The Book Of THoTH

No paradox for time travellers

THE laws of physics seem to permit time travel, and with it, paradoxical situations such as the possibility that people could go back in time to prevent their own birth. But it turns out that such paradoxes may be ruled out by the weirdness inherent in laws of quantum physics.

Some solutions to the equations of Einstein's general theory of relativity lead to situations in which space-time curves back on itself, theoretically allowing travellers to loop back in time and meet younger versions of themselves. Because such time travel sets up paradoxes, many researchers suspect that some physical constraints must make time travel impossible. Now, physicists Daniel Greenberger of the City University of New York and Karl Svozil of the Vienna University of Technology in Austria have shown that the most basic features of quantum theory may ensure that time travellers could never alter the past, even if they are able to go back in time.

The constraint arises from a quantum object's ability to behave like a wave. Quantum objects split their existence into multiple component waves, each following a distinct path through space-time. Ultimately, an object is usually most likely to end up in places where its component waves recombine, or "interfere", constructively, with the peaks and troughs of the waves lined up, say. The object is unlikely to be in places where the components interfere destructively, and cancel each other out.

Quantum theory allows time travel because nothing prevents the waves from going back in time. When Greenberger and Svozil analysed what happens when these component waves flow into the past, they found that the paradoxes implied by Einstein's equations never arise. Waves that travel back in time interfere destructively, thus preventing anything from happening differently from that which has already taken place (www.arxiv.org/quant-ph/0506027). "If you travel into the past quantum mechanically, you would only see those alternatives consistent with the world you left behind you," says Greenberger.

"This is a very nice idea," says physicist Avshalom Elitzur of the Weizmann Institute in Rehovot, Israel, who also suggests that further work in the area could help to clarify the nature of time itself. "Time is a very mysterious thing."

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According to our model, if you travel into the past quantum mechanically, you would only see those alternatives consistent with the world you left behind you. In other words, while you are aware of the past, you cannot change it. No matter how unlikely the events are that could have led to your present circumstances, once they have actually occurred, they cannot be changed. Your trip would set up resonances that are consistent with the future that has already unfolded.

This also has enormous consequences on the paradoxes of free will. It shows that it is perfectly logical to assume that one has many choices and that one is free to take any one of them. Until a choice is taken, the future is not determined. However, once a choice is taken, and it leads to a particular future, it was inevitable. It could not have been otherwise. The boundary conditions that the future events happen as they already have, guarantees that they must have been prepared for in the past. So, looking backwards, the world is deterministic. However, looking forwards, the future is probabilistic. This completely explains the classical paradox. In fact, it serves as a kind of indirect evidence that such feedback must actually take place in nature, in the sense that without it, a paradox exists, while with it, the paradox is resolved. (Of course, there is an equally likely explanation, namely that going backward in time is impossible. This also solves the paradox by avoiding it.)

Ghostly Ripples in Space

Neutrinos are tiny particles that can plow through the center of the Earth without even a hint of a whisper. Every second, billions of them rain down—and up—from outer space.

Some of these cosmic ghosts were witnesses to the first seconds of the universe. Now, for the first time, density fluctuations have been observed in these oldest of neutrinos.

"We know there is an ocean of neutrinos out there, but what we have shown is that there are ripples on it," Roberto Trotta of the University of Oxford told SPACE.com in a telephone interview.

The detection provides additional confirmation of the standard cosmological picture, as well as the fundamental theory of particle physics.

Neutrinos are subatomic particles with no electric charge and very little mass. They are extremely hard to study because they have a very low probability of interacting with the rest of the world.

According to the standard theory, neutrinos arose in large numbers out of the fires of the Big Bang. These so-called "background" neutrinos still exist: with 2,500 of them inhabiting every cubic inch of the universe.

Trotta and Alessandro Melchiorri of La Sapienza University in Rome have found evidence in the universe's distant past for wrinkles in this neutrino background.

"It has been one of the holy grails of cosmology to find the neutrinos that were made in the Big Bang," said Scott Dodelson from Theoretical Astrophysics Group at Fermilab. "This is another piece of indirect evidence."

Besides supporting the Big Bang theory, the ripples also provide a unique test of neutrino physics.

Elusive prey

To detect individual neutrinos, scientists use huge detectors—sometimes made of water or ice. Billions and billions of neutrinos pass through these particle traps unfazed, but every so often one of the elusive quarry gets snagged.

However, these detected neutrinos—coming from the Sun, or from cosmic rays, or occasionally from distant explosions—have millions of times more energy than the background neutrinos.

Having lost much of their energy as the universe expanded and cooled, the background neutrinos are impossible to see in any current—or even proposed—detector, so Trotta and Melchiorri looked at the effect that neutrinos would have on another relic from the early universe—the cosmic microwave background.

Ripples and clumps

The cosmic microwave background (CMB) is a snapshot of the universe as it looked 380,000 years after the Big Bang. Back then it was a fairly boring place, with no galaxies or stars—just minor fluctuations (one part in 100,000) in the density of matter.

This clumpiness left its imprint in temperature variations in the CMB. Light emitted from a dense clump would appear—after traveling billions of light years to reach us—as a hot spot in the microwave sky. Astronomers made a detailed map of these CMB spots with the Wilkinson Microwave Anisotropy Probe (WMAP) in 2003.

Although the matter clumps were small in the beginning, gravity has congealed them over time to form galaxy clusters, planets, and everything in between.

Neutrinos are a relatively small player in this part of the story because they rarely interact with the structure-forming material. But collectively, the gravitational pull of the ripples can influence the clumps.

"Neutrinos can’t talk to the matter and photons directly, but they can affect the CMB through their gravity," Dodelson said.

Trotta and Melchiorri showed that there is evidence for neutrino ripples in the WMAP data. Basically, the neutrino density tends toward being smooth, so fluctuations do not to last very long. As "neutrino wrinkles are ironed out," said Trotta, they drag on the matter—effectively reducing the number of small-sized clumps.

Very big down to very small

The ripple detection conflicts with theories that predict certain neutrino interactions, which are not part of the standard model of particle physics. These now-disfavored theories would have expected a smoother neutrino density due to these extra interactions.

Remarkably, the ripples—which at the time of the CMB were around 100,000 light years wide—tell us something about the physics at scales smaller than the atom.

"We are using the whole universe as a particle physics experiment," Trotta said.

Measuring neutrino ripples is currently the best way to probe this area of neutrino physics. The research will be published in the journal Physical Review Letters.

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Relativity violations may make light
A new theory suggests light arises because relativity's rules can be broken at the quantum level.


Physicists picture light and other electromagnetic radiation as arising from underlying symmetries in subatomic particles and force fields. But Robert Bluhm of Colby College in Maine and Alan Kostelecky of Indiana University say light quantum-scale violations of relativity may also create light.

"In this picture, light has a strange beauty, and its origin is tied into minuscule violations of Einstein's relativity in a profound and general way," Kostelecky says.

Special relativity holds that the speed of light is the same in every direction. This aspect of the theory, called Lorentz symmetry, says the laws of physics stay the same no matter how a physical system — a spaceship or a molecule — changes its speed or orientation. Current experiments verify the reality of Lorentz symmetry to within a few parts in 10 million billion.

"Although Einstein's theory has lasted 100 years, many physicists believe it will someday be superceded by a more fundamental theory, because general relativity is not compatible with quantum physics," Bluhm tells Astronomy. "Looking for Lorentz violation is one way of searching for hints of a more fundamental theory."

Finding evidence that the speed of light varies slightly in different directions — proof of broken Lorentz symmetry — would radically revise scientists' notions of the universe. It would mean that space (or more properly, relativity's four-dimensional space-time) has a kind of texture — a preferred orientation.

"Our paper primarily examines technical issues related to what happens when Lorentz symmetry is broken," Bluhm notes. "The fact that light can emerge purely as a result of Lorentz breaking came somewhat as a surprise to me."

Kostelecky offers a view of space in which aligned arrows exist everywhere. The arrows are force vectors representing space-time's preferred direction. At the very smallest scales, particles would exhibit slightly different behavior depending on their orientation with respect to these vectors. Kostelecky imagines light as a shimmering of this vector field analogous to wind blowing through a field of grain.

The results, published March 22 in the journal Physical Review D, show this new description of light is a general feature of relativity violations: It holds true in empty space as well as in the presence of gravity, something conventional theories of light often ignore.

"This is an alternative, viable way of understanding light, with potential experimental implications. That's what makes it exciting," Kostelecky says. Possible detectable effects include asymmetries between properties of certain particles and antiparticles, and cyclic variations in their behavior as Earth rotates, but any deviations from behavior predicted by conventional ideas will be tiny.

Ronald Walsworth, an experimental physicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says the tests Kostelecky has suggested are valuable and worth pursuing. No matter the final result, such experiments promise to deepen physicists' understanding.

Improved experimental tests that fail to find broken Lorentz symmetry would also be important, Walsworth says, because they would put tighter restrictions on theories attempting to explain dark energy, as well as those trying to reconcile the predictions of quantum mechanics and relativity.

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Russian Space Agency: Solar Sail Launch Failed

MOSCOW (AP) -- A joint Russian-U.S. project to launch a solar sail space vehicle crashed back to Earth when the booster rocket's engine failed less than two minutes after takeoff, the Russian space agency said Wednesday.

The Cosmos 1 vehicle was intended to show that a so-called solar sail can make a controlled flight. Solar sails, designed to be propelled by pressure from sunlight, are envisioned as a potential means for achieving interstellar flight, allowing such spacecraft to gradually build up great velocity and cover large distances.

But the Volna booster rocket failed 83 seconds after its launch from a Russian nuclear submarine in the northern Barents Sea just before midnight Tuesday in Moscow, the Russian space agency said.

Its spokesman, Vyacheslav Davidenko, said that "the booster's failure means that the solar sail vehicle was lost." The Russian Defense Ministry launched a search for debris from the booster and the vehicle, he said.

U.S. scientists had said earlier that they possibly had detected signals from the world's first solar sail spacecraft but cautioned that it could take hours or days to figure out exactly where the $4 million Cosmos 1 was.

The signals were picked up late Tuesday after an all-day search for the spacecraft, which had suddenly stopped communicating after its launch, they said.

"It's good news because we are in orbit—very likely in orbit," Bruce Murray, a co-founder of The Planetary Society, which organized the mission, said before the Russian space agency's announcement.

A government panel will investigate possible reasons behind the failure of the three-stage rocket's first-stage engine, Davidenko said.

Past attempts to unfold similar devices in space have failed.

In 1999, Russia launched a similar experiment with a sun-reflecting device from its Mir space station, but the deployment mechanism jammed and the device burned up in the atmosphere.

In 2001, Russia again attempted a similar experiment, but the device failed to separate from the booster and burned in the atmosphere.

The project involved Russia's Lavochkin research production institute that built the vehicle and was financed by an organization affiliated to the U.S. Planetary Society.

The solar sail vehicle weighed about 242 pounds and was designed to go into an orbit more than 500 miles high. It was designed to be powered by eight 49.5-foot-long sail structures resembling the blades of a windmill.

Each blade can be turned to reflect sunlight in different directions so that the craft can ``tack,'' much like a sailboat in the wind.

Controlled flight would have been attempted early next week, and Cosmos 1 was supposed to operate for at least a month.

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Vortices seen in Fermi gas

22 June 2005

Physicists have found conclusive evidence for superfluidity - flow without resistance - in an ultracold Fermi gas. Although indirect evidence for superfluidity in Fermi gases has been seen before, low-temperature physicists have been searching for definitive evidence in the shape of quantized vortices in a rotating gas. Now Wolfgang Ketterle and colleagues at the Massachusetts Institute of Technology (MIT) have observed these vortices in a gas of lithium-6 atoms (Nature 435 1047). The results could shed new light on systems as diverse as high-temperature superconductors and the quark-gluon plasma.

All atoms are either bosons or fermions depending on the value of their intrinsic angular momentum or "spin", and the difference between the two becomes clear at ultracold temperatures. Bosonic atoms have integer spins in quantum units and can collapse into the same quantum ground state in a process known as Bose-Einstein condensation (BEC). This process is at the heart of both superconductivity - the flow of electric current without resistance - and superfluidity.

Fermions, on the other hand, have half-integer spins and obey the Pauli exclusion principle. This means that two fermionic atoms cannot occupy the same quantum state. However, they can bind together to form a bosonic molecule that can then undergo condensation. Similarly, electrons - which are fermions - can form "Cooper pairs" and undergo condensation in the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity.

It is the possibility of reproducing this Cooper-pairing process in a Fermi gas, and possibly learning more about the mysterious pairing mechanism that underpins high-temperature superconductivity, that has led to intense interest in these systems.

Ketterle and co-workers started with a lithium-6 gas that had been cooled to about 50 nanokelvin and then applied a magnetic field to change the strength of the interactions between the atoms. For certain field strengths the atoms formed molecules that subsequently condensed to form a molecular BEC.

Next, the MIT team increased the strength of the magnetic field to convert the molecular condensate into a Fermi gas with strong interactions between the atoms. Finally, they used a green laser beam as a "spoon" to vigorously stir this gas and make it rotate.


A condensate of Fermion pairs (red) is trapped in the waist of a focussed laser beam (pink). Two additional laser beams (green) rotate around the edges to stir the condensate. Current-carrying coils (blue) generate the magnetic field used for axial confinement and to tune the interaction strength by means of a "Feshbach resonance". After releasing the atomic cloud from the electromagnetic trap, the cloud expands ballistically and inverts its aspect ratio. Resonant absorption imaging yields a density profile of the atomic cloud containing vortices (image and text: Ketterle Group).

In contrast to a normal fluid such as water, a superfluid can only rotate by forming a regular array of quantized vortices, each of which carries part of the total angular momentum of the superfluid. In addition to expelling atoms from their centres to leave a string-like hollow core, the vortices also repel each other to form a regular lattice pattern.

"When we observed the beautiful array of vortices, we instantly knew we had created a new form of matter - a high-temperature superfluid," says Martin Zwierlein, lead author of the paper. "It might sound confusing to call it a high-temperature superfluid, but scaled to the density of electrons in a metal, the transition from a normal gas to a superfluid would occur well above room temperature."


The pictures show vortex lattices on the "BEC-side" of the resonance (left), in the unitary regime on resonance (middle) and on the "BCS-side" of the resonance (right). Image and text: Ketterle Group.

Zwierlein also points out that the size of the Cooper pairs in a superconducting material is fixed, whereas the properties of the atom pairs in a superfluid Fermi gas can be changed by simply changing the magnetic field. "Thanks to this unique level control," says Zwierlein, "superfluid Fermi gases can serve as model systems for high-temperature superconductors and even more exotic forms of matter such as neutron stars or the matter that existed in the early stages of the universe."

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It turns out that Kostelecky wrote an article about this in last September's Scientific American: "The Search for Relativity Violations". Explains Lorentz symmetry pretty well, I think--plus it has plenty of illustrations to drive the points home. Tell me if that clears things up somewhat.