B-theory of time

Based on Wikipedia: B-theory of time

The Past Is Not Gone, and the Future Already Exists

Here's a thought that might keep you awake tonight: What if the year 1850 is just as real right now as this present moment? What if dinosaurs still exist—not in some metaphorical sense, but actually exist, just at a different location in time the way Paris exists at a different location in space?

This is the radical claim of the B-theory of time. And before you dismiss it as philosophical navel-gazing, you should know that this view has powerful support from physics. Einstein's theory of relativity, which has been confirmed by countless experiments, treats time almost exactly this way.

But let's back up. The question of what time actually is has been debated since ancient Greece, and the debate continues in philosophy departments and physics labs today. There are essentially two camps, and their disagreement cuts to the heart of how we understand reality itself.

The Two Theories: A and B

In 1908, a Cambridge philosopher named John McTaggart Ellis McTaggart (yes, that was really his name—his friends called him "McT") published a paper that would shape philosophical discussions of time for the next century. He noticed that we describe events in time in two fundamentally different ways.

The first way uses what he called the A-series. This is how we normally think about time. Events are past, present, or future. Your breakfast this morning is past. Reading this sentence is present. Your dinner tonight is future. These properties change: what's future becomes present, then slides into the past. Time flows.

The second way uses the B-series. Here, events have fixed relationships to each other: earlier than, simultaneous with, or later than. Your breakfast is earlier than your dinner. The moon landing in 1969 is later than the Declaration of Independence in 1776. These relationships never change. The moon landing will always be later than the Declaration of Independence, no matter how much time passes.

McTaggart thought both ways of describing time were ultimately incoherent and concluded that time itself was unreal—a conclusion most philosophers reject. But his distinction between A-series and B-series stuck around, and philosophers eventually organized themselves into two camps: A-theorists and B-theorists.

What B-Theorists Actually Believe

B-theorists make a startling claim: the flow of time is an illusion.

Not a small illusion, like thinking the sun revolves around the Earth. A massive, pervasive illusion that shapes every moment of your conscious experience. When you feel time passing, when you sense the present moment arriving and then slipping away, when you feel yourself moving from past to future—none of that reflects objective reality. It's all happening in your head.

According to B-theory, the universe is more like a block than a flowing river. Imagine a loaf of bread. Each slice represents a moment in time. The entire loaf exists all at once. There's nothing special about any particular slice. The slice we call "now" isn't ontologically privileged—it doesn't exist any more or less than slices in the past or future.

This is sometimes called eternalism, or the "block universe" view. Every moment in history, from the Big Bang to whatever happens billions of years from now, exists in a kind of eternal present. We don't move through time; we're extended through it, the way we're extended through space.

Why Would Anyone Believe This?

The appeal of B-theory comes largely from physics. Einstein's special theory of relativity, published in 1905, showed that simultaneity is relative. Two events that happen at the same time for one observer might happen at different times for another observer moving at a different speed. There's no universal "now" that everyone shares.

This creates a problem for the A-theory. If the present is supposed to be objectively special—if it's the cutting edge of reality where the future becomes the past—then which present are we talking about? My present? Yours? An astronaut's moving at high speed? They're all equally valid from the perspective of physics.

B-theory dissolves this problem elegantly. If there's no objective flow of time, if all moments are equally real, then it doesn't matter that different observers disagree about simultaneity. They're just disagreeing about how to slice the block.

Philosophers like David Hugh Mellor and John Jamieson Carswell Smart have championed B-theory for decades. They argue that we should translate all our talk about past, present, and future into tenseless language. Instead of saying "the sun is shining now," we should say "the sun shines on December 29th, 2025." The first sentence seems to describe something happening in the moment; the second describes an eternal fact about where events are located in the block.

The Ancient Roots of This Debate

This argument is older than Christianity, older than the Roman Empire, older than written philosophy itself.

On one side stood Heraclitus, the Greek philosopher known for his doctrine that everything flows. "You cannot step into the same river twice," he famously said, because by the time you step in again, both you and the river have changed. For Heraclitus, change and flux are the fundamental features of reality. Nothing stays the same; everything is in constant motion.

On the other side stood Parmenides, who argued that change is impossible and reality is timeless and unchanging. What exists, exists. What doesn't exist, doesn't exist. And something can't come from nothing or turn into nothing. Therefore, nothing ever really changes. Our perception of change must be some kind of illusion.

B-theory sides with Parmenides. The apparent flow of time, the sense that the present moment is special, the feeling that the future is open while the past is fixed—all illusions. A-theory sides with Heraclitus. Time really does flow. Things really do change. The present is genuinely different from the past and future.

It's remarkable that we're still having this argument 2,500 years later. But perhaps that shouldn't surprise us. The nature of time may be one of those deep questions that the human mind can formulate but not definitively answer.

The Problem of "Thank Goodness That's Over"

The philosopher Arthur Prior offered one of the most intuitive objections to B-theory. Imagine you've had a terrible headache. The throbbing pain has been with you for hours. Finally, it subsides. You feel the sweet relief of its absence and think: "Thank goodness that's over."

What exactly are you relieved about?

Prior argued that on the B-theory, this relief is inexplicable. The headache exists at a certain region of the spacetime block. It still exists there. It will always exist there. The headache isn't gone in any objective sense—it's just located at times earlier than your current utterance.

Are you relieved that the headache is earlier than this moment? That seems bizarre. You're not specifically thinking about the relationship between your headache and your current utterance. You're relieved that the headache is in the past—that it's over, done, no longer happening.

But "over" and "no longer happening" are tensed concepts. They only make sense if the flow of time is real, if the present moment has special significance. On the B-theory, all you can say is that the headache occupies a different temporal location than your thought of relief. But why should that spatial-like separation make you feel better?

B-theorists respond that the asymmetry in our attitudes toward past and future headaches reflects our psychological makeup, not the metaphysical structure of time. We remember the past and anticipate the future because of how our brains work, because of the thermodynamic arrow of time, because of how information propagates. But this psychological asymmetry doesn't prove that time itself has direction or flow.

It's a coherent response. But many philosophers find it unsatisfying. Our relief that bad things are over seems too fundamental, too deeply woven into our experience, to be explained away as mere psychology.

The Problem of Persistence

Here's another puzzle for B-theorists. How do objects persist through time?

You are the same person who started reading this essay a few minutes ago. Your grandmother, if she's still alive, is the same person who was born decades ago. Objects endure through time while undergoing change. An apple starts fresh and becomes rotten. A caterpillar becomes a butterfly. A child becomes an adult.

There are two main philosophical accounts of how this works: endurantism and perdurantism.

Endurantism says that objects are wholly present at each moment of their existence. When you look at your hand, all of you is there—not just a temporal slice, but the complete, whole you. The same complete you existed yesterday and will exist tomorrow.

Perdurantism says that objects are spread out through time the way they're spread out through space. Just as you have spatial parts (your hand, your head, your foot), you also have temporal parts (you-yesterday, you-today, you-tomorrow). What we call "you" is really a four-dimensional spacetime worm, extended through time.

B-theory tends to pair naturally with perdurantism. If time is like space, if all moments are equally real, then it makes sense that objects would extend through time the way they extend through space.

But perdurantism has some strange implications. Consider a simple experience: hearing a bird call out "Bob White." You hear "Bob," then you hear "White," and you have a unified experience of hearing the whole call. The philosopher Roderick Chisholm pointed out that on endurantism, there's one thing—you—that first hears "Bob" and then hears "White." But on perdurantism, there's you-at-time-1 who hears "Bob" and you-at-time-2 who hears "White." These are different temporal parts. So what creates the unified experience?

Similarly, consider René Descartes, the famous philosopher who lived from 1596 to 1650. On perdurantism, Descartes is a four-dimensional entity extending through those 54 years. But what if Descartes had died in childhood? That shortened Descartes would have almost entirely different temporal parts. Would it really be the same person? On endurantism, we can say that the same whole Descartes could have lived longer or shorter. On perdurantism, changes in temporal extent seem to create an entirely different object.

Relativity Makes Things Worse

The spacetime interpretation of Einstein's relativity adds another wrinkle. In relativity, what counts as simultaneous depends on your frame of reference. If you're moving relative to me, we'll disagree about which events are happening "at the same time."

For endurantism, this creates a problem. If an object is wholly present at each moment, which moment are we talking about? In my reference frame, the object exists as a whole at time T. But in your reference frame, moving relative to mine, that same object might have parts at different times—it's sliced differently by your "now." So is the object wholly present or not? It depends on who's asking, which seems odd for a fact about the object itself.

B-theory handles this better. If objects are four-dimensional worms extended through spacetime, then different observers are just slicing the same worm at different angles. The worm itself doesn't care how you slice it.

This is one reason physicists often find B-theory more natural than A-theory. Relativity seems to demand that we treat time and space symmetrically, and B-theory does exactly that.

Can We Eliminate Tense?

B-theorists want to show that tensed language is not fundamental—that we can say everything we need to say about time using tenseless language. "The sun is shining now" becomes "The sun shines on December 29th, 2025." "I was tired yesterday" becomes "I am tired on December 28th, 2025."

The philosopher Quentin Smith has argued this translation project fails. Consider the word "now." Can we replace it with a date?

The problem is that dates are relative to events. What makes December 29th, 2025 the date it is? It's defined by its relationship to other events—25 years after 2000, 54 years before 2079, and so on. These are B-relations, but they don't capture the indexical meaning of "now." When I say "now," I'm not just describing a position in the temporal order. I'm picking out the present moment from my perspective.

Maybe we can define "now" as "simultaneous with this utterance"? But consider the thought "I am not uttering anything now." This thought can be true—if you're thinking it silently. But "I am not uttering anything simultaneous with this utterance" is self-contradictory. So these can't mean the same thing.

B-theorists typically respond that the untranslatability of tensed sentences into tenseless ones doesn't prove that tense is metaphysically fundamental. Language is shaped by our psychology, and our psychology evolved to track tensed facts because they're practically important. But practical importance isn't the same as metaphysical fundamentality.

The View From Nowhen

Imagine you could step outside of time entirely and look at the universe from a timeless perspective—what philosophers sometimes call the "view from nowhen."

From this perspective, B-theorists argue, you would see all of history laid out at once. The Big Bang at one end, the heat death of the universe at the other, and everything in between. You wouldn't see any moment as specially "present" because you'd be outside the temporal order altogether.

Many B-theorists think this is how God (if God exists) would see things, or how a complete physics would describe reality, or how a perfectly rational being would understand the universe. The A-theorist's flowing time is a feature of experience, not reality.

A-theorists find this picture deeply unsatisfying. Sure, you can imagine stepping outside time, but that's a fantasy. In reality, we're always in time. The present is always with us. And our experience of temporal flow is so fundamental, so utterly pervasive, that explaining it away as mere illusion seems like explaining away the phenomenon we're trying to understand.

First-Person Perspectives and the Mystery of Now

The philosopher Vincent Conitzer has recently connected the debate about time to questions about consciousness and personal identity. He argues that the B-theory has trouble accounting for first-person perspectives.

Why is it now now? Why am I me? These questions seem strange, maybe even meaningless. But there's something genuinely puzzling about them. Of all the moments in history, this one has the special property of being present. Of all the conscious beings in the universe, I have the special property of being me. These seem like brute facts that resist explanation.

Conitzer suggests that arguments for A-theory are strengthened when combined with the view that the first-person perspective is metaphysically privileged. Just as my present moment is special to me, my perspective is special to me. These might be connected features of reality, not just quirks of psychology.

The philosopher Caspar Hare has developed related ideas under the name "egocentric presentism"—the view that the present is real in a way the past and future are not, and that this connects to the special status of one's own perspective. These views are speculative and controversial, but they suggest that the A-theory versus B-theory debate connects to some of the deepest questions in philosophy of mind.

What Would It Mean If B-Theory Were True?

Let's take B-theory seriously for a moment. What would follow?

First, death would be different than we think. If B-theory is true, you don't cease to exist when you die. Your life is a four-dimensional region of spacetime. After death, there's no more of you, but the you that exists continues to exist eternally at earlier times. In a sense, you're immortal—not because you continue forever into the future, but because you exist eternally at the times you occupy.

Second, the past is not lost. Your childhood, if B-theory is true, is still happening. The people who have died still exist at their times. History isn't gone; it's just located elsewhere in the block.

Third, the future already exists. Free will might need to be reconceived. If the future is as fixed as the past, what room is there for genuine choice? B-theorists typically argue that this doesn't undermine free will properly understood—your future choices are yours, they're just located at future times rather than emerging from an open present.

Fourth, change itself is reconceived. Nothing really changes on the B-theory; things just have different properties at different times. The apple doesn't become rotten; the earlier parts of the apple are fresh while the later parts are rotten. This is "Cambridge change" rather than genuine change—similar to how a person can change from being taller than their child to shorter without anything happening to them, just by the child growing.

These implications strike many people as deeply counterintuitive. But B-theorists would say that's just our psychological bias showing. We evolved to track tensed facts because they're practically important. But the universe doesn't care about our intuitions.

Living With Uncertainty

After 2,500 years of debate, from Parmenides and Heraclitus through McTaggart and Prior to contemporary physicists and philosophers, we still don't know whether time really flows or whether its passage is the most elaborate illusion our minds create.

Physics leans toward B-theory but doesn't conclusively establish it. Some interpretations of quantum mechanics might restore a privileged present, though this is controversial. Philosophy offers powerful arguments on both sides. Our intuitions scream that A-theory must be right, but intuitions have been wrong before.

Perhaps the most honest position is humble uncertainty. The nature of time is one of those questions where confident answers should make us suspicious. The universe might be a flowing river or a frozen block or something else entirely that we lack the concepts to describe.

What we can say is that the question matters. If time doesn't flow, then everything that has ever happened or will ever happen exists eternally. If time does flow, then the present is the cutting edge of reality, the only moment that truly is. These are radically different pictures of our place in the cosmos.

And perhaps, in the end, the fact that we can even ask this question—that conscious beings embedded in time can step back and wonder about its nature—is the most remarkable thing of all.