In physics the Planck length is the order of magnitude of the vibrating strings that form the physical particles.
It is then postulated that it is not meaningful to probe below the Planck length. However it seems to me that this masks a considerable degree of philosophical confusion with respect to the very notion of strings.
For if it is not in principle possible to give physical meaning to distances less than the Planck length which is incredibly small at about 10^(- 20) the size of a proton, then how can strings themselves be given a physical meaning?
As I have stated before there are inherent philosophical difficulties with the manner in which physicists attempt to define strings.
Indeed from one valid perspective, what they are attempting to do is to achieve the reductionist fallacy of defining the ultimate constituents of the universe in merely quantitative terms (devoid of any qualitative meaning).
So from this perspective physical particles (which necessarily have qualitative characteristics) come into existence through the vibration of strings.
However such vibration could only have meaning in the context of a pre-existing spacetime environment for the strings. However these same spacetime dimensions are supposed to be in some way derived from the strings!
So really this attempted explanation of the nature of strings lacks any proper coherence.
I have long maintained that the notion of a string has a merely potential - rather than actual - significance. Secondly this notion has two complementary aspects which through interaction are linear and circular with respect to each other.
For convenience we can identify the quantitative aspect of the potential string with the linear aspect. Now this is what the conventional approach likewise defines. However it then makes the untenable reductionist assumption whereby the qualitative is reduced to quantitative interpretation so that the string is misleadingly defined in actual terms.
We can then identify the qualitative aspect of the potential string with the circular aspect. In mathematics is we raise 1 to a fractional power (dimension) its result will lie on the circle of unit radius in the complex plane.
In corresponding fashion to properly relate the linear quantitative aspect of the potential string with its - relatively - corresponding qualitative dimensional aspect, we must employ circular logic with respect to this latter aspect.
It is only through the dynamic interaction of both aspects of the string that physical particles can achieve a manifest actual physical existence (that exhibit both quantitative and qualitative characteristics).
Physicists maintain that the existence of such particles can only occur at or above the Planck length.
Now the very measurement of physical attributes in standard scientific terms assumes that linear type notions can be applied to the physical world e.g. where particles assume a certain independence and distances in space and time have an unambiguous positive meaning. We can then detect - literally - natural particles.
Natural numbers are made up of the primes and other composite numbers (derived from the primes).
In like manner natural phenomena are made up of prime and other composite varieties. It is therefore highly likely that what seem the basic natural particles already involve a high level of composition of more fundamental elements (below the Planck length).
Because the prime and natural are in dynamic terms interrelated they necessarily always co-exist together. However as we approach closer to the emptiness from which all matter emerges, the prime aspect becomes more pronounced, with any natural element arising, of an extremely fleeting short-lived nature.
So it is certainly possible that below the Planck length that a dynamic reality does exist that however in practical terms remains unobservable.
Such a reality would then exist overwhelmingly - not in linear spacetime - but rather in a paradoxical prime spacetime.
I have stated before that the very essence of a prime number is that it combines unique quantitative and qualitative aspects (that are linear and circular with respect to each other). So in extremes, this leads to the direct confusion of linear with circular notions. For example in linear terms a particle exists at some definite location. In circular terms it is however in an undefined location existing holistically everywhere. So prime spacetime involves the attempt to reconcile both notions (which are paradoxical in terms of each other).
We can perhaps portray what is involved here best with reference to psychological notions.
What is most prime i.e. primitive at this level entails the direct confusion of conscious (linear) with unconscious (circular) meaning. Thus the more primitive an instinctive impulse, as in earliest infancy, the more short-lived and fleeting it necessarily is in experience. Thus the attempt to embody what is holistic and universal directly in a limited phenomenal manner leads to the collapse of any dimensional context for the impulse (with the corresponding disappearance of the associated phenomenon).
In like manner at the most prime level of physical reality, the attempt to directly identify a dimensional context of space and time (circular) with a physical phenomenon (linear) leads to the immediate erosion of both aspects (i.e. spacetime and phenomenon).
So at a very low level of composition, prime reality would remain undetectable in phenomenal terms. However as levels of organisation increase, phenomena would attain sufficient stability to become detectable. So they would then emerge into a natural physical environment dictated by increasingly linear notions of space and time.
In this context the inability as yet to detect the graviton (the supposed transmitter of the gravitational force) is revealing.
I have likened before (in physical terms) the gravitational force to the role of the unconscious (from a psychological perspective).
This would entail therefore that any transmission of this force would relate to an extremely early stage of prime spacetime. This insight is also replicated in string theory by the prediction that the graviton corresponds to the lowest possible energy level of the string!
This could therefore imply that the phenomenal element associated with gravity is so minuscule that it will remain impossible to detect (in any physically discernible manner).
Put another way, this is a phenomenon related largely to circular (holistic) rather than linear (analytic) notions.
Moreover a considerable amount of activity is likely to take place below the Planck length governed mainly by circular rather than linear notions. So what we term the fundamental particles may largely represent - simply - what can be phenomenally detected (in linear terms) while concealing many layers of further physical organisation that take place in prime spacetime (below the Planck length).
As to the nature of the emptiness from which prime and natural physical actvity ultimately emerge "God Only Knows".