Julian Barbour
(1937 -- )
Julian Barbour writes:
"I was born in 1937 and grew up in the
village of South Newington in North Oxfordshire, England. As a boy I became very
interested in astronomy. For this reason, I decided to study mathematics at
Cambridge, after which I commenced a PhD in astrophysics in Munich.
However, at this time I became deeply interested in foundational issues in
physics, above all the nature of time. I came to the conclusion that time itself
does not exist. If we did not see objects move and things change, we could never
say that time passes. Time is nothing but a measure of change. But physics had
been developed under the assumption that time exists and flows independently of
the objects in the world. I felt this was quite wrong and that physics must be
recast on a new timeless foundation.
Since 1963 I have been working on this project and
the closely related problem of the origin of inertia. I completed a PhD on the
foundations of Einstein's general theory of relativity at Cologne in 1968 and
then decided to become independent, fearing that an academic environment and the
associated pressure to publish as many research articles as possible would
deflect me from my long-term objectives. For 28 years I supported my family and
research by translating Russian scientific journals. This left me free to
develop, in collaboration with the Italian Bruno Bertotti, a theory of time and
inertia. Another major topic of interest for me has been the implications of
this work in quantum cosmology.
I also became very interested in the historical development of ideas
about time and motion, the subject of my Absolute or Relative Motion?,
Volume 1, The Discovery of Dynamics (now retitled as simply The
Discovery of Dynamics and reprinted as a paperback, see writings). In 1996, I
retired from my translating work and have since then concentrated entirely on my
physics research and on writing. I have summarized my ideas about the
non-existence of time for non-specialists in The End of Time, which has
been published in hardback (1999) and paperback (2000) in the UK and hardback
(2000) in the USA (with paperback to be published fall 2001). Articles about the
main ideas in this book were published in The New Scientist in October
1999 and Discover (December 2000). For details about my main publications
and ideas in physics, see writings and ideas.
My wife Verena comes from the Black Forest in
Germany. We have one son and three daughters."
| Quotations from the book The End of
Time: |
"Two views of the world clashed at the dawn of
thought. In the great debate between the earliest Greek philosophers, Heraclitus
argued for perpetual change, but Parmenides maintained there was neither time
nor motion. Over the ages, few thinkers have taken Parmenides
seriously..."
(Barbour: 2000, p. 1)
"Plato... taught that the only real things are
forms or ideas: perfect paradigms, existing in a timeless realm. In our
mortal existence we catch only fleeting glimpses of these ideal forms. Now each
point - each thing - in these 'countries' I have asked you to imagine could be
regarded as a Platonic form. Triangles certainly are. I shall call the
corresponding 'country' Platonia. The name reflects its mathematical perfection
and timeless landscape. Nothing changes in Platonia. Its points are all the
instants of time, all the Nows; they are simply there, given once and for
all."
(Barbour: 2000, p. 44)
"If, as I think they must be, things are properly
considered in Platonia, Lucy never did leap to catch the swifts. The fact is,
there never was one cat Lucy - there were (or rather are, since Lucy is in
Platonia for eternity, as we all are) billions upon billions upon billions of
Lucys. This is already true for the Lucys in one leap and descent.
Microscopically, her 1026 atoms were rearranged to such an extent that only the
stability of her gross features enables us to call her one cat. What is more,
compared with her haemoglobin molecules the features by which we identified her
- the sharp eyes, the sleek coat, the wicked claws - were gross. Because we do
not and cannot look closely at these Lucys, we think they are one. And all these
Lucys are themselves embedded in the vast individual Nows of the universe.
Uncountable Nows in Platonia contain something we should call Lucy, all in
perfect Platonic stillness. It is because we abstract and 'detach' one Lucy from
her Nows that we think a cat leapt. Cats don't leap in Platonia. They just
are."
(Barbour: 2000, p. 48-49)
"Now consider the dilemma God would have faced when
he created the world. Since all places in absolute space are identical, God
would face an impossible choice. Where would he put the matter? God, being
supremely good and rational, must always have a genuine reason for doing
something - Leibniz called this the 'principle of sufficient reason'... - and
because absolute space offered no distinguished locations, God would never be
able to decide where to put the matter. Absolute time, on the assumption that it
existed, presented the same difficulty. Newton had said that all its instants
were identical. But then what reason could God have for deciding to create the
world at some instant rather than another? Again, he would lack a sufficient
reason. For reasons like these, not all of them so theological, Leibniz argued
that absolute space and time could not exist."
(Barbour: 2000, p. 64)
"(The Austrian physicist Ernst) Mach was interested
in many subjects, especially the nature and methods in science. His
philosophical standpoint had points in common with Bishop Berkeley, but even
more with the ideas of the great eighteenth-century Scottish empiricist David
Hume. Mach insisted that science must deal with genuinely observable things,
and this made him deeply suspicious of the concepts of invisible absolute space
and time. In 1883 he published a famous history of mechanics containing a
trenchant and celebrated critique of these concepts..."
(Barbour: 2000, p. 65)
"(Considering Newton's bucket argument, Ernst Mach
suggested) that it is not (absolute) space but all the matter in the
universe, exerting a genuine physical effect, that creates centrifugal force...
Mach's proposal boiled down to the idea that the law of inertia is indeed, as
Bishop Berkeley believed, a motion realtive to the stars, not space. Mach's
important novelty was that there must be proper physical laws that govern the
way distant matter controls the motions around us. Each body in the universe
must be exerting an effect that depends on its mass and distance. The
(Newton's) law of inertia will turn out to be a motion relative to some
average of all the masses in the universe. For this basic idea, Einstein coined
the expression Mach's principle, by which it is now universally known
(though attempts at precise definition vary quite widely).
Mach's idea suggests that the Newtonian way of thinking about
the workings of the universe, which is still deep-rooted, is fundamentally
wrong..."
(Barbour: 2000, p. 65)
"It would be much more natural to specify our
distances to all objects. They define our position. This conclusion is
very natural once we become aware that nothing is fixed. Everything moves
relative to everything else.
Taking this
further, thinking about the position and motion of one object is artificial. We
are part of Mach's All, and any motion we call our own is just part of a change
in the complete universe. What is the reality of the universe? It is that in any
instant the objects in it have some relative arrangement."
(Barbour: 2000, p. 68)
"The proper way to think about motion is that the
universe as a whole moves from one 'place' to another 'place', where 'place'
means a relative arrangement, or configuration, of the complete
universe...
It does not move in absolute
space, it moves from one configuration to another. The totality of these places
is its relative configuration space: Platonia...
... no Sun rises or sets over that landscape to mark the walker's
progress. The Sun, like the moving parts of any clock, is part of the universe.
It is part of the walker...
There is
nothing outside the universe to time it as it goes from one place to another in
Platonia - only some internal change can do that. But just as all markers are on
an equal footing for defining position, so are all changes for the purposes of
timing...
The history of the universe is
the path. Each point on the path is a configuration of the universe. For a
three-body universe, each configuration is a triangle. The path is just the
triangles - nothing more, nothing less.
With time gone, motion is gone..."
(Barbour: 2000, p. 69)
"In modern theoretical cosmology, distance is
absolute and the universe expands. For reasons that are not yet understood, it
simultaneously becomes more richly structured. In a cosmology without both time
and scale, this would correspond, in a realistic scale-free Platonia, to going
from the bland centre to the more interestingly structured 'instants of time'
situated between it and the frontiers. That is where the mist I introduced
in Chapter 3 must collect most thickly - have the highest intensity - at time
capsules structured so that they seem to record evolution from the symmetric
centre. This would be a cosmology of pure structure..."
(Barbour: 2000, p. 75)
"Leibniz famously asked why, among all possible
worlds, just one should be realized. He suggested, rather loosely, that God -
the supremely rational being - could have no alternative but to create the best
among all possible worlds. For this he was satirized as Dr. Pangloss in
Voltaire's Candide. In fact, in his main philosophical work, the
Monadology, Leibniz makes the more defensible claim that the actual world
is distinguished from other possible worlds by preserving 'as much variety as
possible, but with the greatest order possible'. This, he says, would be the way
to obtain 'as much perfection as possible'."
(Barbour: 2000, p. 110-111)
"After it became clear to me that Platonia was the
arena in which to formulate Mach's ideas, I soon realized that it was necessary
to find some analogue of action that could be defined using structure already
present in Platonia. With such an action it would be possible to identify some
paths in Platonia as being special and different from other paths. In Leibniz's
language, such paths could be actual histories of the universe, as opposed to
merely possible ones."
(Barbour: 2000, p. 113)
"Each (point in Platonia) is a possible
realtive arrangement, a configuration, of all the matter in the
universe..."
(Barbour: 2000, p. 115-116)
"... a 'trial distance' d (between two triangles
in Triangle Land)... is an arbitrary quantity, since the relative positioning
of the two triangles is arbitrary. It is, however, possible to consider all
relative positionings and find the one for which d is minimized. This is a very
natural quantity to find, and it is not arbitrary. Two different people setting
out to find it for the same two triangles would always get the same result. It
measures the intrinsic difference between the two matter distributions
represented by the triangles. It is completely determined by them, and does not
rely on any external structure like absolute space...
Using the intrinsic difference... we can determine 'shortest
paths' or 'histories' in Platonia...However, the intrinsic difference by itself
does not lead to very interesting histories, and it is more illuminating to
consider a related quantity. The potential energy of any matter distribution...
is determined by its relative configuration, and is therefore already
'Machian'... Two nearly identical matter distributions have almost the same
potential... To obtain more interesting histories we can simply multiply the
intrinsic differences by the potential... This will change the definition of
'distance'..."
(Barbour: 2000, p. 116-117)
"Poincare's paper (1898) is very interesting. He
identified two problems in the definition of time.
First he considered duration: what does it mean to
say that a second today is the same as a second tomorrow? He noted that this
question had recently been widely discussed... he then noted a second question,
just as fundamental and in some ways more immediate, which has escaped close
attention. How does one define simultaneity at spatially separated points? This
was the question that Einstein posed and answered seven years later with such
devastating effect. I read the subsequent history of relativity as follows.
Einstein answered his question - Poincare's second - with such aplomb and
originality that it eclipsed interest in the question of duration. It is not
that duration plays no role in relativity - quite the opposite, it plays a
central role. But duration is not derived from first principles. It appears
indirectly."
(Barbour: 2000, p. 123)
"... Carl Neumann and Mach became aware of the need
for new foundations of dynamics. In Poincare's writings of around 1900, one can
see clear hints of how dynamics might have been developed further as the main
stream of research. In particular, an explicit theory of the origin of the
spatiotemporal framework might have emerged. That is more than evident from
Poincare's 1898 paper on time and his 1902 comments...
All this was changed by Einstein's 1905 paper. Because of his
quantum doubts, he distrusted explicit dynamical models ... (and) ...
insight into the nature of time and duration was lost."
(Barbour: 2000, p. 136)
"It is impossible to understand relativity if one
thinks that time passes independently of the world...
the important thing is to get away from the idea that time is
something. Time does not exist..."
(Barbour: 2000, p. 137)
"It is often said that relativity destroyed the
concept of Now... In Newtonian physics ... there is a unique sequence of
instants of time, each of which applies to the complete universe. This is
overthrown in relativity, where each event belongs to a multitude of Nows. This
has important implications for the way we think about past, present and
future.
Even in Newtonian theory we can
picture world history laid out before us. In this 'God's-eye' view, the instants
of time are all 'there' simultaneously. The alternative idea of a 'moving
present' passing through the instants from the past to the future is
theoretically possible but impossible to verify. It adds nothing to the
scientific notion of time. Special relativity makes a 'moving present' pretty
well untenable, even as a logical possibility."
(Barbour: 2000, p. 142)
"...(in relativity) there are just point-like events
in space-time and no extended Nows. At the psychological level, Einstein himself
felt quite disturbed about this... (and) the philosopher Rudolf Carnap wrote:
... So he (Einstein) concluded 'that there is something essential
about the Now which is just outside the realm of science'."
(Barbour: 2000, p. 143)
"I am not claiming that the description of space-time
given by Einstein or Minkowski is wrong. Far from it - they got it right, but
they described the finished product, and the complete story must also include
the construction of the product."
(Barbour: 2000, p. 145)
"In the... (Newtonian worlds), past, present
and future are defined throughout the universe, and present is a single
simultaneity hyperplane. In the... (Einsteinian worlds), they are defined
separately for each event in space-time, and the present is much
larger."
(Barbour: 2000, p. 150)
"It is the implications of the BSW paper (see in
the references the article by Ralph Baierlein, David Sharp and John Wheeler)
that I discussed with Karel (i.e. Karel Kuchar) in 1980. They can be
quickly summarized. The basic problem that BSW considered was what kind of
information, and how much, must be specified if a complete space-time is to be
determined uniquely. This is exactly analogous to the question that Poincare
asked in connection with Newtonian dynamics, and then showed that the
information in three Nows was needed. As we have seen, a theory will be
Machian if two Nows are sufficient. What BSW showed is that the basic
structure of general relativity meets this requirement.
In fact, the all-important Einstein equation that does the work
is precisely a statement that a best-matching condition between the two 3-spaces
does hold. The pairing of points established by it is exactly the pairing
established by the orthogonal struts (Barbour's figure 30 on page 175).
In fact, the key geometrical property of space-times that satisfy Einstein's
equations reflects an underlying principle of best matching built into the
foundations of the theory. I think that Einstein, with his deep conviction that
nature is supremely rational, would have been most impressed had he lived to
learn about it."
(Barbour: 2000, p. 176)
"We have reached a crucial stage, and a summary is
called for. In all three forms of classical physics - in Newtonian theory, and
in the special and general theories of relativity - the most basic concept is a
framework of space and time. The objects in the world stand lower in the
hierarchy of being than the framework in which they move. We have been exploring
Leibniz's idea that only things exist and that the supposed framework of space
and time is a derived concept, a construction from the things.
If it is to succeed, the only possible candidates for
the fundamental 'things' from which the framework is to be constructed are
configurations of the universe: Nows or 'instants of time'. They can exist in
their own right: we do not have to presuppose a framework in which they are
embedded. In this view, the true arena of the world is timeless and frameless -
it is the collection of all possible Nows..."
(Barbour: 2000, p. 177)
"We need to consider the 'official line', known as
the Copenhagen interpretation because it was established by Heisenberg and Bohr
at the latter's institute in Copenhagen shortly after the creation of quantum
mechanics...
... It is a basic
Copenhagen tenet that the probabilistic statements reflect a fundamental
property of nature, not simply our ignorance. It is not that before the
measurement the particle does have a definite momentum and we simply do not know
it. Instead, all momenta in the superposition are present as potentialities, and
measurement forces one of them to be actualized. This is justified by a simple
and persuasive fact. If we do not perform measurement but instead allow * to
evolve, and only later make some measurement, then the things observed later
(like the two-slit fringes) are impossible to explain unless all states were
present initially and throughout the subsequent evolution. Outcomes in quantum
mechanics are determined by chance at the most fundamental level. This is the
scenario of the dice-playing God that so disturbed Einstein."
(Barbour: 2000, p.
202-203-204)
"If anything, the second cardinal fact disturbed him
even more. There seems to be a thoroughgoing indefiniteness of nature even more
radical than the probabilistic uncertainties. As we have seen, one and the same
state can be regarded as a superposition of either momentum or position
eigenstates. It is the way this mathematics translates into physics that is
startling. The experimentalist has complete freedom to choose what is to be
measured: position or momentum. Both are present simultaneously as
potentialities in the wave function. The experimentalist merely has to choose
between set-ups designed to measure position or momentum. Once the choice is
made, outcomes can then be predicted - and one outcome is actualized when the
measurement is made. In fact, the indefiniteness is even greater since other
quantities, or observables as they are called, such as energy and angular
momentum, are also present as potentialities in psi (sorry that there are no
Greek fonts for the letter psi).
Only one
experiment can be made - for position or momentum, say, but not both. Every
measurement 'collapses' the wave function. After the collapse, the wave
function, which could have been used to predict outcomes of alternative
measurements, has been changed irrevocably: there is no going back to the
experiment we opted not to perform. It is a very singular business. Whatever
observable we decide to measure, we get a definite result. But the observable
that is made definite depends on our whim. The many people who, like
Einstein, believe in a real and definite world find this immensely
disconcerting. What is out there in the world seems to depend on mere
thoughts that come into our mind...
The fact that in quantum mechanics one can choose to measure
one but not both of two quantities was called complementarity by Bohr.
Pairs of quantities for which it holds are said to be complementary."
(Barbour: 2000, p. 204)
"Contrary to the impression given in many books,
quantum mechanics is not about particles in space: it is about systems being in
configurations - at 'points' in a Q, or 'hybrid Platonia'. That is something
quite different from individual probabilities for individual particles being at
different points of ordinary space. Each 'point' is a whole configuration - a
'universe'. The arena formed by the 'points' is unimaginably large. And
classical physics puts the system at just one point in the arena. The wave
function, in contrast, is in principle everywhere.
This is what I mean by saying that Schrodinger opened the
door onto a vast new arena..."
(Barbour: 2000, p. 210)
"... The simplest possible illustration is given by
two particles moving on a single line;... together they have a two-dimensional
configuration space...
...Many different
kinds of prediction can be made, but they are often mutually exclusive. In a
very essential way, the predictions refer to the system, not its
parts.
... The important thing is that a
single point in Q corresponds to positions of both particles. Anyone who has not
understood this has not understood quantum mechanics. It is this fact, coupled
with complementarity, that leads to the most startling quantum
phenomena."
(Barbour: 2000, p. 211)
"... innumerable interference phenomena indicate
that, in some sense, the (two) particles (in entangled states or quantum
inseparability) are, before any measurement is made, simultaneously present
whenever * extends. Since there is no restriction on the distance between the
particles, any causal effect on the second particle after the first has been
observed would have to be transmitted instantaneously..."
(Barbour: 2000, p. 217)
"Relativity absolutely prohibits the transmission of
information faster than light. But, curiously, wave-function collapse
does not transmit information. When information about particle 1 has been
obtained by an experimentalist, he or she will know immediately what a distant
experimentalist can learn about particle 2. But there is no way such information
can be transmitted faster than light. There is no conflict with the rules of
relativity, though many physicists are concerned that its 'spirit' is
violated."
(Barbour: 2000, p. 218)
"... if the position of one particle was measured,
the position of the other particle could be immediately established with
certainty because of the perfect correlation. Since the second particle, being
far away, could not be physically affected by the measurement, but it was known
for certain where it would be found, EPR (i.e., Albert Einstein, Boris
Podolsky and Nathan Rosen in formulation of their EPR paradox in 1933)
concluded that it must have had this definite property before the measurement
on the first particle .
... Finally, the
choice between momentum or position measurement is a matter of our whim, about
which the second particle can know nothing. The only conclusion to draw is that
the second particle must have possessed definite position and momentum before any measurements were made at all. However,
according to the fundamental rules of quantum mechanics, as exemplified in the
Heisenberg uncertainty principle, a quantum particle cannot possess definite
momentum and position simultaneously. EPR concluded there must be something
wrong - quantum mechanics must be incomplete.
Niels Bohr actually answered EPR quite easily, though not to
everyone's satisfaction. His essential point was that quantum mechanics predicts
results made in a definite experimental context. We must not think that the
two-particle system exists in its own right, with definite properties and
independent of the rest of the world. To make position or momentum
measurements, we must set up different instruments in the laboratory. Then the
total system, consisting of the quantum system and the measuring system, is
different in the two cases. Nature is holistic: it is not for us to dictate
what Nature is or does. Quantum mechanics is merely a set of rules that
brings order into our observations. Einstein never found an answer to this
extreme operationalism of Bohr, and remained deeply dissatisfied.
I feel sure that Bohr got closer to the truth than
Einstein. However, Bohr too adopted a stance that I believe is ultimately
untenable. He insisted that it was wrong to attempt to describe the instruments
used in quantum experiments within the framework of quantum theory. The
classical world of intruments, space and time must be presupposed if we are ever
to talk about quantum experiment and communicate meaningfully with one
another..."
(Barbour: 2000, p.
218-219)
"Despite the sofistication of all his work, in both
relativity and quantum mechanics, Einstein retained a naive atomistic
philosophy. There are space and time, and distinct autonomous things moving in
them. This is the picture of the world that underlies the EPR analysis. In 1949
Einstein said he believed in a 'world of things existing as real objects'. This
is his creed in seven words. But what are 'real objects'?
To look at this question, we first accept that
distinct identifiable particles can exist. Imagine three of them. There are two
possible realities. In the Machian view, the properties of the system are
exhausted by the masses of the particles and their separations, but the
separations are mutual properties. Apart from the masses, the particles have no
attributes that are exclusively their own. They - in the form of a triangle -
are a single thing. In the Newtonian view, the particles exist in absolute space
and time. These external elements lend the particles attributes - position,
momentum, angular momentum - denied in the Machian view. The particles become
three things. Absolute space and time are an essential part of
atomism."
(Barbour: 2000, p. 220)
"Strong confirmation for quantum mechanics being
holistic in a very deep sense was obtained in the 1960s, when John Bell, a
British physicist from Belfast, achieved a significant sharpening of the EPR
paradox. The essence of the original paradox is the existence of correlations
between pairs of quantities - pairs of positions or pairs of momenta - that are
always verified if one correlation or the other is tested. By itself, some
degree of correlation between the two particles is not mysterious. The EPR-type
correlated states are generally created from known uncorellated states of two
particles that are then allowed to interact. Even in classical physics,
interaction under such circumstances is bound to lead to correlations. Bell
posed a sharper question than EPR: is the extent of the quantum correlations
compatible with the idea that, before any measurement is made, the system being
considered already possesses all the definite properties that could be
established by all the measurements that, when performed separately, always lead
to a definite result?
Bell's question
perfectly reflects Einstein's 'robust realism' - that the two-particle system
ought to consist of two separate entities that possess definite properties
before any measurements are made. Assuming this, Bell proceeded to derive
certain inequalities, justly famous, that impose upper limits on the degree of
the correlations that such 'classical' entities could exhibit (tighter
correlations would simply be a logical impossibility). He also showed that
quantum mechanics can violate these inequalities: the quantum world can be
more tightly correlated than any conceivable 'classical world'. Aspect's
experiments specifically tested the Bell inequalities and triumphyntly confirmed
the quantum predictions..."
(Barbour: 2000, p. 220-221)
"In 1957, Hugh Everett, a student of John Wheeler at
Princeton, proposed a novel interpretation of quantum mechanics. Its
implications are startling, but for over a decade it attracted little interest
until Bryce DeWitt drew wide attention to it, especially by his coinage many
worlds to describe the main idea. Everett had used the sober title 'Relative
state formulation of quantum mechanics'. One well-known physicist was prompted
to call it the 'best-kept secret in physics'. So far as I know, Everett
published no other scientific paper. He was already working for the Weapons
System Evaluation Group at the Pentagon when his paper was published. He was
apparently a chain smoker, and died in his early fifties."
(Barbour: 2000, p. 221-222)
"Everett noted that in quantum mechanics 'there are
two fundamentally different ways in which the state function can change':
through continuous causal evolution and through the notorious collapse at a
measurement. He aimed to eliminate this dichotomy, and show that the very
phenomenon that collapse had been introduced to explain - our invariable
observation of only one of many different possibilities that quantum mechanics
seems to allow - is actually predicted by pure wave mechanics. Collapse is
redundant.
The basis of Everett's
interpretation is the endemic phenomenon of entanglement. By its very nature,
entanglement can arise only in composite systems - those that consist of two or
more parts. In fact, an essential element of the many-worlds interpretation as
it is now almost universally understood is that the universe can and must be
divided into at least two parts - an observing part and an observed part.
However, Everett himself looked forward to the application of his ideas in
the context of unified field theories, 'where there is no question of ever
isolating observers and object systems. They are all represented in a single
structure, the field'..."
(Barbour: 2000, p. 222)
"The measurement problem of quantum mechanics
is this: how does the entangled state of many possibilities collapse down to
just one, and when does it happen? ... You can go on asking quantum mechanics
again and again to say when collapse occurs, but it never gives an answer... All
that the Copenhagen interpretation can say is that collapse occurs at the latest
in the perceptions of the experimentalist. When it happens no one can say - it
can only be said that if collapse does not happen we cannot explain the observed
phenomena.
But must it happen? Everett
came up with a simple - with hindsight obvious - alternative. Collapse does not
happen at all: the multiple possibilities represented in the entangled state
continue to exist. In each possibility the observer, in different incarnations,
sees something different, but what is seen is definite in each case. Each
incarnation of the observer sees one of the possible outcomes that the
Copenhagen interpretation assumes is created by collapse."
(Barbour: 2000, p. 224)
"Everett's proposal raises two questions. If many
worlds do exist, why do we see only one and not all? Why do we not feel the
world splitting? Everett answered both by an important property of quantum
mechanics called linearity, or the superposition
principle. It means that two processes can take place simultaneously
without affecting each other. Consider, for example, Young's explanation
of interference between two wave sources. Each source, when active alone, gives
rise to a certain wave pattern. If both sources are active, the processes they
generate could disturb each other drastically. But this does not happen. The
wave pattern when both sources are active is found simply by adding the two wave
patterns together. The total effect is very different from either of the
individual processes, but in a real sense each continues unaffected by the
presence of the other. This is by no means always the case; in so-called
non-linear wave processes, the wave pattern from two or more sources cannot be
found by simple addition of the patterns from the separate sources acting alone.
However, quantum mechanics is linear, so the much simpler situation
occurs.
As a result, quantum
processes can be regarded as being made up of many individual subprocesses
taking place independently of one another... Each subprocess is, so to speak,
aware only of itself. There is a beautiful logic to this, since each subprocess
is fully described by the quantum laws. There is nothing within the branch as
such to indicate that it alone does not constitute the entire history of the
universe. It carries on in blithe ignorance of the other branches, which are
'parallel worlds' of which it sees nothing. The branches can nevertheless be
very complicated..."
(Barbour: 2000, p. 225)
"Any scientific theory must establish a postulate
of psychophysical parallelism: it is necessary to say what elements of the
physical theory correspond to actual conscious experience..."
(Barbour: 2000, p. 226)
"... quantum mechanics is doubly indefinite. First,
if states of a definite kind are chosen, any state of a composite system is a
unique sum of states of its subsystems. For position states... (the) probability
distribution is spread out over a huge range of possibilities in which one
particle has one definite position and the other particle has one definite
position. Positions are always paired together in this way. Everett resolved the
apparent conflict between our experience of a unique world and this multiplicity
of possibilities by associating a separate and autonomous experience with each.
However, he did not address the second indefiniteness:: the states shown as
positions... could equally well be represented by, for example, momentum states.
Then pairs of momentum states result. Depending on the representation,
different sets of parallel worlds are obtained: 'position histories' in the one
case, 'momemntum histories' in the other. One quantum evolution yields not only
many histories but also many families of different kinds of
history.
It was surprisingly long
before this difficulty was clearly recognized as preferred-basis problem:
a definite kind of history will be obtained only if there exists some
distinguished, or preferred, choice of the basis, by which is meant the
kind of states used in the representation. The preferred basis problem is the
EPR paradox in a different guise. Everett may have instinctively assumed that
the position basis is somehow naturally singled out, but there is little
evidence in his paper to confirm this.
The
first question that must be addressed is surely this: what is real? Everett took
the wave function to be the only physical entity. The price for this
wave-function monism is the preferred-basis problem. Because the wave functions
of composite systems can be represented in so many ways, the application of
Everett's ideas to different kinds of representation suggests that one and the
same wave function contains not only many histories, but also many kinds of
history. It leads to a 'many-many-worlds' interpretation. Some accept this,
but I feel there is a more attractive alternative. "
(Barbour: 2000, p. 226-227)
"... in classical relativity, space-time represents
all reality - the complete universe. In contrast, a quantum state by itself has
no definite meaning until the strategic decision - say, to measure position or
momentum - has been taken. The state acquires its full meaning only in
conjunction with actual measuring apparatus outside the system. The system must
interact with the apparatus to reveal its latent potentialities. At present, its
interaction with an apparatus - essentially the rest of the universe - is not
fully understood. The quantum state by itself is only part of the story. It may
be premature to draw conclusions about the quantum universe from incomplete
quantum descriptions of subsystems of it."
(Barbour: 2000, p. 227-228)
"... quantum mechanics as presently formulated needs
an external framework. Indeed, the most basic observables, those for position,
momentum and angular momentum, all correspond to the 'lent' properties mentioned
in the discussion of the EPR paradox. They could not exist without the framework
of absolute space, and Mach's principle suggests strongly it is determined by
the instantaneous configurations of the universe. Time, moreover, plays an
essential role in quantum mechanics yet stands quite outside the description of
the quantum state. But ... time is really just a shorthand for the position of
everything in the universe, so the configurations of the universe can be
expected to play an essential and direct role in a quantum description of the
universe... This is what leads me to the dualistic picture of Platonia, the
collection of all possible configurations of the universe, and the completely
different wave function, conceived of as 'mist' over Platonia. In the language
of Everett's theory, this introduces a preferred basis. In answer to the
question 'what is real?', I answer 'configurations'. My book is the
attempt to show that they explain both time and the quantum - as different sides
of the same coin."
(Barbour: 2000, p. 228)
"... two equations (that) Schrodinger
discovered in 1926... (according to Dirac)... explained all of chemistry
and most of physics ...
....in every
instant we experience creation directly. Creation did not happen in a Big Bang.
Creation is here and now, and we can understand the rules that govern it.
Schrodinger thought he had found the secret of the quantum prescriptions.
Properly understood, what he found were the rules of creation.
... In quantum mechanics, this (the wave function
psi) is all that does change. Forget any idea about the particles themselves
moving. The space Q of possible configurations, or structures, is given once and
for all: it is a timeless configuration space. The instantaneous position of the
system is one point of its Q. Evolution in classical Newtonian mechanics is like
a bright spot moving, as time passes, over the landscape of Q. I have argued
that this is wrong way to think about time. There is neither a passing time
nor a moving spot, just a timeless path through the landscape, the track
taken by the moving spot in the fiction in which there is time."
(Barbour: 2000, p. 229)
"In quantum mechanics with time... there is no track
at all. Instead, Q is covered by the mists... (illustrating the notion)
of wave functions and the probabilities associated with them. The red and
green mists evolve in a tightly interlocked fashion, while the blue mist,
calculated from the other two, describes the change of the probability. All that
happens as time passes is that the patterns of mist change. The mists come and
go, changing constantly over a landscape that itself never changes.
One of the equations that Schrodinger found governs
this process. If psi is known everywhere in Q at a certain time, you know
what * will be slightly later. From this new value, you can go on another small
step in time, and another, and so on arbitrarily far into the future. The role
played here by the red and green mists, the two primary components of psi, is
quite interesting: the way the red mist varies in space determines the rate of
change of the green mist in time, and vice versa. The two components play a
kind of tennis. This equation is sometimes called the time-dependent
Schrodinger equation because time features in it. This is not in fact the
first equation that Schrodinger discovered.
The first one he found is now usually called the stationary
or time-independent Schrodinger equation. This determines what happens
in certain special cases in which the two components of psi, the red and
green 'mists', oscillate regularly, the increase of one matching the decrease of
the other. This has the consequence, as we have already seen for a momentum
eigenstate, that the blue mist (the probability density) has a frozen value - it
is independent of time (though its value
generally changes over Q). Such a state is called a stationary state. This
explains the name given to the second equation - its solutions are stationary
states. The standard view is that the time-dependent equation is seen as a
special case derived from it...
There are
intriguing hints that in the quantum mechanics of the universe the roles of
these two equations are reversed. The stationary equation (or something like
it) may be the fundamental equation, from which the time-dependent equation is
derived only as an approximation... "
(Barbour: 2000, p. 230)
"All solutions of the time-dependent equation can
be found by adding stationary solutions with different frequencies.
Each stationary solution on its own has regular oscillations of its red and
green mists, but a constant - in fact static - distribution of its blue mist.
But as soon as stationary states with different energies, and hence frequencies,
are added together, irregular oscillations commence - in particular in the blue
mist, the touchstone of true change. All true change in quantum mechanics
comes from intereference between stationary states with different energies. In a
system described by a stationary state, no change takes place."
(Barbour: 2000, p. 231)
"If the Machian approach to classical dynamics is
correct, quantum cosmology will have no dynamics. It will be timeless. It must
also be frameless."
(Barbour: 2000, p. 232)
"(Schrodinger's) stationary equation determines the
structures - indeed, creates the structures - of all these amazing atoms and
molecules that constitute so much of the matter in the universe, our own bodies
included. The equation does it by determining which structures are probable. But
I mean creation not only in this sense of the structure of atoms and molecules,
but in an even deeper one..."
(Barbour: 2000, p. 235)
"... we found an alternative definition of
'distances' that works in the purely relative configuration space - in Platonia
- and owes nothing to absolute space. They are defined by the best-matching
procedure, which uses relative configurations and nothing else. In classical
physics, this makes it possible to create a purely relative and hence
self-contained dynamics. We also found that a sophisticated form of
best-matching lies at the heart of general relativity. Best matching would
appear to be a basic rule of the world. "
(Barbour: 2000, p. 236)
"What ... is curvature? For any given curve, it is the
rate at which its slope changes. But the key thing about a rate of change is
that it is with respect to something. That something is all-important. It
is a kind of 'distance'. The ordinary quantum-mechanical 'distance' is simply
distance in absolute space (times the mass of the particle considered). To
eliminate absolute space in classical physics, we replaced it by the Machian
best-matching distance. There is no reason why we should not do the same in
quantum physics.
... the wave functions
that satisfy the Schrodinger conditions in this Machian case are precisely the
eigenfunctions of ordinary quantum mechanics for which the angular-momentum
eigenvalues are zero. This exactly matches our result in classical mechanics -
that the best-matching condition leads to solutions with angular momentum
zero...
The quantum counterpart of
Machian classical dynamics is a static wave function * on Platonia. The rules
that govern its variation from point to point in Platonia involve only the
potential and the best-matching 'distance'. Both are 'topographic features' of
the timeless arena. Surveyors sent to map it would find them. They would see
that the mists of Platonia respect its topography. It determines where the mist
collects. "
(Barbour: 2000, p. 237)
"... many people came to the same conclusion: there is
a time, but it is hidden in the three degrees of freedom.
According to this insight, the basic framework of quantum
mechanics could be preserved, but the time it so urgently needed would be taken
from the 'world' to which it was to be applied. Putting it in very figurative
terms, one-third of space would become time, while the remaining two-thirds
would become two true quantum degrees of freedom. Because time was to be
extracted from space, from within the very thing that changes, the time that was
to be found was called intrinsic time. The notion of intrinsic time was -
and is - a breathtaking idea. But there was a price to be paid, and there was
also a closely related problem to be overcome: which third of space is to be
time?
... The price and the problem are
one and the same. They presented the quantum theoreticians with a head-on
collision between the basic principles of their two most fundamental theories -
the need for a definite time in quantum mechanics and the denial of a definite
time in general relativity."
(Barbour: 2000, p. 245-246)
"At an international meeting on quantum gravity
held at Oxford in 1980, Karel Kuchar, concluding his review of the subject,
stated that the problem of 'quantum geometrodynamics is not a technical one, but
a conceptual one. It consists in the diametrically opposite ways in which
relativity and quantum mechanics view the concept of time'."
(Barbour: 2000, p. 246)
"... There has to be different way to think about
time.
I believe it was found, perhaps
unintentionally, by Bryce DeWitt in 1967. John Wheeler had strongly urged to
find the fundamental equation of quantum gravity. It was Wheeler's high priority
to find the Schrodinger equation of geometrodynamics...
Dirac's method makes it possible to treat all parts of space on an
equal footing, and simply defers to later the problem of time. DeWitt used
Dirac's method to write the fascinating equation that, as Kuchar noted, he
himself calls 'that damned equation', John
Wheeler usually calls the 'Einstein-Schrodinger equation' and everyone
else calls the 'Wheeler-DeWitt equation'.
... The Wheeler-DeWitt equation is telling us, in its most
direct interpretation, that the universe in its entirety is like some huge
molecule in a stationary state and that the different possible configurations of
this 'monster molecule' are the instants
of time. Quantum cosmology becomes the ultimate extension of the theory of
atomic structure, and simultaneously subsumes time.
We can go on to ask what
this tells us about time. The implications are as profound as they can be. Time
does not exist. There is just the furniture of the world that we call instants
of time."
(Barbour: 2000, p. 246-247)
"I believe in a timeless universe for the childlike
reason that time cannot be seen - the emperor has no clothes. I believe that the
universe is static and is described by something like the Wheeler-DeWitt
equation... I believe that it leads to the rules of creation...
According to many accounts, in both mainstream
science and religion, the universe either has existed for ever or was created in
the distant past. Creation in a primordial fireball is now orthodox science -
the Big Bang. But why is it supposed that the universe was created in the past
rather than newly created in every instant that is experienced? No two instants
are identical. The things we find in one are not exactly the same as the things
we find in another. What, then, is the justification for saying that something
was created in the past and that its existence has continued into the present?
"
(Barbour: 2000, p. 251)
"A classical theory that treats time in a Machian
manner can allow the universe only one value of its energy. But then its quantum
theory is singular - it can only have one energy eigenvalue. Since quantum
dynamics of necessity has more than one energy eigenvalues, quantum dynamics of
the universe is impossible. There can only be quantum statics...
Neither Newton's nor Einstein's equations tell us why
the universe has its present form. They have to be augmented by information
about a past state. We could invoke a deity in the way Einstein was wont, who
goes through two steps in creating the universe. First, laws are chosen, then an
initial condition is added...
The
stationary Schrodinger equation is quite different in this respect. It obviously
cannot have initial conditions, either..."
(Barbour: 2000, p. 253)
"There are no laws of nature, just one law of the
universe. There is no dichotomy in it - there is no distinction between the law
and supplementary initial or boundary conditions. Just one, all-embracing static
equation. We can call it the universal equation.... it attaches a ranking - a
greater or lesser probability - to each conceivable static configuration of the
universe.
... the density of the blue
mist can be used to create a collection of configurations in a bag, a heap even,
from which the most probable atomic configurations can be drawn at random.
Configurations - which are structures - are created as more or less definite
potentialities to the extent that the stationary Schrodinger equation tells us
to put more or less into the heap. Like the individual structures within it, the
heap is static. It is carefully laid up in a Platonic palace, which, since
probabilities play such a mysterious role in quantum mechanics, is a kind of
'antechamber of Being'."
(Barbour: 2000, p. 254)
"DeWitt already clearly saw... the crass
contradiction between a static quantum universe and our direct experience of
time and motion - and hinted at its solution in 1967.... (The key idea of
his paper) is that the static probability density obtained by solving the
stationary Schrodinger equation for one fixed energy can exhibit the
correlations expected in a world that does evolve - classically or quantum
mechanically - in time. We can have the appearance of dynamics without any
actual dynamics..."
(Barbour: 2000, p. 257)
"... if the universe as a whole is described by a
stationary Schrodinger equation and time does not exist at all, how does a
Schrodinger equation with time arise?...
... Two static wave patterns (in a space of arbitrarily many
dimensions) can, under the appropriate conditions, be interpreted as an
evolution in time of the kind expected in accordance with the time-dependent
Schrodinger equation. The appearance of time and evolution can arise from
timelessness."
(Barbour: 2000, p. 258-259)
"... Schrodinger's enigmatic psi (wave function)
covers Platonia. Mist hovers over the eternal landscape. The static mist is a
well-behaved solution - an eigenfunction - of the Wheeler-DeWitt equation. There
is nothing here an unsuspecting bystander could say looks like time. You have
seen mist on a landscape. Did it enter your head that such a thing could explain
time? But it can, in principle. The static wave function, simply by its
well-behaved response to the landscape it finds, may be induced into a regular
wave-like pattern. If so, time can 'emerge' from timelessness...
"
(Barbour: 2000, p. 260)
"When... I find myself in Plato's cave and see his
demesne of Platonia laid out before me, I can, using my vivid memory of the
kingfisher between the banks of the stream where I stood, identify the instant
in which death took me. By 'identify the instant', I mean recognize the
configuration of riverbank, sunlight and shadow, rippled water and kingfisher's
wings - all frozen in the position I last witnessed. As always, I insist that
instant of time simply means configuration of the universe..."
(Barbour: 2000, p. 265-266)
"All interpretations of quantum mechanics face two
main issues. First, the theory implies the existence of far more 'furniture'
in the world than we see. I have suggested that the 'missing furniture' is
simply other instants of time that we cannot see because we experience only one
at a time. The other issue is why our experience suggest so strongly a
macroscopic universe with a unique, almost classical history...."
(Barbour: 2000, p. 268)
"In the 1820s and 1830s, William Rowan Hamilton...
established a fascinating and beautiful cennection between the two great
paradigms of physical thought of his time - the wave theory of light and the
Newtonian dynamics of particles..."
(Barbour: 2000, p. 268)
"Hamilton's work in the 1820s showed... that there
are two seemingly quite different ways of explaining the behaviour of light and
the functioning of optical instruments...
This insight led to the distinction between wave optics
and geometrical optics which uses light rays. Innumerable experiments
show that only wave optics, in which light is described by waves, can explain
certain phenomena. The earlier theory of light rays simply fails under these
circumstances. Equally, there are many cases in which geometrical optics, with
its Keplerian light rays, functions perfectly well. We see here the typical
situation that arises when a new theory supplants an old one. The new theory
invariably uses very different concepts - it 'inhabits a different world' - yet
it can explain why the old theory worked as well as it did and why it is that it
fails where it does. Where the wave pattern becomes irregular, geometrical
optics ceases to be valid."
(Barbour: 2000, p. 270)
"Geometrical optics shows how theories that explain
many phenomena impressively and simply can still give a misleading
picture..."
(Barbour: 2000, p. 271)
"... in a strict wave theory the rays are not really
'there', but they are present as theoretical constructs. And many phenomena
can be explained rather well by assuming that particles really are there. As
John Wheeler would say, one has 'particles without particles', or even
'histories without histories'.
In
fact, work that Hamilton did about ten years after his optical discoveries shows
how apt such a 'Wheelerism' is...
Working
entirely within the framework of Newtonian dynamics, Hamilton introduced
something he called the principal function... it is like the mists
on configuration space: at each point of the configuration space, it has a value
(intensity), the variation of which is governed by a definite
equation."
(Barbour: 2000, p. 272)
"I certainly find it difficult to believe that
there is a material world in which we currently find ourselves, and some other,
quite different, immaterial world we enter after death. Apart from anything
else, modern physics suggests very strongly that so-called gross matter - the
clay from which we are made - is anything but that. It is almost positively
immaterial."
(Barbour: 2000, p. 327)
"You cannot travel back into the past and kill
your parents before your conception. In quantum cosmology, you can travel back
to a parallel universe, and there kill your parents before they conceive you.
However, we have to be careful about the use of 'you'. The person who 'travels'
to these other worlds is not exactly you now.
... I find the idea of time travel boring compared with the
reality of our normal existence. Each time capsule that represents an
experienced Now reflects innumerable other Nows all over Platonia, some of them
vividly. In a very real sense, our memories make us present in what we call the
past, and our anticipations give us a foretaste of what we call the future. Why
do we need time machines if our very existence is a kind of being present
everywhere in what can be? This is very Leibnizian. We are all part of one
another, and we are each just the totality of things seen from our own
viewpoint."
(Barbour: 2000, p. 329)
"... I wonder if, at root, there is that much
difference between the Heraclitan and Parmenidean schools, representing 'verbs'
and 'nouns' respectively. If my definition of an instant of time is accepted, it
becomes hard to say in what respect those two great Pre-Socratics might differ.
The two best-known sayings attributed to Heraclitus are 'Everything flows'
(Panta rei) and the very sentence which, entirely unconsciously, I used to
clinch the argument that the cat Lucy who leapt to catch the swift was not the
cat who landed with her prey: 'One cannot step into the same river twice'. There
is always change from one instant to another - no two are alike. But that is
just what I have tried to capture with the notion of Platonia as the collection
of all distinct instants. Heraclitus argued that the appearance of permanence,
of enduring substance, is an illusion created by the laws that govern
change..."
(Barbour: 2000, p. 330)
Go to Julian Barbour's official home page
