The rest mass energy of the heavy nucleus represented by E1 is greater than the rest mass energy of the fragments represented by E3. But the energy level of the heavy nucleus is to be increased to E2 to get the fission process started according to classical physics.
But according to quantum mechanics, fission can take place even if no external energy is given. Such a fission process is termed as barrier penetration. The amount of energy created and the time for which it is created through a barrier penetration process are related through Heisenberg uncertainty relation.
∆E. ∆t ≈ h/2 π
Where h is the Planck constant
Barrier penetration is possible but is not easy.