In physics, Planck energy (Eₚ) is the unit of energy in the system of natural units known as Planck units. It is derived from the universal physical constants and it is defined as:
Eₚ = √((h×c⁵)/G).
The definition dives the approximate equivalent value of this unit in terms of other unit of energy :
1Eₚ ≈ 1.956×10⁹ Joules ≈ 1.2209×10¹⁹ GeV
An equivalent definition are :
Eₚ = h/tₚ , where tₚ is the Planck time;
Eₚ = mₚ×c² , where mₚ is the Planck mass.
The ultra high energy cosmic ray observed in 1991 had a measured energy of about 50joules, which is equivalent to about 2.5 x 10⁻⁸ Eₚ. Theoretically, the highest energy a photon carries precisely 1Eₚ of energy. Most Planck units are extremely small or extremely large, as in the case of Planck temperature and Planck acceleration.
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