142 Medical Time Travel not yet reversible. But cryonicists would argue that this does not imply death either because death only happens when bio- chemistry becomes irreversibly damaged, and “irreversibility” is technology-dependent. To clarify these issues, cryonicists have proposed the “infor- mation-theoretic criterion” for death. [27] According to this criterion, you are not dead when life stops (we already know that  from  clinical  medicine),  you  are  not  dead  when  bio- chemistry is damaged, you are only dead when biochemistry is so badly damaged that no technology, not even molecular nanotechnology [28], could restore normal biochemistry with your memories intact. By this criterion, someone who suf- fered cardiac arrest days ago in the wilderness is really dead. Someone who suffered only a few minutes of cardiac arrest and  cryoprotectant  toxicity  during  morphological  vitrifica- tion may not be. Whether or not one accepts this information-theoretic cri- terion,  the  modern  cryonics  practice  of  using  life  support equipment  to  resuscitate  the  brain  after  legal  death  raises important issues. Among them is the scientific issue that cry- onics cannot be dismissed simply by calling its subjects ‘dead’. Two minutes of cardiac arrest followed by restoration of blood circulation does not a skeleton make. There should be a rule that no one is allowed to say “dead” when discussing cryonics. It is usually a slur that communicates nothing scientific. Whether cryonics can work depends on biological details of cerebral ischemic injury (brain injury during stopped blood flow), cryopreservation injury, and anticipated future technol- ogy. There is much published literature on cerebral ischemia, and a small, but growing body of writing on relevant future technologies. [29– 33] There is, however, very little informa- tion on the quality of preservation achieved with cryonics. [34;35] It would seem logical to look to cryobiologists for this information.