How to Win a Nobel Prize in Physics: From Einstein to Thorne, Weiss, and Barish

The Nobel Prize in Physics for 2017 was awarded to Kip Thorne, Rainer Weiss, and Barry Barish for detecting a disturbance in the universe so faint that it moved their 4-km-long long detectors by one thousandth of an atom, or by 10–13 meters. While this was predicted by Einstein’s general theory of relativity in 1915, it took more than a hundred years of technological developments to reach this accuracy. Even a couple of years ago, it was thought that the technological precision was impossible to achieve. This article traces the history of this impossible achievement.

Electricity is crucial to our life in untold ways. It is produced by a force that we call the electromagnetic force, a force that describes the interaction of the electrical charge and the magnetic field. Gravity is the other crucial force affecting the large scale structure of the universe. All objects that have mass—essentially, everything around us—attract each other. However, this force is much weaker and is felt only when truly large objects are involved.

Einstein and the Behaviour of Gravity

A little over a hundred years ago, Albert Einstein came up with a theory about the behaviour of gravity. In what we now call the General Theory of Relativity (GR), Einstein proposed that if you are in a sealed chamber and experience a force that keeps you stuck to the bottom, there is no way of knowing whether this is because you are in a rocket that is being accelerated or if you are standing on a planet where the gravity is strong enough to force you to the ground. We all know this experience. When a car suddenly accelerates, we are pushed back in a manner that is identical to what we experience when we jump down. It is as if being pushed back is like experiencing the pull of an object behind us. This theory revealed some strange properties of nature. It showed that gravity will bend light. 

We know that light travels in a straight line. But due to gravity, the nature of space around an object is warped and even though light is travelling in a straight line locally, an observer at a distance will see that the path of light is bent. Gravity changes the very fabric of space and nature of time, together referred to as space-time. Since the effect is immeasurably small for all objects we come across, we do not notice it.

We know that accelerated charged particles produce light. Therefore, it is natural to ask what will happen when mass is accelerated. GR predicts that gravitational waves will be produced when an object is accelerated. Richard Feynman pointed out that gravitational waves would transport energy as gravitational radiation produced by an accelerated mass. 

This was significantly different to the idea of gravity as explained by Isaac Newton whose framework we regularly use for our day-to-day actives. For example, satellite launch and observed planetary trajectories can be adequately explained by Newton’s theory of gravity. The difference between gravity as a force as proposed by Newton and as part of a complex space-time changing entity as proposed by Einstein became a point of contention because both the theories made similar predictions for our routine experience. However, Einstein’s theory made significantly different predictions when the gravity was strong and one was close to a large body. 

In 1919 during a solar eclipse, Arthur Eddington first confirmed that when light from a distant star passes close to the sun, it is bent precisely as Einstein had predicted. It was also shown that the slight difference in the orbit of Mercury, the planet closest to the sun, from Newton’s predictions can be explained by Einstein’s theory. Subsequently, the idea of changes in space-time as predicted by Einstein’s theory were also confirmed by launching extremely accurate clocks on satellites. In fact, the Global Positioning System (GPS), a common feature of our life which is used in phones, cars and other devices, would become useless within an hour if corrections proposed by Einstein’s special and general theories of relativity were not taken into account. 

So Einstein’s GR is now considered a more complete theory of gravity and has withstood many tests. However, the question of the consequences of an accelerated mass remained. GR predicts that a moving mass will disturb the space-time to form waves analogous to the waves produced in a pond when you throw a stone on it. These gravitational waves may be visualised as the ripples in the fabric of our universe, namely Einstein’s space-time. It also predicts that they will propagate as waves moving at the speed of light. The problem in detecting these waves is that they are very weak and were considered to be unmeasurably weak until recently. 

In 1993, Russell Alan Hulse and Joseph Hooton Taylor Jr of the University of Massachusetts Amherst indirectly proved that energy between two pulsars is dissipated exactly as GR had predicted and were awarded the Nobel Prize in 1993.

The Nobel Prize 2017 

This year’s Nobel Prize in Physics was awarded to Kip Thorne, Rainer Weiss, and Barry Barish for detecting these gravitational waves directly. They were quick to acknowledge that the prize truly belongs to a group of about a thousand scientists including about forty from India as the detection had been a result of the joint work of all these scientists and engineers. However, Nobel Prize in the sciences (unlike Nobel Prize for Peace) can only be given to individuals and that too, to a maximum of three individuals. These three scientists had conceived of the idea and technique and demonstrated its feasibility and oversaw the construction of the observatory that confirmed the existence of the gravitational waves. 

The idea they proposed has been around for more than a century. This same technique was used by Albert A Michelson and Edward W Morley to prove that light does not need a medium to move in. They were awarded the Nobel prize in 1907. 

The technique is this. Nature decrees that two directions at a 90-degree angle will not interfere with each other. What happens in one direction will not affect any motion in the direction perpendicular to it. So if you are getting waves coming to you from one direction, it will not affect things that are perpendicular to this direction. So if you had two rods of exactly equal length but perpendicular to each other, the length of the bar along the direction of the incoming wave will be affected by the disturbance but the rod perpendicular to it will not be.  So if you continuously measure the length of both the rods, you will notice that sometimes their lengths change for a short while. One can calculate the nature of disturbance from the temporary change in length of one rod compared to that of the one perpendicular to it. 

The genius of Michelson and Morley was in creating an instrument that can measure these fine changes. Instead of a rod, Michelson and Morely used light and mirrors. We know that light travels in waves which have a sinusoidal shape with troughs and crests. We also know that when two waves meet, they get added to produce a combined wave. If the trough of one wave meets the trough of another wave, together the new trough would be twice as large and if the trough meets a crest, the result will be mutually destructive and result in a null change. 

This is called interference of light. Michelson and Morley took a wave of light and, using a mirror that was 50% efficient, split the wave into two. These light waves travelled in two perpendicular directions to a certain distance, and were reflected back so that they meet again. If the rays in two directions have travelled exactly the same distance (within a few hundred nanometres) they would meet in a manner to reinforce each other producing a unique pattern of maximums and minimums. If one of the waves had travelled a different distance even of a hundred nanometres, the match would visibly change. 

Michelson and Morely were trying to prove that light propagates in a medium which is present everywhere in the universe, called ether. As the earth moved through this ether, the velocity of light would be different in the direction of our motion compared to a direction perpendicular to it. In 1887, they made an instrument that is now known as the Michelson interferometer. The null result of their experiment proved that there is no ether and that the velocity of light is the same, wherever it comes from.  The entire field of physics had to be reworked with the idea that light could propagate without a medium and that the velocity of light was not altered by our movement. The interferometer is now a standard experiment in postgraduate physics laboratories.
 

What Thorne, Weiss, and Barish did

Thorne, Weiss, and Barish Proposed the setting up of what was essentially a Michelson interferometer to catch a moving disturbance due to heavy and fast moving objects in the universe. The problem was that the energy in such waves is very small and you need a truly large mass (at least a few times heavier than the sun) moving very quickly to produce detectable waves. Even then, the numbers were stacked badly against such an experiment. For example, for two bars several kilometres long, the deformation due to a passing wave from two objects several times the size of the sun would be less than a thousandth of the size of an atom! It was thought that this was impossible to measure.

The genius of Thorne, Weiss, and Barish was to work out the mechanism and demonstrate the ability to measure these truly tiny differences. They proposed that if the two arms were 4-kilometres (km)-long, then the difference in their length could be measured using lasers and amplifiers. Possible sources of errors were abundant. A person walking within ten metres from one of the mirrors would produce this kind of change in length. Sea waves several kilometres away or even the movement of the moon could also produce such effects. Most of these effects are periodic and their effects calculable or they can be eliminated. Even then, doubts would remain if only one detector detected a possible signal from the cosmos.

Laser Interferometer Gravitational-Wave Observatory

In the 1960s, American scientists, including Joseph Weber, and Soviet scientists Mikhail Gertsenshtein and Vladislav Pustovoit proposed the design that could detect gravitational waves. In 1967, Rainer Weiss of Massachusetts Institute of Technology published an analysis of interferometer that could be used to measure such small distances. In 1968, Kip Thorne from Caltech pioneered the theoretical efforts to understand gravitational waves and their sources. The experiment was called LIGO (Laser Interferometer Gravitational-Wave Observatory).

LIGO began receiving funds from 1980. It also benefited from the work of cofounder Ronald Drever, who died in March 2017 at the age of 85. Barry Barish, a particle physicist by training and experienced in large collaborative experiments, took charge of the laboratory in 1994 and delivered the experiment. It can be argued that Drever should also have received this recognition but his passing away excluded this—Nobel Prize is given to living persons only. In view of the faint nature of signal and possible errors, it was crucial to establish at least two identical LIGO instruments very far from each other and both should find a disturbance simultaneously. That would confirm that what they saw was genuine. 

Two laboratories were set up, one located at Hanford in north-west United States (US) and at Livingston in south-east USA. They both had two perpendicular arms. Each arm where the light travelled to and fro was exactly 4 km long and placed in a tunnel. It was built between 1994 and 2002. The final instrument with problems sorted out and with improved capabilities, labelled. Advanced LIGO began operating in 2015. In its present state, A-LIGO can detect a change within one thousandth of the size of a nucleus. 

A-LIGO has now been joined by the VIRGO detector in Italy, which is a joint effort of Italy and France with recent inclusion of the Netherlands, Poland and Hungary. With its base in Europe, it will significantly increase the ability to point out sources from which the signal came. India is also expected to join the LIGO effort with one LIGO facility proposed to be set up in Maharashtra.

In 2015, the LIGO–VIRGO collaboration began working. In 2016 it announced the detection of gravitational waves passing through the earth from the merger of two black holes. Since then, the LIGO–VIRGO collaboration reported four events of this kind. 

Understanding Gravity Near Massive Objects

The reason why vast resources are being committed to this project (it is the largest project supported by the National Science Foundation of the US) is that the study of gravitational waves will provide us windows to several strange things happening in the universe that we do not understand and some that we do not even know about.

LIGO has now firmly established that Einstein’s General Theory of Relativity is an accurate description of gravitational interactions. This means that we have an idea about how gravity works not only around us but even near massive objects. This is important since all previous tests of gravity with the Sun, Mercury and even satellites around the earth, were around objects that are small by astronomical standards. The gravitational waves that we have now seen come from the merger of two black holes, which are several tens of times bigger than sun. 

Black holes are strange entities. They come into existence when a large star runs out of its nuclear fuel and explodes in a spectacular manner, called a supernova explosion. It leaves behind an object of unfathomable mass packed into a singularity. Not even light will escape from such objects and hence they are invisible. However, at some distance from the object, light can escape. This is called the “event horizon” as objects closer than this edge (even light) cannot escape and hence any event happening inside this limit are not knowable. However, when things fall onto this event horizon from the outside, they produce their last sigh before they disappear for ever behind the event horizon. This last sigh, being outside the event horizon, is visible to us.

We have seen several such objects in our galaxy that have a mass a few times the mass of the sun. We believe that the centre of each galaxy consisting of hundreds of billions of stars have black holes that are a few million times the size of the sun. The merger of two black holes seen by LIGO–VIRGO were roughly 29 and 36 times the mass of the sun. The event happened some billion light years away implying that the event occurred even before dinosaurs arose on earth and possibly before life arose on earth. It had been travelling since then and produced a disturbance on earth when it reached us last year. Since then LIGO–VIRGO has reported four more such observations, roughly at the rate of one every three months or so. All these events happened a few billion years ago. So why are we not seeing more recent events? For one thing, the events are rare and the possibility of such events is also rare. 

Currently, the precision with which LIGO–VIRGO can pinpoint an event in the sky is very low and hence it is not possible to find the exact location of events that last for only a fraction of a second. With time and with more instruments around the globe, we will be able to find the location and also begin to learn how the merger of two black holes affects their environment, and how it changes the space-time around them. With increasing sensitivity, we may even see less spectacular events of smaller objects merging or see events that are even older. After all, the universe is some 14 billion years old and it will be interesting and important to know if black hole mergers occurred earlier, when the universe was smaller, and when it was easier for a black hole to meet another. 

A lot of science of early universe will be revealed by gravitational wave instruments, giving us completely new ways of understanding the birth pangs of the universe. The making of A-LIGO and VIRGO will also allow us to explore and understand new technologies that will be useful in the future. After all, if we had launched GPS satellites without knowing GR, they would have been useless. New technologies, like new born babies, have a future that is impossible to predict. 

 

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