The many worlds of M-theory

Why is there something rather than nothing?
Why do we exist?
Why this particular set of laws and not some other?


[Stephen Hawking, The Grand Design]

In this latest article in our informal series on the nature of reality — certainly a central issue for anyone who claims to be a realist — we’ll take one last look at The Grand Design. This time, the focus will be the metaphysics of Hawking’s physics: not the usual science question “What do we know?” but the philosophical question “What can we know?” about the world in which we live.

Why turn to a scientist for an answer to a philosophical question? Hawking anticipates the challenge, asserting baldly at the very beginning of The Grand Design that “philosophy is dead”:

How can we understand the world in which we find ourselves? Philosophy has not kept up with modern developments in science, particularly physics. Scientists have become the bearers of the torch of discovery in our quest for knowledge.

The problem for philosophy is that it is ill-equipped to deal with the emerging physical characteristics of the universe — that is, of the universes, most of which must remain unknowable to us. Using a suite of related theories known collectively as “M-theory,” science can describe what philosophy can’t:

According to M-theory, ours is not the only universe. Instead, M-theory predicts that a great many universes were created out of nothing. Their creation does not require the intervention of some supernatural being or god. Rather, these multiple universes arise naturally from physical law. They are a prediction of science.

Why are we aware of this universe, among all the possibilities? Theists like to drag out the anthropic principle at this point, arguing that we are in a universe that has been created to accommodate our existence. Right idea, but entirely backwards. We could not exist in any universe which did not support life of our sort, so it is neither surprising nor particularly marvelous that we are here — it not here, where?

Only a very few [universes] would allow creatures like us to exist. Thus our presence selects out from this vast array only those universes that are compatible with our existence. Although we are puny and insignificant on the scale of the cosmos, this makes us in a sense the lords of creation.

Since the only universe we can possibly observe is one that makes us possible, the key human behaviour is not adoration but observation. Yet we know from previous postings that neither the physical nor the mental realm is provably real — indeed, both appear to be demonstrably unreal, full of swarming quantum potentials and swirling brain activity. So how can we perceive our universe accurately at all? Hawking’s answer is that we can’t, not really, but we can create a useful version of our unapproachable realities, employing a technique he calls “model-dependent realism”:

There is no picture- or theory-independent concept of reality. Instead we will adopt a view that we will call model-dependent realism: the idea that a physical theory or world picture is a model (generally of a mathematical nature) and a set of rules that connect the elements of the model to observations. This provides a framework with which to interpret modern science.

Hawking recognizes that the world we create inside is not the world that exists outside. Hence the notion of model-dependent realism, which is not traditional realism, but a representation based on sensory information:

It is based on the idea that our brains interpret the input from our sensory organs by making a model of the world. When such a model is successful at explaining events, we tend to attribute to it, and to the elements and concepts that constitute it, the quality of reality or absolute truth.

Is this reality or truth “absolute” in the usual sense? No, it isn’t. But don’t despair quite yet. Even though traditional realism doesn’t survive the facts of modern physics — if the “holographic principle” is true, the four-dimensional space-time of current physics doesn’t survive, either! — model-dependent realism provides a satisfying form of concreteness. This concreteness is based not on a meta-physical definition of reality but on a real-world application of observation.

In a model-dependent theory of the world, proposed realities must match our actual observations. That means no angels, no pixies, no fairy dust, no bearded patriarch sitting on a cloud. That means also no word game justifications of this, that, or the other thing. No ontological arguments, no Thomistic proofs, no untestable hypotheses with no more than assumed or asserted existence. If it can’t be observed, if it can’t be tested, if it can’t be replicated, if it is incapable of disproof — then it isn’t “real” in the way that a table is real.

Is a table really a table? Who knows. But we do know that our perception “table” is a more concrete kind of model than is our ideation “heaven.” A model is not “real” or “unreal” but rather “accurate” or “inaccurate.” Point at a table. Now point at heaven. Any difference? In this model-dependent approach, we replace a philosophical category with a set of empirical data. If you ask me, it’s an improvement — a more complex and a more mature way of conceptualizing the world. Hawking writes:

According to model-dependent realism, it is pointless to ask whether a model is real, only whether it agrees with observation. … We make models in science, but we also make them in everyday life. Model-dependent realism applies not only to scientific models but also to the conscious and subconscious mental models we all create in order to interpret and understand the everyday world. There is no way to remove the observer—us—from our perception of the world, which is created through our sensory processing and through the way we think and reason.

What can we conclude? Even if there are many universes and no single reality, there is at any time a single, practical reality for us, a model by which we understand our world — a model that is as precise as possible, as complete as possible, as consistent with our observations (our experiences) as possible. Within the limits of our perception, the model is knowable, testable, and reliable. Airplanes fly. Electric motors work. Quantum computers are coming. None of this would be possible if our theories did not tightly match our observations. There’s no need for classical metaphysics here, no call for a supernatural realm — but there is “reality,” as far as we can know it, and there is “truth,” as far as we can perceive it. It seems that the realists can relax a little. There’s less threat here — and more promise — than at first we thought.

And that’s the real truth.

All the articles in this “reality” series stand alone, but they do complement each other. Here’s the rest of the series:

Peek-a-boo: searching for reality (December 16, 2010)
Hawking’s hidden dice (January 12, 2011)
The one about one (January 19, 2011)
All possible paths: the meta- of physics (January 21, 2011)
Drafting our narratives (January 24, 2011)