Planck-Scale Physics: Facts and Beliefs

Abstract

The relevance of the Planck scale to a theory of quantum gravity has become a worryingly little examined assumption that goes unchallenged in the majority of research in this area. However, in all scientific honesty, the significance of Planck's natural units in a future physical theory of spacetime is only a plausible, yet by no means certain, assumption. The purpose of this article is to clearly separate fact from belief in this connection.

Quarks, Hadrons, and Emergent Spacetime

“…phenomenon that does not have an underlying quantum counterpart [2]. Furthermore, as observed by some authors [3] and as clearly voiced by Meschini [4], the only links between the general idea of an underlying quantized space and the Planck natural units are the dimensional analysis and the theoretical ad hoc considerations of Planck-size black holes. However, as pointed out in [4], dimensional analysis is not a trustworthy tool that could provide us with reliable information on the realm of the unknown.…”

“…Furthermore, as observed by some authors [3] and as clearly voiced by Meschini [4], the only links between the general idea of an underlying quantized space and the Planck natural units are the dimensional analysis and the theoretical ad hoc considerations of Planck-size black holes. However, as pointed out in [4], dimensional analysis is not a trustworthy tool that could provide us with reliable information on the realm of the unknown. This claim is justified in [4] with a few examples from the history of physics which show that dimensional analysis becomes reliable only when the underlying theory is already known.…”

“…However, as pointed out in [4], dimensional analysis is not a trustworthy tool that could provide us with reliable information on the realm of the unknown. This claim is justified in [4] with a few examples from the history of physics which show that dimensional analysis becomes reliable only when the underlying theory is already known. For example, Meschini considers estimates of the size of hydrogen's atom and its binding energy that could have been made on the basis of dimensional analysis before any knowledge of Planck constant and the associated quantum features of reality were available.…”

“…It is therefore more minimal than, for example, the approach of [16] which assumes seven spatial dimensions. 4 In addition, the phase space approach realizes the philosophical condition of a symmetric treatment of things and processes (i.e. positions of things and their changes or, in other words, motions) as advocated by such thinkers as Heraclitus, Leibniz, and -most notably -Whitehead [17,18].…”

MOND and natural scales of distance and mass

“…One may doubt whether (or in what sense) Planck's scales are "natural" for quantum gravity. As clearly demonstrated by Meschini [4], dimensional analysis is not an infallible tool that could provide us with absolutely sound information on the realm of the Unknown. In fact, it becomes reliable only when we know in advance which theory, physical constants, and/or quantities should be chosen as relevant to a given problem.…”