Previously in series: Many Worlds, One Best Guess
Followup to: The Generalized Anti-Zombie Principle

Warning: Mach's Principle is not experimentally proven, though it is widely considered to be credible.

Centuries ago, when Galileo was promoting the Copernican model in which the Earth spun on its axis and traveled around the Sun, there was great opposition from those who trusted their common sense:

"How could the Earth be moving? I don't feel it moving! The ground beneath my feet seems perfectly steady!"

And lo, Galileo said: If you were on a ship sailing across a perfectly level sea, and you were in a room in the interior of the ship, you wouldn't know how fast the ship was moving. If you threw a ball in the air, you would still be able to catch it, because the ball would have initially been moving at the same speed as you and the room and the ship. So you can never tell how fast you are moving.

This would turn out to be the beginning of one of the most important ideas in the history of physics. Maybe even the most important idea in all of physics. And I'm not talking about Special Relativity.

Suppose the entire universe was moving. Say, the universe was moving left along the x axis at 10 kilometers per hour.

If you tried to visualize what I just said, it seems like you can imagine it. If the universe is standing still, then you imagine a little swirly cloud of galaxies standing still. If the whole universe is moving left, then you imagine the little swirly cloud moving left across your field of vision until it passes out of sight.

But then, some people think they can imagine philosophical zombies: entities who are identical to humans down to the molecular level, but not conscious. So you can't always trust your imagination.

Forget, for a moment, anything you know about relativity. Pretend you live in a Newtonian universe.

In a Newtonian universe, 3+1 spacetime can be broken down into 3 space dimensions and 1 time dimension, and you can write them out as 4 real numbers, (x, y, z, t). Deciding how to write the numbers involves seemingly arbitrary choices, like which direction to call 'x', and which perpendicular direction to then call 'y', and where in space and time to put your origin (0, 0, 0, 0), and whether to use meters or miles to measure distance. But once y...

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