Block Universe

Ever since the advent of the special theory of relativity, we have been able to raise numerous philosophical and existential questions. According to special relativity, our conventional ideas about the past, present, and future can be seen as somewhat solipsistic---*in fact, there may be no absolute past, no absolute present, and no absolute future.* It might sound like madness, but this is one of the profound implications of special relativity: there is no absolute time. Indeed, what we call "time" is largely a concept that is biologically, socially, and culturally constructed rather than an absolute, universal entity.

The Relativity of Simultaneity

The relativity of simultaneity tells us that no single event is more "real" in one frame of reference than in another. In fact, the sequence in which events occur, as perceived in one frame of reference, is just as valid as the sequence perceived in another frame of reference. There is no ultimate or absolute ordering of events in the universe.

To illustrate this, consider a thought experiment involving a futuristic space train and a space station:

Let's imagine we find ourselves at the start of the 22nd century. Humanity has achieved such a high level of technological sophistication that interplanetary (and interstellar) travel is commonplace using "space trains." These space trains can travel from Point A to Point B at speeds up to half the speed of light (0.5c), relative to a person standing still on a nearby planet.

Now, let's consider two scientists, Alexander and Frank, who plan to travel from the planet Sielea (A) to the planet Niapra (B) aboard one of these space trains to conduct an experiment. We will act as external observers, controlling their journey from a vantage point far away, off to the side of the tracks that run between the two planets. From our perspective, the train moves from left to right: it departs from the Sielea planet on the left and moves toward the Niapra planet on the right.

In the experiment conducted by Alexander and Frank, Alexander sits at the front (right end) of the train, and Frank sits at the rear (left end). The train travels at 0.5c. After the train passes by our observation point, each scientist activates a device that emits a flash of light. In their own frame of reference on the train, these two flashes occur simultaneously. According to Alexander and Frank's synchronized clocks and direct observations, they press their buttons at the same time, see each other press their buttons at the same time, and watch both flashes of light emerge at the same moment. For them, the event of emitting these two light pulses is perfectly simultaneous in their reference frame.

But from our perspective, observing from a stationary frame (relative to the train's motion), the situation looks very different. As the train speeds past us, we see Frank's light (from the rear of the train, on the left side) reach us first. Only afterward do we detect Alexander's light (from the front of the train, on the right side). In our frame of reference, these two events (the emission of the two light beams) did not occur at the same time. Thus, what is simultaneous in the train's reference frame is not simultaneous in ours. This demonstrates that there is no universal "now" and no absolute sequence of events that the entire universe must agree upon. The ordering of events is just as valid in one frame of reference as it is in another.

What does this tell us about reality? It strongly suggests that our intuitive concept of time as a universally flowing river, where all observers must agree on the order of events, is not correct. Instead, time behaves more like a direction in a four-dimensional fabric---spacetime---where different slices of that fabric reveal different perspectives on which events occur first, second, or simultaneously.

Block Universe Theory

In the "Block Universe" view, the past, present, and future are not separate ontological entities racing into existence one after the other. Instead, the entire history of the universe---every point in space at every moment in time---can be considered as a single four-dimensional structure. Observers moving at different velocities carve up this spacetime structure into "now slices" differently, giving rise to their own unique perceptions of simultaneity and temporal order. Expanding this idea, one might say that all events "already exist" in this grand tapestry; what we call "passing time" is simply our subjective progression through this four-dimensional landscape.

Growing Block Universe Theory

The "Growing Block Universe" is a variation on the block universe idea. It posits that the past and present exist as a "block," while the future is not yet realized and "grows" as new moments come into being. In this view, events accumulate over time, creating an ever-expanding four-dimensional block as the universe evolves. This perspective attempts to reconcile our sense of an unfolding future with the relativistic notion that time is not absolute.

Causality and Light Cones

Causality in relativity is governed by the structure of light cones. Events within your "future light cone" can be influenced by your current actions, while events in your "past light cone" have influenced you. Events outside both cones cannot affect or be affected by you. The geometry of spacetime ensures that causality is preserved: no observer can witness effects happening before their causes. However, which events are deemed simultaneous or which occur "first" can differ depending on how you are moving through the spacetime. This intertwines the geometry of the universe with the causal structure of events, reinforcing the idea that time is not a universal arrow pointing forward but a dimension that, together with space, forms the rich four-dimensional fabric we inhabit.