In 1803 an English scientist named Thomas Young
did a remarkable experiment. Young, who had many interests and
was also involved in the decipherment of Egyptian hieroglyphics,
was exploring the nature of light. His experiment would start
a revolution in physics that would eventually overturn the rules
of motion established by Isaac Newton a century before. It would
also expose one of the great mysteries of the universe: quantum
Indeed, the enigma represented by quantum physics
is truly a profound mystery. If someone were to capture an actual
sea serpent or stumble across a living dinosaur, it would be the
focus of press coverage for months. The discovery would be the
topic of conversation at every water cooler in every office around
the world. Yet, as startling as such a find would be, neither
would change our world view all that much. We know that both large
marine reptiles and dinosaurs existed in the distant past. We
might be amazed that somehow these creatures managed to survive
so long on Earth without being detected by science, but their
discovery would not change the theory of evolution to any great
The mystery at the heart of quantum physics, however,
strikes directly at our perception of whether the universe and
everything in it, including ourselves, is real. Also, while the
idea of living sea serpents or dinosaurs is highly speculative
and unlikely, the theory of quantum physics is one of the most
"battle tested" in science. There is little doubt that, despite
its bizarre features, it is accurate. As physicist Daniel M. Greenberger
put it, "Einstein said that if quantum mechanics were correct
then the world would be crazy. Einstein was right - the world
example of an interferance pattern (Credit:
Patrick Edwin Moran licensed through Wikipedia Commmons).
Double Slit Experiment
The best place to start talking about quantum physics
is with Young's 1803 experiment. At the time, scientists were
trying to figure out if light consisted of some kind of particle
or if it was a wave traveling through some unknown medium (like
waves moving through water). Young's experiment used a tiny source
of light and a screen. In between these two objects he added a
barrier with two narrow, vertical, parallel slits.
Young knew that if light was just a stream of tiny
particles it should just pass through each of the slits and pile
up on the screen behind the holes.
This was, in fact, exactly what occured if he covered
one slit and left the other one open. A vertical sliver of light
showed up on the screen behind the hole. Young might have expected
to see two slivers of light when he uncovered the other slit,
but this is not what happened.
Instead the screen was covered with a series of
vertical bands of light and dark that ran across most of the screen.
Young realized what this meant: The light was traveling like a
wave and passing through both slits. After passing through the
slits, the waves spread out in a semi-circular pattern so that
they ran into each other. As they did, where the two wave tops
met they would re-enforce each other and where a wave trough and
top met, they would cancel out. The result was an alternating
series of light and dark bands. Scientists call this an interference
pattern as it is the result of two waves interfering with
sketch of his experiment showing the light waves emerging
from slit A & B, and interfering to create lines on
the screen at the bottom at C, D, E and F.
So light was indeed a wave. Over the years, though,
scientists looked for the medium that the waves traveled on (which
they referred to as the ether) but couldn't find it. What's
more, there also seemed to be evidence piling up that light did
travel as a kind of particle (which would later be given the name
photon). Eventually, it was decided that photons had a
duel nature, acting as both a wave and a particle. Despite this,
physicists still wondered what would happen if you sent the photons
through the double slit experiment one at a time.
Eventually a light source was invented that would
release only one photon at a time and Young's double slit experiment
was tried again. Instead of a screen, however, photographic paper
was used because a single photon was too dim to be seen on a screen,
but after millions of them had passed through the slits (one at
a time) any pattern would be visible on the film when it was developed.
When the photograph was developed it showed the
same interference pattern as before. Scientists were forced to
conclude that each individual photon traveled as a wave, passed
through both slits at the same time and interfered with
itself, only appearing as a particle at a particular position
when it finally hit the photographic paper. This seemed crazy.
Scientists decided to see what would happen if they
put a photon detector next to one of the slits to see which path
the photon actually took. They indeed managed to do that,
but when they did, the interference pattern disappeared and only
two slivers of light (one behind each hole) appeared on the screen.
The photons seemed to "know" that they were being watched and
changed their behaviors from that of a wave to that of a particle!
Scientists then decided to put the photon detector
on the far side of the screen from the light source so
the photon would not be observed until after it had gone
through the slit. The result was the same, however. The photon
seemed to "know" there would be a detector on the opposite side
of the screen before it got there and then changed to particle
mode before going through the slits.
that demonstrates the double slit experiment. Note that
while this clip was included in the questionable documentary
What the Bleep Do We Know?, it does accurately
demonstrate this particular experiment.
Finally a scientist named John Wheeler proposed
an experiment where the screen could be pulled away at the last
moment before the photons hit to be replaced by a set of optical
detectors that would determine from which slit the photon had
come from. The decision about whether or not to move the screen
would not be made until after the photon passed through the slit.
At the time Wheeler proposed the experiment, it was technically
impossible to do. A few years later, however, the experiment was
actually tried. If the screen was in position the photon acted
as part of an interference pattern, but if the screen moved at
the last moment so the "which slit" information was captured,
that photon did not participate in the interference pattern. The
photon seemed to know how to behave when it reached the slit even
though the decision about whether the screen would be in place
had not yet been made. It seemed that either the photon could
predict the future, or a decision about how to place the screen
could change the past.
It looked like to scientists that in quantum theory
causality seemed to take a hike. Things happening in the present
seemed to alter the past. And that was just the tip of quantum
If this stuff seems to bother you , don't worry.
It bothered a lot of people, including Albert
Light, Star Bright
Tonight, go out and look at the stars. If it is
winter (in the northern hemisphere) you should be able to see
the constellation Orion, the Hunter. It's easy to pick out because
of the three stars in a straight line that make up Orion's belt.
Look at the middle star. It's a blue-white supergiant named Alnilam
1300, light years away. When you look at the star, what happens?
Most books will tell you that one-thousand-three hundred years
ago - the Early Middle Ages in Europe - an excited electron on
a hydrogen atom on the outer fringes of the star released a particle
of energy: a photon.
The photon raced away from Alnilam in the
direction of Earth at the speed of light, about 186,000 miles
per second. Though photons aren't greatly affected by gravity,
the planets, stars and other celestial objects it passed pulled
on it tiny amounts giving it unique path through the emptiness
of space. As it approached Earth, it dodged interactions with
the molecules of the atmosphere. You looked up at the sky at just
the right time to catch it. The photon (along with many others)
stimulated the retina in the back of your eye, sending a signal
to your brain and in your mind you saw the starlight. A rather
amazing course of events; except that according to quantum theory,
that is not what happened. Not at all.
Orion constellation. The star in the center is Alnilam.(Credit:
Mouser Williams licensed through Wikipedia Commmons)
Nobody really knows what is actually happening at
the quantum level. There are, however, several interpretations
of the quantum theory that are designed to help us think about
it in terms that our minds might understand. The best known one
is called the Copenhagen Interpretation because it was
authored mostly by the physicist Neils Bohr who lived in Copenhagen.
For years, scientists and engineers have used Copenhagen as the
standard way of looking at the quantum world. The Copenhagen interpretation
of quantum theory sees your visualization of Alnilam this
About 1300 years ago a photon didn't leave the hydrogen
atom, but a wave of probability. The wave represented not the
probability of where the photon was, but where it might be found
if it was observed. The wave moved outward at the speed of light
not in the direction of Earth, but as a sphere inflating at the
speed of light. The planets, stars and other objects near it affected
the probability of where the photon might appear, but there was
still a chance it could show up anywhere in the expanding sphere.
The wave/sphere expanded for 1300 years until it
was two-thousandsix-hundred light years across - Some 15,250,809
billion miles wide. The wave front swept through Earth's atmosphere
and just at the moment you focused your eye upon Alnilam
and the wave front entangled with the cells on your retina. Then
somewhere, between your retina entangling with the wave and your
mind seeing the star, it happened.
Instantly across the expanse of 2,600 light years
the probability wave collapsed and the photon came into being,
interacting with the retina of your eye. It could have been that
if you hadn't looked up at just the right moment that the photon
could have been collapsed by some alien observer a few seconds
later on a planet a thousand light years away on the other side
of Alnilam. Your observation on Earth, however, forever
removed that possibility.
When you observed that photon a unique history was
created for it. The path that it traveled from that hydrogen atom
at Alnilam to your eye was created.
helped birth Quantum Theory but was very bothered by it.
It might seem that the instantaneous collapse of
anything 2600 light years wide would be impossible as it would
exceed the speed of light. However, this is just one of several
examples where quantum theory seems to defy the cosmic speed limit.
This was also something that bothered Einstein deeply.
It has been said that in the beginning of the 20th
century Einstein birthed two children - two great physics theories.
It is said that he loved one child (Relativity) and hated the
other (Quantum Physics).
What upset him about quantum physics? First it was
non-deterministic. If you were to set up a gun and fire it at
a target it would be very easy to predict the course of the bullet
if you knew the speed and direction that it was going when it
left the barrel. Not so with a photon. As our example with the
light wave traveling from a distant star showed, the photon moves
as a probability wave. The photon could show up anywhere along
the course of the wave, though the chances are better in some
places than others. This caused Einstein to quip that he didn't
think "God plays dice with the Universe."
This second thing that bothered Einstein was the
idea that, according to Copenhagen, until an object was observed,
it only exists as a probability wave. For a photon this might
not seem like a big deal. It's a really, very, very small thing.
However, not only do photons obey the rules of quantum physics,
so do electrons, protons, atoms, and molecules. They are all only
waves until they are observed and a the two slit experiment has
been done with objects as large as fullerene molecules which consist
of 60 carbon atoms each.
In the end when you think about it, our whole world,
including ourselves, is just made up of atoms and molecules. Does
that mean that we are just a big probability waves?
had doubts about the theory that he helped create and
came up with the famous cat thought experiment to show
the theory was incomplete.
The idea that objects in our world do not exist
independent of observation caused Einstein to quip, "I prefer
to think the moon still exists even when I'm not looking at it."
Einstein wasn't the only founder of quantum theory
that had doubts about it. Edwin Schrödinger came up with one of
the key equations for predicting how a quantum system will change
over time. It won him the Nobel prize in 1933. Despite this, he
was bothered by some of quantum physics implications and came
up with an example to demonstrate their absurdness.
Schrödinger's thought experiment involved placing
a cat in a sealed box (note: this was only an example, he never
meant for somebody to try this with a real cat). Into the box
with the cat would go a small piece of "diabolical" machinery
consisting of radioactive material, a geiger counter and a vial.
The radioactive material was sized so that over the course of
an hour there would be a 50 percent chance of it decaying and
giving off a particle that would set off the counter. The counter
was also hooked up so if it detected the particle it would release
a hammer designed to smash the vial filled with deadly hydrogen
At the end of an hour there would be a fifty/fifty
percent chance you would open the box and find the cat dead or
alive. What state is the cat in before the box is opened, however?
Because the decay of an atom is a quantum event, the Copenhagen
Interpretation suggests that until it is observed the atom (as
a probability wave function) is in superposition - both states
simultaneously. This would seem to mean the deadly mechanism and
the cat are also in superposition with the cat both alive and
dead. Schrödinger found this a ridiculous idea and used it to
try and show the problems that quantum theory was either wrong
Another thing that bothered early physicists working
with quantum theory was the idea of an observer collapsing the
wave function. What qualifies as an observer? The geiger counter?
The cat? A conscious human experimenter?
Consciousness seems to some people to be connected
in some strange way with quantum physics. For many physicists,
however, this is an anathema. Since Copernicus first moved the
Earth from the center of the solar system to just one of a number
of planets orbiting the sun, man's place in the cosmos has continually
shrunk until our planet is just a tiny speck in a vast, endless
universe. If quantum effects are directly connected with consciousness,
it would seem to turn 500 years of science on its head. Something
physicists are loath to do.
famous Schrödinger's Cat thought experiment. (Credit:
Dehatfeild licensed through Wikipedia Commmons)
Are there other interpretations of the theory that
resolve these problems? Yes, and we will discuss them in the next
segment. Each alternative interpretation, however, comes with
a price and none can completely escape quantum weirdness.