Aside

The infinite turn and speculative explanations in cosmology

Reza Tavakol, Fabio Gironi
Infinity, in various guises, has been invoked recently in order to `explain’ a number of important questions regarding observable phenomena in science, and in particular in cosmology. Such explanations are by their nature speculative. Here we introduce the notions of relative infinity, closure, and economy of explanation and ask: to what extent explanations involving relative or real constructed infinities can be treated as reasonable?
Read more at http://arxiv.org/pdf/1604.06773v1.pdf

Reflections on the renormalization procedure for gauge theories

Gerard t Hooft
Various pieces of insight were needed to formulate the rules for working with gauge theories of the electro-magnetic, weak and strong forces. First, it was needed to understand how to formulate the Feynman rules. We had to learn that there are many different ways to derive them, and it was needed to know how different formulations of the gauge constraint lead to the same final results: the calculated values of the scattering amplitudes. The rules for dealing with the infinities that had to be subtracted were a big challenge, culminating in the discovery of the Becchi-Rouet-Stora-Tyutin symmetry. Fond recollections of the numerous discussions the author had with Raymond Stora on this topic are memorised here. We end with some reflections on the mathematical status of quantum field theories, and a letter sent by Stora to the author.
… read more at http://arxiv.org/pdf/1604.06257v1.pdf

Aside

Robert Dicke and the Naissance of Experimental Gravity Physics

The photograph in the upper left shows Eotvos and colleagues, likely measuring gravitational field gradients. On the right is an illustration of features common to the two repetitions of the Eotvos experiment in Dicke’s group: the balance placed in a vacuum, the torque detected remotely, and the instrument buried. On the lower left is a sketch of the torsion balance used in the static version (in an early design)

The photograph in the upper left shows Eotvos and colleagues, likely measuring
gravitational field gradients. On the right is an illustration of features common to the two repetitions of the Eotvos experiment in Dicke’s group: the balance placed in a vacuum, the torque detected remotely, and the instrument buried. On the lower left is a sketch of the torsion balance used in the static version (in an early design)

P. J. E. Peebles
The experimental study of gravity became much more active in the late 1950s, a change pronounced enough be termed the naissance of empirical gravity physics. A review of the developments since 1915, and up to the transition to what might be termed a normal and accepted part of physical science in the late 1960s, shows the importance of advances in technologies, here as in all branches of science. The role of contingency is illustrated by Robert Dicke’s decision to change directions in mid-career, to lead a research group dedicated to the experimental study of gravity. One sees the power of nonempirical evidence, which led some in the 1950s to feel that general relativity theory is so logically sound as to be scarcely worth the testing, while Dicke and others argued that a poorly tested theory is only that, and that other nonempirical arguments, based on Mach’s Principle and Dirac’s Large Numbers, suggested it was worth looking for a better theory of gravity. I conclude by offering lessons from this history, some peculiar to the study of gravity physics during the naissance, some of more general relevance. The central lesson, which is familiar but not always well advertised, is that physical theories can be empirically established, sometimes with surprising results.
… Read more at arxiv.org

Back to Parmenides

Henrique de A. Gomes
After a brief introduction to issues that plague the realization of a theory of quantum gravity, I suggest that the main one concerns defining superpositions of causal structures.
This leads me to a distinction between time and space, to a further degree than that present in the canonical approach to general relativity. With this distinction, one can make sense of superpositions as interference between alternative paths in the relational configuration space of the entire Universe.
But the full use of relationalism brings us to a timeless picture of Nature, as it does in the canonical approach (which culminates in the Wheeler-DeWitt equation). After a discussion of Parmenides and the Eleatics’ rejection of time, I show that there is middle ground between their view of absolute timelessness and a view of physics taking place in timeless configuration space.
In this middle ground, even though change does not fundamentally exist, the illusion of change can be recovered in a way not permitted by Parmenides.
It is recovered through a particular density distribution over configuration space which gives rise to ‘records’. Incidentally, this distribution seems to have the potential to dissolve further aspects of the measurement problem that can still be argued to haunt the application of decoherence to Many-Worlds quantum mechanics.
I end with a discussion indicating that the conflict between the conclusions of this paper and our view of the continuity of the self may still intuitively bother us. Nonetheless, those conclusions should be no more challenging to our intuition than Derek Parfit’s thought experiments on the subject…
… Read more at http://arxiv.org/pdf/1603.01574v1.pdf

Aside

NEUTRINOS: Mysterious Particles with Fascinating Features, which led to the Physics Nobel Prize 2015

neutrinos1Alexis Aguilar-Arevalo and Wolfgang Bietenholz
The most abundant particles in the Universe are photons and neutrinos. Both types of particles are whirling around everywhere, since the early Universe. Hence the neutrinos are all around us, and permanently pass through our planet and our bodies, but we do not notice: they are extremely elusive. They were suggested as a theoretical hypothesis in 1930, and discovered experimentally in 1956.
Ever since their properties keep on surprising us; for instance, they are key players in the violation of parity symmetry. In the Standard Model of particle physics they appear in three types, known as “flavors”, and since 1998/9 we know that they keep on transmuting among these flavors.
This “neutrino oscillation” implies that they are massive, contrary to the previous picture, with far-reaching consequences.
This discovery was awarded the Physics Nobel Prize 2015.
Read more at http://arxiv.org/pdf/1601.04747v1.pdf

Aside

The Daniell Cell, Ohm’s Law and the Emergence of the International System of Units

Joel S. Jayson
Telegraphy originated in the 1830s and 40s and flourished in the following decades, but with a patchwork of electrical standards. Electromotive force was for the most part measured in units of the predominant Daniell cell. Each company had their own resistance standard. In 1862 the British Association for the Advancement of Science formed a committee to address this situation. By 1873 they had given definition to the electromagnetic system of units (emu) and defined the practical units of the ohm as 109 emu units of resistance and the volt as 108 emu units of electromotive force. These recommendations were ratified and expanded upon in a series of international congresses held between 1881 and 1904. A proposal by Giovanni Giorgi in 1901 took advantage of a coincidence between the conversion of the units of energy in the emu system (the erg) and in the practical system (the joule) in that the same conversion factor existed between the cgs based emu system and a theretofore undefined MKS system. By introducing another unit, X (where X could be any of the practical electrical units), Giorgi demonstrated that a self consistent MKSX system was tenable without the need for multiplying factors. Ultimately the ampere was selected as the fourth unit. It took nearly 60 years, but in 1960 Giorgi’s proposal was incorporated as the core of the newly inaugurated International System of Units (SI). This article surveys the physics, physicists and events that contributed to those developments.
… Read more at http://arxiv.org/pdf/1512.07306v1.pdf