The are Four Fundamental Forces:

Gravity (the weakest), Electromagnetic, and
Nuclear both weak and strong.

Einstein’s General Theory of Relativity explains
gravity.

Quantum Mechanics explains the other three.

**T**he force that
keeps us all glued to the surface of Earth, gravity, dominates any discussion of
the evolution and fate of the universe. Although it ranks as the weakest of the
Four Fundamental forces, the others pale when you talk about the universe as a
whole because the two nuclear forces act only over very short distances, while
most large objects are electrically neutral and therefore unaffected by the
electromagnetic force.

Isaac Newton first described gravity and had the insight to
realize that the force that holds us to Earth (and makes apples fall) is the
same one that keeps the planets in their orbits around the Sun. He deduced the
mathematical nature of the mutual force and correctly hypothesized that gravity
acts across the entire universe.

Albert Einstein modified this view of gravity by arguing
that the gravitational force is a manifestation of the curvature of space-time.
Although Einstein’s idea is necessary for describing the evolution of the
universe as a whole, Newton’s theory works well enough when gravitational
forces are not extremely strong.

Einstein envisioned gravity
as a curvature of space-time caused by the matter in it, as opposed to
Newton’s idea of a force acting at a distance. Although objects try to move
through space-time in straight lines, this warpage makes their paths appear bent

**A**t the heart of quantum
mechanics—the mathematical theory of the structure and behavior of
atoms—lies a certain degree of unpredictability. As first stated by the German
physicist Werner Heisenberg, the uncertainty principle says you can't
simultaneously measure both the position and velocity of a particle with perfect
accuracy. This means that no one can ever predict precisely the future behavior
of a particle because it’s impossible to measure the particle’s current
state exactly.

The uncertainty principle does not simply state that scientists don’t yet
have the proper equipment to measure positions and velocities: instead, the very
process of performing the measurement changes those quantities. The uncertainty
principle implies that space can never truly be empty. In reality, the quantum
vacuum is filled with particles and antiparticles that briefly appear and then
disappear just as quickly.

**Q**uantum mechanics, the best theory we have for describing the atomic
and subatomic world, comes up with some extraordinary portraits of nature. One
example is the uncertainty principle, which says that the position and velocity
of a particle (its state, as a physicist would say) can’t both be measured
with unlimited accuracy. A related idea is that the state of a particle cannot
be known precisely until the particle is observed; in other words, each and
every particle has a probability of being in any state. It does not exist in a
particular state until an experimenter observes it.

The Austrian physicist Erwin Schrödinger, one of the founders of quantum mechanics, thought of a paradox to show that quantum mechanics doesn’t apply to larger, tangible things. He envisioned a cat locked in a steel chamber with a tiny amount of radioactive material, a Geiger counter, and a diabolical device designed so that if the Geiger counter detects a radioactive decay, it activates a hammer that breaks a flask of acid and poisons the cat. It takes only one decaying atom to kill the cat, but whether an atom decays or not is governed by probability. Applying the rules of quantum mechanics to this system would mean that the cat is neither alive nor dead until a human observer actually looks in the chamber. Schrödinger argued that this was nonsense and merely an example of applying quantum mechanics to situations in which it doesn’t apply. Still, a few physicists defend the idea that the cat doesn’t take on an existence (or lack thereof!) until it’s observed.