Oh, How Exciting!

Quantum particles, atoms and molecules, appear in
two places at once, spin clockwise and anticlockwise
at the same time, instantaneously influence each other
half a universe apart

We are made of atoms and molecules, yet we cannot
do that - at what point does quantum mechanics cease
to apply - the struggle to come up with an answer is
proving to be its own reward

Giving birth to quantum information gaining the attention
of high-tech industries in a pursuit that quantum cynic Albert
Einstein dismissed as a gentle pillow lulling good physicists
to sleep – but quantum theory

Is a Masterpiece: No experiment has ever disagreed with its
predictions, confident it is a good way to describe how the
universe works on smallest scale – the question is:
What does it mean?

*****************************************************

Physicists answer with philosophical speculations and
with experiments about the bedrock beneath quantum theory
in a zoo of interpretations, the such the Copenhagen
theory of Niels Bohr

An electron's location within an atom is meaningless without
its measurement – only when interacting with an electron
by observing it with non-quantum classical device,
confers physical property on it and it becomes
part of reality

The many worlds interpretation explains quantum
strangeness by everything having multiple existences
in a myriad parallel universes

The De Broglie-Bohm interpretation considers
quantum theory incomplete because it lacks
the hidden properties that would make sense
of everything

The transactional interpretation sees particles
travelling backwards in time - a crowded and
noisy quantum zoo

In the Wonderful Copenhagen physicists do not
trouble with philosophy, questions of measurement
or why it induces a change in the fabric of reality, are
ignored - quantum theory simply provides a useful answer

The unquestioning use of Copenhagen is called "shut up and
calculate" - do calculations and apply the results without saying
anything about the fundamental nature of reality

“Entanglement” concerns the use of quantum information
about the properties of sets of quantum particles, shared
between all of them, resulting in action at a distance:

Measuring a property of one particle instantaneously
affects the properties of its entangled partners, no matter
how far apart, the foundation of quantum computing –
a single measurement gives the answer to millions
of calculations done in parallel by quantum particles

Physicists wonder what these phenomena reveal about
the nature of reality - one implication is that information
held in quantum particles lies at the root of reality

The Copenhagen interpretation sees quantum systems as
information carriers; classical measurement registers
change in the information content of the system

Thus measurement updates information the fundamental
component of reality - thus the universe itself is a
vast quantum computer

But if the measurement process of a classical observer is
creating the reality we observe; WHAT performed the
observations that brought the universe
into existence?

This requires an observer outside the system, of there's
nothing outside the universe by definition, cosmologists
prefer physicist Hugh Everett’s many worlds
interpretation quantum mechanics

Reality is NOT bound to measurement – nut a myriad
different possibilities inherent in a quantum system
instead; each manifesting in their own universe

David Deutsch, who drew up the blueprint for the first
quantum computer, said computer operation takes
place in terms of multiple universes

Tim Maudlin, philosopher of science, says when quantum
theory predicts one outcome is ten times more probable
than another, repeated experiments bear it out

The ‘many worlds’ theory says all possible outcomes will
occur given the multiplicity of worlds, but does not explain
why observers see the most probable outcome

There is the "incredulous stare objection" – in many worlds
there are multiple copies of you – people are reluctant to
accept the multiplicity of themselves

Technology using quantum world's stranger aspects in
quantum computers to perform tasks while being in
many states at once, will make these worlds
seem real

Deutsch and Brown claim many worlds is gaining
among cosmologists, string theory, cosmology
and observational astronomy suggest

We live in one of many universes…

The choice is a matter of taste, in most cases
we cannot discriminate experimentally, can
only follow our instincts

Physicists wandering round the quantum zoo,
choosing a favourite on a whim, seems
unscientific, but has not done
any harm so far

Quantum theory transformed the world through
spin-offs like transistor and laser - keeping an
open mind about the meaning of quantum
theory might open up a new field
of physics – oh, how exciting…!

*******************************************************
Knowing something about quantum physics is
a great help in reading the Discworld Series by
Terry Pratchett and the Johnny Trilogy, of
Course.

*******************************************************

“Quantum reality: The many meanings of life”
24 January 2011 by Michael Brooks Magazine
issue 2796 - For similar stories, visit the
Quantum World Topic Guide Editorial:

"Don't fear the f-word in quantum physics"

Really Exciting

Quantum theory is a scientific masterpiece – but physicists
still aren't sure what to make of it

A CENTURY, it seems, is not enough. One hundred years
ago this year, the first world physics conference took place
in Brussels, Belgium. The topic under discussion was how
to deal with the strange new quantum theory and whether
it would ever be possible to marry it to our everyday
experience, leaving us with one coherent description
of the world.

It is a question physicists are still wrestling with today.
Quantum particles such as atoms and molecules have
an uncanny ability to appear in two places at once, spin
clockwise and anticlockwise at the same time, or
instantaneously influence each other when they are
half a universe apart.

The thing is, we are made of atoms and molecules, and
we can't do any of that. Why? "At what point does
quantum mechanics cease to apply?" asks Harvey
Brown, a philosopher of science at the University
of Oxford.

Although an answer has yet to emerge, the struggle
to come up with one is proving to be its own reward.
It has, for instance, given birth to the new field of
quantum information that has gained the attention
of high-tech industries and government spies.

It is giving us a new angle of attack on the problem
of finding the ultimate theory of physics, and it might
even tell us where the universe came from. Not bad
for a pursuit that a quantum cynic - Albert Einstein –
dismissed as a "gentle pillow" that lulls good
physicists to sleep.

Unfortunately for Einstein quantum theory has turned
out to be a masterpiece. No experiment has ever
disagreed with its predictions, we can be confident
that it is a good way to describe how the universe
works on the smallest scales.

Which leaves us with only one problem: what
does it mean? Physicists try to answer this with
"interpretations" - philosophical speculations, fully
compliant with experiments, of what lies beneath
quantum theory. "There is a zoo of interpretations,"
says Vlatko Vedral, who divides his time between
the University of Oxford and the
Centre for Quantum Technologies in Singapore.

No other theory in science has so many different
ways of looking at it. How so? And will any one
win out over the others?

Take what is now known as the Copenhagen interpretation,
for example, introduced by the Danish physicist Niels Bohr.
It says any attempt to talk about an electron's location
within an atom is meaningless without making a measurement
of it.

Only when we interact with an electron by trying to observe
it with a non-quantum or "classical" device does it take on
any attribute that we would call a physical property and
become part of reality.

Then there is the "many worlds" interpretation, where
quantum strangeness is explained by everything
having multiple existences in myriad parallel
universes.

Or you might prefer the de Broglie-Bohm interpretation,
where quantum theory is considered incomplete: we
are lacking some hidden properties that would make
sense of everything.

There are plenty more, such as the Ghirardi-Rimini-
Weber interpretation, the transactional interpretation
(which has particles travelling backwards in time),
Roger Penrose's gravity-induced collapse interpretation,
the modal interpretation... in the last 100 years, the
quantum zoo has become a crowded and noisy place.

But there are only a few interpretations that matter
to most physicists:

Wonderful Copenhagen
The most popular is Bohr's Copenhagen interpretation.
Its popularity is due to the fact that physicists don't
want to trouble themselves with philosophy.

Questions over what constitutes a measurement, or
why it might induce a change in the fabric of reality
can be ignored in favour of simply getting a useful
answer from quantum theory.

That is why unquestioning use of the Copenhagen
interpretation is known as the "shut up and calculate"
interpretation. "Given that most physicists just want
to do calculations and apply their results, the majority
of them are in the shut up and calculate group,"
Vedral says.

This approach has a couple of downsides, though.
First, it is never going to teach anything about the
fundamental nature of reality. That requires a
willingness to look for places where quantum
theory might fail, rather than where it succeeds
(New Scientist, 26 June 2010, p 34).

"If there is going to be some new theory, I don't think
it's going to come from solid state physics, where the
majority of physicists work," says Vedral.

Second, working in a self-imposed box also means
new applications of quantum theory are unlikely to
emerge. The many perspectives we can take on quantum
mechanics can be the catalyst for new ideas.

"If you're solving different problems, it's useful to be able
to think in terms of different interpretations," Vedral says.

Nowhere is this more evident than in the field of quantum
information. "This field wouldn't exist if people hadn't
worried about the foundations of quantum physics," says
Anton Zeilinger of the University of Vienna in Austria.

At the heart of this field is the phenomenon of entanglement,
where the information about the properties of a set of
quantum particles becomes shared between all of them.

The result is what Einstein termed "spooky action at a
distance": measuring a property of one particle will
instantaneously affect the properties of its entangled
partners, no matter how far apart they are.

When first spotted in the equations of quantum theory,
entanglement seemed such a weird idea that the Irish
physicist John Bell created a thought experiment to
show that it couldn't possibly manifest itself in the
real world.

When it became possible to do the experiment, it
proved Bell wrong and told physicists a great deal
about the subtleties of quantum measurement.
It also created the foundations of quantum computing,
where a single measurement could give you the answer
to millions of calculations done in parallel by quantum
particles, and quantum cryptography, which protects
information by exploiting the nature of quantum
measurement.

Both of these technologies have, attracted the attention
of governments and industry keen to possess the best
technologies and to prevent them falling into
the wrong hands.

Physicists are more interested in what these phenomena
tell us about the nature of reality.

One implication of quantum information experiments
seems to be that information held in quantum particles
lies at the root of reality.

Adherents of the Copenhagen interpretation, such as
Zeilinger, see quantum systems as carriers of information,
and measurement using classical apparatus as
nothing special: just a way of registering change
in the information content of the system.

"Measurement updates the information," Zeilinger says.
This new focus on information as a fundamental component
of reality has also led some to suggest the
universe itself is a vast quantum computer.

However, the Copenhagen interpretation requires an artificial
distinction between tiny quantum systems and the classical
apparatus or observers performing measurement on them.

Vedral has been probing the role of quantum mechanics
in biology: various processes and mechanisms in the cell
are quantum at heart – also photosynthesis and radiation-
sensing systems (New Scientist, 27 November, p 42).

"We are discovering that more and more of the world can
be described quantum mechanically - I don't think there
is a hard boundary between quantum and classical,"
he says.

Considering the nature of things on the scale of the
universe has also provided Copenhagen's critics
with ammunition. If the process of measurement by
a classical observer is fundamental to creating the
reality we observe, what performed the observations
that brought the contents of the universe into
existence?

"You need an observer outside the system to make
Sense, but there's nothing outside the universe by
definition," says Brown.

That's why cosmologists are more sympathetic to an
interpretation created in the late 1950s by Princeton
University physicist Hugh Everett.

His "many worlds" interpretation of quantum mechanics
says that reality is not bound to a concept of measurement.
Instead, the myriad different possibilities inherent in a
quantum system each manifest in their own universe.

David Deutsch, a physicist at the University of Oxford
and the person who drew up the blueprint for the first
quantum computer, says he can now only think of the
computer's operation in terms of these multiple universes.
To him, no other interpretation makes sense.

Not that many worlds is without its critics; Tim Maudlin,
a philosopher of science based at Rutgers University
in New Jersey, applauds its attempt to demote
measurement from the status of a
special process.

At the same time he is not convinced that many worlds
provides a good framework for explaining why some
quantum outcomes are more probable than others.

When quantum theory predicts one outcome of a
measurement is 10 times more probable than
another, repeated experiments always bear
that out.

According to Maudlin, many worlds says all possible
outcomes will occur, given the multiplicity of worlds,
but doesn't explain why observers still see the
most probable outcome.
"There's a very deep problem here," he says.

Deutsch says these issues have been resolved in
the last year or so. "The way that Everett dealt
with probabilities was deficient, but over the
years many-worlds theorists have been
picking away at this - and we have
solved it," he says.

However Deutsch's argument is abstruse
and his claim has yet to convince everyone.
Even more difficult to answer is what proponents
of many worlds call the "incredulous stare objection".

The obvious implication of many worlds is there are
multiple copies of you and that Elvis is still performing
in Vegas in another universe. Few people can
stomach this idea.

Persistence will be the only solution here, Brown
reckons. "There is a widespread reluctance to accept
the multiplicity of yourself and others," he says.
"But it's just a question of getting used to it."

Deutsch thinks this will happen when technology
starts to use the quantum world's stranger sides.
Once we have quantum computers that
perform tasks by being in many states
at the same time, we will not be able
to think of these worlds as anything
other than physically real.

"It will be very difficult to maintain the idea that
this is just a manner of speaking," Deutsch says.

He and Brown both claim that many worlds is
already gaining traction among cosmologists.

Arguments from string theory, cosmology and
observational astronomy have led some
cosmologists to suggest we live in one of
many universes.

Last year, Anthony Aguirre of the University
of California, Santa Cruz, Max Tegmark of
the Massachusetts Institute of Technology,
and David Layzer of Harvard University laid
out a scheme that ties together ideas from
cosmology and many worlds
(New Scientist, 28 August 2010, p 6).

But many worlds is not the only interpretation
laying claim to cosmologists' attention.

In 2008, Anthony Valentini of Imperial College
London suggested the cosmic microwave
background radiation (CMB) that has filled
space since just after the big bang, might
support the de Broglie-Bohm interpretation.

In this scheme, quantum particles possess as
yet undiscovered properties dubbed
hidden variables.

The idea behind this is that taking these hidden
variables into account would explain the strange
behaviours of the quantum world, which would
leave an imprint on detailed maps of the CMB.

Valentini says hidden variables could provide
a closer match with observed CMB structure than
standard quantum mechanics does.

Though it is a nice idea, as yet there is no
conclusive evidence that he might be onto
something. What's more, if something
unexpected does turn up in the CMB, it won't
be proof of Valentini's hypothesis, Vedral
reckons: any of the interpretations could
claim that the conditions of the early universe
would lead to unexpected results.

"We're stuck in a situation where we probably
won't ever be able to decide experimentally
between Everett and de Broglie-Bohm," Brown
admits. But that is no reason for pessimism.

"I think there has been progress. A lot of people
say we can't do anything because of a lack of a
crucial differentiating experiment - but it is
definitely the case that some interpretations
are better than others."

For now, Brown, Deutsch and Zeilinger refuse
to relinquish their favourite views of quantum
mechanics. Zeilinger is happy that the debate
about what quantum theory means, shows
no sign of going away.

Vedral agrees. Although he puts himself "in the
many worlds club", which interpretation you choose
to follow is largely a matter of taste, he reckons.

"In most of these cases you can't discriminate
experimentally, so you really just have to follow
your instincts."

The idea that physicists wander round the quantum
zoo, choosing a favourite creature on a whim might
seem rather unscientific, but it hasn't done
any harm so far.

Quantum theory has transformed the world through
spin-offs - transistor and laser, for example - and
there may be more to come.

Having different interpretations to follow, gives
physicists ideas for doing experiments in
different ways. If history is anything to go
by, keeping an open mind about what
quantum theory means might yet
open up another new field
of physics, Vedral says.

"Now that really would be exciting."