Every Clock Is Personal: Time Dilation Between the Puranas and Physics

A short wait in Brahmaloka

The Bhagavata Purana (Book 9, Chapter 3) tells the story of King Kakudmi, also called Raivata, who ruled from the city of Kushasthali. He had a daughter, Revati, of such beauty and accomplishment that no man on Earth seemed worthy of her. So the king did what only a king of legend could do. He took her to Brahmaloka, the realm of the Creator himself, to ask Brahma to choose her husband.

When they arrived, Brahma was listening to a performance by the celestial musicians, the Gandharvas. Father and daughter waited respectfully until the music ended. Then Kakudmi bowed and presented his list of suitable princes.

Brahma laughed. In the short time they had been waiting, he explained, twenty-seven great cycles of ages had passed on Earth. Every prince on the king's list was long dead. So were their sons, their grandsons and the kingdoms they had ruled. Kushasthali itself had passed into history. Brahma advised the king to return and give Revati in marriage to Balarama, the elder brother of Krishna, who now lived on Earth.

The story ends with a detail that feels almost like science fiction. Returning to Earth, Kakudmi and Revati found that human beings had grown smaller over the ages. Revati was much taller than Balarama. With a gentle touch of his plough, he reduced her height, and they were married.

Similar stories appear across the world. In Japan, the fisherman Urashima Taro spends what seems like three days in an undersea palace and returns to find that three hundred years have passed. In Ireland, Oisín returns from Tír na nÓg, the land of eternal youth, to find his companions centuries dead. Humans everywhere seem to have sensed that time might not run the same way in every place.

Today these stories are often forwarded with a triumphant caption: the ancients knew about Einstein's time dilation. Did they? To answer honestly, we first need to understand what time dilation actually is, not as a metaphor, but as one of the most precisely tested facts in physics. It is stranger than any legend, and it is happening to you right now.

Speed slows time

In 1905, a 26-year-old patent clerk in Bern published a paper on the electrodynamics of moving bodies. Albert Einstein began from two simple-sounding principles.

1. The laws of physics are the same for everyone moving at a steady speed. There is no experiment you can do inside a smoothly moving train, with the windows closed, that tells you whether the train is moving or standing still.

2. The speed of light in empty space is the same for every observer (about 299,792 kilometres per second) no matter how fast the source or the observer is moving.

The second principle is the shocking one. If you run towards a ball thrown at you, it hits you faster. If you run towards a beam of light, it still arrives at exactly the same speed. Experiments had been hinting at this since Michelson and Morley's famous null result in 1887. Einstein took it seriously, and followed it wherever it led.

It led to the conclusion that time itself cannot be the same for everyone.

The light clock

The clearest way to see why is a thought experiment Einstein's successors made famous: the light clock.

Imagine a clock made of two mirrors facing each other, one above the other, with a pulse of light bouncing between them. Each round trip is one "tick". Now put this clock on a very fast train.

For a passenger on the train, the light simply goes straight up and straight down. Nothing strange happens. But for someone standing on the platform as the train flashes past, the mirrors are moving sideways while the light travels. To hit the top mirror, the light must travel along a diagonal, and then another diagonal back down. Its path is a zigzag, and a zigzag is longer than a straight up-and-down line.

Here is the crucial step. The observer on the platform must see the light travelling at exactly the same speed as the passenger does: that is Einstein's second principle. A longer path at the same speed takes more time. So, as seen from the platform, each tick of the moving clock takes longer. The moving clock runs slow.

And this is not a quirk of mirrors. If the light clock runs slow, then every other process on the train: the passenger's watch, her heartbeat, the chemistry of her cells, the decay of atoms in her body, must run slow by exactly the same amount. Otherwise she could compare them and detect her own motion, breaking the first principle. It is time on the train that runs slow, not just one kind of clock.

The Lorentz factor

A little geometry with that zigzag gives the exact result. A clock moving at speed v runs slow by a factor called gamma (γ), the Lorentz factorLorentz factor How much a moving clock slows: γ = 1 ÷ √(1 − v²/c²).:

γ = 1 ÷ √(1 − v²/c²)

Time elapsed on the platform = γ × time elapsed on the train

At everyday speeds, γ is so close to 1 that the effect is invisible. It only becomes dramatic close to the speed of light:

Notice the shape of that table. The effect stays small for a long time, then rises steeply as speed approaches the speed of light, growing without limit. A traveller who could reach 99.99% of light speed could spend one year on a spacecraft and return to find that seven decades had passed at home. That is not so far from Kakudmi's experience, but we will come back to that.

Each observer experiences their own time as perfectly normal. The traveller's watch ticks at one second per second, her coffee cools at the usual rate, and her thoughts move at their usual pace. Time dilation is never something you feel. It only appears when two clocks that have travelled different paths are compared.

Gravity slows time too

Special relativity deals with steady motion. It says nothing about gravity. For the next decade Einstein struggled to fold gravity into his picture, and the key came to him in 1907 in what he later called the happiest thought of his life: a person falling freely does not feel their own weight.

Think of an astronaut in the International Space Station. She floats not because there is no gravity up there (at that height gravity is still about 90% as strong as on the ground) but because she and the station are falling together, around the curve of the Earth. Inside, gravity seems to have vanished. Conversely, a person in a sealed rocket accelerating through deep space would be pressed to the floor exactly as if standing on a planet. Einstein proposed that these situations are not just similar but physically identical. This is the equivalence principle.

Now put a light clock in that accelerating rocket. Send a pulse of light from the floor to the ceiling. By the time it arrives, the ceiling has picked up speed, so the light arrives slightly "stretched": its frequency lower. Send light from the ceiling down, and it arrives slightly compressed. Observers on the floor and ceiling will disagree about each other's clocks: the floor clock runs slow compared with the ceiling clock.

By the equivalence principle, the same must be true in a gravitational field. Clocks lower down in gravity run slower than clocks higher up. Close to the Earth's surface the effect is approximately:

Fractional difference in clock rates ≈ g × h ÷ c²

where g is the acceleration due to gravity and h is the difference in height. Because c² is so enormous, the effect is tiny: about one part in 10¹⁶ for every metre of height. But it is real, and it adds up.

• Your head ages faster than your feet. Over a lifetime of about 80 years, the difference is roughly half a millionth of a second.
• Someone who spent seventy years living at the height of Everest's summit would age about 2 milliseconds more than a twin living at sea level.
• The centre of the Earth sits deepest in its own gravity. A 2016 calculation by Ulrik Uggerhøj and colleagues estimated that, over the planet's 4.5-billion-year history, its core has aged about two and a half years less than its surface.

Measuring it

The first laboratory test came in 1959. Robert Pound and Glen Rebka at Harvard sent gamma rays up and down a 22.5-metre tower and, using an exquisitely sensitive nuclear technique called the Mössbauer effectMössbauer effect Gamma rays emitted and absorbed without recoil, so sharp in frequency that tiny shifts show up., detected the tiny frequency shift Einstein's theory predicted. In 1976 the Gravity Probe A mission carried a hydrogen maser clock on a rocket to an altitude of about 10,000 km and confirmed the effect to better than one part in ten thousand.

Today the measurements are almost unbelievably fine. In 2010, a team at the US National Institute of Standards and Technology (NIST) used optical atomic clocks to detect the difference in ticking rate between two clocks whose heights differed by just 33 centimetres. In 2022, researchers at JILA in Colorado measured the gravitational time difference across a single tiny cloud of atoms, over a height of about a millimetre.

In other words, time literally passes at a different rate on the top shelf of your bookcase than on the bottom shelf. We can now measure that.

The evidence: time dilation is not a theory you have to believe

It is common to hear relativity described as "just a theory". In science, a theory is not a guess. It is an explanation that has survived every attempt to break it. Time dilation, in particular, has been tested so many ways, for so long, with such precision, that it is now built into everyday technology. Here are the landmarks.

Particles that live too long

When cosmic rays from space strike the upper atmosphere, about 15 kilometres up, they create showers of particles including muons. A muon at rest lives, on average, only about 2.2 millionths of a second before it decays. Even travelling at 99.5% of the speed of light, that is only long enough to cover around 660 metres. Almost none should reach the ground.

Yet detectors at sea level catch them constantly: enough that some pass through your body every second. The reason is time dilation. At that speed γ is about 10, so the muon's internal clock runs ten times slower from our point of view, and its average range stretches to about 6.6 kilometres, with many travelling much further. In 1963, David Frisch and James Smith counted muons on the summit of Mount Washington in New Hampshire and again at sea level, and found far more surviving the descent than a non-relativistic calculation allowed. In 1977, at CERN in Geneva, muons circulating in a storage ring at a γ of about 29 were found to live about 29 times longer than muons at rest, in agreement with Einstein to around one part in a thousand.

Atomic clocks around the world

In October 1971, physicist Joseph Hafele and astronomer Richard Keating bought round-the-world tickets on ordinary commercial flights, and four extra seats for four caesium atomic clocks. They flew once eastward and once westward, then compared the clocks with reference clocks at the US Naval Observatory.

Both effects were at work. The planes flew high, where gravity is weaker, so their clocks should gain time. Their speed relative to the Earth's centre differed, because flying east adds to the Earth's rotation and flying west subtracts from it. Relativity predicted a loss of about 40 nanoseconds on the eastward trip and a gain of about 275 nanoseconds on the westward one. The clocks showed a loss of about 59 and a gain of about 273 nanoseconds, matching the predictions within the experimental uncertainties.

The satellite in your pocket

The most convincing proof of time dilation is one that billions of people use every day without knowing it. The Global Positioning System works by timing signals from satellites orbiting about 20,200 km above the Earth. A position error of one nanosecond in timing corresponds to about 30 centimetres on the ground, so the clocks must be astonishingly accurate.

Two relativistic effects pull the satellite clocks in opposite directions:

Thirty-eight microseconds sounds like nothing. But light travels about 11 kilometres in that time. Without correcting for relativity, GPS positions would drift by kilometres every day and the system would be useless within hours. The satellite clocks are deliberately tuned before launch to tick very slightly slow, so that once they are in orbit they match ground time. Every time a map app finds your location, Einstein is being proved right again.

Astronauts, slightly younger

Astronauts on the International Space Station travel at about 7.7 km/s. Their speed slows their clocks more than the weaker gravity speeds them up, so overall they age a little more slowly than people on the ground. When Scott Kelly spent 340 days on the station in 2015–16, he aged roughly 8 to 9 milliseconds less than he would have on Earth. The cosmonaut Sergei Krikalev, who spent more than 800 days in orbit over his career, is about one-fiftieth of a second "younger" than he would otherwise be.

That is real, measurable and utterly unnoticeable. It is also a long way from twenty-seven ages of the world.

The twin paradox, resolved

There is a puzzle hidden in special relativity, and thoughtful readers usually spot it at once.

If motion is relative, then each observer sees the other's clock running slow. The passenger sees the platform clock running slow just as surely as the platform sees the train clock running slow. So imagine twins. One stays on Earth. The other flies to a distant star and back at high speed. From Earth, the traveller's clock runs slow, so she should return younger. But from the spacecraft, isn't it Earth that is moving away and back? Shouldn't the Earth twin be younger instead? They cannot both be younger when they meet again and compare birthdays.

This is the famous twin paradox. It is not a contradiction in relativity. It is a lesson in what relativity actually says.

The journeys are not symmetrical

The Earth twin stays in one steady state of motion for the whole story. The travelling twin does not. To come home, she must slow down, turn around and speed up again. During the turnaround she feels it: she is pressed into her seat, and loose objects in her cabin slide across the floor. The Earth twin feels nothing. That difference is physical and undeniable, and it breaks the symmetry.

There is a deeper and more beautiful way to see it. In relativity, space and time are woven into a single four-dimensional fabric called spacetime. Each person's life traces a path through it, called a worldline. The time a person actually experiences (called their proper timeproper time The time a clock actually records along its own path: the time a traveller lives.) is a kind of length measured along their worldline.

In ordinary geometry, a straight line is the shortest path between two points. In the geometry of spacetime, the rule is reversed: the straight, unaccelerated path between two events is the one along which the most time passes. The Earth twin's worldline is the straight one. The traveller's path, with its outward leg, its turn and its return, is bent. So when they meet again, more time has passed for the twin who stayed at home.

An example

Suppose the traveller flies to a star 4 light-years away at 80% of the speed of light, turns around and comes straight back. Ignoring the short periods of acceleration:

• On Earth, the round trip of 8 light-years at 0.8c takes 10 years.
• For the traveller, γ is about 1.67, so only 6 years pass on board.

She left on the same day as her twin. She returns four years younger. Both twins' experiences were perfectly ordinary while they lived them: twelve monthly calendars for every year, the same heartbeats, the same seasons of thought. They simply took different paths through spacetime, and the paths had different lengths in time.

One more twist deepens the strangeness. Relativity also says that observers moving relative to each other will disagree about which distant events happen "at the same time". There is no universal "now" spread across the whole universe. What is happening "at this moment" on a distant star is not a fact that all observers agree on. That is perhaps the most radical idea in the whole theory, and we will return to it at the end.

At the edge: black holes and frozen time

On Earth, gravitational time dilation is a matter of nanoseconds. Near a black hole, it can become almost limitless.

In 1916, only months after Einstein published general relativity, the German astronomer Karl Schwarzschild (writing while serving on the Russian front in the First World War) found an exact solution to Einstein's equations for the space around a single massive point. Hidden in that solution was a boundary that would later be called the event horizonevent horizon The boundary around a black hole from which nothing, not even light, can escape.: a surface around a sufficiently compact mass from which nothing, not even light, can escape.

As a clock is lowered towards that horizon, distant observers see it tick ever more slowly. At the horizon itself, according to their reckoning, it would stop altogether.

Two views of one fall

Imagine an astronaut falling into a large black hole while her partner watches from a safe distance.

• What the partner sees. The falling astronaut's movements grow slower and slower, and her light becomes redder and fainter. She seems to approach the horizon forever without quite reaching it, frozen and fading, until she is effectively invisible.
• What the astronaut experiences. Nothing special at all, at first. Her own watch ticks normally. She crosses the horizon in a finite, ordinary amount of her own time, and continues inwards. (For a small black hole, tidal forces would tear her apart first; for a very large one, she might cross the horizon unharmed.)

Both accounts are correct. They describe the same event from two different paths through spacetime. There is no single "true" clock against which to judge them, which is exactly what relativity has been telling us all along.

Hours that cost years

The 2014 film Interstellar dramatised this with Miller's planet, orbiting so close to a giant spinning black hole that one hour on its surface corresponds to seven years far away. The physicist Kip Thorne, who later shared the 2017 Nobel Prize in Physics for the detection of gravitational waves, worked on the film's science and showed that such an extreme ratio is possible in principle, though only around a black hole spinning at almost the maximum rate nature allows.

We do not need cinema for evidence of the effect near black holes. At the centre of our own galaxy lies Sagittarius A*, a black hole about four million times the mass of the Sun. A star known as S2 swings around it on a sixteen-year orbit, passing as close as about 120 times the Earth–Sun distance. In 2018, astronomers of the GRAVITY collaboration at the European Southern Observatory watched S2 through its closest approach and detected the gravitational stretching of its light predicted by general relativity. The deep gravity of a black hole was visibly slowing the light-clock of a star.

Here, at last, we find ratios of time that begin to resemble the Puranic stories: a short stay in one place, and a long passage of time elsewhere. So it is time to ask the question directly.

Did the ancients know?

Now we can return to King Kakudmi with clear eyes.

Running the numbers

In the Puranic system, one mahāyugamahāyuga A full cycle of the four ages in Puranic time: 4.32 million years. (a full cycle of the four ages) lasts 4.32 million years. Twenty-seven of them come to about 117 million years. Suppose the Gandharva performance that Kakudmi waited through lasted twenty minutes. Then the ratio between Earth time and the king's time is about three trillion to one.

Could physics produce such a ratio? In principle, yes. A traveller would need to move at a speed that differs from the speed of light by about one part in 10²⁵, a number so extreme that no conceivable engine could reach it. Alternatively, she could hover astonishingly close to the event horizon of a black hole, deeper in gravity than any real spacecraft could survive. Relativity does not forbid the ratio. It just makes it absurdly costly.

Where the resemblance ends

But asking whether the ratio is possible misses the more important question: is the idea the same? And here, honesty requires a clear answer. It is not.

• No mechanism. Relativity explains why clocks differ: speed and gravity, connected by precise equations. The Purana gives no mechanism at all. Time differs in Brahmaloka because of the realm's place in a sacred cosmic hierarchy, not because of motion or mass.
• No law. Relativity predicts exact numbers that can be measured and checked, from muons to GPS. The Puranic ratios of divine to human time are fixed by tradition, not derived from anything that could be tested.
• A different purpose. The story is not trying to describe physics. It is teaching anitya, impermanence: the kings, cities and fortunes we cling to are swept away in a moment from a higher perspective. Read that way, the story is profound, and it loses nothing by not being physics.
• A shared human intuition. The same structure appears in Japan, Ireland and many other cultures with no connection to India. That suggests something universal in human imagination about otherworlds, not a lost scientific discovery unique to one tradition.

What is genuinely remarkable is the imaginative leap itself. Most cultures assumed time was a single river flowing at one speed for everyone. The Puranic thinkers, among others, were willing to imagine that time might flow at different rates in different realms. That was an act of philosophical courage. It was not physics, but it was the kind of thinking that makes room for physics.

The checklist

The last row is there for a reason. Not every surprising claim is false. The method (checking each one against evidence) is what matters.

Felt time is a different thing

There is one more confusion worth clearing up, because it appears constantly in spiritual writing about time. People say that time "slowed down" in a car accident, "vanished" in deep meditation, or "flew" during a joyful evening. These experiences are real. But they are not time dilation. They are changes in how the mind measures time, while every clock in the room ticks on exactly as before.

The falling experiment

In 2007, the neuroscientist David Eagleman and his colleagues tested the common claim that time slows down in moments of terror. They dropped volunteers, safely, in a 31-metre free fall into a net. Afterwards, the volunteers judged their own fall to have lasted about a third longer than the falls of others they had watched.

But during the fall, each wore a wrist display flashing numbers slightly too fast to read at normal speed. If their perception had genuinely sped up (if they were "living in slow motion") they should have been able to read the numbers. They could not.

The researchers concluded that fear does not slow our experience of time as it happens. Instead, an intense event is laid down in memory with unusual richness and detail. When we look back, the dense memory feels as if it must have taken longer. The slowing is in the remembering, not the living.

Why holidays feel long and years feel short

The same principle explains a familiar puzzle. A week of travel in a new place seems to fly by while it is happening, yet looking back it feels long and full. A routine month at work feels slow day by day, yet vanishes in memory. The difference is novelty. New experiences create many distinct memories; routine creates few. Many psychologists think this is part of why the years seem to speed up as we grow older: fewer moments are new.

Meditation and timelessness

Contemplative traditions, including the yogic ones, have long described states in which time seems to disappear. Scientists have begun to study how meditation affects time perception. Some experiments suggest that even brief mindfulness practice can make people judge short intervals as lasting longer than they do, perhaps because attention is turned towards the passing moment rather than away from it. The psychologist Marc Wittmann has argued that our sense of time is closely tied to our awareness of our own bodies and of the present moment.

These are fascinating findings, and they matter for how we live. But a meditator's watch, checked against an atomic clock, runs at exactly the same rate as everyone else's in the room. A monk deep in samadhi in a Himalayan cave is, in the physical sense, ageing very slightly faster than a pilgrim at sea level, because of the altitude, by roughly fourteen microseconds a year at a height of 4,000 metres.

Keeping these two meanings separate protects both. Physics loses nothing by admitting that the mind experiences time in rich and variable ways. Spiritual practice loses nothing by admitting that it does not alter the ticking of atoms. Mixing them up does a disservice to both.

I am Time

On the battlefield of Kurukshetra, when Arjuna asks to see Krishna's true form, the vision that appears is overwhelming: worlds rushing into blazing mouths like rivers into the sea. Asked who he is, Krishna answers in one of the most famous lines of the Bhagavad Gita (11.32): kālo'smi, "I am Time", grown mighty, the destroyer of worlds. In the Shaiva tradition, Shiva is worshipped as Mahakala, the Great Time, most famously at the Jyotirlinga of Ujjain: the same city through which classical Indian astronomers drew their prime meridian.

In these traditions, time is not a neutral background. It is the deepest power in the cosmos, before which kings, cities and fortunes dissolve. That is precisely the lesson Brahma laughs into King Kakudmi's ear.

Modern physics arrives at its own, very different, humility about time. It tells us there is no single cosmic clock ticking for everyone. Every object (every muon, satellite, astronaut and person) carries its own proper time along its own path through spacetime. Two people who part and meet again may have lived different amounts of time, and both will be right about their own. Even "now" is not shared across the universe: observers in relative motion disagree about which distant events are simultaneous.

Einstein felt the weight of this. When his lifelong friend Michele Besso died in 1955, Einstein wrote to Besso's family that, for physicists who believe in physics, the distinction between past, present and future is only a stubborn illusion. He died himself a few weeks later. Physicists still debate what he meant, and whether the flow of time is a deep feature of nature or something the brain constructs. The equations do not settle that question. They only insist that time is woven together with space, bent by motion and mass, and personal to every path.

Two kinds of wisdom

So where does that leave the old stories?

The Puranas did not discover relativity. Claiming that they did does them no honour; it turns poetry into a failed physics exam. Their wisdom is of another kind: a moral and contemplative insight that, from a large enough perspective, all our worldly urgency is brief.

Relativity did not discover that wisdom either. Its gift is another kind of truth: exact, testable, built into the satellites overhead and the atoms in our bodies. Every time you check your location on your phone, you rely on the fact that clocks in orbit and clocks on the ground disagree, and on our ability to calculate by how much.

Put side by side, they make a surprisingly good pair. The sage says: do not cling to your moment; it is smaller than you think. The physicist says: your moment is your own; no one else's clock measures it exactly. Both remove us from the centre of the universe. Both invite a kind of humility. Neither needs to borrow the other's authority.

King Kakudmi went to the Creator with a list of princes and returned to a world that had moved on without him. Every astronaut who returns from the space station comes back, by a few thousandths of a second, to a world that has moved on a little further than they have. One story is told in myth, the other in milliseconds. Both remind us that time is not something we stand outside of and measure. It is something each of us is travelling through, on a path that belongs to us alone.

Further reading

Lorentz factors, GPS, ISS, gravitational and Kakudmi figures were calculated for this article from standard formulas.

• Bhagavata Purana 9.3 (the story of Kakudmi and Revati); Bhagavad Gita 11.32
• A. Einstein, "On the Electrodynamics of Moving Bodies" (1905)
• R. V. Pound and G. A. Rebka Jr., "Apparent Weight of Photons", Physical Review Letters (1960)
• D. H. Frisch and J. H. Smith, "Measurement of the Relativistic Time Dilation Using μ-Mesons", American Journal of Physics (1963)
• J. C. Hafele and R. E. Keating, "Around-the-World Atomic Clocks", Science (1972)
• J. Bailey et al., measurements of muon lifetime in the CERN storage ring, Nature (1977)
• C. W. Chou et al., "Optical Clocks and Relativity", Science (2010)
• T. Bothwell et al., gravitational redshift across a millimetre-scale atomic sample, Nature (2022)
• GRAVITY Collaboration, detection of gravitational redshift in the orbit of the star S2, Astronomy & Astrophysics (2018)
• C. Stetson, M. P. Fiesta and D. M. Eagleman, "Does Time Really Slow Down during a Frightening Event?", PLoS ONE (2007)
• U. I. Uggerhøj et al., "The young centre of the Earth", European Journal of Physics (2016)
• Kip Thorne, The Science of Interstellar (2014)
• Carlo Rovelli, The Order of Time (2018)
• Marc Wittmann, Felt Time: The Psychology of How We Perceive Time (2016)