Search FQXi

If you are aware of an interesting new academic paper (that has been published in a peer-reviewed journal or has appeared on the arXiv), a conference talk (at an official professional scientific meeting), an external blog post (by a professional scientist) or a news item (in the mainstream news media), which you think might make an interesting topic for an FQXi blog post, then please contact us at with a link to the original source and a sentence about why you think that the work is worthy of discussion. Please note that we receive many such suggestions and while we endeavour to respond to them, we may not be able to reply to all suggestions.

Please also note that we do not accept unsolicited posts and we cannot review, or open new threads for, unsolicited articles or papers. Requests to review or post such materials will not be answered. If you have your own novel physics theory or model, which you would like to post for further discussion among then FQXi community, then please add them directly to the "Alternative Models of Reality" thread, or to the "Alternative Models of Cosmology" thread. Thank you.

Contests Home

Current Essay Contest

Previous Contests

To Be Announced February 14
February 28, 2023 - April 19, 2023 open to submissions
Fetzer Franklin Fund, and The Peter and Patricia Gruber Foundation

Undecidability, Uncomputability, and Unpredictability Essay Contest
December 24, 2019 - April 24, 2020
Fetzer Franklin Fund, and The Peter and Patricia Gruber Foundation

What Is “Fundamental”
October 28, 2017 to January 22, 2018
The Fetzer Franklin Fund, and The Peter and Patricia Gruber Foundation

Wandering Towards a Goal
How can mindless mathematical laws give rise to aims and intention?
December 2, 2016 to March 3, 2017
The Peter and Patricia Gruber Fund

Trick or Truth: The Mysterious Connection Between Physics and Mathematics
Nanotronics Imaging, The Peter and Patricia Gruber Foundation, and John Templeton Foundation Media Partner: Scientific American

How Should Humanity Steer the Future?
January 9, 2014 - August 31, 2014
The Peter and Patricia Gruber Foundation, John Templeton Foundation, and Scientific American

It From Bit or Bit From It
March 25 - June 28, 2013
The Gruber Foundation, John Templeton Foundation, and Scientific American

Questioning the Foundations
Which of Our Basic Physical Assumptions Are Wrong?
May 24 - August 31, 2012
The Peter and Patricia Gruber Foundation, SubMeta, and Scientific American

Is Reality Digital or Analog?
November 2010 - February 2011
The Peter and Patricia Gruber Foundation and Scientific American

What's Ultimately Possible in Physics?
May - October 2009
Astrid and Bruce McWilliams

The Nature of Time
August - December 2008

Forum Home
Terms of Use

Order posts by:
 chronological order
 most recent first

Posts by the author are highlighted in orange; posts by FQXi Members are highlighted in blue.

By using the FQXi Forum, you acknowledge reading and agree to abide by the Terms of Use

 RSS feed | RSS help

Russ Otter: on 11/15/11 at 21:52pm UTC, wrote Connections The binding of existence This is a story, built upon...

Re Ality (Facebook): on 4/16/11 at 15:40pm UTC, wrote Time and Space Space is an intrinsic attribute of the existent entities...

Anonymous: on 9/19/10 at 21:30pm UTC, wrote I believe the "If/then" statement(the most powerful equation in the...

Kostia: on 6/13/10 at 13:57pm UTC, wrote Dear Craig, I've just read your excellent paper in Scientific American....

Ralph: on 6/6/10 at 4:02am UTC, wrote Flawed relativistic theories aside, time need not flow in a single...

Don Saar: on 5/22/10 at 18:22pm UTC, wrote Time is an accounting of the relative motions in space of bodies without...

robert: on 9/23/09 at 9:42am UTC, wrote moving dimension theory

Narendra Nath: on 1/1/09 at 13:30pm UTC, wrote It seems to me that the curvatures in space and time may be varying both...


David Vognar: "Completeness theorem: If a system’s components can transduce, that system..." in The Entropic Price of...

Georgina Woodward: "On obtaining the singular, relative, measurement product it replaces the..." in The Present State of...

Steve Dufourny: "The paper of Wilczek of course is very relevant considering the idea about..." in The Noise of Gravitons

Georgina Woodward: "Material neuronal structure in which memory is encoded, physical records..." in Quantum Physics and the...

Steve Dufourny: "It is really how we consider the structure of the spacetime, and also how..." in The Noise of Gravitons

Aleksandr Maltsev: "Hi Georgina, Write a letter to" in Quantum Physics and the...

Georgina Woodward: "In quantum experiments using particles, there won't be swapping with a..." in The Present State of...

Aleksandr Maltsev: "I shortened the phrase Zeeya Merali  «Why does time flow….?    How..." in Time's Arrow, Black Holes...

click titles to read articles

The Entropic Price of Building the Perfect Clock: Q&A with Natalia Ares
Experiments investigating the thermodynamics of clocks can teach us about the origin of time's arrow.

Schrödinger’s A.I. Could Test the Foundations of Reality
Physicists lay out blueprints for running a 'Wigner's Friend' experiment using an artificial intelligence, built on a quantum computer, as an 'observer.'

Expanding the Mind (Literally): Q&A with Karim Jerbi and Jordan O'Byrne
Using a brain-computer interface to create a consciousness 'add-on' to help test Integrated Information Theory.

Quanthoven's Fifth
A quantum computer composes chart-topping music, programmed by physicists striving to understand consciousness.

The Math of Consciousness: Q&A with Kobi Kremnitzer
A meditating mathematician is developing a theory of conscious experience to help understand the boundary between the quantum and classical world.

February 6, 2023

CATEGORY: The Nature of Time Essay Contest (2008) [back]
TOPIC: What Makes Time Special by Craig Adam Callender [refresh]
Bookmark and Share
Login or create account to post reply or comment.

Craig Adam Callender wrote on Nov. 14, 2008 @ 15:30 GMT
Essay Abstract

What is the difference between time and space? This paper proposes a novel answer: the temporal direction is that direction on the manifold of events in which our best theories can tell the strongest, most informative “stories.” Put another way, time is that direction in which our theories can obtain as much determinism as possible. I make two arguments. The first is a general one based on an empiricist theory of laws. I argue that according to this theory time is distinguished as the direction of informative strength. The second argument is a more specific illustration of the first: understanding informative strength as having a well-posed Cauchy problem, I show that for a wide class of equations (i.e., second-order linear partial differential equations) the desire for strength does indeed distinguish the temporal direction. Not only that, but the argument rigorously connects three otherwise mysterious connections among temporal features to one another.

Author Bio

Craig Callender is Professor of Philosophy at the University of California, San Diego. He works in the foundations of physics, especially on statistical mechanics, the interpretation of quantum mechanics, and topics in quantum gravity. Much of his work focuses on the nature of time in modern physics. He has published extensively in philosophy and physics journals. In addition, he has edited two books "Philosophy Meets Physics at the Planck Scale" (CUP) and "Time, Reality and Experience" (CUP), and authored the popular science book "Introducing Time" (Icon/Totem).

Download Essay PDF File

Bookmark and Share

Peter Morgan wrote on Nov. 16, 2008 @ 13:59 GMT
Nice paper, Craig. I was struck by your segue between events and PDEs, however, on page 7, as reminiscent of quantum mechanics, while ultimately your argument depends on an argument about PDEs. Given your preference for an empiricist approach, I feel that you don't adequately justify a transition from epistemological issues of what we can measure to what seems to be an ontology of classical fields. To be specific, if we can observe only discrete events, an empiricist has some trouble in justifying even trajectories unless the events are Perfectly Correlated, even more in justifying the existence of (classical) fields as the cause of the events.

Personally, I justify fields pragmatically, as a useful and empirically effective class of models, not as a necessary explanation for events, so I'm curious to know if you're willing to step out of empiricism as far as a model-building methodology of Physics, which I take to be a constructive way to address the post-empiricist critique (you say you favor "versions with an empiricist slant", which is a perfectly nuanced statement except for its lack of commitment to a specific methodology).

Because your approach is so technical, for example in its invocation of the Cauchy problem for PDEs, I feel that it may not be robust to being reformulated in terms of quantum theory. A PDE approach to a /fluctuating/ field theory is immediately problematic, even if it's classical, because a field theory (loosely speaking) becomes not differentiable as soon as we introduce any kind of fluctuations.

Hamiltonian or Lagrangian formalisms for quantum theory clearly have trouble dealing with multiple time-like dimensions of space-time, so we would have to construct a more purely algebraic formalism for quantum or random fields; such a mathematics seems possible, at least for random fields, in which case I suppose we would have to accept that our 1-dimensional time would be contingent (the last part of this may well be incomprehensible unless you look at my "Lie fields revisited", Arxiv:0704.3420, J. Math. Phys. 48, 122302(2007), sorry, though my FQXi paper might be enough to get the idea).

Bookmark and Share

Dr. E (The Real McCoy) wrote on Nov. 16, 2008 @ 17:07 GMT
Love your paper Craig! Thanks so much for it!

You write, "What is the difference between time and space? This question, once a central one in metaphysics, has not been treated kindly by recent history. By joining together space and time into spacetime Minkowski sapped some of the spirit out of this project."

The secret, Craig, is to go back to the scene of the crime. Neither...

view entire post

Bookmark and Share

matthew kolasinski wrote on Nov. 19, 2008 @ 17:52 GMT
Dear Prof. Callendar,

i can't claim to be terribly good at advanced math, but have the impression that what you have done here with your hyperbolas is to re-discover the light cone.

going out on a limb a bit and predicting that the angle of the asymptotes of your hyperbolas will consistently be 45º.

whether or no my empirically intuitive impressions of what you have done prove correct, the relationships described in your paper are very interesting.


"Theories of laws of nature come in many forms, but I've always favored versions with an empiricist slant. Empiricist theories seek to explain the laws given the distribution of actual or observed facts, rather than going the other way round and explaining why the facts are what they are in virtue of the laws."

i've noticed that there appears to be a third category: explaining what the laws are by virtue of other laws.


thank you,

matt kolasinski

Bookmark and Share

Narendra Nath wrote on Nov. 29, 2008 @ 14:02 GMT
Dear Craig,

it is fascinating to see a presentation that emphsizes faith in time as a concept. Both space and time do help describe the phenomenon observed in our Universe. However, i see that spacetime/get joined as the forth dimension through the involvement of the velocity of light and its constancy. It does present problems if the value of c is changing. Some exptal evidence exisits that the velocity of light coming from a 12 billion old source is small degree higher than what we take as constant today. In my own essay, while discussing the first billion years of universe after Big Bang, i propose that the turmoil of those times may well demand significantly higher value of c . Besides, even the relative strengths of the four physical force/fields may well have varied from their currently taken relative values.

Similarly, the relation, E = mc^2, may need a relook too! Thus, the philosophical consequences arise that may affect the currently established physics!

Another interesting point that some authors here have discussed extensively concern the non-physical entity called ' consciousness ' as a possible creator of every thing physical, beginning from Big Bang, as it always exist independent of time. Your thoughts/ideas on htese aspects will be valuable.

Bookmark and Share

Cristi Stoica wrote on Nov. 30, 2008 @ 19:39 GMT
Dear Craig,

I liked your ideas of time as the great informer, and the comparison you make between space and time.

Perhaps the connection between information and Cauchy problem can be made stronger. One question: when you propose a criterion for selecting the time among other directions in spacetime, do you refer at selecting a particular direction, or a class of directions that can play the role of time?

Best wishes,

Cristi Stoica

“Flowing with a Frozen River”,

Bookmark and Share

John Young wrote on Nov. 30, 2008 @ 23:02 GMT
Dear Craig,

I want to tell you how proud the family is of you and how woefully ignorant we are about physics. But, you are not only brilliant but a nice person too.

Your proud Dad's cousin,

John Young

Bookmark and Share

Narendra Nath wrote on Dec. 19, 2008 @ 16:25 GMT
There is a justifiable connection between space dimensions and inertia/motion. On the other hand, there is similar connection between time and energy content. This gets reflected in the two uncertainty relations as the two respective conjugate parameters. As motion is the significant character in the physical universe, we apparently need both space and time as concepts to describe motion and the implicit energy source that gives rise to it is needed too!

Unless alternates to such concepts are evolved, one need to continue to believe till better conceptual picture emerges. The same may as well be true about Quantum Mechanics as explanation for tye motions in the micro-world or for that matter in explaining the anamolous behavior of black holes with intense density of matter/radiation. An alternate explanation can always be on the cards, as the human mind ia capable of reaching the ultimate limits of cosmic knowledge.

Bookmark and Share

Lawrence B. Crowell wrote on Dec. 31, 2008 @ 01:19 GMT
I enjoyed the clarity of your paper. The thesis of your paper is that hyperbolic DE's pose well formed Cauchy data. On this I would agree. There appears to be a converse problem however. Elliptic DE's, four dimensional Laplacian heat or diffusion equations for gauge terms have well behaved moduli spaces. Hyperbolic spacetimes obey the Lorentz group which are modulo Z_2 SU(1,1)xSU(2) as the product of a 3-rotational group and a hyperbolic group of transformations which define boosts. The transformation of the (A_+, A_-, A_3), by the hyperbolic g = e^{-ze_3) results in a nonHausdorf condition on the moduli.

This seems to lead to a curious situation. Hyperbolic Des have well defined Cauchy data, but a moduli space which has an irregular topology such as Zariski. Conversely an elliptic DEs on an open domain do not have well defined Cauchy problems, but a well defined Hausdorff moduli space.

A Hausdorff space obeys a Cauchy convergence condition, so that a sequence of connection terms will converge in a separable manner. However, in general we do not have well defined Cauchy data (same name but different concepts here) for hyperbolic DEs or gauge connections.


L. C.

Bookmark and Share

Narendra Nath wrote on Jan. 1, 2009 @ 13:30 GMT
It seems to me that the curvatures in space and time may be varying both ways as per the demands of the universe evolution. It may also be true that the Big Bang singularity may not be the sole reason fot the Universe we see. Other alternatives like expanding/contracting and /or matter/antimatter pre=existing universes had a role too. Also, at some stage the changing curvatures in space and time may not be coordinated togather, as per the theory of relativity demands on space/time! Unless, accurate comological data becomes available from far into space, the accuracy and precision may remain in uncertainties!

Bookmark and Share

robert wrote on Sep. 23, 2009 @ 09:42 GMT
moving dimension theory

Bookmark and Share

Don Saar wrote on May. 22, 2010 @ 18:22 GMT
Time is an accounting of the relative motions in space of bodies without which neither time nor space can be understood. Or, time is a system of accounting for the relative motion of bodies in space. And how we keep "score" is by means of human - invented clocks and calendars. See: and

Don Saar,, Relativity Calculator,

Bookmark and Share

Ralph wrote on Jun. 6, 2010 @ 04:02 GMT
Flawed relativistic theories aside, time need not flow in a single direction or even flow at all. To assume that WYSIWYG applies to the universe seems a little absurd to me. We perceive time to be linear because that is how we are wired. There is no unified theory because not only are we missing pieces of the puzzle we can't even imagine what the picture actually looks like.

Human perception is limited by what the brain can absorb, our perceptions of time may be the result of one of those limitations.

Bookmark and Share

Kostia wrote on Jun. 13, 2010 @ 13:57 GMT
Dear Craig,

I've just read your excellent paper in Scientific American. I am far from fundamental physics, but the idea of timeless universe resonated with my own thoughts. Since our observed world undergoes constant changes at all scales, it seems to be natural to assume that we need at least 4 dimensions to describe it; whether you call the extra dimension “time” or something else, it doesn't matter. Yet this 4-th dimension is so different from the other 3 coordinates and so dependent on often random interactions between various objects constituting the universe that it is very tempting to accept that time is imaginary and it makes sense only within subsystems, while the universe as a whole lacks this "artificial" property.

Yet, astrophysics theories claim that not only individual components of the universe have been changing since the big bang. The space itself went through "inflation" and continues to expand “over time”. It's hard to get how this concept can be consistent with the timeless universe model.

Bookmark and Share

Anonymous wrote on Sep. 19, 2010 @ 21:30 GMT
I believe the "If/then" statement(the most powerful equation in the universe)necessitates some sort of time continuum< Craig. But I really enjoyed your artical...

Bookmark and Share
report post as inappropriate

Re Ality (Facebook) wrote on Apr. 16, 2011 @ 15:40 GMT
Time and Space

Space is an intrinsic attribute of the existent entities which comprise our reality, time is not. Time should not be reified as if it is another physical dimension of our reality. All existent entities undergo change, and our concept of time is a function of our experience of that change. By putting similar entities in different situations, its value can be altered, but that is a function of the observation process (which we are able to quantify anyway). It is not an inherent characteristic of time, which only has one value. However, utilising time as a measuring tool helps in everyday life and in articulating scientific observations. As always, contemplations about what happens ‘beyond’ our existence might be fascinating, but are irrelevant metaphysical considerations when developing objective understanding.

© Paul Reed

April 2011

Bookmark and Share

Russ Otter wrote on Nov. 15, 2011 @ 21:52 GMT

The binding of existence

This is a story, built upon knowledge, intuition, and speculation. In the end, it is built upon some known theoretically successfully tested truths, and some unknowns conveyed in a formula that I consider trumps any objections – as we ponder the scope of existence. First we know of existence, by way of our self-awareness, coupled with scientific...

view entire post

Bookmark and Share

Login or create account to post reply or comment.

Please enter your e-mail address:
Note: Joining the FQXi mailing list does not give you a login account or constitute membership in the organization.