Two fundamental theories describing our world collide in the tangled halls of black holes. Are there really black holes? Apparently so. Is it possible to solve the fundamental problems that emerge upon closer examination of black holes? Unknown. To understand what scientists are dealing with, you will have to immerse yourself in the history of the study of these unusual objects. And we 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 point of fundamental physics and astronomy, the boundary on which two of the most fundamental theories describing our world collide: the quantum theory and Einstein’s theory of space-time and gravity, aka the General theory of relativity.
Black holes and gravity
These two theories seem incompatible. And it’s not even a problem. They exist in different worlds, quantum mechanics describes the very small, and General relativity describes the very large.
Only when you get to extremely small scales and extreme gravity do these 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 black hole boundary. This is where we meet the most extreme gravity. But there is one problem: no one has ever “seen” a black hole.
What is a black hole?
Imagine all the drama in the physical world unfolding in the theater of space-time, but gravity is the only force that actually changes the theater it plays in.
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 the pressure against gravity. After all, gravity is still capable of exerting force, like this.
Matter collapses and no force in nature can leave this collapse.
In infinite time, the star collapses to an infinitely small point: a singularity, or let’s call it a black hole. But in finite time, of course, the star nucleus collapses into something that has finite dimensions, and will still have a huge mass in an infinitely small region. And it will also be called a black hole.
Black holes don’t suck in everything
It is noteworthy that the idea that a black hole will inevitably suck everything into itself is wrong
In fact, whether you’re spinning around a star or a black hole formed from a star, it doesn’t matter if the mass remains the same. The good old centrifugal force and your angular momentum will keep you safe and will not let you fall.
And only when you turn on your rocket brakes to interrupt the spin will you start falling inside.
However, once you start falling into the 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 ashes, because according to GRT 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 it has two consequences:
- At the point where you reach the speed of light, falling inside, you also need to fly out of this point at an even greater speed, which seems impossible. Hence, ordinary physical wisdom will tell you that nothing can leave the black hole by breaking this barrier, which we also call the “event horizon”.
- It also follows that the basic principles of quantum information preservation are suddenly violated.
Whether this is true and how 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 what arguments we will come to in the end.
Are there black holes?
Obviously, all this excitement would be justified only if black holes really existed in this Universe. So do they exist?
In the last century, it has been convincingly proved that some double 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, probably formed by the fusion of many stars and gas clouds that have sunk into the center of the galaxy.
The evidence is compelling, but circumstantial. Gravitational waves have allowed us to at least” hear “the fusion of black holes, but the signature of the event horizon is still elusive and we’ve never” seen ” black holes before — they’re just too small, too far and, in most cases, too black.
What does a black hole look like?
If you look right into a black hole, you’ll see the darkest darkness you can imagine.
But the immediate environment of a black hole can be quite bright, 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-ray radiation.
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 radiating at different wavelengths.
Ring of fire with black-black center
And yet, at the very center of the event horizon catches, like a bird of prey, every photon that comes too close.
Since the space is curved by a huge mass of 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 effect of the ring of light was calculated already in 1916 by the famous mathematician David Hilbert just a few months after albert Einstein completed his General theory of relativity.
After repeatedly traversing the black hole, some of the rays of light can escape, while others will be in the event horizon. On this intricate path of light, you can literally peek into a black hole. And “nothing” that will appear to your eyes, will be the horizon of events.
If you took a picture of a black hole, you would see a black shadow surrounded by a glowing fog of light. We called this feature the shadow of a black hole.
Notably, this shadow seems larger than you would expect if you take the diameter of the event horizon as the starting point. The reason is that the black hole acts like a giant lens, amplifying itself.
The shadow’s environment will be represented by a tiny “photon ring” because of the light that circles around the black hole almost forever. In addition, you will see more rings of light appearing near the event horizon, but concentrating around the shadow of the black hole due to the lensing effect.
Fantasy or reality?
Can a black hole be a real fiction, which can only be modeled on a computer? Or can it be seen in practice? Answer: maybe.
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 at a distance of 26,000 light-years with a mass of 4 million times the mass of the Sun and a black hole in a giant elliptical galaxy M87 (Messier 87) with a mass of 3-6 billion solar.
M87 is a thousand times farther away, but a thousand times more massive and a thousand times larger, so both objects will have about the same diameter of the shadow projected onto the sky
See the mustard seed in new York from Europe
By coincidence, simple radiation theories predict that for both objects, radiation generated near the event horizon will be emitted at radio frequencies of 230 Hz or higher.
Most of us face these frequencies only when we have to go through a scanner at a modern airport. Black holes are constantly bathed in them.
This radiation has a very short wavelength – about a millimeter-that is easily absorbed by water. In order for the telescope to observe cosmic millimeter waves, it must be placed high on a dry mountain to avoid absorption of radiation in the earth’s troposphere.
In fact, we’re going to need a millimeter telescope that can see an object the size of a mustard seed in new York city from somewhere in the Netherlands. This telescope will be a thousand times sharper than the Hubble space telescope, and for the millimeter wave range, the size of such a telescope will be from the Atlantic ocean or larger.
Virtual telescope the size of the Earth
Fortunately, we don’t need to cover the Earth with a single radio, because we can build a virtual telescope with the same resolution by combining data from telescopes in different mountains all over the Earth.
This method is called aperture synthesis and very long baseline interferometry (VLBI). The idea is quite old and proven for several decades, but only now it has become possible to use it on high radio frequencies.
The first successful experiments have shown that the structures of the event horizon can be investigated in such frequencies. Now there is everything necessary to conduct such an experiment on a large scale.
Work is already underway
The BlackHoleCam project is a European project for the final 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, the Americas, Asia and Africa. Together they want to take the first picture of the black hole.
In April 2017, they observed the galactic center and the M87 using 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.
Black hole photo
If scientists manage to see the horizon of events, they will know that the problems that arise at the junction of quantum theory and GRT are not abstract, but very real. Perhaps that is when they can be resolved.
This can be done if you get a clearer image of the shadows of black holes, or track the stars and pulsars on their way around the black holes, using all available methods for the study of these objects.
Perhaps it is black holes that will become our exotic laboratories in the future.