Many-worlds is an
interpretation of quantum
mechanics that asserts the
objective reality of the
wavefunction, but denies the reality of
wavefunction collapse. It is
also known as
MWI, the
relative state
formulation,
theory of the universal
wavefunction,
parallel universes,
many-universes interpretation or just
many
worlds.
The original
relative state formulation is due to
Hugh Everett who formulated it in 1957.
Later, this formulation was popularized and renamed
many-worlds by
Bryce
Seligman DeWitt in the 1960s and '70s.
Bryce Seligman DeWitt, Quantum
Mechanics and Reality
Could the solution to the dilemma of
indeterminism be a universe in which all possible outcomes of an
experiment actually occur?,
Physics Today,23(9) pp
30-40 (1970) "“every quantum transition taking place on every star,
in every galaxy, in every remote corner of the universe is
splitting our local world on earth into myriads of copies of
itself.”"also April 1971 letters followup
Proponents argue that many-worlds reconciles how we can perceive
non-
deterministic events, such as the
random decay of a radioactive
atom, with the deterministic
equations of
quantum physics. Prior to many-worlds,
reality had been viewed as a single "
world-line". Many-worlds, rather, views reality
as a many-branched tree where every possible quantum outcome is
realised.
In many-worlds, the
subjective
appearance of wavefunction collapse is explained by the
mechanism of
quantum
decoherence. By decoherence, many-worlds claims to resolve all
of the
correlation paradoxes of
quantum theory, such as the
EPR paradox and
Schrödinger's cat, since every
possible outcome of every
event defines or exists in its own "history" or
"world". In layman's terms, there is a very large—perhaps
infinite—number of universes, and everything that could possibly
have happened in our past, but didn't, has occurred in the past of
some other universe or universes.
The decoherence approach to interpreting quantum theory has been
further explored and developed becoming quite popular, taken as a
class overall. MWI is one of many
Multiverse hypotheses in
physics and
philosophy. It
is currently considered a
mainstream
interpretation along with the other
decoherence
interpretations and the
Copenhagen interpretation.
Outline
Although several versions of many-worlds have been proposed since
Hugh Everett's original work, they all
contain one key idea: the equations of physics that model the time
evolution of systems
without embedded observers are
sufficient for modelling systems which
do contain
observers; in particular there is no observation-triggered
wavefunction collapse which the
Copenhagen interpretation
proposes. Provided the theory is
linear with
respect to the wavefunction, the exact form of the
quantum dynamics modelled, be it the
non-relativistic
Schrödinger
equation,
relativistic quantum field
theory or some form of
quantum
gravity or
string theory, does not
alter the validity of MWI since MWI is a
metatheory applicable to all linear
quantum theories, and there is no
experimental evidence for any non-linearity of the wavefunction in
physics. MWI's main conclusion is that the universe (or
multiverse in this context) is composed of a
quantum superposition of very
many, possibly even a
non-denumerable infinitely many, increasingly divergent,
non-communicating parallel universes or quantum worlds.
The idea
of MWI originated in Everett's Princeton
Ph.D. thesis
"The Theory of the Universal Wavefunction", developed under his
thesis advisor John Archibald
Wheeler, a shorter summary of which was published in 1957
entitled "Relative State Formulation of Quantum Mechanics" (Wheeler
contributed the title "relative state"; Everett originally called
his approach the "Correlation Interpretation", where "correlation"
refers to quantum
entanglement). The phrase "many-worlds" is due to
Bryce DeWitt, who was responsible for
the wider popularisation of Everett's theory, which had been
largely ignored for the first decade after publication. DeWitt's
phrase "many-worlds" has become so much more popular than Everett's
"Universal Wavefunction" or Everett-Wheeler's "Relative State
Formulation" that many forget that this is only a difference of
terminology; the content of all three papers is the same.
The many-worlds interpretation shares many similarities with later,
other "post-Everett" interpretations of quantum mechanics which
also use
decoherence to explain the
process of measurement or wavefunction collapse. MWI treats the
other histories or worlds as real since it regards the
universal wavefunction as the "basic
physical entity" or "the fundamental entity, obeying at all times a
deterministic wave equation". The other decoherent interpretations,
such as
many histories,
consistent histories, the
Existential Interpretation etc,
either regard the extra quantum worlds as metaphorical in some
sense, or are
agnostic about their
reality; it is sometimes hard to distinguish between the different
varieties. MWI is distinguished by two qualities: it assumes
realism, which it assigns to
the wavefunction, and it has the minimal formal structure possible,
rejecting any
hidden variables,
quantum potential, any form of a
collapse postulate (i.e.
Copenhagenism) or mental
postulates (such as the
many-minds interpretation
makes).
Decoherent interpretations of many-worlds use
einselection to explain how a small number of
classical pointer states can emerge from the enormous Hilbert space
of superpositions have been proposed by
Wojciech H. Zurek. "Under scrutiny of the environment,
only pointer states remain unchanged. Other states decohere into
mixtures of stable pointer states that can persist, and, in this
sense, exist: They are einselected." These ideas complement MWI and
bring the interpretation in line with our perception of
reality.
Many-worlds is often referred to as a
theory,
rather than just an interpretation, by those who propose that
many-worlds can make testable predictions (such as
David Deutsch) or is falsifiable (such as
Everett) or that all the other, non-MWI, are inconsistent,
illogical or unscientific in their handling of measurements;
Hugh Everett argued that his
formulation was a
metatheory, since it
made statements about other interpretations of quantum theory; that
it was the "only completely coherent approach to explaining both
the contents of quantum mechanics and the appearance of the
world."
Interpreting wavefunction collapse
As with the other interpretations of quantum mechanics, the
many-worlds interpretation is motivated by behavior that can be
illustrated by the
double-slit
experiment. When
particles of light (or
anything else) are passed through the double slit, a calculation
assuming wave-like behavior of light can be used to identify where
the particles are likely to be observed. Yet when the particles are
observed in this experiment, they appear as particles (i.e. at
definite places) and not as non-localized waves.
Some versions of the
Copenhagen interpretation of
quantum mechanics proposed a process of "
collapse" in which an indeterminate
quantum system would probabilistically collapse down onto, or
select, just one determinate outcome to "explain" this phenomenon
of observation. Wavefunction collapse was widely regarded as
artificial and ad-hoc, so an alternative interpretation in which
the behavior of measurement could be understood from more
fundamental physical principles was considered desirable.
Everett's Ph.D. work provided such an alternative interpretation.
Everett noted that for a composite system – for example a subject
(the "observer" or measuring apparatus) observing an object (the
"observed" system, such as a particle) – the statement that either
the observer or the observed has a well-defined state is
meaningless; in modern parlance the observer and the observed have
become
entangled; we can only
specify the state of one
relative to the the other, i.e.
the state of the observer and the observed are correlated
after the observation is made. This led Everett to derive
from the unitary, deterministic dynamics alone (i.e. without
assuming wavefunction collapse) the notion of a
relativity of
states.
Everett noticed that the unitary, deterministic dynamics alone
decreed that after an observation is made each element of the
quantum superposition of the
combined subject-object wavefunction contains two "relative
states": a "collapsed" object state and an associated observer who
has observed the same collapsed outcome; what the observer sees and
the state of the object have become correlated by the act of
measurement or observation. The subsequent evolution of each pair
of relative subject-object states proceeds with complete
indifference as to the presence or absence of the other elements,
as if wavefunction collapse has occurred, which has the
consequence that later observations are always consistent with the
earlier observations. Thus the
appearance of the object's
wavefunction's collapse has emerged from the unitary, deterministic
theory itself. (This answered Einstein's early criticism of quantum
theory, that the theory should define what is observed, not for the
observables to define the theory). Since the wavefunction appears
to have collapsed then, Everett reasoned, there was no need to
actually assume that it had collapsed. And so, invoking
Occam's razor, he removed the postulate of
wavefunction collapse from the theory.
Probability
A consequence of removing
wavefunction collapse from the quantum
formalism is that the
Born rule requires
derivation, since many worlds claims to derive its interpretation
from the formalism. Attempts have been made, by many-world
advocates and others, over the years to
derive the
Born rule, rather than just conventionally
assume it, so as to reproduce all the required statistical
behaviour associated with quantum mechanics. There is no consensus
on whether this has been successful.
Everett, Gleason and Hartle
Everett (1957) briefly derived the
Born
rule by showing that the Born rule was the only possible rule,
and that its derivation was as justified as the procedure for
defining probability in
classical
mechanics. Everett stopped doing research in theoretical
physics shortly after obtaining his Ph.D., but his work on
probability has been extended by a number of people.
Andrew Gleason (1957) and
James Hartle (1965) independently reproduced
Everett's work, known as
Gleason's
theorem which was later, in 1989, extended..
De Witt and Graham
Bryce De Witt and his doctoral student
R. Neill Graham later provided alternative (and longer) derivations
to Everett's derivation of the
Born rule.
They demonstrated that the
norm
of the worlds where the usual statistical rules of quantum theory
broke down vanished, in the limit where the number of measurements
went to infinity.
Deutsch et al
An
information-theoretic
derivation of the Born rule from Everettarian assumptions, was
produced by
David Deutsch (1999) and
refined by Wallace (2003-2009) and Saunders (2004). Deutsch's
derivation is a two-stage proof: first he shows that the number of
orthonormal Everett-worlds after a
branching is proportional to the conventional
probability density. Then he uses game
theory to shows that these are all equally likely to be observed.
The last step in particular has been criticised for circularity.
Other reviews have been positive, although the status of these
arguments remains highly controversial. It is fair to say that some
theoretical physicists have taken them as supporting the case for
parallel universes. In the
New
Scientist article, reviewing their presentation at a September
2007 conference, Andy Albrecht, a physicist at the University of
California at Davis, is quoted as saying "This work will go down as
one of the most important developments in the history of
science."
Wojciech H. Zurek (2005) has produced a derivation of
the Born rule, where
decoherence has
replaced Deutsch's informatic assumptions. Lutz Polley (2000) has
produced Born rule derivations where the informatic assumptions are
replaced by symmetry arguments.
Advantages
- MWI removes the observer-dependent role in the quantum measurement process by replacing
wavefunction collapse with
quantum decoherence. Since the
role of the observer lies at the heart of most if not all "quantum
paradoxes," this automatically resolves a number of problems; see
for example Schrödinger's cat
thought-experiment, the EPR paradox, von
Neumann's "boundary problem" and even wave-particle duality. Quantum cosmology also becomes
intelligible, since there is no need anymore for an observer
outside of the universe.
- MWI is realist, deterministic, local theory, akin to classical physics (including the theory of relativity), at the expense
of losing counterfactual
definiteness. MWI achieves this by removing wavefunction collapse, which is
indeterministic and non-local, from the deterministic and local equations of quantum theory.
- MWI (or other, broader multiverse considerations) provides a
context for the anthropic
principle which may provide an explanation for the fine-tuned universe.
- MWI, being a decoherent
formulation, is axiomatically more streamlined than the Copenhagen and other collapse interpretations; and thus
favoured under certain interpretations of Ockham's razor. Of course there are other
decoherent interpretations that also possess this advantage with
respect to the collapse interpretations.
Common objections and misconceptions
- MWI states that there is no special role nor need for precise
definition of measurement in MWI, yet uses the word "measurement"
repeatedly through out its exposition.
- :MWI response: "measurements" are treated a subclass of
interactions, which induce subject-object correlations in the
combined wavefunction. There is nothing special about measurements
(they don't trigger any wave
function collapse, for example); they are just another unitary time development process. This
is why no precise definition of measurement is required in
Everett's formulation.
- The many-worlds interpretation is very vague about the ways to
determine when splitting happens, and nowadays usually the
criterion is that the two branches have decohered. However, present day
understanding of decoherence does not allow a completely precise,
self contained way to say when the two branches have decohered/"do
not interact", and hence many-worlds interpretation remains
arbitrary. This is the main objection opponents of this
interpretation raise, saying that it is not clear what is precisely
meant by branching, and point to the lack of self contained
criteria specifying branching.
- :MWI response: the decoherence or "splitting" or "branching" is
complete when the measurement is
complete. In Dirac notation a
measurement is complete when:
- :\lang O[i]|O[j]\rang = \delta_{ij}
- :where O[i] represents the observer having detected the object
system in the i-th state. Before the measurement has started the
observer states are identical; after the measurement is complete
the observer states are orthonormal.
Thus a measurement defines the branching process: the branching is
as well- or ill- defined as the measurement is. Thus branching is
complete when the measurement is complete. Since the role of the
observer and measurement per se plays no special role in MWI
(measurements are handled as all other interactions are) there is
no need for a precise definition of what an observer or a
measurement is – just as in Newtonian physics no precise definition
of either an observer or a measurement was required or expected. In
all circumstances the universal
wavefunction is still available to give a complete description
of reality.
- :Also, it is a common misconception to think that branches are
completely separate. In Everett's formulation, they may in
principle quantum interfere
(i.e. "merge" instead of "splitting") with each other in the
future, although this requires all "memory" of the earlier
branching event to be lost, so no observer ever sees two branches
of reality.
- There is circularity in Everett's measurement theory. Under the
assumptions made by Everett, there are no 'good observations' as
defined by him, and since his analysis of the observational process
depends on the latter, it is void of any meaning. The concept of a
'good observation' is the projection postulate in disguise and
Everett's analysis simply derives this postulate by having assumed
it, without any discussion.
- :MWI response: Everett's treatment of observations /
measurements covers both idealised good measurements and
the more general bad or approximate cases. Thus it is legitimate to
analyse probability in terms of measurement; no circularity is
present.
- Talk of probability in Everett presumes the existence of a
preferred basis to identify measurement outcomes for the
probabilities to range over. But the existence of a preferred basis
can only be established by the process of decoherence, which is
itself probabilistic or arbitrary.
- :MWI response: Everett analysed branching using what we now
call the "measurement basis".
It is fundamental theorem of quantum theory that nothing measurable
or empirical is changed by adopting a different basis. Everett was
therefore free to choose whatever basis he liked. The measurement
basis was simply the simplest basis in which to analyse the
measurement process.
- :MWI response: all accepted quantum
theories of fundamental physics are linear with respect to the
wavefunction. Whilst quantum gravity or string theory may be non-linear in this
respect there is no evidence to indicate this at the moment.
- Conservation of energy is
grossly violated if at every instant near-infinite amounts of new
matter are generated to create the new universes.
- :MWI response: Conservation of energy is not violated since the
energy of each branch has to be weighted by its probability,
according to the standard formula for the conservation of energy in
quantum theory. This results in the total energy of the multiverse
being conserved.
- Occam's Razor rules against a
plethora of unobservable universes – Occam would prefer just one
universe; i.e. any non-MWI interpretation.
- :MWI response: Occam's razor actually is a constraint on the
complexity of physical theory, not on the number of universes. MWI
is a simpler theory since it has fewer postulates. See the
"advantages" section.
- Unphysical universes: If a state is a superposition of two
states \Psi_A and \Psi_B, i.e. \Psi = (a \Psi_A + b \Psi_B), i.e.
weighted by coefficients a and b, then if b << a,="" what=""
principle="" allows="" a="" universe="" with="" vanishingly=""
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an="" equal="" footing="" the="" much="" more="" probable="" one=""
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- :MWI response: The magnitude of the coefficients provides the
weighting that makes the branches or universes "unequal", as
Everett and others have shown, leading the emergence of the
conventional probabilistic rules.
- Violation of the principle of
locality, which contradicts special relativity: MWI splitting is
instant and total: this may conflict with relativity, since an
alien in the Andromeda galaxy can't know I collapse an electron
over here before she collapses hers there: the relativity of
simultaneity says we can't say which
electron collapsed first – so which one splits off another universe
first? This leads to a hopeless muddle with everyone splitting
differently. Note: EPR is not a get-out here, as
the alien's and my electrons need never have been part of the same
quantum, i.e. entangled.
- :MWI response: the splitting can be regarded as causal, local
and relativistic, spreading at, or below, the speed of light (e.g.
we are not split by Schrödinger's
cat until we look in the box). For spacelike separated splitting you can't say which
occured first -- but this is true of all spacelike separated
events, simultaneity is not defined for
them. Splitting is no exception; many-worlds is a local
theory.
Brief overview
In Everett's formulation, a measuring apparatus
M
and an object system
S form a composite system,
each of which prior to measurement exists in well-defined (but
time-dependent) states. Measurement is regarded as causing
M and
S to interact. After
S interacts with
M, it is no
longer possible to describe either system by an independent state.
According to Everett, the only meaningful descriptions of each
system are relative states: for example the relative state of
S given the state of
M or the
relative state of
M given the state of
S. In DeWitt's formulation, the state of
S after a sequence of measurements is given by a
quantum superposition of states, each one corresponding to an
alternative measurement history of
S.
Schematic illustration of splitting as
a result of a repeated measurement.
For example, consider the smallest possible truly quantum system
S, as shown in the illustration. This describes
for instance, the spin-state of an electron. Considering a specific
axis (say the
z-axis) the north pole represents spin "up"
and the south pole, spin "down". The superposition states of the
system are described by (the surface of) a sphere called the
Bloch sphere. To perform a measurement
on
S, it is made to interact with another similar
system
M. After the interaction, the combined
system is described by a state that ranges over a six-dimensional
space (the reason for the number six is explained in the article on
the Bloch sphere). This six-dimensional object can also be regarded
as a quantum superposition of two "alternative histories" of the
original system
S, one in which "up" was observed
and the other in which "down" was observed. Each subsequent binary
measurement (that is interaction with a system
M)
causes a similar split in the history tree. Thus after three
measurements, the system can be regarded as a quantum superposition
of 8= 2 × 2 × 2 copies of the original system
S.
The accepted terminology is somewhat misleading because it is
incorrect to regard the universe as splitting at certain times; at
any given instant there is one state in one universe.
Relative state
The goal of the relative-state formalism, as originally proposed by
Everett in his 1957 doctoral dissertation, was to interpret the
effect of external observation entirely within the mathematical
framework developed by
Paul Dirac,
von Neumann and others, discarding
altogether the ad-hoc mechanism of wave function collapse. Since
Everett's original work, there have appeared a number of similar
formalisms in the literature. One such idea is discussed in the
next section.
The relative-state interpretation makes two assumptions. The first
is that the wavefunction is not simply a description of the
object's state, but that it actually is entirely equivalent to the
object, a claim it has in common with some other interpretations.
The second is that observation or measurement has no special role,
unlike in the
Copenhagen
interpretation which considers the wavefunction collapse as a
special kind of event which occurs as a result of
observation.
The many-worlds interpretation is DeWitt's popularisation of
Everett's work, who had referred to the combined observer-object
system as being split by an observation, each split corresponding
to the different or multiple possible outcomes of an observation.
These splits generate a possible tree as shown in the graphic
below. Subsequently DeWitt introduced the term "world" to describe
a complete measurement history of an observer, which corresponds
roughly to a single branch of that tree. Note that "splitting" in
this sense, is hardly new or even quantum mechanical. The idea of a
space of complete alternative histories had already been used in
the theory of probability since the mid 1930s for instance to model
Brownian motion.
Under the many-worlds interpretation, the
Schrödinger equation, or
relativistic analog, holds all the time everywhere. An observation
or measurement of an object by an observer is modeled by applying
the wave equation to the entire system comprising the observer
and the object. One consequence is that every observation
can be thought of as causing the combined observer-object's
wavefunction to change into a quantum superposition of two or more
non-interacting branches, or split into many "worlds". Since many
observation-like events have happened, and are constantly
happening, there are an enormous and growing number of
simultaneously existing states.
If a system is composed of two or more subsystems, the system's
state will be a superposition of products of the subsystems'
states. Once the subsystems interact, their states are no longer
independent. Each product of subsystem states in the overall
superposition evolves over time independently of other products.
The subsystems states have become correlated or
entangled and it is no longer possible
to consider them independent of one another. In Everett's
terminology each subsystem state was now
correlated with
its
relative state, since each subsystem must now be
considered relative to the other subsystems with which it has
interacted.
Successive measurements with
successive splittings
Comparative properties and experimental support
One of the salient properties of the many-worlds interpretation is
that observation does not require an exceptional construct (such as
wave function collapse) to explain it. Many physicists, however,
dislike the implication that there are infinitely many
non-observable alternate universes.
, there are no practical experiments that distinguish between Many-Worlds and Copenhagen. There may be cosmological, observational evidence.
Copenhagen interpretation
In the Copenhagen interpretation, the mathematics of quantum
mechanics allows one to predict
probabilities for the occurrence of various
events. In the many-worlds interpretation, all these events occur
simultaneously. What meaning should be given to these probability
calculations? And why do we observe, in our history, that the
events with a higher computed probability seem to have occurred
more often? One answer to these questions is to say that there is a
probability measure on the space
of all possible universes, where a possible universe is a complete
path in the tree of branching universes. This is indeed what the
calculations give. Then we should expect to find ourselves in a
universe with a relatively high probability rather than a
relatively low probability: even though all outcomes of an
experiment occur, they do not occur in an equal way. As an
interpretation which (like other interpretations) is consistent
with the equations, it is hard to find testable predictions of
MWI.
Quantum suicide
There is a rather more dramatic test than the one outlined above
for people prepared to put their lives on the line: use a machine
which kills them if a random quantum decay happens. If MWI is true,
they will still be alive in the world where the decay didn't happen
and would feel no interruption in their stream of consciousness. By
repeating this process a number of times, their continued
consciousness would be arbitrarily unlikely unless MWI was true,
when they would be alive in all the worlds where the random decay
was on their side. From their viewpoint they would be immune to
this death process. Clearly, if MWI does not hold, they would be
dead in the one world. Other people would generally just see them
die and would not be able to benefit from the result of this
experiment. See
Quantum
suicide.
The universe decaying to a new vacuum state
Any event that changes the number of observers in the universe may
have experimental consequences.
Quantum tunnelling to new vacuum state
would reduce the number of observers to zero (i.e. kill all life).
Some
Cosmologists argue that the
universe is in a
false vacuum state and that consequently the
universe should have already experienced
quantum tunnelling to a true
vacuum state. This has not happened and is
cited as evidence in favour of many-worlds.
Many-minds
The many-worlds interpretation should not be confused with the
similar
many-minds interpretation
which defines the split on the level of the observers' minds.
Reception
There is a wide range of claims that are considered "many-worlds"
interpretations. It is often claimed by those who do not believe in
MWI that Everett himself was not entirely clear as to what he
believed; however MWI adherents (such as
DeWitt,
Tegmark,
Deutsch and others) believe they fully
understand Everett's meaning as implying the literal existence of
the other worlds. Additionally Everett's reported belief in
quantum immortality,
requires belief in the reality of all the many-worlds represented
by the components of the uncollapsed
universal wavefunction.
"Many-worlds"-like interpretations are now considered fairly
mainstream within the quantum physics community. For example, a
poll of 72 leading physicists conducted by the American researcher
David Raub in 1995 and published in the French periodical
Sciences et Avenir in
January 1998 recorded that nearly 60% thought many-worlds
interpretation was "true".
Max Tegmark
also reports the result of a poll taken at a 1997 quantum mechanics
workshop. According to Tegmark, "The many worlds interpretation
(MWI) scored second, comfortably ahead of the
consistent histories and
Bohm interpretations." Other such
polls have been taken at other conferences: see for instance
Michael Nielsen's blog report on one
such poll. Nielsen remarks that it appeared most of the conference
attendees "thought the poll was a waste of time". MWI sceptics (for
instance
Asher Peres) argue that polls
regarding the acceptance of a particular interpretation within the
scientific community, such as those mentioned above,
cannot be used as evidence supporting
a specific interpretation's validity. However, others note that
science is a group activity (for instance,
peer review) and that polls are a systematic way
of revealing the thinking of the scientific community.
A 2005 minor poll on the Interpretation of Quantum Mechanics
workshop at the Institute for Quantum Computing University of
Waterloo produced contrary results, with the MWI as the least
favored.
One of MWI's strongest advocates is
David
Deutsch. According to Deutsch, the single photon interference
pattern observed in the double slit experiment can be explained by
interference of photons in multiple universes. Viewed in this way,
the single photon interference experiment is indistinguishable from
the multiple photon interference experiment. In a more practical
vein, in one of the earliest papers on quantum computing, he
suggested that parallelism that results from the validity of MWI
could lead to "
a method by which certain probabilistic tasks
can be performed faster by a universal quantum computer than by any
classical restriction of it". Deutsch has also proposed that
when reversible computers become conscious that MWI will be
testable (at least against "naive" Copenhagenism) via the
reversible observation of spin.
Asher Peres was an outspoken critic of
MWI, for example in a section in his 1993 textbook with the title
Everett's interpretation and other bizarre theories. In
fact, Peres questioned whether MWI is really an "interpretation" or
even if interpretations of quantum mechanics are needed at all.
Indeed, the many-worlds interpretation can be regarded as a purely
formal transformation, which adds nothing to the instrumentalist
(i.e. statistical) rules of the
quantum mechanics. Perhaps more
significantly, Peres seems to suggest that positing the existence
of an infinite number of non-communicating
parallel universes is highly suspect as it
violates those interpretations of
Occam's
Razor that seek to minimize the number of hypothesized
entities. Proponents of MWI argue precisely the opposite, by
applying Occam's Razor to the set of assumptions rather than
multiplicity of universes. In
Max
Tegmark's formulation, the alternative to many-worlds is the
undesirable "many words", an
allusion to
the complexity of
von Neumann's collapse
postulate.
MWI is considered by some to be
unfalsifiable and hence unscientific because
the multiple
parallel universes are
non-communicating, in the sense that no information can be passed
between them. Others claim MWI is directly testable. Everett
regarded MWI as falsifiable since any test that falsifies
conventional
quantum theory would
also falsify MWI.
According to
Martin Gardner MWI has
two different interpretations: real or unreal, and claims that
Stephen Hawking and
Steve Weinberg favour the unreal
interpretation. Gardner also claims that the interpretation
favoured by the majority of physicists is that the other worlds are
not real in the same way as our world is real, whereas the
"realist" view is supported by MWI experts
David Deutsch and
Bryce DeWitt. However
Stephen Hawking is on record as a saying
that the "other worlds are as real as ours" and
Tipler reports
Hawking saying that MWI is "trivially true"
(scientific jargon for "obviously true")
if quantum theory
applies to all reality.
Roger Penrose
agrees with Hawking that QM applied to the universe implies MW,
although he considers the current lack of a successful theory of
quantum gravity negates the claimed
universality of conventional QM.
Speculative implications
Speculative physics deals with questions also discussed in science
fiction.
Quantum suicide thought experiment
It has been claimed that there is a thought experiment that would
clearly differentiate between the many-worlds interpretation and
other
interpretations of quantum
mechanics. It involves a
quantum
suicide machine and an experimenter willing to risk death.
However, at best, this would only decide the issue for the
experimenter; bystanders would learn nothing. The flip side of
quantum suicide is
quantum
immortality.
Weak coupling
Another speculation is that the separate worlds remain weakly
coupled (e.g. by gravity) permitting "communication between
parallel universes". This requires that
gravity be a classical force and not
quantized.
Similarity to Modal Realism
The many-worlds interpretation has some similarity to
modal realism in
philosophy, which is the view that the
possible worlds used to interpret modal
claims actually exist. Unlike philosophy, however, in quantum
mechanics counterfactual alternatives can influence the results of
experiments, as in the
Elitzur-Vaidman
bomb-testing problem or the
Quantum Zeno effect.
Time travel
The many-worlds interpretation could be one possible way to resolve
the paradoxes that one would expect to arise
if time travel turns out to be permitted by physics
(permitting
closed timelike
curves and thus violating
causality).
Entering the past would itself be a quantum event causing
branching, and therefore the timeline accessed by the time
traveller simply would be another timeline of many. In that sense,
it would make the
Novikov self-consistency
principle unnecessary.
Many-worlds in literature and science fiction
The many-worlds interpretation (and the somewhat related concept of
possible worlds) have been
associated to numerous themes in
literature,
art and
science fiction.
Some of these stories or films violate fundamental principles of
causality and relativity, and are extremely misleading since the
information-theoretic structure
of the path space of multiple universes (that is information flow
between different paths) is very likely extraordinarily complex.
Also see Michael Clive Price's FAQ referenced in the external links
section below where these issues (and other similar ones) are dealt
with more decisively.
Another kind of popular illustration of many-worlds splittings,
which does not involve information flow between paths, or
information flow backwards in time considers alternate outcomes of
historical events. According to the many-worlds interpretation, all
of the historical speculations entertained within the
alternate history genre are realized in
parallel universes.
See also
Notes
Further reading
- Jeffrey A. Barrett, The Quantum Mechanics of
Minds and Worlds, Oxford University Press, Oxford, 1999.
- Julian Brown, Minds, Machines,
and the Multiverse, Simon & Schuster, 2000, ISBN
0-684-81481-1
- Paul C.W. Davies, Other Worlds, (1980) ISBN
0-460-04400-1
- James P. Hogan, The Proteus
Operation (science fiction involving the many-worlds
interpretation, time travel and World War 2 history), Baen, Reissue
edition (August 1, 1996) ISBN 0671877577
- Adrian Kent, One world versus
many: the inadequacy of Everettian accounts of evolution,
probability, and scientific confirmation * Asher Peres, Quantum Theory: Concepts and
Methods, Kluwer, Dordrecht, 1993.
- Andrei Linde and Vitaly Vanchurin, How Many Universes are in the Multiverse?
- Stefano Osnaghi, Fabio Freitas, Olival Freire Jr, The
Origin of the Everettian Heresy, Studies in History and
Philosophy of Modern Physics 40(2009)97–123. A study of the
painful three-way relationship between Hugh
Everett, John A Wheeler and
Niels Bohr and how this affected the
early development of the many-worlds theory.
- Mark A. Rubin, Locality in the Everett Interpretation
of Heisenberg-Picture Quantum Mechanics, Foundations of Physics
Letters, 14, (2001) , pp. 301–322,
- Frank J. Tipler, Testing
Many-Worlds Quantum Theory By Measuring Pattern Convergence
Rates
- David Wallace, Harvey R. Brown, Solving the measurement
problem: de Broglie-Bohm loses out to Everett, Foundations of
Physics,
- David Wallace, Worlds in the Everett Interpretation,
Studies in the History and Philosophy of Modern Physics,
33, (2002), pp. 637–661,
- John A. Wheeler and Wojciech Hubert Zurek (eds),
Quantum Theory and Measurement, Princeton
University Press
, (1983), ISBN 0-691-08316-9
External links
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