|Fields||Theoretical physics, history of science|
|Alma mater||University of Cambridge, University of Cologne|
|Known for||timeless physics (the thesis that time is an illusion)|
Since receiving his Ph.D. degree on the foundations of Einstein's general theory of relativity at the University of Cologne in 1968, Barbour has supported himself and his family without an academic position, working part-time as a translator. He resides near Banbury, England.
His 1999 book The End of Time advances timeless physics: the controversial view that time, as we perceive it, does not exist as anything other than an illusion, and that a number of problems in physical theory arise from assuming that it does exist. He argues that we have no evidence of the past other than our memory of it, and no evidence of the future other than our belief in it. "Change merely creates an illusion of time, with each individual moment existing in its own right, complete and whole." He calls these moments "Nows". It is all an illusion: there is no motion and no change. He argues that the illusion of time is what we interpret through what he calls "time capsules", which are "any fixed pattern that creates or encodes the appearance of motion, change or history".
Barbour's theory goes further in scepticism than the block universe theory, since it denies not only the passage of time, but the existence of an external dimension of time. Physics orders "Nows" by their inherent similarity to each other. That ordering is what we conventionally call a time ordering, but does not come about from "Nows" occurring at specific times, since they do not occur, nor does it come about from their existing unchangingly along the time axis of a block universe, but it is rather derived from their actual content.
Barbour also researches Machian physics, a related field. The Machian approach requires physics to be constructed from directly observable quantities. In standard analytical dynamics a system's future evolution can be determined from a state consisting of particle positions and momenta (or instantaneous velocities). The Machian approach eschews the momenta/instantaneous velocities, which are not directly observable, and so needs more than one "snapshot" consisting of positions only. This relates to the idea of snapshots, or "Nows" in Barbour's thinking on time.
Along with physicist Bruno Bertotti, Barbour developed a technique called "best matching" for deriving gravitational equations directly from astronomical measurements of objects’ spatial relations with each other. Published in 1982, the method describes gravitational effects as accurately as Einstein's general relativity, but without the need for a "background" grid of spacetime. According to physicist David Wiltshire at the University of Canterbury in New Zealand, such a truly Machian or relational approach could explain the appearance of an accelerated expansion of the universe without invoking a causative agent such as dark energy.
Criticism of Barbour's Ideas
Theoretical physicist Lee Smolin repeatedly refers to Barbour's ideas in his books. However Smolin is usually higly critical of Barbour's ideas, since Smolin is a proponent of a realist theory of time, where time is real and not a mere illusion as Barbour suggests.  In a recent book, Time Reborn, Smolin argues for the view that time is both real and fundamnental.  Smolin reasons that physicists have improperly rejected the reality of time because they confuse their mathematical models—which are timeless but deal in abstractions that do not exist—with reality. Smolin hypothesizes instead that the very laws of physics are not fixed, but that they actually evolve over time.
Theoretical physicist Sean Carroll has criticized Barbour and all physicists who adhere to a "timeless-view" of the universe. Carroll does not seem so much opposed to the idea, as he finds it useless for physics:
The problem is not that I disagree with the timelessness crowd, it’s that I don’t see the point. I am not motivated to make the effort to carefully read what they are writing, because I am very unclear about what is to be gained by doing so. If anyone could spell out straightforwardly what I might be able to understand by thinking of the world in the language of timelessness, I’d be very happy to re-orient my attitude and take these works seriously.
Carroll, however, does seem to hold the view that time is real, although he questions if time is a fundamental or emergent property:
The “reality” of something is only an interesting issue if its a well-defined concept whose actual existence is in question, like Bigfoot or supersymmetry. For concepts like “time,” which are unambiguously part of a useful vocabulary we have for describing the world, talking about “reality” is just a bit of harmless gassing. They may be emergent or fundamental, but they’re definitely there. [...] Temperature and pressure didn’t stop being real once we understood them as emergent properties of an underlying atomic description. The question of whether time is fundamental or emergent is, on the other hand, crucially important. I have no idea what the answer is (and neither does anybody else). Modern theories of fundamental physics and cosmology include both possibilities among the respectable proposals. 
- 1999. The End of Time: The Next Revolution in our Understanding of the Universe. Oxford Univ. Press. ISBN 0-297-81985-2; ISBN 0-19-511729-8 (paperback: ISBN 0-7538-1020-4)
- 2001. The Discovery of Dynamics: A Study from a Machian Point of View of the Discovery and the Structure of Dynamical Theories. ISBN 0-19-513202-5
- 2006. Absolute or Relative Motion?. ISBN 0-19-513203-3. Paperback reprinting of The Discovery of Dynamics.
- 1982 (with B. Bertotti). Mach's Principle and the Structure of Dynamical Theories.
- 1994 (with Vladimir Pavlovich Vizgin) Unified Field Theories in the First Third of the 20th Century . ISBN 0-8176-2679-4.
- 1996 (with Herbert Pfister) Mach's Principle: From Newton's Bucket to Quantum Gravity. Birkhaueser. ISBN 0-8176-3823-7.
Scientific work by others bearing on Barbour's theories
- Anderson, Edward (2004) "Geometrodynamics: Spacetime or space?" Ph.D. thesis, University of London.
- Anderson, Edward (2007) "On the recovery of Geometrodynamics from two different sets of first principles", Stud. Hist. Philos. Mod. Phys. 38: 15.
- Baierlein, R. F., D. H. Sharp, and John A. Wheeler (1962) "Three-dimensional geometry as the carrier of information about time", Phys. Rev. 126: 1864–1865.
- Max Tegmark (2008) "The Mathematical Universe", Found. Phys. 38: 101–150.
- Wolpert, D. H. (1992) "Memory Systems, Computation, and The Second Law of Thermodynamics", International Journal of Theoretical Physics 31: 743–785. Barbour argues that this article supports his view of the illusory nature of time.
- "Julian Barbour - Contact". Retrieved 1 October 2011.
- Nature of Time
- It is utterly beyond our power to measure the changes of things by time ... time is an abstraction at which we arrive by means of the changes of things; made because we are not restricted to any one definite measure, all being interconnected. Mach himself was a sceptic about time: "It is utterly beyond our power to measure the changes of things by time ... time is an abstraction at which we arrive by means of the changes of things; made because we are not restricted to any one definite measure, all being interconnected."
- Merali, Zeeya. "Is Einstein's Greatest Work All Wrong—Because He Didn't Go Far Enough?". Discover magazine. Retrieved 4-10-2012. Check date values in:
- Smolin L., (1997) Three Roads to Quantum Gravity (pp. 119–121, 131); (2006) The Trouble with Physics (pp. 321–22); (2013) Time Reborn (pp. 92–5)
- Monk, Ray (June 6, 2013). "Time Reborn by Lee Smolin – review". The Guardian. Archived from the original on July 28, 2013.
- "Time Reborn: a new theory of time - a new view of the world". Royal Society of Arts. May 21, 2013. Archived from the original on July 28, 2013.
- Julian Barbour's WorldSci profile
- Julian Barbour's website
- The End Of Time: A Talk With Julian Barbour
- Discover December 2000 From Here to Eternity
- Killing Time A 25-minute feature about the idea that time is an illusion, filmed by Dutch TV in December 1999 and first shown early in 2000
- Physics World profile
- The End of Time, Chapter One (requires free registration)
- Video (with mp3 available) of Barbour discussion on Bloggingheads.tv
- Does Time Exist? 2012 lecture at the Perimeter Institute for Theoretical Physics