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A child’s cosmic flip-book -

Imagine the universe as a child creating drawings, each one representing a single frozen moment. These images, stacked together in sequence, create what we perceive as the passage of time, where one moment follows into another like frames in a movie.

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One living frame -

In our normal experience, only the current moment ever feels real. The past has faded into memory, and the future hasn't arrived. It's like watching a movie where only the current frame is visible while all others disappear.

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An entire existence -

Now imagine that the child has finished all the pictures and stacked them together. Instead of a flow of time, you get a block—a single structure containing every moment that has ever existed or ever will. Time becomes spatial, like a sculpture.

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The equality of past, present, and future -

If all moments exist together in this “block,” then the distinction between past, present, and future dissolves. They are all equally real, not just memories or predictions but actual existing parts of the universe.

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The illusion of “now” -

People only ever experience the present, and so we instinctively believe it’s the only real moment. But this perception may be misleading. Science suggests that our sense of “now” could be a product of our limited perspective, but that it might not actually be a universal truth.

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Spacetime -

In the early 1900s, Albert Einstein proposed his theory of relativity, which tries to explain how time and gravity work in unison. His theory changed the game by fusing space and time into one fabric known as spacetime, and he asserted that moving through space affects how you move through time. This makes time less absolute than we thought.

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A fractured universe -

There is no universal present. Instead, countless “nows” exist depending on where and how fast you’re moving. What feels like the present to one person might be the past or future for someone else. Relativity permits them all.

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An alien example -

Picture three alien spaceships a million light-years away. One hovers unmoving in space, and so it shares your “now.” Another flies away from the Earth, and its present aligns with our planet’s past. The third spaceship heads toward Earth, and its “now” lines up with our future. All three are real!

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Cosmic perspective -

Now imagine that each alien ship can hypothetically communicate with Earth using an instantaneous internet connection: one with your present, one with your ancestors in 1925, and one with your descendants in 2125. The theory of relativity shows how different times can coexist depending on perspective.

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Cosmic democracy of perspective -

Einstein’s theory of relativity insists that no observer has a privileged viewpoint; each version of “now” is equally correct. This cosmic democracy forces us to accept that past, present, and future must all exist simultaneously.

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Living in a frozen universe -

Under this interpretation, we don’t move through time like actors in a play. Instead, everything that will ever happen already exists. The universe isn’t dynamic but frozen—unchanging, complete, and eternal, like a giant cosmic iceberg.

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The reality of what you cannot perceive -

Just because something is unreachable doesn’t make it unreal. We accept the existence of galaxies beyond our observable horizon. Similarly, even if we can't experience the future yet, it may still be real in an iceberg universe.

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Destiny -

If the future exists, it must already be determined. This contradicts our experience of choosing and changing our paths, and creates tension between what science says about time and what we feel about our own agency.

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The illusion of decision-making -

People always feel as though they’re actively shaping their future with choices, like deciding whether to read a book or to scroll through the internet. But if time is fixed, even that decision was made at the Big Bang, which leaves no room for free will.

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Quantum randomness -

Enter quantum physics, which throws a wrench into determinism. Unlike classical systems, quantum events are fundamentally unpredictable. They’re not random due to ignorance—they are, by nature, uncertain, and even the universe can’t forecast them precisely.

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Radioactive atoms -

A radioactive atom might decay now or in a million years. We can calculate the chances, but not the outcome. This built-in randomness suggests that the future isn’t fully written; it’s probabilistic rather than deterministic.

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Massive consequences -

Quantum particles can cause chain reactions that change the world. A single decay could lead to a mutation in an animal, creating bizarre new lifeforms or preventing them from existing at all. This randomness alters the course of history.

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A quantum wrench in the machine -

If the future depends on random quantum events, then it cannot be set in stone. And if it's not predetermined, then it cannot coexist with the past in the same way. This throws doubt on the idea that all the layers of time are stacked together.

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Personal time -

Although a global “now” is elusive, each object in the universe (whether a human or an atom) has its own personal timeline. You are undeniably existing between your birth and death. In that sense, your own “now” is perfectly clear.

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Relative order -

Even with all the relativity and randomness, one thing remains: no one dies before being born. There is still order in our personal timelines, which provides a consistent, meaningful structure for every individual being.

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The comfort of local experience -

If we stop trying to define time across the whole universe and just focus on local experiences, everything becomes easier. Your “now” is real for you. Every person and object has their own line of time, and each one is as valid as the next.

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Rethinking the block -

But what if we were wrong about the block of stacked pictures drawn by the child? Maybe it doesn’t contain the future. Maybe it only includes the past, and a thin, uneven surface layer that represents the present, continually growing as new events happen.

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The surface of “now” -

The surface of the present wouldn’t be smooth, and would instead be rough and textured, shaped by countless individual experiences of “now.” Each of these personal moments contributes to the formation of time’s leading edge, collectively building the universe’s timeline.

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A growing frontier of reality -

Instead of a static block, we might live in a growing block universe. New moments and new pages are added to the past as uncertain events become certain. Radioactive atoms decay, people make choices, and history is written one second at a time.

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A return to free will -

In this version of time, the future isn’t yet real, so you do have choices. The timeline grows with every decision, every accident, every quantum surprise. Anyone can change the future with even the smallest of decisions.

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Dinosaurs and Mars colonies -

Are the dinosaurs still real somewhere in spacetime? Are Mars colonies of the future already built? Depending on the theory, yes. Or no. Or maybe. The answer depends on how you think time exists (if it does at all).

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Science’s honest shrug -

Truthfully, no one has a definitive answer. Time remains one of the most perplexing concepts in science. We have multiple ways to describe it, but none are fully proven or universally accepted. It’s an ongoing mystery.

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Time as an emergent phenomenon -

Time may not be a basic part of the universe. Like how heat comes from molecular motion, time might emerge from deeper, hidden interactions. Life, consciousness, and time could all be illusions born from complexity.

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Much to learn -

Even if time remains elusive, there’s a universe of things we do understand, and even more waiting to be discovered. There’s still time to learn and explore. And who knows, maybe you’ve already learned all there is to know and your body in the present is simply catching up.

Sources: (Kurzgesagt) (Britannica) (Scientific American)

See also: Where do time zones come from?

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