Mapped: The boundaries of human perception

Thirty-five years ago, the Voyager 1 space probe turned its camera toward Earth and snapped a photograph from 3.7 billion miles away. The now-iconic image, dubbed the “Pale Blue Dot” by the author and astrophysicist Carl Sagan, shows our home planet as the faintest pixel against a vast black canvas, a stark reminder of how little space we occupy in the Universe.

This logarithmic graph does something similar, but for perception. It displays scales of space and time, ranging from subatomic sizes to cosmic distances, from fleeting instants to timespans spanning millions of years. Tucked within this sweeping range is a box labeled “human experience” — the sliver of space and time we can directly perceive.

The graph comes from EUREKA! Physics of Particles, Matter and the Universe (1997) by the late theoretical physicist Roger Blin-Stoyle. He described the graph not as a perfect scientific representation of the limits of human perception but as an approximation, one that highlights just how much of space and time lies outside our natural grasp.

For millennia, we could barely speculate about the extreme phenomena beyond our perception — what the cosmos looks like on the tiniest and grandest of scales. It’s only recently that we began to map it.

Don’t be fooled by the apparent size of the “human experience” box. Each tick on the graph represents a tenfold increase, allowing it to compress enormous differences into a manageable frame. The time axis spans 40 orders of magnitude, from fleeting quantum events (10-23 seconds) to cosmic epochs (1017 seconds). The space axis also covers a wide range, from subatomic distances (10-15 meters) to a scale suitable for measuring the observable universe (1026 meters). If it were drawn to scale linearly, the “human experience” box would be far smaller.

Beyond the barriers of human experience

Evolution tuned our senses for survival. We can effortlessly hear the crack of a tree branch, perceive the slither of a snake in the grass, or catch the flash of doubt in a friend’s eyes. But phenomena like the flitting of electrons and the birth of black holes lie far outside our natural perception.

Only through our curiosity, reason, and invention have we stretched our inherited limitations to explore the edges of space and time. Blin-Stoyle’s graph provides a rough perspective on those boundaries.

Let’s start with its Y-axis, which shows a timescale ranging from 10^-23 to 10¹⁷ seconds. The former is an infinitesimal unit of time. It’s shorter than a zeptosecond (10^-21 seconds) but longer than a yoctosecond (10^-24 seconds) and represents the temporal realm of transient quantum interactions.

The largest unit on the graph’s timescale is equally mind-bending. It’s longer than a petasecond (10¹⁵ seconds), which already rounds out to 31.7 million years. The Universe is approximately 435 petaseconds old. That doesn’t sound like much until you express it in more conventional units: 13.8 billion years.

The X-axis relates to physical distances. At the lower bound is a femtometer (10⁻¹⁵ meters), or about the diameter of a single proton. At the upper end lies a unit of length longer than a yottameter (10²⁴ meters). Yottameters are large enough to measure the observable Universe, which is approximately 8.8 yottameters in diameter.

As extraordinary as the upper and lower bounds of this graph are, they don’t push the extent of what we can measure in the Universe. The Planck length, named after the German physicist Max Planck, is the smallest unit of length currently used by physicists. It comes in at an astonishingly tiny 1.6 x 10^-35 meters. The graph also simplifies things by placing space and time on their own axes. This matches how we intuitively experience them. Yet Albert Einstein’s theories of special and general relativity reveal that space and time are linked into a four-dimensional metric called spacetime. The two intertwine in ways our senses can’t detect, further complicating the reality of the Universe beyond our perception.

“Pale Blue Dot” (Credit: NASA/JPL-Caltech)

Living life a millisecond at a time

We live in Einstein’s Universe, but our everyday experience is more like how Issac Newton saw the physical world. Space and time seem fixed and separate, objects have absolute positions, and causes reliably lead to effects. Our senses are confined to this more instinctive view of reality. While this blinds us to the weirdness playing out on the cosmic and quantum scales, it also helps simplify reality.

Just consider how we perceive time. Our brains can process visual information in as little as 50 milliseconds. We process sounds much faster and can distinguish between two sounds occurring just about a millisecond apart. (That’s pretty quick, but nowhere near the speed of a zeptosecond.)

These varying perceptual speed limits concerned engineers during the early days of television. What if, they wondered, people couldn’t adequately synchronize the picture with the audio? “Then they accidentally discovered that they had around a hundred milliseconds of slop,” the neuroscientist David Eagleman wrote in his 2009 book What’s Next? Dispatches on the Future of Science. “As long as the signals arrived within this window, viewers’ brains would automatically resynchronize the signals.”

Today’s video games run significantly faster than yesterday’s TV programs. With the proper setup, a video game can easily reach a frame rate of 120 frames per second or more (most movies, for comparison, are shot at 24 frames per second, meaning 24 individual images flash past your eye every second). However, research suggests that people can’t discern differences much beyond 60 frames per second, which we can think of as the lower limit of the temporal “human experience” on Blin-Stoyle’s graph.

The upper limit to our temporal experience is less precise. It extends to a lifetime — maybe 100 years, “if we are lucky,” Blin-Stoyle writes in EUREKA! But how we perceive that lifetime depends largely on memory, external cues, and situational context. In 1962, geologist Michel Siffre spent just over two months in an Alpine cave to test the effects isolation had on time perception. When he emerged, he estimated 35 days had passed. Siffre’s self-experiment highlights how perception isn’t a passive recording of time; it’s a story the brain constructs.

When you strip away clocks, sunlight, and human contact, the scaffolding we use to mark time’s passing crumbles. Even what we think of as the present is slightly behind, delayed by the split seconds it takes for our brains to process and unify sensory signals arriving at different speeds. Awareness is always a beat too late — our minds stitching together a rough approximation of what exactly happened.

Our perception of space is just as narrow. Blin-Stoyle suggests that if we’re “being generous,” humans have a sense for things as small as 0.1 millimeters (10⁻⁴ meters). That’s roughly the width of a strand of hair or the thickness of a piece of paper. The edges of our spatial perception could be argued to expand to the diameter of the Earth (12,756 kilometers), but now we’re being extra generous since our intuition falls sharply once distances stretch beyond the horizon.

From a human perspective, space and time are baffling and can be utterly frightening. They flatten us with the sheerness of their dimensions. So perhaps it’s cosmic justice — or rather, cosmic mercy — that we don’t directly perceive the scale of existence all the way to its extremities.

But while evolution tuned us to the scale of the everyday — the objects we can hold and the danger and rewards within sight — our curiosity and reason have allowed us to develop the tools necessary to broaden that scale.

Microscopes have revealed cells and microbes. Particle accelerators have cracked open the strange world of bosons and quarks. Telescopes have mapped planets and galaxies. Even though our senses evolved to operate in the narrow band of space and time we call the prehistoric savannah, our minds have pushed far beyond it, building bridges from the tangible to the infinite.

Heaven in a wildflower

Where perception and technology end, imagination pushes forward. Einstein famously imagined what it would be like to ride alongside a beam of light — a physical impossibility, but a mental leap that helped him develop the theory of relativity and transform our understanding of space and time. He couldn’t perceive such a thing, but he could imagine it.

Herein lies the difference between the Pale Blue Dot picture and this graph of our perceptual horizon. The darkness separating Voyager from its home planet in that famous image feels cold and deadly. But on the spacetime graph, the hatched area outside our tiny experiential box is beckoning. Our minds can travel freely where our bodies can’t and ascend from the zone we can perceive to the one we can only conceive. Time and space can be joyful playgrounds.

The Romantic poet William Blake understood this well when he wrote: 

To see a world in a grain of sand

And a heaven in a wild flower,

Hold infinity in the palm of your hand

And eternity in an hour.

As humans, we paint small strokes on the vast canvas of existence. Our eyes cannot pierce the subatomic veil nor trace the farthest threads of the cosmic web. For all the efforts of scientists and poets to elicit truth from those strange, most obscure corners of the Universe, there remains much more to learn and experience.

But even if we cannot live between distant stars or among the nanoscopic particles, we can still imagine ourselves there. We are the brevity that seeks the eternal. And perhaps this is our place: not to encompass the whole but to reflect it.