you can’t do, however, is force all that water into the drain at the same instant. The smaller the drain opening is, and the more water that you have to put into it, the longer it’s going to take for the drain to accept all of that water. If you don’t want to make a mess, you’ll go slowly, and you’ll pour the water into the drain gradually and smoothly. If you don’t care about making a mess, you might dump the water in all at once, where much of it will splash about and collect around the drain, and only much later will it all make its way onto the other side of the drain.

When water in a sink encounters a drain, the water doesn’t immediately all go into the drain unless the flow is slow, doesn’t overflow the drain, and remains confined to a narrow area that goes directly into the drain. For all other cases, the water will have to flow near and/or around the drain before entering it, and has a more difficult time doing so the smaller the drain is.

If you try and force the water into the drain, at high pressure, for instance, you’ll make the messiest situation of all: the water will splash back at you, and might even wind up splashing up to great heights and large distances. If you have a large amount of suction coming from the other side of the drain, trying to draw the water in, you might be able to increase the rate-of-flow that the drain can accept, but you’ll also increase the speed that any splashed-back water travels at.

And the smaller the opening is to your drain, especially relative to the volume of water you’re trying to force into it, the larger the mess you’ll make. In total:

• the more water you pour in,
• at higher speeds and/or greater pressures,
• into a smaller drain opening,

the more the water will splash, and the greater a mess it will make over a larger distance outside of the drain itself. Anyone who’s ever turned a faucet on full blast, even directly over the sink drain, has had this experience: creating a wet mess, and often resulting in not just drops but large globs of water winding up much farther away from the drain than even the initial faucet was from it.

When a disturbance is created in a pond, such as by dropping a stone into an otherwise still body of water, it will generate ripples that propagate circularly outward. If water falls into an already-existing body of water, even if there’s an open drain at the bottom, that water can get kicked up and splashed out entirely, as though it were ejected from the environment around the drain, rather that getting sucked into the drain.

Credit: Sergiu Bacioiu/flickr

On the other hand, if you only pour the water into the drain slowly and smoothly, you won’t make a mess at all. If you had a drain opening that was enormous compared to the amount of water it was accepting, it wouldn’t make a mess at all. And if you used a device like a funnel, you might not spill a drop, so long as the spout of the funnel was smaller than the drain’s diameter. There are plenty of ways to pour your water into a drain without making a mess: you just have to be thoughtful in your choices of how to do it, and careful in the execution of how you’re doing it.

The reason why is straightforward: unless you pour the water in slowly enough so that the various drops, streams, or “glugs” of water only go directly into the drain itself, there’s going to be a pile-up of “splashable material” (i.e., the water itself) around the drain. That material can interact with itself, with the environment around the drain, and with the sink or tub that surrounds the drain. Under the right conditions, some of that material can even be ejected at high speeds, even though if it were to actually go into the drain, it would never come back out.

Instead of water flowing into a drain, a black hole can have material flowing into its event horizon: the region of space around it that serves as a boundary between what can escape and what can’t escape. From outside the event horizon, infalling material often can pile-up on top of itself, and not all (or even most) of that piled-up material will eventually wind up being devoured by the event horizon itself.

Credit: Big Think / NASA

It’s true that we’re not talking about drains here; we’re talking about black holes. And most of what falls into a black hole isn’t water-like at all, although, being made of normal matter, it can often behave as a fluid or a gas: where the particles within it “flow” in some sense. But what drains and black holes have in common is this: they have an “opening size” inherent to them (a surface area), and material can only flow into them through that finite area. While black holes can be large — supermassive ones can even be larger than Neptune’s orbit around the Sun — the material that typically impacts them is normally strewn across a far greater volume of space.

In other words, the drain opening, or the surface area of the black hole, is very, very small compared to the volume of matter that falls into it. Even if it’s material from, say, a single star that falls in, it’s very unlikely that you’ll have a direct hit: something where the star’s velocity points exactly towards the center of the black hole as it comes in. Most things in the Universe, and in a galaxy or galaxy cluster, have what we know as orbital angular momentum, and so it will instead be the black hole’s tidal forces that gradually or suddenly tear that object apart. When that happens, it winds up occupying a much, much larger volume of space than not only the original star (or stellar remnant) itself, but even larger than the black hole’s event horizon size.

This illustration of a tidal disruption event shows the fate of a massive, large astronomical body that has the misfortune of coming too close to a black hole. It will get stretched and compressed in one dimension, shredding it, accelerating its matter, and alternately devouring and ejecting the debris that arises from it. Black holes with accretion disks are often highly asymmetrical in their properties, but far more luminous than inactive black holes that lack them.

However, because it’s still made up of normal matter — particles that can be built out of the building blocks from the Standard Model — these particles can interact with one another on their way towards the black hole, which acts as a cosmic drain. Just as in the case of the drain with the water, this leads to a pile-up of material around the black hole. Typically, because of two reasons:

• that material has a net amount of angular momentum to it,
• and that it’s likely to be distributed, in three-dimensions, with one axis of the distribution being the shortest/smaller,

that material winds up contracting down into a rotating, two-dimensional structure around the black hole: a disk.

That’s what the term accretion disk is all about: a common term whenever discussions of black holes are had. The material gets largely (but not exclusively) confined to a disk because normal matter doesn’t just bounce off of each other perfectly when it collides with other particles of normal matter; rather it “sticks together” in what we call an inelastic collision. And when matter collides inelastically, it doesn’t just stick together, but by the conservation of energy, it also heats up. (Techincally, there are also “accretion flows” that can be either in-or-out of the disk, but the disk itself typically dominates.)

An illustration of an active black hole, one that accretes matter and accelerates a portion of it outward in two perpendicular jets. The normal matter undergoing an acceleration like this describes how quasars and active galaxies work extremely well. Flows of matter inside the accretion disk can lead to flares in a black hole’s emissions. All known, well-measured black holes have enormous rotation rates, and the laws of physics, particularly the conservation of angular momentum, all but ensure that this is mandatory.

Now, we’re getting somewhere. Even though a lot of this matter will inevitably fall into the drain that it’s circling, in the very process of “circling the drain” it collides and heats up, causing it to get even closer to the drain itself. And when that happens, because more gravitational potential energy is getting converted into kinetic energy, it now orbits the black hole at an even faster speed. Because of the properties of the event horizon, that matter begins moving close to the speed of light as it approaches the event horizon itself, and gets very, very hot indeed.

The temperature equivalent of the material in accretion disks routinely exceeds a million degrees (at this point, it doesn’t really matter which units you use: Fahrenheit, Celsius, or Kelvin), and can even crest 10 million degrees. That’s overkill, because above a few thousand or tens-of-thousands of degrees, all of the atoms you can make become fully ionized: into a sea of free atomic nuclei and a sea of free electrons. As the lightest charged constituents of matter, the electrons wind up moving the fastest, and when you have charged particles moving through space, they inevitably generate magnetic fields.

This illustration shows a model of what powers a microquasar: a downscaled version of a supermassive black hole within an active galaxy. The central black hole gains an accretion disk, which in turn generates its own powerful magnetic field. When an additional source of matter (at left) comes into play, the interaction between that new matter and the existing accretion disk can lead to flares, winds, and the emission of large numbers of charged particles and copious radiation, among other signals.

Magnetic fields, then, are incredibly important because one of the things that happens, in physics, is that if you pass a charged particle through a magnetic field, that magnetic field then accelerates the charged particle by changing its direction. In an accretion disk, with the magnetic field it generates, that accelerates particles in one of two directions: perpendicular to the disk, either “up” and out of the plane of the disk, or “down” and out of the plane of the disk. The disk itself is hot and emits radiation, but it’s the particles that get launched perpendicular to the disk, close to the speed of light, that escape.

This is the source of the majority of signals we see coming from black holes: jets, X-rays, flares, matter-and-antimatter particles, and even neutrinos. The matter that circles the drain:

• makes an accretion disk,
• which heats up and becomes ionized,
• where charged particles move very quickly around the black hole,
• creating magnetic fields,
• which deflect particles up-and-down, out of the plane of the disk,
• and those launched particles then create jets, flares, electromagnetic radiation, neutrinos, and even exotic particles like antimatter. (Where the antimatter, mostly in the form of positrons, then later annihilates, downstream, with electrons, creating gamma-rays.)

This picture was spectacularly confirmed with the discovery of “Fermi bubbles” of gamma rays around the Milky Way’s galactic center, caused by these accelerated particles when the Milky Way’s central black hole was active.

In the main image, our galaxy’s antimatter jets are illustrated, blowing ‘Fermi bubbles’ in the halo of gas surrounding our galaxy. In the small, inset image, actual Fermi data shows the gamma-ray emissions resulting from this process. These “bubbles” arise from the energy produced by electron-positron annihilation: an example of matter and antimatter interacting and being converted into pure energy via E = mc². We are certain that no antimatter signature in our galaxy arises from either antimatter stars or large clumps of antimatter.

This also helps us debunk another big myth about black holes: that they somehow “suck” everything into them. Black holes don’t “suck” anymore than other massive objects in the Universe do: neutron stars, white dwarfs, stars, etc. They simply gravitate, and the objects that orbit them orbit just as they would any theoretical mass at a given orbital radius. If normal matter didn’t “clump” and “stick” and “collide inelastically,” then it wouldn’t heat up, wouldn’t ionize, wouldn’t create magnetic fields, and wouldn’t get accelerated to produce these jets. It’s why only normal matter (and not dark matter) can create these signals!

The key that we have to recognize is that the signals that we see are not coming from within the black hole’s event horizon at all! Even though matter is being absorbed by the black hole, and the black hole is growing in mass over time, the matter that falls into the black hole is not what’s creating the jets, the radiation, the neutrinos, the cosmic rays, etc., that the black hole emits. It’s only the material that’s outside the black hole’s event horizon that creates it: the material that doesn’t fall in, but that spends time in the environment near the event horizon, and then gets accelerated and ejected so that it doesn’t cross the event horizon and fall in at all.

Messier 87, best known as the supermassive galaxy whose black hole was first imaged by the Event Horizon Telescope, has its relativistic jets and the shockwaves created by their material imaged in the infrared by Spitzer, amidst the mass of shining stars (in blue). Messier 87 is the most massive (and second-brightest) galaxy within the entire Virgo cluster of galaxies, and it is the central black hole that generates these relativistic jets.

You might then ask, as a follow up, “okay, if all of this is true, then when something does get tidally torn apart, and forms an accretion disk around a black hole, how much of that mass gets eaten versus how much of that mass gets ejected due to the process we just described?”

The answer, perhaps surprisingly, is that most of that infalling mass gets ejected, and only a small fraction of it crosses over the event horizon to the interior of the black hole. Although estimates vary, it’s typically 90-99% of the mass that encounters a black hole that inevitably escapes from it, and just 1-10% of the total mass that winds up getting swallowed: very different from the picture most people have in their minds!

This is why, when I explain it to children, I tell them that black holes eat like Cookie Monsters do. (Cookie monsters are more fun than drains. At least, they were to me when I was a kid.) Because even though they work to devour as much as they can, most of the material winds up getting strewn about and ejected instead, rather than winding up inside the black hole’s interior. It’s because the matter gets near, but remains outside, of the black hole’s event horizon that the emitted signals you see — jets, radiation, flares, particles, and more — can come into existence!

Send in your Ask Ethan questions to startswithabang at gmail dot com!

This article Ask Ethan: How are black holes active if nothing escapes from them? is featured on Big Think.

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