Coffee table conversations with people thinking about foundational issues. Multiverses explores the limits of knowledge and technology. Does quantum mechanics tell us that our world is one of many? Will AI make us intellectually lazy, or expand our cognitive range? Is time a thing in itself or a measure of change? Join James Robinson as he tries to find out.
Space and time appear in charts as axes oblivious to the points they demarcate. Similarly, we may feel that we, and all the objects of our worlds, are like such points — and spacetime is a container in which we sit.
Julian Barbour is a physicist who has spent six decades arguing against this. He takes the relationist approach of Leibniz and Mach: there is no space without objects and no time without change. Rather space is just the geometric relationships between things.
Julian has pioneered theories that recover the predictions of Newtonian mechanics and General Relativity that drop their invocation of imperceptible space, time, and spacetime. Recently he has taken an iconoclastic approach to the arrow of time — looking to a new measure of structure, complexity, and the expansion of the universe instead of the traditional accounts in terms of entropy.
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Space and time are intimately familiar, without them we experience nothing. In t’s and x’s and μ’s they suffuse the equations of physics. Yet their nature remains the subject of debate and conjecture. Are they entities in and of themselves, or do they simply represent relationships between objects or events?
Our guest this week is Julian Barbour, a physicist, and thinker who has led a program of work over six decades to champion the relationist position. According to this view, there is no space without objects — space is a codification of the geometric relationships between objects, likewise, there is no time without change — time emerges from the varied configurations of objects in different “nows”.
This is in direct contrast to a tradition going back to Isaac Newton:
Absolute, true, and mathematical time, in and of itself and of its own nature, without reference to anything external, flows uniformly and by another name is called duration.Newton’s Scholium on Time, Space, Place and Motion
For this claim, Newton provided scant arguments, however, he was able to make a case that space existed independently of particles. His law of inertia showed that rotating masses would experience angular momentum. This is easily measurable, for example, spinning a bucket of water leads to the surface of the liquid curving. What is it spinning relative to that should cause this? Newton reasoned it had to be absolute space.
Newton’s contemporary and arch-rival, Gottfried Wilhelm von Leibniz pushed back against this absolutist picture but although he offered many good arguments he was unable (or unwilling) to provide an answer to this bucket experiment. Two centuries later the physicist Ernst Mach was deeply uncomfortable with this intangibility of absolute space and proposed that the forces of angular momentum might be caused not by motion relative to the ethereal substance of space but by movement relative to ordinary stuff — in particular the fixed stars.
In 1977, with Bruno Bertotti, Julian made good on Mach’s principle for a universe of N (any finite number) particles and vanishing overall angular moment. That is, the predictions of Newtonian mechanics — including the water in the bucket — were completely recovered without any reference to absolute space. This was an extraordinary achievement.
Julian has gone on to show that, again with some constraints (that the universe is closed, roughly speaking that it curves back on itself), the predictions of General Relativity can also be recovered within a completely relational theory — Shape Dynamics, developed with collaborators Tim Koslwoski and Flavio Mercati. While in General Relativity space and time are presented as malleable substances with a starring role in determining motions, in Shape Dynamics they are gone.
In Julian’s latest work, presented in his book The Janus Point, he has turned to the question of the arrow of time. Why do processes seem to happen only in one direction even though the laws of physics are symmetric? Unsatisfied with the frequently given explanation in terms of entropy increase (and in particular the lack of explanation for the “past hypothesis” — that entropy of the universe needed to have been low) Julian had an insight into an alternative picture.
Once again he explored the Newtonian problem first to show that, for arrangements of particles interacting under gravity with a vanishing or negative total energy, there will be a moment of closest approach from which they will move away. Julian presents strong arguments that on either side of this moment of closest approach, there will be two arrows of time and provides a mathematical proof that a quantity he introduces — the complexity — that measures the amount of structure in the world will inexorably increase as a configuration expands relative to its earlier scale.
Physics is not settled. And Julian freely admits that there could be observations that scupper the relationist ship. But his intuitions and evidence are strong, let us suppose then that the relationists are right. What would it mean?
Stand up, spin, and let your arms fly out. You are feeling the dust clouds, stars, and black holes of the universe.
Watch the unfurling of a leaf and the folding of a paper swan. That too is the cryptic touch of gravity.
Julian’s website: http://www.platonia.com/
Questions I’d ask if I had this conversation again
How did being an independent academic shape his research and collaboration?
Julian chose not to follow a typical academic path. He lives on a small farm near Oxford and has strong relations with members of the Physics and Philosophy faculties there, latterly being a visiting professor, but he has never been a fully paid-up member of the academy. He has commented that the “Publish or perish” culture does not encourage the sort of work he has done — which one may characterize as infrequent, but groundbreaking papers that challenge the foundations of physical thought. I wonder what challenges and opportunities this choice presented him.
What did you learn from translating seventy million words of academic texts?
For income, for many years Julian would translate physics texts from German and Russian for three days a week, the other two he would dedicate to his research. How did he find this rhythm, had he been given the means to spend five days a week on the questions of time and space, would he have taken it?
How would you compare the senses of wonder and insight you get from reading Kepler and Shakespeare?
In The Janus Point, there are more quotes from Shakespeare than from Newton, Leibniz, or Mach. Julian is somewhat unusual in that he is a physicist who reads the original works of natural philosophy — such as Newton’s Principia, Descartes’ The World — I believe he reads them for both pleasure and insight. Has Shakespeare also informed his thinking on time, space, and structure?