It's been a while since we talked about the motion of objects in terms of displacement, velocity, and acceleration. Remember the guy standing on his head? If he were to describe the motion of objects around him, it would be a unique perspective compared to your own, if you were standing on your own two feet, right-side up. The point is, motion is relative, or described in relation to the observer or how the frame of reference is defined. The motion of the object with respect to your point of reference is different than the motion described with respect to someone else.

In this lesson, you will encounter some really strange concepts—mainly because when you start talking about speeds close to the speed of light. At these speeds, everything started behaving differently that our normal view of relativism. In fact, in 1905, along with his famous equation E = mc², Einstein went against the normal understanding of Newtonian physics and said that not only is there an equivalence between mass and energy, but our speed dictates how we experience time and length as well. This was his Special Theory of Relativity.

In 1915, Einstein also expanded on the understanding of Newtonian physics as well in his General Theory of Relativity. This theory is equally as challenging to understand as it involves some pretty high-level mathematics. It basically states the simple truth that motion is relative to the motion between the observer and the object, specifically when dealing with light and gravity. The result is that light behaves similar to objects in that it "bends" when it experiences a gravitational field. Light can also appear to shift frequencies if the object emitting the light or the observer are moving (or both). This concept is used to determine whether distant objects are moving away from us (red shift) or moving toward us (blue shift). We will leave the more in-depth studies of the General Theory of Relativity for your future studies in physics.