Are Black Holes Really Cosmic Vacuum Cleaners?

We've all seen the sucking power of black holes. We've all seen a black hole indiscriminately devour stars, planets, and Hollywood actors in a headlong rush to sate its voracious appetite. The entertainment industry has provided us with a vivid image of a black hole as a cosmic vacuum cleaner.

But what does science say? Is the actual black hole given to us by science similar to the fictional image of black holes presented for our viewing pleasure?

Even though astrophysicists have found indirect evidence that black holes exist elsewhere, no black holes seem to inhabit our neighbourhood of the galaxy. Consequently, we cannot use a "hands-on" approach to discover their properties for ourselves. However, this indirect evidence gives scientists confidence that present theories accurately predict what happens near a black hole. This makes it possible for anyone with access to a (personal) computer to turn their computer room into a virtual lab for black hole experiments!

The animation to the right lets you interactively explore the qestion "Are black holes cosmic vacuum cleaners?" while parked safely in front of your computer screen. In the process, you will uncover fascinating aspects of orbital motion around black holes. Why should scientists have all the fun!

The display shows a particle orbiting a black hole, with the black hole coloured red in order to represent danger. If the black hole has the same mass as the sun, the diameter of the red circle is about 6 kilometres, and the width of the black display area is about 60 kilometres, but a more realistc mass for a black hole that has resulted from stellar collapse is 10 solar masses. Then the diameter of the red circle is about 60 kilometres, and the width of the display area is about 600 kilometres. This scale is used for the animation.

Notice how the particle moves from the edge of the display to a position near the black hole and then moves away without being devoured by the black hole. At the edge of the display the particle is nearly 300 kilometres from the black hole, and at closest approach, the particle is a mere 38 kilometres from the black hole. Some cosmic vacuum cleaner. A real cosmic vacuum cleaner would have a reach of more than a few kilometres.

How, then, did the myth that black holes swallow anything that gets near them get started? Likely from the most fundamental property of a black hole - it is physically impossible for an object to esacpe a black hole once it falls inside a black hole. But falling inside a black is not the same as closely approaching a black hole. With the aid of powerful enough rockets, an object can approach arbitrarily closely to a black hole without fear of being trapped. As seen in the display, even without rockets, freely falling objects moving along appropriate paths can approach quite closely to a black hole without falling inside.

Consider another interesting example: replace the sun by a black hole of the same mass. What happens to the Earth? The swallowing of the Earth by the black hole is one thing that does not happen. Granted, things would get cold quite quickly because of the continuous night, but the Earth's orbit would not be altered.

Virtual Experiments with Black Holes

A set of suggested experiments appears below. The ording of the experiments has a certain consistency, but the experiments are somewhat independent, so you can pick and choose if you want. However, the Warmup Exercises provide helpful familiarity with how to interact with the animation, so everyone should at least glance at these once before trying their hands with the experiments. Also, the difficulty level roughly increases as the experiment number increases.

WARNING: Experiment 8 is substantially more difficult than the other experiments, requires a greater level of comfort with mathematics, and is still in a very sketchy form.

Because some people like a challenge, many of the experiments contain questions and links to answers. Don't attempt to answer these questions unless you really want to, but if you attempt some questions, try to withstand the temptation to look up an answer before making an honest effort. If the temptation proves too strong, ...

Feel free to modify these experiments, and to think up your own experiments.

Have fun!

If the display turns white, checking trail or clicking the Start button will restore the display to black. Unfortunately, any trail that has built up will be lost.

Warmup Exercises

Experiment 1: Falling Into a Black Hole

Experiment 2: Escape Velocity

Experiment 3: Investigation of Stable Circular Orbits

Experiment 4: Investigation of Unstable Circular Orbits

Experiment 5: Investigation of Spiral Orbits

Experiment 6: Gravity Bends Light Rays!

Experiment 7: Boundaries Between Orbits of Various Kinds

Experiment 8: Orbital Precession and Closed Orbits

References

Numerical Methods