Inertial reference frames are reference frames that move relative to each other with constant velocity.

• You’ll learn to work with the positions and times of events.

• All the clocks in an inertial reference frame are synchronized.

《 LOOKING BACK Section 4.3 Relative motion

Einstein’s theory of relativity is based on a simple-sounding principle: The laws of physics are the same in all inertial reference frames. This leads to these conclusions:

• Light travels at the same speed c in all inertial reference frames.

• No object or information can travel faster than the speed of light.

  

  Full Alternative Text

Time is relative. Two reference frames moving relative to each other measure different time intervals between two events.

• Time dilation is the idea that moving clocks run slower than clocks at rest.

• We’ll examine the famous twin paradox.

  

  Full Alternative Text

Distances are also relative. Two reference frames moving relative to each other find different distances between two events.

• Length contraction is the idea that the length of an object is less when the object is moving than when it is at rest.

  

  Full Alternative Text

Einstein’s most famous equation, $E=m{c}^2$, says that mass can be transformed into energy, and energy into mass, as long as the total energy is conserved.

• Nuclear fission converts mass into energy.

• Collisions between high-speed particles create new particles from energy.

  

  Full Alternative Text

Abstract though it may seem, relativity is important in technologies such as medical PET scans (positron-emission tomography) and nuclear energy. And the global GPS system, a technology we use every day, functions only when the signals from precision clocks in orbiting satellites are corrected for relativistic time dilation.