Time
Time is something we constantly measure, but we rarely stop to understand how it actually works. From the spinning of the Earth to the mind-bending physics of light speed, let us explore how humans track the past, present, and future.
Welcome to the Fourth Dimension!
It mimics a sundial! In the Northern Hemisphere, where early civilisations first invented sundials, the shadow cast by the sun moves to the right as the day progresses. Mechanical clocks were designed to mimic this ancient natural motion.
Time passes more slowly for you! According to Albert Einstein, time is relative. If you travel at near light speed, time physically slows down for you. When you return, you will have aged less than the people who stayed on Earth.
Test Your Temporal Instincts
Think about how we measure the passage of events. Pick an answer below to test your knowledge.
The Earth completing one full spin! It takes exactly 24 hours for our planet to spin around once on its axis, giving us our cycle of day and night.
It takes 365 and a quarter days! Since our calendar only has 365 days, those leftover quarter days add up. Every four years, we put them together into one extra day (February 29th) to keep our seasons aligned.
The Mechanics of Time
Tap each card to uncover the cosmic mechanics of time, from planetary orbits to the brilliant physics of Galileo and Albert Einstein.
Key Concepts
Earth's Rotation
Tap to learn moreA standard day is determined by the Earth spinning on its axis. One complete, 360-degree rotation takes exactly 24 hours, which provides our planet with its continuous cycle of daylight and darkness.
Earth's Revolution
Tap to learn moreWhile the Earth spins, it also travels in a massive orbit around the Sun. The time it takes for our planet to complete one full journey around the Sun is what we call a solar year.
The Leap Year
Tap to learn moreAn orbit does not take exactly 365 days; it takes 365 and a quarter days. To stop our seasons from drifting out of sync, we combine those leftover quarters and add an extra Leap Year day to the calendar every four years.
Time Zones
Tap to learn moreBecause the Earth is a sphere, the sun hits different locations at different times. To ensure noon always happens in the middle of the day everywhere, the globe is divided into 24 distinct time zones, each separated by one hour.
The International Dateline
Tap to learn moreThis is an imaginary longitudinal line running vertically through the Pacific Ocean. It acts as the official starting point for the world's clocks. Crossing it means you move forward or backward an entire calendar day.
Pendulum Physics
Tap to learn moreGalileo discovered that a swinging weight, known as a pendulum, always takes the exact same amount of time to swing back and forth, regardless of how high it was dropped. This constant physical rate made the first accurate clocks possible.
The Speed of Light
Tap to learn moreUnlike a thrown ball, light does not speed up or slow down based on how fast you are moving. Light waves travel at a universal constant speed of exactly 300,000 kilometres per second.
Einstein's Relativity
Tap to learn moreBecause the speed of light cannot change, Albert Einstein deduced that time itself must be flexible. If you travel in a spaceship at incredibly high speeds, time will physically pass more slowly for you than for people on Earth.
Interactive: The Relativity Time Machine
Start the clocks to watch time flow. Adjust the throttle to increase your velocity, and watch the live telemetry to see how your spacecraft's clock physically slows down compared to the clock back on Earth.
Apply Your Knowledge
Let us see if you can correctly identify the essential physics terminology associated with time and celestial mechanics.
Match the Concepts
Click a scientific term to select it, then click the matching description to place it.
Autonomous Rover Command
You are a NASA engineer programming a rover on Mars. A sudden sandstorm is approaching the rover, but your radio signal will take 11 minutes to reach Mars, and another 11 minutes for you to confirm the rover received it. Write a brief sequence of logical "If/Then" instructions you would programme into the rover's computer so it can save itself without waiting for your command.
What Has Changed Since This Episode Aired
This episode originally aired in 1997. Since then, physics and space exploration have advanced significantly!
Then (1997): Engineers were just figuring out how to deal with the 10-minute communication delay for early rovers.
Now: Rovers like Curiosity and Perseverance use advanced artificial intelligence to autonomously navigate terrain and select scientific targets entirely on their own!
Then: Scientists relied on standard atomic clocks to keep global time.
Now: Physicists have developed Strontium "optical lattice" clocks that are so precise they will not lose a single second in 15 billion years (which is actually older than the universe itself).
Then: Bill Nye used a theoretical thought experiment about twin astronauts to explain Einstein's theory of relativity.
Now: We use this physics every day! GPS satellites orbit the Earth faster and in weaker gravity than we experience. This causes time to move differently for the satellites. Scientists must programme GPS clocks to artificially adjust by microseconds every day, or modern map applications would completely fail!
Then: We added Leap Years to fix our orbit, and historically we added "Leap Seconds" to account for the Earth's spin slowing down.
Now: Adding random seconds has caused massive crashes in global computer networks. Because of this, global scientists recently voted to completely abolish the "Leap Second" by the year 2035.
Then: Time was viewed purely as a sequence of events moving consistently forward.
Now: Quantum physicists have successfully created "Time Crystals". This is a completely new phase of matter where atoms move and change in a repeating pattern across time, rather than just repeating across physical space!
Test Your Understanding
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Episode Discussion
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