What are gravitational waves?

Shock waves produced when two massive objects collide, like a sonic boom in space High-energy radiation emitted when a massive object enters a black hole Ripples in the curvature of spacetime caused by accelerating massive objects, traveling outward at the speed of light and causing distances to alternately stretch and compress Pulses of gravity emitted by pulsars as they rotate rapidly

Einstein predicted gravitational waves in 1916 but believed they would never be detected. Why did he think they would be undetectable?

Gravitational waves travel faster than light and cannot be measured in real time He calculated that gravitational waves are unimaginably weak — even the most violent astronomical events produce only infinitesimally tiny distortions of space at Earth's distance No massive enough objects existed in the universe to produce detectable gravitational waves Gravitational waves are absorbed by the interstellar medium before reaching Earth

What was the Hulse-Taylor binary pulsar, and why did it win the Nobel Prize?

A single rapidly spinning pulsar whose slowing-down rate confirmed Einstein's GR equations A binary system of a black hole and neutron star whose merger produced the first detected gravitational wave burst Two neutron stars orbiting each other — their orbit was observed to be shrinking at exactly the rate predicted if the system were losing energy to gravitational waves, providing the first indirect evidence that gravitational waves are real Two white dwarfs that produced a gravitational wave signal strong enough for early detectors to register

How does LIGO detect gravitational waves?

It measures the temperature change in a tank of water caused by a gravitational wave heating the molecules It uses a very sensitive pendulum that swings when a gravitational wave passes It uses laser interferometry — a laser beam is split down two 4 km arms at right angles; a passing gravitational wave stretches one arm and compresses the other, changing how the laser beams recombine and revealing the wave It measures tiny changes in Earth's rotation rate caused by the angular momentum carried by a gravitational wave

On September 14, 2015, LIGO detected GW150914 — the first gravitational wave event. What produced this signal?

Two neutron stars merging in our own Milky Way galaxy about 1,000 light-years away Two black holes, one 29 solar masses and one 36 solar masses, merging 1.3 billion light-years away and converting about 3 solar masses of mass entirely into gravitational wave energy A supernova explosion in the Andromeda Galaxy releasing energy in all directions The formation of a neutron star inside a galaxy cluster 500 million light-years away

Why was the neutron star merger GW170817 (August 17, 2017) considered a landmark event in the history of astronomy?

It was the most powerful gravitational wave event ever detected, briefly shaking Earth's crust It was the first gravitational wave detected from inside the Milky Way galaxy It was detected in both gravitational waves AND electromagnetic radiation (gamma rays, optical light, X-rays, and radio waves), opening the era of multi-messenger astronomy and revealing that heavy elements like gold are made in neutron star mergers It was the first event that allowed scientists to directly image the colliding neutron stars with a radio telescope

What is LISA, and how will it differ from ground-based LIGO?

LISA is a larger version of LIGO with 40 km arms instead of 4 km, planned for Nevada LISA is a space-based gravitational wave detector planned for the 2030s, with three spacecraft separated by 2.5 million km forming a triangle, sensitive to much lower gravitational wave frequencies than LIGO can detect LISA is a network of gravitational wave detectors on the Moon that avoids Earth's seismic noise LISA is a proposed update to LIGO that uses quantum-entangled photons to detect gravitational waves

GW170817 allowed scientists to test whether gravity travels at the speed of light. What was the result?

Gravity was found to travel about 10% slower than light, as some alternative theories predicted Gravity was confirmed to travel at the speed of light to within 1 part in 10^15 — the gravitational wave and gamma rays arrived only 1.7 seconds apart despite traveling 130 million light-years Gravity was found to travel instantaneously, as Newton originally assumed The measurement was inconclusive because the two signals arrived too far apart to compare

Two black holes of 30 solar masses each are about to merge. At peak emission, the gravitational wave power output briefly exceeds the light output of all stars in the observable universe combined. How is this possible given that black holes emit no light?

The merger creates a brief loophole in the law of conservation of energy The surrounding gas and dust produces the energy — not the black holes themselves The energy comes from the mass of the black holes themselves (E = mc^2) — the merger converts about 5% of the total mass into gravitational wave energy, which for 60 solar masses is an enormous amount The power only seems greater because gravitational waves carry more information per joule than light does