Contemporary Physics
While 1915 may seem like a long time ago, virtually all physics that has been done
since then (with the exception of atomic and nuclear physics) is beyond the scope
of most introductory physics classes and can be called “contemporary physics.”
While you do not need to know the details of any of the theories in this section,
knowing some of the major results may be useful in answering a few questions on
the test, which may focus either on the major results themselves or the people who
worked on the theories.
The dates in the
discussion that follow
are just for context, not
to be memorized.In 1915, a decade after he published a paper on special relativity, Einstein
published the theory of general relativity, which (as the name suggests) was more
general than the first theory because it could account for accelerated motion and for
motion in the strong gravitational fields near large masses. Indeed, the guiding
principle of general relativity, called the Equivalence Principle, is that it is
impossible to differentiate between an accelerating reference frame and a reference
frame in a gravitational field; such frames are equivalent.
For example, consider an elevator in space (far from Earth) that is accelerating
upward. A person standing in that elevator will feel pressed against the floor as if
pulled down by gravity, even though there is no gravitational field nearby.
Likewise, a person in a stationary elevator on Earth feels pressed against the floor
because of Earth’s gravitational field. Next, consider what the person would see if
the elevator were accelerating very quickly and the person shined a flashlight at the
opposite wall. The light would appear to bend down, since it is going straight as
the person accelerates up. The Equivalence Principle dictates that the same bending
must occur in a strong gravitational field as well. That is, gravity makes light bend.
Einstein went on to point out that this is not simply a property of light. Light bends
because space itself bends. The space near large amounts of mass is bent, and it is
this bending that causes all the gravitational effects that we see. Furthermore, it is
not simply space that bends. Four-dimensional spacetime bends, with the result that
time dilates in a gravitational field just as it does for an object moving at very high
speed.
If the elevator were simply
moving up at constant
speed, special relativity
says that the light would