Time’s Arrow

I shall use the phrase “time’s arrow” to express this one-way property of time which has no analogue in space.

Sir Arthur Stanley Eddington (1882-1944)

The increase of disorder or entropy is what distinguishes the past from the future, giving a direction to time.

Stephen Hawking, in A Brief History of Time (1988)

“(Entropy)…a measure of randomness or disorder in a system. Darrell Ebbingvery usefully suggests thinking of a deck of cards. A new pack fresh out of the box, arranged by suit and in sequence from ace to king, can be said to be in its ordered state. Shuffle the cards and you put them in a disordered state. Entropy is a way of measuring just how disordered that state is and of determining the likelihood of particular outcomes with further shuffles…that’s the general idea.”

Bill Bryson, in A Short History of Nearly Everything (2003) Chapter Eight

Good friends beware! the only life we know/ Flies from us like an arrow from the bow, /the Caravan of life is moving by, / Quick! to your places in the passing show.

From: The Rubáiyát of Omar Khayyám, the title that Edward FitzGerald gave to his 1859 translation from Persian to English of a selection of quatrains (rubāʿiyāt) attributed to Omar Khayyam (1048–1131), dubbed “the Astronomer-Poet of Persia”.

I shot an arrow into the air,

It fell to earth, I knew not where;

For, so swiftly it flew, the sight

Could not follow it in its flight./

I breathed a song into the air,

It fell to earth, I knew not where;

For who has sight so keen and strong,

That it can follow the flight of song?/

Long, long afterward, in an oak

I found the arrow, still unbroke;

And the song, from beginning to end,

I found again in the heart of a friend.

Henry Wadsworth Longfellow

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