Many Worlds Interpretation (Hugh Everett's Theory) MWI: Testability and Objections

SOURCE: Stanford Encyclopedia of Philosophy

Testability:

Despite the name "interpretation", the MWI is a variant of quantum theory that is different from others. Experimentally, the difference is relative to collapse theories. It seems that there is no experiment distinguishing the MWI from other no-collapse theories such as Bohmian mechanics or other variants of MWI.
The collapse leads to effects that are, in principle, observable; these effects do not exist if the MWI is the correct theory. To observe the collapse we would need a super technology, which allows "undoing" a quantum experiment, including a reversal of the detection process by macroscopic devices. See Lockwood 1989 (p. 223), Vaidman 1998 (p. 257), and other proposals in Deutsch 1986. These proposals are all for gedanken experiments that cannot be performed with current or any foreseen future technology. Indeed, in these experiments an interference of different worlds has to be observed. Worlds are different when at least one macroscopic object is in macroscopically distinguishable states. Thus, what is needed is an interference experiment with a macroscopic body. Today there are interference experiments with larger and larger objects (e.g., fullerene molecules C60), but these objects are still not large enough to be considered "macroscopic". Such experiments can only refine the constraints on the boundary where the collapse might take place. A decisive experiment should involve the interference of states which differ in a macroscopic number of degrees of freedom: an impossible task for today's technology.[8]
The collapse mechanism seems to be in contradiction with basic physical principles such as relativistic covariance, but nevertheless, some ingenious concrete proposals have been made (see Pearle 1986 and the entry on collapse theories). These proposals (and Weissman's 1999 non-linear MW idea) have additional observable effects, such as a tiny energy non-conservation, that were tested in several experiments. The effects were not found and some (but not all!) of these models have been ruled out.
In most no-collapse interpretations, the evolution of the quantum state of the Universe is the same. Still, one might imagine that there is an experiment distinguishing the MWI from another no-collapse interepretation based on the difference in the correspondence between the formalism and the experience (the results of experiments).
An apparent candidate for such an experiment is a setup proposed in Englert et al. 1992 in which a Bohmian world is different from the worlds of the MWI (see also Aharonov and Vaidman 1996). In this example, the Bohmian trajectory of a particle in the past is contrary to the records of seemingly good measuring devices (such trajectories were named surrealistic). However, at present, there are no memory records that can determine unambiguously (without deduction from a particular theory) the particle trajectory in the past. Thus, this difference does not lead to an experimental way of distinguishing between the MWI and Bohmian mechanics. I believe that no other experiment can distinguish between the MWI and other no-collapse theories either, except for some perhaps exotic modifications, e.g., Bohmian mechanics with initial particle position distribution deviating from the quantum distribution. There are other opinions about the possibility of testing the MWI. It has frequently been claimed, e.g. by De Witt 1970, that the MWI is in principle indistinguishable from the ideal collapse theory. On the other hand, Plaga 1997 claims to have a realistic proposal for testing the MWI, and Page 2000 argues that certain cosmological observations might support the MWI.

Objections to the MWI:

Some of the objections to the MWI follow from misinterpretations due to the multitude of various MWIs. The terminology of the MWI can be confusing: "world" is "universe" in Deutsch 1996, while "universe" is "multiverse", etc. There are two very different approaches with the same name "The Many-Minds Interpretation (MMI)". The Albert and Loewer 1988 MMI mentioned above should not be confused with
Lockwood’ 1996 MMI (which resembles the approach of Zeh 1981). The latter is much closer to the MWI as it is presented here, see Sec. 17 of Vaidman 1998. Further, the MWI in the Heisenberg representation (Deutsch 2001) differs significantly from the MWI presented in the Schrödinger representation (used here). The MWI presented here is very close to Everett's original proposal, but in the entry on Everett's relative state formulation of quantum mechanics, as well as in his book Barrett 1999, Barrett uses the name "MWI" for the splitting worlds view publicized by De Witt 1970. This approach has been justly criticized: it has both some kind of collapse (an irreversible splitting of worlds in a preferred basis) and the multitude of worlds. Now I consider the main objections in detail.

Ockham's Razor:

It seems that the majority of the opponents of the MWI reject it because, for them, introducing a very large number of worlds that we do not see is an extreme violation of Ockham's principle: "Entities are not to be multiplied beyond necessity". However, in judging physical theories one could reasonably argue that one should not multiply physical laws beyond necessity either (such a verion of Ockham's Razor has been applied in the past), and in this respect the MWI is the most economical theory. Indeed, it has all the laws of the standard quantum theory, but without the collapse postulate, the most problematic of physical laws. The MWI is also more economic than Bohmian mechanics which has in addition the ontology of the particle trajectories and the laws which give their evolution. Tipler 1986 (p. 208) has presented an effective analogy with the criticism of Copernican theory on the grounds of Ockham's razor.
One might consider also a possible philosophical advantage of the plurality of worlds in the MWI, similar to that claimed by realists about possible worlds, such as Lewis 1986 (see the discussion of the analogy between the MWI and Lewis's theory by Skyrms 1976). However, the analogy is not complete: Lewis' theory considers all logically possible worlds, many more than all worlds incorporated in the quantum state of the Universe.

Theorists Reveal Path To True Muonium -- Never-seen Atom

SOURCE

ScienceDaily (June 4, 2009) — True muonium, a long-theorized but never-seen atom, might be observed in future experiments, thanks to recent theoretical work by researchers at the Department of Energy's SLAC National Accelerator Laboratory and Arizona State University. True muonium was first theorized more than 50 years ago, but until now no one had uncovered an unambiguous method by which it could be created and observed.

"We don't usually work in this area, but one day we were idly talking about how experimentalists could create exotic states of matter," said SLAC theorist Stanley Brodsky, who worked with Arizona State's Richard Lebed on the result. "As our conversation progressed, we realized 'Gee…we just figured out how to make true muonium.'"
True muonium is made of a muon and an anti-muon, and is distinguished from what's also been called "muonium"—an atom made of an electron and an anti-muon. Both muons and anti-muons are created frequently in nature when energetic particles from space strike the earth's atmosphere. Yet both have a fleeting existence, and their combination, true muonium, decays naturally into other particles in a few trillionths of a second. This makes observation of the exotic atom quite difficult.
In a paper published this week in Physical Review Letters, Brodsky and Lebed describe two methods by which electron–positron accelerators could detect the signature of true muonium's formation and decay.
In the first method, an accelerator's electron and positron beams are arranged to merge, crossing at a glancing angle. Such a collision would produce a single photon, which would then transform into a single true muonium atom that would be thrown clear of the other particle debris. Because the newly created true muonium atoms would be traveling so fast that the laws of relativity govern, they would decay much slower than they would otherwise, making detection easier. [An artist's impression of this process can be seen at right.]
In the second method, the electron and positron beams collide head-on. This would produce a true muonium atom and a photon, tangled up in a cloud of particle debris. Yet simply by recoiling against each other, the true muonium and the photon would push one another out of the debris cloud, creating a unique signature not previously searched for.
"It's very likely that people have already created true muonium in this second way," Brodsky said. "They just haven't detected it."
In their paper, Lebed and Brodsky also describe a possible, but more difficult, means by which experimentalists could create true tauonium, a bound state of a tau lepton and its antiparticle. The tau was first created at SLAC's SPEAR storage ring, a feat for which SLAC physicist Martin Perl received the 1995 Nobel Prize in physics.
Brodsky attributes the pair's successful work to a confluence of events: various unrelated lectures, conversations and ideas over the years, pieces of which came together suddenly during his conversation with Lebed.
"Once you pull all of the ideas together, you say 'Of course! Why not?' Brodsky said. "That's the process of science—you try to relate everything new to what you already know, creating logical connections."
Now that those logical connections are firmly in place, Brodsky said he hopes that one of the world's colliders will perform the experiments he and Lebed describe, asking, "Who doesn't want to see a new form of matter that no one's ever seen before?"
Journal reference:
Stanley J. Brodsky and Richard F. Lebed. Production of the Smallest QED Atom: True Muonium (µ µ-). Physical Review Letters, 2009; DOI: 10.1103/PhysRevLett.102.213401
Adapted from materials provided by DOE/SLAC National Accelerator Laboratory.

Goal: Developing The Best Atomic Clock In The World

SOURCE

ScienceDaily (June 5, 2009) — They are masters at working with light: the scientists at the newly founded QUEST Institute at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig. And they want to work on some of the most exciting questions relating to physics today: on unimaginably precise methods of measurement for observing the Earth, on the pressing question of the fundamentals of physics, of whether the fundamental constants are really constant, and on the development of the best atomic clock in the world made of a single aluminium atom.

These are just some examples from the catalogue of tasks which the scientists have drawn up for themselves. This gives us an idea of why optics is often described as "the" key technology of the 21st century.
QUEST stands for "Quantum Engineering and Space-Time Research" and it is the name of a whole Excellence Cluster to which the new institute belongs. It is already a prime example of good cooperation between completely different but perfectly complementary partners: Next to PTB as the state research institute of the Federal Ministry of Economics (BMWi) with its special knowledge in the field of metrology, i.e. precise measurement technologies, there are six institutes from the Leibniz University of Hannover, the Laser Zentrum Hannover, the German-British Gravitational Wave Detector GEO600 in Ruthe, the Center of Applied Space Technology and Microgravity (ZARM) in Bremen and the Albert Einstein Institute (Max Planck Institute for Gravitational Physics).
Now strengthened once more by the new QUEST Institute at PTB, the joint research is not only to answer the deep fundamental issues of physics, but also to lead to new high tech products with industry. In its start-up phase the institute is being financed by the German Research Foundation (DFG). After this period of funding, the professorship will be permanently continued by PTB and German Federal Ministry of Economics and Technology (BMWI).
A long, intensive search – this is the meaning of the word "quest". A good secondary meaning for the acronym, which primarily came about as the abbreviation of fields of research which previously existed more in parallel. "Quantum Engineering", the first part of the name stands for a relatively new branch of research, which deals with the control of quantum physics at the engineering level. "Space-Time Research", the second part, is research dealing with all aspects of space and time, for example the development of ever more exact atomic clocks or new processes to explore space, for instance precise measurement procedures in geodesy. Because in atomic clocks as well as in novel quantum sensors the main focus is on nothing more than using quantum phenomena with the aid of state-of-the-art technologies, founding the Excellence Cluster QUEST in November 2007 was a success right from the start. For this made it possible to gather the competence of the specialised scientists and institutions in a completely new way and much more intensively.
This success story is now opening another chapter: at PTB, in the immediate vicinity of the unique measuring equipment and some of the best atomic clocks in the world, a new research group has started working: the QUEST Institute at PTB consists of a Professorship ("Experimental Quantum Metrology"), a Junior Research Group ("Cold Ion Quantum Sensors"), a Research Project ("Sub-hertz Lasers and novel optic resonators“) as well as several so-called Task Groups.
Prof. Dr. Piet O. Schmidt, Head of the Institute, specialises in new methods of spectroscopy. "We use, for example, the exactly identical frequency distances of the spectral lines in an optical frequency comb in order to investigate atomic or molecular systems," explains Schmidt. "What is also very interesting are the methods of quantum logic originally developed for future quantum computers with stored ions, but which can also be used for investigating special classes of atoms or molecules which were not spectroscopically accessible or could only be accessed with difficulty previously." He also wants to use the quantum logic spectroscopy to develop an optical atomic clock on the basis of an extremely narrow transition in a single aluminium ion. "There is a chance that it might become the best atomic clock in the world", says Schmidt.
The clock that Schmidt has in mind is to attain a relative uncertainly of 10-18 (to date the limit at PTB lies at 10-15, held by the so-called caesium fountain clocks). The researchers will, thus, be able to pursue more intensively the question of whether the fine-structure constant, the gravitational constant or the mass ratio between the electron and the proton, change in space or time, as predicted by some modern theories in physics such as the String Theory. But concrete measurement capabilities will also be connected to this.
"The availability of such sensitive clocks will lead to other highly sensitive measuring instruments, which will be able to determine, for example, a change in the gravitational potential of the Earth with an accuracy corresponding to a height difference of one centimetre," explains Schmidt. "Currently the geoid of the Earth is known with an uncertainty of 30 cm to 50 cm." With QUEST, interdisciplinary cooperations between engineers who deal with the development of navigation systems, geodesists as well as researchers in the field of laser cooled atoms, are to be stimulated. This is to lead to the development of novel sensors for geoscience and navigation.
And probably the most important characteristic of QUEST lies right here: the intensive cooperation of thematically related but to date organisationally separate research groups: that is between university research, several federal research institutions and an international project on basic research. Not least, the novel quantum engineering technologies will probably also lead to forward-looking cooperations with industrial partners and, thus, to new commercial applications.
Adapted from materials provided by Physikalisch-Technische Bundesanstalt (PTB).

Researchers make breakthrough in the quantum control of light

SOURCE

This image represents a quantum state with zero, three and six photons simultaneously. The theory is on left and the experiment is on the right. Image: UCSB.
Researchers at UC Santa Barbara have recently demonstrated a breakthrough in the quantum control of photons, the energy quanta of light. This is a significant result in quantum computation, and could eventually have implications in banking, drug design, and other applications.

In a paper to be published in today's issue of the journal Nature, UCSB physics researchers Max Hofheinz, John Martinis, and Andrew Cleland document how they used a superconducting electronic circuit known as a Josephson phase qubit to prepare highly unusual quantum states using microwave-frequency photons. The breakthrough is the result of four years of work in the laboratories of Cleland and Martinis.
The project is funded by the federal agency called the Intelligence Advanced Research Projects Activity, or IARPA. The government is particularly interested in quantum computing because of the way banking and other important communications are currently encrypted. Using large numbers, with hundreds of digits, encryption codes are changed daily and would take years of traditional computing to break. Quantum computing could potentially break those codes quickly, destroying current encryption schemes.
In the experiments, the photons were stored in a microwave cavity, a "light trap" in which the light bounces back and forth as if between two mirrors. In earlier work, these researchers showed they could create and store photons, one at a time, with up to 15 photons stored at one time in the light trap. The research shows that they can create states in which the light trap simultaneously has different numbers of photons stored in it. For example, it can simultaneously have zero, three, and six photons at the same time. Measuring the quantum state by counting how many photons are stored forces the trap to "decide" how many there are; but prior to counting, the light trap exists in a quantum superposition, with all three outcomes possible.
Explaining the paradoxical simultaneity of quantum states, Cleland said that it's like having your cake and eating it -- at the same time.
"These superposition states are a fundamental concept in quantum mechanics, but this is the first time they have been controllably created with light," Cleland said. Martinis added, "This experiment can be thought of as a quantum digital-to-analog converter." As digital-to-analog converters are key components in classical communication devices (for example, producing the sound waveforms in cell phones), this experiment might enable more advanced communication protocols for the transmission of quantum information.
First author Hofheinz designed and performed the measurements. He is a postdoctoral researcher from Germany who has been working at UCSB for the last two years on this project. The devices used to perform the experiment were made by Haohua Wang, a postdoctoral researcher from China, who is second author on the Nature publication.
The scientists said their research is leading to the construction of a quantum computer, which will have applications in information encryption and in solving or simulating problems that are not amenable to solution using standard computers.
Source: University of California - Santa Barbara (news : web)

Theorists Reveal Path to True Muonium

SOURCE

In this artist's depiction of how experimentalists could create true muonium, an electron (blue) and a positron (red) collide, producing a virtual photon (green) and then a muonium atom, made of a muon (small yellow) and an anti-muon (small purple). The muonium atom then decays back into a virtual photon and then a positron and an electron. Overlaying this process is a figure indicating the structure of the muonium atom: one muon (large yellow) and one anti-muon (large purple). Credit: Graphic: Terry Anderson/SLAC
(PhysOrg.com) -- True muonium, a long-theorized but never-seen atom, might be observed in future experiments, thanks to recent theoretical work by researchers at the Department of Energy's SLAC National Accelerator Laboratory and Arizona State University. True muonium was first theorized more than 50 years ago, but until now no one had uncovered an unambiguous method by which it could be created and observed.

"We don't usually work in this area, but one day we were idly talking about how experimentalists could create exotic states of matter," said SLAC theorist Stanley Brodsky, who worked with Arizona State's Richard Lebed on the result. "As our conversation progressed, we realized 'Gee…we just figured out how to make true muonium.'"
True muonium is made of a muon and an anti-muon, and is distinguished from what's also been called "muonium"—an atom made of an electron and an anti-muon. Both muons and anti-muons are created frequently in nature when energetic particles from space strike the earth's atmosphere. Yet both have a fleeting existence, and their combination, true muonium, decays naturally into other particles in a few trillionths of a second. This makes observation of the exotic atom quite difficult.
In a paper published on Tuesday in Physical Review Letters, Brodsky and Lebed describe two methods by which electron-positron accelerators could detect the signature of true muonium's formation and decay.
In the first method, an accelerator's electron and positron beams are arranged to merge, crossing at a glancing angle. Such a collision would produce a single photon, which would then transform into a single true muonium atom that would be thrown clear of the other particle debris. Because the newly created true muonium atoms would be traveling so fast that the laws of relativity govern, they would decay much slower than they would otherwise, making detection easier.
In the second method, the electron and positron beams collide head-on. This would produce a true muonium atom and a photon, tangled up in a cloud of particle debris. Yet simply by recoiling against each other, the true muonium and the photon would push one another out of the debris cloud, creating a unique signature not previously searched for.
"It's very likely that people have already created true muonium in this second way," Brodsky said. "They just haven't detected it."
In their paper, Lebed and Brodsky also describe a possible, but more difficult, means by which experimentalists could create true tauonium, a bound state of a tau lepton and its antiparticle. The tau was first created at SLAC's SPEAR storage ring, a feat for which SLAC physicist Martin Perl received the 1995 Nobel Prize in physics.
Brodsky attributes the pair's successful work to a confluence of events: various unrelated lectures, conversations and ideas over the years, pieces of which came together suddenly during his conversation with Lebed.
"Once you pull all of the ideas together, you say 'Of course! Why not?' Brodsky said. "That's the process of science—you try to relate everything new to what you already know, creating logical connections."
Now that those logical connections are firmly in place, Brodsky said he hopes that one of the world's colliders will perform the experiments he and Lebed describe, asking, "Who doesn't want to see a new form of matter that no one's ever seen before?"

More information: "Production of the Smallest QED Atom: True Muonium," Physical Review Letters
Source: SLAC National Accelerator Laboratory (news : web)

All about Antigravity: 25 very interesting scientific documents

1. arXiv:0904.2394 [ps, pdf, other]
Title: Levitating Dark Matter
Authors: Nemanja Kaloper, Antonio Padilla
Comments: 17 pages LaTeX
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Astrophysical Phenomena (astro-ph.HE); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

2. arXiv:0902.3871 [ps, pdf, other]
Title: Dark energy and the mass of the Local Group
Authors: A.D. Chernin, P. Teerikorpi, M.J. Valtonen, G.G. Byrd, V.P. Dolgachev, L.M. Domozhilova
Comments: 7 pages, 1 figure, submitted to ApJL
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

3. arXiv:0901.4055 [ps, pdf, other]
Title: Extended General Relativity: large-scale antigravity and short-scale gravity with \omega=-1 from five dimensional vacuum
Authors: Jose Edgar Madriz Aguilar, Mauricio Bellini
Comments: 7 pages, no figures
Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Physics - Theory (hep-th)

4. arXiv:0811.1008 [ps, pdf, other]
Title: A constraint on antigravity of antimatter from precision spectroscopy of simple atoms
Authors: Savely G. Karshenboim (Max-Planck-Institut fuer Quantenoptik, Garching and D.I. Mendeleev Institute for Metrology, St.Petersburg)
Subjects: General Relativity and Quantum Cosmology (gr-qc); Atomic Physics (physics.atom-ph)
5. arXiv:0811.0522 [pdf]
Title: Primitive Virtual Negative Charge
Authors: Kiyoung Kim
Comments: 33 pages, 8 figures
Subjects: General Physics (physics.gen-ph)

6. arXiv:0803.2864 [pdf]
Title: Exact 'antigravity-field' solutions of Einstein's equation
Authors: Franklin S. Felber
Comments: 3 pages, 3 figures; this version shows correspondence of exact 'antigravity' field, calculated from a metric first derived by Hartle, Thorne, and Price, with weak 'antigravity' fields calculated from retarded potentials in Ref. [2]; also adds impulse calculation
Subjects: General Physics (physics.gen-ph)

7. arXiv:0710.4316 [src]
Title: Can the new Neutrino Telescopes and LHC reveal the gravitational proprieties of antimatter?
Authors: Dragan Slavkov Hajdukovic
Comments: This paper has been withdrawn by the author. Full version replacing partial results is at: arXiv:gr-qc/0612088v2
Subjects: General Relativity and Quantum Cosmology (gr-qc)

8. arXiv:0706.4171 [ps, pdf, other]
Title: Local dark energy: HST evidence from the vicinity of the M 81/M 82 galaxy group
Authors: A.D. Chernin, I.D. Karachentsev, O.G. Kashibadze, D.I. Makarov, P. Teerikorpi, M.J. Valtonen, V.P. Dolgachev, L.M. Domozhilova
Comments: 17 pages, 1 figure
Journal-ref: Astrophys.50:405-415,2007
Subjects: Astrophysics (astro-ph)

9. arXiv:0706.4068 [ps, pdf, other]
Title: Detection of dark energy near the Local Group with the Hubble Space Telescope
Authors: A.D. Chernin, I.D. Karachentsev, P. Teerikorpi, M.J. Valtonen, G.G. Byrd, Yu.N. Efremov, V.P. Dolgachev, L.M. Domozhilova, D.I. Makarov, Yu.V. Baryshev
Comments: 11 pages, 1 figure
Subjects: Astrophysics (astro-ph)

10. arXiv:0704.2753 [ps, pdf, other]
Title: Local dark energy: HST evidence from the expansion flow around Cen A/M83 galaxy group
Authors: A. D. Chernin, I. D. Karachentsev, D. I. Makarov, O. G. Kashibadze, P. Teerikorpi, M. J. Valtonen, V. P. Dolgachev, L. M. Domozhilova
Subjects: Astrophysics (astro-ph)

11. arXiv:gr-qc/0702142 [src]
Title: Concerning production and decay of mini black holes
Authors: Dragan Slavkov Hajdukovic
Comments: This paper has been withdrawn by the author. Full version replacing partial results is at: arXiv:gr-qc/0612088v2
Subjects: General Relativity and Quantum Cosmology (gr-qc)

12. arXiv:gr-qc/0701168 [src]
Title: Antigravity as the basis for a New Interpretation of the Planck Length
Authors: Dragan Slavkov Hajdukovic
Comments: This paper has been withdrawn by the author. Full version replacing partial results is at: arXiv:gr-qc/0612088v2
Subjects: General Relativity and Quantum Cosmology (gr-qc)

13. arXiv:gr-qc/0612088 [pdf]
Title: Black holes, neutrinos and gravitational proprieties of antimatter
Authors: Dragan Slavkov Hajdukovic
Comments: This new version is four times longer than the first one, and consequently contains much more results
Subjects: General Relativity and Quantum Cosmology (gr-qc); Astrophysics (astro-ph); High Energy Physics - Theory (hep-th)

14. arXiv:gr-qc/0604076 [ps, pdf, other]
Title: 'Antigravity' Propulsion and Relativistic Hyperdrive
Authors: Frankliln S. Felber
Comments: 4 pages, 4 figures, 2 video clips. To be presented at 25th International Space Development Conference, Los Angeles, 4-7 May 2006
Subjects: General Relativity and Quantum Cosmology (gr-qc)

15. arXiv:astro-ph/0603226 [ps, pdf, other]
Title: Non-Friedmann cosmology for the Local Universe, significance of the universal Hubble constant and short-distance indicators of dark energy
Authors: Arthur D. Chernin, Pekka Teerikorpi, Yurij V. Baryshev
Comments: 10 pages, 1 figure, submitted to A&A
Subjects: Astrophysics (astro-ph)

16. arXiv:astro-ph/0602102 [pdf]
Title: Hubble's law and Superluminity Recession Velocities
Authors: Leonid S. Sitnikov
Comments: 7 pages, 3 figures
Subjects: Astrophysics (astro-ph)

17. arXiv:gr-qc/0602041 [pdf]
Title: Testing existence of antigravity
Authors: Dragan Slavkov Hajdukovic
Subjects: General Relativity and Quantum Cosmology (gr-qc)

18. arXiv:gr-qc/0509105 [ps, pdf, other]
Title: Physics of Gravitational Interaction: Geometry of Space or Quantum Field in Space?
Authors: Yurij Baryshev
Comments: 9 pages, to be published in the Proceedings of the 1st Crisis in Cosmology Conference, AIP proceedings series
Subjects: General Relativity and Quantum Cosmology (gr-qc)

19. arXiv:astro-ph/0506070 [ps, pdf, other]
Title: Co-existence of Gravity and Antigravity: The Unification of Dark Matter and Dark Energy
Authors: Xiang-Song Chen
Comments: 3 pages, no figure; discussions added that low-energy gravitons can also serve as both dark matter and dark energy; references added
Subjects: Astrophysics (astro-ph)

20. arXiv:hep-th/0506067 [ps, pdf, other]
Title: Ultra-large distance modification of gravity from Lorentz symmetry breaking at the Planck scale
Authors: D.S. Gorbunov, S.M. Sibiryakov
Comments: 28 pages
Journal-ref: JHEP 0509 (2005) 082
Subjects: High Energy Physics - Theory (hep-th)

21. arXiv:physics/0506017 [pdf]
Title: Zero-point energy of vacuum fluctuation as a candidate for dark energy versus a new conjecture of antigravity based on the modified Einstein field equation in general relativity
Authors: Guang-jiong Ni
Comments: 11 pages,1 figure
Subjects: General Physics (physics.gen-ph)

22. arXiv:gr-qc/0505099 [ps, pdf, other]
Title: Exact Relativistic 'Antigravity' Propulsion
Authors: F. S. Felber
Comments: 4 pages, 3 figures, changed format only, attached 5 AVI files (animated exact solutions of black holes incident on initially stationary payloads)
Subjects: General Relativity and Quantum Cosmology (gr-qc)

23. arXiv:gr-qc/0505098 [pdf]
Title: Weak 'Antigravity' Fields in General Relativity
Authors: F. S. Felber
Comments: 5 pages, 3 figures, 1 table. Updates include: (1) Large Hadron Collider off-line experiment designed to test relativistic gravity; (2) demonstration that weak 'antigravity' fields correspond with new exact solutions calculated from an exact metric first derived by Hartle, Thorne, and Price
Subjects: General Relativity and Quantum Cosmology (gr-qc)

24. arXiv:gr-qc/0411096 [ps, pdf, other]
Title: The warp drive and antigravity
Authors: Homer G. Ellis
Comments: 6 pages, AMSTeX, 1 Encapsulated PostScript figure
Subjects: General Relativity and Quantum Cosmology (gr-qc)

25. arXiv:gr-qc/0411064 [ps, pdf, other]
Title: Symmetry relating Gravity with Antigravity: A possible resolution of the Cosmological Constant Problem?
Authors: Israel Quiros
Comments: 3 pages, no figures, revtex
Subjects: General Relativity and Quantum Cosmology (gr-qc)

Super-efficient Transistor Material Predicted

SOURCE

(PhysOrg.com) -- New work by condensed-matter theorists at the Stanford Institute for Materials and Energy Science at SLAC National Accelerator Laboratory points to a material that could one day be used to make faster, more efficient computer processors.

In a paper published online Sunday in Nature Physics, SIMES researchers Xiao-Liang Qi and Shou-Cheng Zhang, with colleagues from the Chinese Academy of Sciences and Tsinghua University in Beijing, predict that a room temperature material will exhibit the quantum spin Hall effect. In this exotic state of matter, electrons flow without dissipating heat, meaning a transistor made of the material would be drastically more efficient than anything available today. This effect was previously thought to occur only at extremely low temperatures. Now the race is on to confirm the room-temperature prediction experimentally.
Zhang has been one of the leading physicists working on the quantum spin Hall effect; in 2006 he predicted its existence in mercury telluride, which experimentalists confirmed a year later. However, the mercury telluride had to be cooled by liquid helium to a frigid 30 millikelvins, much too cold for real-world applications.
In their hunt for a material that exhibited the quantum spin Hall effect, Zhang and Qi knew they were looking for a solid with a highly unusual energy landscape. In a normal semiconductor, the outermost electrons of an atom prefer to stay in the valence band, where they are orbiting atoms, rather than the higher-energy conduction band, where they move freely through the material. Think of the conduction band as a flat plain pitted with small valence-band valleys. Electrons naturally "roll" down into these valleys and stay there, unless pushed out. But in a material that exhibits the quantum spin Hall effect, this picture inverts; the valence-band valleys rise to become hills, and the electrons roll down to roam the now lower-energy conduction band plain. In mercury telluride, this inversion did occur, but just barely; the hills were so slight that a tiny amount of energy was enough to push the electrons back up, meaning the material had to be kept extremely cold.
When Zhang, Qi and their colleagues calculated this energy landscape for four promising materials, three showed the hoped-for inversion. In one, bismuth selenide, the theoretical conduction band plain is so much lower than the valence band hills that even room temperature energy can't push the electrons back up. In physics terms, the conduction band and valence band are now inverted, with a sizeable difference between them.

"The difference [from mercury telluride] is that the gap is much larger, so we believe the effect could happen at room temperature," Zhang explained.
Materials that exhibit the quantum spin Hall effect are called topological insulators; a chunk of this material acts like an empty metal box that's completely insulating on the inside, but conducting on the surface. Additionally, the direction of each electron's movement on the surface decides its spin, an intrinsic property of electrons. This leads to surprising consequences.
Qi likens electrons traveling through a metal to cars driving along a busy road. When an electron encounters an impurity, it acts like a frustrated driver in a traffic jam, and makes a U-turn, dissipating heat. But in a topological insulator, Qi said, "Nature gives us a no U-turn rule." Instead of reversing their trajectories, electrons cruise coolly around impurities. This means the quantum spin Hall effect, like superconductivity, enables current to flow without dissipating energy, but unlike superconductivity, the effect doesn't rely on interactions between electrons.
Qi points out that, because current only flows on their surfaces, topological insulators shouldn't be seen as a way to make more efficient power lines. Instead, these novel compounds would be ideal for fabricating tinier and tinier transistors that transport information via electron spin.
"Usually you need magnets to inject spins, manipulate them, and read them out," Qi said. "Because the current and spin are always locked [in a topological insulator], you can control the spin by the current. This may lead to a new way of designing devices like transistors."
These tantalizing characteristics arise from underlying physics that seems to marry relativity and condensed matter science. Zhang and Qi's paper reveals that electrons on the surface of a topological insulator are governed by a so-called "Dirac cone," meaning that their momentum and energy are related according to the laws of relativity rather than the quantum mechanical rules that are usually used to describe electrons in a solid.
"On this surface, the electrons behave like a relativistic, massless particle," Qi said. "We are living in a low speed world here, where nothing is relativistic, but on this boundary, relativity emerges."
"What are the two greatest physics discoveries of the last century? Relativity and quantum mechanics." Zhang said. "In the semiconductor industry in the last 50 years, we've only used quantum mechanics, but to solve all these interesting frontier problems, we need to use both in a very essential way."
Zhang and Qi's new predictions are already spurring a surge of experiments to test whether these promising materials will indeed act as room-temperature topological insulators.
"The best feedback you can get is that there are lots of experiments going on," he said.
More information: http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys1270.html
Provided by SLAC National Accelerator Laboratory (news : web)

Ultra-dense Deuterium May Be Nuclear Fuel Of The Future

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ScienceDaily (May 12, 2009) — A material that is a hundred thousand times heavier than water and more dense than the core of the Sun is being produced at the University of Gothenburg. The scientists working with this material are aiming for an energy process that is both more sustainable and less damaging to the environment than the nuclear power used today.

Imagine a material so heavy that a cube with sides of length 10 cm weights 130 tonnes, a material whose density is significantly greater than the material in the core of the Sun. Such a material is being produced and studied by scientists in Atmospheric Science at the Department of Chemistry, the University of Gothenburg.
Towards commercial use
So far, only microscopic amounts of the new material have been produced. New measurements that have been published in two scientific journals, however, have shown that the distance between atoms in the material is much smaller than in normal matter. Leif Holmlid, Professor in the Department of Chemistry, believes that this is an important step on the road to commercial use of the material.
The material is produced from heavy hydrogen, also known as deuterium, and is therefore known as “ultra-dense deuterium”. It is believed that ultra-dense deuterium plays a role in the formation of stars, and that it is probably present in giant planets such as Jupiter.
An efficient fuel
So what can this super-heavy material be used for?
“One important justification for our research is that ultra-dense deuterium may be a very efficient fuel in laser driven nuclear fusion. It is possible to achieve nuclear fusion between deuterium nuclei using high-power lasers, releasing vast amounts of energy”, says Leif Holmlid.
The laser technology has long been tested on frozen deuterium, known as “deuterium ice”, but results have been poor. It has proved to be very difficult to compress the deuterium ice sufficiently for it to attain the high temperature required to ignite the fusion.
Energy source of the future
Ultra-dense deuterium is a million times more dense than frozen deuterium, making it relatively easy to create a nuclear fusion reaction using high-power pulses of laser light.
“If we can produce large quantities of ultra-dense deuterium, the fusion process may become the energy source of the future. And it may become available much earlier than we have thought possible”, says Leif Holmlid.
“Further, we believe that we can design the deuterium fusion such that it produces only helium and hydrogen as its products, both of which are completely non-hazardous. It will not be necessary to deal with the highly radioactive tritium that is planned for use in other types of future fusion reactors, and this means that laser-driven nuclear fusion as we envisage it will be both more sustainable and less damaging to the environment than other methods that are being developed.”
Deuterium – brief facts
Deuterium is an isotope of hydrogen that is found in large quantities in water, more than one atom per ten thousand hydrogen atoms has a deuterium nucleus. The isotope is denoted “2H” or “D”, and is normally known as “heavy hydrogen”. Deuterium is used in a number of conventional nuclear reactors in the form of heavy water (D2O), and it will probably also be used as fuel in fusion reactors in the future.
Adapted from materials provided by University of Gothenburg.

Particles, Molecules Prefer Not To Mix

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ScienceDaily (May 11, 2009) — In the world of small things, shape, order and orientation are surprisingly important, according to findings from a new study by chemists at Washington University in St. Louis.

Lev Gelb, WUSTL associate professor of chemistry, his graduate student Brian Barnes, and postdoctoral researcher Daniel Siderius, used computer simulations to study a very simple model of molecules on surfaces, which looks a lot like the computer game "Tetris." They have found that the shapes in this model (and in the game) do a number of surprising things.
WUSTL chemists headed by Lev Gelb simulated the motions and behavior of particles on a lattice and found "birds of a feather flock together." It's plainly evident that, in this four-component mixture of squares, rods, S shapes and Z shapes, the shapes all make little clusters, rather than completely mixing together. Tetris, anyone?
"First, different shapes don't mix very well with each other; each shape prefers to associate with others of the same kind," Gelb says. "When you put a lot of different shapes together, they separate from each other on microscopic scales, forming little clusters of nearly pure fluids. This is true even for the mirror-image shapes.
"Second, the structures of the pure (single-shape) fluids are quite complex and not what we might have predicted. There is a very strong tendency for some of the shapes, like rods and S- and Z- shapes, to align in the same direction. Finally, how `different looking' the shapes are isn't a good predictor for how well they mix; it turns out that the hard-to-predict characteristic structures of the fluids are more important than the shapes themselves, in this regard."
The researchers used Monte Carlo computer simulations of a simple lattice model (think of the lattice as a checkerboard), on which are placed "tetrominoes," which are S-, Z-, L-, J-, T-, rod- and square-shaped pieces.
Gelb and his colleagues use simulations to develop an atomic-scale understanding of the behavior of complex systems. They want to understand how molecules and nanoparticles of different shapes interact with each other to gain a better understanding of self-assembly, which is important in the development of new, strong materials for one, and designed catalysts for another.
Lining up
Gelb says that there has long been interest in self-assembly and in designing things that will assemble into predictable structures. Most researchers try to hold simple shapes together energetically, using some sort of chemical lock and key, such as DNA or hydrogen bonds. But if the particles have more shape to them, surprising things can happen.
"People have known for a long time when you make round nanoparticles and deposit them on a surface and you do it well, they make a nice, crystalline lattice," Gelb says. "If you do mixtures of two sizes you can get a number of different patterns with them. But if the particles aren't round, if they are short rods or things with more structure, it gets much more complicated quickly, and there's much less known about that."
The chemists also studied all 21 mixtures of two different shapes, as well as many combinations of three or more shapes.
"In all of the binary mixtures you get small-scale phase separation, which is counterintuitive," Gelb says. "It's not that the shapes repel each other. When there's no special repulsion between things or no stronger interaction between things of the same shape, you expect things to mix really well. In fact, that's not what happens."
Using ideas from classical thermodynamics and solution theory, the team was able to understand this separation using two different quantities. One is the virial coefficient, which measures the overlap between two shapes. They found that the shapes adopt alignments that minimize this overlap. Another is the volume of mixing. If you mix two liquids together, the volume of the mixture isn't necessarily the same as the volume of pure liquids you started with. In a mixture of water and ethanol, for instance, the volume of the mixture is smaller by about five percent than the sum of the original volumes. They found that in this model the volume always goes up when mixing different shapes.
Small world
"That's another indication that they don't mix well," Gelb says. "They take up more space when you mix them than when you allow them to be separate."
The model provides information on a very small world.
"If you think of the shapes as molecules sticking to a crystalline surface they would be a few Angstroms wide," says Barnes. "If you relate the model to nanoparticles, the shapes would be much larger, on the scale of tens of nanometers across."
In explaining the alignment phenomenon, Siderius offers the analogy of a roomful of people trying to circulate among each other.
"If they're all randomly placed, they'd bump shoulders frequently," he says. "But if they aligned a bit, everyone could move around more freely, which increases the entropy. In the past, we'd think of an ordered system as being low in entropy, but in this case the ordered state is high entropy."
Does it have anything to do with Tetris?
"Well, it suggests that one of the reasons the game is difficult is that the shapes don't fit together as well as we might think," says Gelb. "That, and they come down too fast."
The results were published in the on-line edition of the journal Langmuir on April 27, 2009
Journal reference:
Barnes et al. Structure, Thermodynamics, and Solubility in Tetromino Fluids. Langmuir, 2009; 090427084503036 DOI: 10.1021/la900196b
Adapted from materials provided by Washington University in St. Louis.

Post-Quantum Correlations: Exploring the Limits of Quantum Nonlocality

This figure shows levels of nonlocality as measured by the CHSH Bell inequality. Classical nonlocal correlations (green) are at 2 and below; quantum nonlocal correlations (red) are above 2 but below Tsirelson’s bound (BQ); and post-quantum nonlocal correlations (light blue) are above and, in some cases, below Tsirelson’s bound. BCC marks the “bound of triviality,” above which correlations are unlikely to exist. In the current study, scientists found that post-quantum correlated nonlocal boxes (dark blue line) are also unlikely to exist, despite some boxes being arbitrarily close to being classical. Image credit: Brunner and Skrzypczyk. ©2009 APS.

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(PhysOrg.com) -- When it comes to nonlocal correlations, some correlations are more nonlocal than others. As the subject of study for several decades, nonlocal correlations (for example, quantum entanglement) exist between two objects when they can somehow directly influence each other even when separated by a large distance. Because these correlations require “passion-at-a-distance” (a term coined by physicist Abner Shimony), they violate the principle of locality, which states that nothing can travel faster than the speed of light (even though quantum correlations cannot be used to communicate faster than the speed of light). Besides being a fascinating phenomenon, nonlocality can also lead to powerful techniques in computing, cryptography, and information processing.

Quantum Limits
Despite advances in quantum research, physicists still don’t fully understand the fundamental nature of nonlocality. In 1980, mathematician Boris Tsirelson found that quantum correlations are bounded by an upper limit; quantum nonlocality is only so strong. Later, in 1994, physicists Sandu Popescu and Daniel Rohrlich made another surprising discovery: a particular kind of correlation might exist above the “Tsirelson bound,” as well as below the bound, in a certain range (see image). These so-called post-quantum correlations are therefore “more nonlocal” than quantum correlations.
“Tsirelson's bound represents the most nonlocal ‘boxes’ that can be created with quantum mechanics,” Nicolas Brunner, a physicist at the University of Bristol, told PhysOrg.com. “Nonlocality here is measured by the degree of violation of a Bell inequality. So, quantum non-locality appears to be limited. The big question is why. That is, is there a good physical reason why post-quantum correlations don’t seem to exist in nature?”
In a recent study, Brunner and coauthor Paul Skrzypczyk, also of the University of Bristol, propose an explanation for why post-quantum correlations are unlikely to exist, which may reveal insight into why quantum nonlocality is bounded, as well as into the underlying difference between quantum and post-quantum correlations.
In their study, Brunner and Skrzypczyk have shown that a certain class of post-quantum correlations is unlikely to exist due to the fact that it makes communication complexity trivial. This triviality occurs due to the fact that the nonlocality of these correlations can be enhanced beyond a critical limit, and - surprisingly - in spite of the fact that some of these correlations are arbitrarily close to classical correlations (they give an arbitrarily small violation of Bell’s inequality). As previous research has suggested, any theory in which communication complexity is trivial is very unlikely to exist.
Beyond Quantum
“’Post-quantum’ means beyond quantum,” Brunner explained. “This term applies to correlations, which are conveniently - and probably most simply - described by ‘black boxes.’ The basic idea is the following: imagine a black box shared by two distant parties Alice and Bob; each party is allowed to ask a question to the box (or make a measurement on the box, if you prefer) and then gets an answer (a measurement outcome). By repeating this procedure many times, and at the end comparing their respective results, Alice and Bob can identify what their box is doing. For instance, it could be that the outcomes are always the same whenever Alice and Bob choose the same questions. This kind of behavior is a correlation; knowing one outcome, it is possible to deduce the other one, since both outcomes are correlated.
“Now, it happens that there exist different types of correlations; basically those that can be understood with classical physics (where correlations originate from a common cause), and those that cannot. This second type of correlation is called nonlocal, in the sense that it cannot be explained by a common cause. A priori it is not obvious to tell whether some correlations are local or not. The way physicists can tell this is by testing a Bell inequality; when a Bell inequality is violated, then the correlations cannot be local; that is, there cannot exist a common cause to these correlations.
“Now, an amazing thing about quantum mechanics is that it allows one to construct boxes that are non-local. This is quantum nonlocality. Now, it happens that not all nonlocal boxes can be constructed in quantum mechanics. Thus there exist correlations which are unobtainable in quantum mechanics. These are called post-quantum correlations. In general, post-quantum correlations can be above Tsirelson’s bound, but in some very specific cases, they can also be below.”
‘Distilling’ Post-Quantum Nonlocality
To demonstrate that post-quantum correlations cannot exist in nature, Brunner and Skrzypczyk developed a protocol for deterministically distilling nonlocality in post-quantum states. That is, the technique refines weakly nonlocal states into states with greater nonlocality. In this context, “distillation” can also be thought of as “purifying,” “amplifying,” or “maximizing” the nonlocality of post-quantum correlations. Since nonlocal correlations are more useful if they are stronger, maximizing nonlocality has significant implications for quantum information protocols. The physicists’ protocol works specifically with “correlated nonlocal boxes,” which are a particular class of post-quantum boxes.
Brunner and Skrzypczyk’s distillation protocol builds on a recent breakthrough by another team (Forster et al.), who presented the first nonlocality distillation protocol just a few months ago. However, the Forster protocol can distill correlated nonlocal boxes only up to a certain point, violating a Bell inequality called the Clauser-Horne-Shimony-Holt (CHSH) inequality only up to CHSH = 3. While this value is greater than Tsirelson’s bound of 2.82, it does not reach the bound of 3.26, which marks the point at which communication complexity becomes trivial.
Taking a step forward, Brunner and Skrzypczyk’s protocol can distill nonlocality all the way up to the maximum nonlocality of the Popescu-Rohrlich box, which is 4. In passing the 3.26 bound of triviality, they show that these post-quantum correlated nonlocal boxes do indeed collapse communication complexity.
The distillation protocol is executed by two distant parties that share two weakly correlated nonlocal boxes. Each party can input one bit into a box to receive one output bit, simulating a binary input/binary output system with local operations. As the scientists explain, a distillation protocol can be viewed as a way of classically wiring the two boxes together. The protocol is a choice of four wirings, one for each input of Alice and Bob. The wiring (algorithm) that determines the outbit bits of the boxes will transform the two nonlocal boxes into a single correlated nonlocal box, which has stronger nonlocality than the two individual boxes.
Importantly, this protocol can distill any correlated nonlocal box that violates the CHSH inequality by less than a limit of 3.26 to more than 3.26. In other words, any correlated nonlocal box that has not previously made communication complexity trivial can be made to do so. Surprisingly, some of these boxes can even be arbitrarily close to being classical (below or equal to 2), and yet, since they can be distilled beyond the “bound of triviality,” they still collapse communication complexity. According to previous studies of triviality, such boxes are very unlikely to exist - even those below Tsirelson’s bound.
Trivial Complexity
Theoretically, when communication complexity is trivial, even the most complex problems can be solved with a minimum amount of communication. In the following example, Brunner explains what would happen in real life if a single bit of information could solve any problem.
“Communication complexity is an information processing task,” Brunner said. “Here is an example. Suppose you and I would like to meet during the next year; so given our respective agendas, we would like to know whether there is a day where both of us are free or whether there is not; doesn’t matter what that day is, we just want to know whether there is such a day or not.
Since we are in distant locations, we must send each other some information to solve the problem. For instance, if I send you the whole information about my agenda, then you could find out whether a meeting is possible or not (and so solve the problem). But indeed that implies that I should send you a significant quantity of information (many bits). It turns out that in classical physics (or, if you prefer, in everyday life), there is no better strategy; I really have to send you all that information. In quantum physics, though there exist stronger correlations than in classical physics (quantum nonlocal correlations), I would still have to send you an enormous amount of communication.
“Now, the really astonishing thing is that, if you have access to certain post-quantum correlations (post-quantum boxes), a single bit of communication is enough to solve this problem! In other words, communication complexity becomes trivial in these theories, since one bit of communication is enough to solve any problem like this one. Importantly, in classical or quantum physics, communication complexity is not trivial. More generally, for computer scientists, a world in which communication complexity becomes trivial is highly unlikely to exist. Previously, it was known that post-quantum boxes with a very high degree of violation of a Bell inequality make communication complexity trivial; now, the astonishing thing about our result is that we show that some correlations with a very small degree of violation of a Bell inequality - but indeed not accessible with quantum mechanics - can also make communication complexity trivial.”
Post-Quantum Future
In the future, Brunner and Skrzypczyk hope to find improved distillation protocols that might work for a wider variety of post-quantum nonlocal boxes, not only correlated nonlocal boxes. More research is also needed to explain why quantum correlations cannot exist in the gap between Tsirelson’s bound and the bound of triviality. Ultimately, this line of research could help make a distinction between quantum and post-quantum correlations, with important theoretic implications.
“The greatest implications of our results are the following,” Brunner said. “First, they give new evidence that certain post-quantum theories allow for a dramatic increase in communication power compared to quantum mechanics, and therefore appear very unlikely to exist in nature. The nice thing, in particular, is that some of these theories allow only for little nonlocality (as measured by the degree of violation of a Bell inequality). Thus our result is a striking demonstration that we still have no clue on how to correctly measure nonlocality. Finally, it is one step further towards an information-theoretic axiom for quantum mechanics.”
More information: Nicolas Brunner and Paul Skrzypczyk. “Nonlocality Distillation and Postquantum Theories with Trivial Communication Complexity.” Physical Review Letters 102, 160403 (2009).
Copyright 2009 PhysOrg.com. All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

New nanocrystals show potential for cheap lasers, new lighting

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For more than a decade, scientists have been frustrated in their attempts to create continuously emitting light sources from individual molecules because of an optical quirk called "blinking," but now scientists at the University of Rochester have uncovered the basic physics behind the phenomenon, and along with researchers at the Eastman Kodak Company, created a nanocrystal that constantly emits light.

The findings, detailed online in today's issue of Nature, may open the door to dramatically less expensive and more versatile lasers, brighter LED lighting, and biological markers that track how a drug interact with a cell at a level never before possible.
Many molecules, as well as crystals just a billionth of a meter in size, can absorb or radiate photons. But they also experience random periods when they absorb a photon, but instead of the photon radiating away, its energy is transformed into heat. These "dark" periods alternate with periods when the molecule can radiate normally, leading to the appearance of them turning on and off, or blinking.
"A nanocrystal that has just absorbed the energy from a photon has two choices to rid itself of the excess energy—emission of light or of heat," says Todd Krauss, professor of chemistry at the University of Rochester and lead author on the study. "If the nanocrystal emits that energy as heat, you've essentially lost that energy."
Krauss worked with engineers at Kodak and researchers at the Naval Research Laboratory and Cornell University to discover the new, non-blinking nanocrystals.
Krauss, an expert in nanocrystals, and Keith Kahen, senior principal scientist of Kodak and an expert in optoelectronic materials and devices, were exploring new types of low-cost lighting similar to organic light-emitting diodes, but which might not suffer from the short lifespans and manufacturing challenges inherent in these diodes. Kahen, with help from Megan Hahn, a postdoctoral fellow in Krauss' laboratory, synthesized nanocrystals of various compositions.
Xiaoyong Wang, another postdoctoral fellow in Krauss laboratory, inspected one of these new nanocrystals and saw no evidence of the expected blinking phenomenon. Remarkably, even after four hours of monitoring, the new nanocrystal showed no sign of a single blink—unheard of when blinks usually happen on a scale of miliseconds to minutes.
After a lengthy investigation, Krauss and Alexander Efros from the Naval Research Laboratory concluded that the reason the blinking didn't occur was due to the unusual structure of the nanocrystal. Normally, nanocrystals have a core of one semiconductor material wrapped in a protective shell of another, with a sharp boundary dividing the two. The new nanocrystal, however, has a continuous gradient from a core of cadmium and selenium to a shell of zinc and selenium. That gradient squelches the processes that prevent photons from radiating, and the result is a stream of emitted photons as steady as the stream of absorbed photons.
With blink-free nanocrystals, Krauss believes lasers and lighting could be incredibly cheap and easy to fabricate. Currently, different color laser light is created using different materials and processes, but with the new nanocrystals a single fabrication process can create any color laser. To alter the light color, an engineer needs only to alter the size of the nanocrystal, which Krauss says is a relatively simple task.
The same is true of what could one day be OLED's successor, says Krauss. Essentially, "painting" a grid of differently sized nanocrystals onto a flat surface could create computer displays as thin as paper, or a wall that lights a room in any desired color.

The Laser Glow of an Atom Cloud

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Random lasers are the "disco ball" version of normal lasers. Instead of beaming in one direction, the light is scattered out in all directions by particles in a powder, for example. Calculations described in the 1 May Physical Review Letters show that a cloud of cold atoms could form a new type of random laser, where individual atoms scatter and amplify the light. The work suggests that the atoms will begin lasing if the cloud can be made opaque, or "cloudy," enough, which the authors think is within reach of current technology. Because such a system is fundamentally simple, it may give insight into some of the mysteries of random lasers.
A normal laser is essentially a gain medium inside a reflective cavity. The light is amplified by the medium as it bounces back and forth between the cavity's mirrors. A random laser has no cavity. Instead, tiny "mirrors," or scatterers, are added to the gain medium, causing photons to bounce around and become amplified by the medium, before escaping in all directions. For example, a container of micron-sized particles floating in water in which a laser dye has also been dissolved can emit laser light if pumped with external light. Random lasers do not require the same precise manufacturing as normal lasers, so they could be inexpensive to produce. Potential applications include digital displays, light emitting paints, and temperature sensors.
There are, however, some uncertainties regarding the spectra and time variability of random lasers [see Focus story: "Photon Marathon"]. So Robin Kaiser of Nice Sophia Antipolis University and the National Center for Scientific Research (CNRS) in France and his colleagues considered a random laser made up of cold atoms. Such a system would not have the absorption and other difficult-to-model complexities of existing random lasers. However, "it was not clear that atoms would be able to play the roles of both gain medium and scatterers," says Nice team member William Guerin. A single atom scatters photons when it's in its ground state and amplifies photons (through stimulated emission) when it's in an excited state, but it can't do both at the same time. So the researchers needed to find the right balance of atoms in both ground and excited states.
The team modeled a two-level atomic system. In their scenario, a complicated three-photon transition--made possible by an external pump laser--keeps a sufficient fraction of the atoms in the excited state (a so-called population inversion) from which they can provide gain. For different pump frequencies and intensities, the researchers calculated the amount of scattering necessary to trap light in the cloud long enough for it to become true "coherent" laser light, with all the photons propagating in synch. They quantified the amount of scattering within the cloud with a number called the optical thickness [1]. A value between 200 and 250 would lead to lasing, they found, which is within reach of state-of-the-art cold atom traps that currently achieve 150, Guerin says.
The researchers performed a simulation showing that the laser light should be detectable over background light coming from the cloud. They are now trying to make such a random laser in the lab using a standard magneto-optical trap to hold the atoms. Guerin says there are tricks to compress the trap to increase the density and therefore the amount of scattering.
There may, however, be experimental challenges involving the non-uniform density of the cloud, says Diederik Wiersma of the European Laboratory for Non-linear Spectroscopy in Florence, Italy. But if it can work, the use of atoms, rather than micron-scale scatterers, "could enrich our understanding of the random laser phenomena," Wiersma says. Moreover, because cold atoms are capable of slowing down light, he speculates that they could be more efficient at trapping light than current techniques. --Michael Schirber Michael Schirber is a freelance science writer in Lyon, France.
References:[1] The optical thickness--a characterization of the "opaqueness" of the atoms--can be thought of as the diameter of the cloud measured in units of the "mean free path," which is the typical distance a photon travels before scattering. This refers to scattering with the pump laser turned off, when there is no lasing.
Related Information:Related info:
Focus story: Photon Marathon (2004)
more explanation of random lasers from Wiersma's lab web site
Threshold of a Random Laser with Cold Atoms Luis S. Froufe-Pérez, William Guerin, Rémi Carminati, and Robin Kaiser Phys. Rev. Lett. 102, 173903 (issue of 1 May 2009)

A Light Touch

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Magnetically encoded information is at the core of much modern technology, and researchers are always looking for better ways to manipulate it. In the 8 May Physical Review Letters, a German team shows that a surprisingly feeble light beam can flip zeros to ones and vice versa, in a special magnetic layer. Although currently limited to very low temperatures, the apparently new effect might one day be extended to improve data storage.
Most familiar magnets are metals. They contain atoms that host tiny magnetic bar magnets, or moments, that can point up or down, and the atoms are surrounded by a sea of electrons. But researchers have long been interested in a different type of magnet, one consisting of widely separated magnetic ions embedded in a semiconductor. Unlike a metal, the number of free electrons in a semiconductor changes when it's exposed to electric current or light, so these materials should provide new ways to influence the magnetic properties, via the electrons. Light can flip the magnetization--the total magnetic moment of atoms in a region--from up to down, for example. But until now, experimenters needed very bright light to weaken the magnetization enough to reorient it.
In the new research, a team led by Laurens Molenkamp of the University of Würzburg in Germany grew a thin, crystalline layer of the common semiconductor gallium arsenide but replaced about one percent of the gallium atoms with the magnetic atom manganese. At temperatures below about 25 Kelvin, this layer acts as a ferromagnet: the magnetic moments on different manganese atoms point in the same direction, either up from the surface or down into it. The magnetization direction persists even when the researchers apply an opposing magnetic field, as long as the field does not exceed a threshold called the coercive field.
But when the team focused the light from a standard red laser on the film, the magnetization in the illuminated spot changed direction to match the magnetic field-- opposite to the rest of the film. The light didn’t change the strength of the magnetization, the team found, but instead reduced the field strength needed to flip it.
A mundane explanation would be that the light simply heats the film, which decreases the coercive field. To check this possibility, the team monitored the size of the magnetically flipped spot over many seconds of illumination. "It grows rather slowly," notes Würzburg team member Georgy Astakhov. "Heat diffusion occurs much, much faster." Instead, the team proposes that electrons liberated by the light (and the "holes" they leave behind) affect the magnetization directly. These charge carriers, they suggest, are quickly trapped in regions with high or low manganese concentration. These trapped charges effectively grease the motion of the "domain wall" that separates regions of opposite magnetization, by smoothing out local variations that would otherwise impede its motion. When the domain wall can move smoothly, a region with magnetization pointing up can more easily spread at the expense of a neighboring region having oppositely-directed magnetization.
Theo Rasing, of Radboud University in Nijmegen, Netherlands, says that more work is needed to confirm this non-thermal explanation, including extending the experiment to more than one sample. He also notes that because the dim light is on for a long time, the energy needed to flip the magnetization is not so different from other experiments that use very bright but short light pulses. Nonetheless, Rasing says that seeing a non-thermal change in magnetism with such a dim beam expands such "opto-magnetic" effects to new materials and mechanisms and should inspire further experiments by others.--Don Monroe Don Monroe is a freelance science writer in Murray Hill, New Jersey.
Related Information:
Physics Viewpoint essay by Molenkamp: Convincing a Magnetic Semiconductor to Work at Room Temperature (December, 2008)
Focus story on another optical magnetization flipping technique: Flipping Atoms Fast (1999)
Nonthermal Photocoercivity Effect in a Low-Doped (Ga,Mn)As Ferromagnetic Semiconductor G. V. Astakhov, H. Hoffmann, V. L. Korenev, T. Kiessling, J. Schwittek, G. M. Schott, C. Gould, W. Ossau, K. Brunne, and L. W. Molenkamp Phys. Rev. Lett. 102, 187401 (issue of 8 May 2009)

Carbon Nanotubes: Innovative Technology Or Risk To Health Or Environment?

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ScienceDaily (May 10, 2009) — Carbon nanotubes have made a meteoric career in the past 15 years, even if their applications are still limited. Recent research results show that – apart from their favorable mechanical and electrical properties – they also have disadvantageous characteristics.

One aspect which has rarely been considered so far is now addressed by researchers of the research center Forschungszentrum Dresden-Rossendorf. “If the application of products and commodities containing carbon nanotubes will increase in the future, then there will be a higher probability for the tubes to get into the environment during their production, usage or disposal, to be distributed there, and to bind pollutants such as heavy metals on their way trough the environment”, says Harald Zaenker, scientist at the FZD.
Via water into the environment
An important way for carbon nanotubes of getting into the environment is the way via the water. In their original state, the flimsy carbon fibers with a diameter of less than 50 nanometers (1 nanometer = 1 millionth of a millimeter) are hardly water-soluble. At first glance, they should therefore not be mobile in groundwater, lakes etc., i.e. they should rapidly settle or deposit. However, carbon nanotubes are able to form colloidal solutions if their surface structure is changed. Changes in the surface structure can be brought about deliberately during the production of the tubes or can be induced by natural processes if the tubes are released into the environment.
A colloidal solution, unlike a true solution of water-soluble substances, is a solution in which the apparently dissolved substance is finely dispersed in the solvent forming tiny particles. These particles are still much bigger than the molecules of a dissolved substance in a true solution. As colloids, carbon nanotubes might be transported anywhere in environmental waters. It is known meanwhile that the tubes can even penetrate cell walls and, thus, might theoretically be able to enter also animal or human cells. In addition, changes in the surface structure of carbon nanotubes cause another effect: their capability to bind heavy metals is increased.
Tubes with changed surface
The scientists investigated carbon nanotubes both in their original state and in a state changed by oxidizing acids (such as a mixture of nitric and sulfuric acid). They found out that solutions of treated carbon nanotubes scatter light more strongly. “This is an indication that colloids have formed which do not settle”, Harald Zaenker says.
The researchers provided evidence for the first time that the heavy metal uranium, which is ubiquitous in the environment and, hence, also in the water, is particularly attached to the surface of treated carbon nanotubes. The scientists found out that the uranium uptake capacity is increased by an order of magnitude in comparison to untreated carbon nanotubes. “Therefore, it is plausible to assume that carbon nanotubes, if released to the environment, influence the transport of uranium in environmental waters and even in biological systems. The possible impact on the environment and on human health has in general been considered too little”, Harald Zaenker says.
On the other hand, the high bonding capacity of carbon nanotubes for uranium and other heavy metals also suggests using them for the removal of heavy metals from waters. However, they are not yet a cost-efficient alternative to classic water purifiers, Zaenker says. “Eventually, it is important to further study the behavior of carbon nanotubes in waters”, the scientist says. “Only then can the positive and negative aspects of carbon nanotubes be better assessed.”
Journal reference:
Schierz et al. Aqueous suspensions of carbon nanotubes: Surface oxidation, colloidal stability and uranium sorption. Environmental Pollution, 2009; 157 (4): 1088 DOI: 10.1016/j.envpol.2008.09.045
Adapted from materials provided by Forschungszentrum Dresden Rossendorf.