Transcript cargese

Ponte di Legno 2003
Causality in Philosophy
and Physics
Mark Fox
Department of Physics and Astronomy
University of Sheffield
Outline
Why ?
• Aristotle’s four causes
• Causality in classical physics
• Causality in quantum physics
• Trying to make sense of it all
The four causes
1. MATERIAL
- what is it made of ?
2. FORMAL
- what is the whole, the
composition, the form ?
3. EFFICIENT
- what is the source of changes ?
4. FINAL
- what is the end, purpose ?
Physics, Book II, Chapter III
Aristotle
384 - 322 BC
An example
1. Material cause: Bronze
(paper or wood would have decayed)
2. Formal cause: Sir John Franklin
(ie not Queen Victoria, David Beckham)
3. Efficient cause: The sculptor, the
crane operator who put in on the plinth
4. Final cause: Born in Spilsby, Arctic
explorer
Spilsby, Lincolnshire
Another example
1. Material cause: sodium atoms
2. Formal cause: gaseous sodium
in a discharge tube
3. Efficient cause: The electrical
current
4. Final cause: Lab experiments,
street lighting
Causality in physics
Material :
atoms
electrons, protons, neutrons
quarks, strings
Formal :
macroscopic: solid, liquid, gas
Microscopic: states of atoms, nuclei
Wave function Y ? *
Efficient :
cause  effect
Final:
?
* N. Gisin, EQEC, Munich, June 2003
Efficient causality
EFFECT: t  L / c
Effect must come
after the cause
CAUSE: t = 0
L
Signals cannot travel
faster than light
Magic
EFFECT:
t<L/c
CAUSE: t = 0
L
v
Observer can see effect
preceding cause
Superluminal propagation ?
Clear message for causality
Physics in Action: December 2003
Experiment confirms that information
cannot be transmitted faster than the speed
of light
Ever since Einstein stated that nothing can travel
faster than light, physicists have delighted in
finding exceptions. One after another,
observations of such "superluminal" propagation
have been made. However, while some image or
pattern- such as the motion of a spotlight
projected on a distant wall - might have
appeared to travel faster than light, it seemed
that there was no way to use the superluminal
effect to transmit energy or information.
Quantum philosophy ?
“We live in a remarkable era in which
experimental results are beginning to
elucidate philosophical questions. In no
domain have the results been more dramatic
than in quantum mechanics.”
Abner Shimony
“The reality of the quantum world”
Scientific American , January 1988
Stern-Gerlach experiment (1924)

non-uniform
magnetic field
I
r
v
+Ze
-e
Atomic magnet
atom beam
or
Force  z Bz'
Einstein-Podolsky-Rosen (1935)
either
or
special
radioactive
source
Interpretations
or
Einstein’s
approach
Bohr’s
approach
Bell’s theorem (1964)
• John Bell, 1928 -1990
• Measure at different angles
• Bohr and Einstein interpretations
predict different results
Modern EPR-Bell experiments
with photons
EPR

D1(1)
S

D1(0)
D2(0)
b
a
BELL
 
D1(1)
D1(0)
D2(1)
S

D2(1)
D2(0)
Aspect experiments (1981-2)
PMT a switch
filter
a'
switch b
 S 
filter
PMT
PMT
b'
PMT
or
Einstein approach
Local hidden variables
inconsistent with the
results
“Spooky”* action at a distance
measurement
t=0
“The EPR experiment is as close to magic as any physical
phenomenon that I know of, and magic should be enjoyed.”
David Mermin, Physics Today, 1985(4), 38
* Einstein, letter to Max Born, 1947
Quantum teleportation
output
|y
ACME
ALICE
|y
input
BOB
Quantum teleportation
machine
“Quantum information processing”
Teleportation of photons in the lab
classical
information
output photon
|y
|F+ |F- |Y+ |Y-
U
BOB
BSM
ALICE

input photon
|y


EPR source
Demonstrated with
single photons across
River Danube 2003
Philosophical implications
Two types of question:
1. If we cannot know what we have before the
measurement is made, is it meaningful to consider
objective reality existing independently of the observer ?
2. How do we preserve the notion of efficient causality
with measurements producing effects instantaneously at
space-time separated locations ?
Bell’s assessment
“Phenomena of this kind made physicists despair of finding any
consistent space-time picture of what goes on on the atomic or
subatomic scale. Making a virtue of necessity, and influenced
by positivistic and instrumentalistic philosophies, many came to
hold not only that it is difficult to find a coherent picture but
that it is wrong to look for one - if not actually immoral then
certainly unprofessional. Going further still, some asserted that
atomic and subatomic particles do not have any definite
properties in advance of observation. There is nothing, that is to
say, in the particles approaching the magnet, to distinguish those
subsequently deflected up from those subsequently deflected
down. Indeed even the particles are not really there.”
J.S. Bell, Journal de Physique (1981), C2, 41-61
Is the moon there when nobody looks ?
*
“After we came out of the church, we stood
talking for some time about Bishop Berkeley’s
ingenious sophistry to prove the non-existence
of matter, and that everything in the universe is
merely ideal. I observed, that though we are
satisfied his doctrine is not true, it is impossible
to refute it. I never shall forget the alacrity with
which Johnson answered, striking his foot with
mighty force against a large stone, till he
Samuel Johnson rebounded from it, ‘I refute it thus’.”
(1709-1784)
from Boswell’s life of Johnson (year 1763)
* A. Einstein
Material or formal causality ?
Does the act of observation create the reality ?
• Wave function Y is the form of the system
• Measurement “collapses the wave function” i.e.
changes the form
• Form is unknown prior to the measurement
• Measurement does not create the matter !
• Teleportation of form not matter (N. Gisin, EQEC 2003)
Efficient causality in quantum physics
• No surprise that quantum physics does not agree with
classical concepts of efficient causality.
•
Narrow definition of efficient causality resolves
contradictions:
 No predictable effects prior to their cause
 No information-carrying signals travel faster than light
 But is this satisfactory ?
•
Form (wave function) of the particle is extended (nonlocal). Measurement changes the form.
Question for the philosophers
Can a cause be simultaneous with its effect ?
“Those causes which are particular and
actual, are and are not simultaneously with
the things of which they are causes.”
Aristotle, Physics II, 3
Summary
• Many issues resolved by distinguishing
clearly between matter and form
• Concept of efficient causality in classical
physics apparently at odds with quantum
experiments. Resolve by narrowing definition
of efficient causality
• New subject of “quantum information
processing” beginning to exploit non-locality
Ponte di Legno 2003
The argument by design
in Physics
Mark Fox
Department of Physics and Astronomy
University of Sheffield
Outline
Why ?
• Final causality
• Teleology: the purpose of the
universe
• Design arguments in physics
The four causes
1. MATERIAL
- what is it made of ?
2. FORMAL
- what is the whole, the composition,
the form ?
3. EFFICIENT
- what is the source of changes ?
4. FINAL
- what is the end, purpose ?
Aristotle, Physics, Book II, Chapter III
Aristotle
384 - 322 BC
Final causality in physics
• Does everything have a
purpose, e.g. atoms, photons,
galaxies, the universe ? If so,
what is it ?
TELEOLOGY
• Is intelligence necessary for
final causality ?
Theme of next conference ?
The argument by design
Thomas Aquinas, Summa Theologiae I, q. 2, a. 3.
The fifth way is taken from things’ being directed.
We see that there are things that have no knowledge,
like physical bodies, but which act for an end.
This is clear in that they always, or for the most part, act
in the same way, and achieve what is best. This shows
that they reach their end not by chance but in virtue of
some tendency.
But things which have no knowledge do not have a
tendency to an end unless they are directed by something
that does have knowledge and understanding.
An example is an arrow directed by an archer.
Therefore there is some being with understanding which
directs all things to their end, and this, we say, is God.
Design arguments in biology
Why does evolution occur ?
Can natural selection by
chance mutation
explain everything ?
Can intelligent life evolve
spontaneously
Evolution of the eye
The God of the gaps in a
clockwork universe
?
• Newton’s “proof” of the existence of God
• Laplace (1798-1857): “We did not need that hypothesis”
The anthropic principle
Planet earth is ideal for humans.
Is this just a co-incidence ?
What conditions are required for human beings to exist ?
• Stable stars
• Planets
• Carbon atoms
• water
• surface temperature between 0 ºC and 100 ºC
These conditions are very hard to achieve !
Carbon nuclear resonance
• Carbon is essential to life. Where does it come from ?
• nucleosynthesis of heavy elements in stars
3 He  C + 2
4
12
• How does this occur ? (Salpeter, 1952, Hoyle 1954)
4
8
*
(a) LONG lifetime
2 He + (99  6) keV  Be
4
He + Be  C  C + 2
8
*
12
*
12
(b) resonance at 7.7 MeV
(c) 16O* level below 7.16 MeV prevents 12C + 4He  16O*
Steps (a), (b), (c) depend critically on: me, mp, h, c, e …..
Planet earth
Is planet earth unique ?
extra-solar planets:
119 planets
104 planetary systems
13 multiple planet systems
• Life takes a very long time to evolve
• Require stable conditions for billions of years
• Importance of the moon
Quantum theory to the rescue
“Although the uncertainties introduced by the quantum picture
are often stressed, this same quantum picture is absolutely vital
for the stability, consistency, and intelligibility of the physical
world. In a Newtonian world, all physical quantities like energy
and spin, can take any values whatsoever... Hence if one were to
form a ‘Newtonian hydrogen atom’ … every pair of electrons
and protons that came together would be different. There could
not exist a well-defined element called hydrogen with universal
properties.”
John Barrow
Theories of everything: the quest for ultimate explanation
(Oxford, 1991)
The $6,000,000 question
WHY ?
Why is there matter at all ?
Why isn’t there nothing ?
Why is there energy in the universe ?
meta - physics