If you’ve read my last few columns, you know that the question of our biological compatibility (or lack thereof) with space travel has been on my mind.
Indeed, the technical challenges are daunting — perhaps even insurmountable. (See my February/March column, “Homo sapiens: making us suited for the stars.”) But I know that at the end of the day, that won’t stop those who are truly determined to expand humanity’s presence beyond Earth. For them, spaceflight is more than a technical achievement; it’s the highest expression of our uniquely human drive to understand our place in the cosmos. And, at the end of the day, we all have the freedom to choose how we spend our limited time.
To bring this idea into focus, I offer this parable of three brothers who shared every milestone, every memory, as if proper time flowed equally through them all. Their story illustrates how our experience of time ultimately depends on how much we must adapt and remember:
Once there were three siblings, triplets, born moments apart. They grew up indistinguishable in every way: same home, same food, same books, same memories. They lived as though time were something shared, a rhythm moving through them equally without favor or exception. To be older was to have spent more time alive, and time was something the universe gave you in equal measure.
But that belief would be challenged.
In adulthood, two of the brothers became astronauts and were chosen for a historic mission: separate 50-year round trips to the star Sirius and back, moving at relativistic speeds. One traveled aboard a basic capsule, experiencing microgravity the entire way, save for daily resistance exercises. The other boarded a rotating von Braun wheel, a spacecraft designed to simulate Earth-like gravity through the apparent resistance to centripetal acceleration. The third stayed on Earth, carrying out an ordinary life.
Einstein’s theory of special relativity already predicted what would happen next: Time moves slower for those in motion relative to those at rest, so the two traveling brothers would return younger than the one who remained on Earth.
And indeed, when they returned, they had aged less — at least, by the calendar. But one of them was dying.
The problem Einstein didn’t solve
The astronaut from the capsule, who had spent five decades in free fall, was breaking down. His bones were brittle. His immune system had collapsed. His muscles had withered. His organs bore the unmistakable mark of accelerated aging. In contrast, his brother from the von Braun wheel was in good health, alert, mobile, resilient. Both had taken the same journey. Both had followed the same relativistic path. Both had measured the same proper time.
So why was only one dying?
Relativity offered no answer. In Einstein’s space-time, they had traced identical geodesics. The equations held. But the human body isn’t a coordinate or a worldline. It is an engine of adaptation, a system of systems, constantly reacting to its own state and surroundings. It changes to survive. And that change, it turns out, is not free.
Time as distinction
We are taught to think of time as something that passes, like wind across a field. But what if time isn’t what happens to us, what if it’s what we register? This is the foundation of epistemic time, the notion that time is not a universal ticking but an internal accumulation of distinction. Here, a system such as the human body experiences time only to the extent that it undergoes informational change — that it senses, remembers, adapts.
In this view, time is not something we travel through. It is something we build, one distinction at a time. (You can read my recent paper on this topic here.)
For the astronaut in free fall, the experience of space was far from passive. His body was forced to compensate continuously: for fluid redistribution, for bone density loss, for cardiovascular drift, for sensory disorientation. Each adaptation required reorganization. Every system was working overtime just to preserve a sense of normalcy in a profoundly abnormal environment.
From the standpoint of epistemic time, these compensations were costly. Every change registered by the body, every internal distinction, was a unit of experienced time. So his biological clock was not ticking slower. It was racing ahead.
The astronaut in the von Braun wheel, meanwhile, faced none of these pressures. The artificial gravity mimicked Earth’s loading, so his body had fewer adaptations to make, fewer disturbances to resolve. His systems remained closer to homeostasis. In epistemic terms, he aged less, because he experienced less disruption.
Epistemic time is the accumulation of meaningful, internal change. It is not measured by clocks but by the number of distinctions a system undergoes, how much it must adapt, remember or reorganize in response to contrast. If nothing changes, no information is processed. If no information is processed, no time is experienced. In this view, time is not what passes in the universe but what a system registers as difference. A perfect vacuum, a frozen cell or a photon in transit does not experience time unless something internal changes in a distinguishable way. This is why epistemic time can differ radically from both proper time and coordinate time. It is the only kind of time that feels like anything.
Rethinking aging
This is the heart of the paradigm shift. Aging is not just the slow unraveling of DNA, nor merely the entropic march of biology. It is the accumulation of irreversible epistemic transitions, changes that leave a mark. A life of trauma, of constant adaptation, of repeated stress, doesn’t just feel longer. It is longer, in informational terms.
The astronaut in the capsule did not age faster because of gravity or acceleration. He aged faster because his body endured a more turbulent stream of informational change. He didn’t just survive the journey — he paid for it with epistemic time.
Cryostasis and the clock that doesn’t tick
This framework solves another paradox. In traditional relativity, someone in cryostasis, frozen for 500 years, would be called “500 years old” by the calendar. But this is absurd. No internal distinctions were made. No memories were formed. No metabolic changes occurred.
From the epistemic standpoint, this person experienced no time. They are, experientially, as young as the moment they were frozen. Because without information, without difference, without change, time does not exist.
Einstein gave us a way to measure time geometrically. He showed us that time is relative to motion and gravity. But epistemic time shows us that true time is relative to experience. The universe may track your proper time, but your biology tracks how hard you’ve had to fight for coherence.
The astronaut who died first didn’t die because of space. He died because of adaptation overload. Because his systems couldn’t hold their shapes against the tide of change. His life wasn’t cut short by relativistic mechanics. It was accelerated by informational turbulence. He was epistemically the oldest of the three.
And in this, we see something profound: Time doesn’t just pass.
It remembers.
Aerospace America publishes a rich variety of opinions relevant to the future of aerospace. The views expressed are those of the author(s) and do not necessarily reflect those of our publisher, AIAA.
