How does matter stick together?

10 January 2017

Question

Donald asks: In a perfect elastic collision, there is conservation of momentum and energy. A ball dropped would return exactly to the height it was dropped from. Gases exhibit this. I can see how one can go from Jupiter-sized to the sun by accretion.  My problem is getting from one helium atom or hydrogen molecule to a football, then to earth mass sized.  It would seem that the perfect elastic collisions would frustrate it sticking together! Both  ydrogen and helium being gases and rarefied in those cosmic conditions. How is the density achieve? It can't be just gravity! Kinetic energy has to be lost! How?

Answer

Chris Smith put Donald's question to Imperial College's Stuart Higgins...

Stuart - So stars and planets are formed in these giant dust clouds and, initially, it’s gravity that starts to bring things together. And the star forms and the gas cloud collapses on itself and the gravity pulls in and, eventually, at some point, nuclear fusion starts the process the force is so great, and a star is born. With planets when you’ve got dust circulating the existing the systems. Now initially, as different atoms pass, they might bind together and form molecules and these might start to aggregate and get into slightly larger dust particles. But, actually, there is a problem here because when dust particles start to get bigger and bigger, when they bash into each other they don’t necessarily stick, they bounce off each other like two pool balls hitting each other, and this is an ongoing puzzle in science. It’s not clear what the answer is. Some people think that it depends on the shape of the dust particles. So, if the dust forms in a very irregular way - they call it a fractal way, it’s a very rough surface and the chance of it sticking when two pieces come together is quite high. Some people think maybe that there’s some organic material in there that’s more sticky that might add to that mix, or maybe ice or frost, but it’s a really big puzzling problem. It’s called the metre-sized problem, which is how do you get from bits of things that are in an order of centimetres up past a metre without them bashing off each other so that they split apart again or that the become too big that they suddenly fall into the star they’re all orbiting?

Chris - Is it not just the fact that when you’ve got millions/billions of particles all in one place that you’ve got billions of goes at the more kind of bashing into each other with different amounts of momentum, coming from different directions and, just by chance, a few are going to cancel each other’s movement out exactly and hang around together And then, once you’ve got fewer of those, a few more of those join in and eventually, with enough rolls of the dice, you get there?

Stuart - Yes. So potentially, part of the problem is that when people study this they literally get dust and fire it at each other and record it with a high speed camera. And that give you some indication; it shows you how things bounce off each other, but it doesn’t necessarily reflect the environment the particles are moving in. So, for example, some people, or some scientists believe that if your large particle is moving through a gas flow and a bit flies off, that small bit that flies off is going to fly straight into a gas stream, be pushed back towards the big particle again. So it all depends and basically it’s complicated and we're still not sure exactly what happens.

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