Big cosmic squeezes
Ken Tapping, 2nd December, 2015
The pressure in the centre of the Earth is huge. It has to be in order to support thousands of kilometres of overlying rock. There are objects out in space much more massive than our world, where the pressures in their cores must be enormously higher. Is there a limit to what rock and the other materials used to make the universe can take? What happens if they are exceeded? Matter is made up of atoms, so these tiny particles bear the brunt of supporting the material making up the various bodies in the universe. What happens to atoms under extreme pressures?
Atoms are made up of a nucleus, consisting of protons and neutrons, surrounded by shells of orbiting electrons. Every electron is specified by a set of parameters, like the address of a resident living in an apartment building. Imagine a multi-storey apartment block with vacant apartments scattered throughout. Increasing pressure can be relieved in many cases by making our apartment block smaller. We can do this by moving residents from upper floors to fill the vacant apartments lower in the building. Then we can make the building smaller by removing some of the upper floors. Similarly, atoms can respond to extreme pressures by making sure all the addresses near the nucleus are filled. Matter in this condition is called degenerate. Just doing this makes atoms a lot smaller, so that a teaspoonful of degenerate matter would weigh over a tonne. The Sun will end up as a white dwarf star, about the size of the Earth, made of degenerate matter but still weighing as much as it did before. There are stars larger than the Sun. What happens to them?
Even in a degenerate state, atoms are still mostly empty space, so there is capacity to shrink even more if we apply enough pressure. We can do this by piling on even more overlying material or by applying an immense, compressive shock. If this is intense enough, the atoms completely collapse, with the electrons and protons being forced together to form neutrons. This can turn a star into a mass of neutrons, where the star is now compressed down to a ball a few kilometres in diameter – a neutron star. The gravity on the surface of one of these objects is so high that mountains may struggle to a height of a few millimetres. We are now pretty sure that neutron stars exist. The observations and the calculations agree with each other. However, there are even more bizarre things out there, such as black holes.
Piling on more and more pressure will eventually cause the neutrons to shrink, and once that starts, the increasing gravitational shrinkage force will always exceed the ability of the neutrons to resist. Is it possible they will shrink indefinitely, creating something very tiny with almost infinite density? In mathematics we call such things singularities. If these exist in nature we cannot observe them; Mother Nature makes them invisible.
Those of us who have climbed high ladders or walked up hundreds of stairs know that climbing against gravity takes work. Light and heat being radiated into space by stars is not exempt from this rule. Getting out into space takes energy. If the energy required is more than the energy of the photons making up the light, they cannot escape. This is exacerbated by the intense gravity around the singularity folding up the fabric of space-time, enclosing the singularity in an “event horizon”. Inside this, light is not stopped from getting out; time is distorted so that it just takes forever.
It is intriguing that the description of the singularities we think lie in the cores of black holes is similar to what we think our universe was like at the moment of the Big Bang: extremely small, incredibly dense and very hot.
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