Definitions
from Wiktionary, Creative Commons Attribution/ShareAlike License.
 adjective Of or pertaining to Max Planck (18581947), German physicist and founder of
quantum theory .
Etymologies
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Examples

Assuming an interaction between both kinds of matter which becomes important at Planckian densities, it is found that the universe undergoes a periodic cycle of contraction and expansion.

This would include: the formation and decay of (nonsupersymmetric) black holes; graviton scattering at hyperPlanckian energies; physically continuous transitions from one topology, through a quantum state with no geometric interpretation, to a different topology; states where a submanifold of spacetime has a noncommutative geometry; states with a variety of apparent geometric singularities, where the physics is nonsingular.

However, I didn´t press to ask the following natural question: would a generic collapsing spacetime (which has growing entropy, in the classical regime) also become lowentropy as it enters the Planckian regime?

Even though the Planck scale was still just as inaccessible, string theory was so good that it became reasonable to hope that we could figure it all out just by using brainpower, even without Planckian accelerators.
Arguments For Things I Don’t Believe, 1: Research on String Theory is Largely a Waste of Time

If inhomogeneities are not magically suppressed, then there is no way they can be neglected when the universe reaches the Planckian densities characteristic of the bounce.

[6] The ones that came to mind were modifications of the gravitational force law on laboratory scales, strings, black holes, and extra dimensions at particle accelerators, cosmic superstrings, and transPlanckian corrections to the CMB.

“Therefore, whether real black holes emit Hawking radiation remains an open question and could give nontrivial information about Planckian physics.”

Later, Richard Easther gave a talk entitled “TransPlanckian Physics and Cosmology”, but I had another meeting to go to and so could not attend.

As, according to Planck, the quantum of energy is the product of the number of light vibrations with the Planckian constant, which is denoted by the letter h, it is possible to calculate the number of vibrations which corresponds to a given passing from one orbit to another.

And indeed Bohr succeeded, in his first treatise on this question, in calculating the Rydberg constant from other known magnitudes, namely the weight of an atom of hydrogen, the Planckian constant, and the value of the electric unit of charge.
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