In September 2015 the LIGO Scientific Collaboration and Virgo Collaboration made the first ever direct detection of gravitational waves from the merger of two black holes more than a billion light years away – a discovery that was awarded the 2017 Nobel Prize for Physics.  Less than five years later the new field of gravitational-wave astrophysics is now firmly established as one of the most vibrant research topics in all of physics – with a dozen more confirmed detections, a further 56 candidate events from the upgraded detectors’ third observing run, and a range of results that are bringing important new insights for cosmology, astrophysics, nuclear physics and relativity.    Join LIGO astrophysicist Professor Martin Hendry, from the University of Glasgow, as he charts the story so far – and what lies ahead – for this exciting new field.  

You can watch the recording of the webinar.

Additionally, Martin has provided a number of supporting links, including links to more in-depth topics via the excellent questions in the Q&A for your information below:

Many thanks for joining last week’s webinar on gravitational-wave astrophysics.  It was a real pleasure to give this talk and to get so many excellent questions.  I include below a list of some useful weblinks to find out more about LIGO and gravitational-wave research.  In the coming days I’ll post some more detailed links about some of the in-depth topics that were touched on in the questions.

 “If a gravitational wave stretches the distance between the LIGO mirrors, doesn't it also stretch the wavelength of the laser light?”

As I mentioned on the webinar, this is such a good question it features on our LIGO “frequently asked questions” page at  The answer there is quite brief, and mainly links to some other resources where it’s explored more fully – including a blog article by my LIGO colleague Amber Stuver:

Amber’s blog links in turn to some other short papers on this topic, one of which is a really nice paper by former LIGO spokesperson Peter Saulson. Peter’s article was published in the American Journal of Physics but a freely available version of it can be found here.

“To what extent do LIGO and Virgo’s observations tell us about the quantum nature of gravitational waves?”

 This is a great question, and I partly addressed it in a short article I wrote for New Scientist that was published in spring 2016, a few months after the first detection GW150914 was announced.  Quoting from that article, I said “Future observations of gravitational waves may lead to further insights about how a quantum gravity theory would work. In truth, though, for most of the sources that LIGO will see, our measurements should be perfectly consistent with the general relativity description. Our data won’t have the detail or “resolution” necessary to allow us to investigate many aspects of quantum gravity.  That said, our results can already be used to constrain aspects of those quantum gravity theories by putting limits on the mass of the graviton, the hypothetical quantum particle of gravity.”

In some of our recent LIGO and Virgo papers on the gravitational-wave detections to data we have explored tests of General Relativity, but as I mentioned in the webinar so far Einstein has passed the test with flying colours! (See links in the slides).   There have also been some very interesting papers exploring the possibilities for future gravitational-wave observations to test some other specific aspects of quantum gravity – see e.g. this paper on “quantum echoes” or this one about quantum effects of the “luminosity distance” to remote gravitational-wave sources (such as those which will be detectable with LISA).  This article in Physics World might also be of interest.

And the quantum properties of *light* are extremely important for the operation of LIGO and Virgo.  For instance, the technique of “quantum squeezing” has been recently implemented to enhance the sensitivity of the advanced detectors.  Here is an early LIGO Virgo Science Summary on this topic, and here is a LIGO news item on more recent developments.

 “What do gravitational waves tell us about dark matter and dark energy?”

This is another question that features briefly on our FAQ page.  Quoting from that short reply: “Dark matter and dark energy have had a big role in the history of the universe expanding (in fact we think dark energy is now causing that expansion to speed up!) and in the formation of galaxies and clusters of galaxies. But we don't expect dark matter to exist in nearly dense enough 'clumps' to produce gravitational waves that could be detected by LIGO.”   There is one intriguing possibility, however, which is that the dark matter *is* clumped, and could perhaps exist as black holes formed in the very early universe – and if these so-called “primordial black holes” exist, could they be detected through gravitational waves?  The jury is still out on this question, but LIGO and Virgo are searching for such (low-mass) black holes and our recent searches are discussed briefly in this Science Summary.  A detailed review article on the theoretical possibilities can be found here.

 One of the ways in which dark matter can influence the propagation of gravitational waves is through gravitational lensing.  We see this phenomenon routinely in *light* when we observe the distant universe, and we can be confident that future gravitational-wave observations may well detect it too.  A recent article on this topic that neatly summarises the current issues and future prospects is here.


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