![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
swestrupIts a long read, but if this experiment proves to be correct, then the Copenhagen Interpretaion of Quantum Mechanics is dead, and I will most happily dance on its grave!
Quantum rebel
New Scientist vol 183 issue 2457 - 24 July 2004, page 30
Has a simple experiment unravelled our most cherished notions of
reality? Marcus Chown investigates
WHEN you look at your reflection in the bathroom mirror every morning,
you could be doing yourself a favour. After all, some physicists believe
that the most fundamental aspects of the universe do not really exist
until they are observed. So you could argue that getting up and
stumbling to the bathroom each day is vital to your well-being.
That is absurd, of course. But is it any more absurd than the standard
interpretation of quantum theory, our most successful description of the
microscopic world of atoms and their constituents? The theory's
weirdness has grown to be accepted as the way things are: a bizarre
scheme in which reality manifests itself in different ways depending on
whether - and how - you measure it.
It now seems that our acceptance of such strangeness may be a mistake.
An audacious and highly controversial experiment suggests that is not
how things are at all. Shahriar S. Afshar, a 33-year-old
Iranian-American physicist, has carried out a novel version of the
"double-slit" experiment held by physicists to embody the central
mystery of quantum theory's weirdness - and he says he has contradicted
the standard result. "According to my experiment, one of our key
assumptions about quantum theory is wrong," says Afshar. It's not a
claim to make lightly. If he is right, it will reopen an argument that
has lain dormant since the birth of quantum theory.
The Danish physicist Niels Bohr claimed that the only way to interpret
the theory - the only way to understand what the mathematics of quantum
theory has to say about how quantum things manifest in the familiar,
classical world of our experiment - is to assume that nothing really
exists until it is measured. To Bohr, it made no sense to talk about an
objective reality independent of observers because our observations make
a difference to what we will see.
This "Copenhagen interpretation" of quantum theory came from Bohr's
conviction that, though the fundamental building blocks of reality might
seem to be both particles and waves - a phenomenon physicists call
wave-particle duality - it is more likely they are something else
entirely, something for which there is no analogue in the familiar
classical world in which we carry out our experiments. When faced with
our classical apparatus these mysterious quantum entities will show
either a particle-like or a wave-like face.
Bohr proposed that the face you see depends on how you set up your
experiment. And, he said, you'll never see both at the same time in one
experiment. He called this the "principle of complementarity". Einstein
took exception to this: he refused to believe that the very fabric of
the observable universe could change depending on our choice of
measuring equipment. But he never managed to find an experimental way to
refute complementarity, and Bohr's influence ensured that it gradually
became the accepted view of how the quantum world will manifest in our
classical experiments. Afshar, however, may have succeeded where
Einstein failed.
His experiment centres on shining laser light onto two nearby apertures.
This light emerges from the apertures as two spreading beams. Where the
beams overlap, they interfere, producing alternating bands, or fringes,
of light and dark. You can easily explain this interference pattern if
you think of the light as waves. When two wave crests meet, they combine
to produce even brighter light; when crests and troughs meet, there is
darkness. The exact geometry of the interference pattern - the width and
separation of the light and dark fringes, for instance - depends on the
position of the slits, the frequency of the laser, and so on.
Nothing mysterious so far; the physicist Thomas Young first demonstrated
this phenomenon in 1801 using sunlight. The problem arises because of
quantum theory, which says you can consider the light beams as streams
of particles called photons. Each photon is a packet of light energy.
How do particles produce an interference pattern?
The short answer is they can't - yet they do. And the mystery deepens
when you turn the laser right down so that only one photon travels
through the apparatus at a time. It takes a lot longer, but the
interference pattern builds up, one photon at a time. For this to
happen, each photon must somehow pass through both slits and interfere
with itself. Observing an interference pattern in these circumstances is
the equivalent of hearing the sound of one hand clapping. The particles
- whose defining characteristic is that they are localised at a
particular point in space - are behaving like waves, which are smeared
through a relatively large region of space. The Caltech physicist
Richard Feynman once called this hybrid behaviour "the only mystery" in
physics.
It certainly is mysterious: if you set the experiment up to follow the
path of the particles - using, say, a photon detector to see which slit
the light goes through - you'll be rewarded with a view of the
particle-like face: your photon detector will register a photon. And if
you look for evidence of waves, by looking for an interference pattern
for example, you'll see that instead. Experiments have shown that
attempts to locate the photon on its way through the apparatus always
result in a washed-out interference pattern. Bohr's interpretation of
the double-slit experiment appears to be right: nature does not permit
us to know which slit a particle passed through - "which way"
information - and also see an interference pattern. This has become the
orthodox view, reprinted in thousands of physics textbooks.
So how come Afshar is claiming Bohr was wrong - that you can track the
photons' paths and not destroy the interference pattern? Because, he
says, he's done it.
He carried out his original experiment at the Institute for
Radiation-Induced Mass Studies, a privately funded organisation in
Boston, where Afshar is principal investigator. The set-up is relatively
simple (see Diagram). Laser light falls on two pinholes in an opaque
screen. On the far side of the screen is a lens that takes the light
coming through each of the pinholes (another opaque screen stops all
other light hitting the lens) and refocuses the spreading beams onto a
mirror that reflects each onto a separate photon detector. In this way,
Afshar gets a record of the rate at which photons are coming through
each pinhole. According to complementarity, that means there should be
no evidence of an interference pattern. But there is, Afshar says.
He doesn't look at the pattern directly, but has designed the experiment
to test for its presence. He places a series of wires exactly where the
dark fringes of the interference pattern ought to be. Then he closes one
of the pinholes. This, of course, prevents any interference pattern from
forming, and the light simply spreads out as it emerges from the single
pinhole. A portion of the light will hit the metal wires, which scatter
it in all directions, meaning less light will reach the photon detector
corresponding to that pinhole.
But Afshar claims that when he opens up the closed pinhole, the light
intensity at each detector returns to its value before the wires were
set in place. Why? Because the wires sit in the dark fringes of the
interference pattern, no light hits them, and so none of the photons are
scattered. That shows the interference pattern is there, says Afshar,
which exposes the wave-like face of light. And yet he can also measure
the intensity of light from each slit with a photon detector, so he can
tell how many photons pass through each slit - the particle-like face is
there too.
"This flies in the face of complementarity, which says that knowledge of
the interference pattern always destroys the which-way information and
vice versa," says Afshar. "Something everyone believed and nobody
questioned for 80 years appears to be wrong."
When Christopher Stubbs of Harvard University invited Afshar to repeat
the experiment as a visiting scientist in Stubbs's lab earlier this
year, the result was the same. Afshar has now submitted his work for
peer-reviewed publication. What ought to happen now is that the
journal's review process will either find some flaw in Afshar's
reasoning or else uphold his position and throw Bohr's ideas onto the
scrap heap.
In reality, things are unlikely to be quite that clear-cut. That is
because Afshar is not only challenging the orthodox interpretation of
quantum theory, he is also challenging the orthodox interpretation of
interpretations.
There are at least half a dozen different interpretations of quantum
theory. Each one is a way of relating the mathematics of quantum theory
to what might be going on in the real world. Most physicists believe
that, because they are derived from the same mathematics, the various
interpretations all predict identical outcomes in all conceivable
experiments: no experiment can rule just one of them out. Nonsense, says
Afshar. "The key phrase here is 'all conceivable experiments'," he says.
"How can you ever say you've considered all conceivable experiments? You
can't. I mean, I've just conceived of one where some interpretations
predict a demonstrably wrong outcome, and my experiment is repeatable
and verifiable."
John Cramer of the University of Washington in Seattle agrees. He says
he used to believe that experiments could never distinguish between
quantum interpretations - right up until he heard about Afshar's
experiment. But he now believes Afshar has found a loophole. By testing
for the interference pattern indirectly while concentrating on the
particle measurement, he has discovered a simple, repeatable experiment
where the Copenhagen interpretation predicts a different outcome from
other interpretations. "Afshar's experiment could actually have been
done at any time since Thomas Young demonstrated the wave nature of
light with a double-slit experiment," says Cramer. "But no one thought
of it."
So what does it mean for quantum theory? Antony Valentini of the
Perimeter Institute for Theoretical Physics in Waterloo, Canada,
believes that Afshar's experiment shows complementarity to be a piece of
historical baggage that should have been discarded many decades ago.
"Bohr's views were at best simplistic," Valentini says. "Some people
have tried to update the principle of complementarity, but it's still a
hopelessly vague idea that's difficult to make sense of - as Afshar's
experiments highlights." He believes it is time to admit that Bohr's
views have no role to play in quantum theory.
Cramer believes Afshar's experiment also falsifies the "many worlds
interpretation". This claims that particles can do many things at once,
such as simultaneously pass through two slits, but they do each in a
separate universe. So when an experimenter determines that a photon has
gone through one slit, it means we are in a universe in which that, and
that alone, has happened. The photon did also go through the other slit,
and it went through both, but those events happened in other, entirely
separate universes. "Afshar has identified a place where the Copenhagen
and the many worlds interpretations are inconsistent with the formalism
of quantum mechanics itself," Cramer says.
However, Afshar is aware that each person's opinion of his experiment
depends on their own view of how quantum theory should be interpreted.
Valentini, for example, believes that there must be something behind
quantum theory, and that things do have properties with well-defined
values (New Scientist, 29 June 2002, p 30), so it is unsurprising that
he finds a refutation of Bohr's ideas so appealing. Cramer, too, has a
vested interest in Afshar's experiment. He has developed his own
interpretation of quantum theory, called the transactional
interpretation. This uses waves that travel backwards in time to allow
quantum particles to interact and, Cramer says, it stands up to Afshar's
experimental test.
Afshar is about to embark on a photon-by-photon version of his
experiment at Rowan University in New Jersey, where he is now a visiting
research professor. Since the detectors can distinguish the origin of
the photons, and since there will be only one photon in the set-up at
any one time, Afshar can glean which-way information about each of the
photons. "The experiment performed at Harvard is essentially the same as
running the single-photon version for a very long time," he says.
He fully expects the experiment to produce the same result. That will be
a relief for many, he says. "Many physicists have found Bohr's ideas
either vague or intolerable, but until now nobody has been able to show
in an experiment that complementarity fails."
Afshar admits that, in the end, he is unsure what his experiment means
in detail for quantum theory. "We are back at the fork in the road
encountered by Bohr and Einstein, and avoided entirely due to Bohr's
ingenious complementarity," he says.
But we've still got a wave that's a particle, and a particle that's a
wave. What are we to make of that? Well, Afshar says, there are two
choices. The first is to shrug your shoulders and concede that human
logic and language will never explain what is going on. The second
option is to conclude that the particle phenomenon isn't really there,
and to use the wave picture for the entire experiment. In this
interpretation, the interference pattern and which-way information are
not logically inconsistent - the waves do go through both slits, while
the "image" each detector sees corresponds to light waves from only one
of the pinholes.
Afshar believes the second option is the simpler and better choice,
which leaves a big question: is there any such thing as a photon?
The photon detectors in Afshar's experiment "click" when they detect a
photon. But if there is no photon, what are they seeing? It comes down
to the interpretation of Einstein's photoelectric effect, the experiment
that "proved" the existence of the photon - and won him the 1921 Nobel
prize. Afshar says the American physicist Willis Lamb and others have
explained these particle-like clicks as a result of the interaction of
unquantised electromagnetic waves and quantised matter particles in the
detector. So although Einstein was right to doubt Bohr's
complementarity, he was "right for the wrong reasons", Afshar says. "In
order to declare Einstein the winner of the Bohr-Einstein debate, we
must take back his Nobel prize. We have no other choice but to declare
the idea of Einstein's photon dead."
Afshar has long doubted the existence of the photon. Indeed, like other
physicists, Afshar brings his own prejudices to the interpretation of
his experiment. For 18 years he has been developing a fundamental theory
of physics designed to unite the incompatible theories of quantum
mechanics and general relativity, in which electromagnetic fields such
as light simply cannot be quantised and there is no such thing as a
photon. "It was to test this that I did my experiment," says Afshar.
If he is right about the photon, where will it end? He has already
designed another experiment that he believes could resolve the light
quantisation issue once and for all. "If in that experiment we find that
there are no photons - quanta of light - then all of us will have to get
back to the drawing board," he says. But that's not the end of it.
Interference experiments using other quantum entities, such as electrons
and atoms, have also been used to support complementarity. A further
goal is to adapt his experiment to show whether these "particles" are
also illusions. "If the same results are obtained in analogous
experiments using particles other than photons then the debate would
cover the whole of quantum mechanics," Afshar says
Quantum rebel
New Scientist vol 183 issue 2457 - 24 July 2004, page 30
Has a simple experiment unravelled our most cherished notions of
reality? Marcus Chown investigates
WHEN you look at your reflection in the bathroom mirror every morning,
you could be doing yourself a favour. After all, some physicists believe
that the most fundamental aspects of the universe do not really exist
until they are observed. So you could argue that getting up and
stumbling to the bathroom each day is vital to your well-being.
That is absurd, of course. But is it any more absurd than the standard
interpretation of quantum theory, our most successful description of the
microscopic world of atoms and their constituents? The theory's
weirdness has grown to be accepted as the way things are: a bizarre
scheme in which reality manifests itself in different ways depending on
whether - and how - you measure it.
It now seems that our acceptance of such strangeness may be a mistake.
An audacious and highly controversial experiment suggests that is not
how things are at all. Shahriar S. Afshar, a 33-year-old
Iranian-American physicist, has carried out a novel version of the
"double-slit" experiment held by physicists to embody the central
mystery of quantum theory's weirdness - and he says he has contradicted
the standard result. "According to my experiment, one of our key
assumptions about quantum theory is wrong," says Afshar. It's not a
claim to make lightly. If he is right, it will reopen an argument that
has lain dormant since the birth of quantum theory.
The Danish physicist Niels Bohr claimed that the only way to interpret
the theory - the only way to understand what the mathematics of quantum
theory has to say about how quantum things manifest in the familiar,
classical world of our experiment - is to assume that nothing really
exists until it is measured. To Bohr, it made no sense to talk about an
objective reality independent of observers because our observations make
a difference to what we will see.
This "Copenhagen interpretation" of quantum theory came from Bohr's
conviction that, though the fundamental building blocks of reality might
seem to be both particles and waves - a phenomenon physicists call
wave-particle duality - it is more likely they are something else
entirely, something for which there is no analogue in the familiar
classical world in which we carry out our experiments. When faced with
our classical apparatus these mysterious quantum entities will show
either a particle-like or a wave-like face.
Bohr proposed that the face you see depends on how you set up your
experiment. And, he said, you'll never see both at the same time in one
experiment. He called this the "principle of complementarity". Einstein
took exception to this: he refused to believe that the very fabric of
the observable universe could change depending on our choice of
measuring equipment. But he never managed to find an experimental way to
refute complementarity, and Bohr's influence ensured that it gradually
became the accepted view of how the quantum world will manifest in our
classical experiments. Afshar, however, may have succeeded where
Einstein failed.
His experiment centres on shining laser light onto two nearby apertures.
This light emerges from the apertures as two spreading beams. Where the
beams overlap, they interfere, producing alternating bands, or fringes,
of light and dark. You can easily explain this interference pattern if
you think of the light as waves. When two wave crests meet, they combine
to produce even brighter light; when crests and troughs meet, there is
darkness. The exact geometry of the interference pattern - the width and
separation of the light and dark fringes, for instance - depends on the
position of the slits, the frequency of the laser, and so on.
Nothing mysterious so far; the physicist Thomas Young first demonstrated
this phenomenon in 1801 using sunlight. The problem arises because of
quantum theory, which says you can consider the light beams as streams
of particles called photons. Each photon is a packet of light energy.
How do particles produce an interference pattern?
The short answer is they can't - yet they do. And the mystery deepens
when you turn the laser right down so that only one photon travels
through the apparatus at a time. It takes a lot longer, but the
interference pattern builds up, one photon at a time. For this to
happen, each photon must somehow pass through both slits and interfere
with itself. Observing an interference pattern in these circumstances is
the equivalent of hearing the sound of one hand clapping. The particles
- whose defining characteristic is that they are localised at a
particular point in space - are behaving like waves, which are smeared
through a relatively large region of space. The Caltech physicist
Richard Feynman once called this hybrid behaviour "the only mystery" in
physics.
It certainly is mysterious: if you set the experiment up to follow the
path of the particles - using, say, a photon detector to see which slit
the light goes through - you'll be rewarded with a view of the
particle-like face: your photon detector will register a photon. And if
you look for evidence of waves, by looking for an interference pattern
for example, you'll see that instead. Experiments have shown that
attempts to locate the photon on its way through the apparatus always
result in a washed-out interference pattern. Bohr's interpretation of
the double-slit experiment appears to be right: nature does not permit
us to know which slit a particle passed through - "which way"
information - and also see an interference pattern. This has become the
orthodox view, reprinted in thousands of physics textbooks.
So how come Afshar is claiming Bohr was wrong - that you can track the
photons' paths and not destroy the interference pattern? Because, he
says, he's done it.
He carried out his original experiment at the Institute for
Radiation-Induced Mass Studies, a privately funded organisation in
Boston, where Afshar is principal investigator. The set-up is relatively
simple (see Diagram). Laser light falls on two pinholes in an opaque
screen. On the far side of the screen is a lens that takes the light
coming through each of the pinholes (another opaque screen stops all
other light hitting the lens) and refocuses the spreading beams onto a
mirror that reflects each onto a separate photon detector. In this way,
Afshar gets a record of the rate at which photons are coming through
each pinhole. According to complementarity, that means there should be
no evidence of an interference pattern. But there is, Afshar says.
He doesn't look at the pattern directly, but has designed the experiment
to test for its presence. He places a series of wires exactly where the
dark fringes of the interference pattern ought to be. Then he closes one
of the pinholes. This, of course, prevents any interference pattern from
forming, and the light simply spreads out as it emerges from the single
pinhole. A portion of the light will hit the metal wires, which scatter
it in all directions, meaning less light will reach the photon detector
corresponding to that pinhole.
But Afshar claims that when he opens up the closed pinhole, the light
intensity at each detector returns to its value before the wires were
set in place. Why? Because the wires sit in the dark fringes of the
interference pattern, no light hits them, and so none of the photons are
scattered. That shows the interference pattern is there, says Afshar,
which exposes the wave-like face of light. And yet he can also measure
the intensity of light from each slit with a photon detector, so he can
tell how many photons pass through each slit - the particle-like face is
there too.
"This flies in the face of complementarity, which says that knowledge of
the interference pattern always destroys the which-way information and
vice versa," says Afshar. "Something everyone believed and nobody
questioned for 80 years appears to be wrong."
When Christopher Stubbs of Harvard University invited Afshar to repeat
the experiment as a visiting scientist in Stubbs's lab earlier this
year, the result was the same. Afshar has now submitted his work for
peer-reviewed publication. What ought to happen now is that the
journal's review process will either find some flaw in Afshar's
reasoning or else uphold his position and throw Bohr's ideas onto the
scrap heap.
In reality, things are unlikely to be quite that clear-cut. That is
because Afshar is not only challenging the orthodox interpretation of
quantum theory, he is also challenging the orthodox interpretation of
interpretations.
There are at least half a dozen different interpretations of quantum
theory. Each one is a way of relating the mathematics of quantum theory
to what might be going on in the real world. Most physicists believe
that, because they are derived from the same mathematics, the various
interpretations all predict identical outcomes in all conceivable
experiments: no experiment can rule just one of them out. Nonsense, says
Afshar. "The key phrase here is 'all conceivable experiments'," he says.
"How can you ever say you've considered all conceivable experiments? You
can't. I mean, I've just conceived of one where some interpretations
predict a demonstrably wrong outcome, and my experiment is repeatable
and verifiable."
John Cramer of the University of Washington in Seattle agrees. He says
he used to believe that experiments could never distinguish between
quantum interpretations - right up until he heard about Afshar's
experiment. But he now believes Afshar has found a loophole. By testing
for the interference pattern indirectly while concentrating on the
particle measurement, he has discovered a simple, repeatable experiment
where the Copenhagen interpretation predicts a different outcome from
other interpretations. "Afshar's experiment could actually have been
done at any time since Thomas Young demonstrated the wave nature of
light with a double-slit experiment," says Cramer. "But no one thought
of it."
So what does it mean for quantum theory? Antony Valentini of the
Perimeter Institute for Theoretical Physics in Waterloo, Canada,
believes that Afshar's experiment shows complementarity to be a piece of
historical baggage that should have been discarded many decades ago.
"Bohr's views were at best simplistic," Valentini says. "Some people
have tried to update the principle of complementarity, but it's still a
hopelessly vague idea that's difficult to make sense of - as Afshar's
experiments highlights." He believes it is time to admit that Bohr's
views have no role to play in quantum theory.
Cramer believes Afshar's experiment also falsifies the "many worlds
interpretation". This claims that particles can do many things at once,
such as simultaneously pass through two slits, but they do each in a
separate universe. So when an experimenter determines that a photon has
gone through one slit, it means we are in a universe in which that, and
that alone, has happened. The photon did also go through the other slit,
and it went through both, but those events happened in other, entirely
separate universes. "Afshar has identified a place where the Copenhagen
and the many worlds interpretations are inconsistent with the formalism
of quantum mechanics itself," Cramer says.
However, Afshar is aware that each person's opinion of his experiment
depends on their own view of how quantum theory should be interpreted.
Valentini, for example, believes that there must be something behind
quantum theory, and that things do have properties with well-defined
values (New Scientist, 29 June 2002, p 30), so it is unsurprising that
he finds a refutation of Bohr's ideas so appealing. Cramer, too, has a
vested interest in Afshar's experiment. He has developed his own
interpretation of quantum theory, called the transactional
interpretation. This uses waves that travel backwards in time to allow
quantum particles to interact and, Cramer says, it stands up to Afshar's
experimental test.
Afshar is about to embark on a photon-by-photon version of his
experiment at Rowan University in New Jersey, where he is now a visiting
research professor. Since the detectors can distinguish the origin of
the photons, and since there will be only one photon in the set-up at
any one time, Afshar can glean which-way information about each of the
photons. "The experiment performed at Harvard is essentially the same as
running the single-photon version for a very long time," he says.
He fully expects the experiment to produce the same result. That will be
a relief for many, he says. "Many physicists have found Bohr's ideas
either vague or intolerable, but until now nobody has been able to show
in an experiment that complementarity fails."
Afshar admits that, in the end, he is unsure what his experiment means
in detail for quantum theory. "We are back at the fork in the road
encountered by Bohr and Einstein, and avoided entirely due to Bohr's
ingenious complementarity," he says.
But we've still got a wave that's a particle, and a particle that's a
wave. What are we to make of that? Well, Afshar says, there are two
choices. The first is to shrug your shoulders and concede that human
logic and language will never explain what is going on. The second
option is to conclude that the particle phenomenon isn't really there,
and to use the wave picture for the entire experiment. In this
interpretation, the interference pattern and which-way information are
not logically inconsistent - the waves do go through both slits, while
the "image" each detector sees corresponds to light waves from only one
of the pinholes.
Afshar believes the second option is the simpler and better choice,
which leaves a big question: is there any such thing as a photon?
The photon detectors in Afshar's experiment "click" when they detect a
photon. But if there is no photon, what are they seeing? It comes down
to the interpretation of Einstein's photoelectric effect, the experiment
that "proved" the existence of the photon - and won him the 1921 Nobel
prize. Afshar says the American physicist Willis Lamb and others have
explained these particle-like clicks as a result of the interaction of
unquantised electromagnetic waves and quantised matter particles in the
detector. So although Einstein was right to doubt Bohr's
complementarity, he was "right for the wrong reasons", Afshar says. "In
order to declare Einstein the winner of the Bohr-Einstein debate, we
must take back his Nobel prize. We have no other choice but to declare
the idea of Einstein's photon dead."
Afshar has long doubted the existence of the photon. Indeed, like other
physicists, Afshar brings his own prejudices to the interpretation of
his experiment. For 18 years he has been developing a fundamental theory
of physics designed to unite the incompatible theories of quantum
mechanics and general relativity, in which electromagnetic fields such
as light simply cannot be quantised and there is no such thing as a
photon. "It was to test this that I did my experiment," says Afshar.
If he is right about the photon, where will it end? He has already
designed another experiment that he believes could resolve the light
quantisation issue once and for all. "If in that experiment we find that
there are no photons - quanta of light - then all of us will have to get
back to the drawing board," he says. But that's not the end of it.
Interference experiments using other quantum entities, such as electrons
and atoms, have also been used to support complementarity. A further
goal is to adapt his experiment to show whether these "particles" are
also illusions. "If the same results are obtained in analogous
experiments using particles other than photons then the debate would
cover the whole of quantum mechanics," Afshar says
