Last month the Nobel Prize for Physics has been awarded to three American physicists for their ” decisive contributions to the LIGO detector and the observation of gravitational waves”
All are retired professors and quite old, not unusual for Nobel Prize winners…:-; More unusual is that this Nobel Prize has been awarded for the observation of gravitational waves in September 2015, only two years ago! The time between a discovery and the Nobel Prize is often 10-20 years and tends to increase
In this case the physics community was pretty sure that the Nobel Prize would go to LIGO, the Laser Interferometer Gravitational-Wave Observatory, where the gravitational waves were observed. Problem is that a Nobel Prize (with the exception of the Peace Prize) can not be awarded to an organisation but only to a maximum of three individuals (and never posthumously). And the article in Physical Review Letters, where the discovery was published in February 2016, has more than 1000(!) authors. Here is the beginning of the author list
In this blog I will explain why these three people were selected. But first I must tell a bit more about gravitational waves, and why physicists are so excited that they have been observed.
Motion takes place in 3-dimensional space as a function of time. Both space and time are absolute concepts, independent of each other.
Newtonian mechanics works extremely well, but there is one disturbing fact, the speed of light c in vacuum turns out to be always the same, no matter how fast the light source is moving itself. Einstein “solved” the problem in 1905 by accepting the constancy of c as a fact, which resulted in his Theory of Special Relativity (TSR)
But it came at a price! Space and time are no longer absolute and independent in this theory, together the three dimensions of space and the single dimension of time form a 4-dimensional continuum, called spacetime .
Gravitation doesn’t play a role in the TSR, but in 1916 Einstein publishes his Theory of General Relativity (TGR). In this theory gravitation is described as a curvature of spacetime. A massive object like the Sun curves the spacetime in its surroundings and a planet like Earth just “follows” this curvature.
A consequence of this theory is that even light would follow this curved spacetime and will be deflected when it passes close to the Sun. This prediction was successfully confirmed only a few years later. During a solar eclipse the stars near the Sun became visible and their position was shifted in complete agreement with the TGR. It was front page news and made Einstein world famous.
Another prediction of the TGR was that (accelerated) motion of massive objects could produce waves and ripples in this fabric of spacetime. Mind you, in spacetime itself ! However, these waves and ripples were estimated to be very small, maybe only measurable if those objects were extremely massive.
For example, two black holes or neutron stars, orbiting each other.
Here is an artist impression of the gravitational waves caused by two orbiting black holes. I have hesitated to include this image, because I find it very confusing, suggesting that the cells of the spacetime fabric are moving up and down, whereas the cells themselves are changing shape, stretching and contracting. But the image comes from LIGO, so who am I…:-)?
After this long(?) introduction it is time to go back to LIGO and the three Nobel Prize winners.
LIGO has a long and complicated history, starting in the 1960! Here are some important dates. The names of the three Nobel Prize winners in blue.
In 1968, almost 50 (!) years ago, Kip Thorne (Caltech) did calculations about the gravitational waves of black holes. Which, by the way, had not yet been discovered at that time, but their existence followed from the TGR! He came to the conclusion that detection should be possible. Also in the 1960s, Rainer Weis (MIT) proposed to use interferometry to detect the incredibly small variations in the fabric of spacetime. See below for more about interferometry.
In 1980, under pressure of the American National Science Foundation (NSF) , MIT and Caltech joined forces in the LIGO project. But progress was slow and funding not easy.
In 1994, Barry Barish (Caltech) was appointed director of the project. He was a good organiser, proposed to build the LIGO detector in two phases. This proposal was approved by NSF and got a budget of USD 395 million, the largest project in NSF history!
In 2002, the first phase of LIGO became operational, but no gravitational waves were detected.
In 2004, funding and groundwork started for the second phase, “Enhanced LIGO”, four times more sensitive than the first phase.
In September 2015, after a 5 year overhaul of USD 200 million was completed, Enhanced Ligo started operating.
Within days, on 14 September at 9:50:45 UTC, Enhanced LIGO detected gravitational waves for the first time in history.
So, what is an interferometer? Here is a sketch of the LIGO interferometer
And who could better explain how it works than Rainer Weis himself?
What may not be fully clear from the video is the huge scale of this LIGO project.
Two “identical” interferometers have been built in the US, about 3000 km apart
Here is an aerial view of the Hanford interferometer, each of the arms is 4 km long!
Both interferometers can be seen easily on Google Earth. Left Hanford, right Livingston.
As Weis explained, gravitational waves cause small differences in the length of the arms. Very, very small. In the order of 10-19 m, that is about 1/10.000 part of the size of a proton. Read that again and again, I still find it difficult to believe..:-)
The sensitivity must be about 1/10.000 part of the size of a proton.
New technology had to be developed to reach this sensitivity. Ultra-high vacuum, very precise mirrors, extremely stable lasers. Noise reduction (thru seismic vibrations, a passing truck, etc) is the main problem. That is also the main reason that two interferometers were built. Accidental noise should be different in both detectors, but a gravitational wave should reach both (with a slight time difference, because of the distance between the two detectors).
Critical is the suspension of the mirrors. They must be absolutely stable. Here two images of the damping and suspension systems. Click here for details
What kind of signal do we actually expect? Let’s concentrate on orbiting back holes (it is called a binary), like Thorne did. As shown in the earlier image, they produce gravitational waves, but those are way too small to be detected. However, the binary will loose energy sending out these waves, as a result over time the two black holes will get closer and closer. Until they come so close that they will merge into one larger black hole, a cataclysmic process that may take less than a second! It is during this phase that the gravitational waves are much stronger and might be detectable.
Here is a computer simulation of the merger of two black holes. The simualtion has been SLOWED down about 100 times, in reality the merger occurs in a split second. The “moving” background stars are a result of the extreme distortion of spacetime.
Click here to see the gravitational waves, sent out during the merger.
You will notice that before merging the two black holes spin faster and faster, distorting the fabric of spacetime more and more. It is a bit similar to a bird chirp: increasing frequency and loudness. After they merge into one, no more gravitational waves.
So, what happened on 14 September 2015? The two interferometers were to start the first research run on 18 September and were already in fully operational “engineering mode”, when at 9:50:45 UTC both detected the typical “chirp” signal. For testing purposes sometimes “fake” signals were injected, to test the alertness of the system and the scientists. It took a few hours before it became clear that this was a real signal and not a test!
Here is the “Nobel Prize winning” signal. The red graph is from Hanford, the blue one from Livingston (the Hanford signal is also shown, inverted and shifted in time) Notice the time scale, the whole merger takes place in a few tenths of a second!
The lower two graphs show a fit to the data, using Numerical Relativity. It is surprising how much information can be extracted from these two graphs. Here is a (partial) result
Two black holes, with a mass of 35 and 30 M☉. (solar mass) , at a distance of about 1.4 billion lightyear away from Earth, merged into a single black hole of 62 M☉. .
The mass difference of 3 M☉ , was radiated during the merger as gravitational waves. That is an awful lot of energy! The estimated peak emission rate was greater than the combined power of all light radiated by all the stars in the observable universe! If you don’t believe me, click here.
This first event has been named GW150914. GW stands for Gravitational Wave and is followed by the detection date 14-9-2015. In the past two years more gravitational waves have been detected, here is a list
If you look at the location, you see that in the first five events the location of the binary is not well-defined. The reason is that you need more detectors to determine the location accurately, two is not enough.
The sixth event, GW170814 was not only detected by LIGO, but also by Virgo , the European counterpart of LIGO. This interferometer is located near Pisa in Italy. Same setup as LIGO, slightly smaller arms (3 km)
Virgo was also designed in two phases. The first phase did not detect gravitational waves. In 2106 Advanced Virgo became operational and is now cooperating with LIGO. Another interferometer will be built in India: INDIGO .
The last event, detected until now, GW170817 (about three months ago), is an interesting one, because it is not a merger of black holes! For the first time a merger of two orbiting neutron stars has been observed. The masses of the two stars are comparable with the Sun and the binary is closer to Earth, although still a respectable 130 million lightyear! It is not sure if the merger resulted in a neutron star or a black hole. But anyway, a merger of neutron stars should result in visible light coming from the debris after the merger.
Because of the detection with three interferometers, it was possible to narrow the region of space where the gravitational waves came from. The location predicted by LIGO/Virgo was still large, about 150 times the area of a full moon. Within hours after detection, alerts were sent to astronomers all over the world and a massive search started.
Here is an image of this elliptical galaxy. The inset shows the light coming from the merger, getting weaker and weaker, as expected.
More interesting discoveries can be expected in the future, this is just the beginning.
When you want to learn more about this fascinating new field of astronomy, you should read the book Ripples in Spacetime, written by Govert Schilling