LIGO

Last Friday I went to an MIT Panel on LIGO which was fascinating. Here are my notes.

Panel

• Moderator Rainer Weiss, Professor of Physics, Emeritus, invented laser interferometer gravitational wave detector, co-founded LIGO, also started COBE to detect Cosmic Microwave Background Radiation
• Lisa Barsotti, Principal Research Scientist,
• Edmund Bertschinger, Professor of Physics, theorist, cosmology and general relativity
• Matthew Evans, Assistant Professor of Physics,
• Salvatore Vitale, Research Scientist, data analysis, extracting signal from data

Rainer Weiss

• LIGO started in the 80s at MIT building 20, now gone
• No one cared about the old building so they could tear down walls, remove wiring, etc.
• Trucks on Vasser St shook building
• 1.5 meter prototype, two grad students
• F&T Deli in old Kendall Sq pre T stop. Ideas discussed at table there because they didn't kick you out. Now plaque there.
• Tons of students at all levels with all kinds of papers and thesis

Edmund Bertschinger

• If a tree falls and no one is there does it make a sound? No, because sound is waves hitting ear
• LIGO is the first direct signal from as close to a black hole as we will ever get
• To see this black hole would need a telescope that can resolve 10⁻²⁰ radians, 12 orders of magnitude smaller than we can
• Gravitational waves are oscillating tides traveling at the speed of light
• The distortion seen is like continents and oceans stretched from the moon tides
• LIGO built not to discover but as an observatory

Salvatore Vitale

• Animation, showed waves
• Different masses produced different signals
• Was 2 black holes of 30 solar masses each
• These were stellar black holes as opposed to those at the center of a galaxy (which would have been much more massive)
• The most massive black holes previously known which were not at center of a galaxy was half this size
• These black holes merged 1 billion years ago and weren't spinning that fast
• Mass tells us about the environment they were in
• Signal didn't deviate from General Relativity formulas
• 62 solar masses is result, 3 solar masses emitted in merger as gravitational waves in 0.2 seconds
• Sun has lost 0.03% of mass in last 5 billion years, so this was HUGE energy and yet it still took something as sensitive as LIGO to detect

Lisa Barsotti

• Amplitude of a gravitational wave (h) is 10⁻²¹
• L is length, so ΔL = h × L
• So try to use larger distances to see more movement, but still really small
• even if we could use the whole earth (L = 6,350km), that would require very precise measurements
• H atom is 10⁻¹⁰
• if L is 4km then it's 4x10⁻¹⁸ = proton diameter / 200
• it's crazy small we are superheroes
• One detector in Livingston, LA and one in Hanford, WA
• Each 4km tubes, 1.2m diameter, 10ms travel time between locations at c
• The more light you have you essentially amplify the wave
• Up to 125W entering interferometer, up to 1MW in each arm at full power (now operating at 100kW)
• Detector hears noise of interferometer + gravitational wave

Matthew Evans

• Three kinds of noise to be minimized and removed from signal:
  • Quantum Mechanics - Photons bouncing off mirror moves mirror makes noise, mainly high frequency noise
  • Thermal Noise - materials aren't at absolute zero, so brownian motion, lower frequency
  • Seismic Noise -, vibrations from outside world, lowest frequency

• incredible isolation, big scary laser, massive super optics
• 120 W laser
• roughly. 50 BBH merges each year I a volume of 1 Gpc³
• about 10 million galaxy's per Gpc³
• Advanced LIGO range now 0.1 - 1 Gpc depending on system mass
• We can expect 5 or more BBH events in next observing run (due later 2016)
• gravitational waves from Big Bang - frequency one over the age of the universe
• Future improvements:
  • Squeezed states of light to reduce quantum noise in the interferometer
  • Thermal noise reduced with better materials and cryogenics
  • Next big leap from a longer interferometer, up to 40km is doable, 400km is not

• Conclusions:
  • General Relativity is correct even in the strong field regime
  • Large stellar mass black holes exist in binaries and merge
  • Direct detection of gravitational waves is possible

Q&A

• Salvatore was first person to see the information come out of the data (the masses of the bodies)
• One location got the data 7ms ahead of the other and they're very proud of it
• Measuring amplitude so goes down with 1/distance, not 1/distance²
• Detector in Pisa Italy come online in 2016
• Another being built in Japan for 2017-2018
• Another copy of LIGO in India
• Not spherical waves. Technically quadropolar. Perpendicular to plane
• Just a sphere expanding won't create gravity waves, need things moving around each other to form a quadrupole system
• Though a lumpy supernova might
• Gravitational waves interact so weakly with matter, not absorbed by planets, etc.