The Architecture of Eternity: Do Past, Present, and Future All Exist at Once?

In March 1955, a month before his own death, Albert Einstein learned that his longtime friend Michele Besso had died.

In a letter of condolence to the Besso family, he wrote a line that still fuels debate among physicists and philosophers of science today: that the distinction between past, present, and future is "only a stubbornly persistent illusion."

It wasn't just a comforting thought for a grieving family.

Einstein's special theory of relativity, published half a century earlier, had already shown mathematically that the flow of time depends on how an observer moves through space.

Philosopher of science Tim Maudlin, however, warns that nothing in relativity itself proves that the past and future are just as real as the present. That, he argues, is a popular but mistaken reading of Einstein's words.

Two People on a Street and a Galaxy 2.5 Million Light-Years Away

To understand why physicists take seriously the idea that the past and future are just as real as the present, it helps to revisit a certain thought experiment.

Physicist Roger Penrose described it in his 1989 book The Emperor's New Mind, building on an argument that physicist C. W. Rietdijk and philosopher Hilary Putnam had each formulated independently years earlier.

Picture two people walking side by side down a street. One is moving in the direction of the Andromeda galaxy; the other is walking the opposite way.

According to special relativity, that tiny difference in walking speed shifts what each of them would consider "now" on Andromeda by several days, purely because of the difference in the direction they're moving.

For one of them, a hypothetical decision by an advanced alien civilization to head for Earth has already been made. For the other, that same decision hasn't happened yet.

The only way to reconcile this within physics is to accept that both events, the deliberation and the decision, already exist somewhere in spacetime, since relativity doesn't allow for a single, universal "now" for the entire universe.

The Architecture of the Block: A Loaf of Bread, Not a Film

Physicists call this static structure the Block Universe, or eternalism.

Instead of picturing the cosmos as a film unspooling frame by frame, eternalists compare it to a loaf of bread that's already been sliced: one end is the Big Bang, the other is the heat death of the universe, and every moment is just a single slice.

Some popularizers sharpen the image even further, describing it as a vast, static crystal in which every moment is forever locked into its own coordinates.

In this model, every person, from birth to death, exists as one long, four-dimensional thread stretched across spacetime.

Physicists often bring this identity to life with a worm analogy: the tail of that thread is your baby self, the head is the version of you on your deathbed, and every cross-section along the way represents a single moment of life.

All of these moments, according to the eternalist view, exist equally, just at different coordinates.

Eternalism, however, is not a consensus position in physics or the philosophy of science.

The Stanford Encyclopedia of Philosophy notes that the link between relativity and eternalism remains disputed: some philosophers argue that accepting the spacetime framework necessarily implies eternalism, while others reject that connection.

The opposing position, presentism (the idea that only the present exists), runs into serious trouble when confronted with relativistic physics, but it hasn't been formally disproven.

Why We Feel Time Passing at All

If the universe really is a static block, an open question remains: why don't people experience it that way? Part of the answer lies in thermodynamics.

The second law of thermodynamics, the only physical law that distinguishes past from future, holds that disorder in a closed system always increases: a shattered cup never reassembles itself.

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The brain exploits that asymmetry to build memories, spending energy to create order within itself while increasing disorder in its surroundings.

We remember the past because it left a measurable trace behind; we don't remember the future because, thermodynamically speaking, no such trace exists yet.

Neuroscience research further shows that our "present moment" isn't instantaneous at all.

Experiments using deliberately shuffled film clips, published in the journal PLOS ONE, show that the brain integrates sensory information within a window of roughly two to three seconds before we consciously register it as a single, unified experience.

Anything shorter than that never reaches consciousness; anything longer already becomes memory.

Many Worlds, One Equation

The Block Universe rests mainly on relativity, but the twentieth century also gave physics quantum mechanics, a theory in which particles exist only as probabilities before they're measured.

A young doctoral student named Hugh Everett tried to reconcile those two views at Princeton University, in a thesis he defended in 1957.

Everett's interpretation, later named the many-worlds interpretation, holds that the wave function never collapses into a single outcome.

Every time a quantum measurement has more than one possible result, the universe, according to this idea, branches: every outcome happens, each in its own branch of reality.

John Wheeler, Everett's mentor at Princeton, was thrilled with the work at first.

In May 1956, he personally traveled to Copenhagen to present Everett's ideas there, but the reaction from the physics community was cold.

Everett himself didn't reach Copenhagen until 1959, at Wheeler's urging, to meet with Niels Bohr, but that meeting didn't improve matters either.

Nearly seventy years later, the many-worlds interpretation remains a serious, but experimentally unconfirmed, interpretation of quantum mechanics.

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It makes identical measurement predictions to competing interpretations, meaning there's currently no laboratory way to determine whether the universe really does branch.

Critics such as physicist Adrian Kent dispute even the mathematical derivation of the rule used to determine the probability of outcomes within the theory itself.

Where the Science Stops and the Speculation Begins

Up to this point, the story stands on fairly solid ground: relativity, thermodynamics, and quantum mechanics, even allowing for contested interpretations.

From here on, popular literature about the "crystal universe" often makes leaps that physics doesn't support.

Neuroscience generally explains déjà vu as a brief memory glitch, one in which the brain mistakenly files a current experience as something already lived through.

Some authors, drawing on physicist David Bohm's idea of the "implicate order," a 1980s concept holding that the universe might resemble a hologram in which every fragment is connected to the whole, speculate that déjà vu could be a brief "glimpse" into other parts of spacetime.

Bohm was a respected and serious physicist, but applying his idea to déjà vu remains speculation without a shred of experimental evidence, and even within physics, many regard his implicate-order theory as more of a philosophical metaphor than a testable model.

A similar fate has befallen Wheeler's delayed-choice experiment, often cited in popular writing as proof that a decision made in the present can affect how a particle behaved in the past.

A detailed mathematical analysis of this experiment shows the opposite: the results can be fully explained by standard quantum mechanics, with no signal traveling backward through time whatsoever.

What the experiment actually demonstrates is that a particle has no defined property before it's measured, not that the future changes the past.

Near-death experiences are no exception to this pattern either.

The life review, in which a dying person reportedly sees their entire life at once, has been documented since 1892, when Swiss geologist Albert Heim collected accounts from mountain climbers who survived falls. The phenomenon is real and well documented in the medical literature.

Its explanation, however, remains a matter of ongoing research within neuroscience itself, not evidence of anything supernatural.

Studies link these experiences to disrupted sensory processing and heightened activity in the brain's temporal lobes during extreme stress, not to any "lifting of filters" that supposedly conceal a broader reality.

A common claim in popular literature holds that near-death experiences prove death is simply an exit from the illusion of time. No peer-reviewed scientific paper, however, has confirmed this.

What Remains: Comfort Without Proof

Add it all up, and the boundary becomes clearer than it first appears.

That there's no single, universal "now" for the entire universe: that's established physics, confirmed by hundreds of experiments since 1905.

That relativity necessarily means the past and future exist in the same way as the present: that's an influential and seriously defended, but still contested, philosophical interpretation of that physics, not its inevitable consequence.

Everything else, déjà vu as a leak in time, quantum mechanics as proof of immortality, near-death experiences as an escape from illusion, belongs to speculation, however comforting it may be.

What speculation like this actually offers isn't proof, but the thought that the moments we've lost haven't been erased, just placed somewhere beyond our reach.

I've read Einstein's letter to the Besso family several times. The physics in it is beyond dispute. The comfort many people draw from it, the idea that none of us ever truly disappears, remains, for now, beyond what any experiment can confirm.

SOURCES

• Quote Investigator – the origin of Einstein's quote from his letter to the Besso family
• Tim Maudlin, "Einstein didn't think time is an illusion", IAI TV, 2022. – link
• Stanford Encyclopedia of Philosophy – "Being and Becoming in Modern Physics"
• APS Physics Magazine – "Quantum Milestones, 1957: Sprouting Parallel Universes"
• Scientific American – "The Many Worlds of Hugh Everett"
• Springer, Quantum Studies: Mathematics and Foundations – "Why delayed choice experiments do Not imply retrocausality"
• PLOS ONE / PMC – "Temporal Integration Windows for Naturalistic Visual Sequences"
• PMC – "Near death experience: neuroscience perspective"