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December 2, 2016 to March 3, 2017
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Trick or Truth: The Mysterious Connection Between Physics and Mathematics
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Media Partner: Scientific American


How Should Humanity Steer the Future?
January 9, 2014 - August 31, 2014
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It From Bit or Bit From It
March 25 - June 28, 2013
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Questioning the Foundations
Which of Our Basic Physical Assumptions Are Wrong?
May 24 - August 31, 2012
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Is Reality Digital or Analog?
November 2010 - February 2011
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What's Ultimately Possible in Physics?
May - October 2009
Contest Partners: Astrid and Bruce McWilliams

The Nature of Time
August - December 2008

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My Emilly: on 4/28/18 at 6:40am UTC, wrote Thank you for your post, I look for such article along time, today i find...

Kevin Ingram: on 3/17/18 at 10:48am UTC, wrote Yeah, time is change and we learn new things and I have found apexessays...

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Fuzzballs v Black Holes
A radical theory replaces the cosmic crunchers with fuzzy quantum spheres, potentially solving the black-hole information paradox and explaining away the Big Bang and the origin of time.

Whose Physics Is It Anyway? Q&A with Chanda Prescod-Weinstein
Why physics and astronomy communities must take diversity issues seriously in order to do good science.

Why Time Might Not Be an Illusion
Einstein’s relativity pushes physicists towards a picture of the universe as a block, in which the past, present, and future all exist on the same footing; but maybe that shift in thinking has gone too far.

The Complexity Conundrum
Resolving the black hole firewall paradox—by calculating what a real astronaut would compute at the black hole's edge.

Quantum Dream Time
Defining a ‘quantum clock’ and a 'quantum ruler' could help those attempting to unify physics—and solve the mystery of vanishing time.

June 24, 2018

CATEGORY: The Nature of Time Essay Contest (2008) [back]
TOPIC: What Makes Time Special by Craig Adam Callender [refresh]
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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

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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).

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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...

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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

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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.

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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”,

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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

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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.

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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.

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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!

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

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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,

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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.

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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.

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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...

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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

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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...

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Tina Blackwood wrote on Jun. 27, 2017 @ 11:46 GMT
My colleagues are more than interested in this contest. The are the academic writers and would love to try themselves in creative writing. Should I also write my essay if I am a professional writer or it would not be fair?

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sweety 31 wrote on Aug. 19, 2017 @ 07:32 GMT
yalla shoot streamingFittingly, fans showered the 28-year-old Durant with chants of “M-V-P, M-V-P” when he arrived in New Delhi last Thursday — the first of many outpourings of support for the 2013-14 regular-season MVP in a country where basketball is the second-fastest-growing sport among boys and girls.

“It’s been overwhelming, this much love,” Durant said during the trip. “It’s definitely appreciated.”The eight-time All-Star showed his appreciation on Friday when he became the first active NBA player to visit The NBA Academy India, an elite basketball training centre for the country’s top male and female prospects located in the Delhi National Capital Region.

The academy, which opened in May, builds on the NBA’s basketball and youth development initiatives in India. The Reliance Foundation Jr. NBA program has reached more than six million youth nationwide since its launch in 2013. In addition, the first two NBA Basketball Schools — a network of tuition-based development programs around the world open to international male and female players ages 6-18 — launched in Mumbai in April and in New Delhi in June.

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sweety 31 wrote on Aug. 19, 2017 @ 07:48 GMT
yalla-shoot.comAt the Academy, Durant led prospects through a series of shooting, passing, dribbling and defensive drills before conducting a large-scale basketball lesson for 3,459 youth from the Reliance Foundation Jr. NBA program. Between the youth on site and those who participated virtually from Bengaluru, Chennai, Hyderabad and Kolkata via satellite, the clinic set a Guinness World Record for the largest basketball lesson at multiple venues.

“Sharing my love of basketball with thousands of boys and girls across India was an unbelievable experience,” Durant said. “It was an honour to coach these amazing kids and to be a part of this special day.”

Durant’s day also featured the unveiling of two new basketball courts built for the Ramjas School as part of the Kevin Durant Charity Foundation.

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