6. Relativity:
            Space, Time Spacetime
            The speed of light
            Some “things” can go faster than light, but not things that carry information.
            E = mc^2
            Mass changes the rate that clocks run.
            Mass changes distance intervals in a vacuum.
            The expansion of the universe in space and time.

 6. Relativity
Space, Time Spacetime
            The speed of light as the ultimate speed limit for traveling mass energy or information.
            “Things” without mass, energy or information can travel faster than light.
            If information can travel faster than light then it will travel back in time.
            The speed of light limit means that looking out in space is looking back in time.
            We see the moon as it was 1.5 seconds ago, we see the sun as it was 8.5 minutes ago
            We see the nearest star beyond the sun as it was 4 years ago.
            And we see the most distant light detectable by the human eye from the galaxy Andromeda after it has traveled 2 million years.
            The speed of light is the same in all reference frames, this provides a framework for all of relativity which looks for things which remain constant independent of reference frame.
            Einstein wanted to revise the name of his theory of relativity to the theory of invariants but the original name stuck.
            Another example of an invariant is the rest mass of an object.

            The theory of relativity not only combines space and time, it also shows that energy and mass are related by the famous equation E = mc^2.
            The coupling between space and time means that the measured length of an object shortens if the object is measured in motion, it also means that moving cocks run more slowly, and it means that two events that are simultaneous in one frame of reference are not simultaneous in a moving frame.
            In 1915 Einstein expanded his Special Theory of Relativity to the General Theory of Relativity. In the general theory Einstein shows how mass changes space, and how warped space changes the motion of objects.
            Mass changes the rate at which clocks run.
            Mass changes the distance intervals between two objects.
            Einstein’s general relativity also describes the expansion of the universe in space and time. And provides a framework for the understanding of Dark Energy which is thought to be the driving force for the expansion of the universe.