What Is Time? Why Physics Still Can't Pin It Down
What is time? A calm walk through how physics measures it — relativity, the arrows of entropy, the quantum-gravity problem of time — while its deeper nature stays unsettled.
Physics has settled what time *does*. It has not settled what time *is*. Across the last century physicists worked out how clocks slow with speed and gravity, why memory points backward and not forward, and how the universe carries time outward as it expands. The older question — what time IS at base — physics has handed to philosophy and to quantum gravity.
This Almanac Notes essay walks through the four layers physics addresses time at, each with its own epistemic status. We open at relativity (*settled*): special relativity's Lorentz factor (γ ≈ 22 at v = 0.999c) and the cosmic-ray muon evidence; general relativity's gravitational time dilation, measured at the table-top scale by Chou et al. (NIST 2010) between two aluminum-ion clocks separated by a single foot; and the +38 microseconds per day GPS satellites correct for every day. We move through the three arrows of time (*mostly settled*) — thermodynamic, cosmological, psychological — and Sean Carroll's discussion of David Albert's "past hypothesis": the universe began in a state of extraordinarily low entropy, and from there the second law gives entropy only one direction to go. Why entropy was initially so low remains an open question.
We then cross into the is question. J. M. E. McTaggart's 1908 paper "The Unreality of Time" defined the A-series (tensed past/present/future) and B-series (untensed earlier/later) that still organize the debated presentism vs eternalism question. Einstein's March 1955 condolence letter to Michele Besso's family — "the distinction between past, present, and future is only a stubbornly persistent illusion" — sits at the heart of the eternalist position, though relativity only nudges eternalism, it does not strictly entail it. Finally we look at speculative terrain: the Wheeler-DeWitt equation (HΨ = 0) and the candidate quantum-gravity resolutions to the problem of time — Carlo Rovelli's relational time, the Page-Wootters mechanism (Page and Wootters 1983; tabletop quantum simulations since), and loop quantum gravity wrestling with the same gap. No consensus framework.
The essay closes on Augustine, who in Confessions XI (c. 400 AD) put the gap exactly: "if no one asks me, I know. If I wish to explain it to him that asks, I know not." Sixteen hundred years later, the question is still where he left it.
📌 In this video:
The Lorentz factor γ — special relativity and the cosmic-ray muon evidence (1905)
Gravitational time dilation at the table-top scale — Chou et al. NIST 2010 (~4 × 10⁻¹⁷ at one foot)
GPS as a continuous relativity experiment — +38 μs/day net correction
Three arrows of time and Carroll's discussion of Albert's past hypothesis
McTaggart 1908: A-series vs B-series, and the foundational argument against the reality of time
Presentism vs eternalism — Einstein's 1955 letter to Besso's family, in scope
Wheeler-DeWitt (1967): HΨ = 0 and the problem of time in quantum gravity
Rovelli's relational time and the Page-Wootters mechanism
Augustine's question, sixteen hundred years on
🎯 Recommended for:
Curious adults wanting a careful walk through what physics knows (and does not know) about time
Students of physics, philosophy of physics, or quantum gravity
Readers of Aeon, Nautilus, or Quanta who prefer their science with restraint and precision
⏱️ Timestamps:
00:00 — Physics has settled what time does
00:37 — The four layers
01:20 — Relativity (SR · GR · GPS)
03:30 — Three arrows and the past hypothesis
06:07 — McTaggart, presentism vs eternalism
07:33 — Wheeler-DeWitt and the problem of time
08:56 — The picture as it stands
09:40 — Augustine, sixteen hundred years later