An old mechanical alarm clock ticks on the shelf. You hear the steady sound, see the hands moving, and you're certain time is passing. But if you take the clock apart, you won't find a single part labeled "time." Only springs and gears. Our world is much the same: in everyday life, time flows relentlessly, but physicists, diving into the very foundations of matter, discover equations that don't need time. In quantum gravity, the attempt to glue the theory of gravity together with quantum mechanics, time seems to dissolve.
From this arises a bold thought: maybe time isn't fundamental but emerges from something deeper, like ripples on water, which aren't present in the water molecules themselves. The author doesn't say whether this hunch is correct. Instead, he offers an evaluative tool — a criterion of physical coherence. The idea is simple: if a program claims to derive time from timelessness, it must show a clear chain of steps that doesn't undermine itself along the way. Beautiful words aren't enough: you need a working scheme where each stage builds on the previous one, not on a hidden assumption that time already exists.
Two well-known attempts served as a testing ground. The first is semiclassical time: here, quantum matter obeys laws without time, while gravity is described by the old, Einsteinian geometry, where time is still present. The result is a hybrid in which time emerges as an approximation, like the average temperature in a room where invisible molecules dart about. The second is thermal time: its flow is set not by the ticking of a cosmic clock but by the growth of disorder, entropy. The more the states of a system mix, the more noticeable the arrow of time becomes — much like cooling tea inexorably moves toward the temperature of the table.
Both programs face difficulties. Semiclassical time runs into the question: where does the geometry itself, which carries time, come from? Thermal time requires explaining why the growth of disorder has a direction at all in a world without an initial "before" and "after." The author doesn't pass judgment: he shows that the coherence criterion works like litmus paper: it reveals weak spots and suggests where to look next. Instead of declaring time an illusion or a miracle, physicists are invited to honestly trace the entire chain — from the timeless depths to the familiar ticking on the wrist.
So the old alarm clock proves to be a useful metaphor once again. Its hands move not because time is trapped inside, but because the spring unwinds, transmitting force through the gears. Perhaps the Universe's time is not a primordial element but a rhythm born from the play of deeper forces. There's only one way to test this hunch: to build an impeccable mechanism where every step is clear and requires no magic. The coherence criterion is not an answer but an invitation to such work.
as written by the authors arXiv:2605.24970
It is often said that time vanishes in quantum gravity. One general approach to quantum gravity accepts this fundamental timelessness but seeks to derive time's emergence at a non-fundamental level. To better assess such approaches, I develop the criterion of physical coherence and situate it in context by applying it to two programs for time's emergence, drawing from recent works by Chua and Callender (2021) and Chua (2025): semiclassical time and thermal time. Unlike some recent arguments for the metaphysical incoherence of time's emergence, which rule out all claims of time's emergence 'from on high' once we've fixed a definition of metaphysical emergence, my criterion of physical coherence leaves open the possibility that some programs in quantum gravity may succeed on their own terms in providing a physically coherent derivation of time from no-time. This sets a challenge for proponents of time's emergence to clarify the conceptual foundations of their program, while at the same time acting as a litmus test for a program's success.
In quantum gravity, equations are often written so that the time variable disappears entirely, as if a cake recipe didn't specify how many minutes to keep it in the oven.
In Ted Chiang's novella "Story of Your Life" (which inspired the film "Arrival"), aliens perceive time differently, as a unified whole where past, present, and future exist simultaneously, reminiscent of the timeless reality of quantum gravity.