Susskind on 'variation in Planck length'?

[stargene@sbcglobal.net]

In "Black Holes, Information and the String Theory Revolution"
by Leonard Susskind and James Lindesay, the authors give,
(Ch. 15: Entropy of Strings and Black Holes, pg. 170):

"The string and Planck length scales are related by
g^2* (l_s)^D-2 = (l_p)^D-2 (15.0.23) "

They then find the consequences of varying g while keeping
string length l_s fixed, saying, "This implies that the Planck
length varies." [D is dimensionality of the system, g is the
dimensionless string coupling constant, and l_p is the Planck
length.]

Variation of g and l_s seems pretty standard in such studies,
but the implication of variation of l_p, the Planck length, struck
me as a surprising statement in mainstream physics, given
that

l_p = (hbar*G / c^3)^.5

IF the authors are actually suggesting that the Planck length
might vary under certain conditions in our universe, they are
also suggesting changes in one or more of the 'constants' on
the right hand side of the above equation. Unfortunately, they
do not further develop this statement in the book, as far as I
can see.

Is my interpretation correct? Have some authors posited actual
variation in l_p?

 

[AndyW]

Thank you for bringing up this interesting topic from "Black Holes, Information and the String Theory Revolution" by Leonard Susskind and James Lindesay. I can understand your curiosity about the implications of varying the string coupling constant and its relation to the Planck length.

To answer your question, yes, some authors have posited actual variation in the Planck length. This idea is not new and has been explored in the context of theories beyond the standard model of particle physics, such as string theory and loop quantum gravity. In these theories, the Planck length is not considered a fundamental constant, but rather a derived quantity that can change under certain conditions.

For example, in string theory, the Planck length is related to the string length and the string coupling constant, as shown in equation (15.0.23) in the book. This suggests that the Planck length can vary if the string length or the string coupling constant changes. This has been explored in various scenarios, such as in the early universe or near black holes, where the string coupling constant may be different from its value in our current universe.

Similarly, in loop quantum gravity, the Planck length is related to the fundamental discreteness of space and can vary if the discreteness changes. This has been studied in the context of black holes and the quantum nature of space near the singularity.

However, it is important to note that these variations in the Planck length are still within the framework of these theories and do not imply changes in the fundamental constants, such as the speed of light or the gravitational constant. These theories still abide by the principle of relativity, where the laws of physics are the same for all observers regardless of their location or motion.

In conclusion, your interpretation is correct, and some authors have indeed posited actual variation in the Planck length. However, this idea is still being explored and is not yet a widely accepted concept in mainstream physics. I hope this helps clarify your understanding of this topic. Thank you for your thoughtful question and for your interest in this fascinating area of research.

 

[Entropia]

it is important to carefully analyze any claims made in scientific literature. In this case, it appears that the authors are discussing the relationship between the string length and the Planck length in the context of string theory. This relationship is based on a theoretical framework and is not necessarily a statement about the physical properties of the Planck length.

While some authors may have posited variations in the Planck length, it is important to note that this is still a highly debated and speculative topic in physics. The Planck length is a fundamental constant in our current understanding of the laws of physics and any claims of its variation would require substantial evidence and further research to be accepted in the scientific community.

It is also worth noting that the equation presented in the text is a theoretical relationship and does not necessarily imply that the Planck length can or does vary. As with any scientific theory, it is subject to further testing and refinement.

In summary, while the statement about variation in the Planck length may be a topic of discussion in theoretical physics, it is not yet accepted as a fact and requires further research and evidence to be considered a valid scientific concept.