10th Anniversary of the first gravitational wave detection.

Graham writes ...

Credit: Source unknown.

Welcome everyone to another year of blog posts by John and myself. I hope you all had a lovely time anticipating and celebrating Emmanuel – God with us – over the Christmas period! And now that the calendar has clicked over to 2026, it is customary to wish all our readers a blessed and peaceful New Year. I don’t normally bother with New Year resolutions but this year I made one which was to try to make these blog posts shorter – however, I think I may have broken it already with this post?  Anyway, enjoy!
 
If you have read any of our past posts, or indeed this one, please leave a greeting or a comment at the end of this one – thank you.

The topic today involves another anniversary – 10 years since the first gravitational wave detection. However, that’s not quite accurate since the first wave to be detected hit our instruments at 10.51 UTC on 14 September 2015. It then took an army of researchers from 80 Institutions in 15 countries until February 2016 to work out what had been ‘seen’ with sufficient confidence before going public. So, it is approximately a decade since the first rippling ‘whisper’ in spacetime was announced, an event that is now catalogued, among many others, simply as GW150914.  So, this year we mark the 10th anniversary of this historic event, and reflect on what this discovery means, how it transformed astrophysics, and what lies ahead in the new era of gravitational-wave astronomy.

The signals from the two US LIGO systems, confirming the existence of gravitational waves, has hung in my office for 10 years! Credit: GrahamSwinerd.

In the above, I described the signal as a ‘whisper’, as it was extremely weak. This is a consequence of the event being very distant (1.3 billion light-years away) and also because, of the known fundamental forces, gravity is 10 to the power of 36 times weaker than electromagnetism (the other long-range force). Detecting the wave in this instance came down to measuring a change in distance of the order of a fraction of the diameter of a proton – a task that seemed impossible for the many decades since gravitational waves were discovered theoretically by Albert Einstein. Interestingly, Einstein published his discovery in 1916, so it took the experimentalists exactly 100 years to catch up with him!
 
For me, it was one of those events when you remember where you were and what you were doing when the news broke. So how come this rather esoteric happening registered as so significant for me? Many years ago in 1975 I graduated with a PhD on the topic of gravity waves in Einstein’s theory, and I honestly thought that the detection of such events would not happen in my lifetime. When the first historic detection of gravity waves occurred, I was at Lee Abbey, Devon (LAD) co-hosting a conference with John. For those of you who are familiar with LAD, 2015 into 2016 was the period when a major refurbishment of the main house was underway, and as a consequence the conference was held in the neighbouring youth activity centre called the Beacon – great days.

The Italian LIGO, located near Pisa, illustrates the vast L-shape laser interferometer characteristic of current gravity wave observatories. Credit: the VIRGO collaboration.

At the time of the discovery there were only two gravity wave observatories (referred to as LIGO, standing for ‘Laser Interferometer Gravitational wave Observatory’) operating in the world and both of these were in America, one in Livingston, Louisiana, and the other in Hanford, Washington State. This meant that it was not possible to triangulate the position of the event in the night sky, but the attenuation of the energy allowed the distance to be estimated. So, what was it that caused the subtle ‘chirp’ in the detectors? Quoting the Executive Director of the LIGO at that time, David Reitze - "Take something about 150 km in diameter, and pack 30 times the mass of the Sun into that, and then accelerate it to half the speed of light. Now, take another thing that's 30 times the mass of the Sun, and accelerate that to half the speed of light. And then collide them together. That's what we saw here. It's mind boggling."  Basically he’s describing two monster black holes spiralling around each other, getting closer and closer to each other due to the huge amount of orbital energy being lost in the form of gravitational waves.  One has a mass of 30 solar masses (1 solar mass = the mass of our Sun) and the other about 35 solar masses.  In the moments just before they impact and coalesce they are orbiting each other several tens of times per second.  At the moment when their event horizons merge and they become one, the event produces a pulse of pure radiate energy in the form of gravitational waves equivalent to 3 solar masses (E equals m c squared!).  It is the huge energy of this pulse that allowed the LIGO systems to detect the event, even though the black holes were so far away.  To put this pulse of gravity wave energy into perspective, in that brief moment of impact the energy produced was more than the combined luminous output of all the stars in the Universe! Extraordinary …!

Ten years have seen a great improvement in reducing noise and increasing sensitivity of the the LIGO systems. Credit: source unknown.

Way back in February 2016, I was blogging on a promotional website for another book (1), this one about spaceflight for lay persons, and of course I had to write about the events described above. There is a whole lot of interesting detail explained there, hopefully in an accessible way, that I wouldn’t want to repeat here. So, if you are sufficiently interested, please have a look at that post which can be found here. I think it conveys nicely my excitement at the time, and covers things like:
 

• what are gravitational waves,
• how Einstein discovered them in his gravity theory, and
• how LIGO works.

 
For those of you not keen on taking this diversion, however, how each LIGO works can be explained briefly by the following summary. Each LIGO is effectively a laser interferometer, where a high-power laser produces a light beam which is divided by a beam splitter.  Each beam is then directed at right angles to each other down two 4 km long evacuated tunnels that are arranged in an L-shaped configuration. The two beams are then bounced back and forth by mirrors, before eventually returning to their starting point.  If the passage of gravity waves has disturbed the curvature of space-time in the observatory there will be a difference in the length of the light paths of the two beams, which is estimated by analysis of the interference between the beams in the detector. This difference in the length of the light paths is anticipated to be miniscule – less than the diameter of a sub-atomic particle, as mentioned above!

The Japanese LIGO is built underground to reduce noise. Credit: ICRR, University of Tokyo.

Ten years on from the first detection there is now a global network of gravitational wave detectors, LIGO (USA), Virgo (Italy) and KAGRA (Japan), so it is now possible to determine the position of each event. This allows events to be observed by both gravitational wave detectors and across the electromagnetic spectrum (gamma- and X-rays, optical and radio) – an activity referred to as multi-messenger astronomy. In that time nearly 300 gravitational wave events have been catalogued including neutron star (2) collisions. Gravitational wave astronomy has matured into a science which has unveiled a Universe full of intriguing, violent events which were previously unforeseen. It has also allowed us to test our current physical theories, especially Einstein’s gravity in the strong field regime. Remarkably, after 110 years Einstein still stands, but he has a long time to go to beat the 200-odd years that Newton’s theory reigned before being falsified.
 
KAGRA (Kamioka Gravitational Wave Detector) is the most recent detector to became operational (February 2020) in Japan, and is the first gravitational wave detector built underground and the first to utilize cryogenic mirrors, which help reduce thermal noise and improve sensitivity. Looking beyond this, next-generation observatories like the Cosmic Explorer (USA) and the Einstein Telescope (Europe) promise an order-of-magnitude leap in reach and precision.

An impression of the US proposal for a next-generation observatory, Cosmic Explorer. Credit: MIT.

An impression of the Einstein Telescope, Europe's proposal for a future GW detector. Credit: ASPERA.

So, what opportunities for future research does the relatively new science of gravitational wave astronomy offer? As well as providing new tests for our theoretical understanding of gravity, the main avenues foreseen at present include:
 

• Gravitational wave cosmology.

The ‘Crisis in Cosmology’
There has recently been much research concerning the evaluation of Hubble’s constant Ho, which describes the rate of expansion of the Universe. A variety of existing techniques has produced results which are not consistent with each other – a situation that has been labelled ‘the Hubble Tension’ or the ‘Crisis in Cosmology’ (see the blog post for June 2024 for details – click on the relevant date displayed on the right hand side of this page). Gravitational wave events, particularly neutron star mergers, offer an independent way to measure the Universe's expansion which is a useful addition to the debate. The distance d to the merger can be estimated from the gravity wave signal’s amplitude and its recessional speed v from the redshift of its host galaxy's light, hence giving an estimate of Ho ( Ho = v/d).
 
Probing the early Universe
For thousands of years after the Big Bang the Universe comprised a very hot and dense ‘fireball’, which was opaque to the transmission of electromagnetic radiation. Then, about 380,000 years after the initial event, matter and radiation ‘decoupled’ and the light we now see as the cosmic microwave background (CMB) was free to propagate throughout the Universe. As a consequence ‘conventional’ astronomy, using the electromagnetic spectrum, is effectively barred from ‘seeing’ the creation event near time zero. However, we do know that cosmic inflation (see blog post for May 2023 for details), if it happened, would have produced copious amounts gravitational radiation which would still exist today as a gravitational wave background.  And I suspect that such a background would be saturated with information about the creation event, in the same way that the microwave background is packed with information about the decoupling era of the early Universe. I have no idea what such a gravity wave background observatory might look like, but gravity wave cosmology may be the only means of acquiring direct observations of the early events that gave birth of the Universe we see today.
 

• Multi-messenger astronomy

The benefits of the synergy of gravity wave astronomy with electromagnetic and neutrino astronomy are unknown at present, but there is every prospect of unlocking richer physical insights concerning the more exotic objects that populate our Universe.

To finish I just wanted to share a poem composed by a good friend Carol Plunkett in 2016. She is not in any way a scientist, but she was nevertheless inspired to write this by the discovery of gravitational waves.

The Only Echo
 
Did man just hear the echo
of the Universe’s start?
When something out of nothing
oscillated from a Heart?
When Life, vast and unfathomable
occurred when it was not,
and Space and Time immeasurable
commenced its divine plot.
So that, in some millennia,
a knowledge would arise
and render beings capable
of listening to the skies.
And while with probing instruments
they scoured the realms above
they almost tuned their frequency
to that first Word
Of LOVE.

Carol Plunkett
© Carol Plunkett, February 2016.

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The 2015 detection was like hearing a whisper from the cosmos — subtle, fleeting, yet transformative. Over the past decade, that whisper has grown into a chorus, with each gravity wave event telling a story of cataclysm, transformation and cosmic evolution. As we celebrate ten years, the journey is far from over. The detectors will get better, the observations richer, and who knows — perhaps in another decade we will trace gravitational waves back to the very origin of the Universe itself. I think I was born too early!
 
 
Graham Swinerd
 
Southampton, UK
January 2026
 
(1)   How Spacecraft Fly – spaceflight without formulae, Graham Swinerd, Spinger Science, 2008.
(2)  Neutron stars are formed as the remnant of a dying star. At the end of life, the star runs out of fuel which causes a catastrophic collapse. To become a neutron star, it needs an initial mass between roughly 8 and 25 solar masses, leading to a collapsed core mass (after a supernova) of about 1.4 to 3 solar masses. The result of this process produces a neutron star, a super-dense object comprised of neutrons. Anything more massive than about 3 solar masses will collapse further to form a black hole.