[tt] NS 2759: Seven wonders of the quantum world (series)
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Tue May 11 22:51:34 CEST 2010
NS 2759: Seven wonders of the quantum world (series)
* 05 May 2010 by Michael Brooks
From undead cats to particles popping up out of nowhere, from
watched pots not boiling--sometimes--to ghostly influences at a
distance, quantum physics delights in demolishing our intuitions
about how the world works. Michael Brooks tours the quantum effects
that are guaranteed to boggle our minds.
1. Corpuscles and buckyballs
2. The Hamlet effect
3. Something for nothing
4. The Elitzur-Vaidman bomb tester
5. Spooky action at a distance
6. The field that isn't there
7. Superfluids and supersolids
And finally: Nobody understands
Michael Brooks was the Science party candidate for the constituency
of Bosworth in the UK general election this week
1. Corpuscles and buckyballs
IT DOES not require any knowledge of quantum physics to recognise
quantum weirdness. The oldest and grandest of the quantum mysteries
relates to a question that has exercised great minds at least since
the time of the ancient Greek philosopher Euclid: what is light made
History has flip-flopped on the issue. Isaac Newton thought light
was tiny particles--"corpuscles" in the argot of the day. Not all
his contemporaries were impressed, and in classic experiments in the
early 1800s the polymath Thomas Young showed how a beam of light
diffracted, or spread out, as it passed through two narrow slits
placed close together, producing an interference pattern on a screen
behind just as if it were a wave.
So which is it, particle or wave? Keen to establish its reputation
for iconoclasm, quantum theory provided an answer soon after it
bowled onto the scene in the early 20th century. Light is both a
particle and a wave--and so, for that matter, is everything else. A
single moving particle such as an electron can diffract and
interfere with itself as if it were a wave, and believe it or not,
an object as large as a car has a secondary wave character as it
trundles along the road.
That revelation came in a barnstorming doctoral thesis submitted by
the pioneering quantum physicist Louis de Broglie in 1924. He showed
that by describing moving particles as waves, you could explain why
they had discrete, quantised energy levels rather than the continuum
predicted by classical physics.
De Broglie first assumed that this was just a mathematical
abstraction, but wave-particle duality seems to be all too real.
Young's classic wave interference experiment has been reproduced
with electrons and all manner of other particles (see diagram).
We haven't yet done it with a macroscopic object such as a moving
car, admittedly. Its de Broglie wavelength is something like 10^-38
metres, and making it do wave-like things such as diffract would
mean creating something with slits on a similar scale, a task way
beyond our engineering capabilities. The experiment has been
performed, though, with a buckyball--a soccer-ball-shaped lattice
of 60 carbon atoms that, at about a nanometre in diameter, is large
enough to be seen under a microscope (Nature, vol 401, p 680).
All that leaves a fundamental question: how can stuff be waves and
particles at the same time? Perhaps because it is neither, says
Markus Arndt of the University of Vienna, Austria, who did the
buckyball experiments in 1999. What we call an electron or a
buckyball might in the end have no more reality than a click in a
detector, or our brain's reconstruction of photons hitting our
retina. "Wave and particle are then just constructs of our mind to
facilitate everyday talking," he says.
2. The Hamlet effect
A WATCHED pot never boils." Armed with common sense and classical
physics, you might dispute that statement. Quantum physics would
slap you down. Quantum watched pots do refuse to boil--sometimes.
At other times, they boil faster. At yet other times, observation
pitches them into an existential dilemma whether to boil or not.
This madness is a logical consequence of the Schrödinger equation,
the formula concocted by Austrian physicist Erwin Schrödinger in
1926 to describe how quantum objects evolve probabilistically over
Imagine, for example, conducting an experiment with an initially
undecayed radioactive atom in a box. According to the Schrödinger
equation, at any point after you start the experiment the atom
exists in a mixture, or "superposition", of decayed and undecayed
Each state has a probability attached that is encapsulated in a
mathematical description known as a wave function. Over time, as
long as you don't look, the wave function evolves as the probability
of the decayed state slowly increases. As soon as you do look, the
atom chooses--in a manner in line with the wave function
probabilities--which state it will reveal itself in, and the wave
function "collapses" to a single determined state.
This is the picture that gave birth to Schrödinger's infamous cat.
Suppose the radioactive decay of an atom triggers a vial of poison
gas to break, and a cat is in the box with the atom and the vial. Is
the cat both dead and alive as long as we don't know whether the
decay has occurred?
We don't know. All we know is that tests with larger and larger
objects--including, recently, a resonating metal strip big enough
to be seen under a microscope--seem to show that they really can be
induced to adopt two states at once (Nature, vol 464, p 697).
The weirdest thing about all this is the implication that just
looking at stuff changes how it behaves. Take the decaying atom:
observing it and finding it undecayed resets the system to a
definitive state, and the Schrödinger-equation evolution towards
"decayed" must start again from scratch.
The corollary is that if you keep measuring often enough, the system
will never be able to decay. This possibility is dubbed the quantum
Zeno effect, after the Greek philosopher Zeno of Elea, who devised a
famous paradox that "proved" that if you divided time up into ever
smaller instants you could make change or motion impossible.
And the quantum Zeno effect does happen. In 1990, researchers at the
National Institute of Standards and Technology in Boulder, Colorado,
showed they could hold a beryllium ion in an unstable energy
configuration rather akin to balancing a pencil on its sharpened
point, provided they kept re-measuring its energy (Physical Review
A, vol 41, p 2295).
The converse "anti-Zeno" effect--making a quantum pot boil faster
by just measuring it--also occurs. Where a quantum object has a
complex arrangement of states to move into, a decay into a
lower-energy state can be accelerated by measuring the system in the
right way. In 2001, this too was observed in the lab (Physical
Review Letters, vol 87, p 040402).
The third trick is the "quantum Hamlet effect", proposed last year
by Vladan Pankovic of the University of Novi Sad, Serbia. A
particularly intricate sequence of measurements, he found, can
affect a system in such a way as to make the Schrödinger equation
for its subsequent evolution intractable. As Pankovic puts it: to be
decayed or not-decayed, "that is the analytically unsolvable
3. Something for nothing
"NOTHING will come of nothing," King Lear admonishes Cordelia in the
eponymous Shakespeare play. In the quantum world, it's different:
there, something comes of nothing and moves the furniture around.
Specifically, if you place two uncharged metal plates side by side
in a vacuum, they will move towards each other, seemingly without
reason. They won't move a lot, mind. Two plates with an area of a
square metre placed one-thousandth of a millimetre apart will feel a
force equivalent to just over a tenth of a gram.
The Dutch physicist Hendrik Casimir first noted this minuscule
movement in 1948. "The Casimir effect is a manifestation of the
quantum weirdness of the microscopic world," says physicist Steve
Lamoreaux of Yale University.
It has to do with the quantum quirk known as Heisenberg's
uncertainty principle, which essentially says the more we know about
some things in the quantum world, the less we know about others. You
can't, for instance, deduce the exact position and momentum of a
particle simultaneously. The more certain we are of where a particle
is, the less certain we are of where it is heading.
A similar uncertainty relation exists between energy and time, with
a dramatic consequence. If space were ever truly empty, it would
contain exactly zero energy at a precisely defined moment in time -
something the uncertainty principle forbids us from knowing.
It follows that there is no such thing as a vacuum. According to
quantum field theory, empty space is actually fizzing with
short-lived stuff that appears, looks around a bit, decides it
doesn't like it and disappears again, all in the name of preventing
the universe from violating the uncertainty principle. For the most
part, this stuff is pairs of photons and their antiparticles that
quickly annihilate in a puff of energy. The tiny electric fields
caused by these pop-up particles, and their effect on free electrons
in metal plates, might explain the Casimir effect.
Or they might not. Thanks to the uncertainty principle, the electric
fields associated with the atoms in the metal plates also fluctuate.
These variations create tiny attractions called van der Waals forces
between the atoms. "You can't ascribe the Casimir force solely
either to the zero point of the vacuum or to the zero point motion
of the atoms that make up the plates," says Lamoreaux. "Either view
is correct and arrives at the same physical result."
Whichever picture you adopt, the Casimir effect is big enough to be
a problem. In nanoscale machines, for example, it could cause
components in close proximity to stick together.
The way to avoid that might be simply to reverse the effect. In
1961, Russian physicists showed theoretically that combinations of
materials with differing Casimir attractions can create scenarios
where the overall effect is repulsion. Evidence for this strange
"quantum buoyancy" was announced in January 2009 by physicists from
Harvard University who had set up gold and silica plates separated
by the liquid bromobenzene (Nature, vol 457, p 170).
4. The Elitzur-Vaidman bomb-tester
A BOMB triggered by a single photon of light is a scary thought. If
such a thing existed in the classical world, you would never even be
aware of it. Any photon entering your eye to tell you about it would
already have set off the bomb, blowing you to kingdom come.
With quantum physics, you stand a better chance. According to a
scheme proposed by the Israeli physicists Avshalom Elitzur and Lev
Vaidman in 1993, you can use quantum trickery to detect a
light-triggered bomb with light--and stay safe a guaranteed 25 per
cent of the time (Foundations of Physics, vol 23, p 987).
The secret is a device called an interferometer. It exploits the
quantumly weird fact that, given two paths to go down, a photon will
take both at once. We know this because, at the far end of the
device, where the two paths cross once again, a wave-like
interference pattern is produced (see "Quantum wonders: Corpuscles
To visualise what is going on, think of a photon entering the
interferometer and taking one path while a ghostly copy of itself
goes down the other. In Elitzur and Vaidman's thought experiment,
half the time there is a photon-triggered bomb blocking one path
(see diagram). Only the real photon can trigger the bomb, so if it
is the ghostly copy that gets blocked by the bomb, there is no
explosion--and nor is there an interference pattern at the other
end. In other words, we have "seen" the bomb without triggering it.
Barely a year after Elitzur and Vaidman proposed their bomb-testing
paradox, physicists at the University of Vienna, Austria, had
brought it to life--not by setting off bombs, but by bouncing
photons off mirrors (Physical Review Letters, vol 74, p 4763).
In 2000, Shuichiro Inoue and Gunnar Bjork of the Royal Institute of
Technology in Stockholm, Sweden, used a similar technique to show
that you could get an image of a piece of an object without shining
light on it--something that could revolutionise medical imaging.
"It would be very useful for something like X-ray scanning, if there
were no radiation damage to the tissue because no X-rays actually
hit it," says physicist Richard Jozsa of the University of
Josza is the brains behind perhaps the most eye-rubbing of such
tricks: using a quantum computer to deliver the output of a program
even when you don't run the program. As the team that implemented
his idea in 2005 showed, quantum physics does at least retain some
semblance of classical decency: to deliver a sensible answer, the
computer does need to be switched on (Nature, vol 439, p 949).
5. Spooky action at a distance
ERWIN SCHRÖDINGER called it the "defining trait" of quantum theory.
Einstein could not bring himself to believe in it at all, thinking
it proof that quantum theory was seriously buggy. It is
entanglement: the idea that particles can beed in such a way
that changing the quantum state of one instantaneously affects the
other, even if they are light years apart.
This "spooky action at a distance", in Einstein's words, is a
serious blow to our conception of how the world works. In 1964,
physicist John Bell of the European Organization for Nuclear
Research (CERN) in Geneva, Switzerland, showed just how serious. He
calculated a mathematical inequality that encapsulated the maximum
correlation between the states of remote particles in experiments in
which three "reasonable" conditions hold: that experimenters have
free will in setting things up as they want; that the particle
properties being measured are real and pre-existing, not just
popping up at the time of measurement; and that no influence travels
faster than the speed of light, the cosmic speed limit.
As many experiments since have shown, quantum mechanics regularly
violates Bell's inequality, yielding levels of correlation way above
those possible if his conditions hold. That pitches us into a
philosophical dilemma. Do we not have free will, meaning something,
somehow predetermines what measurements we take? That is not
anyone's first choice. Are the properties of quantum particles not
real--implying that nothing is real at all, but exists merely as a
result of our perception? That's a more popular position, but it
hardly leaves us any the wiser.
Or is there really an influence that travels faster than light?
Cementing the Swiss reputation for precision timing, in 2008
physicist Nicolas Gisin and his colleagues at the University of
Geneva showed that, if reality and free will hold, the speed of
transfer of quantum states between entangled photons held in two
villages 18 kilometres apart was somewhere above 10 million times
the speed of light (Nature, vol 454, p 861).
Whatever the true answer is, it will be weird. Welcome to quantum
6. The field that isn't there
HERE'S a nice piece of quantum nonsense. Take a doughnut-shaped
magnet and wrap a metal shield round its inside edge so that no
magnetic field can leak into the hole. Then fire an electron through
There is no field in the hole, so the electron will act as if there
is no field, right? Wrong. The wave associated with the electron's
movement suffers a jolt as if there were something there.
Werner Ehrenberg and Raymond Siday were the first to note that this
behaviour lurks in the Schrödinger equation (see "Quantum wonders:
The Hamlet effect "). That was in 1949, but their result remained
unnoticed. Ten years later Yakir Aharonov and David Bohm, working at
the University of Bristol in the UK, rediscovered the effect and for
some reason their names stuck.
So what is going on? The Aharonov-Bohm effect is proof that there is
more to electric and magnetic fields than is generally supposed. You
can't calculate the size of the effect on a particle by considering
just the properties of the electric and magnetic fields where the
particle is. You also have to take into account the properties where
Casting about for an explanation, physicists decided to take a look
at a property of the magnetic field known as the vector potential.
For a long time, vector potentials were considered just handy
mathematical tools--a shorthand for electrical and magnetic
properties that didn't have any real-world significance. As it turns
out, they describe something that is very real indeed.
The Aharonov-Bohm effect showed that the vector potential makes an
electromagnetic field more than the sum of its parts. Even when a
field isn't there, the vector potential still exerts an influence.
That influence was seen unambiguously for the first time in 1986
when Akira Tonomura and colleagues in Hitachi's laboratories in
Tokyo, Japan, measured a ghostly electron jolt (Physical Review
Letters, vol 48, p 1443).
Although it is far from an everyday phenomenon, the Aharonov-Bohm
effect might prove to have uses in the real world--in magnetic
sensors, for example, or field-sensitive capacitors and data storage
buffers for computers that crunch light.
7. Superfluids and supersolids
FORGET radioactive spider bites, exposure to gamma rays, or any
other accident favoured in Marvel comics: in the real world, it's
quantum theory that gives you superpowers.
Take helium, for example. At room temperature, it is normal fun: you
can fill floaty balloons with it, or inhale it and talk in a squeaky
voice. At temperatures below around 2 kelvin, though, it is a liquid
and its atoms become ruled by their quantum properties. There, it
becomes super-fun: a superfluid.
At room temperature, helium is normal fun. Close to absolute zero,
though, it becomes super-fun
Superfluid helium climbs up walls and flows uphill in defiance of
gravity. It squeezes itself through impossibly small holes. It flips
the bird at friction: put superfluid helium in a bowl, set the bowl
spinning, and the helium sits unmoved as the bowl revolves beneath
it. Set the liquid itself moving, though, and it will continue
That's fun, but not particularly useful. The opposite might be said
of superconductors. These solids conduct electricity with no
resistance, making them valuable for transporting electrical energy,
for creating enormously powerful magnetic fields--to steer protons
around CERN's Large Hadron Collider, for instance--and for
levitating superfast trains.
We don't yet know how all superconductors work, but it seems the
uncertainty principle plays a part (see "Quantum wonders: Something
for nothing"). At very low temperatures, the momentum of individual
atoms or electrons in these materials is tiny and very precisely
known, so the position of each atom is highly uncertain. In fact,
they begin to overlap with each other to the point where you can't
describe them individually. They start acting as one superatom or
superelectron that moves without friction or resistance.
All this is nothing in the weirdness stakes, however, compared with
a supersolid. The only known example is solid helium cooled to
within a degree of absolute zero and at around 25 times normal
Under these conditions, the bonds between helium atoms are weak, and
some break off to leave a network of "vacancies" that behave almost
exactly like real atoms. Under the right conditions, these vacancies
form their own fluid-like Bose-Einstein condensate. This will, under
certain circumstances, pass right through the normal helium lattice
--meaning the solid flows, ghost-like, through itself.
So extraordinary is this superpower that Moses Chan and Eun-Seong
Kim of Pennsylvania State University in University Park checked and
re-checked their data on solid helium for four years before
eventually publishing in 2004 (Nature, vol 427, p 225). "I had
little confidence we would see the effect," says Chan. Nevertheless,
researchers have seen hints that any crystalline material might be
persuaded to perform such a feat at temperatures just a fraction
above absolute zero. Not even Superman can do that.
8. Nobody understands
It is tempting, faced with the full-frontal assault of quantum
weirdness, to trot out the notorious quote from Nobel prize-winning
physicist Richard Feynman: "Nobody understands quantum mechanics."
It does have a ring of truth to it, though. The explanations
attempted here use the most widely accepted framework for thinking
about quantum weirdness, called the Copenhagen interpretation after
the city in which Niels Bohr and Werner Heisenberg thrashed out its
ground rules in the early 20th century.
With its uncertainty principles and measurement paradoxes, the
Copenhagen interpretation amounts to an admission that, as classical
beasts, we are ill-equipped to see underlying quantum reality. Any
attempt we make to engage with it reduces it to a shallow classical
projection of its full quantum richness.
Lev Vaidman of Tel Aviv University, Israel, like many other
physicists, touts an alternative explanation. "I don't feel that I
don't understand quantum mechanics," he says. But there is a high
price to be paid for that understanding--admitting the existence of
In this picture, wave functions do not "collapse" to classical
certainty every time you measure them; reality merely splits into as
many parallel worlds as there are measurement possibilities. One of
these carries you and the reality you live in away with it. "If you
don't admit many-worlds, there is no way to have a coherent
picture," says Vaidman.
Or, in the words of Feynman again, whether it is the Copenhagen
interpretation or many-worlds you accept, "the 'paradox' is only a
conflict between reality and your feeling of what reality ought to
Take The Next Step
The Copenhagen interpretation was an attempt by a generation of
stunned scientist to bridge the difference between our classical
sense of the universe and its quantum reality. It distorts the
reality of quantum behavior and results in confusion, apparent
paradoxes and general misunderstanding.
"If you don't admit many-worlds, there is no way to have a coherent
picture," says Vaidman.
The "coherent picture" that Vaidman refers to is reconciling
Einstein's sense of causality with entanglement. Although the
Multi-World interpretation (MWI) is an improvement over the
Copenhagen interpretation, it still creates confusion and
misunderstanding by failing to accept the truth of the non-locality
of wave-functions. It generates fantasies of alternate co-existing
realities where every choice is explored. "What if Hitler won?"
scenarios. In its detail, the MWI doesn't actually support this
idea, but as a bad explanation, it spawns them.
Take the next step and move past Einstein's causality. Embrace
wave-function non-locality and what it means. Space and time are not
fundamental properties of the universe, they are emergent.
Take The Next Step
Mon May 10 09:23:45 BST 2010 by Liza
I've read your previous comment on the according to you false
particle-wave duality, and you mentioned locality as well. Still,
most books on physics for non-physicists still mention the duality
as an established theory. Do you have any real reason to assume the
duality is false, and non-locality is normal, except for your
personal opinion? What you say may make sense or could just as well
be nothing but wild speculation, but since I'm not a physicist, I
don't have the knowledge to make up my mind.
Take The Next Step
Mon May 10 17:08:56 BST 2010 by David Allen
@Liza "...most books on physics for non-physicists still mention the
duality as an established theory."
Yes, most books are based on the dominant. and very entrenched,
interpretation of quantum mechanics (QM), the Copenhagen
Duality is an explanation, not a theory, QM is the theory. Duality
attempts to explain why under some conditions we see "classical"
particle-like behavior and under others we see QM wave-like
@Liza "Do you have any real reason to assume the duality is false,
and non-locality is normal, except for your personal opinion?"
Yes I have reasons to reject duality as an explanation:
The word "duality" implies equivalence, which isn't true. Particle
like models of QM systems can't describe everything that is seen in
experiments, however the wave-function models can. In other words
the wave-function models are complete, they can describe both the
wave-like behavior and the particle-like behavior. Particle
interpretations are incomplete, and unnecessary.
Wheeler's delayed choice experiment can't be explained by duality
and wave-function collapse. It can only be explained by the
Both duality and wave-function collapse are bad general
explanations. In the right circumstances however, the ideas behind
them can be used to simplify the math, making real-world problems
As for non-locality:
The issues surrounding non-locality are harder to tease apart, so my
conclusions are certainly just my opinion. This will give you some
idea of the different approaches:
The Copenhagen interpretation doesn't reject non-locality, in fact
it needs it to support wave-function collapse. The Many-Worlds
interpretation (MWI) does reject non-locality, but other than that
is basically in line with my wave-only perspective.
MWI's rejection of non-locality appears to be based on a desire to
avoid an explanation that contradicts special relativity. I claim
however that special relativity only applies to energy in space, and
not to quantum information. The non-local behavior of entanglement
does not contradict it. I also claim that non-local entanglement may
not necessarily allow simultaneity, or specific ordering to be
established between frames of reference. This would also avoid a
contradiction with special relativity. However I think that there
may be a way to falsify this second claim.
The wild speculation I engage in is that the Universe has no true
spacial dimensions, time and space are emergent from quantum
decoherence. The flow of time and expansion of space are tied to the
rate of decoherence, but the rate of decoherence slows down as space
expands due to fewer interactions. The relative rates of growth in
space vs. flow of time makes the universe appear to grow at an
exponential rate to observers within the universe. When there is no
more matter and all energy stretches to the ultimate quantum levels,
the dominant quantum interactions become entanglement. The universe
begins to collapse, and as it does the rate of entanglement
increases. To observers inside the universe, this collapse would
appear to be inversely exponential. The distortions in observer time
mean that at the point of maximum expansion, the universe appears to
have just been created, exploding almost instantly to its current
size. At the point of maximum density, the universe appears to have
gone on almost forever in this high density state of slowing
Take The Next Step
Tue May 11 15:53:59 BST 2010 by Liza
Hey thanks! I had to read your comment thrice in order to understand
what you are explaining (no fault of yours), and it's surely
interesting. True, if the wave-function models can explain all
what's observed, and the particle-based models can't, there's no
real reason to hang on to the particle idea except that it's easier
to visualise and understand for most people. I guess non-locality
gets rejected because it seems beyond what we can comprehend,
magical even. The whole decoherence/dimensions speculation is
fascinating, but have you ever tried to make some calculations to
see if it holds up?
PS: it's pretty nice to read comments on this topic from people who
make sense, rather than the usual Zephir-style nonsense.
Quantum Wonders: Nobody Understands
Sun May 09 15:28:00 BST 2010 by andwor
in order to understand the quantum world it is important to take the
next quntum leap. Specifically it is important to look at the
quantum world at a very much smaller scale.
Please see a recently published article (currently online) on
exactly this topic. Entitled "The formulation of harmonic
quintessence and a fundamental energy equivalence equation" Physics
Essays 23: 311-319
Quantum Wonders Nobody Understands
Mon May 10 00:00:32 BST 2010 by Julian Mann
I do not agree with Vaidman that we are forced into accepting
parallel universes. See my comments on the quantum Hamlet Effect and
the existence of Classical Time, Anti-time(Quantum World) and Nul
time.These concepts suffice to explain all the anomolies in Quantum
Mechanics, reconcile it to relativity etc. I have noticed that when
scientists do not understand something in Physics, they invoke such
concepts for which there is no experimental evidence for existence
Quantum Wonders Nobody Understands
Mon May 10 15:20:16 BST 2010 by andwor
Thank you for your insightful comments. I am also exploring the
concept of what you term "anti-time".
Specifically I was considering an adaptation to the Wheeler and
Feynman theory, where time is symmetric. This effectively means that
electromagnetic processes go backwards in time as well as forwards.
But in the presence of the "perfect reflector " in the past, i.e.
The Big Bang, then the half that goes backwards in time gets
refelected forwards in time to arrive when it left, and continues on
its forward journey. This means that the whole signal effectively
goes forward in time.
This requires also that the future is the "perfect absorber" and
since the recent discovery of the accelerating Universe then we do
have our "perfect" absorber
Of course entanglement is the archetypal example of where this might
be happening. Equally well the electomanetic signal does not need to
go all the way back to the BIG Bang it just needs to go back to the
point in time when the electromanetic effect/signal was created,
exactly as in entangled particles/photons.
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