Talks

The standard way to do this is to use the chain rule to backpropagate gradients through layers of neurons. I shall briefly review a few of the engineering successes of backpropagation and then describe a very different way of getting the gradients that, for a while, seemed a lot more plausible as a model of how the brain gets gradients. Consider a system composed of binary neurons that can be active or inactive with weighted pairwise couplings between pairs of neurons, including long range couplings. If the neurons represent pixels in a binary image, we can store a set of binary training images by adjusting the coupling weights so that the images are local minima of a Hopfield energy function which is minus the sum over all pairs of active neurons of their coupling weights. But this energy function can only capture pairwise correlations. It cannot represent the kinds of complicated higher-order correlations that occur in images. Now suppose that in addition to the "visible" neurons that represent the pixel intensities, we also have a large set of hidden neurons that have weighted couplings with each other and with the visible neurons. Suppose also that all of the neurons are asynchronous and stochastic: They adopt the active state with a log odds that is equal to the difference in the energy function when the neuron is inactive versus active. Given a set of training images, is there a simple way to set the weights on all of the couplings so that the training images are local minima of the free energy function obtained by integrating out the states of the hidden neurons? The Boltzmann machine learning algorithm solved this problem in an elegant way. It was proof of principle that learning in neural networks with hidden neurons was possible using only locally available information, contrary to what was generally believed at the time.

I will present a calculation of the vacuum energy that reproduces the observed value and that points to a completely new understanding of quantum gravity as gravitized quantum theory. In that context the fixed Born rule becomes dynamical. After a short conceptual background, I will discuss experimental consequences of this new viewpoint involving intrinsic triple and higher order quantum interferences that are impossible in quantum mechanics and quantum field theory. Also, I will indicate how the observed masses of quarks and leptons (including predictions for the neutrino masses) as well as their mixing angles could be understood in the same context. At the very end I also plan to comment on a time-varying dark energy as another prediction of quantum gravity viewed as gravitized quantum theory.
[This talk is based on the following papers: https://arxiv.org/pdf/2212.00901 https://arxiv.org/pdf/2407.06207 https://arxiv.org/pdf/2501.19269 ]