▁negative ▁energy ▁may ▁actually ▁be ▁possible ▁due ▁to ▁the ▁Cas imir ▁effect ▁in ▁quantum ▁physics . ▁Although ▁early ▁calculations ▁suggested ▁that ▁a ▁very ▁large ▁amount ▁of ▁negative ▁energy ▁would ▁be ▁required , ▁later ▁calculations ▁showed ▁that ▁the ▁amount ▁of ▁negative ▁energy ▁can ▁be ▁made ▁arbitr arily ▁small . ▁ ▁In ▁ 1 9 9 3 , ▁Matt ▁Vis ser ▁argued ▁that ▁the ▁two ▁mouth s ▁of ▁a ▁w orm hole ▁with ▁such ▁an ▁induced ▁clock ▁difference ▁could ▁not ▁be ▁brought ▁together ▁without ▁indu cing ▁quantum ▁field ▁and ▁gravit ational ▁effects ▁that ▁would ▁either ▁make ▁the ▁w orm hole ▁collapse ▁or ▁the ▁two ▁mouth s ▁rep el ▁each ▁other . ▁Because ▁of ▁this , ▁the ▁two ▁mouth s ▁could ▁not ▁be ▁brought ▁close ▁enough ▁for ▁caus ality ▁viol ation ▁to ▁take ▁place . ▁However , ▁in ▁a ▁ 1 9 9 7 ▁paper , ▁Vis ser ▁hypoth es ized ▁that ▁a ▁complex ▁" R oman ▁ring " ▁( named ▁after ▁Tom ▁Roman ) ▁configuration ▁of ▁an ▁N ▁number ▁of ▁w orm hol es ▁arranged ▁in ▁a ▁symmetric ▁polygon ▁could ▁still ▁act ▁as ▁a ▁time ▁machine , ▁although ▁he ▁con cludes ▁that ▁this ▁is ▁more ▁likely ▁a ▁f law ▁in ▁classical ▁quantum ▁gravity ▁theory ▁rather ▁than ▁proof ▁that ▁caus ality ▁viol ation ▁is ▁possible . ▁ ▁Other ▁approaches ▁based ▁on ▁general ▁relativ ity ▁ ▁Another ▁approach ▁involves ▁a ▁dense ▁spin ning ▁cyl inder ▁usually ▁referred ▁to ▁as ▁a ▁Ti pler ▁cyl inder , ▁a ▁GR ▁solution ▁discovered ▁by ▁Wille m ▁Jacob ▁van ▁Stock um ▁in