The idea of black holes has captured the imagination of astronomers, physicists, and sci-fi fanatics and has stirred up several questions and theories. To this day, many inquiries remain unanswered, and many concepts are still unknown. However, thanks to brilliant astrophysicists and decades of observations, we are getting closer to grasping the processes that created our universe. Black holes are one of these mysteries.
We have had a talk with Ole Eggers Bjælde about this interesting yet mysterious phenomenon.
Let’s get started.
But What Is a Black Hole?
We can imagine a black hole as kind of a super-magnet. Except, instead of magnetism, the ability to attract metals, it possesses extreme gravity, which attracts everything. Gravity is the reason we do not just float off into space every time we jump.
The Earth’s gravity constantly wants to pull us towards its center, stopping once we hit the surface.
For a black hole, however, gravity is much stronger than what we experience here on Earth, and it does not really have a surface to stop you.
That is why it is like falling into a hole – but we will get back to that later.
Most people know that a black hole is created when a large star collapses into itself, becoming extremely dense. The matter of an entire star in a much smaller volume. And – as all mass creates some gravity – the more mass, the more gravity.
This means that, technically, not only is the Earth attracting you, but you are also attracting the Earth. However, the gravity your body creates is much weaker compared to a planet because of the difference in mass, or weight.
That is why astronauts seem “lighter” on the moon. It has less mass than Earth and, therefore, a weaker gravitational pull.
It is still mind-blowing just how compressed a black hole really is. According to Ole Eggers Bjælde, a star with the mass of our Sun may be the size of a small city, once it collapses into a black hole.
It is super, super dense.
To put that into perspective, the Sun has a diameter of roughly 1.4 million kilometers, while a small city may be around 10 kilometers from one side to the other. Imagine the entire Eiffel Tower squeezed into the size of a single grain of rice.
That is a massive reduction!
So, calling them holes is only accurate regarding what they do to their surroundings. Not regarding what they are. That gets really complicated really quickly.
Bear with me as we move on.
If a Black Hole Is Not Really a Hole, What Is It?
Well, technically it is a sphere.
A sphere-shaped volume of space. Black holes are phenomena made up of two aspects: a singularity in the middle and a hypothetical border called the event horizon.
The singularity at the center is a point with a virtually infinite density that has no volume and to which everything is attracted. This is the core and the reason black holes have the properties they do. Theoretically, this point is infinitely tiny and infinitely dense.
If you stood on the North Pole, for example, the northern hemisphere would be attracting you stronger than the southern one, as gravity also depends on distance.
This means that, technically, you are at any moment not experiencing all of Earth’s gravity equally, the parts closer to you have more pull, getting weaker the further away you get. If, however, all this mass was in a much smaller volume, the gravity would be more intense.
- Ole Eggers Bjælde
It is the density that separates a star from a black hole, as their mass would be the same. High density means concentrated mass and, therefore, concentrated gravity.
The event horizon is what we imagine from movies and illustrations as a black hole, a dark circle in space. It is a perimeter around the center, also called a point of no return, which, when crossed, instantly pulls everything right to the center. Even though the event horizon seems like a border or edge, it is not solid, there is nothing there.
”So, it’s not a real surface. But that’s why we call it an event horizon. When we talk about a black hole, we talk about everything that sits within this event horizon. Even if we know that all the mass will be in the center, we still talk about the whole thing as the black hole,” Ole Eggers Bjælde explains.
At this hypothetical edge, you get too close to the singularity itself, and the gravity pulls you in. Even when photons, particles of light with an incredible charge and speed, cross the event horizon, they get pulled in and can’t escape.
With that in mind, black holes are not technically objects as they are mostly made of vacuum.
All the matter is concentrated in the singularity itself. The rest is just empty nothingness that looks black as anything that comes inside falls to the center and becomes part of the singularity in a fraction of a second.
“If we believe in the theory, it is so compressed that everything will be situated in something that has no volume in a point (singularity) in the center of this black hole. So, it is actually not an object as such because the most part of the black hole is actually completely empty,” – Ole Eggers Bjælde
Why Is It Technically Impossible to See the Inside and the Outside of a Black Hole?
We covered that anything past the event horizon can never leave the black hole because of the gravitational pull. This makes it impossible to send in any type of sensor, probe, or brave astronaut inside as there is no way any information or signal could escape the gravity.
More significantly, not even light, and its photons, can leave a black hole.
That is why the “surface” of the event horizon looks jet black as all light is absorbed. To be able to see anything, either with our eyes or a camera, the light must travel to the object and bounce off back towards us to get the image.
We can represent the travelling photons as throwing a paper plane in windy weather. The strong wind blowing in one direction will be our gravity. Throwing it with the wind is easy and fast, but if you asked a friend to throw it back to you against the wind, no matter how much force was used, it would probably not get back to you.
Any type of fancy high-tech camera, such as thermal or infrared, that can capture spectrums invisible to the naked eye would also not work. By definition there is nothing that can be taken or pulled out of a black hole, a picture included. And since the event horizon is not a real surface, it also can`t reflect light and be seen.
A black shirt looks black because it absorbs most of the visible light spectrum, which makes up a fraction of all electromagnetic waves. A black hole absorbs the entire electromagnetic spectrum, from radio waves and microwaves all the way to radioactive gamma rays.
That is more black than black itself.
That means looking at a black hole is the true definition of seeing nothing, as no light can make its way back to your eyes.
- Ole Eggers Bjælde
So How Come There Are Pictures?
We have only managed to observe directly and photograph two black holes. The pictures of M87*, and a more detailed image of Sgr A* became public in 2022. All other ones were only detected because of their impacts on their surroundings. As they are invisible with the current understanding of physics, we can only see mass spiraling around and inside them, deducing that a black hole must be causing this.
However, as gasses are pulled in by the gravitational field, they rotate around and into the black hole, almost like water draining out of a bath, but instead, it happens in three dimensions.
As this happens, the gas is heated to millions of degrees, turning into plasma and letting off a glow. And as this happens before the hot gasses cross the event horizon, light and x-rays can escape into space, later to be detected by our telescopes. That is why a picture of a black hole can exist. We can see plasma spiraling around a black dot and disappearing.
It would be impossible to see a dark sphere against dark space, but thankfully, this super-heated disc of plasma acts as somewhat of a backdrop light. This allows us to see the majestic images of a black hole “shadow”.
The black hole is only “visible” if it is attracting and consuming these super-heated cosmic gasses. Once everything has been absorbed, it goes back to being literally invisible.
Black holes are such a fascinating and complicated topic. This article just scratches the surface of what astrophysicists know about them.
If you are interested in learning more about how they affect time, whether traveling through wormholes is possible, or how astronomers study our universe, tune into episode 3 of our podcast, Big Ideas Only.