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According to special (& galilean) relativity, there is no difference between constant movement or not moving at all but it is possible to detect acceleration and rotation is acceleration.

The earth rotates around the sun and the sun rotates around Sagetarius A* (the black hole in the center of the milky way).

So, there should be small accelerations directed to the center of rotation all the time.

Are those accelerations detectable? I think that it should be easily possible to detect the earths rotation around the sun but what about the rotation of the milky way?

Also, does the milky way accelerate in any direction or is it moving constantly?

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  • $\begingroup$ I don't know the right way to explain this, but the Earth orbits the Sun, and the Solar System orbits the center of the galaxy, and an orbiting body is a freely falling body. $\endgroup$
    – Ohm's Lawman
    Commented Jul 29, 2020 at 16:50
  • $\begingroup$ Simply said, there is no acceleration as those exactly cancel out? $\endgroup$
    – dan1st
    Commented Jul 29, 2020 at 16:55
  • $\begingroup$ Is it possible to detect if something orbits something else or just moves at a constant speed in a constant direction? Is spacetime curved in a way that it is the same? $\endgroup$
    – dan1st
    Commented Jul 29, 2020 at 16:58

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For objects moving under gravity, the centre of mass is in free fall so one cannot directly use an accelerometer to detect any acceleration of the centre of mass. If, however, the gravitational field is not unform (as is the case of the earth orbiting the sun) and the body is stuck together so as to move as a whole (as is the earth), then some parts of the body are not in free fall and the mismatch between their dragged-along motion and local free fall is detectable. We call the resulting effects tidal forces as they are the origin of the ocean tides.

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  • $\begingroup$ So, is it possible to e.g. detect if the milkey way (and other galaxies) or even the whole universe rotate around an object we don't know? $\endgroup$
    – dan1st
    Commented Jul 29, 2020 at 17:24
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    $\begingroup$ The steady tidal effects from anything other than the sun and moon are so small as to be impossible to detect. We can detect transient tidal forces using interferometers like LIGO, as these transient tidal forces are gravitational waves. $\endgroup$
    – mike stone
    Commented Jul 29, 2020 at 17:27
  • $\begingroup$ So it is impossible to detect such an object without actually finding the object (or similar)? $\endgroup$
    – dan1st
    Commented Jul 29, 2020 at 17:29
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I have read that the Milky Way and Andromeda are falling toward each other and that the two of them are falling together toward a region of the sky which contains a large cluster of galaxies. It occurs to me that if you assume that the galaxies are uniformly distributed throughout the universe, then the net force on any one of them would be zero. That means that gravity would not oppose the expansion of the universe.

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  • $\begingroup$ I know that galaxies move differently (but at constant speed or not? Or ist space moving?) but that does not answer my question. $\endgroup$
    – dan1st
    Commented Jul 29, 2020 at 20:03
  • $\begingroup$ It does indicate that the Milky Way is accelerating. $\endgroup$
    – R.W. Bird
    Commented Jul 30, 2020 at 12:37

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