Understanding importance of Planck energy

Question

Planck length is considered to be smallest length possible in the universe. Planck time is smallest time interval possible.

Similarly what is importance of Planck energy because it is neither smallest nor largest energy possible in the universe?

Answer 1

Please see this:

The Planck time is the unique combination of the gravitational constant G, the special-relativistic constant c, and the quantum constant ħ, to produce a constant with dimension of time. Because the Planck time comes from dimensional analysis, which ignores constant factors, there is no reason to believe that exactly one unit of Planck time has any special physical significance. Rather, the Planck time represents a rough time scale at which quantum gravitational effects are likely to become important. This essentially means that while smaller units of time can exist, they are so small their effect on our existence is negligible. The nature of those effects, and the exact time scale at which they would occur, would need to be derived from an actual theory of quantum gravity.

Italics mine.

As for the Planck length:

The Planck length is the scale at which quantum gravitational effects are believed to begin to be apparent, where interactions require a working theory of quantum gravity to be analyzed

So your assumptions of possibilities are wrong, they are not a limit that could not be bypassed with better experiments and theories.

Planck energy is also in the natural units of Planck. The significance of the natural units

In particle physics and physical cosmology, Planck units are a set of units of measurement defined exclusively in terms of five universal physical constants, in such a manner that these five physical constants take on the numerical value of 1 when expressed in terms of these units.

It makes computational sense and allows theoretical calculations in a manner independent of specific experiments.

The term Planck scale refers to the magnitudes of space, time, energy and other units, below which (or beyond which) the predictions of the Standard Model, quantum field theory and general relativity are no longer reconcilable, and quantum effects of gravity are expected to dominate

So this is the same for Planck energy, one needs to extend the present theories mainly to include quantization of gravity. The units altogether have the same significance.

Answer 2

The Planck length and Planck time are not the smallest length and time possible, just the smallest length and time that can be observed. There is a nice discussion of this in the answer to the question Is the Planck length the smallest length that exists in the universe or is it the smallest length that can be observed?

So the Planck length represents a limit to the smallest distance we can observe and the Planck time represents a limit to the smallest time interval we can observe. And this leads naturally to the idea of the Planck energy being the highest particle energy we can observe.

To understand why this is consider that the particles we observe have to be created by some interaction. For example the LHC creates particles by colliding two protons (actually by colliding two quarks) and the centre of mass energy is 14 TeV. So the most energetic particle we could observe coming from a collision in the LHC is 14 TeV. If we increase the collision energy we can generate more and more energetic particles, but we can't do this indefinitely. When the collision energy reaches around the magnitude of the Planck energy we will start creating microscopic black holes instead of particles. That's why we can't create a particle with an energy higher than the Planck energy.

Answer 3

The other answers correctly state the importance of the Planck length and time, I will answer your question about the Planck energy.

In everyday life, the Planck energy would not be so special at all, for example, a car's gas tank full of gasoline (stores as much energy as) is about the Planck energy, but this is just coincidence.

Theoretically, the highest energy photon carries about 1 EP of energy (see Ultra-high-energy gamma ray). Most Planck units are extremely small, as in the case of Planck length or Planck time, or extremely large, as in the case of Planck temperature or Planck acceleration. For comparison, the Planck energy is approximately equal to the energy stored in an automobile gas tank (57.2 L of gasoline at 34.2 MJ/L of chemical energy).

https://en.wikipedia.org/wiki/Planck_energy

A more important thing is if you look at cosmic rays of light, and you will right away see that the Planck energy is set where the ultra-high energy rays end (where we end to scale them).

Ultra-high-energy gamma rays are gamma rays with photon energies higher than 100 TeV (0.1 PeV). They have a frequency higher than 2.42 × 1028 Hz and a wavelength shorter than 1.24 × 10−20 m. The existence of these rays were confirmed in 2019.[1] Ultra-high-energy gamma rays are of importance because they may reveal the source of cosmic rays.

https://en.wikipedia.org/wiki/Ultra-high-energy_gamma_ray