Transcript PhysicsAndReality_V02

Is Anything Real?
Have Physicists Lost
Their Grip on Reality?
Edward L Bosworth, PhD
TSYS School of Computer Science
Columbus State University
Columbus, GA
What is Reality?
Reality, also called “ontological reality”
may be defined as follows:
That which exists independently of our
existence and experience, but
which gives rise to our experience.
This is also called “deep reality”.
What is Knowledge?
This is an old question, dating back to Plato.
One definition is that it is “what cannot be
wrong”. By this definition, it is timeless,
universal, necessary, and certain.
Another definition is that it is what cannot be
reasonably doubted at present. Here it is
particular, context-dependent, and probable.
In What Sense is a Theory True?
For many it suffices that a theory is
“empirically adequate” in that it makes
verifiable predictions and has not yet been
falsified.
Others want a theory to be true because
it presents a true picture of reality.
Observations cannot prove a theory.
Theories and Truth
A theory is of the logical form
If A then B.
If B is false, then A is false.
A theory can be falsified by one result.
If B is true, nothing can be said about A.
Even an impressively large collection of
results cannot show a theory logically true.
Sir Karl Popper on Theory
It is too easy to obtain confirmations or
verifications for a theory – if we just look.
Confirmation counts only if the theory predicts
something entirely unexpected.
A theory is no good unless it can be refuted.
Every genuine test of a theory is an attempt to
falsify it.
An Ancient Egyptian Theory
If Pharaoh does his job well,
the sun will rise in the east every day.
The sun rose in the east today.
So, Pharaoh must be doing a good job.
Kick Out the Philosophers
How many physicists think.
Is Newtonian Mechanics true? Yes.
Is Einstein's Mechanics true? Yes.
They give rise to conflicting predictions.
It is not possible for both to be true.
So what is your problem?
A Question on Truth
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The Ptolemaic
system was used for
centuries.
Can one say that it
was ever true?
Can we say people
had knowledge of it.
Is it false? Was it
ever false?
The Three Great World Systems
Galileo named two. There were three views
of the planets and the sun at his time.
1. The Ptolemaic model – Not even Ptolemy
claimed this was true. It just predicted.
2. The Copernican system – now also this is
known to be false. It predicts falsely.
3. Tycho Brahe's system – also false, but not
distinguishable at the time from Copernican.
Why do some call Copernicus true?
A Matter of Gravity
We seek an explanation of the orbits
of the planets around the sun.
Newton's theory leads to predictions
that are very impressively verified.
Does it explain gravity? What is gravity?
What is the reality behind the name “gravity”?
Newton: “I don't know. This just works.”
Did Newton discover gravity or invent it?
Neptune: Problem and Solution
In the 19th century, there were problems in
analyzing the orbit of Uranus.
Either Newton's mechanics were bad, or
there was another planet out there.
Newton's theory predicted a planet at a
specific location.
When we looked, it was there!
It is hard to say such a theory is not true.
Don't Take the Model Seriously
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The model is supposed to be an aid to
thinking and not represent reality.
To take the model seriously is bad physics.
Particles that Spin
A classical spinning object can have any
value of spin, and all components of that
spin may be simultaneously measured.
A quantum object has “spin”, but should not
be considered to be spinning. This spin is a
name for results of experiments.
The analogy to a spinning top may be a help
to thought, but it will give bad results if
taken too literally.
Is Quantum Mechanics True?
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By this, I mean the latest version of the
theory, possibly Quantum Electrodynamics.
QED is certainly empirically adequate. It is
an extremely accurate predictor of the
results of experiments.
Some predictions have been verified to
twelve decimal digits.
To be this good, it must be true.
QM on Reality
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One clear result of quantum mechanics is
the need to distinguish two types of reality.
Ontological reality is that which exists
independently of our existence.
Empirical reality is the set of phenomena
that human experience, aided by science,
yields access to.
NOTE: This is a result of experimental
physics.
QM and Pragmatism
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Some think of the standard view of QM as
a pragmatic interpretation.
Any QM theory is viewed as a method for
generating numbers to be compared with
results of experiment.
The pragmatist refuses on principle to
speculate about deep reality, as it has no
direct expression in experiment.
No comment on the properties of an
unmeasured electron.
Richard Feynman on QM
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“I think that it is safe to say that nobody
understands quantum mechanics.”
“Do not keep saying to yourself, if you can
possibly avoid it, 'but how can it be like
that?' because you will go 'down the drain'
into a blind alley from which nobody has yet
escaped.”
“Nobody knows how it can be like that.”
Heisenberg's Uncertainty Principle
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Measurements come in pairs, such as
position and momentum.
Either quantity in a pair can be measured
with any desired precision, at the cost of
precision in the conjugate quantity.
Example: p x  h/2
It is reported that Heisenberg came to this
after several months of intense work.
The Disturbance Model
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In the Disturbance Model approach to
Heisenberg's Uncertainty Principle, it is the
process of measurement itself that gives
rise to the uncertainty in the measurement.
In this view, an atom's actual position and
momentum are always definite, but an
accurate position measurement will disturb
the momentum.
It is the measured position and momentum
that cannot be accurately predicted.
Disturbance is False.
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While the disturbance model is appealing, it
is false, in the sense that it gives rise to
predictions that contradict experimental
results.
The key experiments are called “Renninger
style measurements” after Mauritius
Renninger, who developed them.
What can be learned from the absence of a
detection event? The photon not seen?
Case Study: Electrons
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To what extent does an unmeasured
electron have attributes?
All electrons have the same charge and
mass; an unmeasured electron has these.
Does an unmeasured electron have a
definite position and a definite momentum?
Those who believe in hidden variable
theories say “Yes”. Bohr says “No”.
Electron Spin
The electron is a Fermion; it has spin ½.
When measured along any given axis in
space, only two values of spin may be
measured +1/2 and -1/2 (units of h/2).
Basic result: An electron cannot have definite
values for measurements along two
perpendicular axes at the same time.
We call these axes X and Z.
Bohr on an Unmeasured Electron
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A pragmatist would refuse on principle to
comment on the existential status of an
unmeasured electron's attributes.
Bohr claims not that such attributes are
meaningless, but that they are nonexistent.
Bohr: “It is like asking the color of the
number 5. The question is meaningless.”
Hidden Variables
The idea of hidden variables is that an
unmeasured electron does have definite
attributes; they just have not been
measured.
The hidden variable theory seems to imply
that deep (ontological) reality is meaningful,
only that we have not yet observed it.
Bohr would not agree with that.
Locality
The idea of locality is that what happens to a
particle depends only on its own variables
and its immediate circumstances.
The idea of locality is easily expanded to
include distant events, if signals from those
events could arrive at the particle at the
time of the measurement.
The theory of relativity speaks to this issue.
Entanglement
As a result of an interaction, two particles may
exist in an entangled state.
In an entangled state, neither particle has a
definite quantum state, but the pair does.
In entanglement, the pair should be viewed as
a single quantum object.
This has nothing to do with present distance
between the particles.
The EPR Paper
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This is an outgrowth of work by Einstein,
Podolsky, & Rosen, published in 1935 as
“Can Quantum Mechanical Description of
Physical Reality be Considered Complete”.
Einstein called quantum entanglement
“spooky action at a distance.”
Einstein thought QM to be an incomplete
theory and proposed a thought experiment.
The EPR Thought Experiment
Consider two electrons in an entangled state
with total spin zero. Call them P1 and P2.
Before the spin of P1 is measured, neither
has a definite spin value.
When the spin of P1 is measured along an
axis, the spin of P2 instantly gains a value.
This happens even if P2 is at a great
distance, too far for light to travel quickly.
More on EPR
EPR defined reality as follows. “If we can
predict something about a system without
interacting with that system in any way, that
something must be real”.
In the paradox, the spin of P2 can be
predicted without interacting with it.
Thus the spin of P2, not directly measured,
must be real. As QM does not allow this, it
is an incomplete theory of nature.
Bell's Inequality
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John Bell took this paradox and developed
some of its logical consequences.
One was that EPR could be described as
entanglement + locality  hidden variables.
Quantum entanglement is well established.
Bell derived an inequality that any theory
based on locality and hidden variables must
satisfy. This is “Bell’s inequality”.
Bell’s Result
We must either
Give up hidden variables (the idea that the
result of a quantum measurement is
actually predetermined) or
Give up on locality and accept that particles
can influence each other instantly over
great distances (faster than light speed) or
both.
Experimental Results
Bell's inequality has been the subject of a number of
experiments since 1964.
John Clauser was the first to attempt an
experimental verification of Bell's inequality.
In 1972, Clauser reported that his data supported
Bell's inequality. There were a few problems with
his experiment.
In 1982 Alain Aspect published the results of a more
sophisticated experiment. These also supported
Bell's result.
Is Light a Photon Phenomenon
Or a Wave Phenomenon
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Admittedly we may assign a wave length
to any variety of electromagnetic radiation
and use that to make predictions.
The real answer: “Don't ask silly questions.”
Do not confuse a convenient model of
reality with deep reality. This is a
fundamental mistake in physics.
References
R_01 Quantum Reality: Beyond the New Physics, by Nick Herbert, 1987
Anchor Books, ISBN 0 – 385 – 23569 – 3.
R_02 On Physics and Philosophy, by Bernard d’Espagnat, 2006
Princeton University Press, ISBN 978 – 0 – 691 – 11964 – 9.
R_03 Philosophy of Science, by Jeffrey L. Kasser, Course No. 4100, marketed by
The Teaching Company. (www.thegreatcourses.com)
R_04 Science Wars: What Scientists Know and How They Know It,
by Steven L. Goldman, Course No. 1235, marketed by The Teaching Company.
R_05 Quantum Mechanics: The Physics of the Microscopic World,
by Benjamin Schumacher, Course 1240, marketed by the Teaching Company.
R_06 The Age of Entanglement: When Quantum Physics Was Reborn
Louisa Gilder, published by Alfred A. Knopf, 2009,
ISBN 978 – 1 – 4600 – 4417 –7.
R_07 The Elegant Universe, by Brian Greene, published by W. W. Norton, 2003
ISBN 978 – 0 – 393 – 05858 – 1.
R_08 Quantum: Einstein, Bohn, and the Great Debate About the Nature of
Reality, by Manjit Kumar, published by W. W. Norton, 2008,
ISBN 978 – 0 – 393 – 07829 – 9.
R_09 The Structure of Scientific Revolutions, by Thomas S. Kuhn, The University of
Chicago Press, 1970, ISBN 978 – 0 – 226 – 5808 – 3.