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Aescorvo t1_j0gg0dn wrote

Reply to comment by alukyane in Does rotation break relativity? by starfyredragon

It’s because an orbit isn’t a rotating frame - the pull of gravity is always down, you’re just moving sideways so fast you miss the planet, by which time the angle of gravity has shifted so that it’s still down.

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dogninja8 t1_j0gi45t wrote

Isn't orbit always a non-inertial (accelerating) reference frame, since your velocity vector is always changing direction? The only way to keep your velocity vector pointing in the same direction (relative to you) is to be rotating yourself, which is also a non-inertial reference frame.

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alukyane t1_j0ggjgs wrote

It's definitely an accelerating frame, since gravity is acting on it. Probably rotating, too, at least around the planet. In any case I'm mostly interested in how free fall could be distinguished from 0 gravity.

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Aescorvo t1_j0gh3zo wrote

I didn’t say it wasn’t accelerating.

Maybe I guessed wrong at what part you thought was weird. There’s no different between freefall and zero gravity. Although, for the special case of an orbit there are slight differences you can detect at different heights.

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Game_Minds t1_j0grywh wrote

Can't all sublight paths through a relativistic spacetime be characterized as orbits? Even in intergalactic space objects' paths are curved by gravity. There would still be slight angular accelerations on basically any "straight" path even if they even out over time

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Moikle t1_j0gu1xq wrote

Free fall IS zero gravity (although tehnically there is no such thing as zero gravity and the concept doesn't really even make sense physically)

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Alis451 t1_j0ghvj9 wrote

>how free fall could be distinguished from 0 gravity.

The Space station is at 0g, but they aren't far enough outside of the Earth's Gravity well to be at Zero Gravity, because if it were it would start sticking itself, Earth's Gravity supersedes your own in relation to other nearby objects. So YES, there is a VAST difference between 0g and Zero Gravity. The space station is in a constant freefall and needs to continuously adjust.

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Aescorvo t1_j0gs39z wrote

Actually, let me amend what I said. Without looking out the window (metaphorically) you couldn’t tell. However, if you had a clock on board, and and an identical clock far enough away that it was effectively in zero gravity, AND you could view it through a telescope each revolution, you would (eventually) see that the clock runs faster than yours. That would at least tell you that it was experiencing lower gravity than you were.

This is akin to one of the effects that we have to account for with GPS satellites. Putting a clock in orbit (with a big enough display) would let us see the discrepancy compared to an identical clock on the surface, deeper in the gravity well.

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alukyane t1_j0h92dr wrote

We then seem to agree that the top-level claim above about acceleration is wrong: you can't actually tell whether you're in an inertial or accelerating frame, if the acceleration is the same for all observable objects. Right?

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