The Rovelli Point of Wrong Turn

[OwlHoot]

I'm not convinced a wrong turn has been made. I think theoretical physicists are, in their various and most likely equally sound ways, struggling with various ramifications of more fundamental and probably embarrasingly simple principles, rather as a number theorist might make heavy weather of some investigation of a polynomial before noticing that it could be factorized and that considering the irreducible factors makes the problem more amenable and perhaps even trivial, or some alien trying to analyze a chess game in its advanced stages without knowing all the basic moves, or even being aware that there is a set of basic moves.

If this is correct then the basic principles should eventually become evident almost inevitably by a process of working backwards and elimination, taking the intersection and common features of various plausible and consistent theories, rather as the basic principle of debugging software is that once sufficient information is available then the reason for the bug becomes obvious.

 

[Chronos]

I like the chess analogy. I think is safe to say we do not know all the rules of the physics game so the possible outcomes from a given set of initial conditions are often devishly hard to predict. Trying to derive them through reverse engineering is probably no less a challenge than to devine the pieces and location in a chess game say 10 moves prior to the existing position. There are, of course, numerous possible prior chess configurations [think of them as initial conditions] that could lead to the same end position - which sounds a lot like QT. BTW, in the chess landscape all of the possible prior positions were played.

I think quantum theory is the problem child, mostly because we seem unable to reconcile it with general relativity. GR is an elegant theory that emerges from a simple, underlying theme and has unparalleled predictive power, at least at the macroscopic level. QT, while spectacularly successful in its own right, often behaves erratically and the rule book has been regularly appended to accommodate the exceptions. To me, this signals that QT is not a complete theory - the underlying theme that connects all the dots has not been derived.

 

[vanesch]

The trouble is that the wavefunction is an ontological contradiction of terms: the square root of the probability that something might be here or there? Can we say that a probability is "real"?

If you view the wavefunction as real, you do not see it of course as a "square root of probability" but as a physical entity, in the same way as the spacetime manifold is seen as a physical entity in general relativity. In the same way as one could say that the classical electric field is a kind of "square root of probability" for a photon detector to click. This is not the case: the field is really there, its square is an intensity, and it now happens that a photodetector has a clicking rate proportional to this intensity. But it is not because we can derive a probability from the classical electric field, that the electric field is somehow "the square root of probability" and hence void of physical meaning.

 

[vanesch]

I think quantum theory is the problem child, mostly because we seem unable to reconcile it with general relativity. GR is an elegant theory that emerges from a simple, underlying theme and has unparalleled predictive power, at least at the macroscopic level. QT, while spectacularly successful in its own right, often behaves erratically and the rule book has been regularly appended to accommodate the exceptions. To me, this signals that QT is not a complete theory - the underlying theme that connects all the dots has not been derived.

Well, it is not entirely true that quantum theory is not build upon some simple principles. It has: it is the superposition principle. The problem with quantum theory is that it allows for a vast variety of possible models: you are entirely free to fix your model, as long as you respect the basic postulates. One model is the one of non-relativistic point particles. When you apply the principles of quantum theory to it, you obtain non-relativistic quantum theory. Another model is the one of relativistic fields over Minkowski space. When you apply the principles of quantum theory to it, you get QFT. Yet another model is the one of relativistic one-dimensional objects in Minkowski space. When you do so, you get string theory. All these are just different models to which the rules of quantum theory are applied, but these rules haven't really changed.

Of course, GR is vastly simpler and more elegant - when you limit yourself to 4-dim gravity. But we know that there is more to the world than that, and then you have to introduce auxilliary fields with their own, arbitrary dynamics too.

 

[Chronos]

I entirely agree with your points, Vanesch. The bone I have to pick is QT does not accommodate time. QT works well, and often superbly well under many circumstances, but without a time constraint, it looks suspiciously unphysical.

 

[Mike2]

If you view the wavefunction as real, you do not see it of course as a "square root of probability" but as a physical entity, in the same way as the spacetime manifold is seen as a physical entity in general relativity. In the same way as one could say that the classical electric field is a kind of "square root of probability" for a photon detector to click. This is not the case: the field is really there, its square is an intensity, and it now happens that a photodetector has a clicking rate proportional to this intensity. But it is not because we can derive a probability from the classical electric field, that the electric field is somehow "the square root of probability" and hence void of physical meaning.

When they say "probability" that something might exist or be the case, they are clearly making a distinction between existence/reality and what could have been the alternative. A probability in itself does not exist.

And even if it did, then a probability only exists if the imaginary complex wavefunction also exists, since the probability is equal to the wave function times the complex conjugate of a wavefunction. Again it is a contradiction of terms to say that a purely complex number is real.

All we really have is just an engineering approximation used to fit the data to some curve or another. We don't really know WHY the math is the way it is. The fact that we are now trying to reconcile the math of QM to GR is also an indication that we took a wrong turn sometime early in the process.

 

[vanesch]

We don't really know WHY the math is the way it is.

We never knew that, and we will never know that.