Gravitational waves

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Gravitational waves

A new tool for observing the Universe through ripples in spacetime

Daniel Grumiller

Institute for Theoretical Physics TU Wien

Public lecture

Monat der freien Bildung, TU Wien, Mai 2016

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Executive summary

I 1915: Einstein’s General Relativity predicts gravitational waves R_µν = 0

I 1916: Schwarzschild constructs first black hole solution

I 2015: LIGO detects gravitational waves

I 2016: LIGO announces results; interpretation as black hole merger

I 2115: gravitational waves standard tool in astrophysics & cosmology

I 2116: someone pays taxes for gravitational waves

Daniel Grumiller — Gravitational waves 2/20

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Executive summary

I 1915: Einstein’s General Relativity predicts gravitational waves

I 1916: Schwarzschild constructs first black hole solution ds² =−(1−2M/r) dt²+ dr²

1−2M/r +r² d²Ω_S2

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Executive summary

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Executive summary

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Executive summary

Left: gravitational waves sensitive to early Universe, Right: Supernova

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Executive summary

Disclaimer: quote above is commonly cited, but probably not authentic

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Outline

Theory

Technology

Observation

Applications

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Outline

Theory

Technology

Observation

Applications

Daniel Grumiller — Gravitational waves Theory 4/20

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Warm-up: electromagnetism

I Electromagnetism: theory describing dynamics of charges

I Unifies electricity, magnetism, optics and special relativity

I Quantum electrodynamics tested with amazing precision and accuracy

I Without charges: still have lightwaves

I Lightwaves solutions of vacuum Maxwell’s equations dF = 0 = d∗F ⇒ A_µ= 0

I Light propagates in spacetime

I In vacuum light propagates with the biggest velocity possible

I First observation of light by humankind: about 2 million years ago

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E =mc²

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I Quantum electrodynamics tested with amazing precision and accuracy Example: gyromagnetic factor of electron

Experiment (2008):

g^expe

2 = 1.00115965218073±0.00000000000028 Theory (2012):

g^the_e

2 = 1.00115965218178±0.00000000000077

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I Lightwaves solutions of vacuum Maxwell’s equations dF = 0 = d∗F ⇒ Aµ= 0

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General Relativity

I General Relativity: theory describing dynamics of masses

I Unifies gravity and special relativity; curves spacetime

I Quantum gravity not tested with any experiment so far

I Without masses: still have gravitational waves

I Gravitational waves solutions of vacuum Einstein’s equations R_µν = 0 ⇒ h_µν = 0

I Gravity waves propagate in spacetime and deform it

I Gravitational waves propagate with the biggest velocity possible

I First observation of gravitational waves by humankind: about 200 days ago

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General Relativity

I Gravitational waves solutions of vacuum Einstein’s equations Rµν = 0 ⇒ hµν = 0

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General Relativity

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General Relativity

I Gravitational waves solutions of vacuum Einstein’s equations R_µν = 0 ⇒ h_µν = 0

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General Relativity

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General Relativity

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General Relativity

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General Relativity

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Gravitational wave sources

I Any acceleration that is not spherically or cylindrically symmetric produces gravitational waves

I Examples: supernovae, black hole merger, big bang, me, ...

I Focus here on black hole merger

I Simple calculation to estimate gravitational wave energy Take two Schwarzschild black holes of equal mass m

Call final black hole massM and gravitational wave energy E Energy conservation:

M+E =m+m⇒E = 2m−M

Use Hawking’s area theorem (with area∝mass squared): A_final≥A_initial⇒M² ≥2m²

Get upper bound on gravitational wave energy E ≤(2−√

2)m≈29% of initial energy Energy released by 10³⁴−10³⁶ Nagasaki bombs!

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Use Hawking’s area theorem (with area∝mass squared): Afinal≥Ainitial⇒M² ≥2m²

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Get upper bound on gravitational wave energy E≤(2−√

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M+E =m+m⇒E = 2m−M Use Hawking’s area theorem (with area∝mass squared):

A_final≥A_initial⇒M² ≥2m²

Get upper bound on gravitational wave energy E≤(2−√

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M+E =m+m⇒E = 2m−M Use Hawking’s area theorem (with area∝mass squared):

A_final≥A_initial⇒M² ≥2m² Get upper bound on gravitational wave energy

E≤(2−√

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Gravitational waves as the last known puzzle piece

Big questions: what is the Universe made of? what are the fundamental forces?

I All known forces of Nature described with amazing precision through the Standard Models of particle physics and Cosmology

I 5 years ago: all particles predicted by Standard Model discovered, except Higgs & graviton (or classical counterpart, gravitational waves)

I July 2012: Higgs particle detected at LHC (CMS and ATLAS)

I September 2015: Gravitational waves detected by LIGO

I Standard Models now complete as far as their known parts are concerned

I Is there something else missing?

I Yes: the dark side of the Universe! (dark matter, dark energy) We may be lucky and witness not only the completion of the Standard Models, but also a first glimpse into the dark side of the Universe within our lifetimes!

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Amazing achievement of humanity from late 1600 till early 2000!

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I Yes: the dark side of the Universe! (dark matter, dark energy) Understanding the dark side may take a couple of decades — interesting times for fundamental physics!

We may be lucky and witness not only the completion of the Standard Models, but also a first glimpse into the dark side of the Universe within our lifetimes!

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Outline

Theory

Technology

Observation

Applications

Daniel Grumiller — Gravitational waves Technology 9/20

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How to measure gravitational waves?

I In principle easy!

I In practice nearly impossible!

I Laser interferometry to the rescue

I LaserInterferometer Gravitational-WaveObservatory

I Can resolve subnuclear distances with laser interferometry

I Good news: sensitive enough to resolve gravitational waves!

I Bad news: sensitive to background noise (tiny seismic activities, thermal noise, passing trucks, ocean waves, ...)

I Reduce errors by having two identical experiments, one at West coast (Hanford, Washington), one at East coast (Livingston, Louisiana)

I Reduce local errors by suspension system to isolate mirrors from shaking

Suspension system works (in principle) like that of a (very advanced) car — wheels feel bumps, but are decoupled from car

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Typical gravitational waves change spatial distances by a small fraction of the size of a proton!

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(Advanced) LIGO needed 25 years of development and 500 million $ investment, involving more than 900 scientists and engineers

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LIGO suspension system

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Outline

Theory

Technology

Observation

Applications

Daniel Grumiller — Gravitational waves Observation 12/20

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September 14, 2015, 5:51am (Boston time)

I East and West coast data compatible with each other

I Gravitational wave signal significantly above background

I Matches very precisely predictions from black hole merger

Daniel Grumiller — Gravitational waves Observation 13/20

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Outline

Theory

Technology

Observation

Applications

Daniel Grumiller — Gravitational waves Applications 14/20

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Theoretical implications of gravitational wave discovery

I General relativity confirmed in qualitatively new way

I To be more precise: 2-body problem amazingly complicated in

General Relativity (100 thousands of CPU hours necessary to describe merger process of two black holes)

I Gravitational waves propagate with the speed of light

I Gravitons (like photons) are massless

I Black holes confirmed in a qualitatively new way

I Black holes hard to observe since nothing escapes (by definition)

Having said all this:

I gravitational waves were expected to exist

I interest therefore mostly in experimental applications!

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Experimental applications: gravitational wave astronomy

A new way to experience the Universe!

I Humanity is no longer deaf to gravitational waves

I Humanity is no longer deaf to gravitational waves!

I Humanity is no longer deaf to gravitational waves!!!

I Beginning of the era of gravitational wave astronomy

I You can hide from light in the shadows, but not from gravity

I Example 1: new properties of black holes, neutron stars, or other massive objects

I Example 2: supernova early warning system

I Example 3: early Universe (light blind to anything before Universe was 370.000 years old)

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Industrial applications

I Curiosity driven research not focused on industrial applications

I This is ultimately reason why major industrial breakthroughs come from curiosity driven research

“Electric bulb was not created in an attempt to improve on candles”

I Two classes of applications: spin-offs and direct applications

I Famous spin-off example: wwwfrom CERN research laboratory

I Direct application of general relativity: GPS

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I It took 8 decades to develop the first industrial general relativity application

I Patience needed in fundamental research on big questions

I Funding must come from public sources, not from industry

I Conversely: Public funding should go to fundamental research, not to industry (Austrian funding agency FWF in dire straits)

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Congratulations to the Advanced LIGO team at MIT and 90 other institutions!

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I hope you enjoyed my talk!

... any questions?

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Backup slide

Educational video by LIGO