Monday, February 22, 2016


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


  • 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


  • 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.


Steven Rios said...
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Darcy said...

Hey Howard - I play volleyball with Matt and Lisa (well, not lately - they've been pretty busy!).