**G J Milburn**

All clocks, classical or quantum, are open non equilibrium irreversible systems subject to the constraints of thermodynamics. Using examples I show that these constraints necessarily limit the performance of clocks and that good clocks require large energy dissipation. For periodic clocks, operating on a limit cycle, this is a consequence of phase diffusion. It is also true for non periodic clocks (for example, radio carbon dating) but due to telegraph noise not to phase diffusion. In this case a key role is played by accurate measurements that decrease entropy, thereby raising the free energy of the clock, and requires access to a low entropy reservoir. In the quantum case, for which thermal noise is replaced by quantum noise (spontaneous emission or tunnelling), measurement plays an essential role for both periodic and non periodic clocks. The paper concludes with a discussion of the Tolman relations and Rovelli’s thermal time hypothesis in terms of clock thermodynamics.

Read more at

# Category Archives: History and Philosophy of Physics

# The subtle sound of quantum jumps

**Antoine Tilloy**

Could we hear the pop of a wave-function collapse, and if so, what would it sound like? There exist reconstructions or modifications of quantum mechanics (collapse models) where this archetypal signature of randomness exists and can in principle be witnessed. But, perhaps surprisingly, the resulting sound is disappointingly banal, indistinguishable from any other click. The problem of finding the right description of the world between two completely different classes of models — where wave functions jump and where they do not — is empirically undecidable. Behind this seemingly trivial observation lie deep lessons about the rigidity of quantum mechanics, the difficulty to blame unpredictability on intrinsic randomness, and more generally the physical limitations to our knowledge of reality.

Read more at https://arxiv.org/abs/2007.15420

# James Chadwick: ahead of his time

**Gerhard Ecker**

James Chadwick is known for his discovery of the neutron. Many of his earlier findings and ideas in the context of weak and strong nuclear forces are much less known. This biographical sketch attempts to highlight the achievements of a scientist who paved the way for contemporary subatomic physics.

Read more at https://arxiv.org/abs/2007.06926

# Physics and the Pythagorean Theorem

**James Overduin, Richard Conn Henry**

Pythagoras’ theorem lies at the heart of physics as well as mathematics, yet its historical origins are obscure. We highlight a purely pictorial, gestalt-like proof that may have originated during the Zhou Dynasty. Generalizations of the Pythagorean theorem to three, four and more dimensions undergird fundamental laws including the energy-momentum relation of particle physics and the field equations of general relativity, and may hint at future unified theories. The intuitive, “pre-mathematical” nature of this theorem thus lends support to the Eddingtonian view that “the stuff of the world is mind-stuff.”

Read more https://arxiv.org/abs/2005.10671

# Memory and entropy

**Carlo Rovelli**

I study the physical nature of traces (or memories). Surprisingly, (i) systems separation with (ii) temperature differences and (iii) long thermalization times, are sufficient conditions to produce macroscopic traces. Traces of the past are ubiquitous because these conditions are largely satisfied in our universe. I quantify these thermodynamical conditions for memory and derive an expression for the maximum amount of information stored in such memories, as a function of the relevant thermodynamical parameters. This mechanism transforms low entropy into available information.

Read more at https://arxiv.org/pdf/2003.06687.pdf

# Roland Eotvos: scientist, statesman, educator

**András PATKÓS**, Institute of Physics, Eötvös University

This lecture recalls the memory of Baron Roland Eötvös, an outstanding figure of the experimental exploration of the gravitational interaction and “funding father” of applied geophysics. Beyond the scientific achievements his contribution to the development of the modern Hungarian schooling and higher educational system, most importantly, the foundation of an innovative institution of teacher’s training did not lose its contemporary significance. This lecture has been invited by the organizers of this Conference in response to the decision of UNESCO to commemorate worldwide the death centenary of the most outstanding Hungarian experimental physicist of modern times.

Read more at https://arxiv.org/ftp/arxiv/papers/2002/2002.05743.pdf

# Joseph Polchinski: A Biographical Memoir

**Raphael Bousso, Fernando Quevedo, Steven Weinberg**

Joseph Polchinski (1954-2018), one of the the leading theoretical physicists of the past 50 years, was an exceptionally broad and deep thinker. He made fundamental contributions to quantum field theory, advancing the role of the renormalization group, and to cosmology, addressing the cosmological constant problem. Polchinski’s work on D-branes revolutionized string theory and led to the discovery of a nonperturbative quantum theory of gravity. His recent, incisive reformulation of the black hole information paradox presents us with a profound challenge. Joe was deeply devoted to his family, a beloved colleague and advisor, an excellent writer, and an accomplished athlete.

Read more at https://arxiv.org/pdf/2002.02371.pdf

# Maxwell’s Demon and Its Fallacies Demystified

**Milivoje M. Kostic**

A demonic being, introduced by Maxwell, to miraculously create thermal non-equilibrium and violate the Second law of thermodynamics, has been among the most intriguing and elusive wishful concepts for over 150 years. Maxwell and his followers focused on ‘effortless gating’ a molecule at a time, but overlooked simultaneous interference of other chaotic molecules, while the demon exorcists tried to justify impossible processes with misplaced ‘compensations’ by work of measurements and gate operation, and information storage and memory erasure with entropy generation. The illusive and persistent Maxwell’s demon fallacies by its advocates, as well as its exorcists, are scrutinized and resolved here. Based on holistic, phenomenological reasoning, it is deduced here that a Maxwell’s demon operation, against natural forces and without due work effort to suppress interference of competing thermal particles while one is selectively gated, is not possible at any scale, since it would be against the physics of the chaotic thermal motion, the latter without consistent molecular directional preference for selective timing to be possible. Maxwell’s demon would have miraculous useful effects, but also some catastrophic consequences.

Read more at https://arxiv.org/ftp/arxiv/papers/2001/2001.10083.pdf