Shining a light on black holes
One year ago, the world was able to see the first image ever of a black hole. Due to the joint efforts of the Event Horizon Telescope (EHT) team, a worldwide scientific collaboration to simulate an Earth-sized telescope, we were able to obtain images of the black hole located at the centre of the M87 galaxy. This footage, not only showing the success of international scientific cooperation, brings the material proof of the existence of black holes, one hundred years after the publication of the general relativity theory by Albert Einstein.
In this post, I will bring you along a journey to the most mysterious places in the universe, and how this bizarrerie of nature came into light through the theories of physicists such as Albert Einstein, John Wheeler or Stephen Hawking.
An historical journey
One hundred years ago, Albert Einstein published his general relativity theory. This theory was meant to refine the ideas of Isaac Newtown on gravitation, providing a unified description of gravity as a geometric property of space and time.
According to this theory, every object in the universe is distorting the space-time structure around it. This distortion is directly proportional to the energy (or mass) and the momentum of the object of interest. To visualise this better, let’s imagine that we would ask two persons to maintain a blanket, horizontally flat, in the air. We would then ask a third person to come and place a bowling ball at the centre of this blanket. Can you visualise what’s going to happen to the blanket ? This is exactly what’s happening to the space-time structure (our blanket), with every single object in the universe.
From this starting point, we could theoretically imagine an object so dense that the curvature of space-time would be infinite. Einstein was strongly against this idea, as it would mean the end of space and time, at this very specific place in the universe.
However, Robert Oppenheimer, and other scientists, showed in 1939 that a massive enough star (for example, 20 times the mass of our sun) could actually collapse under its own gravity to a point of infinite density. Such a point of infinite density is called a singularity. Indeed, during its life, a star is balancing the effect of its own gravity that is pulling the matter inwards with nuclear fusion. The nuclear fusion during which a star is converting its hydrogen into helium, and later other heavier elements, creates an outwards thermal pressure that compensates this gravitational effect. However, at the end of its life, when a star has combusted all of its nuclear fuel, this balance no longer exists and the star will start collapsing under its own gravity. With enough mass, a star will have enough gravitation strength to completely collapse and concentrate all its mass into a singularity.
After 30 years of disinterest from the scientific community due to a focus on nuclear physics, John Wheeler, driven by the recent discovery of quasars, decided to investigate more those mysterious remnants of collapsed stars that he called ‘black holes’.
Black hole’s exhibit
Those black holes, as we know it already, have a singularity at their core, a place of infinite density where the space-time curvature is infinite. Black holes also possess a boundary, called an event horizon, that separates them from the rest of the universe. There, the escape velocity required to escape the gravitation pull of the black hole is equal to the speed of light. In other words, once you’ve passed this event horizon, there is no way to escape as nothing can travel faster than the speed of light. The radius of the event horizon, called the Schwarzschild radius, is directly proportional to the mass of the black hole. This surface has the particularity to evolve in size during the lifetime of a black hole. For example, it will increase each time matter or radiation falls into the black hole (which happens each time an object comes too close to the black hole and doesn’t maintain a stable orbit).
Besides its mass, a black hole is also characterised by its angular momentum and its electric charge. This is unfortunately a very low amount of information to be able to retrieve the original object which gave birth to a specific black hole. Indeed, a specific black hole could have been created from the collapse of a star as we know it (it is called a stellar black hole), but it could also have been created by the collapse of a giant gas cloud (we suspect them to be at the origin of the supermassive black holes at the centre of every galaxies), amongst many other possible combinations. This loss of information (for example, the shape of the object at the origin of the black hole) during the collapsing into a singularity is actually one of the biggest controversy of modern cosmology and there are huge debates regarding the fact that this information is either irremediably lost, or simply hidden from us inside the event horizon.
The heat is on!
Until 1974, it was commonly admitted that nothing could escape, nor be emitted by a black hole as nothing can escape its event horizon. However, it has been discovered that black holes have a temperature that is directly proportional to their surface gravity, and inversely proportional to their mass. Stephen Hawking discovered that black holes are actually able to emit particles and radiations. This phenomenon is known as Hawking radiation.
According to the uncertainty principle of quantum mechanics, pairs of particles and antiparticles are constantly materialising, then mutually annihilating each other in the vacuum. This phenomenon happens everywhere in the universe, including on the edge of black holes. In those extreme places, with extremely high gravity, it might occur, on a random basis, that one member of the pair falls into the black hole, while the other is ejected in outer space. This event would thus prevent those particles to annihilate each other and the rejected particle would then be seen as a radiation emitted by the black hole.
Earlier, I mentioned that when matter or radiation fall into a black hole, the mass and the radius of this black hole increase. The opposite is also true. As the black hole emits Hawking radiation, its mass and its radius will decrease, increasing even more the rate at which it emits radiation, and eventually leads the black hole to completely evaporate.
Space explorers and spaghettis
One question that people usually have in regard of black holes is the following: what would happen if ones would encounter one of those object, and worse, fall into one ? The question has been addressed in a very interesting way in the movie Interstellar but has to be nuanced.
First of all, the answer depends on the mass of the black hole. As we’ve seen, mass and radius are proportional when it comes to black holes. Thus, stellar black holes, with a few dozen times the mass of our sun, will have a very small radius compared to supermassive black holes at the hearth of the galaxies (they can reach billions of times the mass of our sun). As opposed to ones could expect, this makes the smaller black holes far more dangerous than the supermassive ones. Indeed, the small radius of the stellar black holes induces a way stronger tidal effects on any space explorer who would come too close to its event horizon compared to its supermassive counterpart. Due to a much higher gravitational pull on his feet compared to his head, this space explorer would be stretched out longways and squashed in sideways way before reaching the event horizon. This process is being called, in all its irony, spaghettification, and ends up with every single atom composing the body to be ripped apart from each other.
An explorer travelling towards a supermassive black hole, like M87, would be luckier, at least for a while. As the Schwarzschild radius of this black hole is extremely larger, the tidal effects that our space explorer would undergo orbiting this object would be extremely smaller compared to those of a stellar black hole (in the order of quintillions of times). This should allow our astronaut, under the condition that the black hole is not currently active (meaning that it is not absorbing some large amount of matter), to go through the event horizon. Indeed, when objects are being absorbed by black holes, they are being accelerated, around the event horizon, to a speed close to the speed of light due to gravitational effects. Those objects are then being heated at extremely high temperatures due to the collision of particles at such a speed, and are releasing extremely dangerous radiations. This body of matter around the black hole is called an accretion disk.
On his way to the event horizon, our explorer will start seeing everything around him with a bluish tone. This is due to the fact that the light trying to escape the black hole has a shorter wavelength. After travelling a bit longer, the astronaut should reach what is called the photon sphere. This is the place where light is orbiting the black hole. This means that the light reflected by the body of the astronaut would orbit around the black hole and come back to the astronaut. In other words, the astronaut would possibly be able to see himself by looking around him.
Due to time dilatation effects (remember Interstellar ?), an external observer (let’s say, the mother ship) who would see our astronaut going closer and closer to the black hole would actually see him going slower and slower until he appears frozen in time at the event horizon. Our astronaut would then appear redder and redder to the observer, before completely vanishing forever. This is due to the fact that light cannot escape the black hole.
Our explorer has now reached a point of no return, as he has now passed the event horizon. Every direction that he would now follow will irremediably lead him towards the singularity. This is a place where most of our current knowledge of physics are breaking down. It is thus impossible to tell with certitude what would happen to our explorer. However, it is still possible to make guesses.
While our courageous explorer would progress towards the singularity at a speed close to the speed of light, he might be able to see the outside universe as a sphere, getting brighter and smaller at the same time until he reaches a point of everlasting darkness. Soon, as he is getting closer to the singularity, the tidal effects that we have been mentioning when we were speaking about stellar black holes are going to be again a problem. Unfortunately for our explorer, his fate might likely be linked to spaghettification here as well. Unless …
It is theorised that a rotating black hole might not be able to create a classic infinitely dense singularity. Instead, the singularity could be a ring. This specific type of object is called a Kerr black hole. If this black hole would be massive enough, and inactive, it could potentially be possible to travel inside it without suffering the tidal effects associated with an infinitely dense singularity. What ones might encountered while traveling through this ring is left to imagination. Could our astronaut be transported in another point of space and time within our universe ? Or even inside another universe ? The possibilities are endless and fascinating!
