A change of pace this week folks, let’s take a look at one of the more far-out theories that has come from Einstein’s theories. Have you ever woken up in the morning and wondered what actually happened to yesterday? Not what happened yesterday, but what happened to it now that it is over and in the past. If you haven’t, take a moment now and consider a simple question: Where is yesterday? Where did it go?

To the majority of us, the answer is instinctively simple. It has gone, it’s over, yesterday has vanished into the past, tomorrow has not yet arrived, and only the present moment is real, what we are doing in the now exists. This is a view that is so deeply woven into human experience that it feels beyond dispute. That there is no other way to answer that question. But, there is another theory, and it is one of the most intriguing ideas to emerge from modern physics, and this theory suggests that intuition may be wrong.

According to the Block Universe Theory, also known as Eternalism, the past has not  actually disappeared, the future is not awaiting creation, and the present is not uniquely real. Instead, all moments in time, every heartbeat, every supernova, every historical event, and every future conversation, exist together within that vast four-dimensional structure we know as spacetime. If this is true, the universe is not a story being written as it expands and events unfold naturally. In actuality, it is a completed manuscript.

To understand the Block Universe, imagine a DVD containing your favourite movie. On that DVD, every scene exists simultaneously, the opening scene, the climax, and the ending are all physically present and the characters experience events in sequence, but the entire film already exists. Supporters of the Block Universe argue that reality may be similar. Rather than viewing reality as a three-dimensional universe moving through time, physicists describe the universe as a four-dimensional spacetime, where time is treated as another dimension alongside length, width, and height. In this picture, your birth exists, your current moment exists, and your future exists, all these events occupying different locations within spacetime itself, and the sensation time is flowing may simply be the way our consciousness experiences moving through this structure.

Now then, a common misconception is that the Block Universe somehow contradicts Einstein’s theories, whereas in reality, many physicists believe the idea arises naturally from relativity.

This is a principle known as the relativity of simultaneity. I’ve mentioned this in my Special Relativity article remember? It’s the throwing the ball on the moving train analogy. This is the core concept from that theory that states that whether two separate events happen at the same time depends on the observer’s point of view, that is, two events that look simultaneous to one person can happen at different times for another person in motion. Einstein’s Special Relativity shows that observers moving relative to one another can disagree about which distant events are occurring “right now.” There is no universal cosmic clock that defines a single objective present for the entire universe.

Let’s consider two observers moving differently through space. Now then, where one observer may regard two distant events as simultaneous, another observer may calculate that one event happened earlier than the other. Neither perspective is privileged as both are permitted by the laws of physics. However, this then creates a profound problem for the everyday notion of “the present.” If there is no universally agreed-upon present moment, then what exactly separates the real present from the unreal future?

Many philosophers and physicists argue that the simplest answer to this conundrum is that all moments exist equally, and that the distinction between past, present, and future becomes a matter of perspective rather than the fundamental reality.

Einstein himself famously remarked that the distinction between past, present, and future is “a stubbornly persistent illusion,” a statement often associated with block theory enthusiasts as  an interpretation of spacetime. Importantly, though, Einstein’s equations do not explicitly state that the Block Universe is true. Rather, they describe spacetime in a way that many researchers believe is compatible with, and perhaps even suggestive of, a block-like reality

Now it’s time for the pesky teeny tiny stuff to make its appearance as quantum mechanics makes its entry, and as friendly as relativity may appear to the Block Universe, quantum mechanics brings with it complications. 

Quantum theory describes a world fundamentally different from the clockwork universe envisioned by classical physics. We know, from my two-parter, that particles can exist in superpositions of states, where outcomes often appear probabilistic rather than predetermined. Entangled particles exhibit correlations that seem astonishingly non-local. With this in mind, quantum mechanics appears to threaten the entire Block Universe concept.

If the future already exists, how can quantum events be genuinely uncertain? And the answer to that question depends on which interpretation of quantum mechanics you adopt.

First off, we have the deterministic view, and that is that some interpretations, such as the Many-Worlds Interpretation, preserve a deterministic mathematical evolution of the quantum wave function. In these interpretations, reality may still be represented as a vast block-like structure, although it is one that is far stranger than originally imagined.

Next, we have the indeterministic view, which is that other interpretations treat quantum events as involving genuine randomness. Here the future may not be completely fixed until measurements occur, and some physicists see this as direct evidence against a fully formed Block Universe.

And finally, we have the current situation, which is that in actuality, there is no consensus as modern physics has not yet produced a universally accepted synthesis of quantum mechanics and general relativity. It is because of this that the relationship between quantum theory and the Block Universe remains an active area of investigation rather than a settled question.

So, in short, quantum mechanics does not definitively rule out the Block Universe, but it doesn’t conclusively prove it, either.

That leads us on to the problem of free will, and no consequence of the Block Universe generates more debate than its apparent attack on free will. If your future already exists, are your choices truly choices?

Let’s suppose that somewhere within the four-dimensional structure of spacetime there already exists a future version of you deciding whether to accept a job, move cities, or buy that really fast car you want.

Has this decision already been made? If you think about it, at first glance, the answer appears somewhat disturbing, and many critics argue that a Block Universe implies a fixed future and therefore eliminates genuine freedom. That is, if every event is already part of spacetime, then the sensation of choosing may merely be an experience within the block.

However, defenders of free will offer several responses.

Compatibilists argue that free will does not require the future to be open as a decision can still be free if it arises from your values, beliefs, reasoning, and desires, even if that decision is a permanent feature of spacetime. In this view, your future choice exists, but it is still your choice.

Another argument arises from the observer’s perspective. This is because while the future may exist, you don’t have access to it. From within time, uncertainty remains unavoidable, and you deliberate, and evaluate options, and make decisions exactly as before. The difference here is that the existence of future events in spacetime would not grant anyone knowledge of them.

Then we have the libertarian objections. Philosophers who believe genuine free will requires alternative possibilities often reject the Block Universe entirely. They argue that a future that already exists cannot be meaningfully changed and therefore cannot accommodate true freedom.

And like many of these conflicting theories, the debate remains very much unresolved.

One reason the Block Universe fascinates both scientists and science-fiction writers (I have just completed a novel using this very theory as the premise) is that it appears naturally compatible with some forms of time travel. If all moments exist, then travelling to another time may be conceptually similar to travelling to another place. And that is an excellent tool for writing into fiction as General Relativity contains solutions to Einstein’s equations that permit unusual spacetime geometries. Although whether such structures could exist physically obviously remains, like most things in this field, uncertain. That said, the Block Universe does not automatically enable time travel, but it provides a conceptual framework in which the idea becomes easier to imagine.

The Block Universe is not merely a theory about clocks and calendars. It is a new view of reality, as it challenges some of humanity’s most fundamental assumptions. Is time really flowing? Does the future already exist? Is consciousness responsible for our experience of temporal passage? Can free will survive in a universe where every moment is equally real? How does quantum uncertainty fit into the picture?

And once again, my faithful few, more than a century after Einstein revolutionised our understanding of space and time, these questions remain very much unanswered.

Perhaps time truly flows, and the future has yet to be written. Or perhaps, at this very moment, every chapter of your life already exists within an immense four-dimensional cosmic structure, not waiting to happen, but simply waiting to be experienced.