Spin dizzy

I had an interesting tweet the other day from a reader of my book Gravity. Richard Atkinson said: 'reading your gravity book. P3 "the whole thing is rotating" why? If uni started from a single point what made it spin?' It's a thought provoking question.

I ought to start by clarifying the quote. The 'whole thing' I was referring to was the cloud of dust and gas from which the solar system formed, not the whole universe. Whether the universe as a whole is spinning is a whole different question, where the mind struggles to get around the concept of 'spinning with respect to what' given that there may be nothing else. It is possible it does spin (see this article) but we certainly don't know for sure.

But let's get back to the actual quote, about the solar system. What made that spin? Pretty well everything within the universe spins. Overall the angular momentum of the galaxies seem to pretty much cancel out, but for any particular galaxy, as it forms, the slight rotation produced by linear motion combined with gravitational attraction inwards gets magnified as the galaxy condenses (the old 'skater spins faster as (s)he draws her/his arms in' effect). In principle, if all the bits that coalesced to form a galaxy started off still and symmetrical, a galaxy (or solar system) could form without spinning, but in practice there is always movement and asymmetry to start with and the result is to produce a rotation, which then increases with time.

Similarly as the solar system forms, the cloud of gas and dust picks up an increasing spin, which produces the flattened disc shape of both galaxies and solar systems. Yet again, planets spin increasingly quickly as they condense under gravity. The real oddity here is Venus, which rotates in the opposite way to expectation. There is a theory that this could be due to a massive collision, but there is no good evidence for this (other than the spin), so at the moment it is arguably an intriguing mystery.

If you fancy a little mental challenge yourself, you can take a look at Gravity on Amazon.co.uk and Amazon.com.