- Interlude: From Newtonian to Quantum Views of Nature
Adapted from The Fascination of Physics
by Jacqueline D. Spears and Dean Zollman © 1986,1996. Reprinted
with permission of the authors.
- More than 50 years have passed since the wave and particle
models merged to become a new model of the physical world. In
the early days of this century, physicists voiced strong arguments
for and against the wave function and its interpretations. Now,
the arguments have become less emotional; the concepts less unsettling.
Passing years and new generations of physicists have a way of
turning a revolutionary thought into a tradition; the new physics
into the old physics. In the midst of this settled acceptance
of modern physics, we must realize the enormous impact quantum
mechanics and wave functions have upon a physicists view
of reality. We pause briefly to examine the remarkable
transformation from the physics of Newton to that of the modern
- When Isaac Newton introduced his three laws of motion, he
provided a structure within which we could understand all motion
from the falling apple to the orbiting planet. Once we
knew all the forces acting on an object, we could predict all
future motions with complete accuracy. By placing certainty squarely
within the grasp of human intelligence, Newton created an enormously
comforting view of our universe. This feeling of certainty was
stated well by the French mathematician Pierre LaPlace:
An intelligence which at a given instant knew all the forces
acting in nature and the position of every object in the universe
if endowed with a brain sufficiently vast to make all
necessary calculations could describe with a single formula
the motions of the largest astronomical bodies and those of the
smallest atoms. To such an intelligence, nothing would be uncertain;
the future, like the past, would be an open book.
- Newtons model created an image of a rational world
proceeding in a rational way a world view eagerly embraced
by philosophers, theologians, and physicists alike.
- Beneath this world view lie two very important assumptions.
The first is that all events are ordered, not random. To Newton
and his contemporaries, all motion was completely determined
by whomever or whatever started the universe. These motions obeyed
and would continue to obey a series of orderly rules that could
be discovered by the careful observer. The second assumption
was that the physicist acts as an objective observer of events.
Newton and his contemporaries believed that Beneath this world
view lie two very important assumptions. The first is that all
events are ordered, not random. To Newton and his contemporaries,
all motion was completely determined by whomever or whatever
started the universe. These motions obeyed and would continue
to obey a series of orderly rules that could be discovered by
the careful observer. The second assumption was that the physicist
acts as an objective observer of events. Newton and his contemporaries
believed that while the measurer does have some impact on the
events he or she measures, this impact is minimal and predictable.
Events continue, according to a system of ordered rules, with
an existence independent of the observer. All that remained was
for science to discover the rules.
- During the eighteenth and nineteenth centuries, when Newtons
laws were applied to objects as small as molecules, this world
view prevailed. In principle, physicists believed, once they
knew the momentum and position of each molecule, they could predict
all future motions of all molecules. Completing these measurements
and calculations for a gram of water, let alone the entirety
of the universe, was not humanly possible, so statistical or
probabilistic descriptions were adopted. Consistent with Newtons
world view, probabilities were needed only to compensate for
an information overload, not because of the inherent unknowability
- What does the new world view have to say to us about our
knowledge? Implicit in the probabilistic interpretation now given
to matter waves is the assumption that, on the microscopic level,
events are random. Wave descriptions provide us with information
about the probabilities associated with this random behavior;
particle measurements convert these probabilities into brief
certainties. Further, objective observers have become active
participants in the world that they are trying to describe. Physicists
now acknowledge that the types of measurements they undertake
affect the observations and models they subsequently construct.
Words like particle, position, and path have no meaning apart
from the way in which the experimenter measures them. These words
describe our way of ordering the events we see, not a true underlying
structure of nature. Newtons view of an orderly nature
that exists independent of how we observe it exists no more.
- For many physicists the radical departure from more traditional
ideas was difficult to accept. Erwin Schrödinger, whose
equations were the Newtons laws of quantum mechanics, remained
uncomfortable with the probabilistic interpretation given to
matter waves. Albert Einstein, whose quantum explanation of the
photoelectric effect won a Nobel Prize, also remained unconvinced.
He felt that quantum theory was only a stepping stone to a more
complete understanding of matter. In this view, probabilities
do not represent nature but rather, peoples limited ability
to comprehend nature. In a letter to Max Born in 1926, Einstein
summarized his and perhaps many others feelings:
Quantum theory is certainly imposing. But an inner voice tells
me that it is not yet the real thing. The theory says a lot,
but does not really bring us closer to the secrets of the old
one. I, at any rate, am convinced He is not playing at dice.
- Only time will tell whether Einsteins inner voice was
the voice of wisdom or the voice of a past, unwilling to give
way to the future.