Why would cosmologists, philosophers, astronomers and particle physicists gather to talk about symmetry? It sounds odd. But symmetry in a law of nature implies something extremely powerful and beloved of physicists: conservation.
It is a difficult concept though. To make life easier, consider Newton鈥檚 classical mechanics, via an experiment of throwing a ball. We get the same result in London as in Sydney, showing Newton鈥檚 ideas have 鈥渢ranslational symmetry鈥� 鈥� it doesn鈥檛 matter where we move the experiment.
The work of mathematician Emmy Noether, who pioneered the idea of symmetry in fundamental physics, tells us that this means momentum is conserved. Turn to the right and repeat your experiment, demonstrating rotational symmetry, which implies conservation of angular momentum. Astoundingly, your experiment being the same today as tomorrow implies energy conservation.
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Other symmetries are hidden within the laws of quantum mechanical wave function, such as electric charge. The resultant conservation law in that case tells us that every time a positive charge is created, a negative charge also appears, resulting in a universe that is, overall, electrically neutral.
This is useful. Without it, a net charge could build up in regions of the universe, and, as electromagnetism is far stronger than gravity, this would prevent gas being pulled together to form stars and planets. This perfect conservation, derived from perfect symmetry, is beneficial for life, which relies on stars and planets.
A lucky break?
However, a perfectly symmetric universe would also be bad for life, and some breaks in symmetry are essential for it to take root. How this came about is one of science鈥檚 biggest mysteries. One of the most important examples is the dominance of matter over antimatter.
While the presence of matter is taken for granted, particle physics tells us that the creation and destruction of matter should be precisely balanced by that of antimatter. If this were perfect, matter and antimatter would have completely annihilated one another in the cooling early universe, leaving a featureless sea of radiation in their wake. So, at some time in the very early universe, this symmetry was broken at a level of one part in a billion, and matter emerged victorious.
Why some symmetries are broken and not others is a mystery we are still grappling with, although promising ongoing experiments at particle accelerators in Europe and the US will this year be looking for possible differences in neutrinos and anti-neutrinos that might shed light on why matter dominates.
The degree of smoothness of matter emerging from the big bang is yet another vital example of cosmological imperfection, as a perfectly smooth universe at the start would be destined to remain smooth forever, preventing the formation of stars and galaxies. Some initial lumpiness is essential, but only a little, as too much would have dragged all of the matter together to form black holes. Like Goldilocks鈥檚 porridge, the universe鈥檚 lumpiness had to be just right.
Our very existence depends upon the presence of fractured physics, set among perfection, providing the conditions for life to flourish on at least this one small planet. That is why understanding this physics is so important. The source of these imperfections might be written into the ultimate laws of the universe. Or they might be the signature of our place in the multiverse, our universe being one of many, each with its own peculiar symmetries.
Some of these universes will be too perfect for life to form, others too chaotic, but how many could potentially be habitable remains an unanswered question. Symmetry is clearly key to understanding our universe, but it isn鈥檛 too symmetric for us to be here to appreciate it.
Read more: Top 10: Weirdest cosmology theories; The five greatest mysteries of antimatter
