One clock with two times: When quantum mechanics meets general relativity

ScienceDaily (Oct. 20, 2011) — The unification of quantum mechanics and Einstein's general relativity is one of the most exciting and still open questions in modern physics. General relativity, the joint theory of gravity, space and time gives predictions that become clearly evident on a cosmic scale of stars and galaxies. Quantum effects, on the other hand, are fragile and are typically observed on small scales, e.g. when considering single particles and atoms. That is why it is very hard to test the interplay between quantum mechanics and general relativity.

Now theoretical physicists led by Caslav Brukner at the University of Vienna propose a novel experiment which can probe the overlap of the two theories. The focus of the work is to measure the general relativistic notion of time on a quantum scale.

The findings were recently published in Nature Communications.

Time in general relativity

One of the counterintuitive predictions of Einstein's general relativity is that gravity distorts the flow of time. The theory predicts that clocks tick slower near a massive body and tick faster the further they are away from the mass. This effect results in a so-called "twin paradox": if one twin moves out to live at a higher altitude, he will age faster than the other twin who remains on the ground. This effect has been precisely verified in classical experiments, but not in conjunction with quantum effects, which is the aim of the newly proposed experiment.

Quantum interference and complementarity

The Viennese group of researchers wants to exploit the extraordinary possibility that a single quantum particle can lose the classical property of having a well-defined position, or as phrased in quantum mechanical terms: it can be in a "superposition." This allows for wave-like effects, called interference, with a single particle. However, if the position of the particle is measured, or even if it can in principle be known, this effect is lost. In other words, it is not possible to observe interference and simultaneously know the position of the particle. Such a connection between information and interference is an example of quantum complementarity -- a principle proposed by Niels Bohr. The experimental proposal now published in "Nature Communications" combines this principle with the "twin paradox" of general relativity.

Einstein's "twin paradox" for a quantum "only child"

The team at the University of Vienna considers a single clock (any particle with evolving internal degrees of freedom such as spin) which is brought in a superposition of two locations -- one closer and one further away from the surface of Earth. According to general relativity, the clock ticks at different rates in the two locations, in the same way as the two twins would age differently. But since the time measured by the clock reveals the information on where the clock was located, the interference and the wave-nature of the clock is lost. "It is the twin paradox for a quantum 'only child', and it requires general relativity as well as quantum mechanics. Such an interplay between the two theories has never been probed in experiments yet" -- says Magdalena Zych, the lead author of the paper and member of the Vienna Doctoral Program CoQuS. It is therefore the first proposal for an experiment that allows testing the genuine general relativistic notion of time in conjunction with quantum complementarity.

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Journal Reference:

Magdalena Zych, Fabio Costa, Igor Pikovski, Caslav Brukner. Quantum interferometric visibility as a witness of general relativistic proper time. Nature Communications, 2011; 2: 505 DOI: 10.1038/ncomms1498

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Light from galaxy clusters confirms general theory of relativity

ScienceDaily (Sep. 28, 2011) — All observations in astronomy are based on light (electromagnetic radiation) emitted from stars and galaxies and, according to the general theory of relativity, the light will be affected by gravity. At the same time all interpretations in astronomy are based on the correctness of the theory of relatively, but it has been difficult to accurately test Einstein's theory of gravity on scales larger than the solar system. Now astrophysicists at the Dark Cosmology Centre at the Niels Bohr Institute have managed to measure how the light is affected by gravity on its way out of galaxy clusters. The observations confirm the theoretical predictions.

The results have been published in the scientific journal, Nature.

Observations of large distances in the universe are based on measurements of the redshift, which is a phenomenon where the wavelength of the light from distant galaxies is shifted more and more towards the red with greater distance. The redshift indicates how much the universe has expanded from when the light left until it was measured on Earth. Furthermore, according to Einstein's general theory of relativity, the light and thus the redshift is also affected by the gravity from large masses like galaxy clusters and causes a gravitational redshift of the light. But the gravitational influence on light has never before been measured on a cosmological scale.

"It is really wonderful. We live in an era with the technological ability to actually measure such phenomena as cosmological gravitational redshift," says astrophysicist Radek Wojtak, Dark Cosmology Centre under the Niels Bohr Institute at the University of Copenhagen.

Galaxy clusters in the searchlight

Radek Wojtak, together with colleagues Steen Hansen and Jens Hjorth, has analysed measurements of light from galaxies in approximately 8,000 galaxy clusters. Galaxy clusters are accumulations of thousands of galaxies, held together by their own gravity. This gravity affects the light being sent out into space from the galaxies.

The researchers have studied the galaxies lying in the middle of the galaxy clusters and those lying on the periphery and measured the wavelengths of the light.

"We could measure small differences in the redshift of the galaxies and see that the light from galaxies in the middle of a cluster had to 'crawl' out through the gravitational field, while it was easier for the light from the outlying galaxies to emerge," explains Radek Wojtak.

Then he measured the entire galaxy cluster's total mass and with that got the gravitational potential. By using the general theory of relativity he could now calculate the gravitational redshift for the different locations of the galaxies.

"It turned out that the theoretical calculations of the gravitational redshift based on the general theory of relativity was in complete agreement with the astronomical observations. Our analysis of observations of galaxy clusters show that the redshift of the light is proportionally offset in relation to the gravitational influence from the galaxy cluster's gravity. In that way our observations confirm the theory of relativity," explains Radek Wojtak.

New light on the dark universe

The discovery has significance for the phenomena in the universe that researchers are working to unravel. It is the mysterious dark universe -- dark matter and dark energy.

In addition to the visible celestial bodies like stars, planets and galaxies, the universe consists of a large amount of matter, which researchers can work out that it must be there, but which cannot be observed as it neither emits nor reflects light. It is invisible and is therefore called dark matter. No one knows what dark matter is, but they know what the mass and thus the gravity must be. The new results for gravitational redshift do not change the researchers' modelling for the presence of dark matter.

Another of the main components of the universe is dark energy, which according to the theoretical models acts like a kind of vacuum that causes the expansion of the universe to accelerate. According to the calculations, which are based on Einstein's theory of relativity, dark energy constitutes 72 percent of the structure of the universe. Many alternative theories try to explain the accelerating expansion without the presence of dark energy.

Theory tested on a large scale

"Now the general theory of relativity has been tested on a cosmological scale and this confirms that the general theory of relativity works and that means that there is a strong indication for the presence of dark energy," explains Radek Wojtak.

The new gravitation results thus contribute a new piece of insight to the understanding of the hidden, dark universe and provide a greater understanding of the nature of the visible universe.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Copenhagen, via EurekAlert!, a service of AAAS.

Journal Reference:

Radoslaw Wojtak, Steen H. Hansen, Jens Hjorth. Gravitational redshift of galaxies in clusters as predicted by general relativity. Nature, 2011; 477 (7366): 567 DOI: 10.1038/nature10445

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Light from galaxy clusters confirms general theory of relativity

ScienceDaily (Sep. 28, 2011) — All observations in astronomy are based on light (electromagnetic radiation) emitted from stars and galaxies and, according to the general theory of relativity, the light will be affected by gravity. At the same time all interpretations in astronomy are based on the correctness of the theory of relatively, but it has been difficult to accurately test Einstein's theory of gravity on scales larger than the solar system. Now astrophysicists at the Dark Cosmology Centre at the Niels Bohr Institute have managed to measure how the light is affected by gravity on its way out of galaxy clusters. The observations confirm the theoretical predictions.

The results have been published in the scientific journal, Nature.

Observations of large distances in the universe are based on measurements of the redshift, which is a phenomenon where the wavelength of the light from distant galaxies is shifted more and more towards the red with greater distance. The redshift indicates how much the universe has expanded from when the light left until it was measured on Earth. Furthermore, according to Einstein's general theory of relativity, the light and thus the redshift is also affected by the gravity from large masses like galaxy clusters and causes a gravitational redshift of the light. But the gravitational influence on light has never before been measured on a cosmological scale.

"It is really wonderful. We live in an era with the technological ability to actually measure such phenomena as cosmological gravitational redshift," says astrophysicist Radek Wojtak, Dark Cosmology Centre under the Niels Bohr Institute at the University of Copenhagen.

Galaxy clusters in the searchlight

Radek Wojtak, together with colleagues Steen Hansen and Jens Hjorth, has analysed measurements of light from galaxies in approximately 8,000 galaxy clusters. Galaxy clusters are accumulations of thousands of galaxies, held together by their own gravity. This gravity affects the light being sent out into space from the galaxies.

The researchers have studied the galaxies lying in the middle of the galaxy clusters and those lying on the periphery and measured the wavelengths of the light.

"We could measure small differences in the redshift of the galaxies and see that the light from galaxies in the middle of a cluster had to 'crawl' out through the gravitational field, while it was easier for the light from the outlying galaxies to emerge," explains Radek Wojtak.

Then he measured the entire galaxy cluster's total mass and with that got the gravitational potential. By using the general theory of relativity he could now calculate the gravitational redshift for the different locations of the galaxies.

"It turned out that the theoretical calculations of the gravitational redshift based on the general theory of relativity was in complete agreement with the astronomical observations. Our analysis of observations of galaxy clusters show that the redshift of the light is proportionally offset in relation to the gravitational influence from the galaxy cluster's gravity. In that way our observations confirm the theory of relativity," explains Radek Wojtak.

New light on the dark universe

The discovery has significance for the phenomena in the universe that researchers are working to unravel. It is the mysterious dark universe -- dark matter and dark energy.

In addition to the visible celestial bodies like stars, planets and galaxies, the universe consists of a large amount of matter, which researchers can work out that it must be there, but which cannot be observed as it neither emits nor reflects light. It is invisible and is therefore called dark matter. No one knows what dark matter is, but they know what the mass and thus the gravity must be. The new results for gravitational redshift do not change the researchers' modelling for the presence of dark matter.

Another of the main components of the universe is dark energy, which according to the theoretical models acts like a kind of vacuum that causes the expansion of the universe to accelerate. According to the calculations, which are based on Einstein's theory of relativity, dark energy constitutes 72 percent of the structure of the universe. Many alternative theories try to explain the accelerating expansion without the presence of dark energy.

Theory tested on a large scale

"Now the general theory of relativity has been tested on a cosmological scale and this confirms that the general theory of relativity works and that means that there is a strong indication for the presence of dark energy," explains Radek Wojtak.

The new gravitation results thus contribute a new piece of insight to the understanding of the hidden, dark universe and provide a greater understanding of the nature of the visible universe.

Recommend this story on Facebook, Twitter,
and Google +1:

Other bookmarking and sharing tools:

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Copenhagen, via EurekAlert!, a service of AAAS.

Journal Reference:

Radoslaw Wojtak, Steen H. Hansen, Jens Hjorth. Gravitational redshift of galaxies in clusters as predicted by general relativity. Nature, 2011; 477 (7366): 567 DOI: 10.1038/nature10445

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Physicists apply Einstein's general theory of relativity to superconducting circuits

ScienceDaily (June 11, 2011) — In recent years, UC Santa Barbara scientists showed that they could reproduce a basic superconductor using Einstein's general theory of relativity. Now, using the same theory, they have demonstrated that the Josephson junction could be reproduced. The results are explained in a recent issue of the journal Physical Review Letters.

The Josephson junction, a device that was first discovered by Brian David Josephson in the early 1960's, is a main ingredient in applications of superconductivity.

Gary Horowitz, professor of physics at UCSB, said that Einstein's general theory of relativity -- which was developed as a theory of gravity and is extremely successful in explaining a wide variety of gravitational phenomena -- is now being used to explain several aspects of non-gravitational physics.

"The basic phenomenon with Josephson junctions is that you can take two superconductors, separate them by a little gap, and still find current going across it, in a specific way," said Horowitz. "And that has found many applications. So the Josephson junction is something we've reproduced using general relativity."

Horowitz said that he and his co-authors used tools from string theory to develop the gravity model of a superconductor. He explained that it was surprising to be able to link Einstein's general theory of relativity to a totally different area of physics. He said he hoped that the new tools would one day be able to shed light on new types of superconductors.

"Most materials, if you cool them down sufficiently, will actually conduct electricity without any resistance," said Horowitz. "These are superconductors. There is a standard theory of superconductivity, discovered about 50 years ago, that has worked well for most of the so-called conventional superconductors."

A new class of materials was discovered 25 years ago. These are superconductors that have zero resistance at somewhat higher temperatures. Physicists are still working on understanding the mechanism.

This new class of materials involves copper-oxygen planes. Another new class of superconductors, based on iron instead of copper, was discovered a couple of years ago. These materials, called iron nictides, also have the property of superconducting at a higher temperature.

"There is a lot of activity and interest in understanding these materials," said Horowitz. "Ultimately, the goal is to have a room-temperature superconductor, which, you can imagine, would have lots of interesting applications."

Horowitz and his research team found what could be called a gravitational model, or a gravitational dual -- a dual description of a superconductor using gravity, black holes, and all of the traditional ingredients of general relativity. "This came as quite a surprise because this is a totally different area of physics, which is now being connected to this condensed matter area," said Horowitz.

The co-authors of the paper are postdoctoral fellow Jorge E. Santos and graduate student Benson Way.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of California - Santa Barbara.

Journal Reference:

Gary Horowitz, Jorge Santos, Benson Way. Holographic Josephson Junctions. Physical Review Letters, 2011; 106 (22) DOI: 10.1103/PhysRevLett.106.221601

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Physicists apply Einstein's general theory of relativity to superconducting circuits

ScienceDaily (June 11, 2011) — In recent years, UC Santa Barbara scientists showed that they could reproduce a basic superconductor using Einstein's general theory of relativity. Now, using the same theory, they have demonstrated that the Josephson junction could be reproduced. The results are explained in a recent issue of the journal Physical Review Letters.

The Josephson junction, a device that was first discovered by Brian David Josephson in the early 1960's, is a main ingredient in applications of superconductivity.

Gary Horowitz, professor of physics at UCSB, said that Einstein's general theory of relativity -- which was developed as a theory of gravity and is extremely successful in explaining a wide variety of gravitational phenomena -- is now being used to explain several aspects of non-gravitational physics.

"The basic phenomenon with Josephson junctions is that you can take two superconductors, separate them by a little gap, and still find current going across it, in a specific way," said Horowitz. "And that has found many applications. So the Josephson junction is something we've reproduced using general relativity."

Horowitz said that he and his co-authors used tools from string theory to develop the gravity model of a superconductor. He explained that it was surprising to be able to link Einstein's general theory of relativity to a totally different area of physics. He said he hoped that the new tools would one day be able to shed light on new types of superconductors.

"Most materials, if you cool them down sufficiently, will actually conduct electricity without any resistance," said Horowitz. "These are superconductors. There is a standard theory of superconductivity, discovered about 50 years ago, that has worked well for most of the so-called conventional superconductors."

A new class of materials was discovered 25 years ago. These are superconductors that have zero resistance at somewhat higher temperatures. Physicists are still working on understanding the mechanism.

This new class of materials involves copper-oxygen planes. Another new class of superconductors, based on iron instead of copper, was discovered a couple of years ago. These materials, called iron nictides, also have the property of superconducting at a higher temperature.

"There is a lot of activity and interest in understanding these materials," said Horowitz. "Ultimately, the goal is to have a room-temperature superconductor, which, you can imagine, would have lots of interesting applications."

Horowitz and his research team found what could be called a gravitational model, or a gravitational dual -- a dual description of a superconductor using gravity, black holes, and all of the traditional ingredients of general relativity. "This came as quite a surprise because this is a totally different area of physics, which is now being connected to this condensed matter area," said Horowitz.

The co-authors of the paper are postdoctoral fellow Jorge E. Santos and graduate student Benson Way.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of California - Santa Barbara.

Journal Reference:

Gary Horowitz, Jorge Santos, Benson Way. Holographic Josephson Junctions. Physical Review Letters, 2011; 106 (22) DOI: 10.1103/PhysRevLett.106.221601

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here