In a Scientific American essay based on their new book A Grand Design, Stephen Hawking and Leonard Mlodinow are now claiming physicists may never find a theory of everything. Instead, they propose a "family of interconnected theories" might emerge, with each describing a certain reality under specific conditions.
Most of the history of physics has been dominated by a realist approach. Scientists simply accepted that their observations could give direct information about an objective reality. In classical physics, such a view was easily defensible, but the emergence of quantum mechanics has shaken even the staunchest realist.
In a quantum world, particles don't have definite locations or even definite velocities until they've been observed. This is a far cry from Newton's world, and Hawking/Mlodinow argue that - in light of quantum mechanics - it doesn't matter what is actually real and what isn't, all that matters is what we experience as reality.
As an example, they talk about Neo from The Matrix. Even though Neo's world was virtual, as long as he didn't know it there was no reason for him to challenge the physical laws of that world. Similarly, they use the example of a goldfish in a curved bowl. The fish would experience a curvature of light as its reality and while it wouldn't be accurate to someone outside the bowl, to the fish it would be.
Scientific American: The Elusive Theory of Everything
"In our view, there is no picture or theory-independent concept of reality. Instead we adopt a view that we call model - dependent realism: the idea that a physical theory or world is a model (generally of a mathematical nature) and a set of rules that connect the elements of the model to observations. According to model - dependent realism, it is pointless to ask whether a model is real, only whether it agrees with observation. If two models agree with observation, neither model can be considered more real than the other. A person can use whichever model is more convenient in the situation under consideration."
This view is a staunch reversal for Hawking, who 30 years ago argued that not only would physicists find a theory of everything, but that it would happen by the year 2000. In his first speech as Lucasian Chair at Cambridge titled "Is the end in sight for theoretical physics?," Hawking argued that the unification of quantum mechanics and general relativity into one theory was inevitable and that the coming age of computers would render physicists obsolete, if not physics itself.
Of course, Hawking has become rather well known for jumping way out on a limb with his public remarks and for decades he embraced supergravity as having the potential to solve theoretical physicist's ills, even hosting a major conference on it in 1982. However, but Hawking has never harbored allegiances to theories that describe a physical reality.
So, while two well-known physicists coming out against a theory of everything is compelling, it really shouldn't seem like anything new for Hawking.
"I take the positivist view point that a physical theory is just a mathematical model and that it is meaningless to ask whether it corresponds to reality. All that one can ask is that its predictions should be in agreement with observation."
Stephen hawking, The Nature of Space and Time (1996)
