It is sometimes attempted to provide an intuitive picture of virtual particles based upon the Heisenberg energy-time uncertainty principle : Δ Δ E Δ Δ t ≥ ≥ ℏ ℏ 2 , (where Δ E and Δ t are energy and time variations, and ħ the Planck constant divided by 2 π) arguing along the lines that the short lifetime of virtual particles allows the "borrowing" of large energies from the vacuum and thus permits particle generation for short times.