Step by Step
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What gravitational waves are
Einstein predicted in 1916 that accelerating masses create ripples in spacetime itself, traveling outward at the speed of light.
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How LIGO detects them
LIGO's detector arms are 4 km long, sensitive enough to detect length changes of just 10⁻¹⁸ meters — about 1/10,000th the diameter of a proton — as a passing gravitational wave stretches and compresses space itself.
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The historic first detection
On September 14, 2015, LIGO detected gravitational waves for the first time, from the merger of two black holes located about 1.3 billion light-years away — a discovery that earned the 2017 Nobel Prize in Physics.
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GW170817 and future detectors
GW170817, the 2017 detection of a neutron star merger, was observed simultaneously in gravitational waves, gamma rays, and optical light — launching the new field of multi-messenger astronomy. LISA, a future space-based detector, will be able to detect gravitational waves from supermassive black hole mergers, opening an entirely new observational window on the universe.
Applied Walkthrough
1
Einstein's general relativity predicted in 1916 that accelerating massive objects should create ripples in spacetime itself — gravitational waves — propagating outward at the speed of light, though he doubted they'd ever actually be detected given how incredibly weak the effect would be.
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Nearly a century later, on September 14, 2015, LIGO's incredibly precise detectors — sensitive to length changes smaller than 1/10,000th the width of a proton — finally confirmed this prediction, detecting waves from two merging black holes 1.3 billion light-years away.
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In 2017, an even more remarkable detection followed: GW170817, a neutron star merger observed simultaneously through gravitational waves, gamma rays, and visible light — the birth of multi-messenger astronomy, combining multiple types of signals from the same cosmic event.
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Looking ahead, LISA, a planned space-based gravitational wave detector, will extend this capability even further, aiming to detect waves from supermassive black hole mergers — events too large and slow for ground-based detectors like LIGO to observe.
Exam Application
Exams test whether you know Einstein's original 1916 prediction and how long it took to confirm, whether you know the significance of the first 2015 detection and GW170817, and whether you understand what multi-messenger astronomy means.
⚠ Common Trap
The most common trap is confusing the 2015 detection (two merging black holes, gravitational waves only) with the 2017 GW170817 detection (a neutron star merger, observed in gravitational waves PLUS gamma rays and optical light) — these are two distinct, historically significant events with different scientific implications.
✓ Quick Self-Check
1. Who predicted gravitational waves, and when?
Einstein, in 1916.
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2. What did LIGO first detect, and when?
Gravitational waves from two merging black holes, on September 14, 2015.
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3. What was significant about the GW170817 detection?
It was a neutron star merger observed simultaneously in gravitational waves, gamma rays, and optical light — launching multi-messenger astronomy.
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4. How sensitive are LIGO's detectors?
Sensitive enough to detect length changes of about 10⁻¹⁸ meters, roughly 1/10,000th the diameter of a proton.
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5. What will LISA, a future space-based detector, be able to detect that ground-based LIGO cannot?
Gravitational waves from supermassive black hole mergers.
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