In Kurt Vonnegut’s novel Slaughterhouse-Five, Billy Pilgrim becomes “unstuck in time.”
Instead of experiencing his life normally — one moment following another — Billy finds himself moving unpredictably between different moments of his existence.
The alien Tralfamadorians experience time even more strangely.
To them, a person who is dead at one moment is still perfectly alive at many others. They see different moments rather as we might see different places.
It is science fiction.
But behind Vonnegut’s strange vision lies a genuine philosophical question raised by modern physics:
What if the present is not the only moment that exists?
Perhaps the past has not disappeared and the future is not waiting to come into existence.
Perhaps every event — yesterday, today and tomorrow — occupies its own location within a four-dimensional universe of space and time.
This idea is commonly called the block universe.
And if it is correct, our ordinary understanding of time may be profoundly misleading.
How We Normally Imagine Time
Our everyday picture of time seems obvious.
The past existed, but it has gone.
The present exists now.
The future does not exist yet.
Reality therefore seems to be continually changing.
New moments come into existence as the future becomes the present. They then disappear as the present becomes the past.
This philosophical view is known as presentism.
Only the present is fully real.
It fits remarkably well with experience.
You can remember yesterday, but you cannot return to it.
Tomorrow feels open and undecided.
And there seems to be something uniquely real about right now.
But modern physics creates a problem for this apparently obvious picture.
Einstein and the End of Universal Time
Before Einstein, Isaac Newton’s picture of time largely matched common sense.
Time could be imagined as a universal clock ticking steadily throughout the universe.
There was one present moment everywhere.
Then Einstein’s theory of special relativity changed the picture.
One of its consequences is that observers moving relative to one another can disagree about whether distant events happen at the same time.
This is known as the relativity of simultaneity.
Imagine two events happening far apart.
One observer may judge them to occur simultaneously.
Another observer moving differently may judge one event to happen before the other.
Neither observer is simply mistaken.
There is no single universal answer independent of the observer’s state of motion.
And that creates a surprisingly deep problem.
Where Is “Now”?
Imagine that somewhere in a distant galaxy two events occur.
Which one is happening right now?
Our intuition tells us there must be an answer.
But relativity does not provide the universe with a single master clock defining one objective present everywhere.
Different observers can divide spacetime into “past”, “present” and “future” differently when considering sufficiently distant events.
So what could it mean to say that only the present exists?
Whose present?
If reality consists only of everything happening “now”, relativity makes it surprisingly difficult to identify which distant events belong to that privileged moment.
This is one reason some philosophers and physicists have been attracted to a radically different picture.
Instead of reality being continually created moment by moment, perhaps all events belong to a larger structure.
The Universe as a Four-Dimensional Block
Einstein’s theories encouraged physicists to think in terms of spacetime.
We normally describe the position of something using three spatial dimensions.
Where is it from left to right?
Where is it from front to back?
How high is it?
But identifying an event also requires another piece of information:
When did it happen?
An event therefore has a position in both space and time.
Together these can be represented as four-dimensional spacetime.
The block universe takes this seriously.
Instead of imagining the universe as three-dimensional space continually changing through time, imagine the entire history of the universe as one four-dimensional structure.
The formation of Earth occupies one region.
The dinosaurs occupy another.
Your birth occupies another.
You reading this sentence occupies another.
Events tomorrow would occupy other locations within the structure.
Nothing needs to travel from the future into the present and then disappear into the past.
All the events simply have different locations in spacetime.
Your Life as a Worldline
Physics has a useful way of representing an object’s history through spacetime: a worldline.
Imagine following your location through your entire life.
Your birth begins one part of the line.
Every journey you make changes its path through space.
Every second advances it through time.
Eventually the line reaches your death.
Your entire life can therefore be represented as a path through spacetime.
This produces a strange way of thinking about yourself.
The five-year-old you occupies one region of your worldline.
The person reading this page occupies another.
An older version of you may occupy another.
We normally think that the child you once were has disappeared.
The block universe suggests another possibility.
Perhaps that child has not been erased from reality.
They simply exist at a different location in spacetime.
Bournemouth Does Not Disappear When You Leave It
An analogy can make this easier to imagine.
Suppose you travel from Bournemouth to London.
Once you reach London, Bournemouth does not cease to exist.
It is simply somewhere you are not currently located.
You cannot see it from where you are, but it remains part of the geographical landscape.
We do not believe that places spring into existence as we approach them and disappear once we leave.
The block universe asks whether our attitude towards time should be similar.
Perhaps yesterday has not ceased to exist merely because we are no longer located there.
And perhaps tomorrow does not need to spring into existence when we reach it.
Past and future might be more like distant places than we normally imagine.
There is an important difference, of course.
We can turn around and travel back to Bournemouth.
As far as we know, we cannot simply turn around and travel back to yesterday.
The analogy helps us picture eternalism; it does not make space and time identical in every respect.
Eternalism
The philosophical position most closely associated with the block universe is called eternalism.
According to eternalism, past, present and future events are all equally part of reality.
This does not mean they all happen at the same time.
That would contradict the idea.
The Battle of Hastings happened in 1066. You are reading this much later.
Eternalism says that both events belong to spacetime, just as London and Rome both belong to the Earth despite occupying different locations.
From this perspective, the present is not a special feature of the universe.
It may instead be something about our perspective within it.
And that raises perhaps the strangest question of all.
Why Does Time Feel Like It Is Passing?
If the universe is a block, why don’t we experience our entire lives at once?
Why does consciousness seem to occupy a moving present?
You remember being younger.
You experience now.
You anticipate the future.
It feels as though a spotlight called the present is moving forwards along your life.
But where is that moving spotlight in physics?
It is difficult to find.
One possibility is that the sensation of temporal passage arises from consciousness.
Our brains contain records of earlier states — memories — but not comparable records of future states.
Each conscious moment therefore contains information pointing towards what we call the past.
Perhaps this creates the experience of moving from past to future.
But this remains an area where physics, neuroscience and philosophy meet.
Explaining the structure of spacetime is not necessarily the same as explaining why it feels like time passes.
Does the Future Already Exist?
This is where the block universe becomes unsettling.
If eternalism is correct, future events may be just as much part of spacetime as past events.
Your breakfast tomorrow.
A conversation next year.
Events that happen long after you die.
From our present perspective they have not happened yet.
But from the block-universe perspective, “not yet” may describe our location in spacetime rather than the non-existence of those events.
This sounds remarkably like saying that the future is already written.
But we need to be careful.
Does the Block Universe Destroy Free Will?
Suppose tomorrow you choose between tea and coffee.
If tomorrow already exists within the block universe, then whichever drink you choose is already part of spacetime.
Does that mean you never really had a choice?
Not necessarily.
This depends partly upon what we mean by free will.
There is an important distinction between saying:
an event exists at a particular location in spacetime
and saying:
something external forced that event to happen regardless of your decision.
Your decision-making process would itself be part of the block.
Your thoughts, hesitation, reasons and eventual choice would all be events within your worldline.
From one perspective, the choice is part of the four-dimensional structure.
From another, it occurs because you considered the alternatives and made the decision.
Whether that counts as genuine freedom takes us back to the philosophical debate between determinism, libertarian free will and compatibilism.
The block universe does not make that debate disappear.
It makes it stranger.
Is the Block Universe the Same as Determinism?
No.
These ideas are often confused.
Eternalism concerns which times or events exist.
Determinism concerns whether the state of the universe at one time, together with the laws of nature, completely determines what happens at other times.
Those are different questions.
A block universe can represent a history containing events governed by deterministic laws, probabilistic quantum events or perhaps other forms of physical process.
Simply saying that an event occupies a location in spacetime does not by itself explain why that event occurs.
So the block universe should not automatically be translated into “everything was predetermined from the beginning”.
What About Quantum Physics?
Quantum mechanics complicates the picture further.
At very small scales, physics does not resemble a simple clockwork universe in which every future event can straightforwardly be calculated from the past.
Quantum theory describes probabilities and produces deep disagreements about how those probabilities should be interpreted.
Some interpretations involve branching possibilities. Others treat quantum states differently.
Exactly how quantum mechanics should be reconciled with our understanding of time remains part of a much larger debate in fundamental physics.
This is another reason to avoid claiming that Einstein simply proved that the future already exists.
He did not.
Relativity provides powerful reasons for questioning an absolute universal present.
What that ultimately tells us about the reality of past, present and future involves additional philosophical interpretation.
Is the Block Universe Proven?
No.
This distinction matters.
Relativity is extraordinarily well-supported physics.
Time dilation, the relativity of simultaneity and the four-dimensional spacetime framework are not merely imaginative speculation.
The block universe, however, goes further.
It is a way of interpreting what the structure revealed by relativity might mean about the nature of time and existence.
Many philosophers regard eternalism as fitting naturally with relativity.
Others continue to defend versions of presentism or alternative theories of time.
There is also an intermediate idea known as the growing block universe.
According to this view, the past and present exist, but the future does not yet exist.
Reality is like a block continually gaining new slices as new events occur.
That preserves something closer to our intuition that the future is genuinely open.
But it faces its own philosophical difficulties.
There is no universally accepted answer.
And Then There Is Death
This is where Slaughterhouse-Five becomes especially thought-provoking.
The Tralfamadorians regard death very differently from humans.
A person may be dead at one particular moment, but there are countless other moments in which that person is alive.
Something resembling this idea appears in the block-universe picture.
Imagine someone who lives for eighty years.
Their worldline extends across those eighty years of spacetime.
At later times, they are dead.
But if eternalism is correct, does that mean the earlier parts of their life have ceased to exist?
Perhaps not.
The birthday party when they were ten.
Their first love.
A conversation with a friend.
An ordinary Tuesday afternoon when they were forty-three.
All would occupy particular locations within spacetime.
From the eternalist perspective, those events do not become unreal simply because they are earlier than our present experience.
That is a remarkable idea.
But it is important not to claim more than it says.
Is the Block Universe an Afterlife?
No.
The block universe does not provide scientific evidence that consciousness continues after death.
It does not show that a dead person is currently experiencing earlier moments.
It does not establish the existence of a soul.
And it does not imply that we can return to previous moments and meet someone who has died.
Those would be additional claims requiring additional evidence.
What eternalism potentially changes is something subtler.
It challenges what we mean when we say that the past has ceased to exist.
Perhaps a life is not something that appears, exists briefly and is then completely erased from reality.
Perhaps it is a finite structure extending through spacetime.
Birth and death would mark its temporal boundaries.
But the events between those boundaries would remain part of the history — and perhaps the structure — of the universe.
Whether that should provide any comfort is a personal and philosophical question rather than a conclusion of physics.
But it certainly gives us a different way to think about what it means for something to be past.
Vonnegut’s Universe
This helps explain why Slaughterhouse-Five is such an interesting companion to the philosophy of time.
Vonnegut was not writing a physics textbook.
The Tralfamadorian view of time is a literary device within a novel concerned with war, trauma, memory, death and the strange ways human beings try to make sense of terrible events.
Billy does not experience time in the ordinary human way.
His life is not simply a journey from birth towards death.
Its moments remain available to him.
Someone may be dead in one moment and alive in another.
The fictional idea should not be confused with relativity or treated as evidence for eternalism.
But fiction can sometimes do something philosophical arguments struggle to achieve.
It allows us to imagine what a radically different view of reality might feel like.
Once we have imagined time in the Tralfamadorian way, the ordinary assumption that only the present is real suddenly seems less inevitable.
Which Picture Is Correct?
We are left with several possibilities.
Perhaps only the present exists, and reality continually changes as new moments come into existence.
Perhaps the past and present exist, while the future is continually being added.
Or perhaps past, present and future are all parts of a four-dimensional spacetime, and the passage of time belongs more to our experience than to reality itself.
Physics places important constraints on these possibilities.
Philosophy asks what those constraints mean.
And consciousness introduces another mystery entirely.
Why does one moment feel like now?
Why do we remember only one temporal direction?
Why do we experience ourselves moving towards a future we cannot remember?
Perhaps understanding the brain will eventually explain much of this.
Perhaps understanding spacetime more deeply will change the question.
Or perhaps our everyday concepts of past, present and future are simply inadequate for describing reality at its deepest level.
A Different Way to Picture a Life
The block universe offers an extraordinary change of perspective.
Normally we imagine a human life as something gradually disappearing.
Childhood is gone.
Yesterday is gone.
This morning is gone.
Eventually the entire life is over.
But eternalism invites us to imagine a life differently.
Not as a point of existence continually appearing and disappearing, but as a path through spacetime.
Every moment has its location.
The first breath.
The childhood summer.
The ordinary breakfast.
The journey.
The person loved.
The mistake regretted.
The question asked.
The final moment.
Whether this picture ultimately describes reality remains debated.
But once you have imagined your life as a worldline through a four-dimensional universe, it is surprisingly difficult to return completely to the simple idea of a universal clock carrying everything relentlessly from future to present to oblivion.
And that leaves us with an intriguing thought.
Perhaps when we say that a moment has gone, we should ask what exactly we mean.
It is no longer our present.
But does that necessarily mean it is no longer part of reality?
That question takes us directly from the nature of time to another of humanity’s oldest mysteries:
What happens when we die?