Time is perhaps the most familiar yet enigmatic concept in physics. We experience it as an inevitable flow from past to future, but this intuitive perception does not have any ground in physics. As Albert Einstein famously noted, "the distinction between past, present and future is only a stubbornly persistent illusion." Einstein's equations simply envisage time as another dimension of the fabric of our universe, no different from space, which means you could theoretically move back and forth along the time dimension just as you move in space. This view is not only paradoxical to our intuition but also seems to contradict the one law in physics through which the arrow of time manifests itself: the irreversible increase of entropy in closed systems.
To tackle this paradox, we need to zoom out. A lot. The system we're considering here is the entire universe. This Astronomy article provides a helpful overview of some of the main theories regarding the metaphysics of time.
The Block Universe: time as a dimension
The widely accepted theory aligned with Einstein's general relativity is the Eternal Block Universe theory. In this framework, there is no privileged "now" - past, present, and future exist simultaneously as coordinates in a 4D space-time continuum. This theory has been validated by countless observations and experiments covering anything above the quantum scale, including at scales relevant to human perception. Yet it remains deeply puzzling.
Our difficulty in accepting the Eternal Block Universe isn't just about our inability to visualize four dimensions. We face two major conceptual challenges. The first is philosophical: it implies that we have no free will. The second is mathematical: concepts like irrational numbers or infinity don't really fit into a framework with 4D coordinates. This mathematical paradox is explored in my all-time favorite article by QuantaMagazine revealing the intriguing connection between Physics and Information Theory, and questioning what it means for finite spaces to contain objects with infinite information.
Another issue with general relativity is that it breaks down at quantum scale. At this scale, the Standard Model takes over as our best description of reality even though it fails to explain some crucial phenomena - including gravity itself, dark matter, or cosmic inflation. Given gravity is understood to affect how we experience time, the disconnect between our best theories at different scales adds to the mystery about the nature of time.
Alternative perspectives: time as a boundary
Two alternative theories offer very different perspectives: "Presentism", in which only the present moment exists, and the “Evolving Block Universe” or "Crystallizing Block Universe," which aligns more closely with our everyday experience of time - a fixed past, a "now", but an unformed future.
As enticing as these theories look, we should be wary of relying too heavily on our intuitions when dealing with physical phenomena at extreme scales (very large or very small). Wolfram points out that we are "computationally bounded" beings who can only perceive the big picture. That makes us prone to confusing correlation with causation. This limitation of human intuition was actually a major theme in early 20th-century physics and mathematics, where breakthrough discoveries required moving away from intuitive frameworks toward abstract, axiomatic approaches1. The mathematics worked, even when it defied everyday experience, and intuitionists had to surrender to axiomatics.
David Hilbert, the great German mathematician, espoused the now-standard view that real numbers exist and can be manipulated as completed entities. Opposed to this notion were mathematical “intuitionists” led by the acclaimed Dutch topologist L.E.J. Brouwer, who saw mathematics as a construct. [...] Although Brouwer’s intuitionist framework compelled and fascinated the likes of Kurt Gödel and Hermann Weyl, standard math, with its real numbers, dominates because of ease of use.
QuantaMagazine - Does Time Really Flow?
The Crystallizing Block Universe seems to resurrect some of the intuitionist spirit. In this framework, the present acts as a boundary between two regimes:
- Within the boundary: our familiar 3D space with defined states
- Beyond the boundary: a cloud of quantum possibilities without fixed coordinates in space or time
The boundary concept offers a new way to think about the arrow of time. Instead of viewing time primarily as the direction of increasing entropy, we might reverse it and consider instead the process by which quantum possibilities collapse into definite states - i.e encoding information into the fabric of a 3D universe. You need to imagine this process happening at the boundary of every unit of 3D space.
Modern theories: computing the universe
In a sense time represents “computational progress” in the universe, while space represents the “layout of its data structure”
Stephen Wolfram - On the Nature of Time
The idea of a boundary can be conceptualized in several modern theoretical frameworks. Wolfram's Physics Project, for instance, approaches the universe as a hypergraph of "branchial spaces" - a network that branches and merges to represent all possible events. In this model, time isn't a continuous dimension of space but rather the “progressive application of rules that continually modify the abstract structure that defines the contents of the universe”.
Wolfram’s hypergraph shares similarities with Loop Quantum Theory (LQT), where our three-dimensional space consists of discrete nodes connected in networks. These networks evolve through sequences that add and subtract links and nodes, forming a structure called "spin foam". It is effectively a 3+1-dimensional structure, where +1 represents the direction of the sequence of changes in the networks.
Both Wolfram's and LQT frameworks align naturally with the Evolving Block Universe or Crystallizing Block Universe perspective, where the future doesn't really exist in any concrete form. Wolfram suggests that future states could theoretically be computed if we knew all the rules2, but in practice, such computation is far beyond our reach.
Information, energy, and the expanding universe
All these theories - even relativity - share a common thread : it is always easier to access information about the past than information about the future (assuming future information even exists or can be calculated). You can think about “easier” in computational terms - it requires less energy. However, there is no free lunch. Less energy is required to retrieve information about the past because energy has already been spent encoding that information into physical reality. And each time this happens, it increases the overall entropy of the system.
In order to leave a trace, it is necessary for something to become arrested, to stop moving, and this can happen only in an irreversible process — that is to say, by degrading energy into heat. This is true when meteorites leave their impact on the ground […] for computer hard drives, which heat up when data is written to them. [And] every memory you have was created because it takes energy to create a memory pathway, and this recording of information both heats your brain and increases its entropy.
Astronomy citing Carlo Rovelli
In LQT, energy is needed to establish or break links on the spin foam, and Wolfram's theory literally treats time as computational steps. Following this logic, for every unit of 3D space, there ought to be evidence of an inherent quantity of energy as the encoding occurs. This energy would need to be everywhere, seemingly sourceless. If you stick to the boundary analogy, you understand that this energy can't simply be diluted if the 3D-space grows - each new unit comes with its own inherent energy. In other words, if the three-dimensional space expands, the total energy in the universe must increase.
Now if the boundary between past and future continuously encodes new information into the three-dimensional space and if there is no mechanism to remove information, wouldn't the universe need to expand at one point to accommodate it all? And since each new unit contributes to adding more information, wouldn't this expansion necessarily accelerate over time?3.
The relationship between information and energy offers a new way to think about dark energy - the mysterious force driving the expansion of the universe. There's a glaring issue with this idea though : expansion should accelerate exponentially unless there is a counterbalancing force that removes information from the universe. Before trying to address this issue, let’s ask ourselves how this approach can otherwise fit with relativity?
Bridging classical and modern perspectives
Wolfram provides an elegant bridge between his framework and relativity. According to relativity, observers moving at different speeds can experience different sequences of the same events, which Wolfram explains as different perspectives into the same underlying hypergraph. He also proposes that the density of events in the hypergraph determines the density of energy (and mass) in physical space, potentially explaining gravitational effects. What’s interesting with the hypergraph hypothesis is that, when several branches merge into the same event, that computational step effectively deletes information.
Following the intuitionistic logic further, a potential connection between gravity and information removal inevitably points toward black holes and their information paradox4. But perhaps we should step back from that particular rabbit hole - there's only so far an amateur should venture.
Without pretending to “solve” anything, wandering through these alternative perspectives offers new ways to think about time and its relationship to the fundamental structure of our universe. Whether you find yourself in the relativity camp trying to understand dark energy, or you're intrigued by the time-as-encoding-step perspective, the origin of all this energy remains puzzling. There's something comforting in knowing that everyone shares this puzzlement, from "normal" people like me to theoretical physicists.
The coming years may validate or invalidate these theories, and new ones will surely emerge. I hope to see a breakthrough in my lifetime and, hopefully, I’ll be able to understand it. The big picture at least. Even if time is ultimately proven to be just a continuous dimension to space, making its passage an illusion, I'll likely continue approaching it in my daily life as the process by which possibility becomes reality. It's a comforting intuition for optimistic dreamers.
PS: if you understood any of this and are confident that you can explain to a non-physicist why it is utter non-sense, I'd love to hear your thoughts.
