When was Strominger and Vafa 's black hole theory published ?
Using the thermodynamic relationship between energy , temperature and entropy , Hawking was able to confirm Bekenstein 's conjecture and fix the constant of proportionality at formula_2 : formula_3 where formula_4 is the area of the event horizon , calculated at formula_5 , formula_6 is Boltzmann 's constant , and formula_7 is the Planck length .