Nobel Prize in Physics 2017: LIGO and the First Gravitational Wave Detection

This note covers the Nobel Prize in Physics 2017: who won it, what gravitational waves are and why Einstein doubted they could ever be measured, how the LIGO laser interferometer caught one for the first time, how the forty-year hunt for the discovery unfolded, why it matters for astronomy, and quick facts for exams.

What was the Nobel Prize in Physics 2017 awarded for?

The Royal Swedish Academy of Sciences gave the Nobel Prize in Physics 2017 "for decisive contributions to the LIGO detector and the observation of gravitational waves". This is the exact official citation, and it names two things together: building the detector, and actually seeing a signal with it.

In plain words, the prize rewards the people who designed and led a machine sensitive enough to notice a gravitational wave, a tiny ripple in space and time, and who then used that machine to record one for the very first time.

The waves themselves were predicted a century earlier by Albert Einstein's general theory of relativity, but nobody had direct proof they existed until 2015.

One half of the 9 million Swedish kronor prize went to Rainer Weiss for his pioneering work on the detector's design, and the other half was shared between Barry C. Barish and Kip S. Thorne for turning the project into a working, internationally staffed observatory and for the theory behind what it should expect to see.

Who are the laureates?

Three scientists shared this prize, reflecting the different jobs needed to turn an idea on paper into a working instrument that could catch a signal travelling across the universe.

Rainer Weiss

Rainer Weiss was born on 29 September 1932 in Berlin, Germany, and died on 25 August 2025 in Cambridge, Massachusetts, USA.

At the time of the award he was affiliated with the LIGO/VIRGO Collaboration and with the Massachusetts Institute of Technology (MIT), Cambridge, where he was Professor of Physics. He received one half of the prize.

In the mid-1970s Weiss had already worked out the kinds of background noise that would disturb a gravitational-wave measurement, and he designed a laser-based interferometer built to overcome that noise, laying the practical groundwork for LIGO's instruments.

Barry C. Barish

Barry C. Barish was born on 27 January 1936 in Omaha, Nebraska, USA. At the time of the award he was affiliated with the LIGO/VIRGO Collaboration and was the Linde Professor of Physics at the California Institute of Technology (Caltech), Pasadena.

He received one quarter of the prize. Barish took over as LIGO's leader in 1994 and grew what had been a team of roughly 40 researchers into a large international collaboration of more than a thousand scientists, the scale needed to finish the project.

Kip S. Thorne

Kip S. Thorne was born on 1 June 1940 in Logan, Utah, USA. At the time of the award he was affiliated with the LIGO/VIRGO Collaboration and was the Feynman Professor of Theoretical Physics at Caltech.

He received one quarter of the prize. Thorne worked out the theoretical predictions for what signals different cosmic events would produce, work that guided both the design of LIGO and the analysis used to recognise a real signal when it finally arrived.

What problem was gravitational-wave astronomy trying to solve?

Einstein's general theory of relativity, published in 1915 and from which he derived the existence of gravitational waves in a follow-up paper in 1916, describes gravity not as a pulling force but as a curvature of spacetime caused by mass.

One consequence of this theory is that whenever a mass accelerates, such as an ice-skater pirouetting, a star exploding, or a pair of black holes circling each other, it should send out ripples in spacetime called gravitational waves, travelling outward at the speed of light.

The trouble was that Einstein himself was convinced the waves could never be measured, believing the interaction between a passing wave and ordinary matter would be too weak to detect directly. He even had serious doubts about whether the waves were mathematically real at all, and in 1936 he submitted a paper, with his assistant Nathan Rosen, arguing that they did not exist; a referee found serious errors in it and it was rejected before publication.

A key theoretical push came from physicist Richard Feynman, who in 1957 described a thought experiment in which a passing wave would cause beads on a rod to move and heat the rod by friction, convincing many sceptical experts that the waves could, in principle, be detected with a sufficiently sensitive instrument.

That argument inspired Joseph Weber at the University of Maryland to build the very first gravitational-wave detector in the 1960s: a solid aluminium cylinder with a mass of about 1.5 tonnes, fitted with crystals that converted tiny vibrations into voltages, and tuned to vibrate at a resonant frequency of around 1657 Hz.

In 1969 Weber claimed to have recorded matching signals on two such bars situated 1000 kilometres apart, but no other laboratory could repeat his results, and his claimed detections are now considered false alarms. A fundamentally more sensitive, broadband approach was required.

For decades the only accepted evidence remained indirect: in 1974 the American astronomers Joseph Taylor and Russell Hulse observed a binary pulsar, a pair of extremely dense, rapidly spinning stars, whose orbit was shrinking at exactly the rate general relativity predicted if the system were losing energy as gravitational waves.

That discovery earned them the Nobel Prize in Physics in 1993, but it was still not a direct observation of a wave passing through a detector on Earth. Physicists needed an instrument able to notice a change in distance many thousands of times tinier than the width of an atomic nucleus, because that is how weak a gravitational wave becomes by the time it reaches Earth from a distant cosmic collision.

How does a laser interferometer catch a gravitational wave?

LIGO, the Laser Interferometer Gravitational-Wave Observatory, is built as two giant L-shaped instruments, one near Hanford, Washington, and one near Livingston, Louisiana, about three thousand kilometres apart. Each arm of the L is four kilometres long, with a heavy mirror hanging at each end and at the corner.

The basic idea is an interferometer: a laser beam is split at the corner and sent down both arms, bounced off the end mirrors, and brought back together.

If both arms are exactly the same length, the two returning light waves cancel each other out where they meet.

A passing gravitational wave stretches one arm very slightly while squeezing the other, so the light waves no longer line up perfectly, and a flicker of light appears at the detector.

1. A stabilised laser beam, with a power of around 20 watts, is split into two identical beams at the corner of the L-shaped instrument.
2. Each beam travels down one four-kilometre arm and enters a Fabry-Pérot cavity, bouncing many times between heavy mirrors held in an ultra-high vacuum, which stretches the effective light path to well over a thousand kilometres.
3. Both beams return to the corner and recombine; with no wave passing, they cancel out and almost no light reaches the detector.
4. A passing gravitational wave stretches one arm and compresses the other by a tiny amount, so the recombined light no longer cancels out completely.
5. The resulting flicker of light is recorded by a photodetector, and the pattern is compared against the signal from the twin detector 3,000 kilometres away to rule out local disturbances such as passing lorries or small earthquakes.

Reaching the needed sensitivity took new laser technology, ultra-stable mirrors, and some of the largest vacuum systems ever built. Each test mass mirror weighs about 40 kilograms and is made of fused silica, hung by a quadruple pendulum suspension on an actively controlled platform that filters out ground tremors and tidal motion of the Earth's crust.

The mirrors had to stay almost perfectly still against everyday vibration while remaining free to swing with a passing wave, a balance that demanded new materials and precision engineering well beyond anything attempted before.

Draw an L-shape with two arms at right angles meeting at a corner.

Place a laser and beam-splitter at the corner, a heavy suspended mirror at the far end of each arm, and a photodetector at the corner where the two beams recombine.

Label one arm as stretching and the other as compressing as a gravitational wave passes.

What happened when gravitational waves were finally detected?

On 14 September 2015, just before Advanced LIGO's first official observing run was due to begin, both detectors registered a brief signal.

The wave reached the Livingston detector first and arrived at Hanford about seven milliseconds later, exactly as expected for a wave moving at the speed of light between the two sites.

A young physicist named Marco Drago, working at the Max Planck Institute for Gravitational Physics in Germany, was one of the first to notice the curves on his screen and wonder whether it was a real signal or one of the occasional hidden test alarms used to check the team's readiness.

The signal, later named GW150914, matched the pattern expected from two black holes spiralling together and merging into one.

Researchers worked out that the probability of such a signal occurring by chance, from random background noise in the two detectors, was less than about two in ten million, corresponding to a false alarm rate of roughly one event in 203,000 years, which gave the team confidence the signal was genuine.

Even so, researchers did not announce the discovery until 11 February 2016, after months of careful checking, because such an extraordinary claim needed extraordinary certainty.

For a fraction of a second, the merging black holes radiated more power as gravitational waves than the combined light of every star in the visible universe.

Since that first event, LIGO went on to confirm two further events, named GW151226 and GW170104, both again interpreted as mergers of black holes, showing that the first detection was not a one-off fluke.

On 14 August 2017 the European VIRGO detector near Pisa joined LIGO to observe a signal with all three instruments together, which let scientists narrow down, for the first time, the direction in the sky from which the waves had come.

How did the discovery unfold?

Turning Einstein's prediction into a confirmed observation took more than four decades of theory, prototypes and large-scale engineering.

Why does this discovery matter?

Before 2015, astronomers had only ever studied the universe using light and other electromagnetic radiation, along with particles such as cosmic rays and neutrinos.

Gravitational waves are described by the prize committee as a completely new and different kind of signal, direct testimony to disturbances in spacetime itself rather than to light given off by an object.

This opens a way to study events that give off little or no light, such as two black holes merging in total darkness, which could never be seen through an ordinary telescope.

The detection also confirmed, in the most extreme regime tested so far, that Einstein's century-old prediction held up exactly as his equations said it should, including the precise pattern by which the signal's frequency and strength rose, peaked and died away as the black holes merged.

The discovery has already been followed by further detections and by new partnerships, including VIRGO's joint observation with LIGO in August 2017, and an earlier sister project, GEO600 near Hannover in Germany, which had already shared data and technology with LIGO for years.

New observatories are being built or planned in other countries, including KAGRA in Japan and a planned LIGO detector in India, which will help pin down more precisely where in the sky each signal comes from by comparing arrival times at several widely separated detectors, a technique called triangulation.

Looking further ahead, scientists have also proposed a space-based detector, the Laser Interferometer Space Antenna, and a much larger ground-based successor, the Einstein Telescope, both intended to catch gravitational waves from events too distant or too faint for the current generation of instruments.

The press release noted that "a wealth of discoveries awaits those who succeed in capturing the waves and interpreting their message," attributed to the Royal Swedish Academy of Sciences, signalling that this was seen as the start of an entirely new field of astronomy rather than a one-off result.

How does this connect to what you study?

School physics usually introduces waves through sound and light, including the idea that a wave carries energy without carrying matter along with it.

Gravitational waves follow the same basic pattern, a disturbance spreading outward from its source, except that what is "waving" is spacetime itself rather than air or an electromagnetic field.

The idea that gravity bends spacetime, central to this prize, extends the simpler picture of gravity as a force pulling two masses together that most students meet first.

Here, a mass does not just attract another mass; an accelerating mass can send ripples outward, in the same way an accelerating electric charge sends out electromagnetic radiation.

The interferometer itself is a direct, scaled-up application of a classic optics idea: splitting a light beam, sending the parts on different paths, and recombining them to see whether the paths matched.

Any student who has studied interference patterns in light is looking at the same underlying principle that let LIGO notice a change in length far smaller than an atomic nucleus.

How did LIGO grow from an idea into a global collaboration?

The route from Rainer Weiss's early sketches to a working observatory took more than four decades and depended on steadily larger organisation and funding, not only on good physics.

In the early 1970s, while at the Massachusetts Institute of Technology, Weiss wrote a detailed report analysing the many kinds of background noise, including seismic vibrations, thermal motion of the mirrors and fluctuations in the laser itself, that would have to be overcome before a laser interferometer could ever record a real gravitational wave.

In 1974 he applied to the US National Science Foundation for funds to enlarge his existing prototype to arms nine metres long, as a first step towards a much larger instrument.

1. During the 1970s, similar interferometer prototypes were also built in Germany, at the Max Planck Institute, and in Glasgow, Scotland, by Ronald Drever and colleagues, who later moved to Caltech to work alongside Kip Thorne.
2. In October 1983 a detailed feasibility and costing study, known as the Blue Book, recommended building two long-baseline interferometers separated by thousands of kilometres.
3. LIGO was formally initiated in 1984 as a joint project of MIT and Caltech, led together by Weiss, Drever and Thorne.
4. In 1990 the National Science Board approved construction of the full-scale observatory, at an expected cost of close to 300 million US dollars, to be built at Hanford, Washington, and Livingston, Louisiana.

Running a project of this size needed a different kind of leadership than a university physics department could usually provide. In 1994 Barry Barish took over as LIGO's director and grew what had been a team of roughly 40 researchers into a large international collaboration, eventually bringing in researchers from more than twenty countries.

Barish planned the observatory in two stages: an initial version to prove the technology worked, and a far more sensitive upgrade, Advanced LIGO, that would make a real detection probable rather than merely possible. He also created the LIGO Scientific Collaboration in 1997 to coordinate the growing international effort, with Weiss serving as its first spokesperson.

By the time Advanced LIGO came online in 2015, the collaboration had grown to roughly a thousand researchers, drawing on skills in optics, vacuum engineering, materials science, seismic isolation and data analysis that no single institution could have supplied alone.

Quick facts for exams

The Nobel Prize in Physics 2017 was announced on 3 October 2017 by the Royal Swedish Academy of Sciences. One half went to Rainer Weiss and the other half jointly to Barry C. Barish and Kip S. Thorne, "for decisive contributions to the LIGO detector and the observation of gravitational waves". The award recognised the LIGO project's detection of gravitational waves from two colliding black holes on 14 September 2015, confirming a prediction Einstein had made a century earlier in his general theory of relativity.

The total prize amount was 9 million Swedish kronor, shared in proportion to each laureate's role in the discovery.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Gravitational wave — a ripple in spacetime created when a mass accelerates, travelling outward at the speed of light.
• General theory of relativity — Einstein's 1915 theory describing gravity as the curvature of spacetime caused by mass.
• Spacetime — the combined four-dimensional fabric of space and time that general relativity treats as flexible, not fixed.
• Interferometer — an instrument that splits a light beam into two paths and recombines them to detect tiny differences in distance.
• LIGO — the Laser Interferometer Gravitational-Wave Observatory, twin detectors in Hanford and Livingston, USA.
• VIRGO — a gravitational-wave detector near Pisa, Italy, that joined LIGO in observations from August 2017.
• Black hole — an object so dense that nothing, not even light, can escape its gravity.
• Binary pulsar — a pair of dense, rapidly spinning stars orbiting each other, used as indirect evidence for gravitational waves.
• Solar mass — a unit of mass equal to the mass of the Sun, used to describe very heavy objects like black holes.
• Light-year — the distance light travels in one year, used to measure very large cosmic distances.
• Prize share — the fraction of the Nobel Prize money and recognition given to each laureate.
• Royal Swedish Academy of Sciences — the independent body that awards the Nobel Prize in Physics each year.

Common errors and misconceptions

• Misconception: Gravitational waves are the same as sound waves or electromagnetic waves. Correct: They are ripples in spacetime itself, a completely different kind of wave from light or sound.
• Misconception: LIGO is an ordinary telescope. Correct: LIGO does not collect light; it measures tiny changes in the length of its arms caused by passing gravitational waves.
• Misconception: The whole prize was for a single person's idea. Correct: The citation credits "decisive contributions" from all three laureates, covering detector design, theory and project leadership.
• Misconception: Einstein proved gravitational waves existed in 1915. Correct: He predicted them that year in general relativity, but he himself doubted they could ever be measured, and direct proof came only in 2015.
• Misconception: The 1993 Nobel Prize to Hulse and Taylor was for directly detecting gravitational waves. Correct: That prize recognised indirect evidence from a binary pulsar's shrinking orbit, not a direct detection.
• Misconception: LIGO has only one detector. Correct: LIGO uses twin detectors, in Hanford, Washington, and Livingston, Louisiana, so that a real signal appears at both, ruling out local disturbances.
• Misconception: The first detected event involved stars colliding. Correct: GW150914 came from the merger of two black holes, not visible stars.

Exam-style questions with model answers

Q1. In which year was the Nobel Prize in Physics 2017 announced? [1 mark]

1. The Royal Swedish Academy of Sciences announced the Nobel Prize in Physics 2017 on 3 October 2017, recognising the LIGO detector and the observation of gravitational waves.

Q2. State the official citation for the Nobel Prize in Physics 2017. [2 marks]

1. The citation reads "for decisive contributions to the LIGO detector and the observation of gravitational waves," awarded jointly to Rainer Weiss, Barry C. Barish and Kip S. Thorne.

Q3. Name the three laureates of the Nobel Prize in Physics 2017 and their shares of the prize. [3 marks]

1. Rainer Weiss received one half of the prize for his pioneering detector design work, while Barry C. Barish and Kip S. Thorne each received one quarter, Barish for leading LIGO into a large international collaboration and Thorne for his theoretical predictions of gravitational-wave sources.

Q4. Explain, in your own words, how a LIGO interferometer detects a gravitational wave. [4 marks]

1. A laser beam is split into two beams that travel down two perpendicular four-kilometre arms and bounce between mirrors before recombining at a detector. Normally the recombined beams cancel out exactly. When a gravitational wave passes, it stretches one arm very slightly while compressing the other, so the two light paths become unequal and the beams no longer cancel completely. This produces a tiny flicker of light that is recorded and compared with the signal from a second, distant detector to confirm that the disturbance was real and not a local vibration.

Q5. Describe how indirect evidence for gravitational waves was found before 2015, and why it was not enough on its own. [4 marks]

1. In 1974 American astronomers Joseph Taylor and Russell Hulse observed a binary pulsar, a pair of dense orbiting stars, whose orbital period was shrinking exactly as expected if the system were losing energy by emitting gravitational waves. This matched theoretical calculations closely and earned them the Nobel Prize in Physics in 1993. However, it was an inference from a changing orbit rather than a direct measurement of a passing wave reaching Earth, so physicists still needed an instrument capable of recording the wave itself.

Q6. Discuss why the first direct detection of gravitational waves, GW150914, is considered a landmark in physics and astronomy. [5 marks]

1. GW150914, detected on 14 September 2015, was the first time a gravitational wave had ever been directly observed, confirming a prediction Einstein had made a century earlier in general relativity, even though he himself had doubted it could ever be tested. The signal came from two black holes, of 29 and 36 solar masses, spiralling together and merging into a single black hole of about 62 solar masses, 1.3 billion light-years away, releasing energy equivalent to roughly three solar masses as gravitational radiation in a fraction of a second. Unlike every previous astronomical observation, which relied on light or particles, this discovery gave direct evidence of a disturbance in spacetime itself, opening an entirely new way of studying violent cosmic events such as black-hole mergers that give off no light. It also demonstrated that decades of patient detector design, theoretical prediction and large-scale international organisation could succeed where earlier attempts, such as Joseph Weber's resonant bar detectors, had failed, paving the way for further detections and new observatories planned in other countries.

Q7. What role did Barry C. Barish play in LIGO's success, and why was this role necessary? [3 marks]

1. Barish became LIGO's director in 1994 and grew what had been a team of roughly 40 researchers into a large international collaboration of more than a thousand scientists. This larger scale and leadership were necessary because building and operating two highly sensitive, kilometre-scale detectors required far more resources, expertise and coordination than a small university team could provide.

Q8. Who awards the Nobel Prize in Physics? [1 mark]

1. The Royal Swedish Academy of Sciences awards the Nobel Prize in Physics every year, and in 2017 it chose to honour the LIGO detector and the observation of gravitational waves.

Key takeaways

• The Nobel Prize in Physics 2017 went to Rainer Weiss, Barry C. Barish and Kip S. Thorne for the LIGO detector and the first observation of gravitational waves.
• Gravitational waves are ripples in spacetime, predicted by Einstein's general theory of relativity from 1915.
• Einstein himself doubted gravitational waves could ever be directly measured because the effect is so weak.
• LIGO uses laser interferometers with four-kilometre arms to detect changes smaller than an atomic nucleus.
• The first direct detection, GW150914, came from two merging black holes on 14 September 2015.
• Twin detectors 3,000 kilometres apart, in Hanford and Livingston, confirm that a signal is real, not local noise.
• Indirect evidence for gravitational waves came earlier, from Hulse and Taylor's 1974 binary pulsar observations.
• VIRGO in Italy joined LIGO in 2017 to help locate sources more precisely in the sky.

Test yourself

Who shared the Nobel Prize in Physics 2017?

Rainer Weiss, Barry C. Barish and Kip S. Thorne shared the Nobel Prize in Physics 2017 for the LIGO detector and the observation of gravitational waves.

What theory predicted gravitational waves, and when?

Albert Einstein's general theory of relativity, from 1915, predicted that accelerating masses would create gravitational waves rippling through spacetime.

Where are the two LIGO detectors located?

The two LIGO detectors are near Hanford, Washington, and Livingston, Louisiana, in the USA, about 3,000 kilometres apart.

What event produced the first directly detected gravitational wave?

Two black holes, of 29 and 36 solar masses, spiralled together and merged, producing the signal GW150914 on 14 September 2015.

Why does LIGO use two detectors instead of one?

Using two widely separated detectors lets scientists confirm that a signal is a real gravitational wave and not a local disturbance like a passing truck.

What role did Barry C. Barish play from 1994?

Barry C. Barish led LIGO from 1994, turning it from a small group into a large international collaboration able to finish the project.

What indirect evidence for gravitational waves existed before 2015?

Joseph Taylor and Russell Hulse's 1974 binary pulsar observations showed an orbit shrinking exactly as gravitational-wave theory predicted, earning them the 1993 Nobel Prize.

Which second detector joined LIGO in 2017?

The VIRGO detector, near Pisa in Italy, joined LIGO's observations and helped the teams jointly detect a signal on 14 August 2017.