9.7 Gravitational Waves

Gravitational waves are ripples in the fabric of spacetime, predicted by Albert Einstein's General Theory of Relativity (1916). They are generated by some of the most violent and energetic processes in the universe, such as the acceleration of massive objects. As these waves travel outward from their source at the speed of light, they stretch and compress spacetime itself, momentarily altering the distances between objects.

Nature of Gravitational Waves

Gravitational waves are disturbances in the curvature of spacetime, propagating like ripples on the surface of a pond. They are generated by accelerating massive objects, especially those moving asymmetrically.

Primary Sources of Detectable Gravitational Waves:

• Two black holes orbiting each other and merging.
• Binary systems of neutron stars spiraling into each other.
• The asymmetric explosion of a star (a supernova).

Effect on Spacetime

As a gravitational wave passes through a region of space, it causes spacetime to stretch in one direction while simultaneously compressing in the perpendicular direction. This is called a quadrupolar distortion.

• The stretching and compressing alternate as the wave oscillates.
• By the time gravitational waves from distant cosmic events reach Earth, their amplitude is minuscule — far smaller than the diameter of an atom.
• This is why all accelerating objects (including people and cars) technically produce gravitational waves, but these are far too weak to detect.

The Interferometer: Detecting the Ripples

What is an Interferometer?

An interferometer is a highly sensitive instrument that works by splitting a beam of light into two paths, then recombining them to create an interference pattern. Any change in the path length of the light beams will alter this pattern, making interferometers ideal for measuring incredibly small changes in distance.

LIGO: Laser Interferometer Gravitational-Wave Observatory

The primary tool for detecting gravitational waves is a laser interferometer. The most famous example is LIGO, a massive international scientific collaboration with contributions from the USA, India, Germany, Australia, and the UK.

How LIGO Works:

1. A powerful laser beam is split into two identical beams.
2. These beams travel down two long, perpendicular arms (each 4 km long).
3. The beams reflect off mirrors at the end of each arm and return to recombine.
4. Normally, the arms are of equal length, so the returning light waves interfere destructively — no light reaches the detector.
5. When a gravitational wave passes, it stretches one arm while compressing the other. This tiny change in arm length (as small as 1/10,000th the width of a proton) causes the light waves to no longer cancel perfectly.
6. This results in a flicker of light at the detector — the signal of a passing gravitational wave.

LIGO made its first successful detection of gravitational waves in 2015, originating from a binary black hole merger, marking a new era in astronomy.

Summary Table