Section 1.2 - Special Relativity

Albert Einstein

The term "Einstein" has become synonymous with genius, but most people don't have even a rudimentary understanding of his life's work.
Special Relativity? General Relativity? Gravitational Waves? $E =mc^2$?

Special Relativity

Special relativity is a theory of spacetime proposed by Einstein in 1905, explaining the nature of motion, light, and time. This theory is grounded on two facts that are contradictory under classical mechanics but that we must accept under the new theory

  • There is no such thing as absolute motion or absolute rest. Objects move relative to each other (this is consistent under Newtonian mechanics)
  • The speed of light is the same regardless of the relative motion of the light source or the observers (experimentally proven by the Michelson-Morley experiment in 1887)

Do you see the clear contradiction?? Let me provide an example

Scenario 1: Baseball

Scenario 2: Light

The baseball scenario makes sense. The blue guy sees the baseball moving at $20 \thinspace km/hr$, the green guy observes it moving at $100 \thinspace km/hr$

But for the light scenario, the Michelson-Morley experiment tells us that both the blue guy and the green guy should observe the speed of light as $1.08 * 10^9 \thinspace km/hr$. This contradicts Newtonian physics, which would tell us the green guy would observe it as $1.08 * 10^9 + 80  \thinspace km/hr$


Einstein resolve the problem as follows:

  1. $speed =\frac{distance}{time}$
  2. Speed of light has been proven constant
  3. The light from the flashlight is traveling at an increased distance due to the combined motion of the bus and the light itself
  4. The only way to maintain the equation is for time to increase. Thus, time slows down for the blue guy (in motion) relative to the green guy (at rest)

However, Einstein's calculations showed that the time dilation wasn't quite enough to offset the increased distance. He demonstrated that the object in motion warps space by contracting, slightly reducing distance

Thus, Einstein unified the previously separate concepts of space and time into one entity - spacetime