Title. Author(s)Greve, Ralf. Issue Date Doc URL. Type. Note. File Information. A material called spacetime

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1 Title A material called spacetime Author(s)Greve, Ralf Issue Date Doc URL Type lecture Note Colloquium of Mechanics, Study Center Mechanics, Dar File Information Greve_2017_Spacetime.pdf Instructions for use Hokkaido University Collection of Scholarly and Aca

2 A material called spacetime Credit: bornscientist.com Ralf Greve Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan

3 The classical concept of space and time Space: Empty space = three-dimensional nothing, but can be filled with matter. Time: Describes the continued progress of existence and events. Exists independently of anything else. Progresses uniformly from to +. Ralf Greve: A material called spacetime 2/21

4 Concept of absolute space and time works well in everyday life Newtonian mechanics in general, celestial mechanics, continuum mechanics Ralf Greve: A material called spacetime 3/21

5 Galilei transformation Frames of reference: Credit: Wikimedia Commons Transformation equations for any point ( event ) P: Ralf Greve: A material called spacetime 4/21

6 The nature of light Particle theory vs. wave theory: Credit: Wave theory triumphs: Young, Fresnel (early 19 th century): Newton s rings, double-slit experiments interference! Light is a transverse wave. Credit: U. Wikimedia Commons Ralf Greve: A material called spacetime 5/21

7 Aether Waves need a medium for propagation aether : Filling entire space. Invisible, massless, no viscosity. Much more rigid than steel to support the high frequencies ( Hz!) of light waves.?????? Ralf Greve: A material called spacetime 6/21

8 Electromagnetic theory Faraday, Maxwell (mid-19 th century): Light is an electromagnetic wave. Doesn t need a mechanical medium, propagates through empty space. Credit: E. Wikimedia Commons Ralf Greve: A material called spacetime 7/21

9 Maxwell s (macroscopic) equations Gauss s law: Gauss s law for magnetism: Faraday s law of induction: Ampère s law: E: electric field D: electric displacement field ρ f : free charge density B: magnetic B-field (flux density) H: magnetic H-field J f : free current density Ralf Greve: A material called spacetime 8/21

10 Closure requires constitutive equations In vacuum (= empty space): ε 0 : permittivity of free space μ 0 : permeability of free space Wave equations: c: speed of light in free space (c = 299,792,458 m/s) Ralf Greve: A material called spacetime 9/21

11 Special relativity Maxwell s equations are invariant in all inertial systems In particular, c is the same for all observers Credit: Wikimedia Commons Lorentz transformation: Independent existence of space and time spacetime Ralf Greve: A material called spacetime 10/21

12 Minkowski space = flat 4D spacetime of special relativity Four-position: Invariant line element: Metric tensor: (time-positive) Ralf Greve: A material called spacetime 11/21

13 General relativity Einstein tensor: Energy momentum (stress energy) tensor: (describes curvature of spacetime) w : energy density (often ρc 2 ) S i : energy flux density G ij = σ ij + ρv i v j : absolute stress tensor ( relative stresses + advection) Ralf Greve: A material called spacetime 12/21

14 Einstein field equations Mass / energy / momentum act as source for curvature of spacetime: G μν : Einstein tensor g μν : Metric tensor T μν : Energy momentum (stress energy) tensor G : Newton s gravitational constant ( Λ : Cosmological constant ) Can be interpreted as a constitutive equation of spacetime. Ralf Greve: A material called spacetime 13/21

15 Geodesic equation Force-free motion along geodesics of the curved spacetime: x μ : Four-position (ct, x, y, z) τ : Proper time (measured when following the motion) Γ μ αβ : Christoffel symbol [ = fct(g μν + derivatives) ] Gravity: Not external force, but consequence of curved spacetime. Credit: Wikimedia Commons Ralf Greve: A material called spacetime 14/21

16 Gravitational waves Einstein field equation [ G μν = (8πG/c 4 )T μν ] + some simplifying assumptions wave equation for the metric tensor. Solutions: Gravitational waves Ripples in the curvature of spacetime. Travelling at the speed of light c. Transverse waves (like electromagnetic waves). Radiated by accelerated masses (not if motion is spherically of rotationally symmetric). Ralf Greve: A material called spacetime 15/21

17 Indirect detection of gravitational waves Hulse Taylor binary Binary pulsar, discovered in Gradual decay of its 7.75-hour orbit due to radiation of gravitational waves (power ~ W). (Solar system only ~ 5 kw) Credit: Shane L. Larson, NASA/GSFC 1993 Nobel Prize in Physics Ralf Greve: A material called spacetime 16/21

18 Direct detection of gravitational waves Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) 2 observatories in the US LIGO Livingston (LA) Observatory. Credit: Ralf Greve: A material called spacetime 17/21

19 Direct detection of gravitational waves Ralf Greve: A material called spacetime 18/21

20 Direct detection of gravitational waves 20xx Nobel Prize in Physics? Ralf Greve: A material called spacetime 19/21

21 Summary Spacetime itself can be considered a material: physical properties, constitutive equations. Alternative interpretation: spacetime = aether. Open questions, speculations Physics of black holes & Big Bang: General relativity + quantum mechanics. Other spacetimes (or something completely different)? Ralf Greve: A material called spacetime 20/21

22 Thank you THANK YOU Credit: dansalisbury.com