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When I first learned about black holes, I was scared that one would fly through our
solar system and eat us all 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.
First things first, 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.
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. 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. 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.
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.
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 one. Most physicists believe that such a theory, if we had it, would replace
the singularity with something else. How do black holes form?
We presently know of four different ways that black holes may form. 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.
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.
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.
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. All these observations are compatible with the explanation that they are caused
by black holes, and no similarly good alternative explanation exists.
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. 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. 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. 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
If you have any questions about black holes that this video did not answer, please leave
me a comment. And don’t forget to subscribe!