Tuesday, 17 April 2012

Black holier than thou

When I give talks on the universe I often get questions about black holes. Everyone loves a black hole. They are the movie stars of the cosmos with a whole mythology of their own, which is pretty amazing when you consider that they are basically theoretical concepts that have never been directly detected (although we have good reason to believe they're out there).

Here's a quick black hole primer, if you haven't quite got the hang of them:

Although quantum physics is mostly about the very small, there are some hypothetical large scale quantum objects. Perhaps the best know is the universe at the point of the Big Bang - but it is pretty much rivalled by the black hole.

In a crude form, black holes were dreamed up in the 1700s when British astronomer John Michell imagined the way escape velocity - the velocity needed to get away from a planet - getting bigger and bigger as the planet got more massive. With a heavy enough star, Michell realized, the escape velocity would be bigger than the speed of light, and light would never get out. The result would be a dark star, or as the astronomer John Wheeler first called them in 1969, a black hole.

The modern idea of the black hole came from Einstein’s general relativity, which considers gravity to be a warp in space and time. The more massive a body, the more it bends spacetime. With enough mass in a small enough volume, the warp would be so great that nothing - light included - would get out. To get the Sun, a middling-sized star 1.4 million kilometers across, compressed enough to go black it would have to be condensed to just 3 kilometers in diameter.

Normally when a star is active, the outward pressure from the nuclear reactions that power it keeps the star “fluffed up”, but as nuclear fuel runs low, pressure drops and the star begins to collapse. Now another force comes into play - a quantum feature called the Pauli Exclusion Principle that means that similar particles of matter that are close in distance must be different in velocity. This will counter the gravitational collapse - unless the star is too massive. The mass required for this is around one and a half times that of the Sun. Some such stars explode as a supernova. But if this fails to happen, the star should contract, getting smaller and smaller until it becomes a black hole.

In theory, the contraction will continue until there is a singularity, a point of infinite density, at the center of the black hole. This singularity is a quantum object. I say 'in theory' because at a singularity the maths breaks down and this could mean that something completely unexpected happens.

There are several inaccurate myths about black holes. Their gravitational pull is nothing special for a star. If you were orbiting a star that became a black hole, the pull would get no stronger. It's just that you can get much closer to one than an ordinary star, so can experience much more dramatic forces that way. Also they're not totally black. They are expected to give off faint 'Hawking radiation.' And they aren't gateways to another universe. Get in a black hole and you're stuck.

To finish off, a few fun black hole factoids:
  • If you flew towards a black hole, the difference in gravitational pull between your feet and your head would be so big that you would be stretched out long and thin like a piece of spaghetti. This process is actually known as spaghettification - who says physicists don't have a sense of humour?
  • General relativity says the stronger the gravitational pull, the slower time runs as seen from the outside. If we watched an object travelling into a black hole it would get slower and slower before stopping forever at the event horizon (the point beyond which no light escapes). It should take an infinite amount of time (from our viewpoint) for the object to get any further.
  •  Technically, the singularity at the heart of a black hole is a point in time, not a point in space. Onve you have passed the event horizon (which from your viewpoint is no problem) you will inevitably reach the singularity at a particular point in time.
Image from Wikipedia


  1. I once heard something on why light can't escape that went something along the lines of that space was being stretched at velocity c. Any comment/correction?

  2. That seems an overly complex description. I prefer to think of it that a massive body warps space (and time), and the more concentrated that mass the tighter the warp. The event horizon is the point at which spacetime is warped so much that anything trying to get, light included simply heads back in again.

  3. Yes, that's the image I've always had - something akin to watching a flare on the sun be dragged back (is how I sort of see it). Still, I was quite taken with this - as it was so shockingly new ... having slept on it, I think it was Brian (it's amazing) Cox I heard give that explanation.

  4. There's one thing wrong with your 'dragged back' image, Peet. That suggests that the light is being pulled in some direction. It's not - it's going in a straight line. It just happens that the spacetime it is going in a straight line through is extremely warped.