Field
Time
Why clocks disagree, and why the past is not the future.
19 entries
Clocks disagree on purpose
GPS satellite clocks run faster than identical clocks on Earth’s surface by about 38 microseconds per day, as general and special relativity predict. The system corrects for it or it fails.
Rocks keep time by decay
Radioactive isotopes decay at measured rates. Isochron and concordia methods date rocks from thousands to billions of years.
The future is the higher-entropy direction
The thermodynamic arrow of time — why we remember the past and not the future — is the universe’s low-entropy beginning plus the second law.
A day is not 24 hours
Relative to the stars, Earth rotates once in 23 hours, 56 minutes, and about 4 seconds. The 24-hour day is noon to noon, which includes the orbit.
Coral kept the time of ancient days
Growth bands in Devonian corals show about 400 days in a year. The year had more days because each day was shorter.
You are moving thirty kilometres a second
Earth orbits the sun at about 29.8 kilometres per second. Sitting still is a description of the chair, not of the ride.
A particle lives longer because it is moving
Cosmic-ray muons reach the ground in numbers that only make sense if their clocks run slow. Their half-life in flight matches special relativity.
The second is a number of wiggles
Since 1967 the second has been defined as 9,192,631,770 periods of the radiation from a particular transition in caesium-133.
Andromeda is coming
The Andromeda galaxy is moving toward the Milky Way at about 110 kilometres per second. A collision in roughly four to five billion years follows from that velocity.
There is no universal now
In special relativity, whether two distant events are simultaneous depends on the motion of the observer. Nature does not supply one shared present.
The ground at the equator is moving at 1,670 kilometres an hour
Earth’s equatorial circumference and its sidereal day give a ground speed of about 1,670 km/h. A point near the pole barely moves.
The day is getting longer
Tidal friction slows Earth’s spin. The length of the day increases by a few milliseconds per century, measured with ancient eclipses and with lasers.
A year does not divide into days
The tropical year is about 365.2422 days. The fraction is why the calendar needs leap days, and why no integer calendar stays honest forever.
Your head is older than your feet
A clock higher in a gravitational field runs faster. Over a human life the difference between head and feet is less than a microsecond.
Ten days were skipped
In the Catholic world in 1582, Thursday 4 October was followed by Friday 15 October. Ten dates were removed to realign the calendar with the Sun.
The Sun you see is already eight minutes old
Sunlight takes about eight minutes and twenty seconds to reach Earth. The disc in the sky is the Sun as it was that long ago.
Some minutes have 61 seconds
Since 1972, UTC has occasionally inserted a leap second, so that 23:59:60 is a real timestamp and noon stays near the Sun.
The week is an invention
The year, the month, and the day are anchored to astronomy. The seven-day week is not. No celestial cycle completes in seven days.
A photon's journey takes no proper time
Along a light ray the proper time is zero. There is no inertial frame in which a photon sits still and wears a clock.