11-15-2025, 10:09 AM
Chapter 14 — Gravitational Waves
Gravitational waves are ripples in the fabric of spacetime.
They were predicted by Einstein in 1916, but only directly detected in 2015.
Their discovery opened a new way of observing the universe.
With gravitational waves, astrophysicists can detect events that produce no light at all —
colliding black holes, merging neutron stars, and violent cosmic phenomena.
---
14.1 What Are Gravitational Waves?
According to Einstein’s General Relativity:
Mass warps spacetime.
Moving mass creates ripples.
These ripples are gravitational waves, travelling outward at the speed of light.
They stretch and compress space itself as they pass through.
You cannot feel this, because the stretching is incredibly tiny —
even massive waves change distances by less than the width of a proton.
---
14.2 What Creates Gravitational Waves?
Only extremely massive, accelerating objects can create waves strong enough to detect:
• Merging black holes
• Colliding neutron stars
• Spinning neutron stars with uneven mass
• Supernova explosions
• Possibly the Big Bang itself
Black hole collisions produce the strongest and cleanest signals.
---
14.3 The First Detection (2015)
On September 14, 2015, LIGO detected gravitational waves for the first time.
The signal came from:
• Two black holes
• 1.3 billion light-years away
• Merging into a single black hole
The energy released in that moment was greater than the light of all stars in the universe combined —
for less than a second.
This detection confirmed one of Einstein’s last predictions.
---
14.4 How Gravitational Wave Detectors Work
The main detectors are:
• LIGO (USA)
• Virgo (Italy)
• KAGRA (Japan)
These detectors use:
Laser Interferometry
They split a laser beam into two paths and measure tiny differences in length caused by passing waves.
A change as small as 10⁻²¹ in length is enough to signal a gravitational wave.
---
14.5 What Gravitational Waves Tell Us
Gravitational waves reveal information that light alone cannot:
Masses of black holes
Because the signal strength depends on mass.
Distances to collisions
Wave amplitude decreases with distance.
Nature of neutron star matter
Mergers show how “stiff” or “squishy” neutron star material is.
Expansion rate of the universe
By combining gravitational waves with light signals.
Evidence of new physics
Some theories predict additional gravitational wave types.
---
14.6 Neutron Star Mergers
In 2017, LIGO detected waves from two merging neutron stars.
This event also produced:
• gamma-ray bursts
• visible light
• heavy elements (gold, platinum)
It confirmed:
Neutron star collisions create many of the universe’s heavy elements.
This was one of the biggest scientific breakthroughs of the decade.
---
14.7 Black Hole Mergers
Most detections so far involve merging black holes.
These events:
• emit no light
• occur billions of light-years away
• produce clean gravitational wave “chirps”
Gravitational waves are the only way to observe these collisions.
---
14.8 Future of Gravitational Wave Astronomy
The next generation of detectors will transform the field:
LISA (2030s):
• A space-based detector
• Will detect supermassive black hole mergers
Einstein Telescope (Europe):
• More sensitive
• Detects earlier universe signals
Cosmic Explorer (USA):
• 10× longer arms
• Detects smaller black holes, further away
We may eventually detect:
• Gravitational waves from the Big Bang
• Signals from dark matter
• New physics beyond relativity
---
Chapter Summary
• Gravitational waves are ripples in spacetime created by massive accelerating objects.
• Einstein predicted them; LIGO detected them a century later.
• Black hole and neutron star mergers are the main sources.
• Detectors use laser interferometry to measure tiny disturbances.
• Gravitational waves reveal information light cannot.
• The future of gravitational wave astronomy is extremely promising.
---
Practice Questions
1. What creates gravitational waves?
2. How did LIGO detect the first gravitational wave?
3. Why are neutron star mergers scientifically important?
4. What information can gravitational waves reveal that light cannot?
5. What is LISA, and what will it detect?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive
Gravitational waves are ripples in the fabric of spacetime.
They were predicted by Einstein in 1916, but only directly detected in 2015.
Their discovery opened a new way of observing the universe.
With gravitational waves, astrophysicists can detect events that produce no light at all —
colliding black holes, merging neutron stars, and violent cosmic phenomena.
---
14.1 What Are Gravitational Waves?
According to Einstein’s General Relativity:
Mass warps spacetime.
Moving mass creates ripples.
These ripples are gravitational waves, travelling outward at the speed of light.
They stretch and compress space itself as they pass through.
You cannot feel this, because the stretching is incredibly tiny —
even massive waves change distances by less than the width of a proton.
---
14.2 What Creates Gravitational Waves?
Only extremely massive, accelerating objects can create waves strong enough to detect:
• Merging black holes
• Colliding neutron stars
• Spinning neutron stars with uneven mass
• Supernova explosions
• Possibly the Big Bang itself
Black hole collisions produce the strongest and cleanest signals.
---
14.3 The First Detection (2015)
On September 14, 2015, LIGO detected gravitational waves for the first time.
The signal came from:
• Two black holes
• 1.3 billion light-years away
• Merging into a single black hole
The energy released in that moment was greater than the light of all stars in the universe combined —
for less than a second.
This detection confirmed one of Einstein’s last predictions.
---
14.4 How Gravitational Wave Detectors Work
The main detectors are:
• LIGO (USA)
• Virgo (Italy)
• KAGRA (Japan)
These detectors use:
Laser Interferometry
They split a laser beam into two paths and measure tiny differences in length caused by passing waves.
A change as small as 10⁻²¹ in length is enough to signal a gravitational wave.
---
14.5 What Gravitational Waves Tell Us
Gravitational waves reveal information that light alone cannot:
Masses of black holes
Because the signal strength depends on mass.
Distances to collisions
Wave amplitude decreases with distance.
Nature of neutron star matter
Mergers show how “stiff” or “squishy” neutron star material is.
Expansion rate of the universe
By combining gravitational waves with light signals.
Evidence of new physics
Some theories predict additional gravitational wave types.
---
14.6 Neutron Star Mergers
In 2017, LIGO detected waves from two merging neutron stars.
This event also produced:
• gamma-ray bursts
• visible light
• heavy elements (gold, platinum)
It confirmed:
Neutron star collisions create many of the universe’s heavy elements.
This was one of the biggest scientific breakthroughs of the decade.
---
14.7 Black Hole Mergers
Most detections so far involve merging black holes.
These events:
• emit no light
• occur billions of light-years away
• produce clean gravitational wave “chirps”
Gravitational waves are the only way to observe these collisions.
---
14.8 Future of Gravitational Wave Astronomy
The next generation of detectors will transform the field:
LISA (2030s):
• A space-based detector
• Will detect supermassive black hole mergers
Einstein Telescope (Europe):
• More sensitive
• Detects earlier universe signals
Cosmic Explorer (USA):
• 10× longer arms
• Detects smaller black holes, further away
We may eventually detect:
• Gravitational waves from the Big Bang
• Signals from dark matter
• New physics beyond relativity
---
Chapter Summary
• Gravitational waves are ripples in spacetime created by massive accelerating objects.
• Einstein predicted them; LIGO detected them a century later.
• Black hole and neutron star mergers are the main sources.
• Detectors use laser interferometry to measure tiny disturbances.
• Gravitational waves reveal information light cannot.
• The future of gravitational wave astronomy is extremely promising.
---
Practice Questions
1. What creates gravitational waves?
2. How did LIGO detect the first gravitational wave?
3. Why are neutron star mergers scientifically important?
4. What information can gravitational waves reveal that light cannot?
5. What is LISA, and what will it detect?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive