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Hawking's black hole 'blunder' shows how little we know about gravity

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By Bob McDonald, Quirks & Quarks

Gravity isn't just a pain when you're climbing up a long flight of stairs or careening out of control down a hill on skis - it's also a nuisance for scientists such as Stephen Hawking, who are trying to understand how this force works in the universe.

Gravity is the most far-reaching force known to man. It's what holds you in your chair and reaches across billions of light years of space to hold clusters of galaxies together. It's everywhere. So, you would think that something so common and familiar would have been figured out by now. In fact, its workings are still a mystery, and that mystery is a wrench in the scientific gears - or rather, an obstacle for physicists trying to come up with a single theory to describe the universe.

What we do know is that as soon as any kind of mass comes together in one place, gravity appears and pulls the whole system towards the centre. The more mass you have, the more gravity will appear. That's why astronauts walking on the moon felt lighter. The moon is smaller and less massive than the Earth. That's why stars and planets are round. A sphere is the smallest shape possible when gravity pulls towards the middle.

But while we can see and measure the effects of gravity, scientists are still perplexed over what causes it.
 
Einstein described how gravity works on a very large scale, and came up with the revolutionary idea that gravity is more than just a force; it is a bending of space and time. In other words, space is flexible, like the surface of a trampoline. A massive object, such as a star, is like a bowling ball making a depression in the centre of a trampoline. Something smaller, such as a ping-pong ball, will roll around the larger object, riding the walls of the gravity well, the way the Earth goes around the sun. That's how Einstein saw gravity's effect on space.

That model works very well on the large cosmic scale, but it doesn't seem to work at the quantum level, the micro-universe within the atom.

Quantum physicists do not see any effect of gravity down at the scale of quarks, leptons and other exotic particles inside a nucleus. And that is a problem, because they and the cosmologists would like to find a common theory that works in both realms, a unified theory of the entire universe.

At the moment, they are stymied by the fact that gravity just won't fit in.

However, in nature, there are two places where the large and small do come together. One was at the very beginning of the Big Bang, when the entire universe was squeezed into an unimaginably dense, hot, tiny space. The other is in a black hole. We can't go back to the beginning of time to look at the Big Bang, but black holes are around today, which makes them natural laboratories to sort this problem out.
 
The classic picture of a black hole is an area in space where so much mass has accumulated that it fell in on itself and gravity shot up to infinity. If you approach a black hole, it will begin to pull you in. If you change your mind and try to back up, you will have to move really fast to escape that powerful gravity, which gets stronger as you get closer. At one point, called the event horizon, that escape velocity is the speed of light. Since Einstein said nothing can travel faster than light, you are doomed to fall into the hole and disappear forever from this universe.

But the quantum scientists say your identity, the information about you, cannot simply vanish. In their quantum world, information has to be conserved somehow. So, Houston, we have a problem. The two theories are in conflict.
 
That's why Stephen Hawking delivered a short paper this week to try to resolve the issue, by redefining the event horizon. Maybe it's not such an ultimate boundary after all. Perhaps there is an escape route for that information.

It may seem like an esoteric waste of time, pondering such bizarre things as black holes, which have nothing to do with everyday life. In fact, this is an attempt to understand the most fundamental force that affects almost everything we do ... the force of gravity.

Imagine the possibilities if we could control gravity and lift the centre of that trampoline upwards, so you fall away from a massive object rather than towards it. Falling up would certainly make space travel a lot easier.

And since gravity affects both space and time, perhaps we could begin travelling in time. We won't know until we understand how it works.

So, think about that the next time you drop something. The annoying force that pulled your glass to the floor and made it shatter is still a mystery. Don't you think that's astounding in the 21st century?