Julian Barbour is known for a bold idea: what we call “time” may not be a basic ingredient of reality. Instead, time may be something we infer from patterns in how the universe is arranged, and how different arrangements relate to each other.
In this post we explore the core idea in plain terms, then explain the main concepts (Nows, “Platonia”, time capsules), the physics motivations (Machian dynamics, quantum gravity, shape dynamics), the “Janus point” arrow of time proposal, and how these ideas are received by the scientific community.
The main points in plain terms
- Time is not a thing that flows. What exists are complete “snapshots” of the universe, each fully specified in the present.
- A “history” is not something the universe moves through. It is a way we order or relate those snapshots using patterns and correlations.
- We experience a past because some snapshots contain records: memories, messages, fossils, photos, and other traces that look like evidence of earlier states.
- There is no universal background time that everything ticks against. What we call time is built by comparing one change to another, using some regular process as a reference.
- If fundamental physics can be written without a time variable, that might be a clue that time is emergent rather than fundamental.
“Nows” rather than a moving present
Barbour often describes reality as a set of complete instantaneous states. He calls these “Nows”. In this view, there is no extra ingredient that makes one Now turn into the next. The feeling of flow is something we construct.
A simple way to picture this is to imagine a huge library full of complete still photographs of the entire universe. Each photo is a full snapshot: where everything is, what everything is doing, what every measuring device reads, and what every brain state is like.
In everyday thinking, we assume the universe is like a movie reel: the photos are arranged in one real sequence, and the universe plays them in order. Barbour is challenging that assumption. He is saying the snapshots are the fundamental items, and the movie reel is something we add as a way of organizing or describing them.
So when we draw a “path” through a space of possible configurations, we are taking many snapshots and linking them together into a story that looks like motion and change.
But Barbour wants to go further than “this is a useful picture.” His deeper claim is that the story itself is not built into reality as an extra ingredient over and above the snapshots. The snapshots exist as complete states. The ordering into a single history is something we impose because some snapshots fit together in a very coherent, story-like way.
“Platonia” and configuration space
To organize these Nows, Barbour points to a mathematical idea: configuration space, the space of all possible instantaneous arrangements of the universe. He uses the nickname “Platonia” for this arena of possible Nows.
A familiar time-ordered story can be represented as a path through this space. You can imagine taking a sequence of snapshots that differ only slightly from one to the next, and then drawing a line that connects them. That connected line is the “path”, and it looks like what we normally mean by a history: first this state, then that state, then the next.
But on Barbour’s view, the path is not a separate object that exists in the world in addition to the snapshots. It is more like a way of sorting and grouping snapshots that already exist. If we pick one Now and then look for other Nows that match it closely and also contain consistent “records”, we can link them into a chain that reads like a storyline.
So the path is a description we build because many Nows fit together neatly, not a fundamental track that the universe is traveling along. The basic ingredients are the Nows themselves, and “a history” is our way of expressing relationships among them, especially when those relationships form a smooth, movie-like sequence.
“Time capsules” and the appearance of a past
If there is no literal past, why do we have memories and evidence of earlier times? Barbour’s answer is that some Nows contain rich internal structure that acts like a record. He calls these special Nows “time capsules”.
Some snapshots are not just random arrangements. They include lots of internal clues that point to a consistent story of “what came before”.
Consider a single present moment in your life. In that one moment, your brain contains memory traces, your phone contains photos with timestamps, your inbox contains messages, your body contains scars, and your surroundings contain objects that look like they have a history. Even without watching a video of your past, the present contains a web of records that strongly suggest a past sequence.
Barbour’s point is that a single Now could contain exactly that kind of web. It can include:
- brain states that feel like memories of earlier events
- written records, photos, and digital files
- physical traces like erosion, fossils, or broken objects
- correlations among many subsystems that line up as if they were produced by a chain of earlier causes
In that situation, the Now functions like a time capsule: it is one complete snapshot that carries evidence, inside itself, that seems to refer to other snapshots. That internal evidence is a big part of why we experience ourselves as living in a world with a past.
This is also where people push back: critics ask whether “having records” is enough to recover the full idea of a single objective history, rather than many possible story lines that could fit the records.
The technical physics behind the idea
So far we have stayed at the level of the big picture story. Underneath it, there is also a more technical motivation: certain ways of doing classical mechanics and quantum gravity make time look less like a basic ingredient and more like something that can be derived, traded away, or reconstructed from relationships and change. The next sections sketch the main ideas Barbour and others point to when they argue that the timeless perspective is not just philosophy, but something we can connect to real physics.
Machian dynamics and “time from change”
A big influence on Barbour is a Mach-inspired, relational approach to physics. The basic instinct is: define motion and change using only relationships among objects, not relative to an invisible background.
Machian dynamics is the idea that motion and inertia should be defined using only relations between things, not relative to absolute space or an absolute clock.
A common intuition is: if you are spinning, you are doing so relative to what? In a Machian view, it should be relative to the rest of the matter in the universe, not relative to “absolute space”.
In practice, Machian approaches try to build laws so that only relational facts matter. For instance:
- distances between objects
- angles between objects
- ratios of distances
- the overall shape of a configuration
Time can also be treated in a Machian way. Instead of assuming a universal clock ticking in the background, we define time through the pattern of change. A clock is just one system whose change is regular, and we use it to compare other changes.
Barbour’s broader point is that once we focus on relations and change, an external time parameter can start to look like convenient bookkeeping rather than a fundamental ingredient.
Quantum gravity and the “problem of time”
A major motivation comes from quantum gravity. In canonical approaches to quantum gravity, the core equations can look “timeless”, with no explicit time parameter. This leads to the famous “problem of time”: if the fundamental equation does not include time, how do we recover the everyday notion of evolution?
Different researchers interpret this in different ways. Some see it as a sign we are using the wrong variables. Others treat it as a clue that time, as we normally think of it, is not fundamental. Barbour leans toward the second reading and tries to make sense of physics in a way that starts without time.
Shape dynamics
In later work with collaborators, Barbour helped develop shape dynamics, an alternative way of formulating gravity that puts different symmetries front and center.
In simple terms:
- Standard general relativity treats spacetime as the main object and allows time and space to mix depending on the observer.
- Shape dynamics emphasizes the evolving shape of space itself, focusing on relational spatial geometry, and it trades some of general relativity’s symmetry for a different symmetry related to local rescaling of sizes.
Here is the intuition. Imagine two drawings of a triangle. In one drawing, the triangle is just a blown-up version of the other: every side is twice as long, but the angles are the same and the side-length ratios match. Those two triangles are different in overall size, but they have the same shape.
Shape dynamics leans into that distinction. It says that when describing the universe, the most fundamental information is often the pattern of spatial relationships: how distances compare, what the geometry looks like, and how those relational patterns evolve. The “overall size” of the whole picture is treated as less fundamental, more like a choice of scale or a way of labeling the same underlying relational situation.
Crucially, this does not mean shape never changes. In the triangle analogy, shape changes when the angles change or when the ratios of the sides change. Shape dynamics is interested in exactly that kind of change: the evolving geometry and relational structure of space. It tries not to assume the niverse comes with one perfect, universal ruler built into the background.
In essence:
- it offers a new angle on the problem of time
- it can make some conceptual issues look cleaner in certain settings
- It offers a different set of mathematical tools for thinking about gravity and the universe, while still matching general relativity in the situations where general relativity has been well tested.
Shape dynamics is not the mainstream default language of gravity, but it is a serious research track that fits naturally with Barbour’s relational vision.
The Janus point and arrows of time
Barbour also argues for a “Janus point” picture in certain cosmological models. The core suggestion is that an arrow of time can emerge from how structure builds up, without requiring a single uniquely special beginning in the usual way.
A Janus point is a special moment in a model universe where a chosen measure of overall structure or complexity reaches a minimum.
From that minimum, the model can evolve so that structure increases in both directions. Because many arrow-of-time cues are tied to the growth of structure and the formation of records, observers on either side would naturally label the direction away from the Janus point as the direction toward the future.
In some simplified models of the universe, if we track a measure of how “clumpy” or “structured” things are, we can get a surprising pattern. Instead of complexity always rising from a special low-complexity beginning, the model can produce a moment that is lowest in complexity in the middle, with complexity growing in both directions away from it.
Think of it like an hourglass shape, but in terms of structure rather than sand. Near the middle, matter is more evenly spread and the overall state looks simple. Move away from that middle in either direction and we tend to get more structure: more clustering, more distinct subsystems, more stable records, and more processes that can support an arrow-of-time experience.
Barbour calls that middle moment the Janus point, after the Roman god Janus who is often depicted with two faces looking in opposite directions. From that middle, we can define two “future” directions, one on each side, because in both directions we see increasing structure and the growth of record-building processes.
Observers on either side would say time points away from the Janus point, but they would disagree about which direction is “the past” relative to the full model. Put simply:
- there is a central moment where the universe is least structured
- as we go away from it, structure increases on both sides
- on each side, processes that create records become more common
- that makes each side look like it has its own arrow of time, pointing away from the center
This is not the standard textbook story of cosmology. In the usual picture, the arrow of time is tied to a very special early universe and concepts like low entropy. Barbour’s Janus point story comes from particular simplified models and from using a particular way of measuring “how structured” the universe is.
That means two things. First, it may not apply to every realistic cosmological scenario, even if it is interesting in the cases where it works. Second, the conclusion can depend on definitions: “complexity” is not a single universally agreed number. Different reasonable measures of structure can behave differently, so critics ask whether the Janus point arrow is a deep feature of nature or a feature of the chosen model and the chosen measuring stick.
The scientific community’s view
Barbour is widely seen as provocative and serious, with real technical contributions, but his most radical claims about time are not something most physicists agree with.
What is broadly appreciated
- The problem of time in quantum gravity is widely recognized. Timeless-looking equations are part of the landscape, even if interpretations vary.
- Barbour’s relational and Machian instincts have genuine technical content and have influenced discussions in foundations of dynamics and quantum gravity.
- Shape dynamics is viewed by many as an interesting alternative formulation with nontrivial results. It remains niche compared to standard approaches, but it is a live research direction.
Where many researchers are skeptical
- Interpretation leap: many physicists accept that some formalisms can be written without time, but they do not accept that time literally does not exist. They may treat timelessness as a feature of a particular formalism, not a final ontological verdict.
- Explanatory gaps: records and time capsule stories face hard questions. How do you recover the full causal and relativistic structure used in physics? Why do we experience one stable, movie-like storyline, instead of a jumble of possible storylines that could all fit the same evidence? How do probability and typicality work in a timeless setting?
- Testability: critics often say the proposal is more interpretive than a distinct empirically testable theory. If two pictures make the same predictions, many scientists will see the difference as philosophical taste, unless the new framing suggests new calculations or new measurable effects.
A practical way to think about the debate
It helps to separate two claims:
- Modest claim: time might not be fundamental in some deep theories, and the familiar notion of time could emerge in the right limit.
- Strong claim: there is no time at all, only Nows, and the sense of history is fully explained by record-like structures inside certain Nows.
The modest claim has broad sympathy in quantum gravity circles. The strong claim is where disagreement sharpens.