mistaken. The “modern” physics of the past century focuses on phenomena so far beyond
the scope of ordinary experience that Newton and friends can hardly be blamed for failing
to notice them. Modern physics looks at the fastest-moving things in the universe, and at
the smallest things in the universe. One of the remarkable facts about the technological
advances of the past century is that they have brought these outer limits of nature in
touch with palpable experience in very real ways, from the microchip to the atomic bomb.
One of the tricky things about modern physics questions on SAT II Physics is that your
common sense won’t be of very much use: one of the defining characteristics of modern
physics is that it goes against all common intuition. There are a few formulas you are
likely to be tested on—E = hf in particular—but the modern physics questions generally
test concepts rather than math. Doing well on this part of the test requires quite simply
that you know a lot of facts and vocabulary.
Special Relativity
Special relativity is the theory developed by Albert Einstein in 1905 to explain the
observed fact that the speed of light is a constant regardless of the direction or velocity of
one’s motion. Einstein laid down two simple postulates to explain this strange fact, and,
in the process, derived a number of results that are even stranger. According to his
theory, time slows down for objects moving at near light speeds, and the objects
themselves become shorter and heavier. The wild feat of imagination that is special
relativity has since been confirmed by experiment and now plays an important role in
astronomical observation.
The Michelson-Morley Experiment
As we discussed in the chapter on waves, all waves travel through a medium: sound
travels through air, ripples travel across water, etc. Near the end of the nineteenth
century, physicists were still perplexed as to what sort of medium light travels through.
The most popular answer at the time was that there is some sort of invisible ether
through which light travels. In 1879, Albert Michelson and Edward Morley made a very
precise measurement to determine at what speed the Earth is moving relative to the
ether. If the Earth is moving through the ether, they reasoned, the speed of light should
be slightly different when hitting the Earth head-on than when hitting the Earth
perpendicularly. To their surprise, the speed of light was the same in both directions.