Table of Contents
Chapter 1 - What is Time?1
The Nature of Time2
Direction of Time2
Time Never Stops3
Speed of Time3
Time Appears to be Continuous4
Quantum Time: Time May be Discrete4
Does Time Simply Exist, or is it Caused by Something?6
Time Seems Infinite6
Does Time Have a Beginning or End?7
Chapter 2 - How do we measure time?9
The History of Measuring Time9
Why 60 Seconds? Why 60 Minutes? Why 24 Hours?12
How Scientists Define Time14
Calendars15
Leap Seconds17
Time of Day19
Before Mechanical Clocks Were Invented19
Mechanical Clocks29
Pendulum Clocks: A Giant Leap30
Spring-Driven Clocks: Time Becomes Portable30
Electric Clocks: Powering Time31
Quartz Clocks: a Crystal Revolution31
Atomic Clocks: the Master Timekeepers31
Chapter 3 - Time Gets Weirder33
Time is Relative33
The Hafele-Keating Experiment (1971)34
Gravity and Time: Clocks on Mountains34
Watching Muons35
Because Time is Relative36
How GPS works38
Time Can Bend and Stretch42
Time Bends When You Move Very Fast42
What Does It Mean for Time to Stretch?43
Why Does Time Stretch at High Speeds?44
Consequences of Time Stretching44
Time Bends When Gravity is Strong46
Conclusion47
Is Time Just Part of a Simulation?47
Time Can Not be Undone49
Entropy and the Arrow of Time49
New Scientific Theories About Reversing Time50
Cause and Effect are Linked to Time51
What Is Cause and Effect?52
Why Time Matters for Cause and Effect52
What Happens in the Strange World of Quantum Physics?53
Conclusion53
Cyclical time54
Ancient Beliefs About Cyclical Time54
Natural Examples of Cyclical Time55
Modern Scientific Ideas About Cyclical Time55
Conclusion56
Block Time: Is the Universe Frozen?56
How Precise Clocks May Warp Time58
Imaginary Time60
How Is Imaginary Time Different?60
Why Do Scientists Use Imaginary Time?61
Is Imaginary Time Real?61
Conclusion62
Is Time Real?62
Chapter 4 - Perception of Time63
How Does the Brain Measure Time?64
Why Does Time Feel Different Sometimes?64
How Scientists Study Time Perception65
Special Tricks of the Mind66
Conclusion67
Chapter 5 - Time travel67
Scientific Thought on Time Travel67
Cosmic Strings: Threads of the Early Universe70
Movies That Illustrate Time Travel72
Back to the Future (1985):72
Interstellar (2014):73
The Adam Project (2022):73
The Time Travel Paradox74
Time Travel by Telescope75
Conclusion76
Chapter 6 - Weird Thoughts About Time76
Does the Future Already Exist?76
Living in Space-Time.77
What if We Lived in Space Without Time?78
What if We Lived in Time Without Space?79
What if We Lived With Neither Space nor Time?79
Learning Activities80
Thought Experiments80
Twin Time Activity80
Block Time Activity83
No Space, No Time Activity85
Backward Time Activity87
Paused Time Activity89
Discrete Time Activity92
Simple Experiments94
Stick Clock Activity94
Sundial Activity96
Hour Glass Activity98
Candle Clock Activity100
Water Clock Activity103
Explore Your Perception of Time105
Creative Activities106
Draw Time106
Invent a New Clock106
Time Capsule107
Simulated Day107
Appendix107
Measurement of Time107
Quotes About Time110
Riddles About Time112
Jokes About Time114
Glossary116
