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UTC is a Lie
If you’re reading this in North America during the week of March 11, you’re probably not at your best. With the exception of a few regions, all of us in the US and Canada have just had our clocks shifted forward by one hour, and are experiencing a stationary form of mild but consequential jet lag. Daylight Saving Time shifts are known to be detrimental to human health and productivity, and, for a short period following the shift, associated with increased risks of accidents. Sleep disruptions are bad for us, and even if for any given person it’s unlikely to cause more than a bit of grumpiness for a couple of days, on a population level, it results in measurable harm.
So why do we do it? There are historical reasons for the time shift tradition that I won’t go into, but the short version is that the inclination of the Earth on its axis moves the timing of dawn and dusk around significantly over the course of the year, and Daylight Saving Time was invented to try to keep the numbers on the clock aligned as much as possible with when people have enough sunlight to be awake and active. Of course, the total amount of actual daylight is still determined by celestial mechanics, which is entirely indifferent to our Earthly clocks. And no matter what we do, clocks in different parts of the world will never fully synch up. The human desire to match “noon” with the sun being high in the sky and “midnight” with the middle of the actual night is going to mean that we’ll always have to account for regional differences in what our clocks are showing us.
But if we wanted to really synchronize all our clocks, once and for all – to create a true Coordinated Universal Time (UTC) – could we do that?
The answer is no. Not in Einstein’s universe.
There are two different flavors (for lack of a better word) of relativity, and they each mess with time in different ways. Special relativity is all about how movement through space affects time. It says that when you’re moving faster through space, you move more slowly through time. If you put a precise clock on a very fast jet and send it on a high-speed flight for a couple of hours, when the jet lands, that clock will be just a little behind an identical one that stayed on the ground. General relativity, on the other hand, is about gravity. The big insight in GR is that gravity is the result of the curvature of spacetime, causing things like orbits and gravitational attraction, but also causing time to pass differently for anyone living in the curved space near massive objects. GR tells us that if you keep one clock on the ground, and put the other on the top of a very tall tower for a while, when you bring them back together, the one on the ground will be a little bit behind, because time moves more slowly when deeper in the gravitational well of the Earth. In either scenario, the relativistic slowing of time is called “time dilation.”
(Those two effects cancel each other to some degree for things that are in orbit. When you work it all out, you find that for astronauts on the Space Station, the special relativity time-slowing wins, and they come back just a little younger than they would have been if they’d stayed. It’s not a large effect, though. For a year in space, it only comes out to a difference of a few milliseconds, and is unfortunately probably more than made up for by the negative health impacts of microgravity and cosmic radiation. It’s a much bigger deal for GPS satellites, which orbit far enough out that the GR effect is more important – their time moves faster than ours – and both effects have to be accounted for or else your map position will be noticeably wrong within minutes.)
The point is that in a relativistic universe, how time moves depends on where you are and what you’re doing. Even if you could synchronize every clock in the cosmos right now, those clocks would be out of synch immediately in different directions for different circumstances and environments. And, perhaps worse than that, observers in different locations or moving through space at different speeds might not even agree on the order of events in a timeline. Two stars going supernova at the same time as seen by one observer might be seen as occurring A first and then B to another observer, and B first and then A to a third.
Even if we didn’t have to consider time dilation, it would be tricky to define “now” in the cosmos, just because of the finite speed of light. Whenever we’re referring to a cosmic event, be it a distant supernova explosion or a spacecraft landing on Mars, we have to be careful about how we talk about when exactly it occurred. We might, when considering how long light takes to travel across the cosmos, say that the supernova we see today went off millions of years ago. But the causal limits on spacetime (the rules preventing faster-than-light travel for anything, including information) are so rigid that it was physically impossible for the knowledge of that explosion, or any consequence of it, to have reached us before the moment the photons reach our eyes. So saying it happened millions of years ago is a bit misleading. For all practical purposes (and even some deeply impractical physics-related purposes), especially since there’s no universal “now,” there’s no way to assign that event to any other point in our own timeline. Up until the moment it was observed, that supernova had the same physical status as a hypothetical future event.
All this is, of course, to say: the numbers on the clock are not real. Time is squishy and contingent, the past and the future are matters of perspective, and the workings of the cosmos refuse to abide by puny humans’ chronological constraints.
And go easy on yourself this week. Time travel is rough on the soul.
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