**10 things you should know about black holes**

**Sabine Hossenfelder**

[[Notice
the mysteries bvelow.]]

When
I first learned about black holes, I was scared that one would fly through our
solar system and eat us up. That was 30 years ago. I'm not afraid of black
holes anymore but I am afraid that they have been misunderstood. So here are 10
things that you should know about black holes.

1. What is a black hole?1. What is a black hole?

A
black hole contains a region from which nothing ever can escape, because, to
escape, you would have to move faster than the speed of light, which you
can’t. The boundary of the region from which you cannot escape is
called the “horizon.” In the simplest case, the horizon has the form of a
sphere. Its radius is known as the Schwarzschild radius, named after Karl
Schwarzschild who first derived black holes as a solution to Einstein’s General
Relativity.

**2. How large are black holes?**

The
diameter of a black hole is directly proportional to the mass of the black
hole. So the more mass falls into the black hole, the larger the black hole
becomes. Compared to other stellar objects though, black holes are
tiny because enormous gravitational pressure has compressed their mass into a
very small volume. For example, the radius of a black hole with the approximate
mass of planet Earth is only a few millimeters.

**3. What happens at the horizon?**

A
black hole horizon does not have substance. Therefore, someone crossing the
black hole horizon does not notice anything weird going on in their immediate
surroundings. This follows from Einstein’s equivalence principle, which implies
that in your immediate surrounding you cannot tell the difference between
acceleration in flat space and curved space that gives rise to gravity.

However, an observer far away from a black hole who watches somebody fall in would notice that the infalling person seems to move slower and slower the closer they get to the horizon. It appears this way because time close by the black hole horizon runs much slower than far away from the horizon.

However, an observer far away from a black hole who watches somebody fall in would notice that the infalling person seems to move slower and slower the closer they get to the horizon. It appears this way because time close by the black hole horizon runs much slower than far away from the horizon.

That’s
one of these odd consequences of the relativity of time that Einstein
discovered. So, if you fall into a black hole, it only takes a finite amount of
time to cross the horizon, but from the outside it looks like it take forever.

What
you would experience at the horizon depends on the tidal force of the
gravitational field. The tidal forces is loosely speaking the change of the
gravitational force. It’s not the gravitational force itself, it’s the
difference between the gravitational forces at two nearby places, say at your
head and at your feet.

The tidal force at the horizon is inversely proportional to the square of the mass of the black hole. This means the larger and more massive the black hole, the smaller the tidal force at the horizon. Yes, you heard that right. The larger the black hole, the smaller the tidal force at the horizon.

Therefore, if the black hole is only massive enough, you can cross the horizon without noticing what just happened. And once you have crossed the horizon, there is no turning back. The stretching from the tidal force will become increasingly unpleasant as you approach the center of the black hole, and eventually rip everything apart.

The tidal force at the horizon is inversely proportional to the square of the mass of the black hole. This means the larger and more massive the black hole, the smaller the tidal force at the horizon. Yes, you heard that right. The larger the black hole, the smaller the tidal force at the horizon.

Therefore, if the black hole is only massive enough, you can cross the horizon without noticing what just happened. And once you have crossed the horizon, there is no turning back. The stretching from the tidal force will become increasingly unpleasant as you approach the center of the black hole, and eventually rip everything apart.

In
the early days of General Relativity many physicists believed that there is a
singularity at the horizon, but this turned out to be a mathematical mistake.

**4. What is inside a black hole?**

**Nobody really knows.**General relativity predicts that inside the black hole is a singularity, that’s a place where the tidal forces become infinitely large. But we know that General Relativity does not work nearby the singularity because there, the quantum fluctuations of space and time become large.

**To be able to tell what is inside a black hole we would need a theory of quantum gravity – and we don’t have one. Most physicists believe that such a theory, if we had it, would replace the singularity with something else.**

**5. How do black holes form?**

We
presently

**know**of four different ways that black holes may form. [[She doesn’t really mean this – see below. DG]] The best understood one is stellar collapse. A sufficiently large star will form a black hole after its nuclear fusion runs dry, which happens when the star has fused everything that could be fused. Now, when the pressure generated by the fusion stops, the matter starts falling towards its own gravitational center, and thereby it becomes increasingly dense. Eventually the matter is so dense that nothing can overcome the gravitational pull on the stars’ surface: That’s when a black hole has been created. These black holes are called ‘solar mass black holes’ and they are the most common ones.
The
next common type of black holes are ‘supermassive black holes’ that can be
found in the centers of many galaxies. Supermassive black holes have masses
about a billion times that of solar mass black holes, and sometimes even more.
Exactly how they form still is not entirely clear. Many astrophysicists think
that supermassive black holes start out as solar mass black holes, and, because
they sit in a densely populated galactic center, they swallow a lot of other
stars and grow. However, it seems that the black holes grow faster than this
simple idea suggests, and exactly how they manage this is not well understood.
[[So this is not another way that we know black holes form. DG]]

A
more controversial idea are primordial black holes. These are black holes
that might have formed in the early universe by large density fluctuations in
the plasma. So, they would have been there all along. Primordial black holes
can in principle have any mass. While this is possible, it is difficult to find
a model that produces primordial black holes without producing too many of them,
which is in conflict with observation. [[So this is not another way we know
black holes form. DG]]

Finally,
there is the very speculative idea that tiny black holes could form in
particle colliders. This can only happen if our universe has additional
dimensions of space. And so far, there has not been any observational evidence
that this might be the case. [[So again this is not another way we know black
holes form. DG]]

**6. How do we know black holes exist?**

We
have a lot of observational evidence that speaks for very compact objects with
large masses that do not emit light. These objects reveal themselves by their
gravitational pull. They do this for example by influencing the motion of other
stars or gas clouds around them, which we have observed.

We
furthermore know that these objects do not have a surface. We know this because
matter falling onto an object with a surface would cause more emission of
particles than matter falling through a horizon and then just vanishing.

And since most recently, we have the observation from the “Event Horizon Telescope” which is an image of the black hole shadow. This is basically an extreme gravitational lensing event.

And since most recently, we have the observation from the “Event Horizon Telescope” which is an image of the black hole shadow. This is basically an extreme gravitational lensing event.

**All these observations are compatible with the explanation that they are caused by black holes, and no similarly good alternative explanation exists.****7. Why did Hawking once say that black holes don’t exist?**

Hawking
was using a very strict mathematical definition of black holes, and one that is
rather uncommon among physicists.

If
the inside of the black hole horizon remains disconnected forever, we speak of
an “event horizon”. If the inside is only disconnected temporarily, we speak of
an “apparent horizon”. But since an apparent horizon could be present for a
very long time, like, billions of billions of years, the two types of horizons
cannot be told apart by observation. Therefore, physicists normally refer to
both cases as “black holes.” The more mathematically-minded people, however,
count only the first case, with an eternal event horizon, as black hole.

What Hawking meant is that black holes may not have an eternal event horizon but only a temporary apparent horizon. This is not a controversial position to hold, and one that is shared by many people in the field, including me. For all practical purposes though, the distinction Hawking drew is irrelevant.

What Hawking meant is that black holes may not have an eternal event horizon but only a temporary apparent horizon. This is not a controversial position to hold, and one that is shared by many people in the field, including me. For all practical purposes though, the distinction Hawking drew is irrelevant.

**8. How can black holes emit radiation?**

Black
hole can emit radiation because the dynamical space-time of the collapsing
black hole changes the notion of what a particle is. This is another example of
the “relativity” in Einstein’s theory. Just like time passes differently for
different observers,

**depending on where they are and how they move, the notion of particles too depends on the observer, on where they are and how they move. [[!!]]**
Because
of this, an observer who falls into a black hole thinks he is falling in
vacuum, but an observer far away from the black hole thinks that it’s not
vacuum but full of particles. And where do the particles come from? They come
from the black hole.

This
radiation that black holes emit is called “Hawking radiation” because Hawking
was the first to derived that this should happen. This radiation has a
temperature which is inversely proportional to the black hole’s mass: So, the
smaller the black hole the hotter. For the stellar and supermassive black holes
that we know of, the temperature is well below that of the Cosmic microwave
background and cannot be observed.

**9. What is the information loss paradox?**

The
information loss paradox is caused by the emission of Hawking radiation. This
happens because the Hawking radiation is purely thermal which means it is
random except for having a specific temperature. In particular, the radiation
does not contain any information about what formed the black hole.

But
while the black hole emits radiation, it loses mass and shrinks. So,
eventually, the black hole will be entirely converted into random radiation and
the remaining radiation depends only on the mass of the black hole. It does not
at all depend on the details of the matter that formed it, or whatever fell in
later. Therefore, if one only knows the final state of the evaporation, one
cannot tell what formed the black hole.

**Such a process is called “irreversible” — and the trouble is that there are no such processes in quantum mechanics. Black hole evaporation is therefore inconsistent with quantum theory as we know it and something has to give. Somehow this inconsistency has to be removed. Most physicists believe that the solution is that the Hawking radiation somehow must contain information after all.**

**10. So, will a black hole come and eat us up?**

It’s not impossible, but very unlikely.

Most
stellar objects in galaxies orbit around the galactic center because of the way
that galaxies form. It happens on occasion that two solar systems collide and a
star or planet or black hole, is kicked onto a strange orbit, leaves one solar
system and travels around until it gets caught up in the gravitational field of
some other system.

But
the stellar objects in galaxies are generally far apart from each other, and we
sit in an outer arm of a spiral galaxy where there isn’t all that much going
on. So, it’s exceedingly improbable that a black hole would come by on just
exactly the right curve to cause us trouble. We would also know of this long in
advance because we would see the gravitational pull of the black hole acting on
the outer planets.

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