h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Why do Planets and everything else in the universe seem to spin. and if there is no universal frame of reference what do they spin relative too?

Do you mean 'spin' or 'orbit'. Obviously if they spin they they spin relative to their own centre and I don't think that all planets do spin (Mercury for example).

Mercury spins,like the moon..but at the same rate as it orbits the sun
so it keeps the same face to the sun

Even with no frame of reference you can tell that some things are
spinning..eg a bucket of water spinning in space...the water is
thrown out to the sides by centripetal force,even with no gravity.

If I'm inside a huge (say a kilometer) hollow sphere,
floating far from any gravity,I will float freely,no matter how far
I am from the side of the sphere.
If the sphere is spinning,I could stand on its inside surface.
If I'm floating free inside,would I know if the sphere was spinning?
Would I be attracted by the force that allows me to stand?

"If I'm floating free inside,would I know if the sphere was spinning?"

If the sphere was a perfectly uniform colour and smooth texture, and lit evenly from somewhere within, then no. If there was, say, a stripe painted round the internal equator, you'd be able to see that moving.

"Would I be attracted by the force that allows me to stand? "

No. Interesting question.

The "force" that allows you to stand, strictly speaking, is electromagnetic force. Specifically, the repulsive force between the atoms of your feet and the atoms of the interior of the sphere. That's the only "force" at work, given that there are four fundamental forces: gravity, the strong nuclear force, the weak nuclear force, and electromagnetic force.

What you're feeling when you're standing on the interior of the sphere - assuming you're standing on the equator - is not "attraction", it's repulsion. You're being pushed towards the centre by the surface. Your feet push back with the same force.

There's an interesting moment in Iain M Banks's first Culture novel, "Consider Phlebas". A bunch of space pirates are exploring an Orbital (a massive spinning ring shaped structure). A previous scene set on a planet has established that these pirates get about using anti-gravity belts, that locally reduce gravity and allow them to make huge leaps safely. One of the characters arrives on the orbital and, out of habit, leaps over the side of a building intending to make a soft landing on the ground below. He falls to his death, and one of the other characters observes that he confused a gravity field with a rotating frame of reference.

If you were floating within a sphere with a reference (a stripe on the inside), but were spinning at the same rate as the sphere, you could find out that you were spinning by changing your angular momentum, like a skater moving their arms in or out to affect their spin rate.

You'd need to be at the dead centre of the sphere for it to be an issue. Consider the sphere as hollow and the surface as opaque and of negligible thickness, and additionally consider there is sufficient light inside the sphere.

Consider the following cases:
1. The sphere is unmarked. You are at the dead centre. You are stationary, as is the sphere. You perceive no motion, and there is none. Fine.

2. The sphere is marked. You are dead centre. You are stationary, as is the sphere. You perceive no motion, but you're surer you're right, because you can see that the mark is stationary with respect to you.

3. The sphere is marked. You are dead centre. The sphere is spinning, you are not. You can perceive the motion of the sphere relative to you. Nothing you do changes it.

4. The sphere is marked. You are dead centre. The sphere is spinning, and so are you. You perceive the relative motion, but by moving your arms in an out you can affect it. You are therefore able to deduce that you are spinning.

5. The sphere is marked. You are dead centre. The sphere is stationary, but you are spinning. You perceive the relative motion, and by moving your arms in and out you can affect it. You are therefore able to deduce that you are spinning... but in this and the previous case, you can't tell whether the sphere is spinning too. It might just be you.

6. The sphere is UNmarked. You are halfway along a radius - equidistant from the centre of the sphere and the inner surface. The sphere is stationary, as are you. You perceive no motion, and there is none.

7. The sphere is unmarked. You are halfway along a radius. The sphere is spinning, you are not. You can not perceive any motion.

8. The sphere is marked. You are halfway along a radius. The sphere is spinning, you are not. You perceive the motion of the sphere. You can tell it's the sphere that's moving, and not you, because moving your arms in and out has no effect.

9. The sphere is unmarked. You are halfway along a radius. You are spinning. You immediately perceive the motion, because the surface of the sphere seems to advance and retreat from you as you spin. It would do this whether or not it was marked, and whether or not it was spinning, and your ability to affect the rate of spin by adjusting your angular momentum indicates that YOU are spinning. You would be able to tell whether then sphere was spinning if it was marked, by coordinating your adjustments of your angular momentum and seeing whether they affected the rate of progression of the mark, or only your position relative to it.

Like the circles that you find in the windmill of your mind