“Ladies and gentlemen, we have detected gravitational waves. We did it!” – David Reitze, Executive Director of LIGO
[Updated February 14 and 21, 2016 to include additional links and references. Original posting on February 11, 2016.]
Last year, I wrote of the centenary of Einstein’s theory of general relativity. This morning, it was announced that the last remaining prediction of general relativity had been experimentally validated.
At a press conference this morning, the Laser Interferometer Gravitational-Wave Observatory (LIGO) Collaboration announced the detection of gravitational waves, the validating the last major prediction of general relativity. What’s more, the detected signal precisely matched theoretical predictions of the waves which would be produced by the merger of a binary black hole system. And, as if that were not enough, the waves are in the acoustic frequency range, making it possible to translate the signal into sound such that we can actually “hear” two black holes merging. (Okay, it is more of a “boop,” but it came from over a billion light years away.)
Rumors of this discovery had been circulating for a few months. Today’s press conference was timed to coincide with the publication of their results in a peer-reviewed paper in Physical Review Letters. The twin Advanced LIGO detectors (located at Hanford, WA and Livingston, LA) had officially come online on September 18, 2015, but this particular detection was made by both detectors (see graphic above) on September 14 during a “shakedown” run. Some of the early rumors regarding the detection had been called into question since it was known that the detectors had been designed to randomly inject false test signals into the experiment (in a manner to which the experimenters are blind) to evaluate their handling of the data. But, as it turned out, this signal was real. And now, after a whirlwind of rumors, the official announcement is out, along with the paper.
Here is a video of the press conference:
Background: What Are Gravitational Waves?
Einstein predicted the existence of gravitational waves as a direct result of general relativity in a paper published in 1916 (English translation here). Just as the acceleration of electrical charges causes the propagation of electromagnetic waves, Einstein predicted that the acceleration of mass would cause ripples in space-time. However, these predicted ripples would be so minute, even for powerful events, he despaired of them ever being detected.
As it turned out, this is one of a handful of things Einstein turned out to be wrong about. Gravitational waves were first indirectly detected back in 1974 by radio telescope observations of the binary pulsar PSR B1913+16 by Hulse and Tayler, work which netted the pair a Nobel Prize in 1993. (Sabine Hossenfelder has more of that story here.)
How LIGO Works
But that was an indirect observation, obtained by studying the decaying orbits of a binary pair of neutron stars. Direct observation is far more challenging, and is accomplished by means of laser interferometry.
Here is what happens in LIGO’s twin interferometers. Light from a laser is divided into two beams by a beam splitter, with the two beams going out at a 90 degree angle to one another. The beams travel along a four kilometer long path, strike mirrors at the end of the path, and return to the source location. There, the reflected light beams are re-combined into a single path, where constructive or destructive interference takes place depending upon the relative phases of the two beams, which in turn depends upon how far the beams have travelled. Subtle changes in this interference are used to measure minute changes in the distances that the two beams have travelled.
How minute? In the case of the LIGO results being discussed, the difference in the travel path is a mere fraction of the diameter of a proton!
Of course, the biggest challenge for the LIGO team is to prevent ambient vibrations from adding noise to the data. One of the most intriguing aspects of the experimental design of the LIGO detectors is the use of glass fiber quadruple-pendulums to suspend the optical elements, thus isolating them from local ambient vibrations.
It has been a long road since the first iteration of LIGO came online back in 2002, filled with hard work and innovation. Now, champagne corks are a-popping.
For More Information:
- LIGO’s Press Release
- APS announcement (with link to paper)
- Observation of Gravitational Waves from a binary black hole merger | LIGO
- Gravitational waves detected 100 years after Einstein’s prediction | NSF
- NSF’s LIGO Has Detected Gravitational Waves | NASA
- GRAVITATIONAL WAVES DETECTED, NEW ERA OF COSMOLOGY BEGINS | Perimeter Institute
- GRAVITATIONAL WAVES 101 | Perimeter Institute
- Einstein’s gravitational waves found at last | Nature
- Everything you need to know about gravitational waves | BackReaction
- Gravitational Waves Exist: The Inside Story of How Scientists Finally Found Them | The New Yorker
- Gravitational waves detected 100 years after Einstein’s prediction | Phys.org
- Scientists glimpse Einstein’s gravitational waves | Phys.org
- Detection of gravitational waves would open new window on universe | Phys.org
- IT’S OFFICIAL: GRAVITATIONAL WAVES HAVE BEEN FOUND | Popular Science
- The Dawn of a New Era in Science | The Atlantic
- Gravitational Waves Found in 1.3 Billion Year Old Black Hole Collision | NOVA Next | PBS
- How LIGO Detected Gravitational Waves | NOVA Next | PBS
- Not Just Waves but Black Holes (Go LIGO!!) | Physics Buzz | Physics Central
- LIGO: What You Need to Know | Physics Buzz | Physics Central
- In Milestone, Scientists Detect Waves In Space-Time As Black Holes Collide | NPR
- Scientists Detect Gravitational Waves, Proving Einstein Right | New York Times
- IT’S OFFICIAL: Gravitational waves have been detected, Einstein was right | ScienceAlert
- Scientists make first direct detection of gravitational waves | MIT News
- The First Detection Of Gravitational Waves Validates Einstein In A Whole New Way! | Forbes
- LIGO Sees First Ever Gravitational Waves as Two Black Holes Eat Each Other | Bad Astronomy | Slate
- Advance Thoughts on LIGO | Of Particular Significance
- LIGO detects first ever gravitational waves – from two merging black holes | IOP
- Gravitational waves detected 100 years after Einstein’s prediction | Max Planck Society
- Gravitational Waves at Last | Preposterous Universe
- We’ve Found Gravitational Waves. Now What? | Gizmodo
- Finding Beauty in the Darkness | New York Times
- Is Gravitational-Wave Claim True? And Was It Worth the Cost? | Scientific American
- The universe has a high (but not infinite) Sleep Number | Shtetl-Optimized
- LIGO LIVINGSTON OBSERVATORY | Atlas Obscura
- Einstein, A Hunch And Decades Of Work: How Scientists Found Gravitational Waves | NPR
- Growing Old With Einstein: The Long Wait For Detection Of Gravitational Waves | NPR
- Century-Long Search Leads to Landmark Gravitational Wave Discovery | Science Friday
- Gravitational waves are detected for first time | PHYSICS TODAY
- A faint ripple shakes the World | Quantum Diaries
- All Physics Is Local | The Atlantic
- CERN congratulates the discoverers of gravitational waves | CERN
- Gravitational waves found, black-hole models led the way | Phys.org
- Detection of gravitational waves would open new window on universe | Phys.org
- GRAVITATIONAL WAVES AND HOW THEY DISTORT SPACE | Universe Today
- Why You Should Be Excited about This Week’s Big LIGO Announcement | Scientific American
- Why It’s Hard for Black Holes to Get Together | NAUTILUS
- After 100 years, scientists are finally closing in on Einstein’s ripples | Ars Technica
- What will it mean if LIGO sees gravitational waves? | Starts With A Bang
- Can LIGO Test Quantum Gravity? | Forbes
- A common misconception about LIGO detectors of gravitational waves | ArXiv
- Gauge invariance and the detection of gravitational radiation | ArXiv
- GRAVITATIONAL WAVE PHYSICS | Kostas D. Kokkotas
- Fermi GBM Observations of LIGO Gravitational Wave event GW150914 | NSSTC/NASA
- Fermi-LAT OBSERVATIONS OF THE LIGO EVENT GW150914 | ArXiv
- The Role of Gravitation in Physics |Report from the 1957 Chapel Hill Conference
- My Thoughts On The LIGO-VIRGO Result | A Quantum Diaries Survivor
- Guest Post: Daniel Hoak, Gravitational Waves: How We Did It | A Quantum Diaries Survivor
- Henri Poincaré Predicted The Existence Of Gravitational Waves As Early As June 5, 1905 | Relativity and Beyond It