I'm struggling with the notion of an inertial frame of reference. I suspect my difficulty lies with the difference between Newtonian and relativistic inertial frames, but I can't see it.
I've read that Newton's laws apply in any non accelerating frame of reference, which are called inertial frames. So, if I play pool on a train moving with uniform velocity, the balls behave in the same way as if I were playing pool in a pool hall. So the train is (to a good approximation, ignoring forces caused by the Earth's rotation and movement around the Sun etc) an inertial frame.
I've also read that in an inertial frame, Newton's first law is satisfied. So, if I slide a rock on a sheet of ice and I could somehow eliminate the frictional forces between the rock and the ice, the rock would carry on sliding for ever, as predicted by Newton's first law.
Question 1 – is the sheet of ice also therefore a good approximation to an inertial frame?
Question 2 – are these two definitions of inertial frames saying the same thing in different ways?
Both these examples occur in gravitation fields, which doesn't seem to matter as both the train and the ice sheet are presumably good approximations to inertial frames.
Question 3 – is gravity irrelevant when defining these two inertial frames?
In special relativity, I've read (Foster and Nightingale) that an inertial frame is also one where Newton's first law holds. But as it's special relativity there cannot be an inertial frame if there's a gravitational field, so I'm assuming the above two examples are not inertial frames in special relativity.
Question 4 – how come you can use the same definition of an inertial frame (satisfied Newton's first law) but in the case of special relativity, the train and the ice sheet aren't inertial frames. Is this to do with not being able to synchronize clocks in a gravitational field?
In general relativity, I've read (Schutz) that a freely falling frame is (on Earth) the only possible (and local) inertial frame. So again the first two examples would not be inertial frames in general relativity.
Question 5 – am I right to think that my two examples aren't even approximate approximations to an inertial frame in relativity?
Question 6 – have I missed anything else that might be useful?
Many thanks
Edit. On reflection, am I right in thinking that because Newtonian mechanics assumes universal, absolute time we don't need to worry about synchronizing clocks in a Newtonian inertial frame. Therefore we don't need to worry about gravity in a Newtonian inertial frame, because in such a frame gravity does not affect time.
This is not the case in spacetime, as here gravity does affect time and the only way to have synchronized clocks in an inertial frame in spacetime is:
1. have no gravitational field, or
2. use a local, freely falling frame.
Am I on the right track here?
Best Answer
"But as it's special relativity there cannot be an inertial frame if there's a gravitational field, so I'm assuming the above two examples are not inertial frames in special relativity." SR simply doesn't apply. That doesn't mean that these examples are noninertial frames according to SR; it just means that SR can't discuss the situation at all. (Of course, the gravitational field might be small enough to ignore for the purposes of analyzing a given situation using SR.)
The definition of an inertial frame in SR is essentially the same as in Newtonian mechanics.
"am I right to think that my two examples aren't even approximate approximations to an inertial frame in relativity?" Yes in GR. No in SR.