Horizon Complementarity

Sean Carroll had an interesting post at the Cosmic Variance blog. The post discusses the idea, outlined in a couple of recent papers, of finding a concordance between the multiverses which exist according to some speculative cosmological models and the many-worlds interpretation of quantum mechanics. Carroll provides a sketch of his own thoughts about how this might work. (The referenced papers are by Nomura and Bousso & Susskind).

I have some thoughts about this broad question, but for now I want to highlight one key notion utilized in the discussion, which is that of “horizon complementarity”.

I was familiar with the holographic principle, which says roughly that the information about what is inside a region of space-time can be encoded on the surface boundary of the region. This idea developed from the study of black holes, where it was earlier theorized that black hole entropy was proportional to the area of its event horizon. Horizon complementarity is likewise an extension of another idea which was developed in the study of black hole entropy/information paradox. Here’s a lengthy excerpt from Carroll (who is skilled in explaining difficult topics to a general audience - see the original for embedded links):

Is the Universe 2-Dimensional at Short Distances?

We don't have a theory of quantum gravity, but we have a number of research programs on the case. Steve Carlip has a paper (The Small Scale Structure of Spacetime) which discusses an intriguing fact: many of these otherwise disparate programs display or imply the idea that our familiar four dimensional (3 spacelike + 1 timelike) spacetime may be two dimensional (1+1) at high energies/short distances.

If 4 dimensional spacetime is an emergent phase, and the more fundamental physics is comprised of elementary, causally linked, quantum bits of some sort, you might expect this kind of dimensionality.

Here is the physics arxiv blog article on the paper.

Quantum Measurement in an Infinite Universe

Anthony Aguirre, Max Tegmark, and David Layzer have an intellectually stimulating paper on Arxiv called “Born in an Infinite Universe: a Cosmological Interpretation of Quantum Mechanics”. They seek to show that if eternal inflation has led to an infinite, statistically uniform universe which therefore contains innumerable exact copies of our local region, then this leads to a new interpretation of quantum mechanics. Specifically they say we can associate the Born rule probabilities of QM with the actual frequency of measurement outcomes realized across the identical spatially distributed experiments. In other words, when we do an experiment, the uncertainty in the outcome is a result of our ignorance of which copy we are. (Layzer has a related paper posted here).

No Final Theory

I read Marcelo Gleiser’s book, A Tear at the Edge of Creation. Gleiser is a physicist who has begun to wax reflective about the methods and limits of science. His main message is that the idea of a final theory is misguided and counterproductive (Gleiser blogs at the NPR 13.7 blog; he has a recent talk online here; hat tip goes to Peter Woit’s blog).  [UPDATE: 25 June 2010:  Prof. Gleiser responds to some criticisms here.]

Order Underpins Everything

I discovered the work of Kevin H. Knuth, and took a dive into his papers and this recent talk given at the Perimeter Institute. The theme of his research is that a simple ordering relation among elements is more fundamental than, and can be used to derive, more familiar theories. The talk is entitled “The Role of Order in Natural Law”, and was part of a workshop on the topic of laws of nature.

Quantum Interactions Create Space-time

The notion that spacetime is an emergent phenomenon is, by my reckoning, being proposed by an increasing number of thinkers. Physicists and philosophers working in quantum gravity and quantum foundations are turning to the idea that the spacetime of relativity is not fundamental, but rather something which arises from a more fundamental world of quantum mechanical systems and their interactions.

I just saw a reference to one such argument which was made a few years ago in an article by Avshalom C. Elitzur and Shahar Dolev called “Quantum Phenomena Within a New Theory of Time”. This was published in the 2005 collection Quo Vadis Quantum Mechanics?, Avshalom C. Elitzur, Shahar Dolev, Nancy Kolenda, Eds.

Elitzur and Dolev examine several puzzles over the nature of time in quantum mechanics and are led to the hypothesis that quantum interactions (measurements) themselves are responsible for the creation of spacetime.

A couple of quotes from section 17.10, titled “An Outline of the Spacetime Dynamics Theory”:

Another Argument for Emergent Gravity

I have followed with interest a growing body of opinion among physicists that gravity (and space itself) is best thought of as an emergent phenomenon (most recently here).  Erik Verlinde has a paper, called On the Origin of Gravity and the Laws of Newton, which presents a heuristic case for gravity as emergent.

Unlike most of the other research papers I've blogged about, this is not a quantum gravity theory, but rather uses a number of concepts in mainstream physics (thermodynamics, the holographic principle) to derive emergent gravity.  He says that if one coarse grains a microscopic theory (whose precise dynamics need not be known), and applies the holographic principle to measure information on partition screens between particles, the information on the screens will give rise to an entropic force - this is gravity.

The paper has engendered discussion (I first saw it mentioned by Peter Woit here;  there is some appreciation here, and criticism here -- Verlinde responds here).  The main criticisms are that Verlinde's points are either not new, or that they embody circular reasoning (since concepts from Newtonian and post-Newtonian physics are used to derive Newtonian gravity).  Verlinde responds that he is bringing out a new insight which should help convince people that gravity is not a fundamental force, but is emergent.

I can't adjudicate the disagreements, but I think it's very suggestive that the argument for emergence continues to gain adherents.

I also think it is interesting to note that in Verlinde's model the microscopic theory, while not defined in any detail, must have a well-defined asymmeterical time dimension, as in the emergent quantum gravity theories I've reviewed.  "Time is fundamental, while space is not".

[UPDATE 22 Jan.2010:  A couple of more related links (HT).  A New Scientist article, and an illuminating preprint from Lee Smolin, who works through a Verlinde-type derivation in a different way, utilizing ideas from Loop Quantum Gravity research (altho note the specifics of LQG are actually not very important to the analysis).  He does a very good job placing the Verlinde work in context of other research and shows where it seems to add new value.]

A Crystallizing Universe

The use of a phase transition to describe reality pops up in this paper by George Ellis and Tony Rothman: “Time and Spacetime: The Crystallizing Block Universe.”

I had previously read Ellis’ contribution to the FQXi contest on time: “On the Flow of Time”. In that essay, Ellis criticized the notion of picturing the universe as an unchanging four-dimensional space-time block, and proposed a model of an “Evolving Block Universe”, which includes the indispensable notion of time flow. In this new paper, Ellis and Rothman fine-tune this idea.

They associate the flow of time with the transition from a quantum future to a classical past: this transition is marked by the time-irreversible process of quantum state-vector reduction (measurement). They note, however, that phenomena displayed in certain quantum set-ups (delayed choice and quantum eraser schemas) show that the transition process doesn’t take place uniformly. This non-uniform nature of the transition inspired the crystallization metaphor.

I like that Ellis takes quantum measurement seriously as a natural process (actualizing potentialities) and links this transition to the experienced flow of time.

Why Hard Questions are Hard: The Cosmos as a Phase of Being

Why is reality mysterious? Why should difficult questions persist for so long despite the successes of physical science?

An answer to these meta-questions may lie in the concept of a phase transition. As discussed in prior posts (like this recent one), a school of quantum gravity research has arisen which explores the idea that the visible cosmos (of matter bound in space-time geometry) arises at lower energies from a more fundamental quantum world. This more fundamental level is usually characterized by a network of quantum systems, subject to a directional causal arrow, but otherwise connected in a highly non-local fashion (little or no recognizable spatial geometry).

This model is inspired by the myriad examples of phase transitions observed in nature, and particularly those in the field of condensed matter physics (which utilizes the toolkit of quantum field theory to describe the phenomena). Superconductors, superfluids, etc. display remarkable emergent features which arise under certain pressure/temperature conditions.

Picture our familiar physical cosmos as a portion of reality which condensed into a “classical” phase, but retained subtleties in its nature which reflected its pre-transition roots. If this analogy works, then it would explain our situation: while classical explanations usually work well, some phenomena defy such analysis because their foundations go deeper. This could be the case, for instance, for the arrow of time and for conscious experience itself.

String Theorist Turns to Emergent Gravity Approach

Inspired by phase transitions displayed in condensed matter physics, Petr Hořava has constructed a model where the time dimension is decoupled from space at high energy/short distance while the space-time characteristic of relativity (and Lorentz invariance) emerges at low energy/long distances. The model, which is a quantum field theory, is able to be renormalized in a way GR itself cannot be.  The paper, “Quantum Gravity at a Lifshitz Point,” sets out the theory.

Re-defining Where We Live

(A short rant for a Friday)

If you’re like me, you learned the following story.

The universe, or cosmos, consists of a four-dimensional space-time continuum which contains matter and energy. It all began with a big bang singularity: time as well as space started then, so it doesn’t make sense to ask what happened “before”. The universe probably extends beyond what is observable, but the same physical laws prevail everywhere. Nothing exists outside the universe.

Every statement in that paragraph is likely wrong.

Physics Links and Notes

Here are three interesting things I read recently.

1. Lee Smolin has an article titled “The unique universe” in physicsworld (hat tip: Not Even Wrong). It covers some of the same ground as the video I had earlier posted here. In it he argues against some ideas which have been recently popular among physicists when considering the shape of the next fundamental theory of cosmology. First, many now argue that our universe is just one of a vast or infinite number of others: the multiverse. Also, it is argued that the fundamental theory will be timeless, since they see our experience of the flow of time as an emergent local phenomenon. This leaves us with a vision of a timeless and static multiverse.

Smolin says advocates of this vision are led by mistaken reasoning. One problem arises when physicists take the essentially Newtonian schema we use to evaluate systems within the universe (deterministic laws + initial conditions) and try to apply it to the entire cosmos. This leads them to try to describe a process for selecting our universe from a landscape of many universes (anthropically or otherwise). Smolin argues that we would do better to explore theories which take time to be fundamental, and where laws can vary in a process of cosmic evolution.

2. Many people are optimistic that an information-theoretic perspective will lead to new insights in exploring the foundations of quantum mechanics, and this multi-authored paper, called “A new physical principle: Information Causality”, is an interesting effort in this regard. Information causality is, according to the authors, a principle which helps pick out QM from a space of possible theories which, like QM, feature entangled correlations but allow no faster than light signaling. While the principle seems simple when stated (“communication of m classical bits causes information gain of at most m bits”), the fact that other (hypothetical) theories which feature strong correlations don’t meet it is notable. Hat tip goes to this post by the Quantum Pontiff which has some helpful discussion.

3. I had come across the essay “Free will, undecidability, and the problem of time in quantum gravity” by Rodolfo Gambini, which was submitted to the FQXi contest (see here), but I didn’t immediately catch on to his arguments. But now having reviewed two papers on Arxiv by Gambini and colleagues (here and here) I have a better idea what his program is. The key starting point is this: the mathematics of quantum mechanics treat time as an external infinitely divisible classical variable; Gambini et.al. think that fundamental limitations on the practical measurement of time within the physical world have implications for how we should interpret the problem of quantum measurement. For instance, if we look at decoherence theory, we see that a quantum superposition involving a system, a measuring device and the environment can evolve such that the degrees of freedom responsible for interference are dispersed. But decoherence itself says nothing about a measurement taking place -- the system is still evolving unitarily. Gambini et.al. argue that a point comes where no possible mechanism is available to tell whether or not a measurement outcome (or event) has or has not taken place. They think this undecidability threshold can be seen as the marker for when an event has occurred. (Then, in the essay, Gambini waxes philosophical and speculates that this undecidability between evolution and collapse might create space for free will.) A thread about this in physicsforums is here.

I liked reading Gambini’s papers, but I think the calculations regarding the undecidability point are controversial, given that a full explanation would require a theory of quantum gravity. And if my preferred approach to QG is right -- where time is a fundamental aspect of a pre-gravity microscopic quantum theory, and the particles and space-time geometry of current theories are emergent regularities -- then I suspect that the constraints on describing a physical clock would not arise in the same way as it does here.

More on the Fundamental Status of Time

Here are two more links to arguments for why time is fundamental:

A very good talk by Lee Smolin at a Perimeter Institute conference last fall. I thought he did a very good job in explaining how physics got into the practice of viewing time in a geometric fashion (with no important role for the present moment) and why this will not work when formulating a theory of the universe as a whole.

Here's George Ellis in another entry from the FQXi essay contest explaining why it is a mistake to try to describe the universe without time asymmetry.

Markopoulou: Time is Fundamental, Space is Not

The Foundational Questions Institute has run an essay contest on "The Nature of Time" and received a wide variety of responses. These come from well known physicists, other academics, and amateurs alike. Because of time contraints I've only read a few, beginning with authors I recognized (there are likely some "diamonds in the rough" if one plows through all the contributions).

Fotini Markopoulou of the Perimeter Institute, whose work I mentioned in the last post (and several older ones), wrote: "Space does not exist, so time can." She has a talent for writing clearly about these deep concepts, and I find her arguments persuasive (even if her work toward a full theory of quantum gravity still has a long road ahead). So I highly recommend the essay.

Kudos also to cosmologist and blogger Sean Carroll for his nice essay: "What if Time Really Exists" (here is the Cosmic Variance post introducing it). While I don't like some of his specific suggestions (associating time's arrow with macroscopic entropy considerations), I liked the stance he takes in the essay.

For countervailing views you can read the contributions of Carlo Rovelli and Julian Barbour.

UPDATE (7 January 2009): I just found this interesting post by Scott Aaronson - "Time: Different from space" which includes his computer science-derived insight on why time (causal structure)is fundamental.

Aether Makes a Comeback

The nineteenth century version of the ancient concept of the aether (or ether) was killed by the Michelson-Morley experiment and the success of Einstein’s theory of special relativity. Electro-magnetic radiation needed no substance to support wave propagation. Of course we did not revert to a view of space as an void sprinkled with a few solid objects. In modern particle theory, space-time is pictured as filled with matter fields. And in general relativity, space-time is revealed as a dynamic actor, not just a backdrop. Still, space-time remains distinct from matter/energy, and is geometric, rather than substantive. It thus retains a bit of the conceptual flavor of an empty container (a related discussion on the blog is here).

I was surprised to see the number of physics papers on arxiv which invoke the concept of aether (or ether) in the context of theoretical proposals to solving outstanding problems (e.g. dark energy). For me, aether was brought to mind by certain quantum gravity research programs.These propose that the space-time of general relativity is not fundamental: it emerges (along with the matter fields of the standard model) from something more basic – an underlying network of elementary quantum systems. This underlying network is not itself defined against a spatial backdrop and lacks the usual notions of distance or locality. Both space-time geometry and matter as we know them are constituted by the quantum systems: they arise from the aether.

For an example of this kind of work, here’s the second “quantum graphity” paper from Fotini Markopoulou and colleagues (the authors do not invoke the term aether, so don’t blame them!*). The introduction does a good job of discussing the stance they are taking toward the space-time of general relativity, and places this in the context of how other quantum gravity research programs approach the issue.

* Although they do link their work to the model described in this paper: “Quantum ether: photons and electrons from a rotor model” by Levin and Wen.

{UPDATED 19 November, 2008: Minor edits; 8 December 2008: Sean Carroll at Cosmic Variance just posted about his collaboration on aether field models.}

What Lies Beyond the Big Bounce

We don’t have a fully developed theory of quantum gravity yet, but there is one consequence of the theory we already know: it will banish general relativity’s space-time singularities from our conception of the universe. In particular, the idea of the big bang needs to be retired after decades of dominating professional and popular views of cosmology: the observed universe did not begin as a singularity but rather grew out of a pre-existing reality – a “big bounce”.

Martin Bojowald had a nice article in SciAm recently ("Follow the Bouncing Universe" in the print edition). Bojowald is a loop quantum gravity theorist: while loop theory has not produced an adequate theory for quantum gravity (and I think it probably won’t), it has produced formalisms that may be useful for constructing models which offer insight into the question of what will replace singularities in QG. This work goes under the rubric “loop quantum cosmology (LQC)”. I also noticed that Bojowald’s senior colleague Abhay Ashtekar has a paper out summarizing the results of work in LQC.

What intrigues me is their exploration of what the region on the other side of the big bounce might be like.

In his article, Bojowald first outlines the idea that space-time in QG is not a continuum, but rather has a fine-scale fundamental structure. These space-time “atoms” follow the rules of quantum mechanics and therefore the physics that prevails at high energies/short distances will differ from general relativity (GR). Specifically, in the loop model, a repulsive force comes into play at high energy densities, preventing singularities. In the case of the big bang, one scenario is that the initial high density state arose when a pre-existing universe collapsed (hence – a “bounce”). Bojowald describes an early, simplified, model which seemed to imply that the pre-existing universe was similar to our own. However, Bojowald says his own subsequent work found that quantum effects would have dominated the immediately pre-existing world:

“So the bounce was not a brief push by a repulsive force, like the collision of billiard balls. Instead, it may have represented the emergence of our universe from an almost unfathomable quantum state – a world in highly fluctuating turmoil.”

Bojowald finishes by discussing how we might learn more about the pre-existing universe from astronomical clues.

Ashtekar’s paper discusses the same research more formally; in addition he also deals with LGC models for black holes, where again singularities are replaced by quantum regions (somewhat surprisingly to me, black holes are somewhat more difficult to model than the big bang itself). He concludes his discussion of the big bang/bounce this way: “Big bang is not the Beginning nor the big crunch the End. Quantum space-time appears to be vastly larger than what general relativity had us believe!”

My takeaway is that a realm of quantum possibilia extends beyond and surrounds us our island of observable cosmos. The old idea of the universe as a relatively straightforward, neatly bounded space-time container must be discarded.

3 Links

I'm currently reading Theism and Ultimate Explanation: The Necessary Shape of Contingency by Timothy O'Connor. I'm very interested in the cosmological argument from contingency, and this book is an up-to-date take on that and related metaphyical issues. I hope to have a post on this at some point but in the meantime here is a review from a naturalist's perspective by Graham Oppy (HT: sideblog at FQI).

I really enjoyed this insightful cartoon posted at Cosmic Variance along with the comments by Sean Carroll (the original source for the cartoon is here). No offense to cosmologists, but for purely philosophical reasons I think it is best to identify the actual world with the observable or causally connected universe (not that I think that's all there is, but because the regions we assume exist beyond the observable have a different ontological as well as epistemological status - see a related post here).

Finally, I want to post a friendly link to the discussion forum at Panendeism.org. Panendeism, as I understand it, is meant to be like Panentheism, but with the "deism" label stressing that this is a worldview arrived at through reason, without reliance on authority or revelation.

Reduce Everything to Space-time?

[UPDATED 25 Sept.2009: Fixed Links]
I want to quickly comment on an interesting post by Justin at Panexperientialism. In it he reviews a book by Freya Mathews (called The Ecological Self) and also discusses a draft paper by Jonathan Schaffer (Spacetime the One Substance). Please check out his post, which discusses many aspects of Mathews’ ideas in particular beyond what I’m picking up on here (I have not read the book).

Both Mathews and Schaffer advocate a monistic metaphysical view where matter is effectively reduced to space-time.

I agree with these authors that the dual scheme of {space-time container plus material objects} must be rejected, but think they are slightly off-track in wanting to reduce the properties of matter fields to space-time (at least space-time anything like we currently think of it).

These brief comments focus on the relationship of this idea to the work of theoretical physicists. Mathews acknowledges that an early attempt to derive this reduction from general relativity failed (Wheeler’s Geometrodynamics), but still likes the metaphysical vision for philosophical reasons. In his paper Schaffer argues toward a similar goal, but along the way I think he overstates the degree to which GR and (especially) quantum field theory as we know them are congenial to this vision. QFT has matter fields housed in a separate space-time container. In GR the matter and space-time are dynamically intertwined, but the fact that you can model the geometry while leaving out matter shows that they remain distinct.

In some ways the quest for a theory of quantum gravity can (should?) be viewed as a quest for a monistic theory which is rid of the dual scheme. I continue to try to follow the different theories as a layperson to see how they come down on this issue.

String theory: originally an extension of QFT which retained the feature of having fields on a background space-time. Has evolved in many ways over the years and maybe can overcome this starting point (?).

Loop Quantum Gravity and Causal Dynamical Triangulations: these seek to formulate a quantum version of space-time with the promise of integrating matter into the picture later. I’m not sure if this promised integration would be more monistic than GR.

Causal Sets; Quantum Causal Histories/Geometrogenesis; Internal Relativity; Quantum Computing and Condensed Matter-based approaches: these programs seem best on this question as they try to specify a monistic underlying micro-theory from which space-time and matter fields as we know them may simultaneously emerge.

I would note that if the latter sort of approach works, it doesn’t support Schaffer’s advocacy of priority monism (see my previous post on this topic). The underlying network would not be a very coherent whole, but a fairly ill-behaved evolving pluralism of micro-events. Even though Schaffer wants to overcome the container/object scheme, his view of space-time as the holistic fundamental object still has a bit of a hangover from the container idea in my opinion.

Group Field Theory and Emergent Space-Time

This paper by Daniele Oriti includes some ambitious ideas toward a theory of quantum gravity. In its first sections, he introduces his preferred formalism, called Group Field Theory (GFT). He shows how this formalism offers a framework general enough to incorporate aspects of other quantum gravity approaches. He then draws some lessons from these other approaches to suggest a path toward a successful theory by which space-time may be seen to emerge from a discrete quantum micro-structure using a GFT. Interestingly, in light of my last QG post, he takes inspiration from condensed matter theory in advocating his ideas. (My thanks to the anonymous commenter who suggested I look at this paper).

I had come across Oriti’s work before, and my first casual impression was that if GFT was a generalization of quantum field theory which hoped to incorporate gravity, then it might not be too interesting. I had taken to heart the criticisms that approaches which start by extending QFT (like the original string theory) were flawed by not being “background-independent”. Field theory is formulated against a flat space-time background, so how can you get space-time back out of it? As Oriti describes the formalism, while it is a true species of QFT, the way he uses it can be interpreted as modeling pre-geometric discrete quantum gravity elements. If so, then the QFT origin of the mathematical structure may not be an issue. In any case, I’m in no position to make judgments about the merits of the formalism, so I’ll just try to summarize here some the interesting ideas which arise as Oriti explores this framework.

He says the GFT can describe a quantum field in terms of fundamental variables which can be represented either as spin network vertices or elementary (d-1) simplices. Therefore he can draw connections to both the loop quantum gravity/spin foam and dynamical triangulations research programs. He says while there are open issues here, it appears that the GFT formalism can be seen to incorporate enough of these theories (and quantum Regge calculus as well) that he can draw some new lessons from examining certain features of these models from within the GFT framework.

Let me try to see if I can relate what he says the main lesson is (section 3.4 of the paper). These theories have tried to get dynamics from path integrals of the discrete structures they start with. Oriti says what results are the physics of (only) “few-particles”; these approaches lack a way to get interesting large –scale “many-particle” physics which would offer a chance to reveal an emergent space-time “continuum”. GFT offers a way to do a second quantization and field-theoretic analysis of the same starting structures in order to study the complex features which come in the many-particle regime. It is in this regime where we would hope to find an approximation of the continuum space-time described by General Relativity.

One exception to these perceived limitations of the other theories is the Causal version of Dynamical Triangulations (my post on this is here). In this approach, the micro-variables are stripped down to include only causally ordered ones, and the resulting path integral analysis has given interesting results in terms of an emergent four dimensional structure. Oriti suspects, though, that the strict limitations put imposed in CDT may lead one to again prefer analyzing the more general results which can come from using the GFT approach.

Oriti says that condensed matter physics shows the usefulness of field-theoretic and 2nd quantization approaches to studying the collective behavior and statistical properties of many-particle physics. He thinks we should consider quantum space-time as a condensed matter system, with the discrete structures of the GFT formalism as the atoms of space-time, and the continuum space-time as an emergent collective regime. General Relativity would be a hydrodynamic effective description of a quantum space-time fluid. Condensed matter techniques, themselves based on QFT, can point the way for how to research this possibility within GFT. Toward the end of the paper, Oriti offers a speculation that the Bose-Einstein condensate may be the specific analogue to look at (section 7 of the paper). His outline for how this would work is hard for me to follow. Some of the choices one makes in setting the terms in the GFT model seem important, but I can’t offer any opinions on this.

As I’ve said before, I like the idea of having a theory where a discrete quantum micro-physics leads to the space-time of GR in an emergent regime. So Oriti’s work is one I will try to follow as I have the other programs which have this feature. I also like that he wants to incorporate condensed matter physics as a guide to how this works. The parallels between condensed matter physics and fundamental physics are so suggestive that this link should be pursued. I still have a residual worry about the use of a field-theoretic approach which has space and time coordinates in the configuration of the micro-theory. I have this idea that a causal network of elementary quantum systems with absolutely no space-like metric would be a philosophically more appealing starting point. But perhaps this will turn out to be an unfounded worry. I look forward to reading more from Oriti in the future.

Emergent Quantum Gravity Research Series (in chronological order):

What’s New in Quantum Gravity
A section of Lee Smolin’s recent book discusses new approaches.

Causality First
Rafael Sorkin’s Causal Sets and Fotini Markopoulou’s Quantum Causal Histories.

Emerging From the Noise
More on Markopoulou’s approach.

Caution: Universe under Construction
The Causal Dynamical Triangulation program.

Geometrogenesis
More papers from Markopoulou and colleagues.

In the Beginning was the Qubit
Seth Lloyd’s quantum computing-inspired take on quantum gravity.

Dreyer's Internal Relativity
Olaf Dreyer's approach to finding emergent gravity from a quantum mechanical base.

The Superfluid Universe
Grigory Volovik looks for the answers to fundamental physics in the surprising phenomena displayed in condensed matter physics.

Multiverses -- Physical and Metaphysical

I feel I’ve been inconsistent. On the one hand, wearing a philosophy hat, I’ve endorsed a form of modal realism, where our actual world is a subset of a full expanse of metaphysical possibilities. On the other hand, when I wrote about multiverse models offered by theoretical physicists, I downplayed the "actual" reality of the distant regions described in such theories.

I’m thus returning to the old question of what, if any, is the relationship between the philosopher’s space of possible worlds, and the multiverse described by some physical theories. What should one’s stance be toward each of these? I think I have a clearer thought on this; but first let me digress briefly to say what led me to it.

For awhile I’ve wanted to identify the “actual” world with the region we have causal contact with. The reason for this is motivated by my preferred model of causation and ontology, which says that the familiar concrete world consists of events which are actualized possibilities. In an earlier post I stated this and thus proposed that physical models which contained models of the universe or multiverse beyond this region were describing things which were not actual, but only possible. The clear implication to the reader is that the regions so described were “less real” than our neighborhood. Clark, his comments, questioned whether I could justify discounting all the various multiverse theories. Alejandro described my stance as anti-Copernican: the idea being that our particular neighborhood shouldn’t be viewed as special in this way, given a sound theory which describes our region as well as points beyond. I think this was good criticism.

Now, however, in the context of modal realism, I have this idea that it is consistent with my ideas that “actual” be considered an indexical term (see prior post). In this case I can still denote our causal region as actual, to fit my idea of causation as the process of actualizing possibilities or propensities. However, there is no intent to say our region is special. The terms actual and possible are relative to a local point of view. There is nothing special about our region – from the point of view of an observer elsewhere, the concrete events familiar to us are unactualized. And I think this stance can be appropriate in the context of physical multiverse models as well as in the metaphysical context where it arose. The actual world has a different status then the regions beyond our contact, but this is a relational distinction, not an absolute one, and thus not in itself a rejection of these attempts to model distant reality.

So, given that conclusion, what can be said about the relationship between the metaphysical and physical multiverses? The philosophically motivated idea is that of a space where every metaphysical possibility exists. This is equivalent to saying every logical possibility exists if we endorse modal rationalism. Our actual world is one island in this expanse. I see multiverse theories offered by physicists as attempts to model particular subsets of this space by allowing for the extension or variation of our physical events and laws (as best we know them so far). Such theories widen the realm of nomological possibility – and as you widen this scope more and more you begin to converge toward the ultimate space of metaphysical/logical possibility.