The Discovery of Gravitational Waves – Merging Black Holes and Advanced LIGO

An animation illustrating two colliding and merging black holes in outer space.Black Holes Far Ago Have Been Causing a Stir…

You know how when you throw a rock into a pool, that makes ripples in the water?  And how Einstein once upon a time predicted that the very mass of stars and planets should warp spacetime?  Although we have had a justified inkling that Einstein was right for quite some time, we had never before detected such a phenomenon.  Until THIS happened…

In nerd language, it’s what’s called a “Wibbly Wobbly Timey Wimey”!  The very fabric of spacetime “wobbled”.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) actually discovered ripples in space-time created by two merging black holes in a galaxy far faraway, a long time ago.

 

An aerial view showing the surface structure of LIGO Hanford Gravity Observatory.
Aerial view of LIGO Hanford Observatory Source: ligo.org

LIGO

The first direct detection of gravitational waves is without a doubt one of the most remarkable breakthroughs of our time.  The Advanced LIGO laboratories located in the American states of Washington and Louisiana have actually managed to trace the warping of space from the merger of two black holes about 1.3 billion light-years from Earth.

A diagram explaining the basic design of the LIGO interferometer apparatus.
The Basic Design of a LIGO Interferometer Source: ligo.org

LIGO (Laser Interferometer Gravitational-wave Observatory) is a joint project between scientists at MIT, Caltech, and many other colleges and universities – a large-scale physics experiment and observatory, specially designed for the purpose of detecting gravitational waves.  Co-founded in 1992 by Kip Thorne, Ronald Drever of Caltech and Rainer Weiss of MIT, the project and analysis of the data for gravitational-wave astronomy are organised by the LIGO Scientific Collaboration, which includes more than 900 scientists worldwide, as well as 44,000 active Einstein@Home users.

And here’s how it works.

 

Is This “For Real”?

A diagram explaining the basic design of the advanced LIGO configuration.
The Advanced LIGO optical configuration is a bit more complicated than a basic Michelson interferometer. Source: Aasi et al. 2015

The Michelson interferometer is a common configuration for optical interferometry.  The Michelson interferometer produces interference fringes by splitting a beam of light.

The Michelson-Morley experiment was performed in 1887 to compare the speed of light in perpendicular directions, in an attempt to detect the relative motion of matter through the stationary luminiferous aether (“aether wind”).  Although the result of the experiment was negative, the Michelson-Morley experiment was a turning point for the theoretical aspects of the Second Scientific Revolution and soon initiated a line of research that eventually led to special relativity.

 

Laser Interferometry

The idea of laser interferometry is to split a high-powered laser beam and send its light on separate paths along two vacuum tunnels, arranged in an L-shaped configuration.  The two beams are bounced back and forth by mirrors, before eventually returning back to their starting point where the beam is reconstructed and sent to detectors.

If gravitational waves pass through the lab, the expectation is that the light paths will be ever so slightly offset.

 

Advanced LIGO

The Advanced LIGO optical configuration differs somewhat from the basic Michelson interferometer.  The laser beam starts from the left, passing through subsystems to make sure it is stable.

It is split in two to pass into the interferometer arms at the top and right of the diagram.  Then, the laser is bounced many times between the mirrors to build up sensitivity.

The interference pattern is read out at the bottom.

Normally, the light should interfere destructively, so the output is dark.  A change to this indicates a change in length between the arms.  That could be because of a passing gravitational wave.

According to Aasi et al. 2015 of the LIGO Scientific Collaboration, the Advanced LIGO gravitational wave detectors are second-generation instruments designed and built for the two LIGO observatories in Hanford, Washington and Livingston, Louisiana.  The two instruments are identical in design, and are specialized versions of a Michelson interferometer with four-kilometre-long arms. 

As in Initial LIGO, Fabry–Perot cavities are used in the arms to increase the interaction time with a gravitational wave, and power recycling is used to increase the effective laser power.

 

Notable Improvements

Signal recycling has been added in Advanced LIGO to improve the frequency response.

In the most sensitive frequency region around 100 Hz, the design strain sensitivity is a factor of 10 better than Initial LIGO.  Additionally, the low frequency end of the sensitivity band is moved from 40 Hz down to 10 Hz.

All interferometer components have been replaced with improved technologies to achieve this sensitivity gain.  Much better seismic isolation and test mass suspensions are responsible for the gains at lower frequencies.

Higher laser power, larger test masses and improved mirror coatings lead to the improved sensitivity at mid and high frequencies.  Data collecting runs with these new instruments began in mid-2015.

When gravitational waves pass through the Earth, the space and time our planet occupies should alternately stretch and squeeze.  The Advanced LIGO interferometers have been searching for this stretching and squeezing for over a decade, gradually improving the sensitivity of their equipment.

 

Weak Effect

Ah, yes!  The effect is VERY weak.

The expectation was that their experiments would need to detect disturbances no bigger than a fraction of the width of a proton – a tiny particle present at the heart of all atoms.  Consequently, only the biggest of masses moving at the greatest of accelerations are expected to warp their surroundings to any appreciable degree.

Astrophysical events that have the potential to radiate gravitational energy at the speed of light include:

  • supernovae explosions

  • neutron stars collisions

  • black holes mergers.

 

There is substantial noise to contend with.

For even the stillest of objects are vibrating on the smallest scales. Additionally, there are eathquakes, and even the “hum” of the Earth from coastal waves crashing worldwide.

Even when the equipment is damped by hanging the mirrors on suspensions, the whole set-up is still moving. But after years of research, gravitational-wave astronomers have modelled by a gravitational wave signal looks like.

The waves have a telltale frequencies.

 

And yet, the first detection of a gravitational wave was announced today.  The recorded data fits perfectly with the modelled expectation.

It was simultaneously detected by both LIGO experiments, barring a small delay which was explained by the 3,000 kilometre physical separation of the two sets of scientific apparatus.

 

A photographic animation showing a stone thrown into a pool of water and creating ripples.Gravitational Waves – Like Ripples in a Big Pool…

You know how when you throw a rock in a pool and it makes ripples in the water?  And how if you throw bigger rocks in, they make even bigger ripples?  One hundred years ago, physicist Albert Einstein said stars and planets and matter should make ripples in space, like the rocks in water, and he set up to demonstrate mathematically why he thought that:

Matter bends spacetime.

Mass follows the curvature of spacetime.

A graphic animation illustrating how matter bends spacetime according to Einstein's relativity.
According to Einstein’s General Relativity, any mass bends the fabric of spacetime.

Albert Einstein predicted the existence of gravitational waves in 1916, as part of his theory of General Relativity.  He said massive objects moving in space would cause “ripples” in spacetime – or gravitational waves.

Although the Maths all checked out, we had until now never been able to see those ripples. 

This new discovery is very significant.  It is the last great confirmation of Einstein’s ideas on General Relativity.

 

First Direct Evidence of Black Holes

This is also the first direct confirmation of the existence of black holes.

As scientists are all too aware, the gravitational influence of black holes on their surroundings, is so great that not even light can escape their stronghold.  Although it means they do not emit light, we know them to be out there, because we can see light coming from nearby stellar material being shredded and accelerated to high speed as it passes close to a singularity.

Gravitational waves are a signal coming directly from these “dark” objects themselves, and carry information about them.  In this sense, the discovery is also the first confirmed detection of black holes.

Astrophysics and cosmology are now entering a new era.

The technique opens the door to an entirely new way of investigating the Universe.

But this is not just about black holes…

 

A diagram explaining how the bending of light rays in space can be used for observational purposes in the technique of gravitational lensing.
Wherever matter bends spacetime, the path of light follows the curvature of spacetime to reach us.

Proving Einstein Right… Again!

Until now, we had no way in which to see beyond a certain point in the Universe.  This is because much of the Universe that we theorise to be out there does not radiate any electromagnetic radiation in any form (whether gamma-rays or ultraviolet, visible light or radio waves) or emit any particles.

In effect, the early Universe is opaque, and it is impossible to see across space before 380,000 years after the Big Bang.  At this point in time, the Universe had not cooled sufficiently to permit light to propagate.

Gravitational waves cannot be blocked or even deflected.

 

Unlike light or other particles, gravitational waves cannot be blocked or even deflected.  They pass through matter unhindered.

Theoretically then, there ought to be background gravitational waves washing over towards us from the earliest moments of our fast-expanding Universe.  This may well provide scientists with a free pass to explore cosmological phenomena that were previously out of bounds.

 

A Dark Universe

An artist's illustration of warp speed acceleration.
What really happened at time t = 0?

We can now set out to explore this “dark” Universe

We can now push back the frontiers of human astrophysical exploration and try to find out what happened right after the Big Bang.

Will there be a Nobel prize?  That’s a dead cert.

But the LIGO Collaboration involved over 1000 researchers working across a variety of fields using a range of complex technologies worldwide.  Who among the theorists and experimentalists will be deemed as having made the most significant contribution to that discovery?

 

Gravitational-Wave Observatories: What’s Next?

Future spaceborne gravitational-wave observatories may thus be able to detect the remnant signal that may bring us closer to understanding what happened at the very instant of the Big Bang, when time equals zero (t = 0).

What happened at time t = 0?

 

The two LIGO facilities will eventually operate in tandem with a third Italian lab, Virgo.  All three setups will be recording future events together, but researchers will be able to use their different positions and signal timings to pinpoint more accurately the location of its original source in the Universe.

Still Einstein WAS wrong when he predicted that such a disturbance in the force could never be detected…  You win some, you lose some.

So erm…  Take that, Einstein!