to unite it with relativity. Special relativity was successfully united with quantum
mechanics in quantum field theory and the Standard Model of Particle Physics.
Quantum field theory adequately describes three of the four fundamental forces of
nature: the electromagnetic force, the strong nuclear force, and the weak nuclear
force. The electromagnetic force is responsible for almost all of the forces that we
see in daily life, including friction, the normal force, and many others. The strong
nuclear force, as mentioned earlier, is what holds the nucleus of an atom together,
since protons, which are positively charged, repel each other electrically but are
bunched together in the nucleus of an atom. The weak nuclear force mediates
radioactive decay.
However, general relativity and the force of gravity have remained difficult to
relate to quantum mechanical effects. This only matters when examining objects that
are so dense that they can, on the one hand, have enough mass that general relativity
is significant (remember that general relativity has only tiny effects even in the near
vicinity of the Sun, so a very great deal of mass is necessary), and on the other
hand, are small enough that quantum mechanical effects are important (remember
that quantum mechanics most regularly describes atomic structure and atomic
interactions). What has the mass of the Sun or more but is the size of an atom or
smaller? In the present universe, only a black hole has such densities. As a result,
physicists are investigating black holes and what can be learned about them without
seeing them.
The very early universe, before it had expanded very much, also involved
extraordinarily high densities, and light emitted extremely long ago that is just now
reaching us (because it was emitted very far away) can help in investigating current
problems in physics. One major source of information is the Cosmic Microwave
Background (CMB). In the early universe, a great deal of matter and a great deal of
energy (including light) were in a very small space, and as a result, photons that
were emitted from one source just hit particles of matter and were absorbed. As the
universe expanded, the density of the universe decreased, or, equivalently, the
space between particles increased, such that light could travel farther and farther
before hitting matter and being absorbed. Eventually, at a certain point, light
became able to travel almost freely without bumping into matter anymore, and that
light, which happened to be in the microwave area of the electromagnetic spectrum
because of the prevailing temperature of the universe at the time, is still moving
through the universe today. Since this light was everywhere in the universe at the
time, it is everywhere today as well, so it forms a background to everything in the
cosmos (hence the name Cosmic Microwave Background). Measurements of the
CMB are also evidence of the expanding universe, since the explanation for the
CMB is that the universe was once much smaller than it presently is and has been
expanding ever since, and it is hoped that more and more precise measurements of
the CMB will yield answers to questions about how the universe came to be in its