3 ms·

For example by comparing two extremely good clocks. Or the same way the earth's gravitational potential is mapped - by measuring the relative height and the gra

by fsh 7y ago

For example by comparing two extremely good clocks. Or the same way the earth's gravitational potential is mapped - by measuring the relative height and the gravitational acceleration in many points and numerically integrating along the way.

ars 7y ago
I feel like we are speaking different languages - you are in free fall. There is no gravitational acceleration to measure. It's zero.
- fsh 7y ago
  Yes, you seem to have a fundamental misunderstanding about what free fall means. In the absence of other forces, a body in a gravitational field experiences a constant acceleration that only depends on the strength of the field. In the accelerating reference frame of the object, there are no net forces. You could for example measure the gravitational acceleration of earth by jumping from a plane and measuring how fast the ground is coming towards you.
  - ars 7y ago
    You are inside a box, you do not see the ground, so how can you use it to measure anything (to measure "how fast the ground is coming towards you")? Time dilation of a clock does not depend on the clock being able to reference some external object, like the earth. (I do recognize you can not measure time dilation until you compare to some other clock.) So, let's reframe with a thought experiment and maybe you can answer: A clock in the center of the earth, and a clock dropped from a tall tower on earth (ignore the rotation of the earth). Each clock sends two laser pules, exactly 1/100 of a second apart toward the other clock. (The falling clock sends its pulses moments after being dropped, so it has essentially zero speed relative to the clock in the center of the earth.) The partner clock then measures how far apart in time it received the two pules. After that we collect the reports from the clocks and evaluate. Will the two clocks each report the same time interval? For an even better experiment lets add a third clock, this one resting on the surface of the earth.
    - fsh 7y ago
      Now you are confusing a number of different things. Einstein used a free-falling sealed box to derive the equivalence principle. This does not imply that any free-falling system is a sealed box. Thinking about clocks makes absolutely no sense without having a way of comparing them. In your example, the clock in the center of the earth will run slower than the one falling from the tower due to gravitational time dilation. Note that Einstein's "light clock" thought experiments do not capture this effect, since it stems from General Relativity, not Special Relativity.

by fsh 7y ago

ars 7y ago

I feel like we are speaking different languages - you are in free fall. There is no gravitational acceleration to measure. It's zero.

fsh 7y ago
Yes, you seem to have a fundamental misunderstanding about what free fall means. In the absence of other forces, a body in a gravitational field experiences a constant acceleration that only depends on the strength of the field. In the accelerating reference frame of the object, there are no net forces. You could for example measure the gravitational acceleration of earth by jumping from a plane and measuring how fast the ground is coming towards you.
- ars 7y ago
  You are inside a box, you do not see the ground, so how can you use it to measure anything (to measure "how fast the ground is coming towards you")? Time dilation of a clock does not depend on the clock being able to reference some external object, like the earth. (I do recognize you can not measure time dilation until you compare to some other clock.) So, let's reframe with a thought experiment and maybe you can answer: A clock in the center of the earth, and a clock dropped from a tall tower on earth (ignore the rotation of the earth). Each clock sends two laser pules, exactly 1/100 of a second apart toward the other clock. (The falling clock sends its pulses moments after being dropped, so it has essentially zero speed relative to the clock in the center of the earth.) The partner clock then measures how far apart in time it received the two pules. After that we collect the reports from the clocks and evaluate. Will the two clocks each report the same time interval? For an even better experiment lets add a third clock, this one resting on the surface of the earth.
  - fsh 7y ago
    Now you are confusing a number of different things. Einstein used a free-falling sealed box to derive the equivalence principle. This does not imply that any free-falling system is a sealed box. Thinking about clocks makes absolutely no sense without having a way of comparing them. In your example, the clock in the center of the earth will run slower than the one falling from the tower due to gravitational time dilation. Note that Einstein's "light clock" thought experiments do not capture this effect, since it stems from General Relativity, not Special Relativity.