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> create artificial gravity. To those that claim 'acceleration is different from gravity', take your physics course again. The physics course, at least if it i

by pdonis 2y ago

> create artificial gravity. To those that claim 'acceleration is different from gravity', take your physics course again. The physics course, at least if it includes General Relativity, will tell you that what the human body is used to is not gravity but acceleration. The astronauts in free fall in the ISS are fully responding to Earth's gravity; the difference between them and the rest of us stuck here on the Earth's surface is that we are feeling acceleration, because the Earth's surface is pushing upward on us, and they are not. The potential issue with having a spaceship habitat swinging in a circle to create acceleration is the Coriolis effect; that would make it feel at least somewhat different from being on the Earth's surface, even if the magnitude of the acceleration were 1 g. That might cause something like motion sickness in some people--but then again, so does free fall. (I myself don't even like roller coasters, so I would probably be a very poor candidate for being a free-fall astronaut.)

nine_k 2y ago
AFAICT, the equivalence of gravitational and inertial masses is not a completely solved problem in modern physics. Earth also rotates, and thus the Coriolis effect is present. It's not as noticeable as in a fast-rotating habitat in space, but air navigation, naval navigation, and artillery all make adjustments for it, among other factors, especially at high latitudes.
- 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.
fouc 2y ago
There is a gravity gradient in the centrifuge, for a 2m tall person, if the radius is only 10m, gravity gradient ranges from 20% to 100% from head to toe, meaning it's quite noticeable. But at 100m radius it's only 2%, and presumed to not be noticeable (I'm not sure if they've actually put that to the test). https://ntrs.nasa.gov/api/citations/20070001008/downloads/20070001008.pdf https://ntrs.nasa.gov/api/citations/20070001008/downloads/20...
- pdonis 2y ago
  > a gravity gradient More precisely an acceleration gradient, at least in the terminology I was using in my previous post. Yes, this is a good point.
- JoeAltmaier 2y ago
  So use a 1km tether. Lots of room in space.

by pdonis 2y ago

nine_k 2y ago

AFAICT, the equivalence of gravitational and inertial masses is not a completely solved problem in modern physics. Earth also rotates, and thus the Coriolis effect is present. It's not as noticeable as in a fast-rotating habitat in space, but air navigation, naval navigation, and artillery all make adjustments for it, among other factors, especially at high latitudes.

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.

fouc 2y ago

There is a gravity gradient in the centrifuge, for a 2m tall person, if the radius is only 10m, gravity gradient ranges from 20% to 100% from head to toe, meaning it's quite noticeable. But at 100m radius it's only 2%, and presumed to not be noticeable (I'm not sure if they've actually put that to the test). https://ntrs.nasa.gov/api/citations/20070001008/downloads/20070001008.pdf https://ntrs.nasa.gov/api/citations/20070001008/downloads/20...

pdonis 2y ago
> a gravity gradient More precisely an acceleration gradient, at least in the terminology I was using in my previous post. Yes, this is a good point.
JoeAltmaier 2y ago
So use a 1km tether. Lots of room in space.