Dienstag, 5. April 2011
Sechsten Symmetrietyp gefunden?
klauslange,00:10h
Davon berichtet zumindest science daily: hier.
Zitat:
Before the publication of this paper, scientists and engineers had five different types of symmetries to use as tools for understanding the structures of materials whose building blocks are arranged in fairly regular patterns. Four types of symmetries had been known for thousands of years -- called rotation, inversion, rotation inversion, and translation -- and a fifth type -- called time reversal -- had been discovered about 60 years ago. Now, Gopalan and Litvin have added a new, sixth, type, called rotation reversal. As a result, the number of known ways in which the components of such crystalline materials can be combined in symmetrical ways has multiplied from no more than 1,651 before to more than 17,800 now. "We mathematically combined the new rotation-reversal symmetry with the previous five symmetries and now we know that symmetrical groups can form in crystalline materials in a much larger number of ways," said Daniel B. Litvin, distinguished professor of physics, who coauthored the study with Venkatraman Gopalan, professor of materials science and engineering.
The new rotation-reversal symmetry enriches the mathematical language that researchers use to describe a crystalline material's structure and to predict its properties. "Rotation reversal is an absolutely new approach that is different in that it acts on a static component of the material's structure, not on the whole structure all at once," Litvin said. "It is important to look at symmetries in materials because symmetry dictates all natural laws in our physical universe."
The most simple type of symmetry -- rotation symmetry -- is obvious, for example, when a square shape is rotated around its center point: the square shows its symmetrical character by looking exactly the same at four points during the rotation: at 90 degrees, 180 degrees, 270 degrees, and 360 degrees. Gopalan and Litvin say their new rotation-reversal symmetry is obvious, as well, if you know where to look.
The "eureka moment" of the discovery occurred when Gopalan recognized that the simple concept of reversing the direction of a spiral-shaped structure from clockwise to counterclockwise opens the door to a distinctly new type of symmetry. Just as a square shape has the quality of rotation symmetry even when it is not being rotated, Gopalan realized that a spiral shape has the quality of rotation-reversal symmetry even when it is not being physically forced to rotate in the reverse direction. Their further work with this rotation-reversal concept revealed many more structural symmetries than previously had been recognized in materials containing various types of directionally oriented structures. Many important biological molecules, for example, are said to be either "right handed" or "left handed," including DNA, sugars, and proteins.
"We found that rotation-reversal symmetry also exists in paired structures where the partner components lean toward each other, then away from each other in paired patterns symmetrically throughout a material," Gopalan said. These "tilting octahedral" structures are common in a wide variety of crystalline materials, where all the component structures are tightly interconnected by networks of shared atoms. The researchers say it is possible that components of materials with rotation-reversal symmetry could be engineered to function as on/off switches for a variety of novel applications.
The now-much-larger number of possible symmetry groups also is expected to be useful in identifying materials with unusual combinations of properties. "For example, the goal in developing a ferroelectric ferromagnet is to have a material in which the electrical dipoles and the magnetic moments coexist and are coupled in the same material -- that is, a material that allows electrical control of magnetism -- which would be very useful to have in computers," Gopalan said. The addition of rotation-reversal symmetry to the materials-science toolbox may help researchers to identify and search for structures in materials that could have strong ferroelectric and ferromagnetic properties.
Zitat:
Before the publication of this paper, scientists and engineers had five different types of symmetries to use as tools for understanding the structures of materials whose building blocks are arranged in fairly regular patterns. Four types of symmetries had been known for thousands of years -- called rotation, inversion, rotation inversion, and translation -- and a fifth type -- called time reversal -- had been discovered about 60 years ago. Now, Gopalan and Litvin have added a new, sixth, type, called rotation reversal. As a result, the number of known ways in which the components of such crystalline materials can be combined in symmetrical ways has multiplied from no more than 1,651 before to more than 17,800 now. "We mathematically combined the new rotation-reversal symmetry with the previous five symmetries and now we know that symmetrical groups can form in crystalline materials in a much larger number of ways," said Daniel B. Litvin, distinguished professor of physics, who coauthored the study with Venkatraman Gopalan, professor of materials science and engineering.
The new rotation-reversal symmetry enriches the mathematical language that researchers use to describe a crystalline material's structure and to predict its properties. "Rotation reversal is an absolutely new approach that is different in that it acts on a static component of the material's structure, not on the whole structure all at once," Litvin said. "It is important to look at symmetries in materials because symmetry dictates all natural laws in our physical universe."
The most simple type of symmetry -- rotation symmetry -- is obvious, for example, when a square shape is rotated around its center point: the square shows its symmetrical character by looking exactly the same at four points during the rotation: at 90 degrees, 180 degrees, 270 degrees, and 360 degrees. Gopalan and Litvin say their new rotation-reversal symmetry is obvious, as well, if you know where to look.
The "eureka moment" of the discovery occurred when Gopalan recognized that the simple concept of reversing the direction of a spiral-shaped structure from clockwise to counterclockwise opens the door to a distinctly new type of symmetry. Just as a square shape has the quality of rotation symmetry even when it is not being rotated, Gopalan realized that a spiral shape has the quality of rotation-reversal symmetry even when it is not being physically forced to rotate in the reverse direction. Their further work with this rotation-reversal concept revealed many more structural symmetries than previously had been recognized in materials containing various types of directionally oriented structures. Many important biological molecules, for example, are said to be either "right handed" or "left handed," including DNA, sugars, and proteins.
"We found that rotation-reversal symmetry also exists in paired structures where the partner components lean toward each other, then away from each other in paired patterns symmetrically throughout a material," Gopalan said. These "tilting octahedral" structures are common in a wide variety of crystalline materials, where all the component structures are tightly interconnected by networks of shared atoms. The researchers say it is possible that components of materials with rotation-reversal symmetry could be engineered to function as on/off switches for a variety of novel applications.
The now-much-larger number of possible symmetry groups also is expected to be useful in identifying materials with unusual combinations of properties. "For example, the goal in developing a ferroelectric ferromagnet is to have a material in which the electrical dipoles and the magnetic moments coexist and are coupled in the same material -- that is, a material that allows electrical control of magnetism -- which would be very useful to have in computers," Gopalan said. The addition of rotation-reversal symmetry to the materials-science toolbox may help researchers to identify and search for structures in materials that could have strong ferroelectric and ferromagnetic properties.
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Samstag, 2. April 2011
Largest Black Holes
klauslange,17:44h
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Donnerstag, 31. März 2011
'Survival of the fittest' nicht allgemeingültig
klauslange,00:34h
Eine der Hauptlehrsätze der darwinschen Evolution ist jener vom Überleben der Bestangepassten in einer gegebenen Umgebung. In 'Nature' wurde nun ein Artikel veröffentlicht, dass diese Behauptung eindrücklich anhand von Bakterienkulturen zu widerlegen in der Lage ist: Evolution: Not Only the Fittest Survive
Zitat:
Conventional wisdom has it that for any given niche there should be a best species, the fittest, that will eventually dominate to exclude all others.
This is the principle of survival of the fittest. Ecologists often call this idea the `competitive exclusion principle' and it predicts that complex environments are needed to support complex, diverse populations.
Professor Robert Beardmore, from the University of Exeter, said: "Microbiologists have tested this principle by constructing very simple environments in the lab to see what happens after hundreds of generations of bacterial evolution, about 3,000 years in human terms. It had been believed that the genome of only the fittest bacteria would be left, but that wasn't their finding. The experiments generated lots of unexpected genetic diversity."
This test tube biodiversity proved controversial when first observed and had been explained away with claims that insufficient time had been allowed to pass for a clear winner to emerge.
The new research shows the experiments were not anomalies.
Professor Laurence Hurst, of the University of Bath, said: "Key to the new understanding is the realization that the amount of energy organisms squeeze out of their food depends on how much food they have. Give them abundant food and they use it inefficiently. When we combine this with the notion that organisms with different food-utilizing strategies are also affected in different ways by genetic mutations, then we discover a new principle, one in which both the fit and the unfit coexist indefinitely."
Dr Ivana Gudelj, also from the University of Exeter, said: "The fit use food well but they aren't resilient to mutations, whereas the less efficient, unfit consumers are maintained by their resilience to mutation. If there's a low mutation rate, survival of the fittest rules, but if not, lots of diversity can be maintained.
"Rather nicely, the numbers needed for the principle to work accord with those enigmatic experiments on bacteria. Their mutation rate seems to be high enough for both fit and unfit to be maintained."
Zitat:
Conventional wisdom has it that for any given niche there should be a best species, the fittest, that will eventually dominate to exclude all others.
This is the principle of survival of the fittest. Ecologists often call this idea the `competitive exclusion principle' and it predicts that complex environments are needed to support complex, diverse populations.
Professor Robert Beardmore, from the University of Exeter, said: "Microbiologists have tested this principle by constructing very simple environments in the lab to see what happens after hundreds of generations of bacterial evolution, about 3,000 years in human terms. It had been believed that the genome of only the fittest bacteria would be left, but that wasn't their finding. The experiments generated lots of unexpected genetic diversity."
This test tube biodiversity proved controversial when first observed and had been explained away with claims that insufficient time had been allowed to pass for a clear winner to emerge.
The new research shows the experiments were not anomalies.
Professor Laurence Hurst, of the University of Bath, said: "Key to the new understanding is the realization that the amount of energy organisms squeeze out of their food depends on how much food they have. Give them abundant food and they use it inefficiently. When we combine this with the notion that organisms with different food-utilizing strategies are also affected in different ways by genetic mutations, then we discover a new principle, one in which both the fit and the unfit coexist indefinitely."
Dr Ivana Gudelj, also from the University of Exeter, said: "The fit use food well but they aren't resilient to mutations, whereas the less efficient, unfit consumers are maintained by their resilience to mutation. If there's a low mutation rate, survival of the fittest rules, but if not, lots of diversity can be maintained.
"Rather nicely, the numbers needed for the principle to work accord with those enigmatic experiments on bacteria. Their mutation rate seems to be high enough for both fit and unfit to be maintained."
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Freitag, 18. März 2011
Merkur - Sonde im Orbit!
klauslange,23:04h
Erstmalig ist es gelungen eine Raumsonde in den Orbit des Merkur zu platzieren. Es gab in der Vergangenheit Vorbeiflüge am Merkur, aber noch keine Sonde im Orbit des sonnennächsten Planeten.
Berichte von
astronews.com
raumfahrer.net
Berichte von
astronews.com
raumfahrer.net
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Samstag, 5. März 2011
Mögliche Wurmlöcher können Sterne miteinander verbinden
klauslange,16:31h
Eine neue Möglichkeit zur Existenz von Wurmlöchern haben zwei Kasachische Forscher berechnet, wie physorg.com berichtet:
http://www.physorg.com/news/2011-02-scientists-possibility-wormholes-stars.html
Darin
Wormholes are one of the stranger objects that arise in general relativity. Although no experimental evidence for wormholes exists, scientists predict that they would appear to serve as shortcuts between one point of spacetime and another. Scientists usually imagine wormholes connecting regions of empty space, but now a new study suggests that wormholes might exist between distant stars. Instead of being empty tunnels, these wormholes would contain a perfect fluid that flows back and forth between the two stars, possibly giving them a detectable signature.
http://www.physorg.com/news/2011-02-scientists-possibility-wormholes-stars.html
Darin
Wormholes are one of the stranger objects that arise in general relativity. Although no experimental evidence for wormholes exists, scientists predict that they would appear to serve as shortcuts between one point of spacetime and another. Scientists usually imagine wormholes connecting regions of empty space, but now a new study suggests that wormholes might exist between distant stars. Instead of being empty tunnels, these wormholes would contain a perfect fluid that flows back and forth between the two stars, possibly giving them a detectable signature.
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Mittwoch, 2. März 2011
150 Millionen Weltraumtrümmer
klauslange,20:45h
Aufgrund von Raumfahrtaktivitäten, die ich sehr befürworte, befinden sich schon 150 Millionen Trümmer von Weltraumschrott im erdnahen Raum:
http://www.n-tv.de/wissen/Immer-mehr-Muell-im-All-article2721536.html
Rund 150 Millionen Trümmerteile, jedes mindestens einen Millimeter groß, rasen als Weltraumschrott um die Erde. Und es werden immer mehr. Für Satelliten und bemannte Raumfahrzeugen wird die Gefahr von Kollisionen größer. "Noch ist das Risiko nicht dramatisch", meint Prof. Peter Vörsmann vom Institut für Luft- und Raumfahrtsysteme an der Technischen Universität (TU) Braunschweig. Aber wenn man in der Raumfahrt so weiter mache wie bisher, dann werde es künftig sicher mehr Kollisionen geben.
Braunschweiger Forscher sind neben der US-Raumfahrtbehörde NASA weltweit führend, was die Untersuchung des Mülls im All betrifft. Sie haben im Auftrag der Europäischen Raumfahrtagentur ESA ein Simulationsmodell erarbeitet, dass die Verteilung und Bewegung des Weltraumschrotts heute und in Zukunft darstellt.
ISS weicht regelmäßig aus
Es wird von Organisationen und Unternehmen der Raumfahrt weltweit für Raumfahrtmissionen genutzt. Beispielsweise müssen schon heute viele der rund 900 aktiven Satelliten im All und auch die bemannte Internationale Raumstation ISS regelmäßig Ausweichmanöver wegen nahender Trümmerteile fliegen.
"Heute liegt die Wahrscheinlichkeit, dass ein Satellit bei sieben Jahren Lebensdauer von einem gefährlichen Objekt getroffen und zerstört wird, bei zwei Prozent", hat Instituts-Mitarbeiter Carsten Wiedemann ausgerechnet. Doch das Risiko wachse. Jede Kollision im All produziert wie bei einer Kettenreaktion neue Trümmer und wahrscheinlich neue Kollisionen.
Bisher sind vermutlich vier Satelliten getroffen und zerstört worden. Vor drei Jahren haben die Chinesen als Militärtest einen eigenen Satelliten auf seiner Erdumlaufbahn in 800 Kilometern Höhe mit einer Rakete abgeschossen. Danach sei dort die Trümmerzahl um 50 Prozent gestiegen - und das auf einer Bahn, auf der ohnehin schon der meiste Müll herumfliege und auf der die meisten Beobachtungssatelliten stationiert seien, sagt Vörsmann. "Es war das trümmerreichste Ereignis der Raumfahrtgeschichte."
Satelliten von Einschlägen übersät
Doch der meiste Müll entstand bisher durch unbeabsichtigte Explosionen von ausgebrannten Raketen-Oberstufen, in denen sich Resttreibstoffe entzündeten, oder durch chemische Reaktionen in nicht entladenen Batterien. Es schwirren aber auch Millionen von teilweise zentimetergroßen Schlackepartikeln herum, die bei Bahnmanövern aus Feststofftriebwerken entweichen, oder Millionen von Metalltropfen, die von Kühlmitteln aus Kernreaktoren stammen, die die Russen in den 80er Jahren in Satelliten benutzt haben.
Gefährlich für die Raumfahrt seien vor allem die rund 600.000 Müllobjekte, die größer als ein Zentimeter sind, sagt Wiedemann. Die könnten bei einem Tempo von zehn Kilometern pro Sekunde jede noch so dicke Schutzwand zerschlagen. "Die wirken wie eine Handgranate." Kleinere Teilchen können Satelliten immerhin beschädigen. Schon heute seien praktisch alle Satelliten im All von Einschlägen übersät.
http://www.n-tv.de/wissen/Immer-mehr-Muell-im-All-article2721536.html
Rund 150 Millionen Trümmerteile, jedes mindestens einen Millimeter groß, rasen als Weltraumschrott um die Erde. Und es werden immer mehr. Für Satelliten und bemannte Raumfahrzeugen wird die Gefahr von Kollisionen größer. "Noch ist das Risiko nicht dramatisch", meint Prof. Peter Vörsmann vom Institut für Luft- und Raumfahrtsysteme an der Technischen Universität (TU) Braunschweig. Aber wenn man in der Raumfahrt so weiter mache wie bisher, dann werde es künftig sicher mehr Kollisionen geben.
Braunschweiger Forscher sind neben der US-Raumfahrtbehörde NASA weltweit führend, was die Untersuchung des Mülls im All betrifft. Sie haben im Auftrag der Europäischen Raumfahrtagentur ESA ein Simulationsmodell erarbeitet, dass die Verteilung und Bewegung des Weltraumschrotts heute und in Zukunft darstellt.
ISS weicht regelmäßig aus
Es wird von Organisationen und Unternehmen der Raumfahrt weltweit für Raumfahrtmissionen genutzt. Beispielsweise müssen schon heute viele der rund 900 aktiven Satelliten im All und auch die bemannte Internationale Raumstation ISS regelmäßig Ausweichmanöver wegen nahender Trümmerteile fliegen.
"Heute liegt die Wahrscheinlichkeit, dass ein Satellit bei sieben Jahren Lebensdauer von einem gefährlichen Objekt getroffen und zerstört wird, bei zwei Prozent", hat Instituts-Mitarbeiter Carsten Wiedemann ausgerechnet. Doch das Risiko wachse. Jede Kollision im All produziert wie bei einer Kettenreaktion neue Trümmer und wahrscheinlich neue Kollisionen.
Bisher sind vermutlich vier Satelliten getroffen und zerstört worden. Vor drei Jahren haben die Chinesen als Militärtest einen eigenen Satelliten auf seiner Erdumlaufbahn in 800 Kilometern Höhe mit einer Rakete abgeschossen. Danach sei dort die Trümmerzahl um 50 Prozent gestiegen - und das auf einer Bahn, auf der ohnehin schon der meiste Müll herumfliege und auf der die meisten Beobachtungssatelliten stationiert seien, sagt Vörsmann. "Es war das trümmerreichste Ereignis der Raumfahrtgeschichte."
Satelliten von Einschlägen übersät
Doch der meiste Müll entstand bisher durch unbeabsichtigte Explosionen von ausgebrannten Raketen-Oberstufen, in denen sich Resttreibstoffe entzündeten, oder durch chemische Reaktionen in nicht entladenen Batterien. Es schwirren aber auch Millionen von teilweise zentimetergroßen Schlackepartikeln herum, die bei Bahnmanövern aus Feststofftriebwerken entweichen, oder Millionen von Metalltropfen, die von Kühlmitteln aus Kernreaktoren stammen, die die Russen in den 80er Jahren in Satelliten benutzt haben.
Gefährlich für die Raumfahrt seien vor allem die rund 600.000 Müllobjekte, die größer als ein Zentimeter sind, sagt Wiedemann. Die könnten bei einem Tempo von zehn Kilometern pro Sekunde jede noch so dicke Schutzwand zerschlagen. "Die wirken wie eine Handgranate." Kleinere Teilchen können Satelliten immerhin beschädigen. Schon heute seien praktisch alle Satelliten im All von Einschlägen übersät.
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Montag, 28. Februar 2011
Zwei Planeten auf der gleichen Umlaufbahn
klauslange,21:45h
Das ist das erste Mal, dass so etwas nachgewiesen wurde:
http://www.weltderphysik.de/de/4245.php?ni=2508
Das System KOI-730 besteht aus vier Planeten. Zwei von ihnen kreisen im gleichen Abstand um den zentralen Stern und brauchen 9,8 Tage für einen Umlauf. Die Entfernung zwischen ihnen entspricht einem Winkel von 60 Grad gemessen vom Mittelpunkt des Sterns. Genau diese Anordnung macht es möglich, dass das Paar nicht kollidiert. Astronomen nennen die möglichen Aufenthaltsorte mehrerer Körper in einem Orbit auch "Librations-" oder "Lagrange-Punkte". Von ihnen gibt es insgesamt fünf, doch nur die Positionen in einem Winkel von 60 Grad vor oder hinter einem Planeten garantieren eine stabile Bahn. Bei Jupiter in unserem Sonnensystem bewegen sich an diesen beiden Stellen Gruppen von Asteroiden, die so genannten "Trojaner".
"Systeme wie diese sind ungewöhnlich, es ist das einzige, das wir entdecken konnten", berichtet Jack Lissauer vom Ames Research Center der NASA in Kalifornien. Theorien hatten bereits vorhergesagt, dass zwei Planeten auf der gleichen Bahn entstehen können. Die weitere Untersuchung von KOI-730 ist allerdings schwierig, da der zugehörige Stern nur schwach leuchtet.
Forscher der Princeton University haben die Daten von KOI-730 weiter untersucht und berechnet, dass die beiden Planeten noch mindestens zwei Millionen Jahre weiter kreisen können, ohne sich in die Quere zu kommen.
http://www.weltderphysik.de/de/4245.php?ni=2508
Das System KOI-730 besteht aus vier Planeten. Zwei von ihnen kreisen im gleichen Abstand um den zentralen Stern und brauchen 9,8 Tage für einen Umlauf. Die Entfernung zwischen ihnen entspricht einem Winkel von 60 Grad gemessen vom Mittelpunkt des Sterns. Genau diese Anordnung macht es möglich, dass das Paar nicht kollidiert. Astronomen nennen die möglichen Aufenthaltsorte mehrerer Körper in einem Orbit auch "Librations-" oder "Lagrange-Punkte". Von ihnen gibt es insgesamt fünf, doch nur die Positionen in einem Winkel von 60 Grad vor oder hinter einem Planeten garantieren eine stabile Bahn. Bei Jupiter in unserem Sonnensystem bewegen sich an diesen beiden Stellen Gruppen von Asteroiden, die so genannten "Trojaner".
"Systeme wie diese sind ungewöhnlich, es ist das einzige, das wir entdecken konnten", berichtet Jack Lissauer vom Ames Research Center der NASA in Kalifornien. Theorien hatten bereits vorhergesagt, dass zwei Planeten auf der gleichen Bahn entstehen können. Die weitere Untersuchung von KOI-730 ist allerdings schwierig, da der zugehörige Stern nur schwach leuchtet.
Forscher der Princeton University haben die Daten von KOI-730 weiter untersucht und berechnet, dass die beiden Planeten noch mindestens zwei Millionen Jahre weiter kreisen können, ohne sich in die Quere zu kommen.
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Samstag, 26. Februar 2011
Beobachtung bestätigt MOND - Theorie
klauslange,16:02h
Eine der eventuellen Alternativen zur Dunklen Materie ist eine modifizierte newtonsche Dynamik, kurz MOND - Theory genannt. Beobachtungen bezüglich 47 Galaxien zeigen nun, dass sie mit der Theorie einer Dunklen Materie nicht vereinbar sind, wohl aber mit dieser MOND Theorie.
science daily berichtet:
hier
A few theories that would modify our understanding of gravity have been proposed. One of these is Modified Newtonian Dynamics (MOND), which was hypothesized in 1983 by Moti Milgrom a physicist at the Weizmann Institute of Science in Rehovot, Israel. One of MOND's predictions specifies the relative relationship between the mass of any galaxy and its flat rotation velocity. However, uncertainties in the estimates of masses of stars in star-dominated spiral galaxies (such as our own Milky Way) previously had precluded a definitive test.
To avoid this problem, McGaugh examined gas rich galaxies, which have relatively fewer stars and a preponderance of mass in the form of interstellar gas. "We understand the physics of the absorption and release of energy by atoms in the interstellar gas, such that counting photons is LIKE counting atoms. This gives us an accurate estimate of the mass of such galaxies," McGaugh said.
Using recently published work that he and other scientists had done to determine both the mass and flat rotation velocity of many gas rich galaxies, McGaugh compiled a sample of 47 of these and compared each galaxy's mass AND rotation velocity with the relationship expected by MOND. All 47 galaxies fell on or very close to the MOND prediction. No dark matter model performed as well.
"I find it remarkable that the prediction made by Milgrom over a quarter century ago performs so well in matching these findings for gas rich galaxies," McGaugh said. "
MOND vs. Dark Matter -- Dark Energy
Almost everyone agrees that on scales of large galaxy clusters and up, the Universe is well described by dark matter -- dark energy theory. However, according to McGaugh this cosmology does not account well for what happens at the scales of galaxies and smaller.
"MOND is just the opposite," he said. "It accounts well for the 'small' scale of individual galaxies, but MOND doesn't tell you much about the larger universe.
Of course, McGaugh said, one can start from the assumption of dark matter and adjust its models for smaller scales until it fits the current finding. "This is not as impressive as making a prediction ahead of [new findings], especially since we can't see dark matter. We can make any adjustment we need." This is rather like fitting planetary orbits with epicycles," he said. Epicycles were erroneously used by the ancient Greek scientist Ptolemy to explain observed planetary motions within the context of a theory for the universe that placed Earth in its center.
"If we're right about dark matter, why does MOND work at all?" asks McGaugh. "Ultimately, the correct theory -- be it dark matter or a modification of gravity -- needs to explain this."
science daily berichtet:
hier
A few theories that would modify our understanding of gravity have been proposed. One of these is Modified Newtonian Dynamics (MOND), which was hypothesized in 1983 by Moti Milgrom a physicist at the Weizmann Institute of Science in Rehovot, Israel. One of MOND's predictions specifies the relative relationship between the mass of any galaxy and its flat rotation velocity. However, uncertainties in the estimates of masses of stars in star-dominated spiral galaxies (such as our own Milky Way) previously had precluded a definitive test.
To avoid this problem, McGaugh examined gas rich galaxies, which have relatively fewer stars and a preponderance of mass in the form of interstellar gas. "We understand the physics of the absorption and release of energy by atoms in the interstellar gas, such that counting photons is LIKE counting atoms. This gives us an accurate estimate of the mass of such galaxies," McGaugh said.
Using recently published work that he and other scientists had done to determine both the mass and flat rotation velocity of many gas rich galaxies, McGaugh compiled a sample of 47 of these and compared each galaxy's mass AND rotation velocity with the relationship expected by MOND. All 47 galaxies fell on or very close to the MOND prediction. No dark matter model performed as well.
"I find it remarkable that the prediction made by Milgrom over a quarter century ago performs so well in matching these findings for gas rich galaxies," McGaugh said. "
MOND vs. Dark Matter -- Dark Energy
Almost everyone agrees that on scales of large galaxy clusters and up, the Universe is well described by dark matter -- dark energy theory. However, according to McGaugh this cosmology does not account well for what happens at the scales of galaxies and smaller.
"MOND is just the opposite," he said. "It accounts well for the 'small' scale of individual galaxies, but MOND doesn't tell you much about the larger universe.
Of course, McGaugh said, one can start from the assumption of dark matter and adjust its models for smaller scales until it fits the current finding. "This is not as impressive as making a prediction ahead of [new findings], especially since we can't see dark matter. We can make any adjustment we need." This is rather like fitting planetary orbits with epicycles," he said. Epicycles were erroneously used by the ancient Greek scientist Ptolemy to explain observed planetary motions within the context of a theory for the universe that placed Earth in its center.
"If we're right about dark matter, why does MOND work at all?" asks McGaugh. "Ultimately, the correct theory -- be it dark matter or a modification of gravity -- needs to explain this."
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Donnerstag, 24. Februar 2011
Geschichten und Prognosen zum Universum
klauslange,23:06h
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Mittwoch, 23. Februar 2011
Commercial Hope
klauslange,22:15h
Über die Raumfahrtpläne von Privatfirmen weiß raumfahrer.net zu berichten. Eine interessante Entwicklung:
http://www.raumfahrer.net/news/raumfahrt/22022011231045.shtml
http://www.raumfahrer.net/news/raumfahrt/22022011231045.shtml
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