5 ms·

Now, here's the thing that confuses me. Say some advanced civilization can move around some large mass - an asteroid, a planet, a black hole, whatever, we'll j

Now, here's the thing that confuses me. Say some advanced civilization can move around some large mass - an asteroid, a planet, a black hole, whatever, we'll just call it "the big mass". They can, at will, fly it back and forth between two distinct positions we'll call 0 and 1. And let's say some distance away - a light-year, say - they have an facility where they can measure with excruciating precision the force of gravity on a test mass. After isolating out all other known gravity sources, they can use the remaining vector of gravitation force to compute the current position of the original big mass - and whether it's at position 0 or position 1. Assuming they can drag around the big mass from one position to the other in a short amount of time, shouldn't the people at the remote facility be able to detect where the big mass is long before light could reveal its position? Couldn't they use those observations to receive a low-bandwidth, but faster-than-light message? How does relativity prevent that?

ars 13y ago
It takes a while for the test mass to notice that the big mass moved. So the test mass is pulled in a direction where the big mass no longer is, but rather where it was.
- platz 13y ago
  to say it's "pulled in a direction where the big mass no longer is" seems incorrect, because above we have "whenever a gravitating object moves inertially, the gravitational acceleration vector at a point removed actually points at where the object actually is at a given instant", due to the cancellaction and abberation effects. "Indeed, the vector (2.5) does not point toward the “instantaneous” position of the source, but only toward its position extrapolated from this retarded data" (which is pretty much the same location)
  - ars 13y ago
    > whenever a gravitating object moves inertially The object is not moving inertially. Someone is forcing it to change direction, which is the exact opposite of inertially.
fragsworth 13y ago
The gravitational field points at where the object would be, given its current velocity. This is not to say that the object will actually end up there. If the velocity changes, the gravitational field changes direction according to the new velocity. The change propagates at the speed of light.
- jimmahoney 13y ago
  Yes. This is a case when a picture (well, a GIF animation) is worth lots of words. Look at http://jimmahoney.net/pulse_acceleration_field_lines.gif http://jimmahoney.net/pulse_acceleration_field_lines.gif . (I made this from the "Moving Charge" java physics applet at http://www.cco.caltech.edu/~phys1/java/phys1/MovingCharge/MovingCharge.html http://www.cco.caltech.edu/~phys1/java/phys1/MovingCharge/Mo... . It's a demo of the physics of electric charges, not general relativity, but the essence of the idea is the same even if the field equations are different. This follows directly from the central idea of relativity: the physics of uniform constant motion is the same as no motion, if you're moving along at the same speed.) The point charge (mass for gravity) in the center is first moving slowly to the right, then suddenly changes direction to move slowly to the left. (The small red vector indicates velocity.) The white lines represent the direction of the field, that is, the direction an object would be pulled. Near the point source, the field lines point to where it is. Far away, the news hasn't arrived yet that it's motion has changed, and so they point to where it would have been. During the brief acceleration when the source changed direction, the field lines connecting these two regions are "kinked" strongly - that's the radiation, which propagates outward at the speed of light. A second object would feel a sideways pulse as the wave (kink) passed by.
hmsimha 13y ago
I think this is actually a brilliant exercise in though. I have no idea what the answer is, and it's likely impossible to ever cancel out the effects of every other source of gravity within a 1 light-year radius, but it seems like in theory, if the force due to gravity propagates faster than the speed of light, your suggestion may hint at a method of communication that exceeds the speed of light.
- stcredzero 13y ago
  No. The change in gravitational fields propagates at the speed of light. The gravitational force vector for a star in the sky N light years away points to where the star was N years ago.
  - acchow 13y ago
    > The gravitational force vector for a star in the sky N light years away points to where the star would be if kept moving at constant speed for N years. FTFY.
    - hmsimha 13y ago
      Interesting. I don't know enough about physics to have a clue one way or the other, but even if it was the prior case (we base our model on where the star is for gravitational computations based on the light we are receiving), that's just what it is -- a model. So if there is a lapse in our understanding that doesn't take into account that the star's gravitational effect on us actually depends on its current position then that may need to be corrected.
    - fubari13 13y ago
      Constant speed is irrelevant in space-time calculations.
    - stcredzero 13y ago
      > FTFY You learn something new everyday!
gnaritas 13y ago
> Assuming they can drag around the big mass from one position to the other in a short amount of time, shouldn't the people at the remote facility be able to detect where the big mass is long before light could reveal its position? No, the remote facility won't notice the thing moved until the gravity wave hits and that only moves at the speed of light. If it's a light year away, it'll take a year before the remote facility notices the shift.

ars 13y ago

It takes a while for the test mass to notice that the big mass moved. So the test mass is pulled in a direction where the big mass no longer is, but rather where it was.

platz 13y ago
to say it's "pulled in a direction where the big mass no longer is" seems incorrect, because above we have "whenever a gravitating object moves inertially, the gravitational acceleration vector at a point removed actually points at where the object actually is at a given instant", due to the cancellaction and abberation effects. "Indeed, the vector (2.5) does not point toward the “instantaneous” position of the source, but only toward its position extrapolated from this retarded data" (which is pretty much the same location)
- ars 13y ago
  > whenever a gravitating object moves inertially The object is not moving inertially. Someone is forcing it to change direction, which is the exact opposite of inertially.

fragsworth 13y ago

The gravitational field points at where the object would be, given its current velocity. This is not to say that the object will actually end up there. If the velocity changes, the gravitational field changes direction according to the new velocity. The change propagates at the speed of light.

jimmahoney 13y ago
Yes. This is a case when a picture (well, a GIF animation) is worth lots of words. Look at http://jimmahoney.net/pulse_acceleration_field_lines.gif http://jimmahoney.net/pulse_acceleration_field_lines.gif . (I made this from the "Moving Charge" java physics applet at http://www.cco.caltech.edu/~phys1/java/phys1/MovingCharge/MovingCharge.html http://www.cco.caltech.edu/~phys1/java/phys1/MovingCharge/Mo... . It's a demo of the physics of electric charges, not general relativity, but the essence of the idea is the same even if the field equations are different. This follows directly from the central idea of relativity: the physics of uniform constant motion is the same as no motion, if you're moving along at the same speed.) The point charge (mass for gravity) in the center is first moving slowly to the right, then suddenly changes direction to move slowly to the left. (The small red vector indicates velocity.) The white lines represent the direction of the field, that is, the direction an object would be pulled. Near the point source, the field lines point to where it is. Far away, the news hasn't arrived yet that it's motion has changed, and so they point to where it would have been. During the brief acceleration when the source changed direction, the field lines connecting these two regions are "kinked" strongly - that's the radiation, which propagates outward at the speed of light. A second object would feel a sideways pulse as the wave (kink) passed by.

hmsimha 13y ago

I think this is actually a brilliant exercise in though. I have no idea what the answer is, and it's likely impossible to ever cancel out the effects of every other source of gravity within a 1 light-year radius, but it seems like in theory, if the force due to gravity propagates faster than the speed of light, your suggestion may hint at a method of communication that exceeds the speed of light.

stcredzero 13y ago
No. The change in gravitational fields propagates at the speed of light. The gravitational force vector for a star in the sky N light years away points to where the star was N years ago.
- acchow 13y ago
  > The gravitational force vector for a star in the sky N light years away points to where the star would be if kept moving at constant speed for N years. FTFY.
  - hmsimha 13y ago
    Interesting. I don't know enough about physics to have a clue one way or the other, but even if it was the prior case (we base our model on where the star is for gravitational computations based on the light we are receiving), that's just what it is -- a model. So if there is a lapse in our understanding that doesn't take into account that the star's gravitational effect on us actually depends on its current position then that may need to be corrected.
  - fubari13 13y ago
    Constant speed is irrelevant in space-time calculations.
  - stcredzero 13y ago
    > FTFY You learn something new everyday!

gnaritas 13y ago

> Assuming they can drag around the big mass from one position to the other in a short amount of time, shouldn't the people at the remote facility be able to detect where the big mass is long before light could reveal its position? No, the remote facility won't notice the thing moved until the gravity wave hits and that only moves at the speed of light. If it's a light year away, it'll take a year before the remote facility notices the shift.