You probably looked at the time today without giving it much thought.
Seconds become minutes. Morning becomes afternoon. Today becomes tomorrow.
Time seems to move continuously forward, carrying everything with it.
But what exactly is moving?
We can measure time extremely accurately, yet explaining what time actually is turns out to be surprisingly difficult.
Does time really flow? Does only the present exist? Why can we remember yesterday but not tomorrow? And could the past and future somehow be just as real as the moment you are experiencing now?
Time is where everyday experience begins to collide with some of the strangest discoveries in philosophy and physics.
Does Time Really Flow?
Our ordinary experience of time seems straightforward.
The past has happened.
The present is happening now.
The future has not happened yet.
We imagine ourselves travelling through these three stages as though the present were moving forwards along a timeline.
But try to identify what is actually flowing.
A river flows because water changes position relative to its banks.
If time flows, what does it flow relative to?
We sometimes say that time passes at one second per second. But that does not really explain anything.
Perhaps the feeling that time flows is not a property of time itself.
Perhaps it is something produced by the way our minds experience change.
Could There Be No Time Without Change?
This question fascinated philosophers long before modern physics.
Imagine a universe in which absolutely nothing changed.
No planets moved.
No atoms vibrated.
No thoughts occurred.
Nothing happened at all.
Would time still be passing?
One possibility is that time exists independently, like an invisible container within which events occur.
Another is that time is fundamentally a way of describing relationships between changes.
Without change, perhaps there would be no meaningful way of saying that time had passed.
This raises an intriguing possibility.
Maybe clocks do not actually measure time in the way a ruler measures a physical object.
Perhaps clocks simply compare one regular process — a swinging pendulum, a vibrating quartz crystal or an atomic transition — with other changes happening in the universe.
Newton’s Universal Clock
Isaac Newton imagined time as something that existed independently of everything else.
In this picture, the universe possesses a kind of invisible master clock.
Time passes uniformly everywhere, whether anything happens or not.
If two perfectly accurate clocks were placed at opposite ends of the universe, they should ultimately agree about how much time had passed.
This seems very close to common sense.
For centuries it also fitted remarkably well with physics.
Then came Einstein.
Einstein Changes Time
Albert Einstein discovered that time does not behave like a universal clock.
How quickly time passes depends upon motion and gravity.
Imagine two identical clocks.
One remains on Earth while the other travels through space at extremely high speed before returning.
When the clocks are compared, they need not show the same amount of elapsed time.
The travelling clock can have recorded less time.
This is not because the clock malfunctioned.
Less time actually elapsed along its journey.
This effect is known as time dilation.
At everyday speeds the difference is tiny. At speeds approaching the speed of light, it becomes dramatic.
Time is not separate from the physical universe in the simple way Newton imagined.
It is woven into it.
Gravity Changes Time Too
Speed is not the only thing that affects time.
Gravity does as well.
A clock closer to a massive object runs slightly more slowly than a clock farther away.
Even clocks at different heights above the Earth experience tiny differences.
This sounds fantastical, but it is a real physical effect and has been measured with extremely precise clocks.
It also matters practically.
Satellites experience time differently from clocks on Earth’s surface. Systems such as GPS have to account for relativistic effects to maintain their accuracy.
So there is no single cosmic clock ticking identically everywhere.
Different observers can genuinely experience different amounts of time.
Is There a Universal Present?
Things become stranger still.
We naturally imagine that somewhere, right now, something is happening on a distant planet.
But Einstein’s theory of relativity challenges the idea of a universal now.
Observers moving differently can disagree about whether distant events occurred simultaneously.
There may therefore be no single present moment shared across the entire universe.
Your experience still contains a present.
You are reading these words now.
But physics does not provide an obvious universal boundary dividing everything in existence into past, present and future.
This has encouraged some philosophers and physicists to consider a very different picture of reality.
Do the Past and Future Already Exist?
One view is called presentism.
According to presentism, only the present truly exists.
The past existed but no longer does.
The future will exist but does not exist yet.
This closely resembles ordinary experience.
Another possibility is sometimes called the block universe.
Imagine the history of the universe as an enormous four-dimensional structure containing space and time.
Your birth is somewhere within that structure.
So is this moment.
So, perhaps, are events you currently call your future.
From this perspective, time may not literally flow. Different moments may simply occupy different positions in spacetime, rather as Bournemouth and London occupy different positions in space.
We do not normally say London comes into existence when we travel towards it.
Could moments in time be something like that?
It is a deeply counterintuitive idea.
If the future already exists in some sense, what happens to free will?
And why does consciousness experience one moment after another rather than the whole structure at once?
Physics alone does not make all of these philosophical questions disappear.
Why Does Time Have a Direction?
There is another mystery.
Why does time seem to move only one way?
You can scramble an egg.
You never see a scrambled egg spontaneously reassemble itself and jump back into its shell.
A glass can fall from a table and shatter.
We never see scattered fragments leap from the floor and reconstruct the glass.
Yet many fundamental equations of physics do not contain the obvious distinction between past and future that our everyday lives do.
So where does the direction come from?
One important part of the answer involves entropy.
Entropy and the Arrow of Time
Entropy is often loosely described as disorder, although the scientific idea is more precise.
In many physical systems there are overwhelmingly more ways for matter and energy to be spread out than arranged in certain highly ordered configurations.
This creates a statistical tendency for entropy to increase.
A smashed glass has countless possible arrangements of its fragments.
There are comparatively few arrangements corresponding to an intact glass sitting neatly on the table.
The general increase of entropy gives us what physicist Arthur Eddington called the arrow of time.
It helps explain why we see eggs scramble but not unscramble and why many everyday processes appear irreversible.
But this creates another remarkable question.
If entropy has generally been increasing since the early universe, the universe must once have been in an extraordinarily low-entropy state.
Why did the universe begin that way?
The mystery of time leads directly into the mystery of the universe’s beginning.
Why Can We Remember the Past but Not the Future?
Your mind also has an arrow of time.
You remember yesterday.
You do not remember tomorrow.
That seems so obvious that it hardly needs explaining.
But why should consciousness have this particular orientation?
Memory depends upon physical processes in the brain. Creating memories leaves physical traces, and these processes occur within the same thermodynamic universe in which entropy increases.
Our psychological experience of time may therefore be connected with the physical arrow of time.
Yet that still leaves something unexplained.
Why does there seem to be a moving present at all?
Is the Present Created by the Brain?
The brain does not receive all information instantaneously.
Light, sound and touch travel through different pathways and are processed at different speeds.
The brain must combine this information into a coherent experience.
What feels like a perfectly immediate present may therefore already be a construction.
This does not mean time itself is imaginary.
It means our experience of time and physical time are not necessarily the same thing.
We know this from ordinary life.
Five minutes waiting for something unpleasant can feel much longer than five minutes spent doing something enjoyable.
An hour absorbed in an activity can seem to disappear.
As children, a year can feel enormous. Later in life it may seem to pass remarkably quickly.
The clock measures the same interval.
Conscious experience does not.
Why Does Time Seem Faster as We Get Older?
Almost everyone eventually notices this.
Childhood summers can seem endless.
Decades later, years appear to pass with alarming speed.
Time itself is not accelerating.
One possible explanation involves proportion. A year is a much larger fraction of the remembered life of a ten-year-old than of a sixty-year-old.
Another possibility involves memory.
Periods filled with new experiences create many distinctive memories. Routine periods may leave fewer memorable landmarks and can appear shorter when we look back on them.
This creates a curious distinction between experiencing time and remembering time.
A period that seems to pass quickly while we are enjoying it can later appear rich and substantial because it produced many memories.
Our sense of time is therefore not simply a clock inside the head.
It is intertwined with attention, memory and experience.
Could Time Have Had a Beginning?
If we follow the expansion of the universe backwards, we reach an extremely hot, dense early state around 13.8 billion years ago.
It is tempting to imagine a clock ticking before this and then, suddenly, the universe appearing.
But that picture may be misleading.
In Einstein’s general theory of relativity, space and time are parts of the physical universe itself.
Asking what happened before time began might therefore be like asking what lies north of the North Pole.
Perhaps the question assumes something that does not exist.
But we do not yet possess a complete theory describing the earliest possible state of the universe. Ideas involving quantum gravity, bouncing universes or other cosmological models could change the picture.
So science has not established a simple answer to what, if anything, preceded the earliest universe we can describe.
That question takes us towards another of our Great Questions:
How Did the Universe Begin — and Could Something Come From Nothing?
Could Time Ever End?
If time had a beginning, could it also have an end?
That depends partly upon the ultimate future of the universe.
Stars will not shine forever.
Over immense periods, the universe may become increasingly cold and dilute as usable energy becomes less available.
But even if almost nothing interesting were happening, would time itself continue?
Or does time only have meaning when change occurs?
Once again we arrive at the ancient question with which we began.
Is time a thing in its own right?
Or is time ultimately a relationship between events?
Is Time Real?
There is no simple answer.
Time is certainly not unreal in the everyday sense.
Things change.
We age.
Causes precede effects.
Clocks measure real differences between physical processes.
Relativity demonstrates that elapsed time is an important feature of the physical universe.
But the intuitive picture of a universal clock steadily carrying the whole universe from past to future does not survive modern physics.
Perhaps time flows.
Perhaps only the present exists.
Perhaps past, present and future are parts of a larger spacetime structure.
Perhaps the direction of time emerges from entropy.
And perhaps the sensation of time passing belongs partly to consciousness rather than to the universe itself.
These possibilities are not all equally established, and important questions remain open.
The Strange Thing About Time
Time may be one of the most familiar things in existence.
It is also one of the hardest to step outside and examine.
Everything we do happens within it.
Every thought takes time.
Every memory points towards the past.
Every plan imagines a future.
Even asking “What is time?” takes time.
And perhaps that is part of the difficulty.
We are trying to understand something from which we can never remove ourselves.
The deeper we investigate time, the more connections appear — to space, gravity, entropy, memory, consciousness, free will and the origin of the universe.
A glance at a clock can therefore lead surprisingly quickly to some of the deepest questions about reality itself.
And once we ask where time came from, another question is waiting:
How did the universe begin?