**What is Quantum Physics?
**[Illustrations
from original article are missing – visit the ThinkQuest website – double-click back
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Quantum physics is a branch of science that
deals with discrete, indivisible units of energy called quanta as described by
the Quantum Theory. There are five main ideas represented in Quantum Theory:

- Energy is not continuous, but comes in small but discrete units.
^{1} - The elementary particles behave both like particles
*and*like waves.^{2} - The movement of these particles is inherently random.
^{3} - It is
*physically impossible*to know both the position and the momentum of a particle at the same time. The more precisely one is known, the less precise the measurement of the other is.^{4} - The atomic world is
*nothing*like the world we live in.^{5}

While at a glance this may seem like just
another strange theory, it contains many clues as to the fundamental nature of
the universe and is more important then even relativity in the grand scheme of
things (if any one thing at that level could be said to be more important then
anything else). Furthermore, it describes the nature of the universe as being
much different then the world we see. As Niels Bohr
said, "Anyone who is not shocked by quantum theory has not understood
it." ^{6}

**Particle/Wave Duality**

Particle/wave duality is perhaps the easiest
way to get aquatinted with quantum theory because it shows, in a few simple
experiments, how different the atomic world is from our world.

First let's set up a generic situation to
avoid repetition. In the center of the experiment is a wall with two slits in
it. To the right we have a detector. What exactly the detector is varies from
experiment to experiment, but it's purpose stays the
same: detect how many of whatever we are sending through the experiment reaches
each point. To the left of the wall we have the originating point of whatever
it is we are going to send through the experiment. That's the experiment: send
something through two slits and see what happens. For simplicity, assume that
nothing bounces off of the walls in funny patterns to mess up the experiment.

First try the experiment with bullets. Place
a gun at the originating point and use a sandbar as the detector. First try
covering one slit and see what happens. You get more bullets near the center of
the slit and less as you get further away. When you cover the other slit, you
see the same thing with respect to the other slit. Now open both slits. You get
the sum of the result of opening each slit. ^{7} The
most bullets are found in the middle of the two slits with less being found the
further you get from the center.

Well, that was fun. Let's try it on
something more interesting: water waves. Place a wave generator at the
originating point and detect using a wave detector that measures the height of
the waves that pass. Try it with one slit closed. You see a result just like
that of the bullets. With the other slit closed the result is the same. Now try
it with both slits open. Instead of getting the sum of the results of each slit
being open, you see a wavy pattern ^{8}; in the center
there is a wave greater then the sum of what appeared there each time only one
slit was open. Next to that large wave was a wave much smaller then what
appeared there during either of the two single slit runs. Then the pattern
repeats; large wave, though not nearly as large as the center one, then small
wave. This makes sense; in some places the waves reinforced each other creating
a larger wave, in other places they canceled out. In the center there was the
most overlap, and therefore the largest wave. In mathematical terms, instead of
the resulting intensity being the sum of the squares of the heights of the
waves, it is the square of the sum.

While the result was different from the
bullets, there is still nothing unusual about it; everyone has seen this effect
when the waves from two stones that are dropped into a lake in different places
overlap. The difference between this experiment and the previous one is easily
explained by saying that while the bullets each went through only one slit, the
waves each went through both slits and were thus able to interfere with
themselves.

Now try the experiment with electrons.
Recall that electrons are negatively charged *particles* that make up the
outer layers of the atom. Certainly they could only go through one slit at a
time, so their pattern should look like that of the bullets, right? Let's find
out. (NOTE: to actually perform this exact experiment would take detectors more
advanced then any on earth at this time. However, the experiments have been
done with neutron beams ^{9} and the results were the
same as those presented here. A slightly different experiment was done to show
that electrons would behave the same way ^{10}. For
reasons of familiarity, we speak of electrons here instead of neutrons.) Place
an electron gun at the originating point and an electron detector in the
detector place. First try opening only one slit, then just the other. The
results are just like those of the bullets and the waves. Now open both slits. *The
result is just like the waves! ^{11}*

There must be some explanation. After all,
an electron couldn't go through both slits. Instead of a continuous stream of
electrons, let's turn the electron gun down so that at any one time only one
electron is in the experiment. Now the electrons won't be able to cause trouble
since there is no one else to interfere with. The result should now look like
the bullets. But it doesn't! ^{12} It would seem that
the electrons do go through both slits.

This is indeed a strange occurrence; we
should watch them ourselves to make sure that this is indeed what is happening.
So, we put a light behind the wall so that we can see a flash from the slit
that the electron went through, or a flash from both
slits if it went through both. Try the experiment again. As each electron
passes through, there is a flash in only one of the two slits. So they do only
go through one slit! But something else has happened too: *the result now
looks like the result of the bullets experiment!!* ^{13}

Obviously the light is causing problems.
Perhaps if we turned down the intensity of the light, we would be able to see
them without disturbing them. When we try this, we notice first that the
flashes we see are the same size. Also, some electrons now get by without being
detected. ^{14} This is because light is not continuous
but made up of particles called photons. Turning down the intensity only lowers
the number of photons given out by the light source.^{15}
The particles that flash in one slit or the other
behave like the bullets, while those that go undetected behave like waves^{16}.

Well, we are not about to be outsmarted by
an electron, so instead of lowering the intensity of the light, why don't we
lower the frequency. The lower the frequency the less the electron will be
disturbed, so we can finally see what is actually going on. Lower the frequency
slightly and try the experiment again. We see the bullet curve ^{17}.
After lowering it for a while, we finally see a curve that looks somewhat like
that of the waves! There is one problem, though. Lowering the frequency of
light is the same as increasing it's wavelength ^{18}, and by the time the frequency of the light is low
enough to detect the wave pattern the wavelength is longer then the distance
between the slits so we can no longer see which slit the electron went through ^{19}.

So have the
electrons outsmarted us? Perhaps, but they have also taught us one of the most
fundamental lessons in quantum physics - an observation is only valid in the
context of the experiment in which it was performed ^{20}.
If you want to say that something behaves a certain way or even exists, you
must give the context of this behavior or existence since in another context it
may behave differently or not exist at all. We can't just say that an electron
is a particle, since we have already seen proof that this is not always the
case. We can only say that when we observe the electron in the two slit
experiment it behaves like a particle. To see how it would behave under
different conditions, we must perform a different experiment.

**The ****Copenhagen**** Interpretation**

So sometimes a particle acts like a particle
and other times it acts like a wave. So which is it? According to Niels Bohr, who worked in *is* a
particle. When it looks like a wave, it *is* a wave. Furthermore*, it is
meaningless to ascribe any properties or even existence to anything that has
not been measured*^{21}. Bohr is basically saying
that *nothing is real unless it is observed*.

While there are many other interpretations
of quantum physics, all based on the Copenhagen interpretation, the Copenhagen
interpretation is by far the most widely used because it provides a
"generic" interpretation that does not try to say any more then can
be proven. Even so, the

**The Wave Function**

In 1926, just weeks after several other
physicists had published equations describing quantum physics in terms of
matrices, Erwin Schrödinger created quantum equations based on wave mathematics^{22} , a mathematical system that corresponds to the
world we know much more then the matrices. After the initial shock, first
Schrödinger himself then others proved that the equations were mathematically
equivalent ^{23}. Bohr then invited Schrödinger to ^{24}. Even worse, from Schrödinger's point of view,
particles still jumped from one quantum state to another; even expressed in
terms of waves space was still not continuous. Upon
discovering this, Schrödinger remarked to Bohr that "Had I known that we
were not going to get rid of this damned quantum jumping, I never would have
involved myself in this business." ^{25}

Unfortunately, even today people try to
imagine the atomic world as being a bunch of classical waves. As Schrödinger
found out, this could not be further from the truth. *The atomic world is nothing
like our world*, no matter how much we try to pretend it is. In many ways,
the success of Schrödinger's equations has prevented people from thinking more
deeply about the true nature of the atomic world

**The Collapse of the Wave Function**

So why bring up the wave function at all if
it hampers full appreciation of the atomic world? For one thing, the equations
are much more familiar to physicists, so Schrödinger's equations are used much
more often then the others. Also, it turns out that Bohr liked the idea and
used it in his ^{27}. As we shall
see later, the "damned quantum jumping" insures that there are only a
finite, though large, number of possible routes. When no one is watching, the electron take every possible route and therefore interferes
with itself^{28}. However, when the electron is
observed, it is forced to choose one path. Bohr called this the "collapse
of the wave function"^{29}. The probability that a
certain path will be chosen when the wave function collapses is, essentially,
the square of the path's wave function ^{30}.

Bohr reasoned
that nature likes to keep it possibilities open, and therefore follows every
possible path. Only when observed is nature forced to choose only one path, so
only then is just one path taken ^{31}.

**The Uncertainty Principle**

Wait a minute… *probability???* If we
are going to destroy the wave pattern by observing the experiment, then we
should at least be able to determine exactly where the electron goes.

Well, fine. But how exactly are we to
determine the position and the momentum of the electron? If we disturb the
electrons just in seeing if they are there or not, how are we possibly going to
determine both their position and momentum? Still, a clever enough person, say Albert Einstein, should be able to come up with
something, right?

Unfortunately not. Einstein did actually spend a good deal of his life
trying to do just that and failed ^{32}. Furthermore,
it turns out that if it were possible to determine both the position and the
momentum at the same time, Quantum Physics would collapse ^{33}.
Because of the latter, Werner Heisenberg proposed in 1925 that it is in fact *physically
impossible* to do so. As he stated it in what now is called the Heisenberg
Uncertainty Principle, if you determine an object's position with uncertainty
x, there must be an uncertainty in momentum, p, such that xp
> *h*/4pi, where *h* is Planck's constant ^{34}
(which we will discuss shortly). In other words, you can determine *either*
the position *or *the momentum of an object as accurately as you like, but
the act of doing so makes your measurement of the other property that much
less. Human beings may someday build a device capable of transporting objects
across the galaxy, but no one will *ever* be able to measure both the
momentum and the position of an object at the same time. This applies not only
to electrons but also to objects such as tennis balls and toasters, though for
these objects the amount of uncertainty is so small compared to there size that
it can safely be ignored under most circumstances.

**The EPR Experiment**

"God does not play dice" was
Albert Einstein's reply to the Uncertainty Principle. ^{35}
Thus being his belief, he spent a good deal of his life after 1925 trying to
determine both the position and the momentum of a particle. In 1935, Einstein
and two other physicists, Podolski and Rosen,
presented what is now known as the EPR paper in which they suggested a way to
do just that. The idea is this: set up an interaction such that two particles
are go off in opposite directions and do not interact with anything else. Wait
until they are far apart, then measure the momentum of
one and the position of the other. Because of conservation of momentum, you can
determine the momentum of the particle not measured, so when you measure it's position you know both it's momentum and position ^{36}. The only way quantum physics could be true is if
the particles could communicate faster then the speed of light, which Einstein
reasoned would be impossible because of his Theory of Relativity.

In 1982, Alain Aspect, a French physicist,
carried out the EPR experiment ^{37}. He found that *even
if information needed to be communicated faster then light to prevent it, it
was not possible to determine both the position and the momentum of a particle
at the same time* ^{38}. This does not mean that it
is possible to send a message faster then light, since viewing either one of
the two particles gives no information about the other^{39}.
It is only when both are seen that we find that quantum physics has agreed with
the experiment. So does this mean relativity is wrong? No, it just means that
the particles do not communicate by any means we know about. All we know is
that every particle knows what every other particle it has ever interacted with
is doing.

**The Quantum and Planck's Constant**

So what is that *h *that was so important
in the Uncertainty Principle? Well, technically speaking, it's 6.63 X 10^{-34
}joule-seconds ^{40}. It's
call Planck's constant after Max Planck who, in 1900, introduced it in the equation
E=*hv* where E is the energy of each quantum of
radiation and *v *is it's frequency^{41}. What
this says is that energy is not continuous as everyone had assumed but only
comes in certain finite sizes based on Planck's constant.

At first physicists thought that this was
just a neat mathematical trick Planck used to explain experimental results that
did not agree with classical physics. Then, in 1904, Einstein used this idea to
explain certain properties of light--he said that light was in fact a particle
with energy E=*hv* ^{42}.
After that the idea that energy isn't continuous was taken as a fact of nature
- and with amazing results. There was now a reason why electrons were only
found in certain energy levels around the nucleus of an atom ^{43}.
Ironically, Einstein gave quantum theory the push it needed to become the valid
theory it is today, though he would spend the rest of his lift trying to prove
that it was not a true description of nature.

Also, by combining Planck's constant, the
constant of gravity, and the speed of light, it is possible to create a quantum
of length (about 10^{-35} meter) and a quantum of time (about 10^{-43}
sec), called, respectively, Planck's length and Planck's time ^{44}.
While saying that energy is not continuous might not be too startling to the
average person, since what we commonly think of as energy is not all that well
defined anyway, it is startling to say that there are quantities of space and
time that cannot be broken up into smaller pieces. Yet it is exactly this that
gives nature a finite number of routes to take when an electron interferes with
itself.

Although it may seem like the idea that
energy is quantized is a minor part of quantum physics when compared with ghost
electrons and the uncertainty principle, it really is a fundamental statement
about nature that caused everything else we've talked about to be discovered.
And it is always true. In the strange world of the atom, anything that can be
taken for granted is a major step towards an "atomic world view".

**Schrödinger's Cat**

Remember a while ago I said there was a
problem with the *is* to be able to discuss what it *isn't*, and by
far the biggest thing it isn't is complete. Sure, the math seems to be
complete, but the theory includes absolutely nothing that would tie the math to
any physical reality we could imagine. Furthermore, quantum physics leaves us
with a rather large open question: *what is reality?* The *it* is measured. The problem, which is known as
the measurement problem, is when does the cycle stop?

Remember that when we last left Schrödinger
he was muttering about the "damned quantum jumping." He never did get
used to quantum physics, but, unlike Einstein, he was able to come up with a
very real demonstration of just how incomplete the physical view of our world
given by quantum physics really is. Imagine a box in which there is a
radioactive source, a Geiger counter (or anything that records the presence of
radioactive particles), a bottle of cyanide, and a cat. The detector is turned
on for just long enough that there is a fifty-fifty chance that the radioactive
material will decay. If the material does decay, the Geiger counter detects the
particle and crushes the bottle of cyanide, killing the cat. If the material
does not decay, the cat lives. To us outside the box, the time of detection is
when the box is open. At that point, the wave function collapses and the cat
either dies or lives. However, until the box is
opened, the cat is both dead and alive ^{45}.

On one hand, the cat itself could be
considered the detector; it's presence is enough to
collapse the wave function ^{46}. But in that case,
would the presence of a rat be enough? Or an ameba?
Where is the line drawn ^{47}? On the
other hand, what if you replace the cat with a human (named "Wigner's friend" after Eugene Wigner,
the physicist who developed many derivations of the Schrödinger's cat
experiment). The human is certainly able to collapse the wave function,
yet to us outside the box the measurement is not taken until the box is opened ^{48}. If we try to develop some sort of "quantum
relativity" where each individual has his own view of the world, then what
is to prevent the world from getting "out of sync" between observers?

While there are many different
interpretations that solve the problem of Schrödinger’s Cat, one of which we
will discuss shortly, none of them are satisfactory enough to have convinced a
majority of physicists that the consequences of these interpretation s are
better then the half dead cat. Furthermore, while these interpretations do
prevent a half dead cat, they do not solve the underlying measurement problem.
Until a better intrepretation surfaces, we are left
with the ^{49} Yet the problem doesn't go away; it is just left for
the great thinkers of tomorrow.

**The Infinity Problem**

There is one last problem that we will
discuss before moving on to the alternative interpretation. Unlike the others,
this problem lies primarily in the mathematics of a certain part of quantum
physics called quantum electrodynamics, or QED. This branch of quantum physics
explains the electromagnetic interaction in quantum terms. The problem is, when
you add the interaction particles and try to solve Schrödinger's wave equation,
you get an electron with infinite mass, infinite energy, and infinite charge^{50}. There is no way to get rid of the infinities using
valid mathematics, so, the theorists simply divide infinity by infinity and get
whatever result the guys in the lab say the mass, energy, and charge should be^{51}. Even fudging the math, the other results of QED are
so powerful that most physicists ignore the infinities and use the theory
anyway ^{52}. As Paul Dirac,
who was one of the physicists who published quantum equations before
Schrödinger, said, "Sensible mathematics involves neglecting a quantity
when it turns out to be small - not neglecting it just because it is infinitely
great and you do not want it!". ^{53}

**Many Worlds**

One other interpretation, presented first by
Hugh Everett III in 1957, is the many worlds or branching universe
interpretation^{54}. In this theory, whenever a
measurement takes place, the entire universe divides as many times as there are
possible outcomes of the measurement. All universes are identical except for
the outcome of that measurement ^{55}. Unlike the
science fiction view of "parallel universes", it is not possible for
any of these worlds to interact with each other ^{56}.

While this creates an unthinkable number of
different worlds, it does solve the problem of Schrödinger's cat. Instead of
one cat, we now have two; one is dead, the other alive. However, it has still
not solved the measurement problem ^{57}! If the
universe split every time there was more then one possibility, then we would
not see the interference pattern in the electron experiment. So when does it
split? No alternative interpretation has yet answered this question in a
satisfactory way. And so the search continues…

**Further ****Reading**

If you are interested in learning more about
quantum physics, here are some books that you could try (check the bibliography
for more specific information on the books you are interested in):

Richard Feynman's *Lectures on Physics*
deals with the math associated with quantum physics. If you can understand
basic calculus, then this book is for you. Otherwise, while *Lectures*
still provides some valuable information, you may find yourself lost before you
get too far.

John Gribbin's *In
Search of Schrödinger's Cat* is an excellent non-mathematical treatment of
quantum physics. If you've been watching the footnotes you've seen that much of
the data for this paper came from this book. It includes a good history of
quantum physics. Be advised that the sections on supergravity
and supersymmetry at the end are outdated.

Alastair Rae's *Quantum Physics: Illusion or Reality*
presents the basics of quantum physics in terms of the polarization of light. It's 118 pages, half of which are devoted to a discussion of
the alternate interpretations of quantum physics, can easily be read in an
afternoon. It spends more time on alternate interpretations then Gribbin's book, but is less detailed in almost every other
respect. I suggest reading Gribbin's book first then
this book.

**Bibliography**

Feynman, Richard P.,
Robert Leighton, and Matthew Sands.
*The Feynman Lectures on Physics*. Addison-Wesley, *Quantum Mechanics*.

Gribbin, John. *In Search of
Schrödinger's Cat*. Bantam Books,

Rae, Alastair. *Quantum
Physics: Illusion or Reality?*

1 *In Search of Schrödinger's Cat* pages 41 and
43.

2 *Lectures on Physics* page 1-1.

3 *In Search of Schrödinger's Cat* page 66.

4 *In Search of Schrödinger's Cat* page 156.

5 *In Search of Schrödinger's Cat* page 174.

6 *In Search of Schrödinger's Cat* page 5

7 *Lectures on Physics* pages 1-1 and 1-2.

8 *Lectures
on Physics* pages 1-3 and 1-4.

9 *Quantum Physics: Illusion or Reality* page 13.

10 *Quantum Physics: Illusion or Reality* page 12.

11 *Lectures
on Physics* pages 1-4 and 1-5.

12 *In Search of Schrödinger's Cat* page 170.

13 *Lectures
on Physics* pages 1-6 and 1-7.

14 *Lectures on Physics* page 1-8.

15 *Lectures on Physics* page 1-8.

16 *Lectures on Physics* page 1-8.

17 *Lectures on Physics* page 1-8.

18 *Lectures on Physics* page 1-8.

19 *Lectures on Physics* page 1-9.

20 *In Search of Schrödinger's Cat* page 172.

21 *Quantum Physics: Illusion or Reality* page 50.

22 *In Search of Schrödinger's Cat* pages 113 and 114.

23 *In Search of Schrödinger's Cat* page 114.

24 *In Search of Schrödinger's Cat* page 117.

25 *In Search of Schrödinger's Cat* page 117.

26 *In Search of Schrödinger's Cat* pages 117 and 118.

27 *In Search of Schrödinger's Cat* page 173.

28 *In Search of Schrödinger's Cat* page 173.

29 *In Search of Schrödinger's Cat* page 173.

30 *In Search of Schrödinger's Cat* page 173-174.

31 *In Search of Schrödinger's Cat* page 172.

32 *In Search of Schrödinger's Cat* page 174.

33 *Lectures on Physics* page 1-11.

34 *Quantum Physics: Illusion or Reality* page 11.

35 *Quantum Physics: Illusion or Reality* page 1.

36 *In Search of Schrödinger's Cat* page 182.

37 *Quantum Physics: Illusion or Reality* page 43-45.

38 *Quantum Physics: Illusion or Reality* page 43-44.

39 *Quantum Physics: Illusion or Reality* page 52.

40 *Lectures on Physics* page 1-11.

41 *In Search of Schrödinger's Cat* page 41.

42 *In Search of Schrödinger's Cat* page 47.

43 *In Search of Schrödinger's Cat* page 53.

44 *In Search of Schrödinger's Cat* pages 260 and 261.

45 *In Search of Schrödinger's Cat* pages 203 and 205.

46 *In Search of Schrödinger's Cat* page 205.

47 *In Search of Schrödinger's Cat* page 205.

48 *In Search of Schrödinger's Cat* pages 205 and 207.

49 *In Search of Schrödinger's Cat* page v.

50 *In Search of Schrödinger's Cat* pages 256-258.

51 *In Search of Schrödinger's Cat* pages 257 and 258.

52 *In Search of Schrödinger's Cat* page 258.

53 *In Search of Schrödinger's Cat* page 259.

54 *Quantum Physics: Illusion or Reality* page 75.

55 *In Search of Schrödinger's Cat* page 237.

56 *In Search of Schrödinger's Cat* page 241.

57 *Quantum Physics: Illusion or Reality* page 80.

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