Black Holes: stop making them sound more intimidating than they really are!

Last night’s tv featured an episode of the science programme Horizon, entitles Who’s Afraid of a Big Black Hole?
The programme featured a posse of eminent physics brains such as the ubiquitous Michio Kaku (who by the way should really get his hair cut – he’s starting to look far too image conscious) and Lawrence Krauss (featured in my previous post). These big brains were all saying things like “Black holes – weird!” and “Black holes – mind-boggling!”. It was easy to get the impression that the scientists were hamming it up for the cameras to some extent, and that their contributions were then edited to create even more of a Wow! factor. Such is the nature of popular science programmes.
That’s not a problem of course. In fact it’s probably essential in order to grab the attention of the easily distracted amongst the viewing population, which is most of us.

There is a problem with black holes though.
They are indeed weird and mind-boggling.
But then, what isn’t? Everything in the universe is weird and mind-boggling. It’s just because we’re so familiar with some of it, like tables and chairs and trees and rivers, that we don’t notice.
Remember the saying:

“We only tend to think that reality is weird when we contemplate its extremes,
such as the core of the atom
or the edge of the universe.
But the place is actually weird all the way through.”

Whenever black holes are described in the popular press or popular science literature the emphasis is always on their extraordinary gravitational pull (which is created as a result of their stupendous mass – the fact that they consist of the amount of matter in a star compressed to the size of a pinhead, or smaller). This gravitational pull means that anything that gets sucked into a black hole disappears from our universe due to the fact that even light cannot escape from its clutches. Impressive stuff, true. And scary sounding.
(A black hole is created when a star runs out of fuel and stops generating heat. It was this heat that made the star expand into a glowing sphere – once the heat’s gone the force of gravity that holds the star together is the only force at work, and the star is pulled inwards into an ever tighter ball.)
After the programme someone mentioned to me “What I don’t understand is – why doesn’t the whole universe just get sucked into these black holes, they’re so powerful?” A good question, and a quite understandable one due to the huge amount of hyperbole that surrounds black holes.
Here’s my explanation.

When a star collapses from the size of a star to the size of a pinhead the matter in it becomes compressed and more concentrated (to put it mildly). It doesn’t gain any mass.
For instance, look at the sun. The sun has a mass of 2 nonillion kilograms (2 followed by thirty zeros kilograms). If the sun were to collapse to the size of a pinhead that pinhead would also have a mass of 2 nonillion kilograms.
To take a more earthly example, if you take a big football-sized ball of fluffy cotton wool and scrunch it up so that it’s a dense little golfball size it doesn’t get mysteriously heavier when you scrunch it. It just gets denser, with the strands of cotton wool closer together. So it is with the star that’s compressed to the size of a pinhead. The atoms get closer together (and then the subatomic particles get closer together, and so on).
If the sun were to shrink to the size of a pinhead, and thus become a black hole, what effect would this have on the earth? You may think “Oh my God, we’ll be sucked into it, because that’s what black holes do. We’re all doomed!”
However, you’ve actually got nothing to worry about, you’ll be pleased to hear (For the purposes of this illustration I’m ignoring the inconvenient fact that before the sun can turn into a black hole the earth will inevitably have already been destroyed by other solar activity).
Here’s why there’s nothing to make you lose any sleep.
The earth is in a stable orbit around the sun. The characteristics of this orbit (the distance from the sun: the speed of the orbit) are determined by the gravitational pull of the sun on the earth, which is related to the sun’s mass (2 nonillion kilograms, remember: you don’t have to remember exactly what a nonillion is though). It’s the sun’s mass that’s important, not its size. As long as there’s a 2 nonillion kilogram object at the centre of the solar system we’ll keep orbiting it quite happily. It doesn’t matter whether that object is the size of the sun or the size of a pinhead.

The popular press often gives the impression that once a black hole’s been created it starts hoovering up matter from the rest of the universe willy-nilly, due to its immense gravitational pull, eventually consuming the whole place.
However a black hole doesn’t hoover up the universe any more than a planet hoovers up the universe. Meteors fall to earth never to escape just as objects fall into black holes never to escape. A black hole only hoovers up stuff that’s within its sphere of influence, which is no different to that of the star from which it was formed. To be sucked into a black hole an object has to be quite close to it. If it’s a reasonable distance away from the black hole, the black hole is just like any other massive object as far as its gravitational attraction goes.

Whenever black holes are mentioned there’s almost inevitably a mention of ‘What would happen if you fell into a black hole?’. Well, you’d be annihilated of course. But what do you expect? Remember however that a black hole used to be a star – a large object – that has shrunk to a ridiculously small size. To get close enough to a black hole for it to be a problem for you you’d have to get really close to the black hole – closer to it than the original radius of the star from which it was formed. If the black hole were still a star rather than a black hole you’d be inside the star, so you’d be annihilated anyway. Which is worse? Being pulled apart by the gravity of a black hole or being fried by the heat of a star? But do we worry about being annihilated by stars? Not really. They’re just too familiar.
And they’re shiny and twinkly.
Not black.
Or holes.

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2 Comments

Filed under Science

2 responses to “Black Holes: stop making them sound more intimidating than they really are!

  1. Craig

    Nice article.
    However ad I understand it the gravity does become stronger as gravity is proportionate to mass and density of an object. Shown that light can escape from a star(we can see it) but not from a black hole.

    However anything outside the event horizon would orbit it just as it would any other object with enough mass.

  2. Don Boyd

    I’m not worried about our Sun becoming a black hole and potentially sucking us into nothingness; I’m more concerned about the Sun becoming a red giant and
    frying us alive. As you can tell I tend to worry a lot about things over which I have no control.
    But it was still a fascinating read.

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