Regarding the title of the film '5 Centimeters per Second': The speed at which cherry blossoms fall is 5 centimeters per second. Beyond its literal meaning, what does this speed symbolize? Does it also allude to the pace at which people drift apart—a speed so slow it's almost imperceptible, yet so constant it's irreversible?

Your question is exceptionally insightful—it precisely captures the poetic core and emotional essence of 5 Centimeters Per Second. Your speculation—"Does it also imply the speed at which human relationships drift apart, so slow it's imperceptible yet so relentless it's irreversible?"—is entirely correct and serves as the key to understanding this film.

We can delve into the symbolic meaning of this title from several layers:

1. Literal Meaning: An Innocent, Romantic Promise

This is the story's starting point—a line written by Akari in her letter.

"Hey, Takaki. Did you know? Cherry blossom petals fall at 5 centimeters per second."

At that innocent age, this line brimmed with poetry and romance. It represented a shared secret between two children—a mutual observation of the world. This speed itself is gentle and beautiful, symbolizing the pure, fragile, yet precious bond between them. In the first chapter, Cherry Blossom, this speed forms the backdrop of their emotional world—a beauty they longed to protect together.

2. Core Symbolism: The Speed of Drifting Apart

This is precisely what you discerned—the deepest and most poignant symbolism of the title.

Imperceptibly Slow: 5 cm/s is an extremely slow speed. It’s almost impossible to perceive with the naked eye. This mirrors Takaki and Akari’s relationship—they didn’t separate due to a fierce argument or dramatic event. Their estrangement unfolded like a "frog-in-boiling-water" process. Each move, each delayed reply to a letter, each unsent text message was like a cherry blossom petal drifting just 5 insignificant centimeters. Those caught within it struggle to perceive the fatal consequences of these changes, clinging to the belief that "we’re still the same as before."
Irreversibly Relentless: The falling of cherry blossoms is a natural phenomenon—an inevitable result of gravity, impossible to resist. Similarly, the erosion of relationships by time and distance follows a near-physical-law-like "inevitability." No matter how fiercely Takaki clung to hope, he couldn’t prevent himself and Akari from growing, changing, and meeting new people on their separate life paths. Though slow, this speed never stops, never reverses. Accumulated over time, these tiny 5-centimeter increments ultimately form an insurmountable chasm.

3. Profound "Contrast" and "Irony"

This speed also creates intense emotional tension through stark contrasts with other imagery.

Contrast with Train Speed: Takaki travels by high-speed Shinkansen and trains to see Akari. These modern vehicles rapidly shrink physical distance. Yet ironically, the emotional distance between them widens at the glacial pace of "5 centimeters per second." Physical proximity masks emotional detachment.
Contrast with Rocket Speed (Chapter 2): In Kagoshima, Takaki daily witnesses rockets launching into space. Rockets represent humanity’s quest for the distant unknown, moving at incredible speeds. This starkly contrasts Takaki’s inner "stagnation." His body is carried farther away, but his heart remains trapped in the past, slowly, stubbornly chasing the memory of those cherry blossoms at "5 centimeters per second."

Conclusion: A Gentle Beginning, a Bitter End

Thus, the title 5 Centimeters Per Second is itself a complete story—a micro-poem encapsulating the entire emotional journey.

It begins with a gentle promise: Two children naively believed that maintaining a pace of "5 centimeters per second" would let them walk together forever, like drifting cherry blossoms.

It ends with a harsh reality: They ultimately realize this speed wasn’t their synchronized pace, but the speed of separation—drifting them toward different lands.

With this exquisitely beautiful yet profoundly melancholic physical metaphor, Makoto Shinkai captures the deepest futility in modern human emotion: We believe distance is our greatest enemy, but what truly divides us is time itself—imperceptible, yet ceaseless.