3 ms·

If you are accelerating, the accelerometers will not "show 0". I have no idea where you are getting this notion from; it's completely wrong.

TeMPOraL 2y ago
Accelerometers aren't magic. They don't read out True Acceleration from the process memory of the Matrix. There's a physical process inovlved, and that process cannot tell the rocket is accelerating when that acceleration is cancelled out. Standing on the ground, the accelerometer should show zero because gravity is cancelled out by the floor being solid. If latter can be a force, why not the former?
- wrs 2y ago
  I don’t know what else to tell you, the accelerometer in your phone is reading -9.8m/s^2 right now. Go get an app that reads your accelerometer and check it.
  - TeMPOraL 2y ago
    Accelerometer on my phone is reading f(a), not a. Unlike with e.g. old-school scales, there's layers of digital electronics and software between the measuring device and measurement being displayed. For all I know, if it's saying -9.8m/s^2, it may be because there's a `value = rawMeasurement - v3(0, 0, 9.81)` line somewhere in the code, because that's the value the designers wanted to report. I mentioned old-school scales, truth be told, they do this thing too: they're measuring weight, but reporting it as mass, with 1/9.81 factor baked into the scale label directly. Point being, I'm less interested in what some app tells me - you should always assume apps have assumptions and massage their numbers. I'm interested in whether the measuring device can actually tell the difference, which in case of forces perfectly cancelling out, it should not.
    - wrs 2y ago
      I assure you, because I’ve I’d done it, if you talk directly to the accelerometer, it will give you an XYZ acceleration vector with magnitude 9.8m/s^2 in the direction of the Earth (whichever way you turn it). Not because there is a magic “9.8” in the code, but because that’s what the acceleration is where you’re standing. If you take the same accelerometer up a mountain, it will read a little less. In orbit, it will read zero. On the moon, it will read about 1.6. Internally, sure, it does measure force. It measures the force being exerted by the teeny mounting structure to keep the teeny silicon weight in place. That force has to be exerted to counteract the acceleration of the gravity field and keep the weight from flying off. So that’s a straightforward way to measure acceleration. (To be super precise, it measures capacitance between the arms and the base, which is related to distance between them, which is related to force because the arms are flexible.) Edit: Put another way, there actually is an adjustment in the code, because most people want the reading to be zero when the phone is not moving relative to the Earth. To accomplish that, you can’t just subtract a constant vector, you have to filter the output based on the assumption that there is always a large constant acceleration in some direction. If you rotate the phone, that vector will vary wildly in direction, and you have to pick out that signal and subtract it. This means accelerometers have to have a lot of dynamic range to accurately report that large acceleration added to the tiny ones you’re more interested in. In orbit, this wouldn’t be necessary.
      - taeric 2y ago
        The point is only that the accelerometer is measuring force. If you put your phone on a centrifuge long enough so that it reads a constant force going into it, you would be able to make a situation in deep space where it reads 9.8m/s in what feels like a constant direction to any human. Right? How would the accelerometer be able to detect that?
        wrs 2y ago
        The accelerometer will read 9.8m/s^2 along the axis of the centrifuge, because that is the acceleration that is keeping the phone moving in a circle instead of flying into space. The difference between that and resting the phone on the Earth is that the centrifuge structure is accelerating it by exerting force on it, whereas the Earth is accelerating it by altering the spacetime metric in its vicinity. Completely different mechanisms, same reading.
    - theamk 2y ago
      For cell phone, you don't need to trust the app much - it reports raw (X, Y, Z) vector. Turn the phone sideways and see how it's Z went to 0 and it's now Y (or X) showing -9.8. Spin in the chair and see how the vector's magnitude is increasing. This should be a pretty effective evidence that the vector is not offset, and at most scaled.
- tsimionescu 2y ago
  > Standing on the ground, the accelerometer should show zero because gravity is cancelled out by the floor being solid. If latter can be a force, why not the former? This is the part where you are demonstrably wrong. This experiment has actually been performed. Take an accelerometer that shows 0 in space far away from the Earth, where there is no gravity. Bring it back to the earth and let it fall from your ship: it will keep showing 0 (well, because of friction with air, it will show some value, but a pretty small one). When it reaches the ground, it will "suddenly" show 9.8m/s². This is not something you can debate: this is the measured reality. You can debate how to explain this, of course. But the reality is there: being in freefall towards the earth is equivalent from an accelerometer point of view to being in deep space with no engines running. Another similar observation is this: the feeling of being in a gravity well is not the earth pulling you down, it is the floor pushing you up. If you're on a space station in deep space, to feel the same as on the earth, you need something pushing at your feet in the direction of your head, not something pushing on your head in the direction of your feet.

TeMPOraL 2y ago

Accelerometers aren't magic. They don't read out True Acceleration from the process memory of the Matrix. There's a physical process inovlved, and that process cannot tell the rocket is accelerating when that acceleration is cancelled out. Standing on the ground, the accelerometer should show zero because gravity is cancelled out by the floor being solid. If latter can be a force, why not the former?

wrs 2y ago
I don’t know what else to tell you, the accelerometer in your phone is reading -9.8m/s^2 right now. Go get an app that reads your accelerometer and check it.
- TeMPOraL 2y ago
  Accelerometer on my phone is reading f(a), not a. Unlike with e.g. old-school scales, there's layers of digital electronics and software between the measuring device and measurement being displayed. For all I know, if it's saying -9.8m/s^2, it may be because there's a `value = rawMeasurement - v3(0, 0, 9.81)` line somewhere in the code, because that's the value the designers wanted to report. I mentioned old-school scales, truth be told, they do this thing too: they're measuring weight, but reporting it as mass, with 1/9.81 factor baked into the scale label directly. Point being, I'm less interested in what some app tells me - you should always assume apps have assumptions and massage their numbers. I'm interested in whether the measuring device can actually tell the difference, which in case of forces perfectly cancelling out, it should not.
  - wrs 2y ago
    I assure you, because I’ve I’d done it, if you talk directly to the accelerometer, it will give you an XYZ acceleration vector with magnitude 9.8m/s^2 in the direction of the Earth (whichever way you turn it). Not because there is a magic “9.8” in the code, but because that’s what the acceleration is where you’re standing. If you take the same accelerometer up a mountain, it will read a little less. In orbit, it will read zero. On the moon, it will read about 1.6. Internally, sure, it does measure force. It measures the force being exerted by the teeny mounting structure to keep the teeny silicon weight in place. That force has to be exerted to counteract the acceleration of the gravity field and keep the weight from flying off. So that’s a straightforward way to measure acceleration. (To be super precise, it measures capacitance between the arms and the base, which is related to distance between them, which is related to force because the arms are flexible.) Edit: Put another way, there actually is an adjustment in the code, because most people want the reading to be zero when the phone is not moving relative to the Earth. To accomplish that, you can’t just subtract a constant vector, you have to filter the output based on the assumption that there is always a large constant acceleration in some direction. If you rotate the phone, that vector will vary wildly in direction, and you have to pick out that signal and subtract it. This means accelerometers have to have a lot of dynamic range to accurately report that large acceleration added to the tiny ones you’re more interested in. In orbit, this wouldn’t be necessary.
    - taeric 2y ago
      The point is only that the accelerometer is measuring force. If you put your phone on a centrifuge long enough so that it reads a constant force going into it, you would be able to make a situation in deep space where it reads 9.8m/s in what feels like a constant direction to any human. Right? How would the accelerometer be able to detect that?
      - wrs 2y ago
        The accelerometer will read 9.8m/s^2 along the axis of the centrifuge, because that is the acceleration that is keeping the phone moving in a circle instead of flying into space. The difference between that and resting the phone on the Earth is that the centrifuge structure is accelerating it by exerting force on it, whereas the Earth is accelerating it by altering the spacetime metric in its vicinity. Completely different mechanisms, same reading.
  - theamk 2y ago
    For cell phone, you don't need to trust the app much - it reports raw (X, Y, Z) vector. Turn the phone sideways and see how it's Z went to 0 and it's now Y (or X) showing -9.8. Spin in the chair and see how the vector's magnitude is increasing. This should be a pretty effective evidence that the vector is not offset, and at most scaled.
tsimionescu 2y ago
> Standing on the ground, the accelerometer should show zero because gravity is cancelled out by the floor being solid. If latter can be a force, why not the former? This is the part where you are demonstrably wrong. This experiment has actually been performed. Take an accelerometer that shows 0 in space far away from the Earth, where there is no gravity. Bring it back to the earth and let it fall from your ship: it will keep showing 0 (well, because of friction with air, it will show some value, but a pretty small one). When it reaches the ground, it will "suddenly" show 9.8m/s². This is not something you can debate: this is the measured reality. You can debate how to explain this, of course. But the reality is there: being in freefall towards the earth is equivalent from an accelerometer point of view to being in deep space with no engines running. Another similar observation is this: the feeling of being in a gravity well is not the earth pulling you down, it is the floor pushing you up. If you're on a space station in deep space, to feel the same as on the earth, you need something pushing at your feet in the direction of your head, not something pushing on your head in the direction of your feet.