What is a gravitational wave?

• A. A ripple in spacetime produced by accelerating masses.
• B. A stream of particles emitted by black holes.
• C. A fluctuation in the cosmic microwave background.
• D. An electromagnetic wave propagating through space.

Answer: (A)

Gravitational waves are distortions or ripples in the fabric of spacetime itself that travel at the speed of light. They arise when masses accelerate in a non-symmetric way, changing how mass is distributed in a system—especially the quadrupole moment—so the gravitational field propagates outward as a wave. These waves carry energy away from the source and stretch and squeeze spacetime as they pass, though they don’t consist of matter or light themselves. In the universe, the strongest sources are cataclysmic events like merging black holes or neutron stars, which produce detectable signals that instruments like LIGO can pick up as tiny, alternating stretches and compressions of space. The other ideas describe streams of particles, temperature fluctuations in the early universe, or electromagnetic radiation—phenomena that are distinct from gravitational waves because they involve matter or electromagnetic fields rather than spacetime itself.