Debunking: Black Holes

By: Christopher Hartland

Black holes are, in my personal opinion, one of the most fascinating objects in our universe. It is perhaps, the mystery surrounding them that makes them so interesting. As with many scientific concepts, there are multiple common misconceptions about black holes spread by popular science fiction and various misinterpretations of the facts.


What is a black hole?

When stars come to the end of their lives, the resulting object is determined by how massive the star is. A star with a mass greater than 25 times that of the sun will eventually collapse under its own weight, creating a black hole – an object with an ‘event horizon’ within which the gravitational pull is so strong that nothing, not even light, can escape. Due to this gravitational pull, we cannot see into a black hole, and are only aware of their existence due to their effect (through gravity) on the objects and light around them.

Common misconceptions

The most common misconception about black holes seems to be that they act like giant vacuum cleaners – sucking everything in and not allowing anything to orbit them. It is a mistake that can often be seen in science fiction; one such example being a 2006 episode of Doctor Who, in which a planet is described as ‘impossible’ because it is in orbit around a black hole. This is something that even I, admittedly, used to believe. However, when you think carefully about this idea, it is actually quite easy to see that it is incorrect.

Our planet is not spiralling towards the sun and our galaxy, which has a black hole at its centre, is not shrinking. A strong gravitational pull does not necessarily prevent one object from orbiting another. It is entirely possible for a planet, or any other object, to orbit a black hole. In fact, black holes are surrounded by discs of hot gas – accretion discs – that orbit around the event horizon. If an object in orbit were to be ‘nudged’ off of its orbit and towards the black hole, then once beyond the event horizon it would not be able to escape the pull of the black hole and would be ‘sucked in’. However, as long as it remains in orbit, this will not occur.

Our lack of an ability to see inside a black hole lends itself to speculation about what may happen if a person were to fall inside. One common theory is that anyone who falls into a black hole would exit it in another location entirely (be that another part of space or a parallel universe, a scenario depicted in numerous fictional works such as Event Horizon, Doctor Who and Star Trek). This could well be the result of people confusing black holes with wormholes (an entirely theoretical concept of a sort of gateway from one point in space to another). Of course, no one truly knows what is inside a black hole; perhaps they do form some sort of interdimensional portal, but this seems unlikely and falling into a black hole would be a rather brutal experience in which a person would be stretched by the gravitational pull in a process known as spaghettification. It’s hard to imagine the possibility of someone surviving such a journey, much less finding themselves on an interdimensional adventure.

Perhaps drawn from the ‘vacuum cleaner’ perception of black holes, a conclusion that some people come to is that black holes grow in size as they consume material. The truth is that over time black holes shrink. The theory, proposed by Stephen Hawking, is that black holes ‘evaporate’ by losing mass and energy through Hawking radiation. The idea of radiation escaping a black hole may seem contradictory to the very definition of a black hole, but it is visible in numerous ways. While it is true that no matter can leave the black hole once it is within the event horizon, radiation can be emitted from outside of it. For example, jets of plasma can be flung from the accretion disc due to an electric field generated by a magnetic field within the disc (quasars are thought to be black holes that do this).

Are black holes still interesting if the myths are untrue?

To the best of our knowledge, at the centre of a black hole is a singularity – a single point with infinite density. Singularities violate the accepted laws of physics, meaning that either the ‘invisible’ part of a black hole does not contain a singularity, or that the laws of physics need to be adjusted. ( The fact that there is so much yet to understand about the universe leaves us with a lot of potential; how can anything that we don’t understand not be interesting? Even when it comes to science fiction, an accurate representation of black holes can be just as good, if not better, than an overly-fictionalised version. Take the 2014 film Interstellar as an example of a successful film with a scientifically accurate representation of a black hole (barring the climax of the movie which strays into speculation).

While they may not be giant vacuum cleaners or portals to other universes, black holes remain incredibly intriguing.

Extra exploring:
Black holes and worm holes: physics of the universe
HubbleSite’s awesome “create a black hole” app

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