There is a particular fact about the most precise atomic clocks currently in operation that the wider cultural register has, on the available evidence, not yet adequately absorbed. The fact is that these clocks are now sensitive enough that they can directly measure the difference in the rate at which time passes between two positions in the same room that are separated by, in the most recent demonstrations, approximately one millimeter of vertical distance.
This is, on close examination, considerably stranger than it sounds. The statement is not that the clocks are precise enough to measure other things over which time appears to pass at different rates. The statement is that the clocks are precise enough to measure the actual difference in the local rate of time produced by the gravitational difference between two heights in the same laboratory. The clock at the lower position runs slower than the clock at the upper position. The difference is small. The difference is, however, real, and the clocks can now resolve it.
The precision involved is, by any honest accounting, almost difficult to credit. The most accurate optical atomic clocks in operation lose less than one second across periods comparable to the age of the universe. The reason this precision matters, on close examination, is not the precision itself. The reason is that the precision has finally crossed the threshold at which the predictions of general relativity about how gravity affects time can be tested at scales relevant to ordinary human experience, including the difference between sitting on the floor and standing upright in the same room.
What general relativity actually predicts
It is worth being precise about what general relativity is, in fact, predicting, because the wider register has tended to absorb the prediction in a way that misses its scope.
Einstein’s general theory of relativity, published in 1915, predicts that time runs slower in stronger gravitational fields than in weaker ones. The closer one is to a massive object, the slower time passes for one’s clocks relative to clocks farther away. The effect is called gravitational time dilation, and it is, by every available measurement, a real feature of how the universe operates rather than an artifact of measurement.
The prediction applies, structurally, to any difference in gravitational potential. The difference between the top of a mountain and the bottom of a valley produces a measurable time difference. The difference between a satellite in orbit and the ground station it communicates with produces a measurable time difference, which is in fact what the GPS system has to correct for in order to function. The difference between the floor of a building and the second story produces, in principle, a measurable time difference, though the difference is small enough that, until very recently, no instrument was capable of resolving it.
The structural feature the prediction is pointing at, on close examination, is that there is no single rate of time that applies uniformly across the universe. The rate of time is, more accurately, a function of where one is, and specifically of the gravitational potential at that location. Time is not the universal background of the universe that the standard cultural framing has been treating it as. Time is, more accurately, a local variable whose value depends on the local gravitational conditions.
What the recent measurements actually showed
The most striking recent demonstration of this came from a team at JILA, the joint institute of the National Institute of Standards and Technology and the University of Colorado Boulder, led by physicist Jun Ye. In a paper published in Nature in February 2022, the team reported measuring gravitational time dilation within a single sample of ultracold strontium atoms loaded into an optical lattice. The atoms were distributed across a vertical distance of approximately one millimeter, which is roughly the width of a sharp pencil tip.
The team measured, with sufficient precision to be statistically unambiguous, that the clock formed by the atoms at the bottom of the millimeter-scale sample was ticking at a measurably slower rate than the clock formed by the atoms at the top of the same sample. The difference in elevation was one millimeter. The clocks could resolve the resulting time dilation.
The precision involved was, by every available measure, considerable. The team’s clock measurement precision had reached the level of approximately one part in ten-to-the-twenty-first, which is roughly equivalent to losing less than one second across thirty billion years. The NIST announcement of the result noted that the precision was 50 times better than any previous clock comparison had achieved.
What is striking about this, on close examination, is that the precision has now passed a particular threshold that the wider register has not yet fully absorbed. The threshold is the threshold at which gravitational time dilation becomes measurable at scales of ordinary human experience. The threshold has, until very recently, been the structural limitation on testing general relativity. The threshold has now been crossed. The implications are still being worked out.
Why this matters beyond precision for its own sake
The wider cultural register tends to absorb precision improvements as inherently boring. The standard framing assumes that more precise measurement is, in itself, of interest only to specialists, and that the wider population has no particular stake in whether clocks are accurate to one part in ten-to-the-eighteenth or one part in ten-to-the-twenty-first.
The framing misses, on close examination, what the recent precision improvements actually enable. The precision is not, in itself, the point. The precision is, more accurately, the gateway to a category of measurements that have not previously been possible. The recent improvements have enabled, for the first time, the direct measurement of general relativistic effects at the scales of ordinary human experience. The scales include the scales of human bodies. The scales include the scales of buildings. The scales include the scales of geological features that, until now, could be characterized only by indirect methods.
The applications of this are considerable. Coverage in Physics World has documented some of the directions the research community is now exploring. The clocks could be used as extremely sensitive gravimeters, measuring small differences in the Earth’s gravitational field that correspond to subsurface features such as underground water reservoirs, oil deposits, or geological formations. The clocks could be used in fundamental physics research, searching for deviations from the predictions of general relativity that might indicate new physics beyond the current standard model. The clocks could be used as the basis for new timekeeping standards that would supersede the current definition of the second.
The wider implication, on close examination, is that the precision improvements are quietly opening up a new category of access to the structure of physical reality. The structure was always there. The structure was, until very recently, not measurable at the scales of ordinary experience. The structure is now becoming measurable. The implications of having it measurable have not, on the available evidence, been fully worked out by anyone, including the researchers who have produced the measurements.
What this implies, more philosophically
The structural fact that the JILA research has confirmed, beyond any reasonable doubt, is that the time you experience while sitting on the floor is, in some technically real sense, slower than the time experienced by another person standing upright in the same room. The difference is small. The difference is, however, not zero. The difference is, more specifically, approximately one part in ten-to-the-sixteenth across a meter of height difference, which means that across an average human lifetime of eighty years, the person who has spent more time at greater elevation has experienced approximately a few hundred billionths of a second more time than the person who has spent more time at lower elevation.
This is not, by any practical measure, a significant amount of time. The amount is, however, conceptually interesting. The amount is, more specifically, evidence that the standard cultural assumption that everyone in the same room is experiencing the same time is, on close examination, not quite accurate. The assumption is a useful approximation. The assumption is also, by the available evidence, not literally true. Time is genuinely different at different heights. The differences are now measurable. The wider implications of taking the differences seriously have not yet been adequately absorbed by anyone outside the specialist community working on the measurements.
The acknowledgment this article wants to leave
The most accurate atomic clocks now in operation are precise enough to measure differences in the rate of time produced by gravitational differences across distances as small as one millimeter. The precision involved is approximately one part in ten-to-the-twenty-first, which corresponds to losing less than one second across the entire age of the universe. The precision has been demonstrated in published research, has been independently verified by multiple research groups, and is now sufficient to test the predictions of general relativity at scales relevant to ordinary human experience.
The structural implication is that time is, by the available evidence, genuinely local rather than universal. The rate of time depends on the local gravitational conditions. The difference between sitting on the floor and standing upright in the same room is, in some technically real sense, a difference in the local rate of time. The difference is small. The difference is, however, now measurable. The measurable nature of it has shifted, in some real way, from a feature of theoretical physics to a feature of experimentally accessible reality.
The wider cultural register has not yet, on the available evidence, fully absorbed what this implies. The implications include, among other things, the fact that the universe is, in its detailed structure, considerably stranger than the standard cultural framing has been treating it as. The strangeness has always been there. The strangeness has, until very recently, been below the threshold of direct measurement at ordinary scales. The strangeness is now above the threshold. What we do with the access, on the available evidence, is a question the next several decades of fundamental physics are going to be quietly working out.
