Every fundamental law works identically forwards and backwards, yet eggs unscramble never. The past and future differ, and no equation says why.
Documented
Eddington's label, Boltzmann's dice, the Big Bang's low entropy
Eddington coined the phrase in 1927: the arrow is entropy, the universe sliding from improbable order toward probable disorder. Boltzmann had already shown entropy's rise is statistical, not absolute, which sharpens the puzzle: why did the universe start in such an extraordinarily low-entropy state? The documented answer is the Past Hypothesis, the brute posit that the Big Bang began smooth and ordered, roughly one chance in ten to the ten-to-the-one-hundred-and-twenty by Penrose's estimate.
Assessment: The thermodynamics, the statistics, and the cosmic microwave background's smoothness are documented. The Past Hypothesis is a documented assumption, not an explanation. Physics has the arrow's direction measured to perfection and its cause on a blank prescription pad.
Stories
Time as a river, physics as the dam
The folk picture takes the arrow personally: time flows, we are carried, entropy is decay, decay is sad, therefore physics proves impermanence. Self-help borrows it, grief counseling borrows it, and every documentary scores it with strings. The river metaphor predates thermodynamics by millennia, which tells you it was never a conclusion.
Assessment: The metaphor is documented folklore. Nothing in the second law says anything about meaning, and Boltzmann's statistics allow entropy to decrease locally all the time; refrigerators and embryos do it daily.
Interesting claims
Block universe, no arrow at all
The interesting claim: the arrow is perspectival, not physical. In the block-universe reading, all moments exist equally and the flow of time is something consciousness adds, a user interface illusion; entropy gradient explains memory and cause, and nothing actually moves. Death, in this frame, is a boundary of a region, not a deletion.
Assessment: The block view follows naturally from relativity and is held seriously by many physicists, but the consciousness-adds-the-flow step is untested philosophy. The documented core, symmetric laws plus asymmetric boundary conditions, is real; everything about experience layered on top is interpretive.
Sources and further reading
Eddington, The Nature of the Physical World (1928), the coining
Boltzmann, Lectures on Gas Theory (1896-98), the statistical foundation
Penrose, The Emperor's New Mind (1989), the entropy budget calculation
Price, Time's Arrow and Archimedes' Point (1996), the boundary-condition analysis
Albert, Time and Chance (2000), the Past Hypothesis in its strongest form
Worth asking
If the laws are symmetric, the difference between past and future lives in initial conditions, which means the strangest thing in physics is not death, it is birth. Whose choice was that?