Gravity: What s the big attraction? Dan Wilkins Institute of Astronomy

Overview What is gravity? Newton and Einstein What does gravity do? Extreme gravity The true power of gravity Getting things moving gravitational waves

What is Gravity? Attractive force between all bodies with mass

Newton Universal Law of Gravitation F = GMm r 2

Gravity is a Weak Force! Protons in nucleus of an atom Strong Force Attraction 10,000 N Electrostatic Repulsion 100 N Gravitational Attraction 1x10-34 N Gravity weaker than electric forces by 1,000,000,000,000,000,000,000,000,000,000,000,000x

But...

But... Negative and positive charges Opposites attract and cancel out Large scales no net charge

But... Negative and positive charges Opposites attract and cancel out Large scales no net charge No negative mass Gravity remains on large scales

The Earth is Massive! 6x10 24 kg

Structure in the Universe Gravity is responsible for all large-scale structures Planets Stars Solar System

Structure in the Universe Gravity is responsible for all large-scale structures Galaxies Clusters

Einstein General Relativity R µν 1 2 g µνr = 8πG c 4 T µν

Motion Free particles move in straight lines

Motion Free particles move in straight lines Forces cause them to deviate

Geodesics Shortest path between 2 points Flat space straight lines Free particles follow geodesics Sphere (great) circles

Curved Space Matter curves space around it

Gravity from Curved Space Free particles follow the geodesics Gravitational force towards object

Schwarzschild Spacetime around a non-spinning point mass On the Russian front in World War I

Rotating Masses Drag spacetime around with them

Gravitational Lensing Curved space Light bends around massive object

Gravitational Lensing

Spacetime Space and time are the same thing y Can move through space and time Physics decides your speed through time x

Spacetime Space and time are the same thing t Can move through space and time Physics decides your speed through time x

Time Dilation Matter warps spacetime In stronger gravitational fields, time elapses more slowly

Gravitational Redshift

Gravitational Redshift Light is a wave

Gravitational Redshift Light is a wave Time dilation

Gravitational Redshift Light is a wave Time dilation Emitter near mass

Gravitational Redshift Light is a wave Time dilation Emitter near mass Clock slower

Gravitational Redshift Light is a wave Time dilation Emitter near mass Clock slower Lower frequency

Black Holes Extreme density Extreme curvature Horizon Close enough, even light can t escape

The Power of Gravity

Nuclear Fusion

Gravity Makes it Easy! Stars Gravity pulls gas together High temperature High pressure

The Death of Stars Gravity balanced by gas pressure Fuel runs out, star cools and collapses Releases more energy

Accretion Matter falling (spiralling) onto object Releases energy as it falls in

Efficiency E = mc 2

Efficiency E = mc 2 Chemistry Nuclear Nuclear Accretion Fission Fusion ε = 10-7 ε = 0.01% ε = 0.1% ε ~ 40%

Active Galactic Nuclei Supermassive black hole at centre of galaxy 10 8 solar masses Matter accreting

Energy Emission

Energy Emission Thermal emission as disc warms up

Energy Emission Thermal emission as disc warms up Magnetic fields accelerate particles Inverse Compton (Xrays) Synchrotron emission (radio waves)

Jets and Bubbles Energy released into magnetic fields Accelerate particles to 99.9% speed of light Jets Inflate bubbles in surrounding gas

Gravitational Waves Matter warps spacetime Movement causes ripples Waves in spacetime Tell us about what s moving?

Gravitational Waves Cause tiny movements 2 neutron stars in orbit cause movement 1 part in 10 20 at Earth Difficult to detect Large laser interferometers to try to detect

Conclusions Gravity is an attraction between massive objects Responsible for structure in Universe Gravity as spacetime curvature Black holes extreme gravity The most efficient way of releasing energy Gravitational waves new science!

Thank you Any questions?