Gravitational Waves
Science & Technology › Physics – Modern Physics and Astrophysics · Topic 9.6
Gravitational waves are ripples in the fabric of spacetime that travel at the speed of light and are produced when massive objects accelerate, especially when black holes or neutron stars orbit and merge. Albert Einstein predicted them in 1916 from his general theory of relativity. They were first detected directly on 14 September 2015 by the twin detectors of the Laser Interferometer Gravitational-Wave Observatory, LIGO, in the United States, and announced in February 2016. The signal, GW150914, came from two black holes that merged about 1.3 billion years ago. Rainer Weiss, Barry Barish and Kip Thorne received the Nobel Prize in Physics in 2017. India plans to build a third LIGO detector, LIGO-India, at Aundha in the Hingoli district of Maharashtra.
5 min read · UPSC Prelims · Science & Technology · Physics – Modern Physics and Astrophysics
At a Glance
- Predicted Einstein, 1916
- First detection 14 September 2015 (GW150914)
- Nobel Prize 2017: Weiss, Barish, Thorne
- Detector arms 4 km long (LIGO)
- India LIGO-India, Hingoli, Maharashtra
- Exam link UPSC Prelims, Science & Technology: Physics – Modern Physics and Astrophysics
Where Does It Fit?
This is the sixth topic of the macro on modern physics and astrophysics. It follows black holes, whose mergers are the main source, and it tests general relativity. Previous topic: India-Based Neutrino Observatory. Next topic: Sunspots, Magnetars and Neutron Stars.
How Are Gravitational Waves Made and Detected?
| Item | Detail |
|---|---|
| Cause | Accelerating masses disturb spacetime. Strong waves come from merging black holes and neutron stars, supernovae and the early universe |
| Speed | The speed of light |
| Detector design | A laser interferometer splits a beam into two perpendicular arms 4 kilometres long. A passing wave changes the lengths by a tiny amount, less than one thousandth of the width of a proton |
| LIGO sites | Hanford in Washington and Livingston in Louisiana, which allows a signal to be confirmed and its direction found |
| Other detectors | Virgo in Italy, KAGRA in Japan and GEO600 in Germany. Together they form a global network |
| Space plans | LISA, a mission of the European Space Agency with three spacecraft millions of kilometres apart, is planned for the 2030s |
What Have Gravitational Waves Revealed?
- GW150914: the first detection, from two black holes of about 36 and 29 solar masses merging into one of about 62, with the rest of the mass radiated as waves.
- Neutron star merger of 2017: the signal GW170817 was seen along with a gamma ray burst and light, and the event, called a kilonova, produced heavy elements such as gold and platinum.
- New astronomy: waves let us observe events that give out no light, so astronomers now combine them with light and neutrinos in multi-messenger astronomy.
- Tests of relativity: the waves travel at the speed of light and have the shape predicted by general relativity, as in Theories and Laws of Physics, Including the Theory of Relativity.
- Black hole population: hundreds of mergers have been detected, showing black holes with masses that were not expected from stars alone, as in Black Holes.
- Indirect evidence: the Hulse-Taylor binary pulsar, found in 1974, was losing energy at exactly the rate predicted for gravitational waves, and its discoverers received the Nobel Prize in 1993.
What Is LIGO-India and What Else Should You Remember?
- LIGO-India: a planned detector at Aundha in the Hingoli district of Maharashtra, approved by the Union Cabinet in April 2023 at about Rs 2,600 crore, to be built with the United States, and expected around 2030, according to official statements.
- Agencies: the Department of Atomic Energy and the Department of Science and Technology, with the Raja Ramanna Centre for Advanced Technology, Institute for Plasma Research and the Inter-University Centre for Astronomy and Astrophysics as partners. The United States National Science Foundation supplies the detector components.
- Value of a third site: a detector in India far from the other two improves the accuracy of locating the source in the sky and adds sensitivity.
- Indian scientists: Indian researchers took part in the 2015 discovery through the LIGO Scientific Collaboration.
- Electromagnetic waves are different: they are oscillations of electric and magnetic fields in space, while gravitational waves are oscillations of spacetime itself, as in ionosphere Space…">Electromagnetic Wave Behaviour.
- Pulsar timing: in 2023, pulsar timing arrays including the Indian Pulsar Timing Array, which uses the Giant Metrewave Radio Telescope, reported evidence for a background of very low-frequency waves.
- Exam pattern: year of detection, Nobel Prize, location of LIGO-India and sources.
- Frequency bands: ground detectors like LIGO hear waves of tens to thousands of hertz from stellar mergers, while pulsar timing and LISA target far slower waves from supermassive black holes.
- Why detection took a century: the waves are so weak that only the most violent events in the universe produce signals large enough to detect, even after travelling for billions of years.
Why Does It Matter for the Exam?
- Fact questions: who predicted and who detected, and the 2017 Nobel Prize.
- Current affairs link: progress of LIGO-India and new mergers announced.
- Concept questions: how an interferometer detects tiny changes.
- Science policy link: India joins mega-science projects through international partnership.
- Common errors: mixing gravitational waves with electromagnetic or sound waves.
As an analytical point, gravitational waves are a new way of observing the universe. LIGO-India brings India into this field and builds skills in precision engineering and lasers.
Why Is This Topic Important?
- Facts Dates and places asked
- Concept Interferometer
- News LIGO-India status
What Came Next?
The next topic covers sunspots, magnetars and neutron stars. Next topic: Sunspots, Magnetars and Neutron Stars.
Key People and Terms
- Gravitational wave Ripple in spacetime
- Interferometer Instrument using light interference
- Kilonova Neutron star merger flash
- Binary system Two bodies orbiting each other
- Multi-messenger astronomy Using waves, light and particles
- Strain Fractional change in length
Exam Corner
Points to Remember
- Predicted 1916; detected 2015.
- Nobel Prize: 2017.
- LIGO arms: 4 km.
- LIGO-India: Hingoli.
- Neutron star merger: 2017.
Do Not Confuse With
- Gravitational waves and electromagnetic waves Ripples of spacetime versus field waves
- LIGO-India and INO Gravitational waves versus neutrinos
- Detection and prediction 2015 versus 1916
Memory Hook
Einstein sixteen predicts, twenty-fifteen LIGO hears; four-kilometre arms, a proton’s thousandth; Hingoli will be the third ear.
Mains Angle
Questions ask about the significance of the discovery and of LIGO-India. These points add depth.
- Science New window on the universe
- Technology Precision lasers
- India Third detector
A Question You May Face
An original practice question, not a past paper question.
“What are gravitational waves, and how will the LIGO-India project strengthen India’s role in global astronomy?”
How to Answer
- Waves What and how.
- Detection LIGO.
- Results Mergers.
- India Hingoli.
GS Relevance
Frequently Asked Questions
What are gravitational waves?
Gravitational waves are ripples in spacetime that travel at the speed of light and are produced by accelerating massive objects, such as merging black holes or neutron stars. Albert Einstein predicted them in 1916, and they were first detected directly in September 2015 by the LIGO detectors in the United States.
How does LIGO detect gravitational waves?
LIGO splits a laser beam into two beams that travel along perpendicular arms 4 kilometres long and reflects them back. A passing gravitational wave changes the arm lengths by a tiny amount, which shifts the interference pattern of the returning light and reveals the signal.
What is LIGO-India?
LIGO-India is a planned gravitational wave detector at Aundha in the Hingoli district of Maharashtra, approved by the Union Cabinet in 2023 at about Rs 2,600 crore. It will be built with the United States and join the global network of detectors to locate sources more accurately.
Who got the Nobel Prize for gravitational waves?
Rainer Weiss, Barry Barish and Kip Thorne received the Nobel Prize in Physics in 2017 for decisive contributions to the LIGO detector and the observation of gravitational waves. Weiss received half of the prize and Barish and Thorne shared the other half.
What is a kilonova?
A kilonova is a bright flash produced when two neutron stars merge, which throws out matter that forms heavy elements such as gold and platinum. In 2017 LIGO and Virgo detected gravitational waves from such a merger, and telescopes saw the light from the same event.
PYQ Practice — Statement Analysis
1 Gravitational waves were first directly detected in 2015.
By LIGO.
2 Gravitational waves are a kind of sound wave.
They are ripples of spacetime.
3 LIGO-India is planned in the Hingoli district of Maharashtra.
At Aundha.
4 Einstein predicted gravitational waves in 1916.
From general relativity.
Past Mains Questions on This Topic
This topic comes under Physics – Modern Physics and Astrophysics. UPSC Mains questions asked on the matching syllabus topic, year-wise with marks:
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