The Mysteries of Black Holes

Science fiction has envisioned black holes destroying Earth or enabling humans to travel to other parallel universes. In fact, when the Large Hadron Collider, a particle accelerator in Geneva, began operating in 2008, some scientists suggested that it could generate microscopic black holes. This sparked alarm over the possibility of Earth being consumed by one. But are these concerns justified? What would really happen if we fell into a black hole? And, more simply, what exactly is a black hole?

22 de mayo de 2024

What Is a Black Hole?

Put simply, a black hole is a highly compact object object with a gravitational pull so strong that even light cannot escape from it. To better understand what this means, let’s look at a simpler example: a group of middle school students goes on a field trip. While doing some experiments, they discover an abandoned well. Victor, Tony, and Christina climb onto the edge and start walking like tightrope walkers. When the teacher sees them, he yells, startling them and causing them to fall into the well. Will they be able to get out?

The first thing to know is that when Victor fell, he twisted his ankle and can hardly move it. On the other hand, Christina, who is a national high jump champion, is in very good shape. If the well is shallow, all three can simply reach up and grab the hands of their classmates who are trying to help them.

If the well is a bit deeper, requiring a small jump to reach the top, Victor probably won’t be able to get out. If the well is even deeper, requiring a big jump to reach a hand, only Christina might make it. Finally, if the well is so deep that even Christina can’t reach the top, all three will be trapped inside. Without ropes or any other means to rescue them, they would stay there forever, never reuniting with their families outside. A black hole is like a very deep well that not even the best jumper can escape from.

When Gravity Is Very Intense

Most of you have probably studied that gravity is a force experienced and exerted by all objects simply because they have mass. You might also remember that this force is directly proportional to the mass of the object exerting it and inversely proportional to the distance—specifically, the square of the distance—between the center of that object and the point where we measure it. This means that the more compact or concentrated an object is, the stronger its gravitational field will be on its surface.

Consider, for example, a very massive star. The gravitational field on its surface will be strong, but if we launch a rocket with enough speed, it will escape. Now, suppose the star is nearing the end of its life and begins to contract. Its matter becomes more concentrated and its gravitational field increases. So, to launch our rocket again, we would need to give it a higher speed than before. As the star becomes more compact, the required escape velocity will keep increasing.

There will come a point when it becomes impossible to send any object outward, and the only thing we can do is send light signals, which travel at a very high speed. If the star continues to contract, even the speed of light will no longer be enough to escape from the star. Actually, we should say “what used to be a star” because, at that moment, a black hole has just formed.

What Would Happen If We Fell into a Black Hole?

Near a black hole, the gravitational field can be extremely intense. However, in terms of our safety, that’s not what should worry us the most. Besides being very intense, the gravitational field can change very quickly with distance. So, for example, if we were falling toward the black hole feet first, the pull on our legs would be much stronger than the pull on our head, stretching us out like spaghetti.

This phenomenon, known as tidal force, is more pronounced in small black holes, while it is almost unnoticeable in very large black holes like the one at the center of our galaxy. In fact, we could be entering a black hole right now without even realizing it. What would happen once we reached its center is a different matter, but we’ll discuss that another time.

A BRIEF HISTORY OF TIME

If you want to know more about black holes and other mysteries of the Universe, don’t miss A Brief History of Time by Stephen Hawking, one of the world’s leading experts on the subject.

How Big Are Black Holes?

We’ve seen that a black hole can form when a star collapses. For this to happen, the star must have a sufficiently large mass. For example, the Sun will not turn into a black hole at the end of its life; instead, it will transform into a white dwarf (a type of star).

It is also widely accepted that most galaxies contain a very massive black hole at their center—several experimental observations support this hypothesis, and some have even been photographed. However, their origin is not entirely clear, and it’s possible that they didn’t all form in the same way.

All the examples we’ve discussed involve objects with masses exceeding twice that of the Sun. But can there be smaller black holes? And if so, how would they form?

In principle, black holes cannot exist at just any size. If the mass of a body is very small, to form a black hole, it would have to be concentrated in an extremely tiny volume. But when we consider very small distances, the effects of quantum physics come into play. One of these effects is that it is impossible to confine a certain amount of matter within an overly compact space. This means that black holes below a certain mass value, known as the Planck mass, which is roughly one hundred-thousandth of a gram, are deemed impossible.

Microscopic Black Holes?

For quite a few years now, a significant portion of theoretical physicists has been exploring ways to unify the Theory of Gravitation with quantum physics. Among the various proposals, the one that seems to have the most acceptance so far is String Theory. This theory achieves the desired unification but relies on some rather surprising hypotheses. One of them posits the existence of additional spatial dimensions beyond the three familiar ones (length, width, and height). To explain why we don’t perceive them in our daily lives, it is hypothesized that these extra dimensions are curled up on themselves.

String Theory is still a work in progress, with several versions in circulation. In some of them, the presence of these extra dimensions would make gravity more intense. If you think about it a bit, you’ll realize that one implication is that the volume needed to concentrate a certain amount of matter to form a black hole would not be so small. Or, put another way, it would be possible to have black holes with smaller masses than the Planck mass. The new lower limit would be around one TeV (a unit of measurement used in particle physics), which is over a million times smaller than the Planck mass.

An Experiment That Can Generate Black Holes on Hearth

If, as some theories suggest, black holes with a mass on the order of TeV can exist, then it’s possible to create them in the Large Hadron Collider in Geneva (LHC). After all, the energies at which particles collide in this accelerator are of a similar order of magnitude. Should we be worried?

To answer this question, we can ask another one first. Do black holes simply consume matter without ever emitting anything outward?

Back in the 1970s, Stephen Hawking discovered that this is not the case. When quantum physics is combined with the Theory of Gravitation, even if only incompletely, it reveals that a black hole emits radiation and a multitude of elementary particles. This phenomenon, known as Hawking radiation, causes the black hole to lose mass.

Therefore, we have two competing effects. On one hand, gravitational attraction increases the amount of mass confined within the black hole. On the other, Hawking radiation has the opposite effect. Hawking also demonstrated that the smaller the black hole, the higher the rate at which it radiates.

In microscopic black holes, the rate of radiation is much higher than the rate of matter absorption from the surroundings. This makes these black holes have a very short life and evaporate almost immediately after forming.

Therefore, we can rest assured. Even if the theories predicting the formation of black holes in the LHC were accurate, the lifespan of these entities would be extremely short, and they would have virtually no time to absorb surrounding matter.

WRITTEN BY Sandro Maccarrone
@smaccarrone