Time Is Broken showed time bending near mass, dilating with motion, rippling when black holes collide – always as a thing that exists. This post asks whether it does. 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. Some physicists think time is a shadow of something simpler. A few think it has more dimensions than we can see. A handful think it doesn’t fundamentally exist.
The arrow of time
At the quantum level, the equations of physics are mostly time-symmetric – they work just as well running backwards. Maxwell’s equations, the Schrodinger equation, even the equations of general relativity: none of them distinguish past from future. Run the film backwards and the physics still works. Yet we experience time as having a clear direction. Eggs break but don’t unbreak. You remember yesterday but not tomorrow. What gives time its arrow?
The standard answer involves entropy – roughly, the disorder of a system. Think of it this way: there are astronomically more ways for an egg to be broken than for it to be perfectly intact. A broken egg isn’t going to spontaneously reassemble, not because the laws of physics forbid it, but because the odds against it are absurdly, comically enormous. This is the second law of thermodynamics: things tend to move from ordered states to disordered ones, because disordered states are overwhelmingly more probable.
But this just pushes the question back a step: why does entropy increase? The second law is statistical, not fundamental – it says that higher-entropy states are more probable, so systems tend to evolve toward them. But that only works if the universe started in a low-entropy state – a highly ordered initial condition. Why did it? This is one of the deepest unsolved problems in physics, and it sits at the intersection of cosmology, thermodynamics, and the foundations of quantum mechanics. Roger Penrose has devoted much of his career to it; he estimates the probability of the universe’s initial low-entropy state arising by chance at roughly 1 in 10^(10^123) – a number so absurdly large that writing it out would require more paper than exists in the observable universe.
The arrow of time, on this view, isn’t a property of the equations. It’s a property of the initial condition. The universe was handed an astronomically improbable starting state, and everything since has been the slow unwinding of that order into disorder. Take away that initial condition and the arrow vanishes.
The block universe
At the cosmic level, time is inseparable from space. General relativity describes them as a single four-dimensional fabric – spacetime – that can be curved, stretched, and warped by mass and energy. The notion of “now” is surprisingly hard to define across large distances. In special relativity, simultaneity is relative. Imagine two lightning bolts striking opposite ends of a train simultaneously, from the platform’s point of view. A passenger on the train, moving toward one bolt and away from the other, sees them hit at different times – and according to relativity, both observers are equally right. There is no universal “now”. There is only your now, defined by your position and velocity, and it disagrees with everyone else’s.
This leads some physicists to the block universe interpretation: the idea that past, present, and future all exist equally and simultaneously. The four-dimensional spacetime block simply is – complete and unchanging. What we experience as the flow of time is an artefact of our consciousness moving through this block. In this view, the future is as real as the past; we just haven’t encountered it yet.
It’s a view that Einstein himself appears to have held. After the death of his lifelong friend Michele Besso in 1955, Einstein wrote to Besso’s family: “For those of us who believe in physics, the distinction between past, present, and future is only a stubbornly persistent illusion.”
If the block universe is right, there’s no such thing as the flow of time. There’s only a static four-dimensional structure, and the appearance of passage is something our brains impose on it. Which is uncomfortable, because the passage of time feels like the most obvious thing in the world.
The beginning of time
If time bends near mass and stops at an event horizon, what happened at the Big Bang – the most extreme gravitational event of all? In 1983, Stephen Hawking and James Hartle proposed that the question is malformed. In their no-boundary proposal, as you trace time back toward the Big Bang, the distinction between time and space dissolves. Time doesn’t hit a wall or a starting gun. It smoothly becomes something more like a spatial dimension – rounded off, with no edge and no “before.”
Hawking’s analogy: asking what happened before the Big Bang is like asking what’s south of the South Pole. You can walk south from anywhere on Earth, and at every step there’s more south ahead of you – until you reach the pole, where “south” doesn’t end in a wall. The concept simply stops applying. There’s no sign saying “end of south.” There’s just a smooth surface that curves in a way that makes the question dissolve. Time at the Big Bang, in the Hartle-Hawking model, does the same thing. The universe didn’t begin at a first moment. The geometry of spacetime curves in a way that removes the need for a first moment.
Every possible history, all at once
The no-boundary proposal isn’t just a clever picture. It’s calculated using a technique from quantum mechanics called the path integral – an idea Feynman developed in the 1940s. Here’s the intuition.
Normally, if you want to know how a ball gets from point A to point B, you calculate the one path it takes – the arc through the air that Newton’s laws dictate. Feynman showed that in quantum mechanics, this is wrong. The ball takes every possible path simultaneously – straight lines, spirals, loops, detours through the next room and back. Every path contributes to the outcome. Most of them cancel each other out, and what survives is something that looks very much like Newton’s single arc. But the cancellation is the reason, not the single path.
Now apply this to the universe. In quantum cosmology, the universe didn’t take one history from the Big Bang to now. It took every possible history – every possible geometry of spacetime, every possible arrangement of matter and energy, all at once. Some of those histories have time that looks like ours. Some have radically different causal structures. Some might have multiple time dimensions, or looping time, or no time at all. What we observe is the interference pattern of all of them.
Think of it like a choir. A hundred singers each sing a different note. Most of the notes clash and cancel. What the audience hears isn’t silence – it’s a chord. The chord is our universe. The individual notes are the histories that were summed over to produce it. Our experience of time – flowing forward, one second after another – is the chord that survived the cancellation. It’s not the only note that was sung.
Hawking’s last act
Hawking spent his final years refining this picture with Thomas Hertog. Their 2018 paper – submitted just weeks before Hawking’s death – used the holographic principle (more on that in a moment) to argue that the multiverse, if it exists, is far more constrained than the “anything goes” version popular in science fiction. Different regions of the universe might settle into different vacuum states – different stable configurations of the fundamental fields – and each vacuum state could have different effective physics. Different particle masses. Different force strengths. Possibly different properties of time itself.
This isn’t parallel universes in the Star Trek sense. It’s more like ice forming on a pond. Water can crystallise in different orientations, and different patches of ice have their crystals aligned differently. Same water, same physics, different local structure. Hawking and Hertog proposed that the universe is the same way: one underlying theory, but different regions that “froze” into different configurations. Time in one region might tick with subtly different properties than time in another – not because the laws are different, but because the local vacuum is.
The holographic principle
In 1993, Gerard ‘t Hooft proposed – and Leonard Susskind later developed – an idea that sounds absurd: all the information in a three-dimensional region of space can be encoded on its two-dimensional boundary. Like a hologram on a credit card that looks three-dimensional but is physically flat.
This wasn’t metaphor. It grew out of Hawking’s own work on black holes. Hawking showed in 1974 that black holes radiate – slowly evaporating over astronomical timescales. Jacob Bekenstein had shown that a black hole’s entropy (roughly, the amount of information it contains) is proportional to the area of its event horizon, not its volume. That’s deeply strange. The information content of a room is proportional to its volume – more room, more stuff, more information. But for a black hole, it’s the surface that matters. The interior is, informationally speaking, redundant.
If this holds generally – and there’s strong theoretical evidence that it does – then our entire three-dimensional experience, time included, might be a projection from a lower-dimensional boundary. Think of a shadow puppet show. Puppets move in 3D behind the screen. The audience sees 2D shadows on the wall. The holographic principle says something far stranger: the 2D shadow might be the fundamental reality, and the 3D puppet is the projection. We’re the audience and the shadow, convinced we live in 3D because the projection is so seamless.
What does this mean for time? On the boundary, time might work differently – or might not exist in the form we recognise. The “bulk” (our 3+1 dimensional experience) and the boundary encode the same information, but the encoding is radically different. The best-studied example is the AdS/CFT correspondence, discovered by Juan Maldacena in 1997, which shows an exact mathematical equivalence between a gravitational theory in a curved spacetime and a quantum field theory on its boundary – a theory that has no gravity at all. Same physics. Completely different description. In one description, time curves and dilates near massive objects. In the other, there’s no gravity to curve anything. Both are equally correct. They’re not two approximations of the same thing. They’re two exact descriptions of the same thing.
Two times
If time can be a projection of something simpler, can it also be a shadow of something richer?
Itzhak Bars at the University of Southern California has been developing a framework called two-time physics since the late 1990s. The idea: our universe has not four dimensions (three space, one time) but six – four of space and two of time. We can’t perceive the extra dimensions directly, any more than a shadow on a wall can perceive the lamp behind it. Our 3+1 dimensional experience is a particular projection of the 4+2 dimensional reality.
Here’s what makes it interesting. A 3D object casts different 2D shadows depending on the angle of the light. A cube’s shadow can look like a square, a hexagon, or a diamond. Same object, different projections, each one a valid 2D description. Bars showed that the same 4+2 dimensional physics, projected differently, gives different 3+1 dimensional theories – theories that look completely unrelated but are secretly the same underlying reality seen from different angles. Some of those projections have a time dimension that behaves like ours. Others have time that works differently. All are equally valid shadows of the same six-dimensional object.
This is speculative. There’s no experimental evidence for two time dimensions, and the framework is constructed to be mathematically consistent rather than empirically motivated. But it’s a legitimate research programme, published in peer-reviewed journals, and it demonstrates something important: our assumption that there’s exactly one time dimension is a choice, not a logical necessity. The mathematics works perfectly well with more.
Time loops
General relativity doesn’t just allow time to slow down or speed up. Under certain conditions, it permits time to form closed loops – paths through spacetime that return to their own starting point. Gödel found the first one in 1949. Spinning black holes have them. Wormholes might too. Hawking took the idea seriously enough to propose a law of physics to prevent it. It gets much stranger from there.
Time crystals
Start with the warm-up. Salt is a crystal because its atoms sit in a repeating pattern: atom, gap, atom, gap, atom, gap. Nothing in the laws of physics insists they line up that way – they just do, because the arrangement is stable. The pattern is in space.
In 2012, Frank Wilczek (a Nobel laureate) asked the obvious next question: could a pattern repeat in time instead? Could a system tick, tick, tick forever on its own preferred schedule, in its lowest energy state, with no energy input?
This was controversial. A system oscillating in its ground state would seem to violate the expectation that ground states are static – nothing happening, no change, as boring as physics gets. But in 2017, two teams independently built time crystals in the lab – one at Harvard using a chain of ytterbium ions, another at the University of Maryland using a different approach. The trick was a clever sleight of hand. You can’t just shake something and call it a time crystal, because then it’s only dancing to your beat. The Harvard and Maryland teams drove their systems at one speed and watched them respond at a different, slower speed – tap once a second, tick once every two seconds. That mismatch is the giveaway. The rhythm comes from inside the system, not from the experimenter. Time-translation symmetry – the assumption that the laws of physics are the same from one moment to the next – was broken.
Ordinary crystals break spatial symmetry: space looks the same in every direction, but inside a crystal, some directions are special. Time crystals do the same thing to time: time flows the same way from moment to moment, but inside the crystal, some moments are special. The crystal has a rhythm the underlying laws don’t require. It’s a genuinely new kind of stable arrangement of matter – a new “phase” alongside solid, liquid, gas, and magnet. We didn’t know matter could organise itself in time the way it organises itself in space. Now we know it can.
It’s tempting to read this as evidence that time itself is chunky – that the universe has a preferred beat hidden in it somewhere. It isn’t. The discreteness lives in the system’s state, not in time. Same as salt: atoms sit at specific spots, but the space between them is still a smooth continuum. The pattern is in the matter, not in the stage the matter sits on.
Whether the stage itself has a smallest possible tick – whether time is smooth all the way down, or whether the universe has a frame rate – is a different question entirely.
The smallest tick
Is there a shortest possible moment? A tick so small that “before” and “after” stop meaning anything?
Maybe. It’s called the Planck time, and it’s about 5.4 × 10⁻⁴⁴ seconds. To get a feel for how small that is: the ratio between one Planck time and one second is roughly the same as the ratio between one second and a hundred trillion trillion times the current age of the universe. It’s not a duration anyone has measured or ever will measure. It’s more like a speed limit sign at the edge of the map – our best theories of physics say “beyond here, we don’t know what happens.”
The number comes from combining three fundamental constants – the speed of light, the gravitational constant, and Planck’s constant – in the only way that gives you a unit of time. It’s the scale where quantum mechanics and gravity would both matter simultaneously, and right now we don’t have a theory that handles both at once. Our two best frameworks – quantum mechanics (which explains the very small) and general relativity (which explains the very massive) – give contradictory answers at this scale.
Some physicists think the Planck time is a real boundary – that time is genuinely granular at this level, like pixels on a screen. Below one Planck time, there’s no “shorter.” Others think time is smooth all the way down and the Planck time is just where our equations stop working, not where time itself stops. We don’t know. We’re nowhere near being able to test it. But it’s a striking thought: the universe might have a frame rate.
Does time exist at all?
Some physicists have gone further. Julian Barbour, in The End of Time, argued that time doesn’t fundamentally exist. What we call time is just the way we experience the relationships between configurations of matter. The universe doesn’t evolve through time; it simply is a collection of states, and our brains string them into a narrative.
Carlo Rovelli, in The Order of Time, takes a related but more nuanced position: time as we experience it – flowing, universal, directed – is an emergent property that arises from our limited perspective as macroscopic beings who interact with the world thermodynamically. At the most fundamental level of quantum gravity, the equations may contain no time variable at all.
When physicists try to write down an equation that combines quantum mechanics and gravity – the so-called Wheeler-DeWitt equation – they get something startling: the equation has no time variable at all. It describes a universe where nothing changes. How you get from a timeless equation to our everyday experience of things happening one after another is, to put it mildly, an open question.
This is philosophy as much as physics, and it’s nowhere near settled experimentally. But it illustrates how deep the rabbit hole goes. We started with a simple question – “what time is it?” – and ended up with equations in which time has no place.
Where this leaves us
None of the foundations in this post are settled. The block universe is an interpretation, not a measurement. The no-boundary proposal is a model, not a verdict. The holographic principle has strong theoretical support but no direct experimental test. Two-time physics is consistent mathematics without empirical backing. Time crystals exist, but they’re a curiosity rather than a revolution. The Planck time is a scale we can’t probe. The Wheeler-DeWitt equation has no time variable, and nobody knows what to do about that.
What all of them share is the unsettling implication that the time we experience – flowing, directed, universal, one thing after another – might be a surface feature of something deeper. The equations don’t need the arrow. “Now” isn’t in the maths. The fundamental theories we have either don’t mention time or treat it as a dimension no more special than space.
And yet we live in time. Things happen. The egg breaks and doesn’t unbreak. You remember yesterday and not tomorrow. Whatever time is fundamentally, emergently, or not-at-all, our experience of it is real enough to navigate by.
There’s one more direction the equations let us push, and it’s the direction most people would actually want to use a time machine for. Not forward – forward is easy and we’ve covered it. Backward.
Can You Turn Back Time? is next – and the equations are more permissive than you’d expect.