**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

# Category Archives: History and Philosophy of Physics

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

**Alexis 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

# 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 10^{9} emu units of resistance and the volt as 10^{8} 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

# Lessons from Einstein’s 1915 discovery of general relativity

**Lee Smolin**

There is a myth that Einstein’s discovery of general relativity was due to his following beautiful mathematics to discover new insights about nature. I argue that this is an incorrect reading of the history and that what Einstein did was to follow physical insights which arose from asking that the story we tell of how nature works be coherent.

**Contents**

**1** The lessons of general relativity

**2** Following Einstein’s path

**3** Going beyond the standard model: which legacy to follow?

**4** The search for new principles

**5** Einstein’s unique approach to physics

…Read more at http://arxiv.org/pdf/1512.07551v1.pdf

# The Evolution of Quantum Field Theory, From QED to Grand Unification

**Gerard ‘t Hooft**

In the early 1970s, after a slow start, and lots of hurdles, Quantum Field Theory emerged as the superior doctrine for understanding the interactions between relativistic sub-atomic particles.

After the conditions for a relativistic field theoretical model to be renormalizable were established, there were two other developments that quickly accelerated acceptance of this approach: first the Brout-Englert-Higgs mechanism, and then asymptotic freedom.

Together, these gave us a complete understanding of the perturbative sector of the theory, enough to give us a detailed picture of what is now usually called the Standard Model.

Crucial for this understanding were the strong indications and encouragements provided by numerous experimental findings.

Subsequently, non-perturbative features of the quantum field theories were addressed, and the first proposals for completely unified quantum field theories were launched.

Since the use of continuous symmetries of all sorts, together with other topics of advanced mathematics, were recognised to be of crucial importance, many new predictions were pointed out, such as the Higgs particle, supersymmetry and baryon number violation.

There are still many challenges ahead…

… Read more at http://arxiv.org/pdf/1503.05007v1.pdf

# Ehrenfest’s adiabatic hypothesis in Bohr’s quantum theory

**Enric Pérez, Blai Pié Valls**

It is widely known that Paul Ehrenfest formulated and applied his adiabatic hypothesis in the early 1910s. Niels Bohr, in his first attempt to construct a quantum theory in 1916, used it for fundamental purposes in a paper which eventually did not reach the press.

He decided not to publish it after having received the new results by Sommerfeld in Munich. Two years later, Bohr published “On the quantum theory of line-spectra.” There, the adiabatic hypothesis played an important role, although it appeared with another name: the principle of mechanical transformability. In the subsequent variations of his theory, Bohr never suppressed this principle completely.

We discuss the role of Ehrenfest’s principle in the works of Bohr, paying special attention to its relation to the correspondence principle. We will also consider how Ehrenfest faced Bohr’s uses of his more celebrated contribution to quantum theory, as well as his own participation in the spreading of Bohr’s ideas…

…Read more at http://arxiv.org/ftp/arxiv/papers/1502/1502.03022.pdf

# The Tragic Fate of Physicist Paul Ehrenfest

Physics is not immune from tragedy. Even brilliant minds sometimes grapple with inner demons. The worst situations involve not just the physicists themselves but also their families. Consider the tragic case of Paul Ehrenfest and his son Wassik.

Wassik Ehrenfest was a friendly boy with Down Syndrome who, like many children of his time with that condition, spent much of his life in hospitals and institutions. He lived for some time in a facility in Jena, Germany that was progressive for its age but expensive. Little is known of his life, except through his correspondence with his parents. Encouraged by his teachers, he sent many postcards to his parents to show them what he was learning. When the Nazis rose to power in spring 1933, he was transferred to the Waterink Institute for Afflicted Children in Amsterdam, Holland, founded by educational reformer Jan Waterink….

… Read more at medium.com