This week, as we watch the snow storm sweep across eastern Ontario, we can cozily sit back and reflect on the announcement last week of the ‘storm’ that was observed in the outer reaches of space .. and time… Last week, the LIGO (Laser Interferometer Gravitational Observatory) team announced the confirmation of the first detection of gravitational waves. On September 14 last year, a series of gravitational waves of a very distant black hole merger event first passed through the Livingston detector in Louisiana, then just 7 milliseconds later, passed through the second detector in Hanford Washington. Both instruments shoot lasers through 4-kilometer-long tunnels in a near-perfect vacuum. The laser light bounces off of seismically isolated quartz mirrors. The passing gravitational waves slightly altered the path lengths in the arms of both detectors by about .002 fentometres (fm) (1/1,000 the width of a proton). That slight change created a characteristic interference pattern in the laser light, an event LIGO scientists have classified as GW150914 – the first event, we hope of many…
Source of the Gravitational Waves:
The twin observatories nearly simultaneously detected the gravitational ripple caused by a massive collision of two black holes at an estimated distance of approximately 1.3 billion light years. The astronomical significance lies in the result: it is a product of observation and not just simulations. The signal detection device is not an ‘optical’ telescope capturing electromagnetic radiation but a laser interferometer capable of capturing ripples of gravitational radiation.
LIGO consists of two L-shaped facilities, one near Hanford, Washington, and the other near Livingston, Louisiana. At 5:51 a.m. (EDT) on September 14, 2015, both labs caught the gravitational-wave signature of two colliding black holes to form a single black hole losing 3 solar masses in the collision.
Kip Thorne, the co-founder of the LIGO project described this observation of the binary black hole collision event as a ‘storm in the fabric of spacetime’. Here is the interview
What do we mean by ‘storm in the fabric of space-time’?
By ‘storm‘, we mean the massively energetic collision of two objects that produce strong enough gravitational radiation to cause the length of the LIGO observatory tunnels to shrink and expand by scientifically significant femtometres! The collision produced a mass-defect or loss of 3 solar masses – producing an ENORMOUS source of relativistic energy detectable by the twin LIGO instruments.
By ‘spacetime‘ we apply the general relativity concept that combines space and time into a single entity because the observed rate at which time passes for an object depends on the object’s velocity relative to the observer and also on the strength of changes in a gravitational field that produces gravitational waves
(Schematic shown below) Relativistically, the event happened 1.3 billion years ago as well as 1.3 billion light years away.
Here is a description of the instruments and the analysis of the signal detection:
A schematic of LIGO:
A beamsplitter sends light along two paths perpendicular to each other. Each beam then bounces between two mirrors, one of which
allows a fraction of the light through. When the two transmitted beams meet and interfere, they’ll cancel each other out — if the length of the path each beam has travelled remained constant. But if a gravitational wave passes through, it’ll warp spacetime and change that distance, creating an interference pattern that the system will detect. (In the above diagram, we have a potential wave)
LIGO Signal Analysis:
What we are observing here from the signal extracted from the interference pattern is the actual dimensional change in the length of the tunnel in units of dimensionless strain (ΔL / L). The actual ripple distortion was reported as: .002 femtometers or .002 x 10-15 meters NOTE: LIGO didn’t observe over the whole time-period of the binary black hole interaction but it did see the last few cycles of the inward spiral, the merger itself, and the “ringing” effect as the merged black hole settled into its new form. B. P. Abbott & others, “Observation of Gravitational Waves from a Binary Black Hole”, Physical Review Letters
“Based on the signal’s amplitude (that is, the height of the gravitational wave), team members estimate that the colliding black holes had the masses of about 36 and 29 Suns (standard measure of solar mass), respectively. Milliseconds before they merged, these objects spun around each other at nearly the speed of light. LIGO watched all three predicted phases of the collision: the black holes’ spiral into a merger, as well as the ringing of the merged object as it settled into its singular form… Encoded in these light patterns is the information about the relative length change between the two arms, which in turn tells us about what produced the gravitational waves. The merged black hole contains about 62 solar masses; so it’s short three solar masses — the gravitational waves themselves carried away three solar masses worth of energy. The minuscule difference in the waves’ arrival times at the two facilities was exactly what’s expected for gravitational waves, which travel at the speed of light. The LIGO team claims a 5.1-sigma detection, meaning the odds of the signal occurring by chance are about one in 3.5 million.”- See more at: Sky and Telescope – Gravitational Wave detection – a new era of science
To become more involved with LIGO visit the LIGO Open Science Center. Here is the fact sheet from LIGO Open Science Center .
A reader asks:
“From the time I first heard of this, I wondered if it was just a happy co-incidence that the collision was detected just at the time the LIGO was turned on or are these collisions fairly frequent?”
These collisions are not frequent – but possible ! Martin Hendry – a LIGO team member in Glasgow calculated that it would happen not to long after advanced LIGO was turned on. Here is a wonderful video of his presentation at a TED talk: Martin Hendry discusses LIGO