>Certainly if you were alone in the universe, he thought, there would be no way to tell if you were rotating, so there could be no centrifugal force if you started to spin with respect to, say, another person.
Isn't that... intuitively wrong though? If you were spinning fast enough in space, your body would be ripped apart. If you were spinning and let go of an object near you, instead of floating in place, it would appear to retreat away from you rapidly.
I understand the example of how a person in a falling elevator can't tell if gravity is acting on them or not ... it's intuitive and there aren't any obvious contradictions. I don't get why these two examples are brought up together as similar obviously-true things that inspired relativity. It feels to me like it (the article / Mach / Einstein / physicists) is reaching for a parallel where there isn't one between cases that are only superficially similar.
Maybe the idea is that no other bodies around means that your own body would be uncontested in "frame-dragging" such to define that you aren't rotating? But what happens if you hold your arm out and throw a ball perpendicular to your arm? I can't imagine a type of rules of motion that would result in anything but your body being sent spinning slightly, with you seeing the ball retreat while appearing to orbit you and you being able to feel the blood rush a little more to the ends of your body away from the axis you're spinning on. The blood in your veins has inertia, and by changing the inertia of parts of your body, then of course you're going to get pushback from your blood. Maybe the ball being a separate body is now dragging the frame a bit such that you're now rotating relative to the frame, but if the ball has much less mass than you, it seems odd that the ball would have a great effect on the frame compared to you, and possibly continue to, no matter how far away it is from you. Every other effect at a distance rapidly diminishes with distance! This effect wouldn't be necessarily nonlocal (ie. faster than light), but it would seem to have a disrespect for distance not commonly found in physics. (Maybe the effect just diminishes outrageously slowly over distance, but I'm not sure this assails all of my concerns with the concept.)
I'd agree with your intuition in both cases, but it's worth pointing out the equivalence principle (falling in an elevator) isn't necessarily immediately obvious (which I know is not something you claimed). Feynman complained about philosophers who would claim it's obvious that motion was always relative but then wouldn't understand that the same thing didn't apply to rotational motion.
Let me refine that part: I think that what would physically happen in the elevator example is apparent to most (someone with knowledge of Newtonian physics may agree with the idea that the interior of a falling elevator acts identically with the interior of an unmoving gravity-less elevator), but people without knowledge of relativity and the experiments that lead to it may disagree about what the elevator example means about physics and the world.
But the spinning-in-an-empty-universe example is a situation that I don't even agree with Mach/Einstein about what would happen, much less about what it says about the world. Maybe they're right, but this difference makes the thought experiment a much different kind and arguably a less compelling one than the other.
Thanks for the link, I found it an interesting read!
Isn't that... intuitively wrong though? If you were spinning fast enough in space, your body would be ripped apart. If you were spinning and let go of an object near you, instead of floating in place, it would appear to retreat away from you rapidly.
I understand the example of how a person in a falling elevator can't tell if gravity is acting on them or not ... it's intuitive and there aren't any obvious contradictions. I don't get why these two examples are brought up together as similar obviously-true things that inspired relativity. It feels to me like it (the article / Mach / Einstein / physicists) is reaching for a parallel where there isn't one between cases that are only superficially similar.
Maybe the idea is that no other bodies around means that your own body would be uncontested in "frame-dragging" such to define that you aren't rotating? But what happens if you hold your arm out and throw a ball perpendicular to your arm? I can't imagine a type of rules of motion that would result in anything but your body being sent spinning slightly, with you seeing the ball retreat while appearing to orbit you and you being able to feel the blood rush a little more to the ends of your body away from the axis you're spinning on. The blood in your veins has inertia, and by changing the inertia of parts of your body, then of course you're going to get pushback from your blood. Maybe the ball being a separate body is now dragging the frame a bit such that you're now rotating relative to the frame, but if the ball has much less mass than you, it seems odd that the ball would have a great effect on the frame compared to you, and possibly continue to, no matter how far away it is from you. Every other effect at a distance rapidly diminishes with distance! This effect wouldn't be necessarily nonlocal (ie. faster than light), but it would seem to have a disrespect for distance not commonly found in physics. (Maybe the effect just diminishes outrageously slowly over distance, but I'm not sure this assails all of my concerns with the concept.)