 Verfasst am: 18. Mai 2016 20:26 Titel: |
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In diesem Fall hat man eine spezielle Theorie der Quantengravitation und aus der folgt, dass Gravitation unter bestimmten Bedingungen abstossend sein kann. |
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| Verfasst am: 18. Mai 2016 19:51 Titel: |
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| ich kann mich sogar daran erinnern, daß (sinngemäß) behauptet wurde, daß, sobald die Gravitation in Abstoßung umschlägt, deren Energie so gewaltig wird daß selbst sämtliche Materie des Universum dieser Abstoßung nicht entgegenwirken könne... solche, geradezu fantastischen Behauptungen, merke ich mir besonders^^
Mit etwas Zeit find ich die Passage bestimmt, aber heute vermutlich nicht mehr... |
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| Verfasst am: 18. Mai 2016 19:46 Titel: |
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| hier 2 links zu QuantenGeometry:
https://en.wikipedia.org/wiki/Quantum_geometry https://de.wikipedia.org/wiki/Quantengeometrie Interressanterweise sind die Inhalte & auch manche Schlußfolgerungen anders... leider steht nichts zur Gravitation drinnen. Wobei ich Dir schon glaub, aber woher rührt dann der abstossende Effekt beim BigBounce? Oder mal anders herum gefragt - warum nehmen die an, daß in diesem Modell es überhaupt zu einem Bounce kommt und nicht einfach zu nem Crunch? (damit wären wir direkt bei der SL Singularität-Thematik) |
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| Verfasst am: 18. Mai 2016 19:28 Titel: |
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| https://www.physicsforums.com/threads/conformal-gravity.55804/
http://arxiv.org/PS_cache/gr-qc/pdf/0001/0001011.pdf
Ja, ich glaube daß so gut wie alle der Beiträge dazu irgendwie mit der Schleifenquantengravtheorie zu tun haben. Also gilt es nicht für das Standardmodell aber in einem der obigen Artikel wurde glaube ich zumindest erwähnt daß eine "Kombatibilität" mit der ART da ist - was auch irgendwie logisch erscheint denn sämtliche (?) Theorien zur Quantifizierung der Gravitation wollen ja eine Brücke zwischen ART und QM basteln. Was mich halt in diesem Zusammenjang interressiert - wie die drauf kommen daß die Gravitation plötzlich so stark abstossend wirkt? Wie kann man das von den Naturgesetzen herleiten? Vielleicht verwechsel ich auch da was... in einem anderen Bericht, den ich gerade erst gelesen habe (auf der Suche nach den Quellen) ist die Rede von einer Art VakuumEnergiedichte - welche nicht unendlich komprimierbar ist - und die nahe beim Urknall also einem extrem starkem Druck entsprach (verantwortlich für die inflationäre Expansion) welche dann durch diesen Vorgang stark nachließ... hat aber nichts mit einer sich polarisierenden Gravitation zu tun.... |
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| Verfasst am: 18. Mai 2016 19:16 Titel: |
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| https://en.wikipedia.org/wiki/Big_Bounce
Recent developments in the theory Martin Bojowald, an assistant professor of physics at Pennsylvania State University, published a study in July 2007 detailing work somewhat related to loop quantum gravity that claimed to mathematically solve the time before the Big Bang, which would give new weight to the oscillatory universe and Big Bounce theories.[4] One of the main problems with the Big Bang theory is that at the moment of the Big Bang, there is a singularity of zero volume and infinite energy. This is normally interpreted as the end of the physics as we know it; in this case, of the theory of general relativity. This is why one expects quantum effects to become important and avoid the singularity. However, research in loop quantum cosmology purported to show that a previously existing universe collapsed, not to the point of singularity, but to a point before that where the quantum effects of gravity become so strongly repulsive that the universe rebounds back out, forming a new branch. Throughout this collapse and bounce, the evolution is unitary. |
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| Verfasst am: 18. Mai 2016 19:15 Titel: |
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Das entscheidende findet sich am Anfang des Satzes:
Hier geht es um eine sehr spezielle Theorie der Quantengravitation, genannt "quantum geometry". i.A. ist die Aussage dass Gravitation irgendwann negativ wird/werden kann jedenfalls nicht richtig. |
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| Verfasst am: 18. Mai 2016 19:02 Titel: |
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| komisch, ich jab das .pdf zweimal (der gleiche Titel "Space & Time) aber der obige .pdf ist als Textform und unterscheidet sich vom anderen, wo die Seiten als Bilder gespeichert sind. Beide unterscheiden sich im Text obwohl die Kapitel gleich betitelt sind...
vllt. find ich nen Direktlink dazu aber da steht (nur kurz OCR klappt nicht) Beyond Einstein [...] However, very near the BigBang the approximitation breaks down completely. In Quantum Geometry, spacetime curvature does become very large in this Plank regime, but not infinite. Gravity, in fact, becomes repulsive. General Relativity breaks down. [...] das wurde für Science Journal, Summer 2005 www.science.psu.edu/journal geschrieben oder zusammengefaßt |
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| Verfasst am: 18. Mai 2016 18:46 Titel: |
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| Das ist der Auszug eines .pdfs der auf meiner Festplatte lagert. Ich hab vor ca. nem halben Jahr über dieses Thema etwas gelesen, und die Notation, daß Gravitation abstossend werden kann, mehrfach gelesen, auch wenn ich jetzt Mühe habe, die Quellen wiederzufinden.
Aber ich glaube auch sogar in der englischen Wikipedia in irgendweinem Artikel der darum handelt, die 4 Grundkräfte der Physik in einer Urkraft zu vereinen. Ich dachte eigtl. daß das schon bewiesen sei... aber kA ich kann ja nochmal nachschauen vllt. find ichs. |
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| Verfasst am: 18. Mai 2016 18:42 Titel: |
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| also hier wäre mal einer:
Space and Time: From Antiquity to Einstein and Beyond Abhay Ashtekar1,2,3 1Institute for Gravitational Physics and Geometry, Physics Department, Penn State, University Park, PA 16802, U.S.A. 2Inter-University Centre for Astronomy and Astrophysics Post bag 4, Ganeshkhind, Pune 411 017, India 3 Institute for Theoretical Physics, University of Utrecht, Princetonplein5, 3584 CC Utrecht, The Netherlands [...] IV. BEYOND EINSTEIN A really new field of experience will always lead to crystallization of a new system of scientific concepts and laws.When faced with essentially new intellectual challenges, we continually follow the example of Columbus who possessed the courage to leave the known world in an almost insane hope of finding land again beyond the sea. —W. Heisenberg. Recent Changes in the Foundation of Exact Science General relativity is the best theory of gravitation and space-time structure we have today. It can account for a truly impressive array of phenomena [1, 2] ranging from the grand cosmic expansion to the functioning of a mundane global positioning system on earth. But it is incomplete because it ignores quantum effects that govern the sub-atomic world. Moreover, the two theories are dramatically different. The world of general relativity has geometric precision, it is deterministic; the world of quantum physics is dictated by fundamental uncertainties, it is probabilistic. Physicists maintain a happy, schizophrenic attitude, using general relativity to describe the large scale phenomena of astronomy and cosmology and quantum mechanics to account for properties of atoms and elementary particles. This is a viable strategy because the two worlds rarely meet. Nonetheless, from a conceptual standpoint, this is highly unsatisfactory. Everything in our experience as physicists tells us that there should be a grander, more complete theory from which general relativity and quantum physics arise as special, limiting cases. This would be the quantum theory of gravity. It would take us beyond Einstein.2 At the big-bang and black hole singularities the world of the very large and of the very small meet. Therefore, although they seem arcane notions at first, these singularities are our gates to go beyond general relativity. It is now widely believed that real physics can not stop there. Rather, general relativity fails. We need to dramatically revise, once again, our notions of space and time. We need a new syntax. Creation of this syntax is widely regarded as the greatest and the most fascinating challenge faced by fundamental physics today. There are several approaches. While they generally agree on a broad list of goals, each focuses on one or two features as the central ones, to 2 Contrary to the common belief —rooted in Einstein’s later views on incompleteness of quantum mechanics— he was quite aware of this limitation of general relativity. Remarkably, he pointed out the necessity of a quantum theory of gravity already in 1916! In a paper in the Preussische Akademie Sitzungsberichte he wrote: “Nevertheless, due to the inneratomic movement of electrons, atoms would have to radiate not only electromagnetic but also gravitational energy, if only in tiny amounts. As this is hardly true in Nature, it appears that quantum theory would have to modify not only Maxwellian electrodynamics but also the new theory of gravitation.” be resolved first, in the hope that the other problems ‘would take care of themselves’ once the ‘core’ is well-understood. Here, I will focus on loop quantum gravity which originated in our group some twenty years ago and has been developed by about two dozen groups world wide [5]. It is widely regarded as one of the two leading approaches, the other being string theory [7]. In general relativity, space-time is modelled by a continuum. The new idea is that this is only an approximation, which would break down at the so called Planck scale —the unique length, `pl = p G~/c3 10-33 cm, that can be constructed from Newton’s constant of Gravitation G, Planck’s constant ~ of quantum physics and the speed of light c. This scale is truly minute, some 20 orders of magnitude smaller than the radius of a proton.3 Therefore, even in the highest energy particle accelerators on earth, one can safely work with a continuum. But the approximation would break down in more extreme situations, in particular, near the big-bang and inside black holes. There, one must use a quantum space-time of loop quantum gravity. What is quantum space-time? Look at the sheet of paper in front of you. For all practical purposes, it seems continuous. Yet we know that it is made of atoms. It has a discrete structure which becomes manifest only if you zero-in using, say, an electron microscope. Now, Einstein taught us that geometry is also a physical entity, on par with matter. Therefore, it should also have an atomic structure. To unravel it, in the mid 90’s researchers combined the principles of general relativity with quantum physics to develop a quantum theory of geometry. Just as differential geometry provides the mathematical language to formulate and analyze general relativity, quantum geometry provides the mathematical tools and physical concepts to describe quantum space-times [5, 6]. In quantum geometry, the primary objects —the fundamental excitations of geometry— are one dimensional. Just as a piece of cloth appears to be a smooth, two dimensional continuum although it is obviously woven by one dimensional threads, physical space appears as a three dimensional continuum, although it is in fact a coherent superposition of these one dimensional excitations. Intuitively, then, these fundamental excitations can be thought of as quantum threads which can be woven to create the fabric of space-time. What happens, then, near space-time singularities? There, the continuum approximation fails. The quantum fluctuations are so huge that quantum threads can no longer be frozen into a coherent superposition. The fabric of space-time is ruptured. Continuum physics rooted in this fabric stops. But the quantum threads are still meaningful. Using a quantum generalization of Einstein’s equations one can still do physics, describe what happens in the quantum world. In the absence of a space-time continuum, many of the notions habitually used in physics are no longer available. New concepts have to be introduced, new physical intuition has to be honed. In this adventure, quantum Einstein’s equations pave the way. Using these equations recently the big-bang has been analyzed in some detail (see, e.g., [6, 8]). It turns out that the partial differential equations of Einstein’s, adapted to the continuum, have to be replaced by difference equations, adapted to the discrete structures of quantum geometry. Except very near the big-bang, equations of general relativity provide an excellent approximation to the more fundamental ones. However, the approximation breaks down completely near the big-bang, when the density of matter approaches the Planck density pl = c3/G2~ 1094gm/cc. In quantum geometry, space-time curvature does become very large in this Planck regime, but not infinite. Very surprisingly, quantum geometry effects give rise to a new repulsive force, which is so strong that it overwhelms the usual gravitational attraction. General relativity breaks down. The universe bounces back. But quantum Einstein’s equations enable us to evolve the quantum state of geometry and matter right through this Planck regime. The big bang is replaced by a quantum bounce. FIG. 3: An artist’s representation of the extended space-time of loop quantum cosmology. Time again runs vertically. General relativity provides only the top half of this space-time which originates in the big-bang (see figure 2). Quantum Einstein’s equations extend this space-time to the past of the big-bang. The pre-big-bang branch is contracting and the current post-big-bang branch is expanding. The band in the middle represents the ‘quantum bridge’ which joins the two branches and provides a deterministic evolution across the ‘deep Planck regime’. Reliable numerical calculations have been performed in the strict spatially homogeneous isotropic case. Continuum turns out to be a good approximation outside the Planck regime also on the ‘other side of the big-bang’ [6, 8]. More precisely, in a forward-in-time motion picture of the universe, there is a contracting pre-big-bang branch well described by general relativity. However, when the matter density is approximately 0.8pl, the repulsive force of quantum geometry, which is negligible until then, now becomes dominant. Instead of continuing the contraction into a big-crunch, the universe undergoes a big bounce, joining on to the post-big-bang expanding branch we now live in. Classical general relativity describes both branches very well, except in the deep Planck regime. There the two branches are joined by a quantum bridge, governed by quantum geometry. The emergence of a new repulsive, quantum force has a curious similarity with the repulsive force in the stellar collapse discussed in section III. There, a repulsive force comes into play when the core approaches a critical density, crit 6 × 1016gms/cc, and can halt further collapse, leading to stable neutron stars. This force, with its origin in the Pauli exclusion principle, is associated with the quantum nature of matter. However, as indicated in section III, if the total mass of the star is larger than, say, 5 solar masses, classical gravity overwhelms this force. The quantum geometry repulsive force comes into play at much higher densities. But then it overwhelms the standard gravitational attraction, irrespective of how massive the collapsing body is. Indeed, the body could be the whole universe! The perspective of loop quantum gravity is that it is this effect that prevents the formation of singularities in the real world, extending the ‘life’ of space-time through a quantum bridge. Currently, work is under way to extend these results to more and more sophisticated models which incorporate inhomogeneities of the present day universe. If the above scenario turns out to be robust, there will be fascinating philosophical implications for the issue of the Beginning and the End. For, the very paradigm to pose questions will again be shifted. If the questions refer to the notion of time that Einstein gave us, there was indeed a Beginning. Not at the big-bang though, but ‘a little later’ when space-time can be modelled as a continuum. But if by Beginning one means a firm boundary beyond which physical predictions are impossible, then the answer is very different from that given by general relativity: in the more complete theory, there is no such Beginning. To summarize then, thanks to Einstein, our understanding of space and time underwent a dramatic revision in the 20th century. Geometry suddenly became a physical entity, like matter. This opened-up entirely new vistas in cosmology and astronomy. But a new paradigm shift awaits us again in the 21st century. Thanks to quantum geometry, the big-bang and black hole singularities are no longer final frontiers. The physical, quantum space-time is much larger than what general relativity had us believe. The existence of these new and potentially vast unforeseen domains has already provided a fresh avenue to resolve several long standing, problems concerning both cosmology and black holes in fundamental physics. Even more exciting opportunities arise from new questions and the rich possibilities that this extension opens up. |
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| Verfasst am: 18. Mai 2016 16:03 Titel: |
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| Also aus der ART kommt das mit Sicherheit nicht. Dort gibt es zwar abstoßende Gravitation durch die kosmologische Konstante, aber die macht sich eben bei hohen Energiedichten gerade nicht bemerkbar, sondern nur auf großen Skalen, wo die Energiedichten klein sind. | |
| Verfasst am: 18. Mai 2016 15:50 Titel: Re: Abstossende Gravitation? |
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Könntest Du dazu eine Quelle angeben? |
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| Verfasst am: 18. Mai 2016 15:27 Titel: Abstossende Gravitation? |
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| Anscheinend soll die Gravitation bei sehr hoher Energiedichte abstossend wirken, und zwar extrem abstossend...
Ich konnte das jetzt mehrmals im englischen lesen allerdings wurde es dort nie näher erläutert und bei den Wiki-Artikeln direkt zur Schwerkraft steht nichts darüber... Also weiß jmd. vielleicht wie man darauf kommt, wurde das schon experimentell bestätigt bzw. wenn es nur eine Theorie ist, aus welcher "Ecke" kommt das denn (also ist das Teil der ART oder Strings/QLG oder anderen Quantengravitationstheorien) und welche praktische Bedeutung hätte das schlußendlich? Könnte man mit diesem Effekt evtl. eine kurzfristige Anti-Gravitation erzeugen und diese zur Beschleunigung verwenden? Oder ist die erforderliche Energiemenge nicht im menschlichen Bereich? Und wenn das so ist - warum fliegen dann Schwarze Löcher nicht sofort auseinander? Oder zumindest - die Urknallsingularität - anscheinend lassen sich sämtliche Grundkräfte der Physik zu einer vereinen - und die sich eben bei sinkender Energie sukzessiv abspalten. Wenn ich es richtig verstanden habe war die Gravitation die erste Kraft die sich direkt nach dem Urknall losgelöst hatte - war diese zu diesem Zeitpunk anziehend oder abstossend? Wenn sie abstossend wäre, und tatsächlich auch so stark (im Gegensatz zur relativ schwachen anziehenden Gravitation) müßte das nicht letztendlich bedeuten daß der Urknall zumindest zum Teil als tatsächliche Explosion abgelaufen ist? |
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