Download Physics and the Search for Ultimate BuildingBlocks

The Physical Construction of the
World
Leiden: March 24th, 2010
Richard Healey,
University of Arizona
[email protected]
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Atomic Structure
• Rutherford(1911) discovers the nucleus:
atoms are neither indivisible nor immutable!
• Bohr’s(1912) model of the hydrogen atom
• De Broglie’s(1924) matter waves
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Quantum theory
• Heisenberg(1925): No electronic orbits!
• Schrödinger(1926): Not electronic orbits, but
wave-functions.
• Born’s rule(1926) relates wave-function to
probabilities of measurement outcomes.
• Example: the two-slit experiment
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Source
Wall
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The two-slit experiment
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Feynman on the 2-slit experiment
“if one has a piece of apparatus which is
capable of determining whether the electrons go
through slit A or slit B, then one can say it goes
through either slit A or slit B. [otherwise] one
may not say that an electron goes through either
slit A or slit B. If one does say that, and starts to
make any deductions from the statement, he will
make errors in the analysis. This is the logical
tight rope on which we must walk if we wish to
describe nature successfully.”
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“Elementary” Particles
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Electrons, protons, neutrons:
Positrons, mesons, neutrinos:
Baryons, leptons; photons and other bosons:
None are indestructible---so neither are atoms!
The fundamental interactions: electromagnetic,
strong, weak, gravitational
• Each “carried by” corresponding bosons,
including the photon associated with electric and
magnetic fields whose variations constitute light
and other forms of electromagnetic radiation
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Quantum field theories
• How is the photon associated with light?
• As a minimal amount (a quantum) of energy that may
be exchanged when light interacts with something else:
• The size of this minimal amount depends on the
“color” (wavelength) of the light.
• A photon is a quantum of a quantum field theory of
electromagnetism, while
• electrons (and positrons) are quanta associated with a
second quantum field:
• The forces between them are manifestations of the
interaction of these two quantum fields...
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Interacting quantum field theories
• Quantum electrodynamics (QED—the title of
Feynman’s lovely little book): the quantum field
theory of the electromagnetic interaction of
electrons and photons.
• Quantum chromodynamics (QCD): the quantum
field theory of the strong interaction of hadrons.
(Protons and neutrons are now taken to be
composed of quarks, held together by gluons!)
• Unified electroweak theory: a quantum field
theory incorporating both electromagnetism and
the weak interaction.
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The Standard Model
• Quantum field theories for all the basic
interactions (except gravity).
• Elementary particles:
quarks and leptons, including the electron
(“matter”)
gauge bosons, including photons and gluons
(“forces”)
the Higgs boson (for which the LHC will soon
search).
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Particles or Fields?
Richard Feynman (QED):
“I want to emphasize that light comes in this form: particles.”
“Newton thought light was made up of particles—he called
them ‘corpuscles’—and he was right…”
Robert Wald (Quantum Field Theory…):
“Quantum field theory is a quantum theory of fields, not
particles. Although in appropriate circumstances a particle
interpretation of the theory may be available, the notion of
‘particles’ plays no fundamental role either in the
formulation or interpretation of the theory.”
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Do quantum field theories describe
particles?
Doreen Fraser (2008):
“Quantum field theory (QFT) is the basis of the branch of
physics known as ‘particle physics’. However the
philosophical question of whether quantum field
theories genuinely describe particles is not
straightforward to answer. What is at stake is whether
QFT, one of our current best physical theories, supports
the inclusion of particles in our ontology.”
“because systems which interact cannot be given a
particle interpretation, QFT does not describe
particles.”
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Do quantum field theories describe
fields?
David Baker (2009):
“The notion that QFT can be understood as describing
systems of point particles has been all but refuted by
recent work in the philosophy of physics.”
however
“the most popular extant proposal for fleshing out a field
interpretation is problematic. ..two of the most
powerful arguments against particles are also
arguments against such a field interpretation. … If the
particle concept cannot be applied to QFT, it seems
that the field concept must break down as well.”
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Do quantum field theories describe
anything?
Paul Davies (“Particles do not Exist”, 1984):
“There are quantum states and there are particle
detectors. Quantum field theory enables us to
predict probabilistically how a particular detector
will respond to that state. That is all. That is all
there can ever be in physics, because physics is
about the observations and measurements that
we can make in the world. We can’t talk
meaningfully about whether such-and-such a
state contains particles except in the context of a
specified particle detector measurement.”
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A monism suited to quantum field
theory
Horgan and Potrč (Austere Realism: 2008):
• There is only one object---”the blobject”.
• It has no parts: neither particles, nor localized field-values,
events nor spatiotemporal locations (space-time points).
• But the world (i.e. the blobject) has a very rich structure
• Everything we think of as an ordinary or extraordinary
object emerges as an aspect of this structure.
• The quantum field theories of the Standard Model give us
our only ways of modeling certain details of this structure.
Other scientific theories, as well as everyday beliefs, model
other aspects of the world-structure.
• But none of these systems of thought and experience
ultimately refer to any objects except the blobject itself.
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…with a “top-down” metaphysics
• All nontrivial claims of composition are literally false,
since the blobject has no parts: there are no building
blocks to compose the world
• But aspects of the world-structure themselves display a
structure that warrants us in making compositional
claims that we can agree are true, such as
• A proton is composed of two up quarks and one down
quark or
• Your nose is part of your face
• So a derivative composition relation then emerges as
higher-order structure.
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The Standard Model as Metaphysics?
• The quarks, leptons, gauge bosons and the (as
yet undiscovered) Higgs boson of the Standard
Model cannot play the same metaphysical role
as Democritean atoms
• If anything, the Standard Model comports
better with a monistic metaphysics than with
an atomistic metaphysics
• But…
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The Standard Model is Physics, not
Metaphysics!
• …and there is more to physics than the Standard Model
• There is gravity, whose description by the theory of general
relativity has yet to be reconciled with quantum theory.
• Only about 4% of the total energy density in the universe can
be understood in terms of the Standard Model.
• About 22% is thought to be composed of so-called dark
matter, whose nature remains unknown.
• The remaining 74% is thought to consist of dark energy, an
even stranger component, distributed diffusely in space.
• If this is right, then most of the stuff in the universe is neither
composed of atoms, nor of the “elementary particles” of the
Standard Model.
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Some different parts of matter
• To understand how atoms behave in the aggregate,
condensed matter physicists have introduced many
different kinds of quasiparticle constituents---phonons,
excitons, magnons, polaritons, rotons, etc.
• These are not simply composed of atoms or their
constituents, and play an independent explanatory role
in theoretical models.
• One recent Nobel prize-winner (Robert Laughlin) wrote
“The prefix ‘quasi-’ turns out to be a vestige of historical
battles over the physical meaning of these objects and
conveys no meaning. In private conversations one
drops pretense and refers to the objects as particles.”
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The decompositional strategy
• Decomposing a system into its parts is one of
many strategies physics employs in its quest to
explain as well as predict the behaviour of the
natural world.
• This strategy may be locally successful
whether or not it furthers the global
metaphysical goal of displaying the world as a
single, unified, compositional hierarchy.
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Physics is an unfinished quest
• Since Democritus, the search for more elementary
constituents has repeatedly played a successful role in that
quest
• Atomistic metaphysics has often proved a remarkably
useful heuristic in the history of science
• But our current best theories do not straightforwardly
describe ultimate building blocks---neither particles nor
fields
• They merely permit us to talk that way, with care, in certain
contexts (remember Feynman’s caution about saying
which slit an electron goes through!)
• We can only guess whether atomistic metaphysics will
inspire successful new physics beyond the Standard Model
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Pragmatism, not Metaphysics
• This comports well with a philosophy that views
metaphysics not as a systematic search for truth,
but as a fruitful source of ideas and motivation
for the scientist.
• This is pragmatism
• For the pragmatist, contemporary physics neither
supports nor presupposes atomistic metaphysics:
• Rather, atomistic metaphysics is an intellectual
tool that has proved, and may again prove, useful
in science’s continuing attempt to come to grips
with the natural world.
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