What Happened When Two Black Holes Collided?
On September 14, 2015, two detectors felt space itself stretch by far less than the width of a proton. The cause: two black holes merging into one.
In one paragraph
In September 2015 the two LIGO detectors, in Louisiana and Washington State, recorded the same brief ripple in space 6.9 ms apart. Its shape matched what general relativity predicts for two black holes of about 36 and 29 solar masses spiralling together and merging into one of 62, with about 3 solar masses of energy radiated as gravitational waves. Two independent analyses found the signal above 5.1 sigma. It is the first direct detection of gravitational waves; the sky position is known only to a region of about 600 square degrees.
How do the LIGO detectors catch a gravitational wave?
Each Advanced LIGO site runs a laser interferometer: a beam is split and sent down two arms of 4 km set at right angles, then recombined. A passing gravitational wave stretches one arm and squeezes the other by a minute amount, and the mismatch between the returning beams is the measurement. The quantity recorded is strain, the change in length divided by the length.
To see such small changes the instruments are built around silence. Mirrors hang from multi-stage pendulums on seismic isolation platforms, the light travels in ultra-high vacuum, and arrays of seismometers, microphones, magnetometers and other sensors watch for anything that could fake a signal.
Two detectors far apart are essential. A local disturbance shakes one site; a wave in space shakes both, in order, within the time light needs to cross the gap. The authors analysed 16 days of coincident data with two searches: a generic transient search with few assumptions, and a binary coalescence search that matched the data against about 250,000 predicted waveforms.
What did the first gravitational-wave signal show?
On a September morning in 2015 both detectors recorded a signal that rose in frequency and amplitude over about 0.2 s, from 35 Hz up to 250 Hz, then faded: the inspiral, merger and ringdown of two black holes. It arrived at Livingston first and at Hanford 6.9 ms later, consistent with a wave crossing the Earth.
The best-fitting model describes black holes of 36 and 29 solar masses merging into a single black hole of 62 solar masses. The missing 3.0 solar masses were radiated as gravitational waves, at a peak rate equivalent to 200 solar masses per second. The source lies at a luminosity distance of about 410 megaparsecs.
The binary coalescence search found the event with a combined signal-to-noise ratio of 24 and a significance above 5.1 sigma; the generic transient search found it independently at 4.6 sigma. Three consistency checks against general relativity found no deviation.
Why does the first direct detection of gravitational waves matter?
A fair reading is that the measurement is the smaller part of this result and the new sense is the larger one. Until this event, everything known about the universe beyond the solar system came through light and other electromagnetic radiation. Here the information arrived as a distortion of space itself, from a system that emits no light at all.
The detection also closes a long argument. Einstein predicted gravitational waves long before any instrument could test the idea, and doubted for years whether they were physically real; indirect evidence came from a binary pulsar decades later, but the wave itself had never been measured. This signal matches the strong-field predictions of general relativity, including the mass and spin of the final black hole.
Finally, it establishes that pairs of stellar-mass black holes exist and merge within the age of the universe, opening a field in which such events can be counted and used to study how massive stars live and die.
What does the paper not show?
Read before you quote it
With only the two LIGO detectors observing, the direction of the source is known only to a region of about 600 square degrees, far larger than any constellation. The masses and distance carry wide 90% credible intervals, and the spin of the lighter black hole is only weakly constrained.
One event cannot say how often such mergers happen; the authors' rate estimates range from 2 to 400 per cubic gigaparsec per year. Nor does the paper settle how two black holes this heavy came to orbit each other, listing formation in isolated binaries and in dense stellar environments as possibilities.
The tests of general relativity are consistency checks, not a full test of the theory in every regime, and the analysis covers 16 days of data. A second, weaker candidate in the same period had a false alarm probability of 0.02 and is not claimed as a detection.
- It does not show where in the sky the merger happened beyond a region of about 600 square degrees, or how two black holes this heavy came to orbit each other.
- It does not test general relativity beyond the three consistency checks reported.
- Only the two LIGO detectors were observing, so the sky position is localized to about 600 square degrees.
- The source parameters carry 90% credible intervals, for example 36 (+5, -4) solar masses for the primary; the spin of the secondary black hole is only weakly constrained.
- Rate estimates from one event are broad, from 2 to 400 mergers per cubic gigaparsec per year.
- The analysis covers 16 days of data; the second most significant event had a false alarm probability of 0.02.
Takeaways
- saysA pair of black holes merged, and about three times the mass of our Sun turned into ripples in space in a fraction of a second.
- saysThe ripple lasted a fifth of a second and reached the second detector six point nine milliseconds after the first, as a wave crossing the Earth should.
- saysA pair of detectors cannot place the merger better than a blur of six hundred square degrees, and a single event cannot say how such heavy pairs form.
- explainsA gravitational wave is a ripple in space itself, set off when heavy objects accelerate, that stretches and squeezes everything it passes through.
- interpretsA fair reading is that this detection gave astronomy a second sense: until then the universe could only be watched in light, and here it was felt to move.
Ask the paper
In PaperSays you can ask this paper anything, by text or by saying “Hey Aiden”. Three questions listeners asked:
What caused the signal?
Two black holes of about 36 and 29 solar masses spiralling together and merging into one of 62 solar masses. Hear the full answer in the app →
How sure are the authors it was real?
Two independent searches found it, with a significance above 5.1 sigma, and no environmental sensor saw anything like it. Ask in the app →
Where in the sky did it happen?
Unknown beyond a region of about 600 square degrees, because only two detectors were observing. Ask in the app →
Briefing reviewed by an editor before publication. The paper belongs to its authors; this page summarises it and links to the original. Reviewed by Editor. Paper licence: CC BY 3.0. Corrections: support@papersays.com.