General knowledgeIn depthAsked by Anonymous23 Oct 2026, 4:50 AM ISTUpdated
In one line
LIGO detects gravitational waves by measuring tiny changes in the length of its arms using laser beams.
LIGO — Gravitational wave observatory site LIGO, the observatory that first detected gravitational waves in 2015. Image: WikipediaGravitational wave — Aspect of relativity in physics A computer simulation of two black holes merging, which creates gravitational waves detected by LIGO. Image: Wikipedia
The rule
Gravitational waves are ripples in space-time caused by massive objects moving very fast, like merging black holes. LIGO detects these waves by measuring how they stretch and squeeze space.
Let's understand it simply
Imagine you are standing in a field with two long ropes stretched out straight in front of you, each one 4 km long. You shine a torch (flashlight) down each rope. If something shakes the ground, the ropes will stretch or shrink a tiny bit. LIGO does something similar, but instead of ropes, it uses laser beams and mirrors. When a gravitational wave passes through, it makes the distance between the mirrors change just a tiny bit—less than the width of an atom! The laser helps LIGO notice these tiny changes.
Think of it like rolling out two long measuring tapes at right angles. If an earthquake (or a gravitational wave) passes, the tapes get a bit longer or shorter. LIGO is like a super-sensitive measuring tape for the universe.
How it works
LIGO has two long tunnels (arms) at right angles, each 4 km long.
A powerful laser beam is split into two and sent down both arms.
At the end of each arm, mirrors reflect the laser beams back to where they started.
The two beams come back and are combined. If the arms are exactly the same length, the beams cancel each other out (no light is seen at the detector).
When a gravitational wave passes, it stretches one arm and squeezes the other, changing the distance the lasers travel.
This change makes the beams not cancel perfectly, so some light reaches the detector. LIGO measures this light to see if a gravitational wave has passed.
How LIGO detects gravitational waves
Laser split into two beams
Beams travel down two 4 km arms at right angles
Mirrors reflect beams back
Beams combine at detector
Gravitational wave stretches/squeezes arms
Detector sees a change in light pattern
The story behind it
The idea of gravitational waves comes from Albert Einstein's theory of general relativity in 1916. For a long time, scientists thought these waves would be too small to ever detect. In the 1970s and 1980s, scientists like Rainer Weiss, Kip Thorne, and Ronald Drever worked on ways to build a detector sensitive enough. LIGO (Laser Interferometer Gravitational-Wave Observatory) was built in the USA, with two sites: one in Hanford, Washington, and one in Livingston, Louisiana. In 2015, LIGO made the first direct detection of gravitational waves from two black holes merging—a discovery that won the Nobel Prize in 2017.
Where you see it in real life
LIGO observatories: The actual machines that detected gravitational waves.
Laser interferometers: Used in precise engineering and science labs to measure tiny distances, just like LIGO.
Seismographs: Instruments that measure earthquakes also detect tiny vibrations, but not as small as LIGO can.
GPS satellites: Need to account for effects from general relativity, which is related to gravitational waves.
Medical imaging: Some scanning technologies use similar laser and mirror setups for precision.
Sound waves in air: Gravitational waves are like sound waves, but they travel through space-time instead of air.
Why it matters
Opens a new way to study the universe: We can now 'listen' to cosmic events like black holes merging.
Confirms Einstein's theory: Direct proof that gravitational waves exist.
Helps us learn about extreme objects: Black holes, neutron stars, and things we can't see with normal telescopes.
Inspires new technology: LIGO's methods push the limits of lasers, mirrors, and vibration control.
Worked example
Let's imagine a simplified version:
Given: LIGO's arm length = 4 km = 4,000 m
A gravitational wave stretches one arm by 1 × 10⁻¹⁸ m
Step 1: Laser travels down both arms and back
Step 2: If both arms are the same, beams cancel (no signal)
Step 3: Gravitational wave changes one arm by 1 × 10⁻¹⁸ m
Step 4: The laser beams now travel slightly different distances
Step 5: The detector sees a tiny bit of light (signal)
Step 6: LIGO records this as a gravitational wave event
Common mistakes
Thinking LIGO 'sees' waves like light: LIGO measures changes in length, not pictures.
Believing the arms move a lot: The change is incredibly tiny—thousands of times smaller than an atom.
Forgetting about noise: LIGO must filter out vibrations from earthquakes, trucks, and even wind.
Assuming only one LIGO is enough: Two (or more) detectors are needed to confirm a real gravitational wave and find its direction.
Remember it
LIGO uses lasers and mirrors to catch the tiniest stretch in space when a gravitational wave passes—like a cosmic ruler.
Check yourself
Why does LIGO use two arms at right angles?
So it can detect stretching in one direction and squeezing in the other, which is the signature of a gravitational wave.
What does LIGO actually measure when a gravitational wave passes?
It measures the tiny change in the length of its arms using laser beams.
Why are two LIGO observatories needed?
To confirm the signal is real and to help find where the wave came from.
What would happen if both arms changed by the same amount?
No signal would be detected, because the laser beams would still cancel out as before.
Quick recap
LIGO detects gravitational waves by measuring tiny changes in distance using lasers.
Gravitational waves stretch and squeeze space as they pass through Earth.
LIGO's arms are 4 km long and can detect changes smaller than an atom.
The first detection in 2015 confirmed Einstein's prediction from 1916.
Multiple detectors are needed to confirm and locate the source of the waves.
