Changes

#''The laws of physics are identical in all inertial reference frames.''

#''The laws of physics are identical in all inertial reference frames.''

#''The [[speed of light]] is constant for all (inertial) observers, regardless of their velocities relative to each other.''

#''The [[speed of light]] is the same for all (inertial) observers, regardless of their velocities relative to each other.''

As an example of the first of these, if one performs an experiment—say, with a pendulum of some similar apparatus—in a railroad car traveling uniformly at 50 miles per hour, one would find the results of the experiment (positions, speeds, etc.) to be consistent with the laws of physics in the railroad car.  An outside stationary observer, looking through the window, would also find the results consistent with the laws of physics in his stationary reference frame.  He would simply have to subtract 50 miles per hour from the velocities of the objects in order to get values that agree with the values obtained by the observer on the train.  But the two observations would each be consistent with the laws of physics.  This is all straightforward and obvious to modern people, and has been since the time of Galileo.  This principle is now called "Galilean relativity".  Another way of stating this is:

As an example of the first of these, if one performs an experiment—say, with a pendulum or some similar apparatus—in a railroad car traveling uniformly at 50 miles per hour, one would find the results of the experiment (positions, speeds, etc.) to be consistent with the laws of physics in the railroad car.  An outside stationary observer, looking through the window, would also find the results consistent with the laws of physics in his stationary reference frame.  He would simply have to subtract 50 miles per hour from the velocities of the objects in order to get values that agree with the values obtained by the observer on the train.  But the two observations would each be consistent with the laws of physics.  This is all straightforward and obvious to modern people, and has been since the time of Galileo.  This principle is now called "Galilean relativity".  Another way of stating this is:

*''There is no "preferred" frame of reference.''

*''There is no "preferred" frame of reference.''

Neither observer can claim that his frame of reference is "right".  Of course, the observer on the train would think that the Earth is moving at 50 miles per hour under the wheels.

Neither observer can claim that his frame of reference is "right".  Of course, the observer on the train would think that the Earth is moving at 50 miles per hour under the wheels.