136 Medical Time Travel Can whole people travel through time like preserved organs? Remarkably, the answer seems to be yes. Although it is seldom done, medicine sometimes does preserve people like organs awaiting transplant. Some surgeries on major blood vessels of the heart or brain can only be done if blood circulation through the entire body is stopped. [1;2] Stopped blood circu- lation would ordinarily be fatal within 5 minutes, but cooling to +16°C (60°F) allows the human body to remain alive in a ‘turned off’ state for up to 60 minutes. [3] With special blood substitutes and further cooling to a temperature of 0°C (32°F), life without heartbeat or circulation can be extended as much as three hours. [4] Although there is currently no sur- gical use for circulatory arrest of several hours [5], it may be used in the future to permit surgical repair of wounds before blood circulation is restored after severe trauma. [6] While some biological processes are merely slowed by deep cooling, others are completely stopped. Brain activity is an important example. Brain electrical activity usually ceases at temperatures below +18°C (64°F), and disappears completely in all cases as freezing temperatures are approached. [7] Yet these temperatures can still be survived. In fact, not only can the brain survive being turned off, surgeons often use drugs to force the brain to turn off when temperature alone does not do the trick. [8] They do this because if the brain is active when  blood  circulation  is  stopped,  vital  energy  stores  can become depleted, later causing death. This reminds us that death is not when life turns off. Death is when the chemistry of life becomes irreversibly damaged. Specialized  surgeries  are  not  the  only  cases  in  which  the brain can stop working and later start again. Simple cardiac arrest (stopping of the heart) at normal body temperature also causes brain electrical activity to stop within 40 seconds. [9] Yet the heart can remain stopped for several times this long with no lasting harm to the brain. Anesthetic drugs, such as