From Ethan Siegel: “Quantum Immortality”

Starts with a Bang

This article was written by Paul Halpern, the author of Einstein’s Dice and Schrödinger’s Cat: How Two Great Minds Battled Quantum Randomness to Create a Unified Theory of Physics.

Observers are the necessary, but unliked, bouncers in the elegant nightclub of quantum physics. While, no one is entirely comfortable with having doormen checking IDs, they persist; otherwise everyone and everything gets in, contrary to ordinary experience.

Image credit: AIP Emilio Segre Visual Archives, Physics Today Collection of [Paul]Dirac and [Werner] Heisenberg;

© Los Alamos National Laboratory of [John] von Neumann.

In the late 1920s and early 1930s, Heisenberg, Dirac, and John von Neumann, codified the formalism of quantum mechanics as a two-step process. One part involves the continous evolution of states via the deterministic

Schrödinger equation.
Image credit: Wikimedia Commons user YassineMrabet.

Map out a system’s potential energy distribution — in the form of a well, for example — and the spectrum of possible quantum states is set. If the states are time-dependent, then they predictably transform. That could set out, for instance, a superposition of states that spreads out in position space over time, like an expanding puddle of water.

Yet experiments show that if an apparatus is designed to measure a particular quantity, such as the position, momentum or spin-state of a particle, quantum measurements yield specific values of that respective physical parameter. Such specificity requires a second type of quantum operation that is instantaneous and discrete, rather than gradual and continuous: the process of collapse.

Image credit: A Friedman, via http://blogs.scientificamerican.com/the-curious-wavefunction/2014/01/15/what-scientific-idea-is-ready-for-retirement/.

Collapse occurs when a measurement of a certain physical parameter — position, let’s say — precipitates a sudden transformation into one of the “eigenstates” (solution states) of the operator (mathematical function) corresponding to that parameter — the position operator, in that case.

Image credit: Nick Trefethen, via http://www.chebfun.org/examples/ode-eig/Eigenstates.html.

Then the measured value of that quantity is the “eigenvalue” associated with that eigenstate — the specific position of the particle, for instance. Eigenstates represent the spectrum of possible states and eigenvalues the measurements associated with those states.

We can imagine the situation of quantum collapse as being something like a slot machine with a mixture of dollar coins and quarters; some old enough to be valuable, others shining new.

Image credit: © 2014 Marco Jewelers, via http://marcojewelers.net/sell-buy-silver-gold-coins.

Its front panel has two buttons: one red and the other blue. Press the red button and the coins instantly become sorted according to denomination. A number of dollar coins drop out (a mixture of old and new). Press the blue button and the sorting is instantly done by date. A bunch of old coins (of both denominations) are released. While someone seeking quick bucks might press red, a coin collector might push blue. The machine is set that you are not permitted to press both buttons. Similarly, in quantum physics, according to Heisenberg’s famous uncertainty principle certain quantities such as position and momentum are not measurable at once with any degree of precision.

Over the years, a number of critics have attacked this interpretation.

Albert Einstein
Image credit: Oren Jack Turner, Princeton, N.J., via Wikimedia Commons user Jaakobou.

Suggesting that quantum physics, though experimentally correct, must be incomplete, Einstein argued that random, instantaneous transitions had no place in a fundamental description of nature. Schrödinger cleverly developed his well-known feline thought experiment to demonstrate the absurdity of the observer’s role in quantum collapse. In his hypothetical scheme, he imagined a set-up in which a cat in a closed box, whose survival (or not) was tied to the random decay of a radioactive material, was in a mixed state of life and death until the box was opened and the system observed.

Image credit: retrieved from Øystein Elgarøy at http://fritanke.no/index.php?page=vis_nyhet&NyhetID=8513.

More recently, physicist Bryce DeWitt, who theorized how quantum mechanics might apply to gravity and the dynamics of the universe itself, argued that because there are presumably no observers outside the cosmos to view it (and trigger collapse into quantum gravity eigenstates), a complete accounting of quantum physics could not include observers.

Instead, DeWitt, until his death in 2004, was an ardent advocate of an alternative to the Copenhagen (standard) interpretation of quantum mechanics that he dubbed the Many Worlds Interpretation (MWI).

Image credit: University of Texas of Bryce DeWitt;

Professor Jeffrey A. Barrett and UC Irvine, of Hugh Everett III.

He based his views on the seminal work of Hugh Everett, who as a graduate student at Princeton, developed a way of avoiding the need in quantum mechanics for an observer. Instead, each time a quantum measurement is taken, the universe, including any observers, seamlessly and simultaneously splits into the spectrum of possible values for that measurement. For example, in the case of the measurement of the spin of an electron, in one branch it has spin up, and all observers see it that way; in the other it has spin down. Schrödinger’s cat would be happily alive in one reality, to the joy of its owner, while cruelly deceased in the other, much to the horror of the same owner (but in a different branch). Each observer in each branch would have no conscious awareness of his near-doppelgangers.

As Everett wrote to DeWitt in explaining his theory:

“The theory is in full accord with our experience (at least insofar as ordinary quantum mechanics is)… because it is possible to show that no observer would ever be aware of any ‘branching.’”

If Schrödinger’s thought experiment were repeated each day, there would always be one branch of the universe in which the cat survives. Hypothetically, rather than the proverbial “nine lives,” the cat could have an indefinite number of “lives” or at least chances at life. There would always be one copy of the experimenter who is gratified, but perplexed, that his cat has beaten the odds and lived to see another day. The other copy, in mourning, would lament that the cat’s luck had finally run out.

Image credit: Ethan Zuckerman, from Garrett Lisi’s talk (2008), via http://www.ethanzuckerman.com/blog/2008/02/28/ted2008-garrett-lisi-looks-for-balance/.

What about human survival? We are each a collection of particles, governed on the deepest level by quantum rules. If each time a quantum transition took place, our bodies and consciousness split, there would be copies that experienced each possible result, including those that might determine our life or death. Suppose in one case a particular set of quantum transitions resulted in faulty cell division and ultimately a fatal form of cancer. For each of the transitions, there would always be an alternative that did not lead to cancer. Therefore, there would always be branches with survivors. Add in the assumption that our conscious awareness would flow only to the living copies, and we could survive any number of potentially hazardous events related to quantum transitions.

Everett reportedly believed in this kind of “quantum immortality.” Fourteen years after his death in 1982, his daughter Liz took her own life, explaining in her suicide note that in some branch of the universe, she hoped to reunite with her father.

There are major issues with the prospects for quantum immortality however. For one thing the MWI is still a minority hypothesis. Even if it is true, how do we know that our stream of conscious thought would flow only to branches in which we survive? Are all possible modes of death escapable by an alternative array of quantum transitions? Remember that quantum events must obey conservation laws, so there could be situations in which there was no way out that follows natural rules. For example, if you fall out of a spaceship hatch into frigid space, there might be no permissible quantum events (according to energy conservation) that could lead you to stay warm enough to survive.

Finally, suppose you do somehow manage to achieve quantum immortality — with your conscious existence following each auspicious branch. You would eventually outlive all your friends and family members — because in your web of branches you would eventually encounter copies of them that didn’t survive. Quantum immortality would be lonely indeed!

See the full article here.

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Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible.

From Quanta: “Fluid Tests Hint at Concrete Quantum Reality”

Quanta Magazine

June 24, 2014
Natalie Wolchover

A droplet bouncing on the surface of a liquid has been found to exhibit many quantum-like properties, including double-slit interference, tunneling and energy quantization. (John Bush)

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.”

When light illuminates a pair of slits in a screen (top), the two overlapping wavefronts cooperate in some places and cancel out in between, producing an interference pattern. The pattern appears even when particles are shot toward the screen one by one (bottom), as if each particle passes through both slits at once, like a wave. Bottom: Akira Tonomura/Creative Commons

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.

At the fifth Solvay Conference, a 1927 meeting of the founders of quantum mechanics, Louis de Broglie (middle row, third from right) argued for a deterministic formulation of quantum mechanics called pilot-wave theory. But a probabilistic version of the theory championed by Niels Bohr (middle row, far right) won the day.

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.

The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

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
Double slit droplet experiment.Yves Couder et al.

When a droplet bounces along the surface of a liquid toward a pair of openings in a barrier, it passes randomly through one opening or the other while its “pilot wave,” or the ripples on the liquid’s surface, passes through both. After many repeat runs, a quantum-like interference pattern appears in the distribution of droplet trajectories.

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.

In a groundbreaking experiment, the Paris researchers used the droplet setup to demonstrate single- and double-slit interference. They discovered that when a droplet bounces toward a pair of openings in a damlike barrier, it passes through only one slit or the other, while the pilot wave passes through both. Repeated trials show that the overlapping wavefronts of the pilot wave steer the droplets to certain places and never to locations in between — an apparent replication of the interference pattern in the quantum double-slit experiment that Feynman described as “impossible … to explain in any classical way.” And just as measuring the trajectories of particles seems to “collapse” their simultaneous realities, disturbing the pilot wave in the bouncing-droplet experiment destroys the interference pattern.

Droplets can also seem to “tunnel” through barriers, orbit each other in stable “bound states,” and exhibit properties analogous to quantum spin and electromagnetic attraction. When confined to circular areas called corrals, they form concentric rings analogous to the standing waves generated by electrons in quantum corrals. They even annihilate with subsurface bubbles, an effect reminiscent of the mutual destruction of matter and antimatter particles.

In each test, the droplet wends a chaotic path that, over time, builds up the same statistical distribution in the fluid system as that expected of particles at the quantum scale. But rather than resulting from indefiniteness or a lack of reality, these quantum-like effects are driven, according to the researchers, by “path memory.” Every bounce of the droplet leaves a mark in the form of ripples, and these ripples chaotically but deterministically influence the droplet’s future bounces and lead to quantum-like statistical outcomes. The more path memory a given fluid exhibits — that is, the less its ripples dissipate — the crisper and more quantum-like the statistics become. “Memory generates chaos, which we need to get the right probabilities,” Couder explained. “We see path memory clearly in our system. It doesn’t necessarily mean it exists in quantum objects, it just suggests it would be possible.”

The quantum statistics are apparent even when the droplets are subjected to external forces. In one recent test, Couder and his colleagues placed a magnet at the center of their oil bath and observed a magnetic ferrofluid droplet. Like an electron occupying fixed energy levels around a nucleus, the bouncing droplet adopted a discrete set of stable orbits around the magnet, each characterized by a set energy level and angular momentum. The “quantization” of these properties into discrete packets is usually understood as a defining feature of the quantum realm.

A chaotic path.
As a droplet wends a chaotic path around the liquid’s surface, it gradually builds up quantum-like statistics. (Harris et al., PRL (2013)

If space and time behave like a superfluid, or a fluid that experiences no dissipation at all, then path memory could conceivably give rise to the strange quantum phenomenon of entanglement — what Einstein referred to as “spooky action at a distance.” When two particles become entangled, a measurement of the state of one instantly affects that of the other. The entanglement holds even if the two particles are light-years apart.

In standard quantum mechanics, the effect is rationalized as the instantaneous collapse of the particles’ joint probability wave. But in the pilot-wave version of events, an interaction between two particles in a superfluid universe sets them on paths that stay correlated forever because the interaction permanently affects the contours of the superfluid. “As the particles move along, they feel the wave field generated by them in the past and all other particles in the past,” Bush explained. In other words, the ubiquity of the pilot wave “provides a mechanism for accounting for these nonlocal correlations.” Yet an experimental test of droplet entanglement remains a distant goal.

Subatomic Realities

Many of the fluid dynamicists involved in or familiar with the new research have become convinced that there is a classical, fluid explanation of quantum mechanics. “I think it’s all too much of a coincidence,” said Bush, who led a June workshop on the topic in Rio de Janeiro and is writing a review paper on the experiments for the Annual Review of Fluid Mechanics.

Quantum physicists tend to consider the findings less significant. After all, the fluid research does not provide direct evidence that pilot waves propel particles at the quantum scale. And a surprising analogy between electrons and oil droplets does not yield new and better calculations. “Personally, I think it has little to do with quantum mechanics,” said Gerard ’t Hooft, a Nobel Prize-winning particle physicist at Utrecht University in the Netherlands. He believes quantum theory is incomplete but dislikes pilot-wave theory.

Many working quantum physicists question the value of rebuilding their highly successful Standard Model from scratch. “I think the experiments are very clever and mind-expanding,” said Frank Wilczek, a professor of physics at MIT and a Nobel laureate, “but they take you only a few steps along what would have to be a very long road, going from a hypothetical classical underlying theory to the successful use of quantum mechanics as we know it.”

“This really is a very striking and visible manifestation of the pilot-wave phenomenon,” Lloyd said. “It’s mind-blowing — but it’s not going to replace actual quantum mechanics anytime soon.”

In its current, immature state, the pilot-wave formulation of quantum mechanics only describes simple interactions between matter and electromagnetic fields, according to David Wallace, a philosopher of physics at the University of Oxford in England, and cannot even capture the physics of an ordinary light bulb. “It is not by itself capable of representing very much physics,” Wallace said. “In my own view, this is the most severe problem for the theory, though, to be fair, it remains an active research area.”

Pilot-wave theory has the reputation of being more cumbersome than standard quantum mechanics. Some researchers said that the theory has trouble dealing with identical particles, and that it becomes unwieldy when describing multiparticle interactions. They also claimed that it combines less elegantly with special relativity. But other specialists in quantum mechanics disagreed or said the approach is simply under-researched. It may just be a matter of effort to recast the predictions of quantum mechanics in the pilot-wave language, said Anthony Leggett, a professor of physics at the University of Illinois, Urbana-Champaign, and a Nobel laureate. “Whether one thinks this is worth a lot of time and effort is a matter of personal taste,” he added. “Personally, I don’t.”

Attendees of Hydrodynamic Quantum Analogs IV, a meeting held June 2-6 in Rio de Janeiro. The conference organizer, John Bush, a professor of applied mathematics at MIT, is pictured at left. (John Bush)

On the other hand, as Bohm argued in his 1952 paper, an alternative formulation of quantum mechanics might make the same predictions as the standard version at the quantum scale, but differ when it comes to smaller scales of nature. In the search for a unified theory of physics at all scales, “we could easily be kept on the wrong track for a long time by restricting ourselves to the usual interpretation of quantum theory,” Bohm wrote.

Some enthusiasts think the fluid approach could indeed be the key to resolving the long-standing conflict between quantum mechanics and Einstein’s theory of gravity, which clash at infinitesimal scales.

“The possibility exists that we can look for a unified theory of the Standard Model and gravity in terms of an underlying, superfluid substrate of reality,” said Ross Anderson, a computer scientist and mathematician at the University of Cambridge in England, and the co-author of a recent paper on the fluid-quantum analogy. In the future, Anderson and his collaborators plan to study the behavior of “rotons” (particle-like excitations) in superfluid helium as an even closer analog of this possible “superfluid model of reality.”

But at present, these connections with quantum gravity are speculative, and for young researchers, risky ideas. Bush, Couder and the other fluid dynamicists hope that their demonstrations of a growing number of quantum-like phenomena will make a deterministic, fluid picture of quantum mechanics increasingly convincing.

“With physicists it’s such a controversial thing, and people are pretty noncommittal at this stage,” Bush said. “We’re just forging ahead, and time will tell. The truth wins out in the end.”

See the full article, with video, here.

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Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

From Harvard: “The ever-smaller future of physics”

Harvard University

December 5, 2014
Alvin Powell

If physicists want to find their long-sought “theory of everything,” they have to get small. And Nobel Prize-winning theoretical physicist Steven Weinberg thinks he knows roughly how small.

Nobel winner Steven Weinberg brought his thoughts on a “theory of everything” to the Physics Department’s Lee Historical Lecture. Jon Chase/Harvard Staff Photographer

Weinberg, who spoke at a packed Geological Lecture Hall Monday evening, said there are hints that the answers to fundamental questions will reveal themselves at around a million billionths — between 10­-17 and 10-19 — of the radius of the typical atomic nucleus.

“It is in that range that we expect to find really new physics,” said Weinberg, a onetime Harvard professor now on the faculty at the University of Texas at Austin.

Physicists understand that there are four fundamental forces of nature. Two are familiar in our everyday lives: those of gravity and electromagnetism. The two less-familiar forces operate at the atomic level. The strong force holds the nucleus together while the weak force is responsible for the radioactive decay that changes one type of particle to another and the nuclear fusion that powers the sun.

For decades, physicists have toiled to create a single theory that explains how all four of these forces work, but without success, instead settling on one theory that explains how gravity acts on a macro scale and another to describe the other three forces and their interactions at the atomic level.

Weinberg, who won the 1979 Nobel Prize in Physics, with Sheldon Glashow and Abdus Salam, for electroweak theory explaining how the weak force and electromagnetism are related, returned to Harvard to deliver the Physics Department’s annual David M. Lee Historical Lecture. He was introduced by department chair Masahiro Morii and by Andrew Strominger, the Gwill E. York Professor of Physics, who recalled taking Weinberg’s class on general relativity as a Harvard undergrad.

“I wish I could say I remembered you in Physics 210,” Weinberg said to laughs as he took the podium.

The event also recognized the outstanding work of four graduate students — two in experimental physics, Dennis Huang and Siyuan Sun, and two in theoretical physics, Shu-Heng Shao and Bo Liu — with the Gertrude and Maurice Goldhaber Prize.

Weinberg pointed to several hints of something significant going on at the far extremes of tininess. One hint is that the strong force, which weakens at shorter scales, and the weak and electromagnetic forces, which get stronger across shorter distances, appear to converge at that scale.

Gravity is so weak that it isn’t felt at the atomic scale, overpowered by the other forces that operate there. However, Weinberg said, if you calculate how much mass two protons or two electrons would need for gravity to balance their repulsive electrical force, it would have to not just be enormous, but on a similar scale as the other measurements, the equivalent of 1.04 x 1018 gigaelectron volts.

“There is a strong suggestion that gravity is somehow unified with those other forces at these scales,” Weinberg said.

Weinberg also said there are experimental hints in the extremely small masses of neutrinos and in possible proton decay that the tiniest scales are significant in ways that are fundamental to physics.

“This is a very crude estimate, but the mass of neutrinos which are being observed are in the same ballpark that you would expect from new physics associated with a fundamental length,” Weinberg said. “It all seems to hang together.”

A major challenge for physicists is that the energy needed to probe what is actually going on at the smallest levels is far beyond current technology, something like 10 trillion times the highest energy we can harness now. And new technology to explore the problem experimentally is not on the horizon. Even with all the wealth in the world, scientists wouldn’t know where to begin, Weinberg said.

But the experiment may have already been done, by nature, and there may be a way to look back at it, Weinberg said. During the inflationary period immediately after the Big Bang there was that kind of energy, he said, and it would be evident as gravitation waves in the cosmic microwave background, an echo of the Big Bang that astronomers study for hints of the early universe. In fact, astronomers announced they had found such waves earlier this year, though they are waiting for confirmation of the results.

gravitational waves

CMB per ESA/Planck

ESA/Planck

“The big question that we face … is, can we find a truly fundamental theory uniting all the forces, including gravitation … characterized by tiny lengths like 10-17 to 10-19 nuclear radii?” Weinberg said. “Is it a string theory? That seems like the most beautiful candidate, but we don’t have any direct evidence that it is a string theory. The only handle we have … on this to do further experiments is in cosmology.”

See the full article here.

Harvard is the oldest institution of higher education in the United States, established in 1636 by vote of the Great and General Court of the Massachusetts Bay Colony. It was named after the College’s first benefactor, the young minister John Harvard of Charlestown, who upon his death in 1638 left his library and half his estate to the institution. A statue of John Harvard stands today in front of University Hall in Harvard Yard, and is perhaps the University’s best known landmark.

Harvard University has 12 degree-granting Schools in addition to the Radcliffe Institute for Advanced Study. The University has grown from nine students with a single master to an enrollment of more than 20,000 degree candidates including undergraduate, graduate, and professional students. There are more than 360,000 living alumni in the U.S. and over 190 other countries.

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From livescience: “Parallel Worlds Could Explain Wacky Quantum Physics”

November 19, 2014
Kelly Dickerson

The idea that an infinite number of parallel worlds could exist alongside our own is hard to wrap the mind around, but a version of this so-called Many Worlds theory could provide an answer to the controversial idea of quantum mechanics and its many different interpretations.

Credit: Shutterstock/Juergen Faelchle

Bill Poirier, a professor of physics at Texas Tech University in Lubbock, proposed a theory that not only assumes parallel worlds exist, but also says their interaction can explain all the quantum mechanics “weirdness” in the observable universe.

Poirier first published the idea four years ago, but other physicists have recently started building on the idea and have demonstrated that it is mathematically possible. The latest research was published Oct. 23 in the journal Physical Review X.

Quantum mechanics is the branch of physics that describes the rules that govern the universe on the microscopic scale. It tries to explain how subatomic particles can behave as both particles and as waves. It also offers an explanation about why particles appear to exist in multiple positions at the same time.

This fuzzy clump of possible positions is described by a “wave function” — an equation that predicts the many possible spots a given particle can occupy. But the wave function collapses the second anyone measures the actual position of the particle. This is where the multiverse theory comes in.

Some physicists believe that once a particle’s position is measured, the many other positions it could take according to its wave function split off and create separate, parallel worlds, each only slightly different from the original.

Hugh Everett was the first physicist to propose the possibility of a multiverse — an infinite number of parallel universes that exist alongside our own. He published his Many Worlds theory in the 1950s, but the idea was not well-received in the academic world.

Everett ended his career in physics shortly after getting his Ph.D., but many physicists now take the multiverse and parallel-worlds idea seriously. Poirier reworked the Many Worlds theory into the less abstract Many Interacting Worlds (MIW) theory, which could help explain the weird world of quantum mechanics.

Quantum mechanics has existed for more than a century, but its interpretation is just as controversial today as it was 100 years ago, Poirier wrote in his original paper.

Albert Einstein was not a fan of quantum mechanics. The idea that a particle could exist in a haze of probability instead of a definite location did not make sense to him, and he once famously said, “God does not play dice with the universe.” However, this new MIW theory might have helped to put Einstein’s mind at ease. In the MIW theory, quantum particles don’t act like waves at all. Each parallel world has normal-behaving particles and physical objects. The wave-function equation doesn’t have to exist at all.

In the new study, which builds on Poirier’s idea, physicists from Griffith University in Australia and the University of California, Davis, demonstrate that it only takes two interacting parallel worlds — not an infinite number — to produce the weird quantum behavior that physicists have observed. Neighboring worlds repulse one another, the researchers wrote in the paper. This force of repulsion could explain bizarre quantum effects, such as particles that can tunnel through barriers.

But how can physicists prove we’re living in just one of millions of other worlds, or that these worlds interact? Poirier thinks it will take some time to develop a way to test the idea.

“Experimental observations are the ultimate test of any theory,” Poirier said in a statement. “So far, Many Interacting Worlds makes the same predictions as standard quantum theory, so all we can say for sure at present is that it might be correct.”

The authors of the new paper hope that expanding the MIW theory will lead to ways to test for parallel worlds and further explain quantum mechanics.

Richard Feynman, a physicist who worked on the Manhattan Project, once said, “I think I can safely say that nobody understands quantum mechanics,” but Poirier and his colleagues argue that physicists have much to gain from trying.

See the full article here.

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From NYT: “Is Quantum Entanglement Real?”

The New York Times

NOV. 14, 2014
SundayReview
DAVID KAISER

FIFTY years ago this month, the Irish physicist John Stewart Bell submitted a short, quirky article to a fly-by-night journal titled Physics, Physique, Fizika. He had been too shy to ask his American hosts, whom he was visiting during a sabbatical, to cover the steep page charges at a mainstream journal, the Physical Review. Though the journal he selected folded a few years later, his paper became a blockbuster. Today it is among the most frequently cited physics articles of all time.

Bell’s paper made important claims about quantum entanglement, one of those captivating features of quantum theory that depart strongly from our common sense. Entanglement concerns the behavior of tiny particles, such as electrons, that have interacted in the past and then moved apart. Tickle one particle here, by measuring one of its properties — its position, momentum or “spin” — and its partner should dance, instantaneously, no matter how far away the second particle has traveled.

The key word is “instantaneously.” The entangled particles could be separated across the galaxy, and somehow, according to quantum theory, measurements on one particle should affect the behavior of the far-off twin faster than light could have traveled between them.

Entanglement insults our intuitions about how the world could possibly work. Albert Einstein sneered that if the equations of quantum theory predicted such nonsense, so much the worse for quantum theory. “Spooky actions at a distance,” he huffed to a colleague in 1948.

In his article, Bell demonstrated that quantum theory requires entanglement; the strange connectedness is an inescapable feature of the equations. But Bell’s proof didn’t show that nature behaved that way, only that physicists’ equations did. The question remained: Does quantum entanglement occur in the world?

Starting in the early 1970s, a few intrepid physicists — in the face of critics who felt such “philosophical” research was fit only for crackpots — found that the answer appeared to be yes.

John F. Clauser, then a young postdoctoral researcher at the Lawrence Berkeley National Laboratory, was the first. Using duct tape and spare parts, he fashioned a contraption to measure quantum entanglement. Together with a graduate student named Stuart Freedman, he fired thousands of pairs of little particles of light known as photons in opposite directions, from the middle of the device, toward each of its two ends. At each end was a detector that measured a property of the photon known as polarization.

As Bell had shown, quantum theory predicted certain strange correlations between the measurements of polarization as you changed the angle between the detectors — correlations that could not be explained if the two photons behaved independently of each other. Dr. Clauser and Mr. Freedman found precisely these correlations.

Other successful experiments followed. One, led by the French physicist Alain Aspect, tested the instantaneousness of entanglement. Another, led by the Austrian physicist Anton Zeilinger, considered entanglement among three or more particles.

Even with these great successes, work remains to be done. Every experimental test of entanglement has been subject to one or more loopholes, which hold out the possibility, however slim, that some alternative theory, distinct from quantum theory and more in line with Einstein’s intuitions, may still be salvageable. For example, one potential loophole — addressed by Dr. Aspect’s experiment — was that the measurement device itself was somehow transmitting information about one particle to the other particle, which would explain the coordination between them.

The most stubborn remaining loophole is known as “setting independence.” Dr. Zeilinger and I, working with several colleagues — including the physicists Alan H. Guth, Andrew S. Friedman and Jason Gallicchio — aim to close this loophole, a project that several of us described in an article in Physical Review Letters.

HERE’S the problem. In any test of entanglement, the researcher must select the settings on each of the detectors of the experimental apparatus (choosing to measure, for example, a particle’s spin along one direction or another). The setting-independence loophole suggests that, though the researcher appears to be free to select any setting for the detectors, it is possible that he is not completely free: Some unnoticed causal mechanism in the past may have fixed the detectors’ settings in advance, or nudged the likelihood that one setting would be chosen over another.

Bizarre as it may sound, even a minuscule amount of such coordination of the detectors’ settings would enable certain alternative theories to mimic the famous predictions from quantum theory. In such a case, entanglement would be merely a chimera.

How to close this loophole? Well, obviously, we aren’t going to try to prove that humans have free will. But we can try something else. In our proposed experiment, the detector setting that is selected (say, measuring a particle’s spin along this direction rather than that one) would be determined not by us — but by an observed property of some of the oldest light in the universe (say, whether light from distant quasars arrives at Earth at an even- or odd-numbered microsecond). These sources of light are so far away from us and from one another that they would not have been able to receive a single light signal from one another, or from the position of the Earth, before the moment, billions of years ago, when they emitted the light that we detect here on Earth today.

That is, we would guarantee that any strange “nudging” or conspiracy among the detector settings — if it does exist — would have to have occurred all the way back at the Hot Big Bang itself, nearly 14 billion years ago.

If, as we expect, the usual predictions from quantum theory are borne out in this experiment, we will have constrained various alternative theories as much as physically possible in our universe. If not, that would point toward a profoundly new physics.

Either way, the experiment promises to be exciting — a fitting way, we hope, to mark Bell’s paper’s 50th anniversary.

See the full article here.

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From MIT: “Doctoral students seek quantum control in Paola Cappellaro’s Quantum Engineering Group”

MIT News

November 4, 2014
Peter Dunn | Nuclear Science & Engineering

MIT’s Quantum Engineering Group (QEG) has a challenging but potentially world-changing mission: to harness the quantum properties of matter for use in information technology, metrology, defense, healthcare, and many other fields.

The interdisciplinary laboratory, headed by Department of Nuclear Science and Engineering (NSE) Associate Professor Paola Cappellaro, is working theoretically and experimentally towards achieving quantum control — the ability to control and utilize the behavior of nanoscale particles, which obey the laws of quantum mechanics rather than classical physics. A close-knit group of student researchers plays a central role, developing basic enabling tools that could yield dividends for decades to come.

“In one sense we’re doing fundamental research, but it’s really engineering — we’re seeking practical applications,” explains Cappellaro. The QEG is based in MIT’s Research Lab of Electronics, which brings together faculty and students from many departments (including NSE, physics, materials science, and electrical engineering and computer science) and other quantum-oriented programs at the Institute.

“Everyone’s working from different directions, but the common goal is to get control of quantum systems and be able to exploit them to build quantum devices,” says Cappellaro. “The common theme is using nitrogen-vacancy (NV) centers in diamonds as a promising experimental platform for this goal.” NV centers, a type of crystal defect, allow access to nearby electrons and their intrinsic angular momentum, or spin.

Graduate students Alexandre Cooper-Roy (left), Masashi Hirose (center), and Ashok Ajoy work to harness the quantum properties of matter in MIT’s Quantum Engineering Group. Photo: Susan Young

Applications could include leveraging spin to create quantum bits (qubits) for information processing and storage, which would provide exponential increases in computing power, and using NV-containing diamonds as ultra-responsive sensors, able to map molecular structures or monitor nano-scale magnetic fields like those created by brain functions.

There are many hurdles. One is that spin cannot be completely isolated, and is subjected to deleterious noise. This can lead to the decay of quantum superposition, or decoherence, which represents one of the major challenges to quantum control.

Doctoral student Masashi Hirose hopes to help solve the decoherence problem by learning how to use the electronic spin of NV centers to control the spin of nearby atomic nuclei (nuclear spin). “The nuclear spin is much harder to control directly than the electronic spin, but it’s more resistant to noise and can stay in a superposition state for milliseconds rather than microseconds, a 1,000 times increase,” explains Hirose. “The nuclear spin can act as an auxiliary qubit to protect the electronic spin from decoherence.”

Thus, the nuclear spin is a good candidate for memory functions in quantum computing, while computation could be handled with the electronic spin. Development of a control theory for interactions between the two is a research priority for Hirose, who joined Cappellaro’s lab group in 2009 after undergraduate work in Japan.

“The subject matter is a dream for me, and the QEG environment is very cooperative, everyone shares their problems,” he says. “I meet with Paola at least every couple of days, and she always welcomes new ideas, and helps figure out next steps.”

Fellow doctoral student Alexandre Cooper-Roy is also focused on leveraging the NV center electronic spin, and extending today’s rudimentary control abilities to create quantum resources, like multiple-qubit computing structures or sensors.

“NV centers are interesting because we can use them to control and read out the electronic spins associated with other impurities in the diamond lattice,” says Cooper-Roy. “Now we’d like to be able to put a few of these objects together so that we can do things that aren’t currently accessible, but the dynamics become really complicated.”

Cooper-Roy, who has studied in Japan and France as well as his native Canada, notes that the QEG’s research approach emphasizes work at room temperature using relatively simple equipment, which makes it possible for individuals to manage projects.

“A student can come here and build an experiment from scratch; it’s an easy test bed for exploration of quantum mechanics,” he says. “It’s great for training and for seeking practical applications. MIT is a great experience, because there’s an engineering approach — people really focus on optimizing the process. The final product is very important.”

Meanwhile, another PhD candidate, Ashok Ajoy, is attacking the problem of using quantum sensor technology as a sub-molecular microscope. “We’d like to be able to determine the structure of biomolecules, like proteins,” he explains. “The structure and the function are closely tied, and this would enable, say, the design of antibiotics, or the ability to block or amplify what a molecule does.”

The current goal is to achieve resolution of a few angstroms, which requires a synergistic combination of theory and experimentation. “In our lab we do both; I like it that way,” he says, adding that Cappellaro’s personal mentorship is an important enabler.

Long term, there is the potential to achieve sensing at resolution of a single spin: “That’s the holy grail; no one has been able to measure single external nuclear spins and map out their positions,” says Ajoy, who did his undergraduate studies in India. “With these sensors that we and our community have we’re on the cusp; it’s an exciting threshold, a stepping stone to something that could be very useful going forward in all sorts of applications.”

That broad applicability has attracted funding from blue-chip sources, like the National Science Foundation and the Department of Defense. “All the work is fundamentally enabling,” says Cappellaro. “We develop the tools we use in order to create better devices, but also seek to understand the physics so we can come up with smarter ways to use them.”

See the full article here.

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From Brown: “Can the wave function of an electron be divided and trapped?”

Brown University

October 28, 2014
Kevin Stacey 401-863-3766

Electrons are elementary particles — indivisible, unbreakable. But new research suggests the electron’s quantum state — the electron wave function — can be separated into many parts. That has some strange implications for the theory of quantum mechanics.

New research by physicists from Brown University puts the profound strangeness of quantum mechanics in a nutshell — or, more accurately, in a helium bubble.

Experiments led by Humphrey Maris, professor of physics at Brown, suggest that the quantum state of an electron — the electron’s wave function — can be shattered into pieces and those pieces can be trapped in tiny bubbles of liquid helium. To be clear, the researchers are not saying that the electron can be broken apart. Electrons are elementary particles, indivisible and unbreakable. But what the researchers are saying is in some ways more bizarre.

The electron wave function A canister of liquid helium inside the blue cylinder allowed researchers to experiment with tiny electron bubbles only 3.6 nanometers in diameter. The work suggests that the wave function of an electron can be split and parts of it trapped in smaller bubbles. Photo: Mike Cohea/Brown University

In quantum mechanics, particles do not have a distinct position in space. Instead, they exist as a wave function, a probability distribution that includes all the possible locations where a particle might be found. Maris and his colleagues are suggesting that parts of that distribution can be separated and cordoned off from each other.

“We are trapping the chance of finding the electron, not pieces of the electron,” Maris said. “It’s a little like a lottery. When lottery tickets are sold, everyone who buys a ticket gets a piece of paper. So all these people are holding a chance and you can consider that the chances are spread all over the place. But there is only one prize — one electron — and where that prize will go is determined later.”

If Maris’s interpretation of his experimental findings is correct, it raises profound questions about the measurement process in quantum mechanics. In the traditional formulation of quantum mechanics, when a particle is measured — meaning it is found to be in one particular location — the wave function is said to collapse.

“The experiments we have performed indicate that the mere interaction of an electron with some larger physical system, such as a bath of liquid helium, does not constitute a measurement,” Maris said. “The question then is: What does?”

And the fact that the wave function can be split into two or more bubbles is strange as well. If a detector finds the electron in one bubble, what happens to the other bubble?

“It really raises all kinds of interesting questions,” Maris said.

The new research is published in the Journal of Low Temperature Physics.

Electron bubbles

Scientists have wondered for years about the strange behavior of electrons in liquid helium cooled to near absolute zero. When an electron enters the liquid, it repels surrounding helium atoms, forming a bubble in the liquid about 3.6 nanometers across. The size of the bubble is determined by the pressure of the electron pushing against the surface tension of the helium. The strangeness, however, arises in experiments dating back to the 1960s looking at how the bubbles move.

In the experiments, a pulse of electrons enters the top of a helium-filled tube, and a detector registers the electric charge delivered when electron bubbles reach the bottom of the tube. Because the bubbles have a well-defined size, they should all experience the same amount of drag as they move, and should therefore arrive at the detector at the same time. But that’s not what happens. Experiments have detected unidentified objects that reach the detector before the normal electron bubbles. Over the years, scientists have cataloged 14 distinct objects of different sizes, all of which seem to move faster than an electron bubble would be expected to move.

“They’ve been a mystery ever since they were first detected,” Maris said. “Nobody has a good explanation.”

Several possibilities have been proposed. The unknown objects could be impurities in the helium—charged particles knocked free from the walls of the container. Another possibility is that the objects could be helium ions — helium atoms that have picked up one or more extra electrons, which produce a negative charge at the detector.

But Maris and his colleagues, including Nobel laureate and Brown physicist Leon Cooper, believe a new set of experiments puts those explanations to rest.

New experiments

The researchers performed a series of electron bubble mobility experiments with much greater sensitivity than previous efforts. They were able to detect all 14 of the objects from previous work, plus four additional objects that appeared frequently over the course of the experiments. But in addition to those 18 objects that showed up frequently, the study revealed countless additional objects that appeared more rarely.

In effect, Maris says, it appears there aren’t just 18 objects, but an effectively infinite number of them, with a “continuous distribution of sizes” up to the size of the normal electron bubble.

“That puts a dagger in the idea that these are impurities or helium ions,” Maris said. “It would be hard to imagine that there would be that many impurities, or that many previously unknown helium ions.”

The only way the researchers can think of to explain the results is through “fission” of the wave function. In certain situations, the researchers surmise, electron wave functions break apart upon entering the liquid, and pieces of the wave function are caught in separate bubbles. Because the bubbles contain less than the full wave function, they’re smaller than normal electron bubbles and therefore move faster.

In their new paper, Maris and his team lay out a mechanism by which fission could happen that is supported by quantum theory and is in good agreement with the experimental results. The mechanism involves a concept in quantum mechanics known as reflection above the barrier.

In the case of electrons and helium, it works like this: When an electron hits the surface of the liquid helium, there’s some chance that it will cross into the liquid, and some chance that it will bounce off and carom away. In quantum mechanics, those possibilities are expressed as part of the wave function crossing the barrier, and part of it being reflected. Perhaps the small electron bubbles are formed by the portion of the wave function that goes through the surface. The size of the bubble depends on how much wave function goes through, which would explain the continuous distribution of small electron bubble sizes detected in the experiments.

The idea that part of the wave function is reflected at a barrier is standard quantum mechanics, Cooper said. “I don’t think anyone would argue with that,” he said. “The non-standard part is that the piece of the wave function that goes through can have a physical effect by influencing the size of the bubble. That is what is radically new here.”

Further, the researchers propose what happens after the wave function enters the liquid. It’s a bit like putting a droplet of oil in a puddle of water. “Sometime your drop of oil forms one bubble,” Maris said, “Sometimes it forms two, sometimes 100.”

There are elements within quantum theory that suggest a tendency for the wave function to break up into specific sizes. By Maris’s calculations, the specific sizes one might expect to see correspond roughly to the 18 frequently occurring electron bubble sizes.

“We think this offers the best explanation for what we see in the experiments,” Maris said. We’ve got this body of data that goes back 40 years. The experiments are not wrong; they’ve been done by multiple people. We have a tradition called Occam’s razor, where we try to come up with the simplest explanation. This, so far as we can tell, is it.”

But it does raise some interesting questions that sit on the border of science and philosophy. For example, it’s necessary to assume that the helium does not make a measurement of the actual position of the electron. If it did, any bubble found not to contain the electron would, in theory, simply disappear. And that, Maris says, points to one of the deepest mysteries of quantum theory.

“No one is sure what actually constitutes a measurement. Perhaps physicists can agree that someone with a Ph.D. wearing a white coat sitting in the lab of a famous university can make measurements. But what about somebody who really isn’t sure what they are doing? Is consciousness required? We don’t really know.”

Authors on the paper in addition to Maris were former Brown postdoctoral researcher Wanchun Wei, graduate student Zhuolin Xie, and George Seidel, professor emeritus of physics.

See the full article here.

Welcome to Brown

Rhode Island Hall: Rhode Island Hall’s classical exterior was recently renovated with a modern interiorRhode Island Hall: Rhode Island Hall’s classical exterior was recently renovated with a modern interior

Located in historic Providence, Rhode Island and founded in 1764, Brown University is the seventh-oldest college in the United States. Brown is an independent, coeducational Ivy League institution comprising undergraduate and graduate programs, plus the Alpert Medical School, School of Public Health, School of Engineering, and the School of Professional Studies.

With its talented and motivated student body and accomplished faculty, Brown is a leading research university that maintains a particular commitment to exceptional undergraduate instruction.

Brown’s vibrant, diverse community consists of 6,000 undergraduates, 2,000 graduate students, 400 medical school students, more than 5,000 summer, visiting and online students, and nearly 700 faculty members. Brown students come from all 50 states and more than 100 countries.

Undergraduates pursue bachelor’s degrees in more than 70 concentrations, ranging from Egyptology to cognitive neuroscience. Anything’s possible at Brown—the university’s commitment to undergraduate freedom means students must take responsibility as architects of their courses of study.

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From Perimeter: “The Last Gasp of a Black Hole”

Perimeter Institute

October 17, 2014
No Writer Credit

New research from Perimeter shows that two of the strangest features of quantum mechanics – entanglement and negative energy – might be two faces of one coin.

Quantum mechanics is, notoriously, weird. Take entanglement: when two or more particles are entangled, their states are linked together, no matter how far apart they go.

If the idea makes your classical mind twitch, you’re in good company. At the heart of everything, according to quantum mechanics, nature has a certain amount of irreducible jitter. Even nothing – the vacuum of space – can jitter, or as physicists say, fluctuate. When it does, a particle and its anti-particle can pop into existence.

For example, an electron and an anti-electron (these are called positrons) might pop into existence out of the vacuum. We know that they each have a spin of one half, which might be either up or down. We also know that these particles were created from nothing and so, to balance the books, the total spin must add up to zero. Finally, we know that the spin of either particle is not determined until it is measured.

So suppose the electron and the positron fly apart a few metres or a few light years, and then a physicist comes by to measure the spin of, say, the electron. She discovers that the electron is spin up, and in that moment, the electron becomes spin up. Meanwhile, a few metres or a few light years away, the positron becomes spin down. Instantly. That is the strangeness of quantum entanglement.

Negative energy is less well known than entanglement, but no less weird. It begins with the idea – perhaps already implied by the positron and electron popping out of nowhere – that empty space is not empty. It is filled with quantum fields, and the energy of those fields can fluctuate a little bit.

In fact, the energy of these fields can dip under the zero mark, albeit briefly. When that happens, a small region of space can, for a short span of time, weigh less than nothing – or at least less than the vacuum. It’s a little bit like finding dry land below sea level.

Despite their air of strangeness, entanglement and negative energy are both well-explored topics. But now, new research, published as a Rapid Communication in Physical Review D, is hinting that these two strange phenomena may be linked in a surprising way.

The work was done by Perimeter postdoctoral fellow Matteo Smerlak and former postdoc Eugenio Bianchi (now on the faculty at Penn State and a Visiting Fellow at Perimeter). “Negative energy and entanglement are two of the most striking features of quantum mechanics,” says Smerlak. “Now, we think they might be two sides of the same coin.”

Perimeter Postdoctoral Researcher Matteo Smerlak

Perimeter Visiting Fellow Eugenio Bianchi

Specifically, the researchers proved mathematically that any external influence that changes the entanglement of a system in its vacuum state must also produce some amount of negative energy. The reverse, they say, is also true: negative energy densities can never be produced without entanglement being directly affected.

At the moment, the result only applies to certain quantum fields in two dimensions – to light pulses travelling up and down a thin cable, for instance. And it is with light that the Perimeter researchers hope that their new idea can be directly tested.

“Some quantum states which have negative energy are known, and one of them is called a ‘squeezed state,’ and they can be produced in the lab, by optical devices called squeezers,” says Smerlak. The squeezers manipulate light to produce an observable pattern of negative energy.

Remember that Smerlak and Bianchi’s basic argument is that if an external influence affects vacuum entanglement, it will also release some negative energy. In a quantum optics setup, the squeezers are the external influence.

Experimentalists should be able to look for the correlation between the entanglement patterns and the negative energy densities which this new research predicts. If these results hold up – always a big if in brand new work – and if they can make the difficult leap from two dimensions to the real world, then there will be startling implications for black holes.

Like optical squeezers, black holes also produce changes in entanglement and energy density. They do this by separating entangled pairs of particles and preferentially selecting the ones with negative energy.

Remember that the vacuum is full of pairs of particles and antiparticles blinking into existence. Under normal circumstances, they blink out again just as quickly, as the particle and the antiparticle annihilate each other. But just at a black hole’s event horizon, it sometimes happens that one of the particles is sucked in, while the other escapes. The small stream of escaping particles is known as Hawking radiation.

By emitting such radiation, black holes slowly give up their energy and mass, and eventually disappear. Black hole evaporation, as the process is known, is a hot topic in physics. This new research has the potential to change the way we think about it.

“In the late stages of the evaporation of a black hole, the energy released from the black hole will turn negative,” says Smerlak. And if a black hole releases negative energy, then its total energy goes up, not down. “It means that the black hole will shrink and shrink and shrink – for zillions of years – but in the end, it will release its negative energy in a gasp before dying. Its mass will briefly go up.”

Call it the last gasp of a black hole.

See the full article here.

About Perimeter

Perimeter Institute is a leading centre for scientific research, training and educational outreach in foundational theoretical physics. Founded in 1999 in Waterloo, Ontario, Canada, its mission is to advance our understanding of the universe at the most fundamental level, stimulating the breakthroughs that could transform our future. Perimeter also trains the next generation of physicists through innovative programs, and shares the excitement and wonder of science with students, teachers and the general public.

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From NOVA: “Are White Holes Real?”

NOVA

Tue, 19 Aug 2014
Maggie McKee

Sailors have their krakens and their sea serpents. Physicists have white holes: cosmic creatures that straddle the line between tall tale and reality. Yet to be seen in the wild, white holes may be only mathematical monsters. But new research suggests that, if a speculative theory called loop quantum gravity is right, white holes could be real—and we might have already observed them.

A white whole is, roughly speaking, the opposite of a black hole. “A black hole is a place where you can go in but you can never escape; a white hole is a place where you can leave but you can never go back,” says Caltech physicist Sean Carroll. “Otherwise, [both share] exactly the same mathematics, exactly the same geometry.” That boils down to a few essential features: a singularity, where mass is squeezed into a point of infinite density, and an event horizon, the invisible “point of no return” first described mathematically by the German physicist Karl Schwarzschild in 1916. For a black hole, the event horizon represents a one-way entrance; for a white hole, it’s exit-only.

There is excellent evidence that black holes really exist, and astrophysicists have a robust understanding of what it takes to make one. To imagine how a white hole might form, though, we have to go out on a bit of an astronomical limb. One possibility involves a spinning black hole. According to [Albert] Einstein’s general theory of relativity, the rotation smears the singularity into a ring, making it possible in theory to travel through the swirling black hole without being crushed. General relativity’s equations suggest that someone falling into such a black hole could fall through a tunnel in space-time called a wormhole and emerge from a white hole that spits its contents into a different region of space or period of time.

Though mathematical solutions to those equations exist for white holes, “they’re not realistic,” says Andrew Hamilton, an astrophysicist at the University of Colorado at Boulder. That is because they describe universes that contain only black holes, white holes and wormholes—no matter, radiation or energy. Indeed, previous research, including Hamilton’s, suggests that anything that falls into a spinning black hole will essentially plug up the wormhole, preventing the formation of a passage to a white hole.

But there’s a light at the end of the wormhole, so to speak. General relativity, from which Hamilton draws his predictions, breaks down at a black hole’s singularity. “The energy density and the curvature become so large that classical gravity is not a good description of what’s happening there,” says Stephen Hsu, a physicist at Michigan State University in East Lansing. Perhaps a more complete model of gravity—one that works as well on the quantum scale as it does on large ones—would negate the instability and allow for white holes, he says.

Indeed, a unified theory that merges gravity and quantum mechanics is one of the holy grails of contemporary physics. Applying one such theory, loop quantum gravity, to black holes, theorists Hal Haggard and Carlo Rovelli of Aix-Marseille University in France have shown that black holes could metamorphose into white holes via a quantum process. In July, they published their work online.

Loop quantum gravity proposes that space-time is made up of fundamental building blocks shaped like loops. According to Haggard and Rovelli, the loops’ finite size prevents a dying star from collapsing all the way down into a point of infinite density, and the shrinking object rebounds into a white hole instead. This process may take just a few thousandths of a second, but thanks to the intense gravity involved, the effects of relativity make the transformation appear to take much, much longer to anyone watching from afar. That means that minuscule black holes born in the infant universe could “now be ready to pop off like firecrackers,” forming white holes, according to a report in Nature. Some of the explosions astronomers thought were supernovae may actually be the wails of newborn white holes.

The black-to-white conversion could resolve a nettlesome conundrum known as the black hole information paradox. The notion that information can be destroyed is anathema in physics, and general relativity says that anything, including information, that falls into a black hole can never escape. These two statements are not at odds if black holes simply act as locked safes for any information they slurp up, but Stephen Hawking showed 40 years ago that black holes actually evaporate over time. That led to the disturbing possibility that the information contained within them could be lost too, triggering a debate that rages to this day.

But if a black hole instead turns into a white hole, then “all the information is recovered,” says Haggard. “We are quite excited about this mechanism because it avoids so many of the thorny issues that surround this discussion.”

The new work is preliminary, however, and it is far from clear whether loop quantum gravity is an accurate description of reality. The only glimpse we get of white holes might turn out to be those we model in labs and kitchen sinks. But Carroll says that’s okay. Just thinking about these possibly mythical cosmic creatures can improve physicists’ intuition, “even if the real world is messy and not like those exact situations,” he says. “That’s the way in which white holes are very useful.”

See the full article here.

NOVA is the highest rated science series on television and the most watched documentary series on public television. It is also one of television’s most acclaimed series, having won every major television award, most of them many times over.

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From Princeton via Huff Post: “Physicists Observe New Particle That’s Also Its Own ‘Antiparticle'”

Princeton University

10/03/2014
Macrina Cooper-White

After decades of searching, physicists at Princeton University say they’ve observed an elusive particle that behaves both like matter and antimatter.

Yes, the discovery is an exciting step forward for particle physics, but it may also help advance the creation of powerful quantum computers.

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In the early 20th century, as quantum theory emerged, scientists predicted that most common particles, like electrons, had mysterious “antimatter” counterparts with the same mass and opposite charge. Scientists even thought that if a particle came in contact with its “antiparticle,” the two would annihilate one another.

Italian physicist Ettore Majorana first hypothesized in 1937 that one particle — called the “Majorana fermion” — could serve as its very own antimatter particle, and scientists have been searching for that particle ever since.

An experiment revealing the atomic structure of an iron wire on a lead surface. The zoomed-in portion of the image depicts the probability of the wire containing the Majorana fermion. The image pinpoints the particle to the end of the wire, which is where it had been predicted to based on years of theoretical calculations.

For their study, the Princeton researchers designed a simple experiment to observe what they call “emergent particles” which can be found within a material — rather than in the vacuum of a giant collider, where the Higgs boson was discovered.

“This is more exciting and can actually be practically beneficial,” Ali Yazdani, a physics professor at the university who led the research team, said a written statement, “because it allows scientists to manipulate exotic particles for potential applications, such as quantum computing.”

The researchers placed a thin, long chain of pure magnetic iron atoms on a superconductor made of lead. Then they cooled the materials to -457 degrees Fahrenheit and peered at them through a two-story-tall scanning-tunneling microscope. Just check out the video above.

What did the researchers find? An electrically neutral signal at the ends of the iron wires, which is considered to be the “key signature” of the elusive Majorana fermion. The researchers say the fermion’s observed properties make it a good candidate for building quantum bits in computers.

“One of the first steps in making a quantum computer is to make a quantum bit,” Yazdani said in an email to the Huffington Post, “The ideal quantum bit should [be] one that you can control but it does not interact with its environment, so as to be changed.”

While other scientists find the study intriguing, many believe further research should be conducted to confirm the results.

“We should keep in mind possible alternative explanations — even if there are no immediately obvious candidates,” Jason Alicea, a physicist at the California Institute of Technology, who did not participate in this research, told Scientific American.

The research was published online on Oct. 2 in the journal Science.

See the full article, with video, here.

About Princeton: Overview

Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

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