Tag Archives: Quantum Theory

Measuring the Quantum Jitter

Some physicists are determined to find out if we are mere holograms. Perhaps not quite like the dystopian but romanticized version fictionalized in The Matrix, but still a fascinating idea nonetheless. Armed with a very precise measuring tool, known as a Holometer or more precisely twin correlated Michelson holographic interferometers, researchers aim to find the scale at which the universe becomes jittery. In turn this will give a better picture of the fundamental units of space-time, well beyond the the elementary particles themselves, and somewhat closer to the Planck Length.

From the New Scientist:

The search for the fundamental units of space and time has officially begun. Physicists at the Fermi National Accelerator Laboratory near Chicago, Illinois, announced this week that the Holometer, a device designed to test whether we live in a giant hologram, has started taking data.

The experiment is testing the idea that the universe is actually made up of tiny “bits”, in a similar way to how a newspaper photo is actually made up of dots. These fundamental units of space and time would be unbelievably tiny: a hundred billion billion times smaller than a proton. And like the well-knownquantum behaviour of matter and energy, these bits of space-time would behave more like waves than particles.

“The theory is that space is made of waves instead of points, that everything is a little jittery, and never sits still,” says Craig Hogan at the University of Chicago, who dreamed up the experiment.

The Holometer is designed to measure this “jitter”. The surprisingly simple device is operated from a shed in a field near Chicago, and consists of two powerful laser beams that are directed through tubes 40 metres long. The lasers precisely measure the positions of mirrors along their paths at two points in time.

If space-time is smooth and shows no quantum behaviour, then the mirrors should remain perfectly still. But if both lasers measure an identical, small difference in the mirrors’ position over time, that could mean the mirrors are being jiggled about by fluctuations in the fabric of space itself.

 So what of the idea that the universe is a hologram? This stems from the notion that information cannot be destroyed, so for example the 2D event horizon of a black hole “records” everything that falls into it. If this is the case, then the boundary of the universe could also form a 2D representation of everything contained within the universe, like a hologram storing a 3D image in 2D .

Hogan cautions that the idea that the universe is a hologram is somewhat misleading because it suggests that our experience is some kind of illusion, a projection like a television screen. If the Holometer finds a fundamental unit of space, it won’t mean that our 3D world doesn’t exist. Rather it will change the way we understand its basic makeup. And so far, the machine appears to be working.

In a presentation given in Chicago on Monday at the International Conference on Particle Physics and Cosmology, Hogan said that the initial results show the Holometer is capable of measuring quantum fluctuations in space-time, if they are there.

“This was kind of an amazing moment,” says Hogan. “It’s just noise right now – we don’t know whether it’s space-time noise – but the machine is operating at that specification.”

Hogan expects that the Holometer will have gathered enough data to put together an answer to the quantum question within a year. If the space-time jitter is there, Hogan says it could underpin entirely new explanations for why the expansion of our universe is accelerating, something traditionally attributed to the little understood phenomenon of dark energy.

Read the entire article here.

Non-Spooky Action at a Distance

Albert Einstein famously called quantum entanglement “spooky action at a distance”. It refers to the notion that measuring the state of one of two entangled particles makes the state of the second particle known instantaneously, regardless of the distance  separating the two particles. Entanglement seems to link these particles and make them behave as one system. This peculiar characteristic has been a core element of the counterintuitiive world of quantum theory. Yet while experiments have verified this spookiness, other theorists maintain that both theory and experiment are flawed, and that a different interpretation is required. However, one such competing theory — the many worlds interpretation — makes equally spooky predictions.

From ars technica:

Quantum nonlocality, perhaps one of the most mysterious features of quantum mechanics, may not be a real phenomenon. Or at least that’s what a new paper in the journal PNAS asserts. Its author claims that nonlocality is nothing more than an artifact of the Copenhagen interpretation, the most widely accepted interpretation of quantum mechanics.

Nonlocality is a feature of quantum mechanics where particles are able to influence each other instantaneously regardless of the distance between them, an impossibility in classical physics. Counterintuitive as it may be, nonlocality is currently an accepted feature of the quantum world, apparently verified by many experiments. It’s achieved such wide acceptance that even if our understandings of quantum physics turn out to be completely wrong, physicists think some form of nonlocality would be a feature of whatever replaced it.

The term “nonlocality” comes from the fact that this “spooky action at a distance,” as Einstein famously called it, seems to put an end to our intuitive ideas about location. Nothing can travel faster than the speed of light, so if two quantum particles can influence each other faster than light could travel between the two, then on some level, they act as a single system—there must be no real distance between them.

The concept of location is a bit strange in quantum mechanics anyway. Each particle is described by a mathematical quantity known as the “wave function.” The wave function describes a probability distribution for the particle’s location, but not a definite location. These probable locations are not just scientists’ guesses at the particle’s whereabouts; they’re actual, physical presences. That is to say, the particles exist in a swarm of locations at the same time, with some locations more probable than others.

A measurement collapses the wave function so that the particle is no longer spread out over a variety of locations. It begins to act just like objects we’re familiar with—existing in one specific location.

The experiments that would measure nonlocality, however, usually involve two particles that are entangled, which means that both are described by a shared wave function. The wave function doesn’t just deal with the particle’s location, but with other aspects of its state as well, such as the direction of the particle’s spin. So if scientists can measure the spin of one of the two entangled particles, the shared wave function collapses and the spins of both particles become certain. This happens regardless of the distance between the particles.

The new paper calls all this into question.

The paper’s sole author, Frank Tipler, argues that the reason previous studies apparently confirmed quantum nonlocality is that they were relying on an oversimplified understanding of quantum physics in which the quantum world and the macroscopic world we’re familiar with are treated as distinct from one another. Even large structures obey the laws of quantum Physics, Tipler points out, so the scientists making the measurements must be considered part of the system being studied.

It is intuitively easy to separate the quantum world from our everyday world, as they appear to behave so differently. However, the equations of quantum mechanics can be applied to large objects like human beings, and they essentially predict that you’ll behave just as classical physics—and as observation—says you will. (Physics students who have tried calculating their own wave functions can attest to this). The laws of quantum physics do govern the entire Universe, even if distinctly quantum effects are hard to notice at a macroscopic level.

When this is taken into account, according to Tipler, the results of familiar nonlocality experiments are altered. Typically, such experiments are thought to involve only two measurements: one on each of two entangled particles. But Tipler argues that in such experiments, there’s really a third measurement taking place when the scientists compare the results of the two.

This third measurement is crucial, Tipler argues, as without it, the first two measurements are essentially meaningless. Without comparing the first two, there’s no way to know that one particle’s behavior is actually linked to the other’s. And crucially, in order for the first two measurements to be compared, information must be exchanged between the particles, via the scientists, at a speed less than that of light. In other words, when the third measurement is taken into account, the two particles are not communicating faster than light. There is no “spooky action at a distance.”

Tipler has harsh criticism for the reasoning that led to nonlocality. “The standard argument that quantum phenomena are nonlocal goes like this,” he says in the paper. “(i) Let us add an unmotivated, inconsistent, unobservable, nonlocal process (collapse) to local quantum mechanics; (ii) note that the resulting theory is nonlocal; and (iii) conclude that quantum mechanics is [nonlocal].”

He’s essentially saying that scientists are arbitrarily adding nonlocality, which they can’t observe, and then claiming they have discovered nonlocality. Quite an accusation, especially for the science world. (The “collapse” he mentions is the collapse of the particle’s wave function, which he asserts is not a real phenomenon.) Instead, he claims that the experiments thought to confirm nonlocality are in fact confirming an alternative to the Copenhagen interpretation called the many-worlds interpretation (MWI). As its name implies, the MWI predicts the existence of other universes.

The Copenhagen interpretation has been summarized as “shut up and measure.” Even though the consequences of a wave function-based world don’t make much intuitive sense, it works. The MWI tries to keep particles concrete at the cost of making our world a bit fuzzy. It posits that rather than becoming a wave function, particles remain distinct objects but enter one of a number of alternative universes, which recombine to a single one when the particle is measured.

Scientists who thought they were measuring nonlocality, Tipler claims, were in fact observing the effects of alternate universe versions of themselves, also measuring the same particles.

Part of the significance of Tipler’s claim is that he’s able to mathematically derive the same experimental results from the MWI without use of nonlocality. But this does not necessarily make for evidence that the MWI is correct; either interpretation remains consistent with the data. Until the two can be distinguished experimentally, it all comes down to whether you personally like or dislike nonlocality.

Read the entire article here.

Questioning Quantum Orthodoxy

de-BrogliePhysics works very well in explaining our world, yet it is also broken — it cannot, at the moment, reconcile our views of the very small (quantum theory) with those of the very large (relativity theory).

So although the probabilistic underpinnings of quantum theory have done wonders in allowing physicists to construct the Standard Model, gaps remain.

Back in the mid-1920s, the probabilistic worldview proposed by Niels Bohr and others gained favor and took hold. A competing theory, known as the pilot wave theory, proposed by a young Louis de Broglie, was given short shrift. Yet some theorists have maintained that it may do a better job of reconciling this core gap in our understanding — so it is time to revisit and breathe fresh life into pilot wave theory.

From Wired / Quanta:

For nearly a century, “reality” has been a murky concept. The laws of quantum physics seem to suggest that particles spend much of their time in a ghostly state, lacking even basic properties such as a definite location and instead existing everywhere and nowhere at once. Only when a particle is measured does it suddenly materialize, appearing to pick its position as if by a roll of the dice.

This idea that nature is inherently probabilistic — that particles have no hard properties, only likelihoods, until they are observed — is directly implied by the standard equations of quantum mechanics. But now a set of surprising experiments with fluids has revived old skepticism about that worldview. The bizarre results are fueling interest in an almost forgotten version of quantum mechanics, one that never gave up the idea of a single, concrete reality.

The experiments involve an oil droplet that bounces along the surface of a liquid. The droplet gently sloshes the liquid with every bounce. At the same time, ripples from past bounces affect its course. The droplet’s interaction with its own ripples, which form what’s known as a pilot wave, causes it to exhibit behaviors previously thought to be peculiar to elementary particles — including behaviors seen as evidence that these particles are spread through space like waves, without any specific location, until they are measured.

Particles at the quantum scale seem to do things that human-scale objects do not do. They can tunnel through barriers, spontaneously arise or annihilate, and occupy discrete energy levels. This new body of research reveals that oil droplets, when guided by pilot waves, also exhibit these quantum-like features.

To some researchers, the experiments suggest that quantum objects are as definite as droplets, and that they too are guided by pilot waves — in this case, fluid-like undulations in space and time. These arguments have injected new life into a deterministic (as opposed to probabilistic) theory of the microscopic world first proposed, and rejected, at the birth of quantum mechanics.

“This is a classical system that exhibits behavior that people previously thought was exclusive to the quantum realm, and we can say why,” said John Bush, a professor of applied mathematics at the Massachusetts Institute of Technology who has led several recent bouncing-droplet experiments. “The more things we understand and can provide a physical rationale for, the more difficult it will be to defend the ‘quantum mechanics is magic’ perspective.”

Magical Measurements

The orthodox view of quantum mechanics, known as the “Copenhagen interpretation” after the home city of Danish physicist Niels Bohr, one of its architects, holds that particles play out all possible realities simultaneously. Each particle is represented by a “probability wave” weighting these various possibilities, and the wave collapses to a definite state only when the particle is measured. The equations of quantum mechanics do not address how a particle’s properties solidify at the moment of measurement, or how, at such moments, reality picks which form to take. But the calculations work. As Seth Lloyd, a quantum physicist at MIT, put it, “Quantum mechanics is just counterintuitive and we just have to suck it up.”

A classic experiment in quantum mechanics that seems to demonstrate the probabilistic nature of reality involves a beam of particles (such as electrons) propelled one by one toward a pair of slits in a screen. When no one keeps track of each electron’s trajectory, it seems to pass through both slits simultaneously. In time, the electron beam creates a wavelike interference pattern of bright and dark stripes on the other side of the screen. But when a detector is placed in front of one of the slits, its measurement causes the particles to lose their wavelike omnipresence, collapse into definite states, and travel through one slit or the other. The interference pattern vanishes. The great 20th-century physicist Richard Feynman said that this double-slit experiment “has in it the heart of quantum mechanics,” and “is impossible, absolutely impossible, to explain in any classical way.”

Some physicists now disagree. “Quantum mechanics is very successful; nobody’s claiming that it’s wrong,” said Paul Milewski, a professor of mathematics at the University of Bath in England who has devised computer models of bouncing-droplet dynamics. “What we believe is that there may be, in fact, some more fundamental reason why [quantum mechanics] looks the way it does.”

Riding Waves

The idea that pilot waves might explain the peculiarities of particles dates back to the early days of quantum mechanics. The French physicist Louis de Broglie presented the earliest version of pilot-wave theory at the 1927 Solvay Conference in Brussels, a famous gathering of the founders of the field. As de Broglie explained that day to Bohr, Albert Einstein, Erwin Schrödinger, Werner Heisenberg and two dozen other celebrated physicists, pilot-wave theory made all the same predictions as the probabilistic formulation of quantum mechanics (which wouldn’t be referred to as the “Copenhagen” interpretation until the 1950s), but without the ghostliness or mysterious collapse.

The probabilistic version, championed by Bohr, involves a single equation that represents likely and unlikely locations of particles as peaks and troughs of a wave. Bohr interpreted this probability-wave equation as a complete definition of the particle. But de Broglie urged his colleagues to use two equations: one describing a real, physical wave, and another tying the trajectory of an actual, concrete particle to the variables in that wave equation, as if the particle interacts with and is propelled by the wave rather than being defined by it.

For example, consider the double-slit experiment. In de Broglie’s pilot-wave picture, each electron passes through just one of the two slits, but is influenced by a pilot wave that splits and travels through both slits. Like flotsam in a current, the particle is drawn to the places where the two wavefronts cooperate, and does not go where they cancel out.

De Broglie could not predict the exact place where an individual particle would end up — just like Bohr’s version of events, pilot-wave theory predicts only the statistical distribution of outcomes, or the bright and dark stripes — but the two men interpreted this shortcoming differently. Bohr claimed that particles don’t have definite trajectories; de Broglie argued that they do, but that we can’t measure each particle’s initial position well enough to deduce its exact path.

In principle, however, the pilot-wave theory is deterministic: The future evolves dynamically from the past, so that, if the exact state of all the particles in the universe were known at a given instant, their states at all future times could be calculated.

At the Solvay conference, Einstein objected to a probabilistic universe, quipping, “God does not play dice,” but he seemed ambivalent about de Broglie’s alternative. Bohr told Einstein to “stop telling God what to do,” and (for reasons that remain in dispute) he won the day. By 1932, when the Hungarian-American mathematician John von Neumann claimed to have proven that the probabilistic wave equation in quantum mechanics could have no “hidden variables” (that is, missing components, such as de Broglie’s particle with its well-defined trajectory), pilot-wave theory was so poorly regarded that most physicists believed von Neumann’s proof without even reading a translation.

More than 30 years would pass before von Neumann’s proof was shown to be false, but by then the damage was done. The physicist David Bohm resurrected pilot-wave theory in a modified form in 1952, with Einstein’s encouragement, and made clear that it did work, but it never caught on. (The theory is also known as de Broglie-Bohm theory, or Bohmian mechanics.)

Later, the Northern Irish physicist John Stewart Bell went on to prove a seminal theorem that many physicists today misinterpret as rendering hidden variables impossible. But Bell supported pilot-wave theory. He was the one who pointed out the flaws in von Neumann’s original proof. And in 1986 he wrote that pilot-wave theory “seems to me so natural and simple, to resolve the wave-particle dilemma in such a clear and ordinary way, that it is a great mystery to me that it was so generally ignored.”

The neglect continues. A century down the line, the standard, probabilistic formulation of quantum mechanics has been combined with Einstein’s theory of special relativity and developed into the Standard Model, an elaborate and precise description of most of the particles and forces in the universe. Acclimating to the weirdness of quantum mechanics has become a physicists’ rite of passage. The old, deterministic alternative is not mentioned in most textbooks; most people in the field haven’t heard of it. Sheldon Goldstein, a professor of mathematics, physics and philosophy at Rutgers University and a supporter of pilot-wave theory, blames the “preposterous” neglect of the theory on “decades of indoctrination.” At this stage, Goldstein and several others noted, researchers risk their careers by questioning quantum orthodoxy.

A Quantum Drop

Now at last, pilot-wave theory may be experiencing a minor comeback — at least, among fluid dynamicists. “I wish that the people who were developing quantum mechanics at the beginning of last century had access to these experiments,” Milewski said. “Because then the whole history of quantum mechanics might be different.”

The experiments began a decade ago, when Yves Couder and colleagues at Paris Diderot University discovered that vibrating a silicon oil bath up and down at a particular frequency can induce a droplet to bounce along the surface. The droplet’s path, they found, was guided by the slanted contours of the liquid’s surface generated from the droplet’s own bounces — a mutual particle-wave interaction analogous to de Broglie’s pilot-wave concept.

Read the entire article here.

Image: Louis de Broglie. Courtesy of Wikipedia.

General Relativity Lives on For Now

Since Einstein first published his elegant theory of General Relativity almost 100 years ago it has proved to be one of most powerful and enduring cornerstones of modern science. Yet theorists and researchers the world over know that it cannot possibly remain the sole answer to our cosmological questions. It answers questions about the very, very large — galaxies, stars and planets and the gravitational relationship between them. But it fails to tackle the science of the very, very small — atoms, their constituents and the forces that unite and repel them, which is addressed by the elegant and complex, but mutually incompatible Quantum Theory.

So, scientists continue to push their measurements to ever greater levels of precision across both greater and smaller distances with one aim in mind — to test the limits of each theory and to see which one breaks down first.

A recent highly precise and yet very long distance experiment, confirmed that Einstein’s theory still rules the heavens.

From ars technica:

The general theory of relativity is a remarkably successful model for gravity. However, many of the best tests for it don’t push its limits: they measure phenomena where gravity is relatively weak. Some alternative theories predict different behavior in areas subject to very strong gravity, like near the surface of a pulsar—the compact, rapidly rotating remnant of a massive star (also called a neutron star). For that reason, astronomers are very interested in finding a pulsar paired with another high-mass object. One such system has now provided an especially sensitive test of strong gravity.

The system is a binary consisting of a high-mass pulsar and a bright white dwarf locked in mutual orbit with a period of about 2.5 hours. Using optical and radio observations, John Antoniadis and colleagues measured its properties as it spirals toward merger by emitting gravitational radiation. After monitoring the system for a number of orbits, the researchers determined its behavior is in complete agreement with general relativity to a high level of precision.

The binary system was first detected in a survey of pulsars by the Green Bank Telescope (GBT). The pulsar in the system, memorably labeled PSR J0348+0432, emits radio pulses about once every 39 milliseconds (0.039 seconds). Fluctuations in the pulsar’s output indicated that it is in a binary system, though its companion lacked radio emissions. However, the GBT’s measurements were precise enough to pinpoint its location in the sky, which enabled the researchers to find the system in the archives of the Sloan Digital Sky Survey (SDSS). They determined the companion object was a particularly bright white dwarf, the remnant of the core of a star similar to our Sun. It and the pulsar are locked in a mutual orbit about 2.46 hours in length.

Following up with the Very Large Telescope (VLT) in Chile, the astronomers built up enough data to model the system. Pulsars are extremely dense, packing a star’s worth of mass into a sphere roughly 10 kilometers in radius—far too small to see directly. White dwarfs are less extreme, but they still involve stellar masses in a volume roughly equivalent to Earth’s. That means the objects in the PSR J0348+0432 system can orbit much closer to each other than stars could—as little as 0.5 percent of the average Earth-Sun separation, or 1.2 times the Sun’s radius.

The pulsar itself was interesting because of its relatively high mass: about 2.0 times that of the Sun (most observed pulsars are about 1.4 times more massive). Unlike more mundane objects, pulsar size doesn’t grow with mass; according to some models, a higher mass pulsar may actually be smaller than one with lower mass. As a result, the gravity at the surface of PSR J0348+0432 is far more intense than at a lower-mass counterpart, providing a laboratory for testing general relativity (GR). The gravitational intensity near PSR J0348+0432 is about twice that of other pulsars in binary systems, creating a more extreme environment than previously measured.

According to GR, a binary emits gravitational waves that carry energy away from the system, causing the size of the orbit to shrink. For most binaries, the effect is small, but for compact systems like the one containing PSR J0348+0432, it is measurable. The first such system was found by Russel Hulse and Joseph Taylor; its discovery won the two astronomers the Nobel Prize.

The shrinking of the orbit results in a decrease in the orbital period as the two objects revolve around each other more quickly. In this case, the researchers measured the effect by studying the change in the spectrum of light emitted by the white dwarf, as well as fluctuations in the emissions from the pulsar. (This study also helped demonstrate the two objects were in mutual orbit, rather than being coincidentally in the same part of the sky.)

To test agreement with GR, physicists established a set of observable quantities. These include the rate of orbit decrease (which is a reflection of the energy loss to gravitational radiation) and something called the Shapiro delay. The latter phenomenon occurs because light emitted from the pulsar must travel through the intense gravitational field of the pulsar when exiting the system. This effect depends on the relative orientation of the pulsar to us, but alternative models also predict different observable results.

In the case of the PSR J0348+0432 system, the change in orbital period and the Shapiro delay agreed with the predictions of GR, placing strong constraints on alternative theories. The researchers were also able to rule out energy loss from other, non-gravitational sources (rotation or electromagnetic phenomena). If the system continues as models predict, the white dwarf and pulsar will merge in about 400 million years—we don’t know what the product of that merger will be, so astronomers are undoubtedly marking their calendars now.

The results are of potential use for the Laser Interferometer Gravitational-wave Observatory (LIGO) and other ground-based gravitational-wave detectors. These instruments are sensitive to the final death spiral of binaries like the one containing PSR J0348+0432. The current detection and observation strategies involve “templates,” or theoretical models of the gravitational wave signal from binaries. All information about the behavior of close pulsar binaries helps gravitational-wave astronomers refine those templates, which should improve the chances of detection.

Of course, no theory can be “proven right” by experiment or observation—data provides evidence in support of or against the predictions of a particular model. However, the PSR J0348+0432 binary results placed stringent constraints on any alternative model to GR in the strong-gravity regime. (Certain other alternative models focus on altering gravity on large scales to explain dark energy and the acceleration expansion of the Universe.) Based on this new data, only theories that agree with GR to high precision are still standing—leaving general relativity the continuing champion theory of gravity.

Read the entire article after the jump.

Image: Artist’s impression of the PSR J0348+0432 system. The compact pulsar (with beams of radio emission) produces a strong distortion of spacetime (illustrated by the green mesh). Courtesy of Science Mag.