Copyright � 2026 by Shane
Tourtellotte
By itself, time travel is a massively difficult technical problem. Solving that problem and making it a reality is a historic1 achievement, surpassing the Apollo moon landings done with primitive computers, or the Manhattan Project run on slide rules. For all that, however, its practical usefulness would be next to nothing without the parallel work that produced the inertial anchor.
The item itself goes under several names: mass anchor, gravity anchor, local frame anchor, local frame fix, and others. They all describe the same thing working on the same principle. The inertial anchor is what keeps a time machine, and the wormhole mouths it travels between, at the same position and orientation relative to the Earth as it transits through time.
Earth, of course, is constantly in motion, both rotating on its axis and revolving around the Sun. The Sun isn’t stationary either, moving relative to its neighbor stars in a long orbit around the center of the Milky Way Galaxy. The Milky Way itself moves in relation to other galaxies, which are all mostly receding from each other as the Universe expands. All this is to say that “stationary” isn’t as simple a concept as we would like it to be, and not nearly as simple as would be convenient for time-travelers2.
We may never know how many would-be pioneers in time travel hoped the concept would be simple, or somehow forgiving of those not paying attention to it. The more fortunate ones would have had their unpiloted time machine testbeds disappear and never return, their failures unexplained. The experimental machines might have failed from the hard vacuum and extreme temperature change of appearing in deep space, or survived only to “return” to a place that Earth had left behind in its orbit, never to be found. The less fortunate pioneers would have piloted their time machines themselves.
There are rumors of one early experiment that brought its machine back to the instant after it departed, thus avoiding being left behind by Earth. Its internal chronometer reportedly recorded the length of its stay in the past, but the onboard cameras took only blank images, so the experiment was judged unsuccessful. The theory hatched afterward was that it had traveled back in time, but its photos were of the blackness of space, the exposure times inadequate to resolve any stars that would have provided the vital clue to what had happened. Nobody has ever taken credit, or blame, for this test, so it seems destined to remain a legend.
The inertial anchor employs very sensitive measurements of the gravitational field at its location, not only the overall gradient but the interplay of competing vectors due to discrete mass concentrations in the area. The anchor fixes its own position, and those of the wormhole openings, in that matrix of gravitational vectors. It holds those relative positions in space as the time machine of which it is part changes its location in time. The system is not perfect, but in capable and conscientious hands it is very, very good.
The main reason inertial anchors aren’t perfect is that Earth and its environs don’t stay exactly the same, in the short or long term. Primary among the concerns are the continuously changing positions of the Sun and Moon relative to any point on Earth, and the gravitational influences they exert. Fortunately, their motions are regular enough that they can be calculated for movements of thousands of years into the past or future3. Longer trips, naturally, allow for more error to accumulate, and by the time you reach the Cretaceous, solar and lunar ephemera won’t be doing you any good at all.
Other complicating factors include local terrain changes, such as by erosion or deposition of soil, landslides, seismic movements, human excavations, and so forth. Some of these can be chronicled and accounted for; many cannot. The inertial anchor will sense much of the shift in mass and adjust accordingly. This will suffice in most cases, but not all.
For these reasons, well-designed time machines will include a safety margin in the workings of their inertial anchors. The anchor will station the arrival mouth of the wormhole a small distance above the place the anchor calculates as gravitationally equivalent to its departure point. In short, you appear a fraction of a meter -- ideally a small fraction -- above the ground, to avoid the risk of your machine, or yourself, arriving partly or wholly embedded in the ground or floor. The fall to the ground should not be damaging, unless you appear over a rocky field or your machine doesn’t have decent shock absorbers.
(Tangentially, a short-lived 1980s SF television series called Voyagers all but predicted this. The device the time-traveling protagonists used would always drop them into their latest timeframe from a significant height, not usually injurious though greater than you should see with an adjusted inertial anchor. There is a similar effect in the 1981 time-travel movie Time Bandits, though it uses pre-existing portals rather than time machines.)
Some time machines are more ambitious than merely holding themselves in place. They move the traveler in space as well as time, so you can travel directly to ancient Alexandria from a starting point in suburban Ohio. This is feasible, but with a great deal more complexity and an added layer of risk.
The trick is getting a precise measurement of the distances and directions involved. Modern GPS is greatly helpful, but it’s the start, not the end, of the process. One has to adjust for continental drift and other geological processes at the start and end points, which can add up to meters or even tens of meters just going back to the Classical Era. This doesn’t sound like a huge number, except when contemplating how far below the ground you might appear, but even in horizontal terms it’s serious. If you’re hoping, for instance, to pop into a local cave where you can easily hide your machine, meters count a great deal4. Likewise for hoping to appear within a specific building, even if you can see from the ruins today precisely where it was.
There is also the matter of altering your orientation, so you don’t appear at your distant destination halfway around the world sideways or upside-down. Fortunately, gravity gradient measurements make that fairly simple and reliable -- as long as you remember to program that in.
For the privilege of taking these bigger chances, you’re paying with having to make the inertial anchor systems larger and more energy-intensive. In “containing” machines this isn’t a big problem, but in portable time machines it adds a significant percentage in size, mass, and energy draw. Cutting corners is possible, at the usual price of raising the risk to your carcass. It’s another factor added into the already complex series of trade-offs you face in using a time machine.
While the next section deals with these trade-offs, I won’t prescribe what balance you should strike. I do insist that you realize that you’re making these trade-offs, and that you think carefully about them. The time-space continuum is not for dummies.
Footnotes:
Pun not intended, but effectively unavoidable.
It’s tempting, but pointless, to blame Nicolas Copernicus or Edwin Hubble or other scientists for this muddle. They only discovered the truths: they didn’t invent them. Don’t shoot the messengers.
Future projections assume that we don’t do enough astronomical engineering to mess with the numbers. Humans of the 24th century moving asteroids into Earth orbit for convenient mining could change “almost perfect” to “pretty good.” Set your safety margins with this in mind.
This doesn’t even count whether the cave will be in its current form in the time you’re visiting.
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