The theory of quantum
mechanics was introduced when physicists found that the classical
theories could not describe the behavior of particles on the atomic
scale. Newtonian physics was inadequate and its predictions incorrect
when concerning certain phenomena, such the photoelectric
This disparity between
prediction and reality led Max Plank to formulate the foundations
of quantum theory. Others such as Einstien, Bohr, Schrödinger,
de Broglie,Heisenberg, Born, and Dirac further refined it.
Quantum mechanics describes
several areas that classical physics cannot. Here are some key points
of quantum theory:
It is made of fundamental
units, indivisible packets of energy. This theory was first proposed
by Plank to explain the photoelectric
For instance, humans
are quantized. You can have 1 human, 2 humans, 3 humans, or 100
humans, but you can’t have ½ a human. On the other hand, the number
line is not quantized. It is continuous. It doesn’t matter which
two numbers you pick, there’s always another number between them.
You can divide it into an infinite number of pieces. So something
that is quantized is like a staircase—it must jump up by steps.
But a continuous thing is like a ramp—you can take it in any increments
Spin is quantized. All
particles have "spin", or intrinsic angular momentum.
The properties of this "spin" are just like those of a
spinning, charged object. But you can’t really visualize particles
spinning like a top, because to have the amount of angular momentum
that they have, they would have to be spinning faster than light.
So "spin" is an innate property of matter. It is quantized,
and comes only in increments of 1 and ½. So you can have ½ spin,
1 spin, or 1½ spin, but never ¼ spin. Particles with spins that
are increments of whole numbers (1, 2, 3…) are called bosons.
Particles with increments of ½ are called fermions. All particles
have spin, and thus all are either bosons or fermions.
The most commonly known
quanta are photons, discrete units of light energy.
In the beginning, there
was light. And some said the light was made of particles, while
others said the light was made of waves. Neither theory managed
to encompass the full nature of light or the seemingly contradictory
results of experiments. In some experiments, light would diffract
Grating, indicating a wave nature. In others, it would appear
to be a particle, as the photoelectric
effect indicated. Finally, Louis de Broglie resolved the conflict
by proposing that matter and light have a dual nature. They can
behave as both waves and particles. The wave nature of matter can
be thought of as the probability wave of where the particle could
So…if waves can act as
particles, then can particles act as waves? Absolutely. Electrons
have been shown to diffract like waves under the proper conditions.
All particles, including
you and I, have wavelengths. But you’ll never find a person (or
even your pet rat) being diffracted, because as particles get larger,
their wavelengths get smaller. For particles on a macroscopic scale,
the wavelengths are so small we can’t make slits small enough to
Heisenburg Uncertainty Principle,
This states that it is
impossible to perfectly measure a particle’s position and velocity
at the same time. The more accurately you measure a particle’s position,
the more inaccurate your measure of its velocity, and vice versa.
The reason is that to
measure something, we must affect it. So by measuring, we disturb
the "true nature" of that something, and therefore cannot
with perfect accuracy find out its position and momentum.
Imagine you are in a
dark room, blindfolded. There is a large elephant walking across
the room (it’s a really big room), and the only way that you can
find out its location is by throwing ping pong balls at it. You
can find out pretty accurately where it is, where it is going, and
how fast it is going if you throw lots of ping pong balls at it.
Now, imagine that same
situation, but this time you want to find out information about
a paper airplane that someone threw across the room. You can throw
lots of ping pong balls at it, but when they hit the paper airplane
they are going to move it. So you are faced with a problem. If you
throw lots of ping pong balls you will know almost exactly where
it is, but very little about how fast and in what direction it was
originally going. If you throw just a few ping pong balls at it,
you can figure out its speed and direction fairly accurately, but
not its location. You can’t find out measure both things perfectly
at the same time.
The same sort of thing
occurs on the quantum level. At a macroscopic level, the effects
of the uncertainty principle are not obvious—a few photons bouncing
off your couch aren’t going to move it perceptibly. But on the quantum
level, when the thing you’re looking at isn’t a whole lot bigger
than a photon, you really see the uncertainty principle at work.
For instance, if you
want to find out where an electron is, you have to bounce a photon
off it. When you do so, you are going to move the electron from
its original location. The more photons you bounce off the electron,
the more precisely you’ll know where it was at the moment that the
photon bounced off of it. But because all those photons are going
to affect the motion of the electron, you will have very little
idea what its original velocity (speed and direction) was. Or, you
can shoot very few photons at the electron and get a very good idea
of how fast it is moving and in what direction. But you will not
know with much precision where the electron actually is.
So it is impossible to
perfectly measure a particle’s position and velocity at the same
The Heisenburg Uncertainty
Principle helps explain why some experimenters showed light behaving
as a wave while others showed it behaving as a particle. The more
accurately an experiment tries to measure the wave characteristics
of light, the less accurately it will measure light’s particle characteristics.
The opposite is also true. The clearer the particle characteristics
of light are shown, the less clear the wave characteristics of light
R. A., & Faughn, J. S.
D. “Blackbody Radiation."