Wonders of the Universe - Brian Cox [43]
A water molecule is made up of two chemical elements: oxygen and hydrogen. Oxygen and hydrogen atoms are symmetric when they are alone and uncombined. This particular use of the word symmetric is perhaps unfamiliar; what is meant in this context is that the atoms themselves would look the same no matter what angle you viewed them from. In the language of physics, this is called rotational symmetry. A perfect sphere has perfect rotational symmetry, because whichever way you look at it or spin it around it looks exactly the same. When an oxygen atom combines with two hydrogen atoms to form a water molecule – H2O – this rotational symmetry disappears because the water molecules have a particular shape – there is an angle of 105 degrees between the hydrogen and oxygen atoms. A physicist would say that the symmetry is now broken, because the water molecule has a distinct orientation. We can break the symmetry of water still further by cooling down all the molecules until they stick together and solidify into ice. Now the crystals of ice are beautiful and almost impossibly intricate; full of structure and a complexity that completely hides the perfect symmetry of the original atoms, and also the simple but different symmetry of the water molecules themselves.
Approximately 70 per cent of Earth’s surface is covered by water. At the El Tatio Geysers you can see water in all its three forms. Walking through pools of water on the ground, I held a sheet of glass in the geysers’ steam and watched ice crystals form on it.
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Exactly like the journey of steam to ice, of chaos to order, this was the Universe in transition. A transition where the structure and substance of all the particles of matter emerged for the first time.
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The important point here is that all this complexity emerged when the symmetry was broken, but we did nothing to the water itself to break its symmetry other than cool it down. So although it looks for all the world as if a master sculptor sat down and chiselled out beautiful patterns in the ice, this intricacy and beauty emerged completely spontaneously out of building blocks that are themselves utterly symmetric.
Physicists call this process spontaneous symmetry breaking, and it is this idea that lies at the heart of our understanding of the early Universe
THE BIG BANG
Thirteen billion years ago the Universe began in the event called the Big Bang. We don’t know why. We also don’t know why it took the initial form that it did. This is one of the unsolved mysteries that makes fundamental physics so exciting. The first milestone we can speak of in anything resembling scientific language is known as the Planck Era, a period that occurred a mind-blowing 10–43 seconds after the Big Bang. When written in full, that number has 42 decimal places: 0.00000000000000000000000 0000000000000000001 seconds. That’s not very long at all. This number can be arrived at very simply because it is related to the strength of the gravitational force. It is so incredibly tiny ultimately because gravity is so weak – and we don’t know the reason for that, either! At that time the four fundamental forces of nature that we know of today – gravity, the strong and weak nuclear forces, and electromagnetism – were one and the same force, a single ‘superforce’. There was no matter at this stage, only energy and the superforce. This is what a physicist would call a very symmetric situation.
As the Universe rapidly expanded and cooled it underwent a series of symmetry-breaking events. The first, at the end of the Planck Era, saw gravity separate