4 ms·

>* the equivalence of gravitational and inertial masses is not a completely solved problem in modern physics.* I'm not sure what you mean. In GR the two are eq

by pdonis 2y ago

>* the equivalence of gravitational and inertial masses is not a completely solved problem in modern physics.* I'm not sure what you mean. In GR the two are equivalent, period. There is no open issue there. > Earth also rotates, and thus the Coriolis effect is present. Yes, but on the scale of a human it is far, far too tiny to be detectable. So it doesn't make a difference to what humans experience on Earth, even though, as you note, there are scenarios in which it has to be taken into account. In a space habitat swinging around to provide artificial acceleration, however, I think the Coriolis effect on a human would be large enough to be detectable by the human. At least unless you made the length of the cable it was swinging on of the same order as the radius of the Earth, i.e., thousands of kilometers, which is not likely to be practical.

nine_k 2y ago
In GR, the equivalence of inertial and gravitational masses is assumed, not proven in any way [1]. GR, of course, holds exceptionally well during the last 100+ years of experiments. Indeed, in a small enough rotating habitat, jumping straight up would reveal the direction of rotation, and jumping "forward" and "backward" would have noticeably different results. There's hoping though that the mechanisms which under zero g make the bones lose calcium, make tissues with a lot of small blood vessels swell, etc, won't be as seriously affected by the small variations introduced by the Coriolis effect as they are affected by the lack of weight and the pressure resulting from the weight. Solving 100.00% of a problem would be wonderful, but solving roughly 90% would already be highly beneficial. [1]: https://en.wikipedia.org/wiki/Equivalence_principle#Einstein_equivalence_principle https://en.wikipedia.org/wiki/Equivalence_principle#Einstein...
- pdonis 2y ago
  > In GR, the equivalence of inertial and gravitational masses is assumed No, it's not, it's a necessary part of the way GR models gravity as spacetime curvature with the metric being the only "field" associated with gravity. You can't do that and have inertial and gravitational masses being different. I guess you could call modeling gravity as spacetime curvature with the metric being the only "field" associated with gravity an assumption, but it's an assumption that has a lot more consequences than just the equivalence of inertial and gravitational mass. So the latter is not usefully viewed as an "assumption" in GR by itself. It's one of many consequences of a much broader assumption that is the entire basis of the theory. It is true that there are other theories of gravity that use spacetime curvature, but they all have additional fields besides the spacetime metric that are associated with gravity. And all of them known so far are ruled out by experiment, or at least the only region of their parameter spaces that are compatible with experiment make the extra fields observationally meaningless and the theories become equivalent to GR in all their predictions.

by pdonis 2y ago

nine_k 2y ago

In GR, the equivalence of inertial and gravitational masses is assumed, not proven in any way [1]. GR, of course, holds exceptionally well during the last 100+ years of experiments. Indeed, in a small enough rotating habitat, jumping straight up would reveal the direction of rotation, and jumping "forward" and "backward" would have noticeably different results. There's hoping though that the mechanisms which under zero g make the bones lose calcium, make tissues with a lot of small blood vessels swell, etc, won't be as seriously affected by the small variations introduced by the Coriolis effect as they are affected by the lack of weight and the pressure resulting from the weight. Solving 100.00% of a problem would be wonderful, but solving roughly 90% would already be highly beneficial. [1]: https://en.wikipedia.org/wiki/Equivalence_principle#Einstein_equivalence_principle https://en.wikipedia.org/wiki/Equivalence_principle#Einstein...

pdonis 2y ago
> In GR, the equivalence of inertial and gravitational masses is assumed No, it's not, it's a necessary part of the way GR models gravity as spacetime curvature with the metric being the only "field" associated with gravity. You can't do that and have inertial and gravitational masses being different. I guess you could call modeling gravity as spacetime curvature with the metric being the only "field" associated with gravity an assumption, but it's an assumption that has a lot more consequences than just the equivalence of inertial and gravitational mass. So the latter is not usefully viewed as an "assumption" in GR by itself. It's one of many consequences of a much broader assumption that is the entire basis of the theory. It is true that there are other theories of gravity that use spacetime curvature, but they all have additional fields besides the spacetime metric that are associated with gravity. And all of them known so far are ruled out by experiment, or at least the only region of their parameter spaces that are compatible with experiment make the extra fields observationally meaningless and the theories become equivalent to GR in all their predictions.