Black Hole

Allen Versfeld's picture
 
Fictional Black Hole

Science fiction movies love a good Black Hole.  It's dark, mysterious, dangerous and you can wrap all sorts of interesting plots behind it.  Unfortunately, the traditional bad science penned by script writers gets even worse when black holes are added.  Let's have a look at some of the properties black holes exhibit in fiction:

  1. They are unstoppable celestial vacuum cleaners, sucking up anything that strays close by.
  2. If you fall into a black hole, you will be teleported somewhere else in the universe or even travel through time.  
  3. Black holes are the entrances to wormholes.
  4. Black holes usually have beautiful yet sinister colourful spidery stuff swirling around them.
  5. Black holes are noisy places, with atmospheric rushing sounds and deep dramatic bass tones throbbing throughout space.
So how does this stack up to reality?  What is a black hole really?  To answer the question, you need to properly understand what gravity is and how it works.  Black holes are all about gravity.
 
Isaac Newton

Any object has mass.  Isaac Newton's Theory of Universal Gravitation tells us that when something has mass, it attracts all other objects in the universe towards itself. This attraction is called "Gravity", and the strength of that attraction is proportional to the mass of the object.  Gravity is actually an extremely weak force - the gravity attracting your body to objects in your home is so weak that even the most sensitive equipment cannot easily measure it and you certainly don't notice it's effects.  Even extremely large things like mountains don't have an obvious effect, although scientists in the 19th century were able to measure the gravity of mountains using complicated arrangements of pendulums and heavy counterweights.  It's only when we start looking towards a planetary scale that we start to get a strong gravitational field.  

The second thing that Newton tells us about gravity is that its effect on an object depends on how far away that object is.  If you stand in the region of the Earth's surface (a little under 6400 km from the centre of the earth), Earth's gravity pulls you towards its core with a strength of about 1G  ('G' means, literally, the strength of gravity at the surface of the earth.  Physicists prefer to measure gravity in meters per second squared, referring to the speed at which anything will fall in a given gravitational field).  If you double your distance to 12800 km, you find that gravity has weakened to 0.25G.  Every time you double your distance, gravity weakens by a quarter, if you triple it then gravity drops by a ninth, if you quadruple then it drops by a sixteenth, and so on.  This is called the "Inverse Square Law".
 
As you move back down to the surface, the gravity field becomes stronger according to the same law.  Interestingly, if we were to dig a tunnel down towards the centre of the Earth, we would find gravity actually weakening as we descended.  As we get deeper, the amount of rock above us increases and all that rock over our heads exerts its own gravity which acts to cancel out the gravity of the earth still beneath our feet.  Imagine, though, that we could somehow compress the entire earth, squeezing it into half it's size.  You've got the same mass of rock, but the surface is now only 3200 km from the core.  By the inverse square law, gravity at the new surface is four times stronger than what we're used to.  In practical terms, you would weigh four times as much as you do now.  If we could repeat this compression, we would soon end up with a very small world with unimaginably strong gravity at the surface.  But the most important thing to realise at this point, is that if we were to be suspended on a platform at the original 6400 km height from the core, we would still only experience the familiar gravitational field of 1G.
 
Albert Einstein

If we could continue with our experiment of compressing the Earth and not stop till it was the size of a beach ball, we would start to notice very strange things happening, as predicted by Albert Einstein in his General Theory of Relativity.  Gravity distorts the fabric of space and time (A popular piece of techno-babble in science fiction that also happens to be an analogy used by real life physicists when trying to explain the nature of the universe to people like us) in a manner rather like your own bodyweight stretching the surface of a trampoline.  The heavier you are, the deeper the impression you make, and anybody standing near you on the same trampoline will find themselves leaning in towards you.  It's just an analogy, of course, but it helps to explain the phenomenon of gravitational lensing.  This distortion of space means that a beam of light (which must always travel in a straight line) seems to bend when moving through a strong gravity field.  Of course, it really is moving in a straight line through space, but space itself is bent by gravity.  On the surface of our imaginary beach-ball sized earth, the distortion has become so severe that light gets dramatically distorted, rather like looking through a goldfish bowl (Click the image below for a simulation of this effect - it shows a black hole moving past, with a galaxy in the background).  If we were to continue compressing the earth, smaller and smaller, we would eventually reach a point where the lensing becomes so extreme that a beam of light could never leave the surface -- it would be bent all the way back and return to where it came.  This is called a Black Hole.  Light will leave the surface and reach a maximum distance before being drawn back.  That maximum height marks out a boundary known as the Event Horizon, and this is where things really start getting weird.

 
Animation of gravitational lensing from a simulated black hole passing in front of a galaxy

This distortion of space-time has one truly bizarre consequence:  time dilation.  If we place a clock at various distances from a black hole, and observe it through a telescope from very far away, we will find that as the clock gets closer, it runs more slowly.  This happens because time itself is stretched out by the immense gravity - time moves more slowly for the clock than it does for us.  If we were to swap places, placing ourselves very near the event horizon and looking back at a distant clock, we would see the clock (and the rest of the universe) running along in fast forward.  

 
More conventionally, close to the event horizon, we would feel very strong tidal forces.  The inverse square law comes into play again.  If you're standing on a platform just above the event horizon, the gravity at your feet is much much stronger than the gravity at your head.  If you were not supported by the platform, the gravity difference would be so great that your feet would accelerate faster than your head and you would literally be stretched apart.  In fact, as you get close to the event horizon, the tidal forces are powerful enough to rip molecules apart, and this causes a great deal of energy to be released.  Despite the powerful attraction of the black hole, the matter spiralling in becomes so hot from tidal forces that it beams out ultra-violet light, gamma radiation and X-Rays.
 
Black hole accretion disc, with jets

So what have we established so far?  A black hole is simply the region of space around an object with an extremely powerful gravitational field, bounded by the event horizon.  Objects falling in will experience time dilation (time slows down for them), irresistible tidal forces (ripping their very atoms apart) and a powerful sustained blast of hard radiation.  This causes some problems for the sci-fi predictions.  Let's have a look at them one by one:

  1. Unstoppable vacuum cleaners:  Not really.  As we explained while discussing the inverse square law, the gravity of a black hole is no different to the gravity from any other object.  You can orbit a black hole, a space craft can fly away from it, a black hole can itself orbit other heavier objects.  The only difference is that it's radius is small enough that you can get very close to the centre of gravity.  When this happens, you expose yourself to the effects listed above.
  2. Black Holes are the entrances to wormholes:  If wormholes exist (and this is something quantum physicists have predicted but not verified), they will be extremely small.  Even an atom would be too wide to fit through a wormhole.  Real wormholes have absolutely nothing in common with the popular sci-fi version.  But if we assume that wormholes leading from one point in the universe to another do exist, and that physical objects like space-ships could fit through them, and that their ends are marked by black holes, there's two problems:  Nothing in the universe can survive the conditions near the event horizon of a black hole, and even if future technology allows some sort of shielding to protect an explorer, time dilation means that he will experience time slowing further and further till he reaches the event horizon.  From his point of view, as he passes through the event horizon, the rest of the universe will zip past infinitely quickly.  He will certainly travel in time, but it will be a one-way trip forward, and he will shoot straight to the end of the universe.  Not very useful!
  3. Gorgeous special effects:  Black holes are completely dark of course, but the material falling inwards (called the accretion disk) forms into a flat disk of material spiralling inwards.  As you look closer to the centre, the material (mostly gas) becomes hotter and hotter, moving from infrared to visible light to ultraviolet, and all the way up to the hardest gamma-rays.  Unusually for most sights in space, it would be visibly moving very fast.  It would be quite beautiful, just not quite in the way we see in film and TV.
  4. Noisy black holes:  I don't think I need to explain this one!  Sound requires a medium to travel.  No atmosphere means no sound.  Space is silent.  But this is such a common error in space films that we forgive it automatically.  Even documentaries love to play swooshing sounds as planets pass by.  We can let this one pass.
 
 

 
Comments?  Questions?  Why not mail me at uastronomer@gmail.com
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