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 forums@fqxi.org 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.

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

**Undecidability, Uncomputability, and Unpredictability Essay Contest**

*December 24, 2019 - April 24, 2020*

Contest Partners: Fetzer Franklin Fund, and The Peter and Patricia Gruber Foundation

read/discuss • winners

**What Is “Fundamental”**

*October 28, 2017 to January 22, 2018*

*Sponsored by the Fetzer Franklin Fund and The Peter & Patricia Gruber Foundation*

read/discuss • winners

**Wandering Towards a Goal**

How can mindless mathematical laws give rise to aims and intention?

*December 2, 2016 to March 3, 2017*

Contest Partner: The Peter and Patricia Gruber Fund.

read/discuss • winners

**Trick or Truth: The Mysterious Connection Between Physics and Mathematics**

*Contest Partners: Nanotronics Imaging, The Peter and Patricia Gruber Foundation, and The John Templeton Foundation*

Media Partner: Scientific American

read/discuss • winners

**How Should Humanity Steer the Future?**

*January 9, 2014 - August 31, 2014*

*Contest Partners: Jaan Tallinn, The Peter and Patricia Gruber Foundation, The John Templeton Foundation, and Scientific American*

read/discuss • winners

**It From Bit or Bit From It**

*March 25 - June 28, 2013*

*Contest Partners: The Gruber Foundation, J. Templeton Foundation, and Scientific American*

read/discuss • winners

**Questioning the Foundations**

Which of Our Basic Physical Assumptions Are Wrong?

*May 24 - August 31, 2012*

*Contest Partners: The Peter and Patricia Gruber Foundation, SubMeta, and Scientific American*

read/discuss • winners

**Is Reality Digital or Analog?**

*November 2010 - February 2011*

*Contest Partners: The Peter and Patricia Gruber Foundation and Scientific American*

read/discuss • winners

**What's Ultimately Possible in Physics?**

*May - October 2009*

*Contest Partners: Astrid and Bruce McWilliams*

read/discuss • winners

**The Nature of Time**

*August - December 2008*

read/discuss • winners

Current Essay Contest

Previous Contests

Contest Partners: Fetzer Franklin Fund, and The Peter and Patricia Gruber Foundation

read/discuss • winners

read/discuss • winners

How can mindless mathematical laws give rise to aims and intention?

Contest Partner: The Peter and Patricia Gruber Fund.

read/discuss • winners

Media Partner: Scientific American

read/discuss • winners

read/discuss • winners

read/discuss • winners

Which of Our Basic Physical Assumptions Are Wrong?

read/discuss • winners

read/discuss • winners

read/discuss • winners

read/discuss • winners

Forum Home

Introduction

Terms of Use

RSS feed | RSS help

Introduction

Terms of Use

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

RSS feed | RSS help

RECENT POSTS IN THIS TOPIC

**Cristi Stoica**: *on* 12/17/08 at 16:41pm UTC, wrote Dear Dr. Girelli, Dr. Liberati and Dr. Sidoni, You raise a counterargument...

**Ettore**: *on* 12/8/08 at 11:23am UTC, wrote Thanks for the clear explanation. Ettore

**florian**: *on* 12/8/08 at 7:15am UTC, wrote dear Ettore, thanks for your question. Let us restate it: at first order,...

**Ettore**: *on* 12/3/08 at 14:20pm UTC, wrote Dear Florian, I have read your interesting work. I have a doubt concerning...

**Florian Girelli**: *on* 12/1/08 at 14:21pm UTC, wrote **Essay Abstract** From the Physics point of view, time is now best...

RECENT FORUM POSTS

**Steve Dufourny**: "In your theory wich is an assumption .....and in my theory wich is also an..."
*in* Towards the unification...

**Jonathan Dickau**: "In my theory... Early universe evolution happens under the octonions and..."
*in* Towards the unification...

**John Cox**: ""where work is the coherent component of energy radiated by the qubit." ..."
*in* The energetic cost of...

**Lorraine Ford**: "When are physicists going to stop lying? They CLAIM they believe in..."
*in* Understanding...

RECENT ARTICLES

*click titles to read articles*

**Time to Think**

Philosopher Jenann Ismael invokes the thermodynamic arrow of time to explain how human intelligence emerged through culture.

**Lockdown Lab Life**

Grounded physicists are exploring the use of online and virtual-reality conferencing, and AI-controlled experiments, to maintain social distancing. Post-pandemic, these positive innovations could make science more accessible and environmentally-friendly.

**Is Causality Fundamental?**

Untangling how the human perception of cause-and-effect might arise from quantum physics, may help us understand the limits and the potential of AI.

**Building Agency in the Biology Lab**

Physicists are using optogenetics techniques to make a rudimentary agent, from cellular components, which can convert measurements into actions using light.

**Think Quantum to Build Better AI**

Investigating how quantum memory storage could aid machine learning and how quantum interactions with the environment may have played a role in evolution.

RECENT FORUM POSTS

RECENT ARTICLES

Philosopher Jenann Ismael invokes the thermodynamic arrow of time to explain how human intelligence emerged through culture.

Grounded physicists are exploring the use of online and virtual-reality conferencing, and AI-controlled experiments, to maintain social distancing. Post-pandemic, these positive innovations could make science more accessible and environmentally-friendly.

Untangling how the human perception of cause-and-effect might arise from quantum physics, may help us understand the limits and the potential of AI.

Physicists are using optogenetics techniques to make a rudimentary agent, from cellular components, which can convert measurements into actions using light.

Investigating how quantum memory storage could aid machine learning and how quantum interactions with the environment may have played a role in evolution.

FQXi FORUM

January 27, 2021

CATEGORY:
The Nature of Time Essay Contest (2008)
[back]

TOPIC: Is the notion of time really fundamental? by Florian Girelli [refresh]

TOPIC: Is the notion of time really fundamental? by Florian Girelli [refresh]

From the Physics point of view, time is now best described through General Relativity, as part of space-time which is a dynamical object encoding gravity. Time possesses also some intrinsic irreversibility due to thermodynamics, quantum mechanical effects... This irreversibility can look puzzling since time-like loops (and hence time machines) can appear in General Relativity (for example in the Godel universe, a solution of Einstein's equations). We take this apparent discrepancy as a warning bell pointing to us that time as we understand it, might not be fundamental and that whatever theory, lying beyond General Relativity, may not include time as we know it as a fundamental structure. We propose therefore, following the philosophy of analog models of gravity, that time and gravity might not be fundamental per se, but only emergent features. We illustrate our proposal using a toy-model where we show how the Lorentzian signature and Nordstrom gravity (a diffeomorphisms invariant scalar gravity theory) can emerge from a timeless non-dynamical space.

F. Girelli has done his PhD in Marseille (France). He went to the Perimeter Institute (Canada) and SISSA (Italy) for some postdocs. He is now postdoc at the University of Sydney. S. Liberati has done his PhD in SISSA. He did a postdoc at the University of Maryland (USA) before becoming assistant professor at SISSA. L. Sindoni is currently finishing his PhD at SISSA.

Dear Florian,

I have read your interesting work. I have a doubt concerning the boundary conditions in the evolution problems which arise in your model. For the fields psi I would expect boundary conditions like those for the Laplace equation, namely some condition in the boudary of a closed domain of (4-dimensional) euclidean space. On the contrary for the perturbations phi I would expect different boundary conditions because the evolution equations are hyperbolic (thus the boundary conditions would have to be given on a Cauchy hypersurface). Now, psi'=psi+phi is just another psi type solution, thus it must satisfy the first type of boudary conditions that I mentioned above, but then I would expect phi to be constrained on the boundary of a closed set in 4 dimensional space and not on a open hypersurface. There seems to be a conflict between the evolution problems. How is it solved? Many thanks, Ettore

I have read your interesting work. I have a doubt concerning the boundary conditions in the evolution problems which arise in your model. For the fields psi I would expect boundary conditions like those for the Laplace equation, namely some condition in the boudary of a closed domain of (4-dimensional) euclidean space. On the contrary for the perturbations phi I would expect different boundary conditions because the evolution equations are hyperbolic (thus the boundary conditions would have to be given on a Cauchy hypersurface). Now, psi'=psi+phi is just another psi type solution, thus it must satisfy the first type of boudary conditions that I mentioned above, but then I would expect phi to be constrained on the boundary of a closed set in 4 dimensional space and not on a open hypersurface. There seems to be a conflict between the evolution problems. How is it solved? Many thanks, Ettore

dear Ettore, thanks for your question.

Let us restate it: at first order, the field Psi coincides with the perturbations phi, and in this sense, both of them must satisfy hyperbolic boundary conditions, if the Minkowski metric has emerged. This might seem contradictory with the fact that the field Psi a priori is living in a euclidian space and therefore should have some elliptic boundary conditions.

There is in fact no contradiction:

If one considers the full theory, that is we have nonlinear kinetic terms, there is a mismatch between signature of the metric and hyperbolicity-ellipticity, the latter being determined by a matrix involving the metric AND the derivatives of the fields. More precisely, the signature of the metric tensor determines the hyperbolicity-ellipticity only for canonical kinetic terms, for which the matrix in front of the second derivatives in the PDE is just the metric tensor. For nonlinear kinetic terms (our case), this is not true.

As a consequence, when dealing with the full theory, the chosen boundary conditions can be elliptic or hyperbolic, but when restricted to the (linear) operator constructed at *first* order (so that everything becomes linear), they can definitely be interpreted as hyperbolic boundary conditions, and there is no mismatch.

Hope this helps!

Florian, Stefano, Lorenzo

Let us restate it: at first order, the field Psi coincides with the perturbations phi, and in this sense, both of them must satisfy hyperbolic boundary conditions, if the Minkowski metric has emerged. This might seem contradictory with the fact that the field Psi a priori is living in a euclidian space and therefore should have some elliptic boundary conditions.

There is in fact no contradiction:

If one considers the full theory, that is we have nonlinear kinetic terms, there is a mismatch between signature of the metric and hyperbolicity-ellipticity, the latter being determined by a matrix involving the metric AND the derivatives of the fields. More precisely, the signature of the metric tensor determines the hyperbolicity-ellipticity only for canonical kinetic terms, for which the matrix in front of the second derivatives in the PDE is just the metric tensor. For nonlinear kinetic terms (our case), this is not true.

As a consequence, when dealing with the full theory, the chosen boundary conditions can be elliptic or hyperbolic, but when restricted to the (linear) operator constructed at *first* order (so that everything becomes linear), they can definitely be interpreted as hyperbolic boundary conditions, and there is no mismatch.

Hope this helps!

Florian, Stefano, Lorenzo

Dear Dr. Girelli, Dr. Liberati and Dr. Sidoni,

You raise a counterargument against the fundamental nature of time: the discrepancy between the irreversibility and the possibility of closed timelike curves. Starting from here, and from condensed matter inspired emergent gravity, you construct an interesting toy model, which shows a possible mechanism for the emergence of Lorentz and diffeomorphism symmetries (and time). I think that this idea worth to be explored.

Congratulations,

Cristi Stoica

Flowing with a Frozen River

You raise a counterargument against the fundamental nature of time: the discrepancy between the irreversibility and the possibility of closed timelike curves. Starting from here, and from condensed matter inspired emergent gravity, you construct an interesting toy model, which shows a possible mechanism for the emergence of Lorentz and diffeomorphism symmetries (and time). I think that this idea worth to be explored.

Congratulations,

Cristi Stoica

Flowing with a Frozen River

Login or create account to post reply or comment.