Astronomy and Cosmology

14.       Astronomy , Cosmology

Planet Formation

Nobody knows for sure how planets are formed, because we haven’t been around to watch one. But we have a pretty good idea. The driving factor here, as is often the case, is gravity. If you take a large cloud of gas and debris in space, all the bits of the cloud attract all the other bits together; that is what gravity does. The bits will gradually move towards the ‘centre of gravity’ of the cloud, and eventually form a glob of ‘proto planet’ – if it happens to be near a star, that is. And if it is near a star, it will also be orbiting it, or else it will fall into the star, or disappear into inter stellar space. Technically of course, a planet has to be in orbit around a star, but the process I am describing could apply elsewhere – as indeed it does for star formation, only on a larger scale.

 Gravitational attraction depends on two things – the amount of matter, and the distance between them. The closer things are, the stronger the gravitational attraction.

So as the cloud ‘condenses’, the movement gets faster as the bits get closer, the attraction increases, gets faster, and so on until they all come together in a clump. This is our old friend positive feedback. Even when the bits have stuck together, gravity hasn’t quite finished its work. If the planet is big enough, the gravitational forces will cause the inner planet to heat up. This is partly why the centre of the earth is hot. It is not the only reason, though; atomic decay adds a large part to the heating. This caused a lot of confusion in the early days of evolutionary theory. It was thought that the earth was not old enough to provide enough time for evolution to have done its work, until it was realised that the nuclear decay was the key factor that kept the earth’s core hot.

Novas, Supernovas

When a star grows old, and loses its youthful vitality, it gets weak and tired. It just can’t throw off the energy like it used to (you know, like sunlight). Now a large object like a star has a lot of gravitational attraction between all the bits of stuff inside it. When it can’t throw off enough energy, it starts to lose the battle with gravity (a bit like we do when we get old). It starts to collapse under its own gravitational strain.

Now the thing about gravity is that the closer things get, the stronger the gravitational attraction gets. So as the star starts to shrink, the attraction of its own stuff gets stronger. And as it gets stronger, it collapses more quickly. And as it collapses more, the stuff gets closer, and the gravity gets stronger. Sounds familiar? Its that old positive feedback again. The collapsing strengthens the gravitational attraction, and that quickens the collapse, so much so that the final collapse of a huge star can be a matter of seconds. Perhaps its not quite classic feedback, but its close enough. In a way it is similar to thermal runaway, on rather a large scale.

So what happens? Well, when a big star collapses far enough, it starts to heat up under the strain (sounds a bit like me). The heat energy starts to counter balance the gravitation energy, and when it gets hot enough it just explodes. Depending on how big the star is, it is either a huge explosion (a nova) or a ginormous one (a supernova).

If the star isn’t big enough, it just can’t hack being a nova, so it settles down into a sort of quiet old age as a white dwarf, or a neutron star. Though apparently all is not quite so quiet as you might expect. While doing my research, I came across a reference to thermal runaway on the surface of white dwarfs. You may be glad to know that I am not going into that in depth, but it just goes to show how widespread these fundamental processes are.

Goldilocks

I will leave you with one final (possible) example of feedback in the cosmos. Paul Davies has written a book on The Goldilocks Enigma. The title refers to the difficult puzzle of why it is that many things about the Cosmos and the earth are ‘just right’ for life to form. Many physical constants and processes are remarkably ‘fine tuned’ so that stars can form and produce all the other elements, and the earth is also in the right kind of orbit round the right kind of star so that life can form. There is some debate about just how fine the tuning actually is, but all agree that it remains something of a difficult question. There are several attempts to provide answers, and I am not going to go into all of them here. Many are built around something called the ‘Anthropic Principle’, which comes in various flavours, but basically says that things have got to be the way they are for us to be here asking the question. So don’t worry about it.

Davies doesn’t like this answer, and has come up with another one which is a bit tricky to explain, as it depends on quantum physics. It has been said, and oft repeated, that if you think you understand quantum physics, then you haven’t really understood it. Rather fittingly, there seems to be some uncertainty as to who said this; some say Richard Feynman, some Neils Bohr, or even John Wheeler, who seems to get everywhere, like a quantum particle. Maybe they all said it. Quantum physics was developed in the early part of the twentieth century, and a hundred years later, scientists are still unsure of what it really ‘means’. Like the old joke about economists, if you ask five scientists to explain what quantum physics really tells us, you will get six different answers. There are many popular science books that try and explain these problems, such as John Gribbin’s Schrodinger’s Kittens, so I am not going to go into detail.

The hard part of the quantum world is that it seems that on the very small scale, things just do not behave in any sensible way. Particles are not in any exact place (or maybe even time), but are rather fuzzy, or at least that is one way of looking at it.

Quantum physicists explain the difference between this and the ‘real world’ that we are used to by saying that the fuzziness ‘collapses’ at some rather hard to determine point. Others though say that this is the wrong way of looking at it, and that the universe is actually a ‘multiverse’ consisting of a very large (possibly infinite) number of universes in a sort of tangled super universe all running in parallel. People working on quantum cosmology tend to this view, because that is the way the mathematics leads them when you try and do the maths on the whole universe. This viewpoint was originally proposed by Everett, expanded by De Witt, and championed recently by David Deutsch, among others. One of the strange consequences of this view of things is that there is (according to Davies and others) no such thing as a ‘unique past’. This is pretty hard to get hold of, because I can generally remember things in the past, or at least, I used to be able to before I started having senior moments. The point is, that in the multiverse, different universe strands become entangled and unentangled in a way that makes it impossible to completely separate them.

Now what Davies is proposing is that there is some kind of feedback loop that operates on the whole multiverse so that in some sense the future influences the past, as well as the past influencing the future. There is a nice kind of symmetry in this that I quite like, and in fact I had some ideas along these lines, for totally different reasons, a long time ago. Just because an idea looks nice doesn’t make it right though. Anyway, what Davies is saying is that life, and intelligence, form a fundamental part of the whole scheme of things, and so in a way is responsible for tweaking the multiverse in the right direction for life to form. So it all depends on feedback, life the universe and everything. You can’t top that, so I will stop right here.