Two black holes, with different properties, shown with different rendering choices. Left picture is mine, right is from Interstellar, courtesy of Warner Bros. Entertainment Inc..
A black hole is a region of space that is able to
prevent anything, even light, from escaping. Such a region is
therefore, by construction, completely black, but its silhouette
can be seen it there is some light source that can delineate it.
In the Interstellar movie, the light source is the black
hole accretion disk, i.e., the light emitted by some matter that
is orbiting around the black hole. As it orbits around, friction
between between disk particles produce some heat, which make the
particles slowly spiral toward the black hole, and, more
importantly here, produce some light is sufficiently large amount
so as to be seen. Actually, such an accretion disk can be
extremely hot and thus emits a large amount of visible light and
an even larger amount of X-rays. Such an active disk would a be
deadly to witness by any living being. The accretion disk that is
depicted in the Interstellar movie is therefore in an
unusually non active state, or, in astrophysicists' vocabulary, in
a quiescent state. In what follows, we choose to remove the disk
completely (since such a quiescent state may be physically
possible but very unusual), and to somewhat increase the celestial
sphere luminosity. This is sufficient to delineate very accurately
(at least as accurately as in the movie, I would say), the black
hole silhouette. In practice, the celestial sphere shows around
200,000 stars, which is quite a lot as compared to the few
hundreds of stars that we see in urban areas, or the few thousands
that we can see away from any significant light sources.
Amateur astronomer will recognize, on top from
left to right, Sirius, Orion, Taurus and the Pleiades cluster. We
have significantly enhanced eye sensitivity here, so that we can
see (much) more stars, as well as their actual colors, but all is still familiar here. We are
going to accelerate from rest to 99.5% of the speed of
light. Such acceleration is either very long, or very
dangerous! If you assume a constant acceleration a, then
the time t to get close to the speed of light, c
is such that the quantity a t / c is
greater than but of order of 1. If your acceleration is of order of
Earth surface gravity, then t is several years, and if you
want t to be of order of a few minutes, then the
acceleration is of order of 500,000 Earth gravity, a figure which is, of course
instantaneously deadly. That's why interstellar travel seems
completely impossible, and why in the Interstellar movie
one has to find a way to evade this. Forgetting about this, here
is how the sky aspect changes as we go faster and faster:
Want to see a video of the first phase? Click here (big file, ~150 Mb).
In the video, as well as on the above snapshot, there are a
few numbers. It is pretty easy to figure out what they mean.
A black hole is able to trap light in its interior because of the extreme gravity it produces. But of course, such gravity will also drastically affect its vicinity. The most spectacular consequence of this is that light ray passing close to a black hole will be deflected, possibly by a large amount. If we imagine a black hole that appears in foreground of some celestial sphere, then light ray originating from the celestial sphere will be deflected on their way toward us. This will introduce some significant distortion in the picture. For example, compare the two following images:
Click to enlarge to full resolution (2.5k x 2k)
They show how a region of the sky (around the Large Magellanic Cloud in the southern hemisphere) is distorted by the presence of a foreground black hole. What you should notice after careful inspection is the following:
If you want to have some extra information, you may be interested
dedicated web page that was made a few years ago.
Once you are familiar (more or less) with the
above, you can consider thinking to what happens if you orbit the
black hole, i.e., thinking to a non still version of last picture.
As you orbit the black hole, the stars that are exactly behind it
change all the time, and so does their luminosity and position.
The visual impression is that background stars somehow "dance"
around the black hole. Some people find it nice, and quite
relaxing. Let's hope it will be the same for you.
A snapshot of the movie you can download below (click to enlarge).
Can you recognize a distorted Orion constellation and its even more distorted secondary image? Red Betelgeuse is the key.
As shown on the screen, we are too far to the
black hole to go fast: we only cruise at less than 20% of the
speed of light, which is unambitious. What if we want to go
faster? It suffices to go closer to the black hole:
Just as stars often form binary systems, it is
possible that black hole binaries exist and originate from the
evolution of massive star binaries. In case the two black holes
are sufficiently close to each other, then some interesting
phenomena occur. Unfortunately, an exact description of a close
black hole binary gravitational field is one of the most
challenging problem gravitational physics, so that some
approximations have to be done here. The simplest one consists in
considering non rotating, electrically charged black holes. If
their electric charge is of same sign, then it is possible to tune
it so as the gravitational attraction is exactly counterbalanced
by electrostatic repulsion. Such a pair of black hole cannot form
a binary system since they cannot orbit around each others, but
one can at least have a sketch of the combined set of distortion a
pair of black hole produces. Below we show what an observer at
rest would see when looking at a close pair of black hole that
would be moved with respect to each other by non gravitational
forces, thus mimicking a pseudo-orbital motion. We expect to see
the silhouette of both black hole. But forgetting the nature of a
given black hole, we also expect to see the distorted ghost image of its
silhouette because of the presence of the second black hole, and
vice-versa. So we should see actually four black hole silhouettes.
Can you spot them below?