## Pages

### 2017 Nobel Prize in Physics was awarded to Rainer Weiss, Barry C Barish and Kip S Thorne for their work on LIGO (Light Interferometer Gravitational Wave Observatory) that detected the gravitational waves.

A lot has been said about the gravitational waves. Here, I give a brief idea of gravitational waves but I focus on the story of the heroic efforts of Rainer Weiss, Barry C Barish and Kip S Thorne. Newyorker has a detailed piece but I believe it didn’t cover some aspects. I would be focusing on them.

First, I give a brief idea of theory of gravitational waves. Next, I give a brief history of efforts to detect gravitational waves. This is followed by the story of the story of this year’s Nobel winners — the three challenges they faced and how they dealt with the challenges. The post ends with the next steps for LIGO.

### Theory of Gravitational Waves

Newton gave us the formula for gravitational force telling us about the quantity of force (F = GMm/r²) but didn’t say about the mechanism behind the force. Regarding the mechanism of gravity, in his book Principia, Newton just said “I will leave it to the consideration of the readers”.

In his General Theory of Relativity, Einstein explained how gravity arises. Einstein said that “masses warp the space-time” that causes other bodies to fall towards it. We can understand this using the famous trampoline sheet analogy.

If you stand on a trampoline sheet (rubber sheet), a big dent is formed. Einstein said that “mass” has a similar effect on space. It causes big a dent. Other particles around it fall into this, which we call gravity. This is a simplistic version. We are accustomed to thinking of space as something static but note that Einstein said that it’s not. Space can be bent!

This is for a static object. Now imagine that the object rotates. A rotating creates ripples in space and sends out “gravitational waves”. This is similar to ripples formed on the surface of water.

Why are gravitational waves special?: Because they can reach places in the universe where light can’t reach. So, we can learn a lot about universe!

### History of experiments to detect gravitational waves

All this theory is fine. But can we detect gravitational waves? It’s not easy.

When gravitational waves reach an object, they elongate/contract it. The essential idea of detecting gravitational waves is to detect the change in the length of the objects affected by the gravitational waves.

The problem is that the change in length is astonishingly small. For instance, earth contracts/elongates by a thousandth of the diameter of nucleus due to a gravitational wave produced by collapsing black holes.

Joseph Weber made one of the first attempts to detect gravitational waves.

Joseph Weber’s idea was that when gravitational waves pass through the molecules in aluminium tubes, the connections between molecules extend. Imagine that molecules are connected to each other through springs. As the wave passes through them, the springs expand. After the wave passes through the tube, the extended springs relax and the energy is released in form of sound. Joe Weber’s idea was to detect such sound.

Joe Weber published results saying that he detected gravitational waves but the experiment couldn’t be replicated. Hence, people doubted his research design and the quality of data.

At the same time, Richard Garwin, another famous physicist also worked on an instrument. Richard Garwin is an influential figure. This is from his wiki page.
his work on spin-echo magnetic resonance laid the foundations for MRI; he was the catalyst for the discovery and publication of the Cooley–Tukey FFT algorithm, today a staple of digital signal processing; he worked on gravitational waves; and played a crucial role in the development of laser printers and touch-screen monitors
There are interesting stories of debates between Garwin and Weber. In the interest of the length of this post, we can skip this for a moment.

The bottom line is that gravitational waves couldn’t be conclusively detected.

### Building LIGO

Now enters Weiss, Kip Thorne and Drever. Barish enters the scene later.

Weiss had no idea about either gravity or relativity. One needs knowledge of Tensor analysis to solve equations of relativity. Weiss had no idea of that. As it happened, Weiss was asked to teach a course on relativity. So, Weiss had to learn it and teach. In his class, one of his students asked him — if we could detect gravitational waves. That was the first time, Weiss thought of a design to detect gravitational waves.

Kip Thorne had built a prototype. Drever oversaw its construction.

The three — Weiss, Thorne, and Drever — together applied for funding to build the experiment to detect gravitational waves. This was designed to be an identical experiment setup at two different locations (Louisiana and Washington State). This is important for the purpose of confirming the waves. If the wave is detected simultaneously (within the limit of time taken by waves to reach Louisiana from Washington), then it adds credibility to the detection.

### The three challenges

All wasn’t easy. They had three main challenges before them.

1) Opposition to the project: The opposition was mainly due to skepticism of finding the waves and competition with other projects.

Gravitational waves are extremely weak. Only objects like black holes with strong gravity can cause detectable waves. Black holes weren’t detected by then. Some, in fact, were even skeptical of the existence of black holes.

It means that people weren’t sure of a possible astronomical object whose gravitational waves could be detected. This effort was hence thought to be a waste of money!

2) Readying the templates: One can detect a change in length of an object due to gravitational wave and reconstruct a signal. But how to know — which astronomical object produced it? Suppose if we know beforehand that a two black hole system produces a particular form of wave — we can compare this template to the detected wave. If the detected wave matches with this template, then we can be absolutely sure of the detection, as we now know its source.

The problem is that Einstein’s equations are too complex to solve to obtain such template. One can solve Einstein’s equations for a single static black hole. If you add the condition that the black hole is rotating, it gets complex and if you add another black hole to the system, the situation goes out of hand.

3) Practical challenge involved in designing and building the detector: The idea was to build a L shaped vacuum tube with equal arms and pass laser through both the arms and observe them when they return. Each arm is 4 km in length.

If the laser travels the same distance in both the arms by the time it returns, it produces a particular pattern. If it doesn’t travel same distance (which can happen due to elongation or contraction in one of the arms), it means that the length of the arms was changed (for those familiar with Physics, this is called interference; the word interferometer in LIGO comes from this). The change in length of the arms is the proof for a gravitational wave if it is detected simultaneously at two experiment locations.

The problem is that length could be changed due to several factors — a vehicle nearby applying brakes, some noise and various other things. Remember that they were trying to detect a change in length of the order of a thousandth of a nucleus. At that level, even the vibration of atoms due to temperature also matters. The main challenge was hence to shield the experiment from all these
Also, note that the arm of LIGO is 4 km. Since earth is spherical, bending of earth becomes significant at such lengths. The lasers have to be adjusted to that bending precisely.

### How did they overcome the three challenges?

1) The trio of Thorne, Weiss and Drever overcame the first hurdle of opposition to the project using various ways. Remember Richard Garwin that we noted earlier? He put in a good word to the funding agency. He asked the agency to take an independent opinion of Nobel Laureates. This process helped the project because Nobel Laureates weighed in.

The trio also pitched to the funding agency saying that there are already existing working prototypes, what they just needed was to expand the project. After some effort, the funding agency agreed. But the trio was asked to get a Director to the project, to execute it. Barry Barish enters the scenario now and becomes the director. Barry Barish was a nuclear physicist and was a complete outsider to this field but he took up the challenge. Drever goes off the project. Weiss, Thorne and Barry become the new trio. [Drever died recently in March 2017.]

The hurdle of identifying a possible object that produces detectable gravitational waves was overcome — thanks to the discovery of Hulse-Taylor binary in 1973. Hulse-Taylor binary is a rotating system of a pulsar (radiating neutron star) — neutron star pair. By studying the pulses of the pulsar, Hulse and Taylor realised that the Hulse-Taylor binary was emitting gravitational waves. They didn’t detect gravitational waves but inferred indirectly that they could be emitting gravitational waves. Now, this was given an example of a possible object, which can emit detectable gravitational waves.

2) The second issue of creating a theoretical graph of the gravitational waves was also addressed — thanks to the improved computing facilities that could solve Einstein’s equations and provide us with the graphs (template).

How did computers solve the equations? Solving a system of equations means that you have to find a value that satisfies the set of equations.

There are two ways of solving the equations. One method is to solve them using various transformations. It is called analytic solution. Another way is to just keep trying different values until you find a value that satisfies all the equations. This is called numerical solution.

We resort to numerical solution when we can’t solve analytically. The challenge with numerical solution is that human’s capacity to keep continuously trying various numerical values, is limited. Computers are of help here. With huge computational power, they can do this job of trying different values until they satisfy the equations.

As the computing power increased, it helped solve these equations and produce templates for different configurations of black holes.

3) The third hurdle was to overcome the challenges in design and construction. It ended up being a marathon effort of 22 years! You read it right. They worked on the project for 22 years relentlessly continuously fine-tuning the experiment to reach the required precision.

They built two detectors at different places — this is to cross-check the signal. Gravitational wave will be confirmed only if it is detected by both detectors simultaneously. They used a large combination of innovations in experiment design and complex algorithms to do shield the experiment from the surrounding noise.

They had numerous inbuilt checks. The head researchers would inject a fake signal once in a while to check if everything in the pipeline is working and also to keep people on alert.

Finally, on 14th September 2015, they observed a wave! At first, they didn’t believe. Many thought that this was one of the routine fake injections. But this time the signal was too clear to be faked! This made some hopeful. The head researchers went and checked the logbooks to see if it was an injected signal. It wasn’t.

Being the skeptical physicists that they are — they cross checked it for 6 months, making every possible verification to ensure that it’s really the gravitational wave.

Finally, they officially declared the detection of gravitational wave in February 2016. This wave was from a pair of collapsing black holes 1.3 billion light years away! We essentially detected a signal that originated 1.3 billion years back and travelled for 1.3 billion years! Isn’t it amazing?

The Nobel Prize 2017 for Physics was awarded to these three pioneers Rainer Weiss, Barry Barish, and Kip Thorne for taking the initiative to build the detector, surmounting numerous odds, and for relentlessly working for 22 long years to achieve what they did! They dedicated their whole life to this effort and that’s an enormous risk!
A well-deserved prize indeed.
There is a moral for all in this — persistence pays!

### What next for LIGO?

We are continuously detecting gravitational waves since the first discovery. At the time of the announcement of the Nobel Prize (October 3rd, 2017), four waves were detected, the latest being on September 28/29th 2017.

However, there is a practical limitation to detectors on earth. They can only detect signals above a particular strength. The signal strength can be amplified by increasing the length of the arms but building long arm detectors on the earth is difficult. The solution to that is “space”. We can build long arms there.

LISA is the space version of the experiment. Three spacecrafts relay lasers back and forth and gravitational waves are detected by these. The length of these arms is many orders than that of the detector on the earth — the length of each arm is in millions of km — three times the distance between earth and moon. Such distance amplifies the signal and hence can detect even weaker waves.

If all goes well, we might be able to detect the waves from The Big Bang!

Interested to learn more? Hear from the horse’s mouth! Watch the video below. Most of the story narrated above is from the video below.