Foreword
This page is adapted, with minor
revisions and major excisions (...snip...), from Chapter 17 of a book
— Physics: Power
Tools for Problem Solving — that I wrote
in the late-1980s. It introduces
basic concepts that show the strangeness of wave-particle duality and the
mysteries of quantum physics, which is an essential foundational theory
of modern science that — since all of its theory-based predictions
have been verified by observations — has very strong scientific support.
In this page, most
links (those in italics) are inside-the-page and are fast; non-italicized
links open a new page in a new window, so this page remains open
in this window.
This is the first of two pages. The
second page — Quantum Physics
- New Age Religion & Schrodinger's Cat — critically
examines speculative metaphysical claims about quantum physics. {
And a third page looks at The
Joy of Science illustrated in the history
of quantum physics. }
Sections 17.1 and 17.2 explain
wave-particle duality, and try to convince you that "yes, things really are
strange." Section
17.3 describes what quantum physics is and isn't,
and why things are stable and dependable. Then Section 17.4 discusses
quantum uncertainty, and 17.5 shows that "no,
things are not as strange as some people say they are."
<...snip...> {
note: This snip-symbol indicates the omission of material that was in the original
chapter. }
Before 1905 the scientific model
of light was
simple, logical, easy to understand, and wrong. A long-running argument
about whether light is a wave or a particle was apparently settled in 1801
when the double-slit experiments of Young, described in Section 15.1, provided
convincing evidence for the wave nature of light. In 1864, Maxwell developed
a set of equations describing light as an electromagnetic
wave. But our knowledge
of light was not yet complete*, as shown by the Michelson-Morley experiment
(in 1887) and the photoelectric effect. {* And we don't know it all now,
either! }
In 1905, Einstein published papers explaining the Michelson-Morley experiment
(with the special theory of relativity described
in Chapter 16) and the photoelectric effect (by answering the question "Is light a wave or a particle?" with "yes, yes").
The Photoelectric Effect
In the late 1800's, experimenters discovered that when light shines on
metals, electrons are ejected from atoms at the surface of the metal. For
example,... <...snip...>
These observations cannot be explained satisfactorily using the pre-1905
model of light, which predicts that... <...snip...> The observed results are
just the opposite of the predictions based on a classical theory of light. Why?
To answer this question, Einstein
proposed that light is emitted, transmitted, and absorbed as particles he called photons,
whose energy depends on their wave-frequency: E (of photon) = h f
Notice the wave-particle
duality within this equation: the energy of a photon (a particle of
light) equals h (a constant of nature) multiplied by f (the photon's frequency,
which is
a wave-characteristic).
<...snip...>
note: The following subsection (until "Sometimes Analogy is Inadequate") is a radically revised summary of the corresponding section in the original Chapter 17. It shows the extremely strange behavior of wave-particles.
Below are four experiments and the resulting observations.
T: When we cover the bottom slit so electrons can pass through only the top slit (T), electrons form a one-slit pattern (the T-pattern) on the wall in front of the top slit. This pattern, caused by diffraction of the electron waves, is due to the wave-like nature of electrons.
B: When we cover the top slit, so electrons pass through only the bottom slit (B), we observe a one-slit pattern (the B-pattern) similar to the T-experiment, except that now the pattern is lower because it's now centered in front of the bottom slit.
TB: When
both slits are open, so each electron can pass through both T and B, we observe
a two-slit pattern
(the
TB-pattern).
What happens if we do this two-slit experiment
with the intensity of the electron beam turned down very low, so low that only
one electron at a time passes through the slits? If we let this run for
a long time, we observe the same TB-pattern as when the intensity is high and
many electrons are simultaneously going through the slits.
Based on our everyday expectations, the result of the low-intensity experiment
is surprising, since it shows that each single electron is passing through "both T and B" (not "either T or B")
and the wave-like nature of a single electron will interfere with itself.
But even though it passes through both
slits, each electron hits the wall as a whole electron.
Each electron has electrical charge,
and if this charge was "smeared out" when
the electron moves through both slits simultaneously, we would expect different
parts of this smeared-out electron to repel itself. But there is no
term for electrical self-repulsion in the quantum physics equations for an
electron that is moving through the slits and toward the wall.
a summary: An electron goes through both
slits (like a wave) but interacts with the wall as a whole electron (like a
particle); and it has self-interference (like a wave) but (like a particle)
no self-repulsion. Yes, this is very strange. In
some ways the behavior is what we might expect from a wave (but not a particle),
while
in other ways it is behaving like a particle (but not a wave).
T+B: Imagine that — in response to the crazy claim being made for TB,
that when both slits are open "each electron goes through both slits" — you say, "This is impossible; I want to know which slit it really goes through." To gain this knowledge, you do an experiment that is more sophisticated (I'll omit the details) by adjusting the intensity of the electron source so only one electron is moving through the slits at any time, and shining lights in front of T and B so you can discover which slit each electron actually passes through. When you do this experiment, you find that the electrons passing through T form a one-slit pattern (the T-pattern) when they hit the wall, and those passing through B form a B-pattern. Overall, what you see is a sum of the T-pattern and B-pattern, so I call this a T+B experiment. The two-slit pattern (TB) has been destroyed, since interaction with a photon of light occurs when an electron has gone through "either T or B" instead of going through "both T and B" as
in the TB-experiment above.
a variation: If we reduce the intensity
of light, so that some electrons don't interact with any photons and thereby
pass through undetected (*), these electrons form the two-slit pattern of TB.
* Notice that detection depends on whether an electron has interacted with
a photon, not whether humans have "observed" this and have thereby gained knowledge. The same experimental results (re: the mixture of T, B, and TB patterns) will occur whether or not a human is observing, since photon-electron interactions are the causal factor. The meaning of "observation" is
a central theme of another
paper whose goal is to show that "things
are not as strange as some people say they are."
Sometimes Analogy is Inadequate
An electron does not behave like a particle,
like a wave, or like a combination of wave-and-particle. Its type of
behavior depends on the situation; during interaction with the slits an electron
behaves
exactly as if it was a wave, and during interaction with the wall an electron
behaves exactly as if it was a particle.
To describe the quantum behavior of matter
(or light) we use either a wave-explanation or a particle-explanation, depending
on the situation. The wave and particle aspects are complementary because
both are needed to reach a complete understanding.
The usual reaction to wave-particle duality
is amazement-and-confusion. Why? Because all of your experience has been
with things that act like particles, or like waves, but never both. There
is no familiar object or phenomenon that can serve as a total analogy. To
understand electrons or other wave-particles you must accept this fact: pictures
and descriptions that are adequate for everyday experience may not be adequate
for areas beyond everyday experience. When you consider that a tennis
ball has 1032 times more mass than an electron, the fact that their
behavior isn't
totally analogous shouldn't be too surprising!
The same kind of wave-particle duality
occurs for electrons (or other types of matter: neutrons, protons, atoms, ...)
and photons. To understand this dual nature, you must have imagination and
a freedom from preconceived bias. In Chapter 6 of The Character of Physical
Law, Nobel Prize winning physicist Richard Feynman does an excellent analysis
of the double-slit experiment, and offers this advice:
Now we know how
the electrons and light behave. But what can I call it?
If I say they behave like particles I give the wrong impression; also if I
say they behave like waves. They behave in their own inimitable way,
which technically could be called a quantum mechanical way. They behave
in a way that is like nothing that you have ever seen before. ... Our
imagination is stretched to the utmost, not, as in fiction, to imagine things
which are not
really there, but just to comprehend those things which are there. ... It
will be difficult. But the difficulty really is psychological and exists
in the perpetual torment that results from your saying to yourself, "But how
can it be like that?" which is a reflection of uncontrolled but utterly vain
desire to see it in terms of something familiar. I will not describe
it in terms of an analogy with something familiar; I will simply describe it.
... Do not keep saying to yourself, if you can possibly avoid it, "But
how can it be like that?"...
Nobody knows how it can be like that. { excerpts from pages 127-129 }
And we should be thankful it is "like that" because,
as explained in Section 17.3, this strange wave-particle duality is necessary
for atomic stability
and for life.
A few philosophers and scientists don't think quantum physics is a complete-and-satisfactory theory because it answers some questions with probabilities or says "I don't know." But most scientists think these limitations are necessary because, as discussed in Section 17.4, there do seem to be limitations on "what we can know" about how things behave on the microscopic level. <...snip...>
Strangeness and Normality
Chapter 16 and Section 17.2 emphasize that you must "stretch your imagination" to
understand the behavior of nature in extreme situations, for objects that are
very fast or very small. But it is also important to recognize the limits
of strangeness. In everyday situations, relativity theory and quantum
physics both predict the normal behavior that experience has taught us to
expect.
For example, a rocket at 24300 miles/hour
is fast by normal standards, but slow compared with the speed of light. For
this rocket, most relativistic calculations differ from those of classical
physics by a factor of only... 1.000000007, so the two theories predict almost
identical results.
And quantum effects are significant only for extremely small microscopic objects
like electrons, protons, neutrons, and atoms. For large macroscopic objects
that contain a huge number of atoms, most quantum effects are negligible. <...snip...> Even the smallest 1-celled animal, too small to be seen without a microscope, is considered "very large" by
the standards of quantum physics.
One way to evaluate a theory is the correspondence
principle: If a new
theory is to be judged satisfactory it must be able to correctly account for
the experimentally verified results of older theories. Do you see why special
relativity and quantum physics pass this test? <...snip...>
When it is acceptable — for most everyday
events, which involve objects that are relatively slow and large — it is usually
easier (and more intuitive) to use classical physics than the more complex
methods of relativity or quantum physics.
Energy Quantization and the Existence
of Life
When you first study quantum physics
ideas (proposing that an atom is mostly empty space, and all matter has wave-properties)
it might seem that matter is not very substantial or reliable. But the strange
wave-nature of electrons is what causes energy quantization, and this in turn
produces things that we consider normal, that allow life. In the following
passage, from page 101 of The History of Quantum Mechanics, Victor Guillemin
describes what would happen if Planck's constant was zero, which would mean
that energy was not quantized:
The
deterministic laws of classical mechanics would be universally valid, a highly
desirable state of affairs,
so it would seem. However, if Planck's constant were zero, there would
have been no Planck, and indeed no rational beings, or any forms of life, for
it
is quantization
that accounts for the existence of stability and organization in the atomic
substratum of the universe. Because the energy content of atoms is restricted
to certain discrete values (page 57), an assault of considerable energy is
needed to jolt them out of their normal state, and afterward they return quickly
and precisely to normal. Without quantization there could be no definite
normal state. Any electronic configuration whatsoever would be possible,
and the slightest disturbance could alter this configuration permanently. Atoms
would have no stable and specific properties. There would be no well-defined
organization of atoms into molecules or of molecules into large structures. The
universe would be a formless and meaningless blob without history, plan or
purpose.
In our present earthly environment quantization alone makes atoms act — to
use Newton's words — like the "solid, massy, hard, impenetrable particles" formed
by God in the beginning, "that
nature may be lasting."
This is a good description of why quantization
is necessary for life. But to make a non-quantum universe seem even less desirable,
think about what would happen to protons (with positive electric charge) and
electrons (with negative charge) if there was no wave-particle duality and
quantization: they would attract each other until they came into contact
and formed +- clumps that would be useless as building blocks for life.
But if we use this method to measure the speed of an electron, interaction between a photon and the tiny electron will change the electron's motion in a significant and unpredictable way... so we cannot predict with certainty what it will do next.
During any act of observation there is unavoidable interaction between the observing-instrument and thing-being-observed (a photon and electron, respectively, in the example above). Wave-particle duality produces energy quantization, so the changes that occur during an observation-interaction cannot be reduced below a certain level. This causes a natural limitation on the precision of measurements, a limitation that is called the uncertainty principle. <...snip...>
These limitations are imposed by nature,
not by a lack of technology or cleverness. No matter how carefully we
make measuring instruments and plan experiments, we cannot make measurements
that are more precise than is allowed
by the uncertainty principle. <...snip...>
This
section, radically revised and expanded, is in a new page,
Quantum Physics
— Science, New Age Religion, and Schrodinger's Cat
This website for Whole-Person Education has TWO KINDS OF LINKS:
an ITALICIZED LINK keeps you inside a page, moving you to another part of it, and a NON-ITALICIZED LINK opens another page. Both keep everything inside this window, so your browser's BACK-button will always take you back to where you were. |
|
New Age Speculations about Quantum Physics (by various authors) Quantum Physics: Science versus New Age Speculation (by Craig Rusbult) The Joy of Science (illustrated in the History of Quantum Mechanics) |
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