The First Detection: GW150914

On September 14, 2015, the two Advanced LIGO detectors in the United States - Hanford in Washington and Livingston in Louisiana - made the first direct detection of gravitational waves. The signal, named GW150914, came from the merger of two black holes, about 36 and 29 times the mass of the Sun. They combined into a single black hole roughly 62 solar masses in size. An astonishing three solar masses' worth of energy (about 5 × 10⁴⁷ joules) was released as gravitational waves. This energy rippled across the universe, briefly stretching and squeezing spacetime on Earth by less than a fraction of a proton's diameter.

This event had no electromagnetic counterpart because black holes swallow all light. This means there was nothing to see in telescopes, only the "sound" or frequency recorded by LIGO. Still, this detection confirmed Einstein's century-old prediction and opened a brand-new way of observing the universe.

Since that landmark discovery, dozens of additional gravitational-wave events have been recorded, including systems with neutron stars that do produce light. These later detections paved the way for the first direct link between gravitational waves and optical observations, marking the dawn of multi-messenger astronomy.

To go further:

• GW150914: The first observation of a gravitational wave signal [article]
• Warped Spacetime and Horizons of GW150914 [video]

Why Neutron Stars and Black Holes?

Not all objects make detectable gravitational waves. You need extreme masses moving at extreme speeds in extreme orbits. Neutron stars and black holes are perfect because:

Insane Density: Neutron stars pack 1.4 solar masses into a sphere just 20km across. Black holes condense even more mass into infinitely small points. This concentration of matter warps space-time dramatically.

High Speeds: In their final seconds before merging, these objects orbit each other hundreds of times per second, reaching 30-50% the speed of light. Fast-moving massive objects create powerful gravitational waves.

No Internal Forces: Unlike normal stars held up by gas pressure, black holes and neutron stars can spiral infinitely close together without any force stopping them. This creates the violent, high-frequency waves we can detect.

Earth-based objects - even massive ones like mountains or oceans - are far too light and slow to make detectable gravitational waves.

Short Gamma-Ray Bursts: The Initial Flash & Afterglow

Neutron star mergers also produce short gamma-ray bursts (sGRBs) - the most energetic explosions since the Big Bang. These bursts last less than 2 seconds and release more energy in that time than the Sun will emit in its entire 10-billion-year lifetime. They're created when the merger forms a rapidly spinning black hole or massive neutron star surrounded by an accretion disk. Jets of plasma shoot out along the rotation axis at nearly light speed. If Earth happens to lie along the jet's path, satellites detect an intense flash of gamma rays.

After the initial burst comes the "afterglow":

X-ray Afterglow: High-energy radiation from the jet interacting with surrounding material. Visible for hours to days.

Optical/UV Afterglow: The jet continues to emit light as it slows down, visible to ground-based telescopes.

Radio Afterglow: Can persist for months or years as the expanding shock wave interacts with the interstellar medium.

This is what Kilonova Catcher observes - the optical/UV emission from both the kilonova and the jet afterglow.