Between a Zeptometer and the Planck length

In summary, between approximately 1 X 10 E-21 meters and the Planck Length of 1.6 X 10 E-35 meters, there is not much evidence for the existence of preons. However, the concept of point particles in the Standard Model may break down at these scales and string theory posits that fundamental particles are excitations of tiny superstrings. Observational tests have excluded the possibility of discrete space-time theories, but there are potential workarounds.
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What exists that is smaller then a zeptometer yet larger than the Planck Length?
What exists between approximately 1 X 10 E-21 meters (preon length?) and the Planck Length of 1.6 X 10 E-35 meters?
 
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Hello. The Scale of the Universe 2 https://htwins.net/scale2/ shows top quark and neutrino there but it also says "the length shorter than this (femtometer) are not confirmed."
 
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Probably not much.

Very high frequency ultraviolet photons can have arbitrarily small wavelengths up to Heisenberg uncertainty principle limits.

There is no good evidence that preons exist. Indeed, it would take a major reworking of of understanding of physics for them to work because of the experimental limits on binding energy for them (also here).

On the other hand, the point particle concept that is used in the Standard Model for fundamental particles may break down at sufficiently small distance scales beyond the reach of current experimental detection. Renormalization calculations for quantum field theory sometimes have cutoffs equivalent to distances on that order of magnitude.

This is the premise, for example, of string theory that posits that fundamental particles are excitations of tiny, but finite length superstrings, and isn't really too specific about how long those strings have to be because not a lot of observable consequences flow from that over a fairly wide range of many orders of magnitude.

Observational Lorenz invariance violation tests have excluded it to very fine distance scales, which tend to disfavor some kinds of theories in which space-time is itself discrete along the lines of loop quantum gravity, although there are workarounds for this issue.
 
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