theories:speculative_theories:quantum_gravity [The Physics Travel Guide]

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--- theories:speculative_theories:quantum_gravity [2018/03/24 10:54]
jakobadmin [Overview]
+++ theories:speculative_theories:quantum_gravity [2020/04/03 16:50] (current)
95.168.180.148 Changed table to list structure. In a table the entries in the same row have some relation to each other, that was no the case.
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 ====== Quantum Gravity ======
-<tabbox Why is it interesting?>
-<blockquote>
-One of the big problems in physics — perhaps the biggest! — is figuring out how our two current best theories fit together. On the one hand we have the [[models:standard_model|Standard Model]], which tries to explain all the forces except gravity, and takes quantum mechanics into account.  On the other hand we have [[theories:classical_theories:general_relativity|General Relativity]], which tries to explain gravity, and does not take [[theories:quantum_theory:quantum_mechanics|quantum mechanics]] into account. Both theories seem to be more or less on the right track — but until we somehow fit them together, or completely discard one or both, our picture of the world will be deeply schizophrenic.
-It seems plausible that as a step in the right direction we should figure out a theory of gravity that takes quantum mechanics into account, but reduces to General Relativity when we ignore quantum effects (which should be small in many situations). This is what people mean by "quantum gravity" — the quest for such a theory.
+<tabbox Intuitive>
+Quantum gravity is the generic term for theories that try to combine the principles of [[theories:quantum_mechanics:canonical|quantum theories]] with [[models:general_relativity|general relativity]].
-<cite>[[https://www.edge.org/q2008/q08_5.html#baez|John Baez]]</cite></blockquote>
+So far, no experimentally verified theory of quantum gravity exists.
-<blockquote>At the beginning of the XX century,[[theories:classical_theories:general_relativity| General Relativity]] (GR) and [[theories:quantum_theory:quantum_mechanics|Quantum Mechanics]] (QM) once
-again began reshaping our basic understanding of space and time and, respectively, matter, energy
-and causality —arguably to a no lesser extent. But we have not been able to combine these new
-insights into a novel coherent synthesis, yet. The XX century scientific revolution opened by GR and
-QM is therefore still wide open. We are in the middle of an unfinished scientific revolution. Quantum
-Gravity is the tentative name we give to the “synthesis to be found”.<cite>"[[https://arxiv.org/abs/gr-qc/0604045|Unfinished Revolution" by C. Rovelli]]</cite></blockquote>
 ----
-See also
+  * A great book on quantum gravity for laypersons is "Three Roads to Quantum Gravity" by Lee Smolin
-  * [[http://www.pbs.org/wgbh/nova/blogs/physics/2016/02/why-quantize-gravity/|Why Quantize Gravity?]] By Sabine Hossenfelder
-<tabbox Overview>
-Here's an incomplete list of popular approaches to formulate a theory of quantum gravity:
-^ Traditional Discrete Methods  ^ Traditional Approximate theories  ^ Traditional Unorthodox approaches  ^ New                        ^ Most Popular                                                   |
-| Dynamical triangulations      | Euclidean quantum gravity         | Sorkin’s Posets                    | Non-commutative geometry  | [[theories:speculative_theories:string_theory|String Theory]]  |
-| Regge calculus                | Perturbative quantum gravity      | Finkelstein                        | Null surfaces              | [[theories:speculative_theories:lqg|Loop Quantum Gravity]]     |
-| Simplicial models             | QFT on curved space–times         | Twistors                           | Spin foam models           | [[theories:speculative_theories:asymptotic_safety|Asymptotic Safety]]                                              |
-|                               |                                   |                                    |                            | [[theories:speculative_theories:supergravity|Supergravity]]|
-Table adapted from [[https://arxiv.org/abs/gr-qc/9803024|Strings, loops and others: a critical survey of the present approaches to quantum gravity]] by Carlo Rovelli
-<tabbox Layman>
-  * A great book on quantum gravity for layman is "Three Roads to Quantum Gravity" by Lee Smolin
-<tabbox Student>
+<tabbox Concrete>
 **Recommended Textbooks:**
@@ Line 55: / Line 25: @@
   * [[https://arxiv.org/abs/1010.5822|Gauge Gravity: a forward-looking introduction]] by Andrew Randono
+----
+<blockquote>From a brief analysis of the standard Einstein-Hilbert action, we see that fluctuations of the metric at the Planck scale should become very violent, leading to potential changes in the topology of the spacetime [103, 104]. <cite>https://arxiv.org/abs/1711.00864</cite></blockquote>
-<tabbox Researcher>
+<tabbox Abstract>
   * For a nice overview, see [[https://arxiv.org/abs/1708.07445|Towards the map of quantum gravity]] by Jakub Mielczarek, Tomasz Trześniewski and alos
@@ Line 64: / Line 36: @@
   * [[http://nautil.us/issue/45/power/what-quantum-gravity-needs-is-more-experiments|What Quantum Gravity Needs Is More Experiments - Math won’t solve quantum gravity. Experimentation will.]] by Sabine Hossenfelder
+<tabbox Why is it interesting?>
+<blockquote>
+One of the big problems in physics — perhaps the biggest! — is figuring out how our two current best theories fit together. On the one hand we have the [[models:standard_model|Standard Model]], which tries to explain all the forces except gravity, and takes quantum mechanics into account.  On the other hand we have [[models:general_relativity|General Relativity]], which tries to explain gravity, and does not take [[theories:quantum_mechanics:canonical|quantum mechanics]] into account. Both theories seem to be more or less on the right track — but until we somehow fit them together, or completely discard one or both, our picture of the world will be deeply schizophrenic.
+It seems plausible that as a step in the right direction we should figure out a theory of gravity that takes quantum mechanics into account, but reduces to General Relativity when we ignore quantum effects (which should be small in many situations). This is what people mean by "quantum gravity" — the quest for such a theory.
+<cite>[[https://www.edge.org/q2008/q08_5.html#baez|John Baez]]</cite></blockquote>
+<blockquote>At the beginning of the XX century,[[models:general_relativity| General Relativity]] (GR) and [[theories:quantum_mechanics:canonical|Quantum Mechanics]] (QM) once
+again began reshaping our basic understanding of space and time and, respectively, matter, energy
+and causality —arguably to a no lesser extent. But we have not been able to combine these new
+insights into a novel coherent synthesis, yet. The XX century scientific revolution opened by GR and
+QM is therefore still wide open. We are in the middle of an unfinished scientific revolution. Quantum
+Gravity is the tentative name we give to the “synthesis to be found”.<cite>"[[https://arxiv.org/abs/gr-qc/0604045|Unfinished Revolution" by C. Rovelli]]</cite></blockquote>
+----
+See also
+  * [[http://www.pbs.org/wgbh/nova/blogs/physics/2016/02/why-quantize-gravity/|Why Quantize Gravity?]] By Sabine Hossenfelder
+<tabbox Overview>
+Here's an incomplete list of popular approaches to formulate a theory of quantum gravity:
+Traditional
+  * Discrete methods
+    * Dynamical triangulations
+    * Regge calculus
+    * Simplicial models
+  * Approximate theories
+    * Euclidean quantum gravity
+    * Perturbative quantum gravity
+    * QFT on curved space–times
+  * Unorthodox approaches
+    * Sorkin’s Posets
+    * Finkelstein
+    * Twistors
+New
+  * Non-commutative geometry
+  * Null surfaces
+  * Spin foam models
+Most popular
+  * [[theories:speculative_theories:string_theory|String Theory]]
+  * [[theories:speculative_theories:lqg|Loop Quantum Gravity]]
+  * [[theories:speculative_theories:asymptotic_safety|Asymptotic Safety]]
+  * [[models:speculative_models:supergravity|Supergravity]]
+List adapted from [[https://arxiv.org/abs/gr-qc/9803024|Strings, loops and others: a critical survey of the present approaches to quantum gravity]] by Carlo Rovelli
 <tabbox FAQ>
 --> What's so difficult about Quantum Gravity?#
@@ Line 95: / Line 120: @@
 <--
+-->Why is Gravity Non-Renormalizable?#
+see https://arxiv.org/abs/0709.3555
+<--
 <tabbox History>
@@ Line 103: / Line 131: @@
 <tabbox Criticism>
-<blockquote>“Pursuing different and independent ideas is certainly a good strategy as long as we do not know the final answer, but it is a bit worrisome (at least to me) that the proponents of the different approaches not only base their approaches on very different assumptions, but continue to speak languages that are foreign to one another. Surely, when zeroing in on the ‘correct’ theory there should be a convergence of ideas and concepts: when Schrödinger proposed wave mechanics and Heisenberg formulated matrix mechanics, these were initially regarded as very different, but it did not take long before it became clear that they were just equivalent descriptions of the same theory. Unfortunately, at this time there is no such convergence in existing approaches to quantum gravity – a sign that we are probably still very far from the correct answer! […] In one form or another the existing approaches to quantum gravity suffer from a very large number of ambiguities, so far preventing any kind of prediction with which the theory will stand or fall. Even at the risk of sounding polemical, I would put this ambiguity at 10^500 (or even more) – in any case a number too large to cut down for any conceivable kind of experimental or observational advance. […] **Being exposed to many talks from the different ‘quantum gravity camps’ I am invariably struck by the success stories I keep hearing, and the implicit or explicit claims that ‘we are almost there’. I, for one, would much prefer to hear once in a while that something does not work, and to see some indications of inconsistencies that might enable us to discriminate between a rapidly growing number of diverging ideas on quantum gravity [27, 28].** If, however, the plethora of theory ambiguities were to stay with us I would conclude that our search for an ultimate explanation, and with it the search for quantum gravity, may come to an ignominious end”
+<blockquote>“Pursuing different and independent ideas is certainly a good strategy as long as we do not know the final answer, but it is a bit worrisome (at least to me) that the proponents of the different approaches not only base their approaches on very different assumptions, but continue to speak languages that are foreign to one another. Surely, when zeroing in on the ‘correct’ theory there should be a convergence of ideas and concepts: when Schrödinger proposed wave mechanics and Heisenberg formulated matrix mechanics, these were initially regarded as very different, but it did not take long before it became clear that they were just equivalent descriptions of the same theory. Unfortunately, at this time there is no such convergence in existing approaches to quantum gravity – a sign that we are probably still very far from the correct answer! […] In one form or another the existing approaches to quantum gravity suffer from a very large number of ambiguities, so far preventing any kind of prediction with which the theory will stand or fall. Even at the risk of sounding polemical, I would put this ambiguity at 10^500 (or even more) – in any case a number too large to cut down for any conceivable kind of experimental or observational advance. […] Being exposed to many talks from the different ‘quantum gravity camps’ I am invariably struck by the success stories I keep hearing, and the implicit or explicit claims that ‘we are almost there’. I, for one, would much prefer to hear once in a while that something does not work, and to see some indications of inconsistencies that might enable us to discriminate between a rapidly growing number of diverging ideas on quantum gravity [27, 28]. If, however, the plethora of theory ambiguities were to stay with us I would conclude that our search for an ultimate explanation, and with it the search for quantum gravity, may come to an ignominious end”
 <cite>from Quantum Gravity: The View From Particle Physics by Hermann Nicolai</cite></blockquote>

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