Your head is ageing slightly faster than your feet. It is true, and it has been measured. In 2010 physicists in Colorado lifted one of two atomic clocks by just 33 centimetres, and it ticked measurably faster than the one below. Gravity slows time, and the closer you are to the ground, the slower it runs.
Most of us experience time as something we never have enough of. Scientists experience it as one of the deepest puzzles in physics. Saint Augustine put it best, sixteen centuries ago: if nobody asks me what time is, I know; if I try to explain it, I do not.
“By time, the human being is surely in loss, except those who believe, and do good, and urge one another to the truth, and urge one another to patience.”
Qur'an 103:1-3 · our rendering
Surah al-'Asr is one of the shortest chapters of the Qur'an, three verses long. The great scholar al-Shafi'i is reported to have said that if people reflected on this surah alone, it would be enough for them. It begins with an oath 'by time' (al-'asr can also mean the late afternoon, when the day is running out) and then says what is worth doing with it.
What science shows
The second is now defined by atoms. Since 1967, one second has been exactly 9,192,631,770 oscillations of a particular radiation from a caesium-133 atom. The best modern optical clocks, using strontium or ytterbium atoms, are so steady that they would lose or gain less than a second over the whole age of the universe. Scientists plan to redefine the second around them.
Einstein's theories showed that time is not the same everywhere. Moving clocks run slow (special relativity, 1905), and clocks deeper in gravity run slow (general relativity, 1915). In 1971 Joseph Hafele and Richard Keating flew caesium clocks round the world on airliners, eastwards and westwards, and found the time differences relativity predicted. GPS satellites have to correct for both effects every day.
Why does time only go forwards? The laws of motion work just as well backwards, yet eggs break and never un-break. Physicists link the 'arrow of time' to entropy, a measure of disorder, which tends to increase. That means the universe must have begun in a very orderly, low-entropy state, and nobody yet knows why. The cosmologist Roger Penrose calls this one of the great unsolved problems.
Our days are changing too. Tides act like a brake on the spinning Earth, so the day is getting longer by about 1.7 milliseconds per century on average. Corals that grew 400 million years ago laid down daily and yearly growth bands showing around 400 days in a year. Our Moon and Earth are slowly dancing apart.
Then there is deep time. Measurements of the cosmic microwave background by ESA's Planck satellite give the age of the universe as about 13.8 billion years. The Earth is about 4.54 billion years old, dated from meteorites. If the whole history of the universe were squeezed into one calendar year, all of recorded human history would fit into the last few seconds before midnight on 31 December.
Finally, inside us, time is kept by living clocks. Almost every cell runs a roughly 24-hour circadian rhythm, set by a small cluster of neurons in the brain that responds to light. The 2017 Nobel Prize in Physiology or Medicine went to Jeffrey Hall, Michael Rosbash and Michael Young for discovering the genes that drive it.
What each faith says
Judaism
“To every thing there is a season, and a time to every purpose under the heaven:”
Ecclesiastes 3:1 · JPS 1917
Judaism has been called a religion of time more than of place. The Torah's first holy thing is not a mountain or a temple but a day: God blessed the seventh day and made it holy (Genesis 2:3). Shabbat sets aside one day in seven for rest, and the Jewish calendar weaves the cycles of sun and moon together, adding a leap month seven times in every nineteen years so that the festivals keep to their seasons.
The Bible holds two views of time together: the human sense of seasons and limits (Ecclesiastes 3), and God's view, in which 'a thousand years in Thy sight are but as yesterday' (Psalm 90:4). Jewish philosophers such as Maimonides held that time itself was created along with the world.
Christianity
“But don’t forget this one thing, beloved, that one day is with the Lord as a thousand years, and a thousand years as one day.”
2 Peter 3:8 · WEB
In Book XI of his Confessions (around 400 CE), Augustine of Hippo wrestled with time in a prayer to God. He concluded that God did not create the world in time but created time with the world: there was no 'before' creation for God to wait through. Modern physicists who discuss the beginning of time often mention him.
Christians mark time through the church year, from Advent to Easter to Pentecost, and many pray at fixed hours. The New Testament repeats the Psalms' idea that 'one day is with the Lord as a thousand years' (2 Peter 3:8) and speaks of a right moment, kairos, as well as clock time, chronos.
Islam
“A day with your Lord is like a thousand years of what you count.”
Qur'an 22:47 · our rendering
Islam builds the day around time. The five daily prayers are set by the Sun: dawn, just after noon, mid-afternoon, sunset and nightfall, and their timetables are worked out by astronomy. The Islamic calendar is lunar, so Ramadan moves through all the seasons over about 33 years. The Qur'an says the sun and moon run 'by precise calculation' (55:5).
A hadith qudsi reports God saying: 'The son of Adam wrongs Me when he curses time, for I am time; in My hand is the command, and I turn the night and the day' (Bukhari 4826). Scholars explain that God is not time itself, but its Master: complaining about 'the times' is really complaining about the One who orders them. The Qur'an also speaks of a day with God being 'like a thousand years of your counting' (22:47).
Same and different
All three faiths hold that God is not bound by time as we are, all three use a thousand years as a picture of that, and all three make time holy with fixed days and prayers. They shape it differently: a lunisolar calendar and Shabbat for Jews, a solar church year and Sunday for most Christians, a lunar calendar, Friday prayer and five daily prayers for Muslims.
A moment to reflect
- Augustine said he knew what time was until someone asked him. Can you explain it?
- If time runs at different speeds in different places, what does 'now' mean?
- Why do you think religions set aside special days rather than special places only?
- Surah al-'Asr says people are 'in loss' except for a few things. What would you put on that list?
Watch
A five-minute TED-Ed tour of what physics says time is, and is not.
Kurzgesagt squeezes the history and future of everything into one timeline.
Carlo Rovelli at the Royal Institution on the physics and philosophy of time, for adults.
Veritasium on entropy, the reason time seems to have a direction.
Do it together
Mark out 13.8 metres on a playground for 13.8 billion years. Place the Earth's birth, the first life, the dinosaurs and the first humans along it. Recorded history sits in the last half-millimetre.
Invite neighbours of different faiths to talk about Shabbat, Sunday and Friday prayer: what each day protects, and what modern life does to it. End with a shared meal on a day that suits everyone.
Stand a stick in the garden and mark its shadow every hour on a sunny day. You have made the oldest kind of clock, the one Muslim, Jewish and Christian astronomers all used to fix prayer times.
Go deeper
- Caesium fountain atomic clocks (NIST) · article
- Augustine, Confessions Book XI (Christian Classics Ethereal Library) · scripture
- Surah al-'Asr 103 (Quran.com) · scripture
- Ecclesiastes 3 (Sefaria) · scripture
- The 2017 Nobel Prize in Physiology or Medicine · article
Sources (9)
- Clocks 33 cm apart in height run at measurably different rates: Chou et al., Science 329, 1630 (2010); NIST
- Definition of the second by the caesium-133 transition: BIPM, SI base unit: second
- Optical clocks so stable they would not lose a second over the age of the universe; plans to redefine the second: NIST, optical clocks
- Hafele-Keating experiment (1971): Hafele and Keating, Science 177, 166 (1972)
- Day lengthening of about 1.7-1.8 ms per century: Stephenson et al., Proc. R. Soc. A 472 (2016)
- Devonian corals record about 400 days per year: Wells, Nature 197, 948 (1963)
- Age of universe 13.8 billion years (Planck): ESA, Planck reveals an almost perfect universe
- Earth about 4.54 billion years old: USGS, Age of the Earth
- Nobel Prize 2017 for molecular mechanisms of circadian rhythm: Nobel Prize, 2017
