Who has heard of GW170817 ? | Frederique Marion | TEDxBritishSchoolGeneva

remember when you throw stones in the lake and it caused ripples when I was doing this as a girl I had no idea that this would turn out to be the analogy I would most often use to explain gravitational waves to people who are not scientists gravitational waves are prediction of Einstein's theory of general relativity which is the modern theory of gravity that replaced Newton's classical theory in the early 20th century Einstein's theory is based on geometry masses induce a space-time curvature and that curvature is responsible for attracting other masses now when masses move ripples in the space-time curvature are generated and propagates through space it's a bit like those ripples on the lake but on a much larger scale and what vibrates is space itself and those space-time vibrations what we call gravitational waves or G W is for short and I want to tell you the story of how not seeing gravitational waves led to a groundbreaking scientific discovery the GW sources that we detect are generated in very powerful Astrophysical events like stars colliding and we're interested in both the waves themselves to probe Einstein theory and in the events that create them and in that sense observing GWS is a new type of astronomy and it's a very difficult type of astronomy because GWS are extremely elusive and hard to detect let's take a look at numbers to understand how big a challenge it is when a GW which is earth space is briefly distorted distances change and all detectors try to register that change and the way our detectors work is the used light from a laser source monitor all the distance between suspending mirrors that are several kilometers apart and GW is going to change that distance of several kilometers for a short time by 10 to the minus 19 meter which is an absolutely tiny amount it's a billion times smaller than an atom and it's even ten thousand times smaller than the nucleus in an atom it is so small that it took decades to get detectors that would be sensitive enough to record such a small signal there are three such detectors we called them interferometers around the world the two LIGO detectors in the US and the Virgo detector in Europe Virgo and LIGO operate together they form a network of detectors and a big collaboration of more than a thousand people and we started building LIGO and Virgo twenty five years ago or so and we have kept improving them to make them more and more sensitive until finally on September 14 2015 the first gravitational wave signal was detected by the two LIGO detectors so well ago was not operating at the time but it was a milestone for the collaboration and it was just amazing to see that first beautiful signal received from a fascinating source colliding black holes more than a billion light-years away so it took a long time before GWS were detected and you know what they say that the path is the goal and well it has to be the case when you walk the paths for so long before getting any real results it certainly helps that you don't walk the paths alone collaborative work is a great way to sustain motivation and progress in the long run and probably also helps that you don't know beforehand that the past is going to be so long when I joined the effort in 1993 I was aware that detecting the first gravitational waves would take some years maybe even I don't know maybe ten years I didn't anticipate that having children and raising them would almost start looking like a short-term project in comparison now let's get back to our GW sources there were a few detectives in September 2015 most of them from colliding black holes those black holes are what remains of massive stars once they have exhausted the fuel that keeps them shining and they collapse under their own weight gravity is so strong around black holes that nothing can escape not even light that's why they are black and when two black holes spiral toward each other GWS are emitted which drives the black holes closer together emitting stronger and stronger GWS until the black holes collide and merge and those binary black hole mergers as we called them they were corundum Li and we have detected a handful so far and we named them after the date when the signal was recorded so GW 1509 14 for the very first one on September 14 2015 now of the six black hole mergers reported so far there is one that is very special to us namely GW 1708 14 so it's a very recent one it was detected three months ago on August 14th and on that day not only were the two LIGO detectors operating but viago was too and it was the first signal seen in three detectors the first one ever detected by Virgo now why does it matter so much that a signal is seen by three detectors the reason is that it's the only way that we can locate the source in the sky with some precision a single GW detector cannot tell where a signal came from because the detector sees most of the sky it's more sensitive to signals that come from directly above or beneath the detector and there are some blind directions have to keep this in mind but the detector sees most of the sky at any given time so it's not like a telescope that you have to point to a particular direction it's more like one of our ears which can hear sound no matter where it comes from with the consequence that it's kind of hard to tell where sound is coming from now speaking of sound have you ever heard the sound of GWS let's try to hear one now because we have two ears and a brain that is able to compare the sounds received by each of our ears we can get some sense of where sound is coming from and we do just the same with GW detectors by comparing the signals received in several detectors we can nail the sauce position in the sky and with two detectors the localization is very crude but a third detector is a game-changer it really narrows the position to a small area and that's why it was so important that LIGO and viago worked together and why it was so great to make a triple detection so after GW 1708 14 we had three days to celebrate and recover until another signal came that was even more extraordinary GW 1708 17 came on August 17 and the first remarkable fact about it is that it is from a different type of source instead of black holes colliding it was neutron stars colliding a neutron star is also the remnant of an extinct massive star but one that was not quite massive enough to collapse to a black hole and the other remarkable thing is when neutron stars collide you expect a very powerful blasts to follow a blast that you can hope to see with telescopes provided that you can point the telescopes quickly enough to the right direction in the sky because the light from the blast is dim and it fades away rapidly now on that day are three detectors were operating so we were well equipped to localize the source and ask our astronomer colleagues to move into action trouble was the signal was quite loud in the two LIGO detectors but there was no signal to speak of in Virgo no signal big enough to be identified as a signal while sobering as this was it turned out to be very valuable information because it could only mean one thing that the source was close to one of those directions that the Virgo detector was blind to at that time and that information was key to pinpoint a small area in the sky where the source had to be and asked astronomers to target that area and they did in the night that followed and sure enough for the first time they were able to witness the explosion that followed the neutron star merger and the gravitational wave signal and the amount of data and insight that was collected on and from this one event is just incredible one of the things that we learned is that GWS do propagates through space at the speed of light just as Einstein had predicted how could we check that well the first light from the merger reached us 1.7 seconds after the GW signal which means that gravitational waves and light after traveling for 130 million years arrived to us within two seconds which is pretty good synchronicity so this was the story of how by essentially not hearing GW 1708 17 there go allowed to locate the souls in the sky so precisely that telescopes could rush there and witness the explosion that followed and gather an incredible amount of information and discovery that was the first multi messenger one so a new type of events observed with both gravitational waves and light and I'd like to close by suggesting that you ponder the various time scales that are involved in this story so a century between Einstein's prediction and G W detection a last time to build sensitive detectors 130 million years from the source to the earth and a delay of one point seven seconds between gravitational waves and light upon arrival you [Applause]

Channel: TEDx Talks

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Keywords: TEDxTalks, English, Science (hard), Ideas, Physics, Research, Science, Technology, Time

Id: 3UkNzLIFeH0

Length: 12min 18sec (738 seconds)

Published: Fri Jun 01 2018