Does the Future Already Exist? The Growing Block Universe Explained

On the twenty-first of September, 1908, in a lecture hall in Cologne, a forty-four-year-old mathematician named Hermann Minkowski stood before the Eightieth Assembly of German Natural Scientists and Physicians and delivered a sentence that would, over the next century, dismantle one of humanity’s most basic intuitions about reality. Space by itself and time by itself, he said, were doomed to fade away into mere shadows. Only a kind of union of the two would preserve an independent reality.

Three and a half months later, on the twelfth of January 1909, Minkowski was dead. Appendicitis. He never lived to see the geometric framework he had just unveiled, what we now call the Block Universe, become the standard interpretation of Einstein’s relativity for the next century.

In the Block Universe, time is not a flowing river. Time is a dimension, like the three dimensions of space. All moments are equally real. Your birth, this sentence, your death, the death of the universe trillions of years from now, all of them sit at coordinates in a single four-dimensional manifold. Albert Einstein, in his last years, wrote to the grieving family of his oldest friend Michele Besso that for believing physicists, the distinction between past, present, and future is only a stubbornly persistent illusion. Eighteen days later, Einstein himself was dead.

But the Block Universe carries a consequence that has troubled philosophers for a century. If the future is already real, in what sense is it open? In what sense do your choices matter? In what sense are you the author of your own life rather than a four-dimensional shape stretched across spacetime whose every detail has been set from the moment of the Big Bang?

There is a minority position in the philosophy of time developed precisely to answer this anxiety. It was first articulated in 1923 by a Cambridge philosopher named C.D. Broad. It has been defended in modern times by Michael Tooley, Peter Forrest, Storrs McCall, and Dean Zimmerman. It is called the Growing Block Universe, and its central claim is that the future is not part of the four-dimensional manifold at all. The future, on this view, is literally not there. The universe grows.

This is the case for the Growing Block. The case against. And the deeper question both sides share.

C.D. Broad, 1923: the future as empty space

Charlie Dunbar Broad was a Cambridge philosopher, born in 1887, who spent his career working through the philosophical implications of the new physics that had emerged between 1905 and the early 1920s. In 1923 he published Scientific Thought, a book that has been almost entirely forgotten outside specialist circles but contains one of the most striking metaphysical proposals in the philosophy of time.

Broad accepted the four-dimensional manifold of Einstein and Minkowski. He accepted that the past is real. But he refused to extend the manifold into the future. The future, Broad argued, is not yet anything. It has no ontological status. As time passes, new slices of reality come into being, are added to the leading edge of the manifold, and become part of the permanent past. The universe is not eternal. It is growing.

The metaphor Broad used was geological. The past accretes like sediment, layer by layer. The present is the surface where new material is being laid down. The future is the empty space above that surface, into which the universe has not yet grown.

This is the Growing Block. The name comes later. The idea is Broad’s.

What the Growing Block actually claims

The Growing Block makes three distinct claims that are easy to confuse and important to separate.

First, the past is real. Every event that has happened is still there at its coordinates in the four-dimensional structure. Napoleon’s coronation, the building of the pyramids, the formation of the solar system, all of them remain as features of reality. The past does not vanish.

Second, the present is privileged. There is an objective leading edge of the manifold, a hyperplane of the most recent slice, where the universe is actually growing. The present is not a feature of perspective. It is a structural feature of reality.

Third, the future does not exist. Not yet. The events that will happen tomorrow are not anywhere. They have no coordinates. They are not represented by any structure in the manifold. They will exist when the universe grows into them. Until then, they are nothing.

Compare this to the two alternatives. Presentism says only the present exists. The past has ceased to be. The future has not yet come to be. Reality is a knife-edge. Eternalism, the Block Universe view, says all of time is equally real. Past, present, and future are all there in the manifold, all of them with the same ontological status. The Growing Block is the middle position. It splits the difference. The past is real, the present is privileged, the future is unreal.

This is the position that Michael Tooley defended in his 1997 book Time, Tense, and Causation from Oxford University Press, that Storrs McCall defended in his 1994 book A Model of the Universe from Oxford, and that Peter Forrest has refined through papers in the early 2000s. It is not an idle thought experiment. It is a serious position in contemporary metaphysics.

The argument from causation

Tooley’s defense rests on a single claim about the structure of causation. Causation is asymmetric. Effects depend on their causes. Causes do not depend on their effects. This asymmetry, Tooley argues, is not a feature of how we describe the world but a feature of the world itself.

Now ask what could ground that asymmetry. In the Block Universe, all events sit at fixed coordinates. The relation between a cause and its effect is just a relation between two points in the manifold, neither of which has any priority over the other. The manifold contains the cause and the effect, both equally real, both equally fixed. There is nothing in the geometry that distinguishes the cause as the active partner.

The Growing Block grounds the asymmetry naturally. Causes exist before effects because at the moment a cause occurs, the effect is literally not there yet. The cause produces the effect by contributing to the next slice of the manifold. Causation runs from the existing past into the unmade future. The asymmetry of time is the asymmetry of causation, and both reflect a single underlying fact: reality grows in one direction.

This is the strongest positive argument for the Growing Block. It does not appeal to intuition. It appeals to a precise metaphysical claim about the structure of cause and effect, and argues that the Growing Block is the only ontology that can support it.

The Rietdijk-Putnam attack

The dominant argument against the Growing Block comes from special relativity itself.

In 1966, the Dutch philosopher C. W. Rietdijk published a paper in Philosophy of Science titled “A Rigorous Proof of Determinism Derived from the Special Theory of Relativity.” The following year, the American philosopher Hilary Putnam published “Time and Physical Geometry” in the Journal of Philosophy. The two papers, taken together, are now called the Rietdijk-Putnam argument.

The argument proceeds in three steps. First, relativity tells us that simultaneity is frame-dependent. Two observers in relative motion will disagree about which distant events are happening at the same time as their current moment. Second, this means observers in different states of motion will have different sets of events they consider real if reality is identified with the present hyperplane. Third, this is incoherent. Reality cannot depend on who is looking at it.

Roger Penrose later gave the argument a vivid form now known as the Andromeda Paradox, in his 1989 book The Emperor’s New Mind. Two people walk past each other on a street, both glancing at the Andromeda Galaxy 2.5 million light-years away. Their hyperplanes of simultaneity, when extended to Andromeda, differ by approximately one day. To one of them, an alien fleet has already launched from Andromeda. To the other, the launch has not yet happened. Both stand at the same point on Earth. Both use the same physics. If the future is unreal, which of them is right?

The Block Universe defender has an easy answer. Both walkers are correct. Neither hyperplane is privileged. The launch is real because everything in the manifold is real. The disagreement about temporal order is the natural consequence of a structure in which no slice has metaphysical priority.

The Growing Block defender does not have an easy answer. The Growing Block requires that one of those hyperplanes is the real present. But relativity says there is no privileged frame from which we could pick one. Either the Growing Block must give up the requirement of an objective present, or it must abandon relativity in favor of a neo-Lorentzian alternative with a hidden preferred frame.

This is the central pressure on the Growing Block view, and it has shaped the entire defense for the last half-century.

The neo-Lorentzian escape

Some Growing Block defenders bite the bullet. The Cambridge-trained philosopher William Lane Craig, the late Australian philosopher Quentin Smith, and several others have argued that special relativity may not tell us the final story about the metaphysics of time. The position is called neo-Lorentzianism, after the Dutch physicist Hendrik Lorentz, whose transformations Einstein incorporated into relativity but whom Lorentz himself interpreted differently.

Lorentz believed there was a preferred reference frame, the rest frame of the luminiferous ether, but that the laws of physics conspire to make it undetectable from within. Length contraction, time dilation, and the relativity of simultaneity are all real effects, but they are effects of motion relative to the privileged frame, not consequences of there being no privileged frame at all. On Lorentz’s reading, the privileged frame exists. Physics just cannot find it.

The neo-Lorentzian defender of the Growing Block argues that the privileged frame is whichever frame the universe is actually growing in, and that the failure of physics to detect it is an epistemic limitation rather than a metaphysical fact. Some neo-Lorentzians point to the cosmic microwave background as an empirical candidate. The cosmic microwave background, discovered by Arno Penzias and Robert Wilson in 1964 and confirmed by COBE in 1992, WMAP in 2003, and Planck in 2013, has a frame in which it is isotropic, the same temperature in every direction. Earth, the solar system, and the Milky Way are all moving relative to this frame at roughly 600 kilometers per second. The cosmic microwave background frame is, in a precise sense, empirically distinguished.

The standard relativist response is that the cosmic microwave background frame is empirically distinguished but not metaphysically privileged. It is one of infinitely many inertial frames in which the universe happens to look isotropic. The laws of physics do not pick it out. Calling it the real present is a metaphysical assertion that goes beyond what physics can support.

The neo-Lorentzian Growing Block defender concedes this and argues that metaphysics is not exhausted by physics. The defense is coherent but expensive. It requires committing to an undetectable preferred frame, accepting that the deepest insight of special relativity is misleading, and giving up parsimony for the sake of preserving an intuition about temporal becoming. Most working physicists consider the cost too high.

The Epistemic Objection: how do we know it is now now?

A different challenge to the Growing Block comes not from physics but from epistemology. In 2004, the Australian philosopher David Braddon-Mitchell published a paper in Analysis titled “How Do We Know It Is Now Now?” The argument is short, sharp, and deeply uncomfortable.

If the past is real, and if past slices of the manifold contain conscious observers as detailed and complete as we are, then those observers must also believe themselves to be at the present. Napoleon, in 1804, was conscious. He believed his moment was the present. He was wrong, of course, because the present has now moved on. But at the time, his evidence was indistinguishable from ours.

We have no way to verify that we are at the actual leading edge of the manifold. Our evidence is identical to what a past observer would have. If past slices vastly outnumber the present slice, and they do, since the universe has been growing for around 13.8 billion years and the present is only one slice thick, then we should expect to find ourselves at a past moment with overwhelming probability. By the Growing Block defender’s own framework, the probability that we are at the actual present is vanishingly small.

This is the Epistemic Objection. It does not say the Growing Block is incoherent. It says the Growing Block makes it irrational to believe we are at the present, which seems to be a claim everyone accepts.

Peter Forrest, in a 2004 paper titled “The Real but Dead Past,” developed the standard reply. The past, Forrest argued, is real but causally inert. Past consciousnesses are not currently occurring. They existed, but they are not active. Only the present-slice consciousness is currently happening. So when we ask “am I at the present?”, the answer is yes, because we, the conscious entities asking the question, are currently happening, which means we are at the growing edge.

The reply is ingenious. It is also costly. It requires distinguishing between being real and being currently active, a distinction that has no obvious physical meaning. Past observers are real enough to count as observers but not real enough to currently exist. Trenton Merricks and others have argued that this is metaphysics by stipulation rather than by argument.

The debate continues. A 2025 paper in Theoria by James Ewing argues for what he calls mind-independent and non-dynamic passage of time, a sophisticated attempt to capture what is right about the intuition of becoming without committing to a Growing Block ontology. Samuele Iaquinto’s 2024 essay in IAI TV, “Shattering Einstein’s Block Universe,” articulates the contemporary case for genuine temporal becoming. The field is more active in 2025 than it has been in decades.

The view from quantum gravity

A different challenge to both the Block Universe and the Growing Block comes from fundamental physics. The Wheeler-DeWitt equation, formulated by John Wheeler and Bryce DeWitt in the late 1960s and published in DeWitt’s 1967 Physical Review paper, describes the quantum state of the universe as a whole in canonical quantum gravity. The equation has no time variable. The Hamiltonian operator applied to the wave function of the universe equals zero.

At the deepest level of physics, time may not be a fundamental feature of reality at all. Time might be emergent, arising from the dynamics of physical systems but not present in the basic equations of the universe. This is the position defended by Julian Barbour in his 1999 book The End of Time and by Carlo Rovelli in his 2018 book The Order of Time. Barbour argues that the universe is fundamentally a structure of timeless configurations, and that time is just a way of describing relations among these configurations.

In contrast, Lee Smolin, in his 2013 book Time Reborn, defends the opposite position. Time is fundamental, Smolin argues, and the laws of physics themselves evolve over cosmic history. The Block Universe interpretation has misled physics for a century. Tim Maudlin, in his 2012 book Philosophy of Physics: Space and Time from Princeton, defends a position close to Smolin’s: the passage of time is a fundamental, primitive feature of the world.

The debate is unsettled. Working physicists are divided. The Block Universe remains orthodox among general relativists and cosmologists. The Growing Block remains a minority view among analytic philosophers. The timeless-fundamental-physics position is taken seriously by a sub-community of quantum gravity researchers. The time-fundamental position is defended by another sub-community.

What this does not settle

The Growing Block view was developed to answer a specific anxiety. If the Block Universe is correct, then the future is already real, your choices are already made, and the openness you feel when you deliberate is an illusion. The Growing Block was meant to restore that openness by removing the future from the manifold entirely.

It does not work. Or rather, it does not work by itself.

The reason is this. On the Growing Block view, the future is not real until it comes into being. But when a moment comes into being, what determines what it is like? Two answers are possible. The first answer is that the new moment is fully determined by the prior history of the universe plus the laws of physics. In this case the universe is deterministic, and the Growing Block view does not make the future more open than it is on the Block Universe view. It just delays the actualization of an already-fixed sequence. The second answer is that the new moment is not fully determined by prior history plus laws, that there is genuine novelty, genuine metaphysical creativity. In this case the universe is non-deterministic, and the Growing Block view captures a real metaphysical fact about the world.

The second answer requires positing genuine non-determinism, and that is a substantive metaphysical claim that goes beyond the Growing Block view itself. The Growing Block, by itself, does not establish that the universe is non-deterministic. It just says that the future has not yet come into being. Whether that future, when it does come into being, is determined by the past plus the laws is a separate question.

Theodore Sider, in his canonical defense of four-dimensionalism, makes the symmetric point in the other direction. A Block Universe can be deterministic. A Block Universe can also be non-deterministic. Either way, the manifold is real. Both versions are internally coherent.

So the choice between Block Universe and Growing Block is not, finally, the choice between determinism and openness. It is a choice about whether to read the four-dimensional manifold as eternally existing or as growing. The deeper question about openness is independent of this choice.

This is the part both sides share. Both the Block Universe and the Growing Block assume the universe is metaphysically self-contained, that the question is the future real has its answer entirely inside the four-dimensional manifold. The metaphysics of time, on both views, can be settled by examining the structure of spacetime in isolation.

That assumption is the deeper question. It is the assumption that the classical theistic tradition, beginning with Augustine’s Confessions Book XI around the year 400 and developed through fifteen centuries of philosophy, has been calling into question for a very long time. Whether the assumption is correct, and what follows if it is not, is the subject of the companion documentary on the Sleepy Joe Space YouTube channel.

What stays

What stays from this exploration is a clearer view of the landscape. The Block Universe is the natural reading of Einstein’s relativity, dominant among working physicists, defended on relativistic grounds by Rietdijk, Putnam, Penrose, Sider, and many others. The Growing Block is a minority but serious position, defended on causation-asymmetry grounds by Tooley, on branching-future grounds by McCall, on dead-past grounds by Forrest, and on epistemic-becoming grounds by recent papers in 2024 and 2025. Both views face a deeper challenge from quantum gravity, where the Wheeler-DeWitt equation and the work of Barbour and Rovelli suggest that time itself may be emergent rather than fundamental.

Whichever view is correct, the anxiety that drove Broad in 1923 to articulate the Growing Block remains. It is not finally an anxiety about whether the future exists in the manifold. It is an anxiety about whether the universe could have been different from how it is, about whether your choices contribute to the structure or merely reflect it, about whether reality is contingent or necessary.

The geometry of spacetime, whether Block or Growing Block, is one question. What grounds that geometry is another. The first is the question physics has been asking for a century. The second is the question both views have been postponing.

If the future does not yet exist, your tomorrow is empty space waiting to be created. If the future already exists, your tomorrow is a place you have not yet been. Either way, the more interesting question is who, or what, makes either of those possibilities actual.