Sep 25 2012 12:00pm

Alas For Time Travel: The Leap Second Stands In Its Way

Alas For Time Travel: The Leap Second Stands In Its WayWhen one considers time travel, it is wise to become aware of a set of agencies and policies that are responsible for keeping our clocks on time, and of the consequences of their policies for surfing the chronoscape. A small policy change can cause all sorts of problems.

The current system that ensures that clocks run on time involves coordination between the International Earth Rotation and Reference Systems Service (IERS), the International Bureau of Weights and Measures (BIPM), and the Radiocommuncation Sector of the International Telecommunications Union (ITU-R). The IERS charts the Earth’s movements, the BIPM takes signals from atomic clocks distributed around the globe in order to define a precise clock time, and the ITU-R sets policies and standards. Right now these institutions are engaged in the debate over the future of the leap second.

The use of leap seconds has kept our highly precise atomic time standards in sync with the Earth’s tendency to wobble and create slight irregularities in its rotation. But since leap seconds do not come at regular intervals, they cannot be programmed into software and left alone. Instead, whenever there is a leap second, the time architecture of all time-sensitive software applications must be adjusted. This creates confusion and problems for some systems—enough for there to be a powerful case to eliminate the leap second and go to a time standard decoupled from the Earth’s movement.

One constituency has not been addressed in this debate—either because there is nobody in this constituency, or because they are all presently elsewhen. This constituency is that of time travelers. So I am going to imagine, for a moment, that the resolution to eliminate the leap second has passed—and that the following has appeared in some future Tor.com blog:

O woe to time travelers. Their lot is about to get more difficult.

As any experienced time traveler knows, this type of journey is not merely through time, but also through space. Everything in the universe is in motion, and if you simply travel in time while staying in one spot, then you stay in that spot while everything else is in motion. This has rather ugly results. For instance, if you wanted to travel back to chat with Cicero (rather chatty fellow), and you merely set your coordinates in time, then you’d most likely find yourself in empty space because the Earth isn’t where it used to be in Cicero’s time.

So, in traveling in time, you must know where the Earth was for the moment you are traveling to, and you should know where your goal location is in relationship to the Sun—the Earth is spinning, after all. What’s the point of going to chat with Cicero if you end up in the middle of the Pacific because you did not account for the Earth’s rotation? Glug, glug, glug.

Here’s the rub: to travel successfully, you need to know two kinds of time. One kind of time has to do with the timing of Earth’s relationship to other celestial bodies. This has to do with orbit, rotation, and the movement of the entire solar system through space. The other kind of time has to do with the precise measure of duration. Even if you get your space coordinates right, if you get the duration only slightly wrong, you may find yourself plummeting through the atmosphere or someplace underneath the Earth’s crust.

Alas For Time Travel: The Leap Second Stands In Its Way

Another thing to keep in mind is that the time on your clock does not match the time of the Earth’s rotation. Because the Earth is a tilted rotating orb moving through space, the relationship between a rotation to the same point in relationship to the Sun and a 360-degree rotation is a bit complicated. This is why for many years clock time was known as mean time and set to Greenwich Mean Time—it’s an average of the length of day. Up until now, the relationship between mean time and apparent solar time has been represented with a handy formula known as the equation of time.

These two different times need to be reconciled. As Newton pointed out in his Principia Mathematica, the Earth is not a particularly good timekeeper. It wobbles about, and while these wobbles do not have that serious an effect on mean solar time, they accumulate over time to lead to serious differences. The further backwards or forwards in time you travel, the more you must worry about these wobbles. For the past century or so, these differences have been charted with sufficient accuracy to be of use, but before that you can only estimate, and with regard to the future, since we’re dealing with a wobble rather than a regular pattern, it is risky going too far forward. One really should stop frequently to check what the current stage of the Earth’s wobbliness is.

But as I said, the lot of time travelers is about to get more difficult. Ever since the beginning of the 21st century, there have been those pushing to dismantle the time services that kept clock time (known as Universal Coordinated Time or UTC) and mean solar time coordinated. This practice resulted in the use of leap seconds to keep both time scales synchronized. Since the leap seconds were well-documented and announced, the careful time traveler could take them into account when making calculations. But recently, UTC has been decoupled from mean solar time. To avoid confusion, UTC continues to be signaled but the difference between UTC and mean solar time has to be sought out.

What this means is that you can no longer rely on the equation of time when making your time traveling plans. Basically, you can’t trust your gauges any longer, and have to think for yourself.

Not that I’m against thinking for yourself. One part of me thinks that our time system has for too long cultivated ignorance of its underlying algorithms in its users. It was too easy to just look at the clock and know the time without knowing how that time was determined. But this is now, at least temporarily, going to change. So in your first ventures into the past or future using this new time regime, use caution, and double and triple check your calculations, and don’t trust your software, old gauges, or old clocks.

Except for the fantasizing about time travel, this situation is not too far from the truth. The objects we use to tell time are full of hidden algorithms based on choices made in the past. For instance, the 24-hour day is an inheritance from the Egyptians, the 60 minutes in an hour from the Sumerians, the calendar from the Romans and adjustments by Pope Gregory XIII, mean time is a product of the Enlightenment, atomic timekeeping is not that old, and even more recently, atomic timekeeping was adjusted to take into account Einstein’s theory of relativity. My book Objects of Time: How Things Shape Temporality discusses this state of affairs and explores some of its consequences, as well as discussing how life works under other systems of reckoning time.

Kevin K. Birth is a professor of anthropology at Queen’s College, CUNY and a member of the International Society for the Study of Time. His previous book, Any Time is Trinidad Time (1999), was the subject of a 2002 article in Scientific American. Objects of Time: How Things Shape Temporality is out this month from Palgrave.

Sean Arthur
1. wsean
If you "stay in one spot" while you time travel, what are you staying in one spot relative to? :P
2. radm
It's ok for backwards time travel, as you can just get a file with the info on exactly when all the leap seconds were added. Forward travel is much more difficult, as you don't know when they will be added yet.

Of course, what you really want to avoid doing is accidentally sleeping with/bumping off the grandparent of the person who ends up casting the determining vote on the commitee that decides when to have a leap second.

Because that's when things get _really_ complicated...
3. cmdr
Disagree with the premise, and the reason is Newton's First Law.

When you jump on a train, you don't land smack in the back because the train moved out from under you while you didn't, you land in the same spot because you and the train share inertia.

I would argue the same is true with the time-traveler and Earth. The time traveler is moving through space and rotating around the sun, and spinning on his axis at the same rate as the Earth. Conservation of Inertia applies.

Another force that applies in the favor of the time traveler is gravity. Just because the timetraveler is moving through time does not exempt him from the pull of Earth's gravity, which would keep him in the same relative location to the Earth, spinning the traveler with the Earth, and pulling him around the Sun with it.
Sean Arthur
4. wsean
Yep, that's my take on it as well (re: the pull of gravity; I'd say it's Einstein, not Newton, that's relevant here).

It's one of these sort of self-gotcha things. At first you're not thinking about it at all, just taking for granted that the time machine moves in time but stays in the same place. Then you go, "wait, but the Earth's moving!" Well yes, but that doesn't mean the time machine gets left behind and winds up somewhere in the void, because there's no fixed reference frame that it's attached to.
5. Rob Seaman
A series of meetings has been organized to discuss the impacts and requirements of civil timekeeping on planet Earth. Preprints and slides are available from the 2011 meeting:


and for any time travelers reading this are also available for next year's meeting:


Rob Seaman
National Optical Astronomy Observatory
Joseph Newton
6. crzydroid
@3 and 4: The time-traveller is not "attached to" the Earth or affected by its gravity when outside of time.

I'm glad you mentioned relativity, at least if at the end. As I understand it, gravitational forces affect time. In any case, it seems that the time-traveller would need to develop an entirely new definition of time and its measurements as a nescessity. The problem, as outlined here, would be in trying to reconcile recorded events with this new time system, if those events are of interest. The space-travelling thing, if necessary and not rendered moot by constant inertia, would present a bit of a problem. How does one develop a set of universal (no pun intended) spatial coordinates, when the universe is expanding?
Sean Arthur
7. wsean
Why not? I like the idea that the path through time is affected by gravity as though it were the object itself.

Though that raises the question of whether other objects could be affected by the "out-of-time" object's gravity.

Time travel is fun!
Joseph Newton
8. crzydroid
@wsean, well my idea is basically that the object completely disappears from the space-time continuum and reappears at some other time, and would not be affected by any forces in this space-time continuum. I'm sure this is a concept we'd have difficulty grasping in any case. But I am aware this is just an opinion, and I tried to allude to such later in my previous post. If someone wanted to write a story where the "path through time" is affected by gravity and intertia, I wouldn't object.

Neat idea about the object's own gravity affecting this world in the time in between...could be a great idea for a story about unexplained tidal forces. And if the object is travelling backward in time...wahoo!
9. Goru
I am a time traveller. I am travelling to 30 september 2012. It will be tremendous fun to be there.

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