You’ve probably had this experience: stopped in multiple lanes of traffic, you sense your car unexpectedly rolling backward. A sudden wave of adrenaline washes over you, but then drains away like the surf rolling off a rocky shoreline as you realize it was not you rolling backward, but the car next to you creeping forward.

This happens because motion is only meaningful relative to some external reference. In that momentary lapse in attention inevitably induced by the tedium of a traffic jam, your brain’s frame of reference unconsciously shifted from the broader landscape to the narrow tunnel of vehicles around you.

Relative to the Earth, you were always stationary, foot firmly on the brake. But, relative to the car next to you, one of you was moving. Yet without an external point of reference, it was impossible to tell which.

A Universal Truth

If you were to travel into the near-perfect void of interstellar space, you would experience a much more extreme version of this effect. With the nearest objects lightyears away, there is literally nothing against which you can get your bearings. Stars might be visible, but they would be too distant for you to detect relative motion on any reasonable timescale.

Now, let’s say you have a traveling companion in a separate, nearby spacecraft. The voyage will be pretty lonely if you don’t stick together, so you are a bit concerned when you notice yourself speeding away from your extraterrestrial buddy.

But is that what is actually happening? How can you know?

The answer is as simple as it is final: you can’t.

There is no way to determine, through any experiment conducted within your own spaceship, whether it is you or your fellow space explorer who is in motion—or whether both of you are moving, but at different speeds or in different directions.

When physicists say that motion is only a meaningful concept relative to some artificially chosen reference point, they mean it very literally.

I find even thinking about this mildly disorienting.

Orienting Ourselves

It is very difficult to make sense of the world if you don’t know where you are, or whether you are moving or standing still. We find that experience unpleasant. It’s unnerving, just like the momentary traffic jam panic.

Consequently, throughout human history we have invented ways to keep track of our movement and location. Today, we have elaborate and incredibly sophisticated systems for this purpose—from GPS and the accelerometers in your phone, to the inertial navigation systems in ships and airplanes and the star trackers used in deep space exploration.

Orienting ourselves in the world is an ancient problem, though, and a few millennia before these technologies, we used a much simpler approach: milestones.

A Thousand Paces

Today we use the word “milestone” primarily in the metaphorical sense, but the term is, of course, quite literal. A milestone is just that: a stone marker placed at one mile intervals on the road.

That distance was a mille passus, “a thousand paces,” for the ancient Romans who began placing these inscribed markers along their roadways. If that term rings a distant bell whose toll you have not heard since high school French or Spanish, that’s not a coincidence.

The Latin mille, for “thousand” gives us mille in French and Italian, mil in Spanish, and mile in English.

Now, if you’re doing the math wondering how we got from a thousand paces to 5,280 feet—and I know some of you are—it’s a long story, but you must account for the fact that a Roman pace, passus, was a complete two-step cycle, not one individual footfall, and that ancient Romans were a good bit shorter than we are today. Factor that in and you’ll come up with a number that is at least close to a modern mile.

A rock with a number to mark distance seems so simple, so obvious, but milestones were an innovation that had a far-reaching impact.

Milestones, both literal and metaphorical, are how we know how far we’ve come, and how much road lies ahead of us.

Milestones are how we know we are moving. And that’s important, because measured against life’s ultimate path, the one measured in days, weeks, months, and years, the alternative is not great.

Milestones remind us that we are alive.

From Concrete to Concept

Times have changed considerably since the days of the Caesars marking the expansion of the empire by their growing network of precisely demarcated roads.

In today’s world we set milestones with our minds, not our muscles, and as a result it is much easier to place them haphazardly, move them arbitrarily, and inscribe them with ambiguity instead of anchoring them to reference points that give our journeys meaning.

Milestones used badly become a source of self-delusion and dread when they were meant to inspire and mark progress.

How can we ensure milestones remain a positive way to reflect those behind us and inspire us to step boldly toward the next?

A few reminders: 

They are not “every-so-often stones.”

Milestones were placed predictably, every thousand paces. They were tangible and you knew when to expect them. In uncharted territory, if you did not encounter the next milestone in its expected location, you knew you were off course.

When we make the effort to walk that extra mile, we must be able to count on finding the milestone where it’s supposed to be. The alternative is confusion and discouragement.

And remember, it was no single milestone, but rather the system of milestones that helped an infantry build an empire.

Milestones should help you know where you are.

Milestones are most useful when anchored to a fixed datum, a reference point.

The datum for our modern interstate mile markers is the southern or western state line, with the markers increasing as you move north or east. Traveling south on I-65, when I reach Exit 0, I know I am home.

The milestones placed by the ancient Romans marked the distance from the Milliarium Aureum, the “golden milestone” placed near the Roman Forum by Emperor Augustus in 20 BCE. All roads lead to Rome, after all.

Milestones without a datum can show you that you are moving, but you won’t know where you are or how far you’ve come.

And as we have learned, there is no such thing as an absolute point of reference. We have to create them, and it’s usually best to begin where you are.

Milestones provide decision points.

When finding our way through new territory, a well-placed milestone is a decision point.

Should we continue on this path? Or is it time for a new direction? Without these markers, it is all too easy to confuse forward motion with real progress toward your destination.

Marking consistent effort feels good to us, but that is not quite the same as real progress. We must also remember to pause long enough to ensure we are headed in the right direction.

A Personal Milestone

I ask myself that question often, including now, because this piece was inspired by a personal milestone: This happens to be my one-hundredth newsletter!

I have published an original work every Wednesday, with just a few breaks, for the past 104 weeks—two years, twenty-four months, 103,066 words, and more than a few late Tuesday nights.

Had you asked me in March of 2023 if I could write the equivalent of two non-fiction books over the next two years, I would have responded… unlikely.

But that’s how milestones work. Like the Roman legions who measured their progress in increments of a thousand paces, mille passus, I simply kept moving forward with the next mille verba—a thousand words—each week.

Keep that up for a while and you can cover a lot of territory.

So, is this milestone of milestones a turning point?

Outwardly, from your frame of reference, no; my forward motion will continue as you have always perceived it, with another mille verba predictably arriving in your inbox next Wednesday morning.

But from inside my little spaceship in this big universe, perhaps. With over a hundred-thousand words behind me I am beginning to turn a cognitive corner, toward a place where I know myself as someone capable of writing consistently for the long haul, and that at least a few fellow travelers find value in what I have to say.

Thanks for being one of them. I’ll see you next week.

Greg