What is Quantum
Physics?
By Jim Tucek
"I think I can safely say that nobody understands
quantum mechanics." - Richard
P. Feynman
Does the thought of quantum physics send a chill down
your spine, just like the words calculus, differential equations, and -gasp-
organic chemistry? You may not even think that quantum physics is a serious
science, like the more familiar Newtonian physics. Just relax! I'm sure you're
comfortable with regular physics, which describes the way that matter interacts
with other matter, i.e. gravity, velocity, etc. Quantum physics
is just the physics of the incredibly small. While Newtonian physics can
suitably describe the orbit of the planets or the energy transformations during
a game of pool, quantum physics describes how electrons surround the nucleus of
the atom and other subatomic actions.
At
this point, you may be thinking that there's not that big of a difference
between these two sciences. Hey, both explain how matter interacts with other
matter, so what's the big deal? The difference is that the common laws of
physics begin to deteriorate on small scales. For example, Nippendenso (Japan
Electric) built a car that's only half a millimeters long. One could easily
mistake it for a grain of rice if not for its gold color. At the scale of 1 to
1000, physics is already changing. Oil would now gum up the engine, and the
tires wouldn't have enough traction to move the car.
Quantum
physics tries to explain the behavior of even smaller particles. These
particles are things like electrons, protons, and neutrons. Quantum physics
even describes the particles which make these particles! That's right; the
model of an atom that you were taught in high-school is wrong. The electrons
don't orbit like planets; they form blurred clouds of probabilities around the
nucleus. Protons and neutrons? They're each made of three quarks, each with its
own 'flavor' and one of three 'colors'. Lets not forget the gluons, the even
smaller particles that hold this mess together when they collect and form
glueballs (not a very original name). Why weren't you told about this already?
Were you fluent in calculus when you took general chemistry? The quantum model
of the atom is much more complex than the traditional model, so most teachers
save that stuff for college. (But this doesn't mean that you can't have a basic
understanding and impress your friends!) The reason that quantum physics needs
complex math to explain the behaviors and properties of small particles is that
the world of these subatomic particles is a very bizarre one, filled with
quantum probabilities and organized chaos. For example, the exact position and
velocity of an electron is very hard to find because attempts to
"see" it involve bouncing other particles off of it. By doing this,
you've just changed the electron's velocity, so your data is useless. What
quantum physics does is give us the statistical probability of the electron's
location at any one moment. By learning how these particles act, scientists can
better understand the matter which makes up the universe, and the way it
behaves (or misbehaves). Quantum physics even plays a part in blackholes, where
regular physics is thrown out the window and then some!
The
Origins of Quantum Physics
The
roots of quantum physics reach far into the past. Even Isaac Newton, the father
of classical physics, played a part in the development of quantum physics. He
didn't know it at the time, but one of his most famous arguments was a matter
of Quantum physics. Newton tried to explain the behavior of light in terms of
particles, which he called corpuscles. He was the
founder
of the physics of particles after all, so why shouldn't light be treated like
particles, just like the planets. The Dutch physicist Christiaan Huygens,
however, tried to describe light in the terms of waves. Although the wave and
particle theories of light were both sound, there was one obvious problem with
Huygens' wave theory: when light is obstructed, it creates a shadow with
well-defined edges. If light was a wave traveling through Huygens'
"ether", it would flow around the edges of the obstruction, blurring
the shadow. (This is not the case because the wavelength of light is small
enough to create sharp edges, but this was not considered at the time.) Because
of this flaw, and the fact that Newton was the physics hotshot of his day, the
particle theory was accepted. With quantum physics, though, both of these
theories are right.
The wave
theory did not come up again until an English scientist named Thomas Young
devised an interesting experiment to test it. This experiment, which is explained
under Important Experiments, proved Newton's particle theory of light to be
wrong. The experiment was ignored by most scientists because of Newton's
greatness. Augustin Fresnel, a Frenchman, however, adopted this idea and worked
to create a wave explanation of light. His work also included explaining why a
thin film of oil creates such a colorful reflection. He noticed that a film of
oil is bumpy and uneven, so it reflects the light at different angles. He
theorized that if color was a product of the wavelength of light, and since
waves can mix in ways to either strengthen or dampen the product's wavelength,
the colors produced must be a product of the light bouncing off the uneven
surface and interfering with the other reflected light waves. If light was made
up of particles, it couldn't do this. By the nineteenth century, it had become
accepted that light was made of waves.
Important Experiments
Thomas
Young's experiment consisted of a light source shining on a obstacle with a
small slit in it. If a wave hits such a slit, it spreads out the way shown. The
next two slits on the next obstacle create two more such waves, which are close
enough to interfere with each other. If light truly was a wave, then this
interference would either increase or decrease the intensity of the light when
it hit the screen.
If light were just a particle, and you were able to send just one photon
through, then there would be no pattern on the screen, just a single point of
light. However, it has been found that even if just one photon is sent through,
it creates the same interference pattern, although dimmer. If the light is measured,
or observed, in between the screen and the second barrier, no interference
pattern is formed. Instead, there is the most intense light in between the two
slits, which gets dimmer as it
progresses
away.
This
phenomenon is one of the basic principles of quantum physics, the Heisenberg Uncertainty
Principle. If light is not being observed, it acts
as a wave, but if it is being observed, it has to behave itself and act like
particles. Thomson's experiment with cathode ray tubes which allowed him to
discover electrons was rather simple. He took a cathode ray tube, or a glass
tube with very little gas left inside of it, and sent a current through it. As
expected, it created a glow from one electrode to the other. Next, Thomson took
a magnet, which deflects the flow of electrons (called cathode rays), and put
it next to the cathode ray tube. This deflected the beam slightly. He realized
that the cathode rays were being deflected by the negative end of the magnet,
so the cathode rays must be negatively charged. Because of the flow of
electricity through the tube, these negative particles were being knocked off
the atoms in the gas. This led him to name them electrons, which (to him) where
negatively charged particles embedded in a sphere of positive charge, which
made up the atom.
You
may be wondering how quantum physics applies to the real world. How can anything
so small be of any use to us? What about data storage and processors? The
smaller and denser they get, the better they are. And the way things are headed
in the computer hardware industry, in another twenty years these computer
components will be so small and densely packed together that quantum physics
will actually be a major factor. Still not impressed? Do you want some big
discovery in the
laboratory
which prompts a loudly exclaimed "Eureka!" Quantum physics can do
that too.
In 1995, a team of Colorado scientists managed to cool atoms enough to make
them into a Bose-Einstein Condensate. No, this is not some sort of new
ice-cream flavor. This state is created when the wavelength of the atoms get
large enough to overlap, creating a mass of thousands of atoms which act like
one. Another way to look at it is that as the atoms get colder and thus slower,
their velocity gets easier to obtain. You could pretty much say that they had
none. The Heisenberg Uncertainty
Principle says that the more you know about the
velocity of an electron (or photon), the less you can know about its location,
and vise versa. As the velocity gets easier to find, the position of the
electron gets "fuzzy". If the Gateway Arch were under these same
conditions somehow, it's position would be in downtown St. Louis, give or take
a hundred miles. Because of this give-or-take position, the electrons get
"fuzzy" enough to overlap into one big blob.
The
Bose-Einstein Condensate has some very unique properties. Recently, Danish
physicist Lene Vestergaard Hau and her colleagues made a condensate and sent a
beam of light through it. Nanoseconds later the beam of light had left the
other side, although nearly 90% weaker. It's no big deal that it was weaker.
It's just like a really good pair of sunglasses. The real surprise comes from
the fact that the beam of light should have come out even faster. The light
took its time getting through the condensate (relatively speaking). The
computed speed of the light through the condensate was a mere thirty-eight
miles-per-hour. I've gone faster than that, although not through a
Bose-Einstein Condensate. At that speed, it would take light nearly three
centuries to reach the Earth from the sun, instead of the traditional eight
minutes. That's really slow for light! The researchers predict that they will
soon be able to slow light to a sluggish speed somewhere under ten
miles-per-hour.
This
discovery actually has some useful applications. Because light travels
extremely fast under normal circumstances, it makes an efficient way of moving
data, as it does through fiber-optic cables. The idea of a computer that mostly
relies on light instead of electrons to move data to and from the processor,
etc. has the advantage that it would be extremely fast in its calculations. The
speed which would make calculations rival any super computer also makes a very
tricky way of storing the data as light. A Bose-Einstein Condensate could slow
light enough to put it in storage, much like a hard-drive. Although a
Bose-Einstein Condensate at home is a bit unreasonable today, it may not sound
like such a bad idea a few decades from now.
"Anyone who is not shocked by the quantum
theory has not understood it."
- Niels
Bohr
