In the tangled chambers of black holes, two fundamental theories about our world collide. Do black holes really exist? It seems that yes. Can the fundamental problems that emerge upon closer inspection of black holes be resolved? Unknown. To understand what scientists are dealing with, you will have to dive a little into the history of the study of these unusual objects. And we will start with the fact that of all the forces that exist in physics, there is one that we do not understand at all: gravity.
Gravity is the intersection of fundamental physics and astronomy, the border at which two of the most fundamental theories describing our world collide: quantum theory and Einstein's theory of space-time and gravity, aka general relativity.
Black holes and gravity
These two theories seem to be incompatible. And that's not even a problem. They exist in different worlds, quantum mechanics describes very small, and general relativity describes very large.
It is only when you get to extremely small scales and extreme gravity that the two theories collide and somehow one of them turns out to be wrong. In any case, it follows from the theory.
But there is one place in the universe where we could actually witness this problem, and maybe even solve it: the edge of a black hole. This is where we meet the most extreme gravity. But there is one problem: no one has ever "seen" a black hole.
Promotional video:
What is a black hole?
Imagine that all the drama in the physical world unfolds in the theater of space-time, but gravity is the only force that actually changes the theater in which it is played.
The force of gravity controls the universe, but it may not even be a force in the traditional sense. Einstein described it as a consequence of the deformation of space-time. And maybe it just doesn't fit into the Standard Model of particle physics.
When a very large star explodes at the end of its life, its innermost part collapses under its own gravity, as there is no longer enough fuel to maintain pressure against gravity. After all, gravity is still capable of exerting force, it seems like this.
Matter collapses and no force in nature can leave this collapse.
Over an infinite time, a star collapses into an infinitesimal point: a singularity, or let's call it a black hole. But in a finite time, of course, the stellar core will collapse into something of finite dimensions, and will still have a huge mass in an infinitely small area. And it will also be called a black hole.
Black holes don't suck everything around
Remarkably, the idea that a black hole will inevitably suck everything into itself is wrong.
In fact, whether you are orbiting a star or a black hole formed from a star, it doesn't matter as long as the mass remains the same. Good old-fashioned centrifugal force and your angular momentum will keep you safe and prevent you from falling.
It’s only when you engage your rocket brakes to interrupt the spin that you start to fall inward.
However, as soon as you start falling into black holes, you will gradually accelerate to ever higher speeds until you finally reach the speed of light.
Why are quantum theory and general relativity incompatible?
At the moment, everything is going to pieces, since in accordance with general relativity, nothing can move faster than the speed of light.
Light is a substrate used in the quantum world to exchange forces and transport information to the macrocosm. Light determines how quickly you can connect cause and effect. If you move faster than light, you can see events and change things before they happen. And this has two consequences:
- At the point where you reach the speed of light by falling inward, you also need to fly out of this point at an even higher speed, which seems impossible. Therefore, conventional physical wisdom will tell you that nothing can leave a black hole by breaking this barrier, which we also call the "event horizon."
- It also follows from this that the basic principles of conservation of quantum information are suddenly violated.
Whether this is true and how can we modify the theory of gravity (or quantum physics) are questions that many physicists are looking for answers to. And none of us can say which arguments we will end up with.
Do black holes exist?
Obviously, all this excitement would be justified only if black holes really existed in this universe. So do they exist?
In the past century, it has been conclusively proven that some binary stars with intense X-rays are actually stars that have collapsed into black holes.
Moreover, in the centers of galaxies, we often find evidence of huge, dark mass concentrations. These could be supermassive versions of black holes, likely formed by the merging of many stars and gas clouds that plunged into the center of the galaxy.
The evidence is strong but circumstantial. Gravitational waves allowed us to at least "hear" the merging of black holes, but the signature of the event horizon is still elusive and we have never "seen" black holes until now - they are simply too small, too distant and, in most cases, too black.
What does a black hole look like?
If you look directly into a black hole, you will see the darkest darkness imaginable.
But the immediate surroundings of the black hole can be bright enough as the gases spiral inward - slowing down due to the resistance of the magnetic fields they carry.
Due to magnetic friction, the gas is heated to enormous temperatures of several tens of billions of degrees and begins to emit ultraviolet and X-rays.
Ultra-hot electrons interacting with the magnetic field in the gas begin to produce intense radio emission. Thus, black holes can glow and can be surrounded by a ring of fire emitting at different wavelengths.
Ring of fire with a black-black center
And yet, right in the middle, the event horizon catches, like a bird of prey, every photon that gets too close.
Since space is curved by the huge mass of the black hole, the paths of light also bend and even form almost concentric circles around the black hole, like serpentines around a deep valley. This ring of light effect was calculated as early as 1916 by the famous mathematician David Hilbert just a few months after Albert Einstein completed his theory of general relativity.
After traversing the black hole multiple times, some of the light rays may escape, while others will end up in the event horizon. On this intricate path of light, you can literally peer into a black hole. And the "nothing" that appears to your gaze will be the event horizon.
If you took a picture of a black hole, you would see a black shadow surrounded by a glowing mist of light. We called this feature the black hole shadow.
Remarkably, this shadow appears to be larger than one would expect if we take the diameter of the event horizon as its origin. The reason is that the black hole acts like a giant lens, amplifying itself.
The shadow environment will be represented by a tiny "photon ring" due to the light that swirls around the black hole almost forever. In addition, you will see more rings of light appearing near the event horizon, but concentrating around the black hole's shadow due to the lensing effect.
Fantasy or reality?
Could a black hole be a real invention that can only be modeled on a computer? Or can you see it in practice? Answer: it is possible.
There are two relatively nearby supermassive black holes in the universe that are so large and close that their shadows can be captured using modern technology.
At the center of our Milky Way, there are black holes 26,000 light years away with a mass 4 million times the mass of the Sun and a black hole in the giant elliptical galaxy M87 (Messier 87) with a mass of 3-6 billion solar masses.
M87 is a thousand times farther away, but a thousand times more massive and a thousand times larger, so both objects will have approximately the same diameter of a shadow projected onto the sky.
See a grain of mustard in New York from Europe
Coincidentally, simple radiation theories predict that for both objects, radiation generated near the event horizon will be emitted at radio frequencies of 230 Hz and above.
Most of us only come across these frequencies when we have to go through a scanner in a modern airport. Black holes are constantly swimming in them.
This radiation has a very short wavelength - on the order of a millimeter - which is easily absorbed by water. In order for a telescope to observe cosmic millimeter waves, it must be placed high on a dry mountain to avoid absorbing radiation in the Earth's troposphere.
Basically, we need a millimeter telescope that can see an object the size of a mustard seed in New York from somewhere in the Netherlands. This telescope will be a thousand times sharper than the Hubble Space Telescope, and at millimeter wavelengths the size of such a telescope will be the Atlantic Ocean or larger.
A virtual telescope the size of Earth
Fortunately, we do not need to cover the Earth with a single radio dish, because we can build a virtual telescope with the same resolution, combining data from telescopes in different mountains around the Earth.
This technique is called aperture synthesis and very long basic interferometry (VLBI). The idea is quite old and proven over several decades, but only now it has become possible to apply it at high radio frequencies.
The first successful experiments showed that the structures of the event horizon can be investigated at such frequencies. Now there is everything you need to carry out such an experiment on a large scale.
Work is already underway
The BlackHoleCam Project is a European project for the ultimate image, measurement and understanding of astrophysical black holes. The European project is part of a global collaboration - the Event Horizon Telescope consortium, which includes more than 200 scientists from Europe, America, Asia and Africa. Together they want to take the first picture of a black hole.
In April 2017, they observed the galactic center and M87 with eight telescopes on six different mountains in Spain, Arizona, Hawaii, Mexico, Chile and the South Pole.
All telescopes were equipped with precise atomic clocks to accurately synchronize their data. Scientists recorded several petabytes of raw data, thanks to surprisingly good weather conditions around the world at the time.
Photo of a black hole
If scientists manage to see the event horizon, they will know that the problems that arise at the junction of quantum theory and general relativity are not abstract, but very real. Perhaps that's when they can be resolved.
This can be done by obtaining clearer images of the shadows of black holes, or by tracking stars and pulsars on their way around black holes, using all available methods to study these objects.
Perhaps it is black holes that will become our exotic laboratories in the future.
Ilya Khel