What is Time?
Every clock you own disagrees with every other clock. Here's why that matters — and how we decided what "one second" actually means.
The Chandelier Moment
In 1582, a young Galileo Galilei sat in the Pisa Cathedral, bored during mass. He noticed the chandelier swinging overhead. Using his pulse as a clock, he realized something profound: no matter how wide the swing, it took the same number of heartbeats to complete one oscillation.
This simple observation — that a pendulum's period is constant — would eventually lead to the invention of the pendulum clock 74 years later, revolutionizing timekeeping accuracy from hours to seconds.
What Is Time, Really?
- Time (noun)
- The continued progress of existence and events from past through present to future. In physics, it's one of the four dimensions of spacetime — you can't have space without time, or time without space. [Wikipedia: Time]
But here's the weird part: while we experience time flowing "forward," physics doesn't actually require that. The equations work the same backwards or forwards. The reason we remember the past but not the future? That's entropy — the universe's tendency toward disorder. [Wikipedia: Entropy]
A Quick History of Measuring Duration
Humans have tracked the passage of time for millennia, but "accurate" is a relative term. The journey from ancient sundials to atomic clocks shows our obsession with precision.
Sundials
Egyptians divide the day into 12 hours using shadow-casting obelisks. Problem? Winter hours are shorter than summer hours. [Learn more]
Water Clocks
Clepsydra (water thieves) drip at constant rates. Used in Egyptian and Babylonian courts to limit speaking time. [Wikipedia]
Hourglasses
Sand flows through a narrow neck. Accurate to about 15 minutes over an hour — good enough for sermons and exams. Not as precise as atomic measurements.
Pendulum Clock
Christiaan Huygens builds first working pendulum clock. Suddenly, we're accurate to within 15 seconds per day. Revolutionary! [Wikipedia]
Quartz Clock
Warren Marrison at Bell Labs discovers quartz vibrates at 32,768 Hz when electrified. Accurate to 1 second per year. [Wikipedia]
Atomic Clock
Essen and Parry build the first cesium clock at National Physical Laboratory. Accurate to 1 second in 300 years. [Wikipedia]
Types of Time Measurement
Solar Time
Based on the Sun's position. Noon = Sun at highest point. The time we naturally experience, but Earth's rotation isn't perfectly constant — it's slowing down!
Sidereal Time
Based on distant stars, not the Sun. Astronomers love this because stars return to the same position every 23h 56m 4s. Used for telescope pointing.
Universal Time (UT1)
Based on Earth's rotation relative to distant quasars (not stars anymore). More stable than solar time but still drifts as Earth slows.
Atomic Time (TAI)
Based on cesium-133 vibrations. Stable to 1 second in millions of years. The most precise measurement humans have ever created.
How We Measure Time Today
Modern time measurement relies on atomic vibrations. The SI second is defined as exactly 9,192,631,770 periods of radiation corresponding to the transition between two energy levels of a cesium-133 atom.
That's not an estimate — it's the definition. A second is now a specific number of atomic vibrations, not a fraction of Earth's rotation.
The Leap Second Problem
Atomic clocks never "lose" time. But Earth is slowing down (tidal friction from the Moon adds about 1.7 milliseconds per century). So atomic time and Earth time drift apart.
Solution? Leap seconds. Since 1972, we've added 27 leap seconds to keep atomic time (UTC) within 0.9 seconds of Earth time (UT1). The last one was in 2016.
Frequently Asked Questions
What is time in physics?
In physics, time is a fundamental dimension of spacetime. It measures the sequence and duration of events, flowing from past through present to future. In Einstein's relativity, time and space are interwoven — you can't have one without the other.
How do we measure duration?
We measure duration using recurring natural phenomena: Earth's rotation (day/night cycle), Earth's orbit (year), pendulum oscillations, and atomic vibrations. The SI second is defined by exactly 9,192,631,770 vibrations of a cesium-133 atom.
Why is accurate timekeeping important?
Precise timekeeping enables GPS navigation (which needs nanosecond precision), stock trading timestamps, internet synchronization, and scientific experiments. Without accurate clocks, modern technology would collapse. Your phone's GPS would be off by kilometers within a day!
Will time ever end?
According to current physics, time will continue as long as the universe exists. The "arrow of time" points forward due to increasing entropy (disorder). Some theories suggest time began with the Big Bang and might end in a "Big Crunch" or "Heat Death" scenario billions of years from now.
Related Topics: UTC Explained | Atomic Clocks | Leap Seconds