The Man Who Turned the Universe Into a Triangle
Julian Barbour has spent six decades asking what time is. His answer starts with three points on a table.
Every account of why time runs forward hits the same wall. Eggs break and do not unbreak; hot coffee cools and does not warm itself again. Physics explains each case the same way: things drift from order toward disorder. Followed backward, that explanation needs the universe to have begun in an extraordinarily orderly state, and no law says why it did. Physicists call this assumption the past hypothesis, and most accept it as a starting fact rather than something to explain.
Julian Barbour thinks gravity can do without it. He is a British physicist, born in 1937, who has worked outside universities for most of his life. [Sourced · Encyclopedia.com; Nautilus, "The Country Gentleman of Physics"]
Put three stones on a table. They make a triangle. Slide the arrangement, turn it, or photograph it from farther away, and one thing survives all three moves: the angles. Barbour argues that only what survives is real. Position, direction and overall size are measured against something outside, and the universe has no outside. What remains is shape. The smallest universe that has a shape at all is three points, and its every possible state is a triangle. [Sourced · Barbour, "Shape Dynamics. An Introduction," arXiv 1105.0183]
Some triangles carry more structure than others. Three points evenly spaced, an equilateral triangle, is the plainest shape there is: nothing stands out. Slide two points together and leave the third far off, and the triangle has a pair and a loner. Barbour and his collaborators measure the difference with one number, complexity, which is lowest for the even triangle and highest for the tight pair with a distant third. [Sourced · Barbour et al., "Complexity and Its Creation," arXiv 2405.07480; Lazarovici & Reichert, arXiv 1809.04646]
Now let the three points pull on each other by gravity. Start them near the even triangle and run the motion forward: two points fall into orbit around each other while the third is flung away, and the triangle stretches toward a pair and a loner. Run the same motion backward from that even moment and the same thing happens. The plainest triangle sits in the middle, and structure grows in both directions away from it. Barbour named that middle moment the Janus point, for the Roman god with two faces looking opposite ways. [Sourced · Barbour, Koslowski & Mercati, Physical Review Letters 113, 181101, 2014]
Barbour's picture for the same moment in a full universe is a swarm of bees. Scale the triangle up to many points and the plainest moment is a swarm: particles spread fairly evenly, moving in every direction, with no lasting clusters and no pairs held in orbit. Move away from it either way and gravity does what it did to the triangle, many times over. The swarm as a whole spreads out while inside it particles bind into pairs and clusters. In his model this happens in typical cases, on both sides of the low point. [Paraphrase of The Janus Point, 2020, from the editor's notes]
Each point pulls on every other by ordinary gravity. Start at the middle, where the swarm is plainest, and move either way: the whole group spreads out while pairs and clusters form inside it. Lines join points that stay close; rings mark points bound tight to a partner. Neither side is marked as the past.
Model · Newtonian gravity, total energy and spin set to zero, after Barbour, Koslowski & Mercati, Physical Review Letters 113, 181101 (2014) · 24 points in 2D, gravity softened at very short range · size scaled out, only shape shown · faint curve raw, bold curve smoothed · computed by The Penumbra
That leaves the obvious question: why would a universe pass through such a plain moment at all? Barbour's answer is a dartboard. For three points, imagine a board holding every possible triangle; for a universe, every possible arrangement at its Janus point. Throw a dart blind. Under the measure Barbour uses to weight the board, the dart lands overwhelmingly on plain, evenly spread shapes, because almost all of the board is made of them. The orderly-looking beginning that the past hypothesis has to assume becomes, on this board, the expected result. [Paraphrase of The Janus Point, 2020, from the editor's notes]
Barbour gives the quantity a name, entaxy. High entaxy means uniform, the even triangle and the swarm; low entaxy means structure, the pair and the loner. Moving away from the Janus point, entaxy falls as structure grows, and Barbour puts the direction of time there. On his account, what we experience as time is the growth of structure in a universe with no edge, recorded in the structures themselves. [Paraphrase of The Janus Point, 2020, from the editor's notes; to be confirmed against the print edition]
The strongest objection came from H. Dieter Zeh, a physicist who spent much of his career on the arrow of time. In a 2016 comment, Zeh argued that the Janus point does not remove the special condition; it moves it to the middle, with two arrows pointing away from it instead of one. Barbour's dartboard is his reply, and it shifts the dispute. A throw is only blind relative to how the board is weighted, and that weighting is Barbour's choice to defend. [Sourced · Zeh, arXiv 1601.02790]
A second objection is plainer. Reviewing the book, the novelist and physicist Andrew Crumey noted that what physics abstracts from change is duration, the quantity clocks measure. Lived time, the passing of one moment into the next, is left unexplained. [Sourced · Crumey, Wall Street Journal review, 2020]
Back to the three stones. Barbour's claim is that their every arrangement already holds everything time is: an even triangle at the middle and a pair and a loner at either end, with nothing outside to say which end came first. Whether our universe is weighted like his dartboard is open. The claim worth arguing with is that time is not a stage on which the triangle changes but the name for its change. To follow it further, search for shape dynamics or entaxy.
Disclosure: The Penumbra's editor has corresponded with Julian Barbour by email since October 2026 and plans to speak with him. Nothing from that correspondence is used here.