Testing general relativity Successes and failures History of gravity GR�has�many�other e.g.:�photon�trajectories Testing general testable�predictions deflected�by�massive relativity Shortcomings of GR bodies How not to quantize gravity Is this a problem? Earth “Observable” quantum Sun gravity? GR�prediction Hard to see Inflation as a Planck scale microscope Newtonian�trajectory Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? effect�observed�by Eddington�in�1919 5 / 22
Testing general relativity Successes and failures GR has many other as many other e.g.: photon trajectories e.g.: photon History of gravity Testing general testable predictions ble predictions deflected b deflected by massive disclaimer: can also get light relativity bodies bod Shortcomings of GR de fm ection in Newtonian How not to quantize calculation by using certain gravity Is this a problem? Earth limiting procedures, “Observable” quantum Sun Sun half the GR result gravity? GR prediction Hard to see GR pred Inflation as a Planck scale microscope Ne Newtonian trajectory Only testable QG prediction? Small scale patches Large scale fixes effect observed by Where do we stand? Eddington in 1919 5 / 22
Testing general relativity Successes and failures History of gravity GR�has�many�other e.g.:�radar�signals�are Testing general testable�predictions delayed�when�they�travel relativity Shortcomings of GR near�massive�bodies How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 5 / 22
Testing general relativity Successes and failures History of gravity GR�has�many�other e.g.:�radar�signals�are Testing general testable�predictions delayed�when�they�travel relativity Shortcomings of GR near�massive�bodies How not to quantize gravity Is this a problem? Earth “Observable” quantum Sun gravity? Hard to see Inflation as a Planck scale microscope Only testable QG Cassini prediction? spacecraft Small scale patches Large scale fixes Where do we stand? 5 / 22
Testing general relativity Successes and failures History of gravity GR�has�many�other e.g.:�radar�signals�are Testing general testable�predictions delayed�when�they�travel relativity Shortcomings of GR near�massive�bodies How not to quantize gravity Is this a problem? Earth “Observable” quantum Sun gravity? Hard to see Inflation as a Planck scale microscope Only testable QG Cassini l a prediction? n g i s � r a d a spacecraft r Small scale patches Large scale fixes Where do we stand? 5 / 22
Testing general relativity Successes and failures History of gravity GR�has�many�other e.g.:�radar�signals�are Testing general testable�predictions delayed�when�they�travel relativity Shortcomings of GR near�massive�bodies How not to quantize gravity Is this a problem? Earth “Observable” quantum Sun gravity? Hard to see Inflation as a Planck scale microscope Only testable QG Cassini l a prediction? n g i s � r a d a spacecraft r Small scale patches Large scale fixes Where do we stand? 5 / 22
Testing general relativity Successes and failures History of gravity GR�has�many�other Testing general testable�predictions relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches PSR�1913+16 Large scale fixes Where do we stand? e.g.:�binary�pulsars�emit gravitational�waves�(GWs) 5 / 22
Testing general relativity Successes and failures History of gravity GR�has�many�other Testing general testable�predictions relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck GWs�(wavelike�fluctuations scale microscope in�geometry�of spacetime) Only testable QG prediction? Small scale patches PSR�1913+16 Large scale fixes Where do we stand? e.g.:�binary�pulsars�emit gravitational�waves�(GWs) 5 / 22
Testing general relativity Successes and failures (same�effect�as�in�Maxwell’s History of gravity GR�has�many�other theory:�accelerating�charges Testing general testable�predictions relativity produce�EM�radiation) Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck GWs�(wavelike�fluctuations scale microscope in�geometry�of spacetime) Only testable QG prediction? Small scale patches PSR�1913+16 Large scale fixes Where do we stand? e.g.:�binary�pulsars�emit gravitational�waves�(GWs) 5 / 22
Testing general relativity Successes and failures (same�effect�as�in�Maxwell’s History of gravity GR�has�many�other theory:�accelerating�charges Testing general testable�predictions relativity produce�EM�radiation) Shortcomings of GR How not to quantize gravity GWs�take�energy�out�of Is this a problem? “Observable” quantum the�system�and�cause gravity? stars�to�spiral�together Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches GWs PSR�1913+16 Large scale fixes Where do we stand? e.g.:�binary�pulsars�emit gravitational�waves�(GWs) 5 / 22
Testing general relativity Successes and failures (same�effect�as�in�Maxwell’s History of gravity GR�has�many�other theory:�accelerating�charges Testing general testable�predictions relativity produce�EM�radiation) Shortcomings of GR How not to quantize gravity GWs�take�energy�out�of Is this a problem? “Observable” quantum the�system�and�cause gravity? stars�to�spiral�together Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches GWs PSR�1913+16 Large scale fixes Where do we stand? e.g.:�binary�pulsars�emit gravitational�waves�(GWs) 5 / 22
Testing general relativity Successes and failures History of gravity GR�has�many�other Testing general testable�predictions relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? can�also�test�GR�in�the�lab�by measuring�gravitational attraction�directly 5 / 22
Testing general relativity Successes and failures History of gravity GR�has�many�other Testing general testable�predictions relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? can�also�test�GR�in�the�lab�by measuring�gravitational attraction�directly 5 / 22
Shortcomings of GR Successes and failures e.g.:�galactic�rotation History of gravity however,�GR�doesn’t Testing general curves relativity explain�everything... Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 6 / 22
Shortcomings of GR Successes and failures e.g.:�galactic�rotation History of gravity however,�GR�doesn’t Testing general curves relativity explain�everything... Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? star Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches galaxy Large scale fixes Where do we stand? 6 / 22
Shortcomings of GR Successes and failures e.g.:�galactic�rotation History of gravity however,�GR�doesn’t Testing general curves relativity explain�everything... Shortcomings of GR How not to quantize velocity gravity Is this a problem? observed “Observable” quantum gravity? star Hard to see GR�prediction Inflation as a Planck scale microscope distance�from�center Only testable QG prediction? Small scale patches galaxy Large scale fixes Where do we stand? 6 / 22
Shortcomings of GR Successes and failures e.g.:�galactic�rotation History of gravity however,�GR�doesn’t Testing general curves relativity explain�everything... Shortcomings of GR How not to quantize velocity gravity Is this a problem? observed “Observable” quantum gravity? star Hard to see GR�prediction Inflation as a Planck scale microscope distance�from�center Only testable QG prediction? Small scale patches galaxy Large scale fixes discrepancy�usually Where do we stand? explained�by�the�existence of “dark�matter”�haloes 6 / 22
Shortcomings of GR Successes and failures one�of the�key�predictions History of gravity however,�GR�doesn’t Testing general of GR�is�the�expansion�of relativity explain�everything... the�universe... Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 6 / 22
Shortcomings of GR Successes and failures one�of the�key�predictions History of gravity however,�GR�doesn’t Testing general of GR�is�the�expansion�of relativity explain�everything... the�universe... Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum size�of universe gravity? Hard to see Inflation as a Planck GR scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? time 6 / 22
Shortcomings of GR Successes and failures one�of the�key�predictions History of gravity however,�GR�doesn’t Testing general of GR�is�the�expansion�of relativity explain�everything... the�universe... Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum size�of universe gravity? Hard to see Inflation as a Planck GR scale microscope Only testable QG prediction? Small scale patches in�GR,�gravity�is�an Large scale fixes attractive�force:�so Where do we stand? expansion�should time decelerate 6 / 22
Shortcomings of GR Successes and failures one�of the�key�predictions History of gravity however,�GR�doesn’t Testing general of GR�is�the�expansion�of relativity explain�everything... the�universe... Shortcomings of GR How not to quantize gravity Is this a problem? observations “Observable” quantum size�of universe gravity? Hard to see Inflation as a Planck GR scale microscope Only testable QG prediction? in�reality,�the Small scale patches expansion�is in�GR,�gravity�is�an Large scale fixes accelerating! attractive�force:�so Where do we stand? expansion�should time decelerate 6 / 22
Shortcomings of GR Successes and failures one�of the�key�predictions History of gravity however,�GR�doesn’t Testing general of GR�is�the�expansion�of relativity explain�everything... the�universe... Shortcomings of GR How not to quantize gravity discrepancy Is this a problem? observations “Observable” quantum usually�explained size�of universe gravity? by�existence�of Hard to see Inflation as a Planck exotic�“dark GR scale microscope energy” Only testable QG prediction? in�reality,�the Small scale patches expansion�is in�GR,�gravity�is�an Large scale fixes accelerating! attractive�force:�so Where do we stand? expansion�should time decelerate 6 / 22
Shortcomings of GR Successes and failures History of gravity different�effects�probe however,�GR�doesn’t Testing general gravity�at�different�scales relativity explain�everything... Shortcomings of GR solar�system/ binary�pulsar galactic�rot’n How not to quantize accelerated expansion gravity laboratory curves Is this a problem? tests “Observable” quantum gravity? Hard to see scale Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 6 / 22
Shortcomings of GR Successes and failures History of gravity different�effects�probe however,�GR�doesn’t Testing general gravity�at�different�scales relativity explain�everything... Shortcomings of GR solar�system/ binary�pulsar galactic�rot’n How not to quantize accelerated expansion gravity laboratory curves Is this a problem? tests “Observable” quantum gravity? Hard to see scale Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches GR�works�well Large scale fixes Where do we stand? 6 / 22
Shortcomings of GR Successes and failures History of gravity different�effects�probe however,�GR�doesn’t Testing general gravity�at�different�scales relativity explain�everything... Shortcomings of GR solar�system/ binary�pulsar galactic�rot’n How not to quantize accelerated expansion gravity laboratory curves Is this a problem? tests “Observable” quantum gravity? Hard to see scale Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches no�data�either GR�works�well way Large scale fixes Where do we stand? 6 / 22
Shortcomings of GR Successes and failures History of gravity different�effects�probe however,�GR�doesn’t Testing general gravity�at�different�scales relativity explain�everything... Shortcomings of GR solar�system/ binary�pulsar galactic�rot’n How not to quantize accelerated expansion gravity laboratory curves Is this a problem? tests “Observable” quantum gravity? Hard to see scale Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches no�data�either GR�works�well GR�needs�help way from�the�dark�side Large scale fixes Where do we stand? 6 / 22
Shortcomings of GR Successes and failures History of gravity different�effects�probe however,�GR�doesn’t Testing general gravity�at�different�scales relativity explain�everything... Shortcomings of GR solar�system/ binary�pulsar galactic�rot’n How not to quantize accelerated expansion gravity laboratory curves Is this a problem? tests “Observable” quantum gravity? Hard to see scale Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches no�data�either GR�works�well GR�needs�help way from�the�dark�side Large scale fixes ...or�GR�is�not�the Where do we stand? correct�theory�of gravity�on�large scales 6 / 22
Shortcomings of GR Successes and failures History of gravity different�effects�probe however,�GR�doesn’t Testing general gravity�at�different�scales relativity explain�everything... Shortcomings of GR solar�system/ binary�pulsar galactic�rot’n How not to quantize accelerated expansion gravity laboratory curves Is this a problem? tests “Observable” quantum gravity? Hard to see scale Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches no�data�either GR�works�well GR�needs�help way from�the�dark�side Large scale fixes ...or�GR�is�not�the Where do we stand? is�GR�the�right correct�theory�of theory�on�small gravity�on�large scales? scales 6 / 22
How not to quantize gravity Successes and failures � small scales ⇒ quantum mechanics History of gravity Testing general relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 7 / 22
How not to quantize gravity Successes and failures � small scales ⇒ quantum mechanics History of gravity Testing general � classical GR is governed by a system of nonlinear PDEs relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 7 / 22
How not to quantize gravity Successes and failures � small scales ⇒ quantum mechanics History of gravity Testing general � classical GR is governed by a system of nonlinear PDEs relativity Shortcomings of GR � standard approach: quantize perturbations about a simple How not to quantize gravity solution Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 7 / 22
How not to quantize gravity Successes and failures � small scales ⇒ quantum mechanics History of gravity Testing general � classical GR is governed by a system of nonlinear PDEs relativity Shortcomings of GR � standard approach: quantize perturbations about a simple How not to quantize gravity solution Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes � this works fine for GR at the linear level Where do we stand? 7 / 22
How not to quantize gravity Successes and failures � small scales ⇒ quantum mechanics History of gravity Testing general � classical GR is governed by a system of nonlinear PDEs relativity Shortcomings of GR � standard approach: quantize perturbations about a simple How not to quantize gravity solution Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes � this works fine for GR at the linear level Where do we stand? � adding nonlinear terms back in results in a quantum field theory with non-removable divergences and no predictive power 7 / 22
How not to quantize gravity Successes and failures � small scales ⇒ quantum mechanics History of gravity Testing general � classical GR is governed by a system of nonlinear PDEs relativity Shortcomings of GR � standard approach: quantize perturbations about a simple How not to quantize gravity solution Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes � this works fine for GR at the linear level Where do we stand? � adding nonlinear terms back in results in a quantum field theory with non-removable divergences and no predictive power that is, perturbative quantum gravity is non-renormalizable � 7 / 22
Is this a problem? Successes and failures � why do we even want to quantize gravity? History of gravity Testing general relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 8 / 22
Is this a problem? Successes and failures � why do we even want to quantize gravity? History of gravity Testing general � everything else is quantum, why not gravity? relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 8 / 22
Is this a problem? Successes and failures � why do we even want to quantize gravity? History of gravity Testing general � everything else is quantum, why not gravity? relativity Shortcomings of GR � Einstein field equations state How not to quantize gravity Is this a problem? spacetime curvature ⇔ matter content “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 8 / 22
Is this a problem? Successes and failures � why do we even want to quantize gravity? History of gravity Testing general � everything else is quantum, why not gravity? relativity Shortcomings of GR � Einstein field equations state How not to quantize gravity Is this a problem? spacetime curvature ⇔ matter content “Observable” quantum gravity? Hard to see somewhat inconsistent to treat gravity classically and � Inflation as a Planck scale microscope matter quantum mechanically Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 8 / 22
Is this a problem? Successes and failures � why do we even want to quantize gravity? History of gravity Testing general � everything else is quantum, why not gravity? relativity Shortcomings of GR � Einstein field equations state How not to quantize gravity Is this a problem? spacetime curvature ⇔ matter content “Observable” quantum gravity? Hard to see somewhat inconsistent to treat gravity classically and � Inflation as a Planck scale microscope matter quantum mechanically Only testable QG prediction? GR implies spacetime singularities at the centre of black � Small scale patches holes and at the beginning of the universe (the big bang) Large scale fixes Where do we stand? 8 / 22
Is this a problem? Successes and failures � why do we even want to quantize gravity? History of gravity Testing general � everything else is quantum, why not gravity? relativity Shortcomings of GR � Einstein field equations state How not to quantize gravity Is this a problem? spacetime curvature ⇔ matter content “Observable” quantum gravity? Hard to see somewhat inconsistent to treat gravity classically and � Inflation as a Planck scale microscope matter quantum mechanically Only testable QG prediction? GR implies spacetime singularities at the centre of black � Small scale patches holes and at the beginning of the universe (the big bang) Large scale fixes Where do we stand? many believe quantum gravity will “cure” these � 8 / 22
Is this a problem? Successes and failures � why do we even want to quantize gravity? History of gravity Testing general � everything else is quantum, why not gravity? relativity Shortcomings of GR � Einstein field equations state How not to quantize gravity Is this a problem? spacetime curvature ⇔ matter content “Observable” quantum gravity? Hard to see somewhat inconsistent to treat gravity classically and � Inflation as a Planck scale microscope matter quantum mechanically Only testable QG prediction? GR implies spacetime singularities at the centre of black � Small scale patches holes and at the beginning of the universe (the big bang) Large scale fixes Where do we stand? many believe quantum gravity will “cure” these � these seem like theoretical prejudices � 8 / 22
Is this a problem? Successes and failures � why do we even want to quantize gravity? History of gravity Testing general � everything else is quantum, why not gravity? relativity Shortcomings of GR � Einstein field equations state How not to quantize gravity Is this a problem? spacetime curvature ⇔ matter content “Observable” quantum gravity? Hard to see somewhat inconsistent to treat gravity classically and � Inflation as a Planck scale microscope matter quantum mechanically Only testable QG prediction? GR implies spacetime singularities at the centre of black � Small scale patches holes and at the beginning of the universe (the big bang) Large scale fixes Where do we stand? many believe quantum gravity will “cure” these � these seem like theoretical prejudices � is there any experimental information available? � 8 / 22
“Observable” quantum gravity? Successes and failures how big are quantum gravity effects? � History of gravity Testing general relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 9 / 22
“Observable” quantum gravity? Successes and failures how big are quantum gravity effects? � History of gravity Testing general � the dimensionful constants in the theory: relativity Shortcomings of GR Newton’s constant G � How not to quantize gravity Planck’s constant � � Is this a problem? “Observable” quantum gravity? the speed of light c � Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 9 / 22
“Observable” quantum gravity? Successes and failures how big are quantum gravity effects? � History of gravity Testing general � the dimensionful constants in the theory: relativity Shortcomings of GR Newton’s constant G � How not to quantize gravity Planck’s constant � � Is this a problem? “Observable” quantum gravity? the speed of light c � Hard to see Inflation as a Planck scale microscope these define the Planck mass/energy and length: � Only testable QG prediction? G ∼ 10 19 GeV � � � c � G Small scale patches c 3 ∼ 10 − 35 m M Pl = ℓ Pl = Large scale fixes c 2 Where do we stand? 9 / 22
“Observable” quantum gravity? Successes and failures History of gravity Testing general relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 9 / 22
“Observable” quantum gravity? Successes and failures how big are quantum gravity effects? � History of gravity Testing general � the dimensionful constants in the theory: relativity Shortcomings of GR Newton’s constant G � How not to quantize gravity Planck’s constant � � Is this a problem? “Observable” quantum gravity? the speed of light c � Hard to see Inflation as a Planck scale microscope these define the Planck mass/energy and length: � Only testable QG prediction? G ∼ 10 19 GeV � � � c � G Small scale patches c 3 ∼ 10 − 35 m M Pl = ℓ Pl = Large scale fixes c 2 Where do we stand? � physics intuition: quantum gravity effects will become tangible for (individual particle) energies � M Pl or distances � ℓ Pl 9 / 22
Hard to see Successes and failures This means its not easy to observe quantum gravity effects: History of gravity Testing general relativity Shortcomings of GR ℓ Pl / ℓ typical size ℓ phenomenon How not to quantize 10 3 m 10 − 38 gravity solar mass black hole 10 − 10 m Is this a problem? 10 − 25 hydrogen atom “Observable” quantum 10 − 15 m gravity? 10 − 20 proton Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 10 / 22
Hard to see Successes and failures This means its not easy to observe quantum gravity effects: History of gravity Testing general relativity Shortcomings of GR ℓ Pl / ℓ typical size ℓ phenomenon How not to quantize 10 3 m 10 − 38 gravity solar mass black hole 10 − 10 m Is this a problem? 10 − 25 hydrogen atom “Observable” quantum 10 − 15 m gravity? 10 − 20 proton Hard to see Inflation as a Planck scale microscope Only testable QG prediction? E/M Pl c 2 typical energy E phenomenon 10 − 9 GeV Small scale patches 10 − 28 hydrogen atom Large scale fixes 10 3 GeV 10 − 16 large hadron collider Where do we stand? 10 9 GeV 10 − 8 ultra high energy cosmic rays 10 16 GeV 10 − 3 cosmological inflation 10 / 22
Inflation as a Planck scale microscope Successes and failures History of gravity Testing general relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 11 / 22
Inflation as a Planck scale microscope Successes and failures History of gravity Testing general relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 11 / 22
Inflation as a Planck scale microscope Successes and failures History of gravity Testing general relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 11 / 22
Primordial GWs: testable prediction? Successes and failures History of gravity Testing general relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 12 / 22
Primordial GWs: testable prediction? Successes and failures History of gravity Testing general relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 12 / 22
Primordial GWs: testable prediction? Successes and failures History of gravity Testing general relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches Large scale fixes Where do we stand? 12 / 22
Primordial GWs: testable prediction? Successes and failures History of gravity Testing general relativity Shortcomings of GR How not to quantize gravity Is this a problem? “Observable” quantum gravity? Hard to see Inflation as a Planck scale microscope Only testable QG prediction? Small scale patches detection Large scale fixes announced monday, Where do we stand? march 17 by bicep2! 12 / 22
Successes and failures Small scale patches Solution 1: quantizing the wrong theory Solution 2: right theory but wrong quantization . . . and the answer is Small scale patches Large scale fixes Where do we stand? 13 / 22
Solution 1: quantizing the wrong theory Successes and failures if GR is cannot be consistently quantized, maybe it is not the � Small scale patches correct theory of gravity Solution 1: quantizing the wrong theory Solution 2: right theory but wrong quantization . . . and the answer is Large scale fixes Where do we stand? 14 / 22
Solution 1: quantizing the wrong theory Successes and failures if GR is cannot be consistently quantized, maybe it is not the � Small scale patches correct theory of gravity Solution 1: quantizing the wrong theory � some options: Solution 2: right theory but wrong quantization . . . and the answer is Large scale fixes Where do we stand? 14 / 22
Solution 1: quantizing the wrong theory Successes and failures if GR is cannot be consistently quantized, maybe it is not the � Small scale patches correct theory of gravity Solution 1: quantizing the wrong theory � some options: Solution 2: right theory but wrong quantization . . . and the answer is � string theory: gravitational field is actually induced by Large scale fixes vibrations of higher dimensional objects (strings and Where do we stand? branes) 14 / 22
Solution 1: quantizing the wrong theory Successes and failures if GR is cannot be consistently quantized, maybe it is not the � Small scale patches correct theory of gravity Solution 1: quantizing the wrong theory � some options: Solution 2: right theory but wrong quantization . . . and the answer is � string theory: gravitational field is actually induced by Large scale fixes vibrations of higher dimensional objects (strings and Where do we stand? branes) pros: vibrations of strings governed by 2D conformal � field theory which is nicely behaved 14 / 22
Solution 1: quantizing the wrong theory Successes and failures if GR is cannot be consistently quantized, maybe it is not the � Small scale patches correct theory of gravity Solution 1: quantizing the wrong theory � some options: Solution 2: right theory but wrong quantization . . . and the answer is � string theory: gravitational field is actually induced by Large scale fixes vibrations of higher dimensional objects (strings and Where do we stand? branes) pros: vibrations of strings governed by 2D conformal � field theory which is nicely behaved higher curvature and Horava-Lifshitz theories: GR is � effective version of a more complicated (4D) theory that can be consistently quantized 14 / 22
Solution 1: quantizing the wrong theory Successes and failures if GR is cannot be consistently quantized, maybe it is not the � Small scale patches correct theory of gravity Solution 1: quantizing the wrong theory � some options: Solution 2: right theory but wrong quantization . . . and the answer is � string theory: gravitational field is actually induced by Large scale fixes vibrations of higher dimensional objects (strings and Where do we stand? branes) pros: vibrations of strings governed by 2D conformal � field theory which is nicely behaved higher curvature and Horava-Lifshitz theories: GR is � effective version of a more complicated (4D) theory that can be consistently quantized pros: the theories are selected to avoid divergences � when quantized 14 / 22
Solution 2: right theory but wrong quantization Successes and failures if we try to quantize gravitational perturbations and get � Small scale patches inconsistent results, maybe we ought to try something else? Solution 1: quantizing the wrong theory Solution 2: right theory but wrong quantization . . . and the answer is Large scale fixes Where do we stand? 15 / 22
Solution 2: right theory but wrong quantization Successes and failures if we try to quantize gravitational perturbations and get � Small scale patches inconsistent results, maybe we ought to try something else? Solution 1: quantizing the wrong theory � loop quantum gravity: tries to quantize GR non-perturbatively Solution 2: right theory but wrong quantization . . . and the answer is Large scale fixes Where do we stand? 15 / 22
Solution 2: right theory but wrong quantization Successes and failures if we try to quantize gravitational perturbations and get � Small scale patches inconsistent results, maybe we ought to try something else? Solution 1: quantizing the wrong theory � loop quantum gravity: tries to quantize GR non-perturbatively Solution 2: right theory but wrong quantization . . . and the answer is Large scale fixes Where do we stand? � pros: no exotic classical theory of gravity; succeeds because of choice of variables and non-standard polymer quantization 15 / 22
. . . and the answer is Successes and failures Small scale patches Solution 1: quantizing the wrong theory Solution 2: right theory but wrong quantization . . . and the answer is Large scale fixes Where do we stand? 16 / 22
. . . and the answer is Successes and failures Small scale patches Solution 1: quantizing ? the wrong theory Solution 2: right theory but wrong quantization . . . and the answer is Large scale fixes Where do we stand? 16 / 22
. . . and the answer is Successes and failures in the absence of experimental facts, the only way to distinguish � Small scale patches models is consistency checks/theoretical bias Solution 1: quantizing the wrong theory Solution 2: right theory but wrong quantization . . . and the answer is Large scale fixes Where do we stand? 16 / 22
. . . and the answer is Successes and failures in the absence of experimental facts, the only way to distinguish � Small scale patches models is consistency checks/theoretical bias Solution 1: quantizing the wrong theory � there are criticisms of each approach: Solution 2: right theory but wrong quantization . . . and the answer is Large scale fixes Where do we stand? 16 / 22
. . . and the answer is Successes and failures in the absence of experimental facts, the only way to distinguish � Small scale patches models is consistency checks/theoretical bias Solution 1: quantizing the wrong theory � there are criticisms of each approach: Solution 2: right theory but wrong quantization . . . and the answer is � string theory involves a background higher dimensional Large scale fixes spacetime; i.e. not really a quantization of geometry Where do we stand? 16 / 22
. . . and the answer is Successes and failures in the absence of experimental facts, the only way to distinguish � Small scale patches models is consistency checks/theoretical bias Solution 1: quantizing the wrong theory � there are criticisms of each approach: Solution 2: right theory but wrong quantization . . . and the answer is � string theory involves a background higher dimensional Large scale fixes spacetime; i.e. not really a quantization of geometry Where do we stand? really hard to calculate things in loop quantum gravity; not � even sure how classical GR is recovered 16 / 22
Successes and failures Small scale patches Large scale fixes The dark matter problem The dark energy problem Large scale fixes Other ideas Where do we stand? 17 / 22
The dark matter problem Successes and failures do we need to modify gravity to account for dark matter? � Small scale patches Large scale fixes The dark matter problem The dark energy problem Other ideas Where do we stand? 18 / 22
The dark matter problem Successes and failures do we need to modify gravity to account for dark matter? � Small scale patches � probably not . . . lots of evidence for dark matter other than Large scale fixes galactic rotation curves The dark matter problem The dark energy problem Other ideas Where do we stand? 18 / 22
The dark matter problem Successes and failures do we need to modify gravity to account for dark matter? � Small scale patches � probably not . . . lots of evidence for dark matter other than Large scale fixes galactic rotation curves The dark matter problem The dark energy � for example: weak gravitational lensing lets us map dark problem Other ideas matter distribution in galactic clusters Where do we stand? 18 / 22
The dark matter problem Successes and failures do we need to modify gravity to account for dark matter? � Small scale patches � probably not . . . lots of evidence for dark matter other than Large scale fixes galactic rotation curves The dark matter problem The dark energy � for example: weak gravitational lensing lets us map dark problem Other ideas matter distribution in galactic clusters Where do we stand? however . . . � 18 / 22
The dark matter problem Successes and failures do we need to modify gravity to account for dark matter? � Small scale patches � probably not . . . lots of evidence for dark matter other than Large scale fixes galactic rotation curves The dark matter problem The dark energy � for example: weak gravitational lensing lets us map dark problem Other ideas matter distribution in galactic clusters Where do we stand? however . . . � what is it? � 18 / 22
The dark matter problem Successes and failures do we need to modify gravity to account for dark matter? � Small scale patches � probably not . . . lots of evidence for dark matter other than Large scale fixes galactic rotation curves The dark matter problem The dark energy � for example: weak gravitational lensing lets us map dark problem Other ideas matter distribution in galactic clusters Where do we stand? however . . . � what is it? � � direct searches: observe dark matter interaction with dense materials in isolated labs 18 / 22
The dark matter problem Successes and failures do we need to modify gravity to account for dark matter? � Small scale patches � probably not . . . lots of evidence for dark matter other than Large scale fixes galactic rotation curves The dark matter problem The dark energy � for example: weak gravitational lensing lets us map dark problem Other ideas matter distribution in galactic clusters Where do we stand? however . . . � what is it? � � direct searches: observe dark matter interaction with dense materials in isolated labs � indirect searches: observe by-products of dark matter annihilations in the Milky Way 18 / 22
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