Black holes, Interstellar, etc.

What would we see when orbiting around a black hole?

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. 

Going fast

Unless we can spend huge amounts of energy, it is not possible to be at rest close to a black hole: the thrust we would need to make in order to counterbalance the black hole gravity would be too large by an incredibly huge amount. Therefore, we need to orbit the black hole if we want to make something realistic. But since gravity is huge close to the black hole, he centrifugal force originating from our orbital motion must also be huge, and since the black hole distance is small, our orbital velocity will be very large. Now, forgetting about the black hole, a large velocity (whether or not orbital) does in itself induce several optical phenomena. The most well-known (but actually the least important) is the Doppler shift: a green light source will appear bluer for an approaching observer, and redder for a receding one. But there are some least known although more important phenomena to be taken into account. The photon flux is always greater when one approaches a light source, because one has to wait less time between consecutive photons. Therefore, the approaching observer will experience a brighter flux than a receding one. Finally, it is well-known that when one drives a car as rain is falling, the apparent direction of the water droplets as seen from within the car change with the car velocity, in the sense that the faster the car goes, the more horizontally the water droplets seem to fall. In other words, water droplet seem to originate form a direction that is all the more close to the direction of motion that the car velocity is large. For the very same reasons, such effect, called aberration, also exists for light. This means that the faster you move, the more the landscape you are heading to will shrink. This is very counterintuitive! Here are a few pictures:

Let us start from an observer a rest, looking at the sky we are familiar to.

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:

At one sixth and one third of the speed of light – click to enlarge

At one half and two thirds of the speed of light – click to enlarge

At 83% and 99.5% of the speed of light – click to enlarge

As you can check, the color change is far from being the most spectacular feature one would see (should one survive to the trip). So, should you experience this, interstellar travel should look like:

• An acceleration phase, where the sky would fold into the front direction, becoming there very bright, and very dim elsewhere
• A travel phase, where almost nothing would notably change from the boring bright spot of the last image
• A deceleration phase, where the celestial sphere would unfold, thus revealing a possibly different sky than the one at start in case you travelled far enough.
  

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.

Extreme gravity

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:

• Each star position has changed when we added the black hole. They all tend to be pushed away from the direction toward which we set the black hole.
• This position change goes together with some distortion for extended objects such as the Large Magellanic Cloud.
• More importantly, any star now has a ghost image. The ghost image is (almost) always dimmer than the primary image, and closer to the black hole. The segment joining both images always goes through the geometric center of the black hole silhouette.
  
• Most of the stars luminosity is unchanged, except for a few ones which have a large increase in luminosity (as well as their ghost image). This corresponds to stars which are close to being exactly behind the black hole. Such phenomenon is the so-called gravitational lensing. Any foreground celestial object can produce some gravitational lensing, provided it is seen from a sufficiently large distance so that the deflection of light it makes is larger than its apparent diameter. Only black holes and possibly neutron star can therefore make gravitational lensing when seen from such a small distance.
  

If you want to have some extra information, you may be interested in this dedicated web page that was made a few years ago.

Combining both: orbiting around a black hole

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:

Cruising much closer to the black hole, at 50% of the speed of light. Will you find Orion this time?

Download video here (warning, somewhat big file, ~140 Mb)

Several Black holes?

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?

There is a big black hole to the left, and a small black hole to the right. To the left of the big black hole, one sees the banana-shaped secondary image of the small black hole. And to the right of the small black hole, we see the bean shaped secondary image of the big black hole. That's still easy...

Now, what happens if the small black hole goes behind the bigger one? As we saw, a point like object is scattered on a shell around the foreground black hole. So it happens here!

And of course, we can also put the small black hole in front of the big one:

The shell, aka the big black hole, is thicker than in the previous case, which should sound logical if you are not too confused. When the black hole are aligned with the observer, we now only see one image of each instead of two. So, what soes it look like if we make a movie of this? Click here. But for what reason could one be interested in lookisng at combined distortions made by a black hole pair? Saying more could make this webpage non spoiler free, so let us say that part of the answer lies in M. Thorne's book, The Science of Interstellar.

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Last modified 7 November 2014