We are back this week with the third installment of the series – we are to discover what black holes are, and to finally leave everything prepared for the final chapter: black holes colliding 🙂
Today, many topics could be extended ad infinitum, and quite a few of you have emailed me back asking for more in previous installments, so several footnotes have been added, branching into related topics so that you can either google whatever interests you, or ask me for a couple of ideas around it. Let us begin by remembering the key pieces from the previous two chapters:
1. Time and space are both part of the same thing – spacetime.
They influence each other and thus concepts like simultaneity depend on who is looking – that is, spacetime is relative and there is no “absolute time” or “absolute space”. (Relative, however, does not mean “without rules”!)
2. Energy (and matter, as matter is merely a super-concentrated form of energy) and spacetime are also not independent things: energy tells spacetime how to curve, and spacetime tells matter how to move.
The higher the energy concentration, the bigger the curvature of spacetime. That’s why the Sun manages to have so many objects orbiting around it: the curvature is big enough to prevent all these objects from escaping (they would need to move much faster!).
It is finally the time for us to learn about what a black hole is. So let us get this off the way directly: we have no clue 🙂
Think about this: a black hole represents such a concentration of matter that the curvature of spacetime is so brutal that nothing can escape from it. And nothing means nothing: light that were emitted from inside the black hole would see a spacetime so curved that it would not be able to escape outside, and thus, from our side we would (and do) see nothing but darkness1.
So, how do black holes form? There are two answers for this, as there are two types of black holes: supermassive ones [mass of thousands-to-billions of Suns] typically located in the center of galaxies, and for which the short answer is that we do not know, and stellar black holes. These form when stars die –depending on the mass of the original star, the result might be a white dwarf, a neutron star, or a black hole2. When the star is massive enough, once it runs out of thermonuclear fuel, the pressure is so big that the star will collapse towards its core now that it has no counterbalance from thermonuclear fusion, and send a shockwave that will send out most of its outer material in an incredible burst of light (a supernova), leaving the center remnant: a stellar black hole.
We truly don’t know what goes on inside a black hole: spacetime is so warped inside that nothing can escape –no information is emitted about what is going on inside1. The only thing we know is, in fact, that the black hole is not the black thing. You see, the black shell that covers the black hole is what we call an event horizon. It means that anything happening beyond that boundary cannot possibly affect us: since light is the fastest thing in the universe4, if light cannot make it, then nothing can. Spacetime is so warped beyond the boundary that if you tried to approach the black hole, outside observers would see you forever frozen at the boundary as time slows down completely due to the huge gravitational effects on spacetime. You, however, would see yourself entering normally because your own sense of time would not be affected at all – unfortunately, you would have no way of telling us what happens on the other side, as you would have no way of escaping and we would be puzzled at your image completely standstill on the event horizon3. The next image pictures the “array of possibilities” for a particle standing inside and outside the black hole. Outside, provided you have enough energy to overcome the gravitational pull of the black hole (or, as we learned in the previous chapter, to overcome the curvature of spacetime around you), you can move in any direction you want. Inside, however, the pull is so strong / spacetime is so warped, that moving in any direction that is not towards the black hole is simply not an available choice!
[From the Wikipedia: possible paths]
This second picture uses a small boat to provide a graphical example:
You’ll notice, if you look at the center of the picture, that there is something called singularity in the center of the black hole. This singularity actually represents the true black hole, in the sense that it is the true, final product of the collapse: an infinitely dense point (all the mass of the black hole is concentrated there, at a point with zero volume), in which spacetime would become so warped that it would stop making sense: simply put, the laws of physics should all break down. This is why we stated above that we just don’t know: not only we cannot observe the singularity due to the event horizon, but also we cannot even be sure whether true singularities exist or whether they are the mathematical artifact of a theory5[if you only read one footnote, let it be this one] that has reached the point where it breaks down (in this case, due to incredibly high densities).
This begs for a last question: how big are these things? For example, a stellar black hole with 10 times the mass of the Sun would be just ~60km across. On the other hand, a black hole with the size of the Earth (~12,500 km across) would have a mass of around 4,000 Suns, or about 1 billion Earths. Rather impressive, isn’t it?
Let’s summarize our knowledge about black holes:
- A singularity in the middle, a point with infinite density and all the mass of the black hole where the (known) laws of physics break down and we have no idea of what truly goes on there.
- An event horizon, signaling the beginning of the region where not even light can escape from the black hole. Time completely stops at the boundary.
- In principle, empty space in the middle (or matter absorbed by the black hole going from the event horizon to the singularity).
- Even though you are thinking of these things as voracious matter absorbers, causing destruction at their surroundings, reality is a bit underwhelming: they do indeed swallow surrounding matter, but simply due to their gravitational effect. Our entire galaxy orbits a huge black hole and we are not even close to getting eaten by it – you can orbit a black hole just like you orbit the Sun!
Our next stop would finally be the collision. Unfortunately, we have reached our Newsletter size limit! We will have to wait until December for the series conclusion, when we will finally discover what happens when two black holes collide!
If you made it this far, congratulations once again! The end of the series is now quickly approaching – stay tuned until then!
———————————-footnotes——————————————
1 Actually, quantum theory predicts that black holes must emit so-called Hawking radiation, leading to evaporating black holes – a very small one (e.g., with the mass of our sun), however, would take much longer than the age of the the universe to evaporate, so don’t wait up! The reasoning involves the fact that the vacuum is not actually a vacuum – and weird, quantum stuff happens when you add a black hole to the mix. In any case, this light is too dim for us to observe from the Earth, so experimental proof will probably have to come from an indirect effect of this radiation.
2 Stars die when the thermonuclear power produced in their core is no longer enough to counterbalance the enormous pressure of their own weight. For smaller stars it results in a white dwarf, formed essentially by carbon and oxygen nuclei in a sea of electrons [this is so-called degenerate matter, and all these former atoms now form the quantum equivalent of the classical ideal gas], and which slowly cools down until it no longer emits any light and becomes a black dwarf [these have never been directly observed, as they emit no light, but they are a natural consequence of the formation of a white dwarft]. However, bigger stars have a denser, iron core, and when they collapse they do so into neutron stars: pressure is so large that the atoms themselves cannot withstand it, and electrons and protons fuse together into neutrons. This type of star is so dense that a single teaspoon would contain around ~10 billion tons (for example, the total amount of waste produced by the world in 2015, or the total amount of plastic producted since the sixties). This gives us a better idea of how empty matter actually is: an atom has a size of roughly 0.03-to-0.3 nanometers (i.e., an atom is around as big for us as we are for the Sun), and its nucleus has a size of 0.000001 nanometers (i.e., the nucleus is bigger for us than we are for the entire solar system) – it’s actually incredible that we do not fall through things! For even bigger stars, this collapse will result into black holes –refer to the main body for details.
3 The image would, however, change its wavelength with time, from visible to infrared, to radio, microwave… in an effect that is known as redshift. Furthermore, there are a myriad articles describing what would happen to you if you were to approach a black hole (e.g., depending on its size, tidal effects would stretch you into an spaghetti shape -not in a relative way but in a “dead” way- long before you actually reached the event horizon; always an interesting read.
4 Speed has an upper bound, given by the speed of light, that can only be achieved by particles without mass such as the photon (the particle of light). Accelerating a particle with mass requires increasingly high amounts of energy (that’s why you cannot run as fast as you want – not only is your body not physiologically prepared to do so, your legs cannot exert enough force fast enough for you to accelerate further!), and this energy required becomes infinite once you get “infinitely close” to the speed of light. It’s also the reason the LHC particle accelerator has a diameter of 27 kilometers – if you want to get particles to a speed very close to the speed of light you need to give them massive amounts of energy in a controlled manner, which requires this kind of impressive, cutting-edge infrastructure.
5 Just to be clear: a scientific theory is not a hypothesis. Science works based on the premise that what we know is true based on what we currently know. That is: nothing is ever “proved” to be true. In this sense, while in mathematics (and in real life) a counterexample suffices to prove something wrong, in real life there are no “proofs”. A hypothesis is an assumption, something proposed for the sake of argument to test its veracity. A theory, however, is a principle that has been formed to explain things that have already been substantiated by data, the result of experiments and observations against a hypothesis and that lend it a much higher degree of veracity. So high that it is the highest that one can get in science (no, a “law” is not “higher” on the scale!). There is nothing safe from being proved wrong, and theories need not be universal. For example, Newtonian mechanics (the one with the three laws that most of us studied in high school) is a particular case (the low-speed case) of special relativity, just like special relativity is a particular case (no gravitation) of general relativity. Never fall for the “it’s just a theory” trap – not only it’s a bogus argument, it’s a clear signal that the other person does not understand how science works!
Kike Miralles 
