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Saturday, 4 February 2012

Black holes and the history game

When we’ve exhausted “I Spy” and “Twenty Questions”, my six-year-old son and I sometimes play a game that we invented. It’s a kind of quiz that exercises both our brains. I realise I’m in danger of coming across as one of those pushy, neurotic parents who forcibly over-educate their offspring to prove their superiority and give them a head start in the rat-race. But, cross my heart, we play purely for the fun of it.

Here’s how the game works (and, if you have sprogs of your own, you’re welcome). You, the grown-up, play the role of question-master. Think of three events, inventions or discoveries. Say them in a random order and, if necessary, explain what each one is. Then your diminutive descendant just has to put them into chronological order. Not convinced? Believe me, it’s a lot more fun than it sounds.

Let’s have a go. (Follow the links for clues to the answers.)

- This one won’t tax an adult, but it makes an interesting conversation point. Whether the apple of your eye gets it right or wrong, you’ll probably end up discussing dragons and castles until one of you falls asleep, and forget the rest of the game.

- Yes, they all still exist, but which was invented first? Again, this one’s not exactly challenging, but it might surprise your playmate, especially to learn how very long the first one was invented before the other two.

- Starting to work the grey-cells now?

(4) The wheel; the plough; fire.

- This one will almost certainly surprise the younger player, for whom all three inventions belong to ancient history.

Did you get them all? Congratulations. Now, here comes the tricky one.

(6) Stars; galaxies; black-holes.

Until recently, physicists thought we knew the answer to this one. In the early universe, gravity made hydrogen clump together into big dense regions (proto-galaxies) where stars formed, fusing hydrogen nuclei into helium, and releasing energy. Once those stars had exhausted their hydrogen fuel, they collapsed and, if they were huge enough, kept on collapsing right down to zero size, forming a black hole, where gravity is so intense, it breaks space and swallows light.

That's how we thought it worked, but it was all just conjecture. In fact, we weren't even confident that black holes existed at all. A black hole was a possible solution to the “field equations” of General Relativity, Einstein’s theory of gravity, but there was no evidence of them until the discovery of active galaxies in the 1950s. It became clear that something at the heart of many distant galaxies was firing out incredibly energetic jets of matter and radiation and, because the intensity of the jets was observed to vary quite rapidly, the source had to be something tiny compared with the galaxy. A swirling vortex of gas around a black hole seemed like the only possible culprit. Still, this evidence wasn’t completely compelling. Finally, at the end of the twentieth century, detailed observations of the rapid orbits of stars at the centre of our own Milky Way galaxy settled the debate. Just watch the time-lapse movie of the observations collected between 1992 and 2005 to be convinced that an incredibly massive body is tugging those stars with huge gravitation force. Their motion tells us that the invisible object has a mass 3.7 million times that of the sun, but is much smaller than the orbit of the Earth.

More recently still, astronomers have discovered that most if not all galaxies possess such a super-massive black hole at their centre, and that these black holes could not have formed by the unpredictable chance collisions of stars within the galaxies, because there is a perfect correlation between the size of the galaxy and the size of the black hole. In other words, if you measure the size of the black hole, you can predict exactly the size of the galaxy surrounding it. This suggests the possibility that the black holes were there first, and actually caused the galaxies to form around them, by their gravitational pull.

To add to the mystery, data collected by the Hubble Space Telescope in 2011, to appear in Astrophysical Journal, reveal that supermassive black holes existed in dwarf galaxies in the early universe (as seen by observing very distant galaxies, in order to look back in time). So, perhaps supermassive black holes have always been there. They may be older than any galaxy and, if so, may well have been created at the dawn of time, in the big bang.

It seems such a straightforward question whether black holes, stars or galaxies came first, but the more we learn about the universe, the more it surprises us. The new evidence by no means settles the matter; it only proves that we have not yet understood how or when black holes and galaxies formed.

In case you’re wondering, my six-year-old’s answer was: stars then galaxies then black holes. At the moment, it’s as good a guess as any.


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