Essay Abstract

Are we moving in the right direction on the road to quantum gravity? It seems to me that there is a serious conceptual problem with most current approaches. I discuss a simple guiding principle which I believe to be quite obvious, but which has nevertheless been largely neglected so far. This leads me to argue that the most relevant approach to quantum gravity might be a framework of emergent gravity based on insights from condensed matter physics. Within this framework, I discuss a thought experiment based on the idea to describe quantum gravity in a first approximation simply as a Minkowski spacetime containing nearly-point-like objects or `atoms' of spacetime, from which the geometry of our universe and its physics emerge as an effective, internal description based on collective excitations. Based on this thought experiment, I will try to convince you that there are strong spatiotemporal limits on the knowledge that we can possibly achieve with respect to quantum gravity.

Author Bio

The author has studied electronical engineering, philosophy and fundamental physics at the universities of Leuven (Belgium) and Madrid (Spain), and has recently achieved a PhD in quantum gravity from a condensed-matter perspective.

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Dear Gil Jannes,

My gravitation theory also is based on existence of atoms of space-time. However, these atoms of space-time possess extension property, therefore they cannot be point-like objects. What you think, the space-time has a 'solid' structure or it is a 'soup' of fluctuating atoms? If atoms are virtual particles that disappear and appear continually, then a holes in space-time must exist. If an atom disappears, it creates a vacant place without space-time properties (a hole in space-time). Since a hole does not possess the extension and time properties, one collapses quickly. However, the lifetime of hole is non-zero because environment is not able to fill a hole instantly.

Since you admit the existence of atoms of space-time, you must admit also holes in space-time.

Sincerely, Leshan

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Hi Gil,

I point out that there is a clear distinction to be made between quantum fluctuations and thermal fluctuations, that the former are Lorentz invariant (in consequence of the Lorentz invariant definition of the vacuum state of a quantum field), while the latter are not. This difference is reflected in the difference of units between hbar and kT, action in contrast to energy. For a free quantum field, this can be laid out very clearly, as in my Phys. Lett. A 338, 8-12(2005) [arXiv:quant-ph/0411156]. It has not been clear to me that there is an analogue of Lorentz invariant fluctuations in the condensed matter analogue.

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Dear Gil Jannes,

I found your essay most enjoyable and informative.

"Assume that there exists a condensed matter system of sufficient complexity such that it contains a whole internal universe."

I'm assuming that this condensed matter system has more the nature of a "perfect fluid" than a "bunch of particles".

Then you note, "A second motivation to take emergent gravity seriously, is that it has something sensible to say about the accelerated expansion of the universe, which seems to require the existence of some form of repulsive `dark energy'." This should follow from the original assumptions, at least in the ideal case.

I also liked your observation that "from a direct extrapolation to our universe, one would expect the vacuum energy of the universe to be approximately equal to the energy content of the matter component (baryonic or `normal' matter plus dark matter): a 50-50 distribution. Observations indicate that the distribution is in fact approximately 70% dark energy and 30% matter. By cosmological standards, 50-50 is an excellent prediction, certainly better than simply saying that the vacuum energy is a random value from a range of roughly 10**500 possibilities, as is sometimes claimed."

Although my essay does not directly support yours, I believe that they overlap in some areas. I invite you to read my essay and make any comments you feel appropriate. I also would say that, if you have the same trouble bringing consciousness into physics that many do, simply ignore the consciousness interpretation and focus on the pure physics implied by the existence of the gravi-magnetic field. I believe it may stimulate new ideas about quantum gravity that are not totally divorced from yours.

Edwin Eugene Klingman

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Hi Gil: How do you see the following?

The most elemental/fundamental/deepest way (or manner) in which human thought is [comprehensively and consistently] enmeshed and interactive with physical (and this includes sensory, of course!)experience is the source of our deepest genius and of the deepest and broadest conclusions/unifications that are revealed (and possible) in physics.

This above is in keeping with the FACT that the ability of thought to describe OR reconfigure sense is ultimately dependent upon the extent to which thought is similar to sensory experience.

Electromagnetric space (e.g., photons and the Sun) is both larger and smaller than ordinary or typical space (such as the Earth). When space manifests as gravitational/electromagnetic energy, scale is then balanced, space is particle/wave, invisible/visible, and larger/smaller. Accordingly, space is both repulsive and attractive as well.

Essay author Frank Martin DiMeglio

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