Alan Alderson asked:
Thanks for the show, I find it totally fascinating.
It seems the universe is full of angular momentum. Planets revolve and orbit stars, stars revolve and orbit galactic centres even sub atomic particles spin.
Yet the origin of the universe was the big bang where everything exploded outward froma point source. As I understand it there is a law of conservation of angular momentum saying it cannot be created or destroyed onlt transferred. So I am puzzled where did all the angular momentum come from?
You don't actually have to have any overall rotation in the whole universe for objects within the universe to be rotating in various different directions.
Imagine that you have a large cloud of gas that has no rotation to begin with, but that you split it up into lots of little pieces which go on to form into lots of different galaxies. Each piece by random chance will have some particles which are swirling around in some particular direction, and others which are swirling around in other directions, so it will have some very tiny rotation in some particular direction. Now as that cloud collapses down to form a star or a galaxy, there's an effect called the conservation of angular momentum which means that spin is accelerated as the object gets smaller.
That's similar to the effect when a figure skater is skating on ice and they start off spinning quite slowly, when they pull their arms in you'll see they start spinning incredibly quickly. So, that means that an object, even if it only has a tiny rotation to begin with, can actually end up rotating quite appreciably as we see planets and stars in the universe doing. Certainly, if you look at the rotations of different stars, you will see that smaller stars tend to rotate much more quickly than bigger stars and that is because they're being spun up as they've contracted down.
Angular momentum is a vector. If two bodies have equal amounts of angular momentum, but they are spinning in opposite directions, the total angular momentum of the two bodies is zero. Two bodies can begin motionless beside one another, give each other a push and move apart spinning in opposite directions. That does not change the total angular momentum of the pair. Phractality, Sun, 1st Apr 2012
Brilliant OP question.
Of course the universe may be spinning right now, a so-called Machian Universe. However, this presumes, just as it did very early on, that the universe has a boundary. What is it relatively a boundary between is a new question. Ęthelwulf, Sun, 1st Apr 2012
Although the initial expansion seems to have been very smooth gravitational collapse and supernova explosions rapidly create turbulence at all scales. this should integrate to zero if the universe does not have any net rotation
No, it doesn't 'rotate'.
From Lisalotte who I wish would register here some day :)
If you think of a solar system evolving from a cloud of dust where all particles are moving independently, then there will be a random angular momentum in the cloud of dust about its particular centre of gravity. But as the gravitational forces start to pull the system together, the angular momentum will be conserved, but the mass will gradually accumulate close to the centre. To conserve the angular momentum, the cloud in general, and the central mass in particular will have to rotate faster and faster -- smaller effective radius means larger angular velocity, since j = m * omega * r, and j and m are both constant. (Actually some mass will be lost to the system in the energetic turbulence of the process, but not enough to change this conclusion). Once the rotation speed of the central body gets to a particular level, the remaining material outside it is moving too fast to accrete with it, and so it will orbit. But irregularities and gravitational forces in this material will cause a similar process to repeat in the accumulation of the proto-planets and their satellites. That accounts for the fact that in general nearly every massive body in the solar system rotates and orbits in the same direction, and in nearly the same plane as everything else. But all of that angular momentum arises from a simple random sum of the angular momenta of the material that has accreted in the first place.
That is interesting Wulf :)
Ford explains it to Arthur whilst at the Restaurant at the End of the Universe
I need to reread it :)
The Doc has presented a simple question in a perfectly eloquent fashion. This is a question which has been bugging me for some time and respectfully I suggest none of you have answered it. Not that I expect to either but here goes.