## Let's Summarize

Let's summarize what the Second Postulate has mandated about space and time. Each makes use of two frames of reference which are moving relative to one another at a constant speed: inertial frames of reference. Observers are positioned in each frame equipped with rulers and clocks with which they can determine lengths and time intervals in their and the other frame.

1. Events which are simultaneous in one frame, will not be simultaneous in the other.

   * This means that the two observers cannot synchronize clocks between their two frames.
   * This means that no frame is distinguished from the other in any way – so that there cannot be a privileged frame of reference as implied by the idea of the ether.

2. A clock in the Away frame will appear to run slower as viewed by the Home frame. This is called Time Dilation.
   * This is reciprocal between the two frames since each observer would think of themselves are being in their own Home frame and the other an identically moving Away frame.
3. A length in the Away frame will appear to be shorter as viewed by the Home frame. This is called Length Contraction.
   * Just like Time Dilation, Length Contraction is reciprocal between Home(s) and Away(s).