In What Time Is It? we untangled the human mess of the hour. In What Day Is It? we did the same for the calendar. In Ticks or Tocks? we traced the physics of the second from quartz crystals to optical lattice clocks that won’t lose a tick in the lifetime of the universe. All of those stories treated time as something that flows at the same rate everywhere – a backdrop against which clocks are merely more or less accurate. That assumption is wrong. Time itself bends.
Einstein enters the chat
Einstein’s special theory of relativity, published in 1905, showed that time passes more slowly for objects moving at high speeds relative to an observer. This isn’t a theoretical curiosity – it’s measurable. In 1971, Hafele and Keating flew caesium clocks on commercial airliners around the world and compared them to reference clocks on the ground. The flying clocks disagreed with the ground clocks by exactly the amount relativity predicted (Hafele & Keating, 1972, Science).
The speed of light as a universal speed limit. Nothing with mass can reach the speed of light. As you approach it, time dilation increases without bound. At the speed of light, time stops entirely – from a photon’s frame of reference (to the extent that’s meaningful), no time passes at all. A photon emitted from a star ten billion light-years away has, from its own perspective, arrived at your eye instantaneously.
Muon decay provides one of the cleanest experimental demonstrations. Cosmic ray muons are created in the upper atmosphere and should decay in roughly 2.2 microseconds, which at near-light speed would let them travel only about 660 metres. But we detect them at sea level, roughly 15 kilometres below where they were created. How? At 99% of the speed of light, their time is dilated by a factor of roughly seven. They “live” long enough to reach us. Rossi and Hall first confirmed this in 1941 (Physical Review), and it remains one of the most intuitive demonstrations of special relativity.
The twin paradox
Special relativity produces a result so counterintuitive that it has its own name. Imagine two twins: one stays on Earth, the other takes a round trip to a distant star at near-light speed. When the travelling twin returns, less time has passed for them. They are younger than their sibling. This is not an illusion or an accounting trick – it’s a real, physical difference in elapsed time.
The “paradox” label is misleading. There’s no logical contradiction. The resolution is that the two twins are not in symmetric situations: one of them accelerated (turned around), and that breaks the symmetry. The twin who stayed home followed an inertial path through spacetime – no acceleration, no turning around. In relativity, the straighter your path through spacetime, the more time you experience. It’s counterintuitive – we’re used to thinking that straight lines are shortest, but in spacetime, a straight path is the one that ages you the most. Any acceleration – any turning around – reduces the elapsed time. This is why the travelling twin ages less.
The effect doesn’t require a spaceship. The International Space Station orbits at about 7.7 kilometres per second. Astronauts on the ISS age very slightly slower than people on the ground – roughly 0.01 seconds less per year. Scott Kelly, who spent 340 days aboard the ISS in 2015-2016 while his identical twin Mark stayed on Earth, returned about 5 milliseconds younger than he would have been had he stayed home. Not enough to matter biologically. Enough to prove the physics is real.
Supersonic time travel
Concorde, that beautiful, impractical supersonic airliner, offered a surreal temporal experience. You could leave London at 10:30 AM and arrive in New York at 9:30 AM the same day – arriving before you departed, by clock time. The crossing took about three and a half hours, but the five-hour time difference meant you gained more than you spent.
This wasn’t relativity – it was time zones. But the special relativistic effect was real too, if tiny. Concorde flew at roughly Mach 2 – twice the speed of sound, about 600 metres per second. At that speed, the time dilation factor is approximately 1 + 2 x 10^-12, which means passengers aged about 0.000000002% less than people on the ground per flight. Over a career of flying Concorde, a pilot might have “saved” a few hundred nanoseconds of biological time. Not enough to notice. Enough to measure.
The more interesting effect was the experience itself. Westbound on Concorde, the sun appeared to move backwards in the sky. You were flying faster than the Earth rotates at that latitude. For the duration of the flight, you were outrunning the planet’s spin. It’s the closest any commercial passengers ever came to the intuitive experience of time running in an unusual direction.
Gravity bends time
Einstein’s general theory of relativity, from 1915, added another twist: time passes more slowly in stronger gravitational fields. The closer you are to a massive object, the slower your clock ticks relative to someone further away. A clock on the floor of your house runs very slightly slower than a clock on your roof. The difference is about 10 nanoseconds per year per metre of altitude, which doesn’t affect your morning routine but absolutely matters for GPS.
GPS satellites orbit at about 20,200 km above the Earth. Their clocks tick faster than ground clocks by about 45 microseconds per day due to weaker gravity up there. They tick slower by about 7 microseconds per day due to their orbital speed. The net effect is that satellite clocks gain roughly 38 microseconds per day relative to the ground. If this weren’t corrected, GPS positions would drift by about 10 kilometres per day. Every GPS satellite has its clock rate deliberately adjusted before launch to compensate.
This means that when you use your phone to navigate to a restaurant, you are relying on corrections derived from general relativity. Einstein helps you find pizza.
The gravitational effect has been measured with astonishing precision. In 2010, optical clocks at NIST detected the difference in time flow between two clocks separated by just 33 centimetres of altitude (Chou et al., 2010, Science). Time really does run at different speeds depending on where you are in a gravitational field. There is no single “correct” rate at which time passes. It’s always relative to something.
This has practical consequences beyond GPS. The definition of UTC itself requires a choice: the clocks that contribute to UTC are at different altitudes and latitudes, so they tick at slightly different rates due to gravity. The BIPM corrects all contributing clocks to the rate they would tick at the “geoid” – the mean sea-level gravitational potential of the Earth. A clock in Boulder, Colorado (1,655 metres above sea level) ticks faster than one in London (near sea level) by roughly 15 microseconds per year. Without the geoid correction, the ensemble average would be meaningless – you’d be averaging clocks that are physically keeping different times. The concept of “a second” on Earth is, in a gravitational sense, a political decision about which altitude to use.
The universe’s default clock rate
The geoid is a local compromise – we picked Earth’s mean sea level and called it “the reference.” But zoom out and the same problem applies everywhere. Every mass in the universe – every star, planet, galaxy cluster – sits in a gravitational well where time runs slower. A clock in deep intergalactic space, far from any significant mass, ticks faster than any clock on any planet. That hypothetical far-from-everything clock is as close as you can get to time running “undiluted” – the fastest rate time can flow.
There is no single point where gravity’s influence drops to exactly zero. Gravity has infinite range, and the universe is full of mass, so every location experiences some gravitational time dilation. But the effect falls off sharply with distance. In the great voids between galaxy clusters – regions hundreds of millions of light-years across containing almost nothing – gravitational time dilation is vanishingly small. For all practical purposes, that’s where time runs at its natural rate.
This creates an odd inversion of perspective. We think of time on Earth as “normal” and relativistic corrections as exotic. But from the universe’s point of view, we’re the anomaly. We live at the bottom of a gravitational well. Our clocks are the slow ones. Imagine you grew up in a swimming pool and thought water resistance was just how movement worked. Then someone drained the pool and you felt what running is like without the drag. Deep space is the drained pool. We’ve been wading our whole lives.
The practical consequence is that there’s no privileged clock in the universe. UTC is corrected to the geoid, but the geoid is a human choice, not a physical constant. A civilisation on a neutron star would pick a very different reference – one where “a second” on their surface lasts far longer than ours. Neither civilisation’s second is more correct than the other’s. “How fast does time pass?” isn’t a question with an answer until you specify where.
The young heart of the Earth
We don’t need black holes to see gravitational time dilation at work on a grand scale. The core of the Earth, being under more gravitational stress than the surface, has experienced less elapsed time since the planet formed. The centre of the Earth is roughly 2.5 years younger than the surface – not metaphorically, but in actual measured atomic clock ticks. Time has passed more slowly down there for 4.5 billion years, and it adds up. Feynman mentioned a version of this calculation; it was rigorously computed by Uggerhoj et al. (2016, European Journal of Physics).
This is not a thought experiment. It’s a straightforward consequence of general relativity applied to the known density and gravitational profile of the Earth. If you could somehow place a clock at the centre of the planet when it formed and retrieve it today, it would show a date 2.5 years behind a clock that had spent its life on the surface. The rock beneath your feet is, in a physically meaningful sense, younger than the rock you’re standing on.
Black holes and the edge of time
Near a black hole, gravitational time dilation becomes extreme. At the event horizon – the boundary beyond which nothing, not even light, can escape – time, from an outside observer’s perspective, stops entirely. An object falling toward a black hole appears to slow down asymptotically, growing dimmer and redder, never quite crossing the horizon from the viewpoint of someone watching from a safe distance. The object falling in experiences time perfectly normally from its own point of view. Neither observer is wrong. Time is doing something different in each location.
The mathematics are well-established. Karl Schwarzschild worked out the mathematics of what happens to spacetime around a simple, non-spinning massive object – and he did it in 1916, just months after Einstein published general relativity. His solution predicts that at the event horizon, the gravitational time dilation factor goes to infinity. Time, as experienced by a distant observer, literally ceases to advance for anything at the horizon.
Inside the horizon, things get stranger still. The physics is hard to describe without the maths, but the gist is this: falling toward the centre becomes as unavoidable as the passage of time itself. You can no more stop falling inward than you can stop moving into the future. The singularity at the centre isn’t a place you travel to. It’s a moment you can’t avoid – the future that everything inside the horizon is headed toward.
Time ripples
If gravity bends time, and gravitational fields change – say, when two black holes spiral into each other – then the bending itself should propagate outward as a wave. Einstein predicted this in 1916. It took a century to confirm.
On 14 September 2015, the LIGO detectors in Livingston, Louisiana, and Hanford, Washington, detected gravitational waves from two black holes merging 1.3 billion light-years away (Abbott et al., 2016, Physical Review Letters). What LIGO measured was spacetime itself stretching and compressing as the wave passed through. The arms of the detector – each four kilometres long – changed length by roughly one-thousandth the diameter of a proton. That’s the most precise measurement humans have ever made.
Here’s what that means for time. A gravitational wave doesn’t just stretch space. It stretches spacetime. As the wave from those merging black holes passed through Louisiana, time in the detector was oscillating – running very slightly faster, then very slightly slower, then faster again, hundreds of times per second. The oscillation was absurdly tiny, but it was real. For a fraction of a second, time in Livingston and time in Hanford were running at different rates, because the wave hit them at different moments.
We usually think of time as the background against which things happen. Gravitational waves show that the background itself vibrates. Time has ripples. They’re passing through you right now – from distant supernovae, from colliding neutron stars, from black holes that merged before the Earth existed. You can’t feel them. LIGO can.
So what time is it?
After all of this – the human history of sundials and railways and political time zones, the physics of caesium atoms and clock ensembles, the relativity that bends time near massive objects and at high speeds – the answer is: it depends.
It depends on where you are in a gravitational field. It depends on how fast you’re moving. It depends on which timescale you’ve chosen and why. It depends on whether you care about the sun’s position, or the purity of atomic seconds, or the agreement between your timestamp and everyone else’s.
The phone in your pocket hides all of this heroically. It receives signals from GPS satellites that have been corrected for both special and general relativistic effects. It knows your time zone from your location. It knows about DST transitions from a regularly updated database. It adjusts for leap seconds, or at least it tries to. It presents you with a number that looks simple and authoritative, and you glance at it and get on with your day.
Underneath, it’s leaning on millennia of astronomy, centuries of mechanical engineering, decades of atomic physics, and Einstein. It’s a tower of clever hacks and hard-won compromises, and it’s a miracle it works at all.
But we’ve only covered what time does – how it bends near mass, dilates with motion, ripples across the universe. The harder question is whether time fundamentally exists. The arrow that distinguishes past from future isn’t in the equations. “Now” isn’t a location in spacetime. The equations of quantum gravity may contain no time variable at all.
Does Time Even Exist? is next – a tour of the foundations, from the block universe to the holographic principle and the physicists who think time is a shadow of something simpler.