(From my scientist-friend Chris):

1. What is time? In standard physics it is the 4th dimension. If that is true, why can’t we move along it totally freely, as we can with our 3 known space dimensions? If we do, you come up with the paradox that you could kill your father, and hence cease to exist. That would free up a lot of energy and make a big explosion! Such has never been observed (fortunately). So time is a problem. It is not even agreed upon by two observers moving relative to each other, so is not absolute.

2. Causality seems to be a better bet, because we can identify events and their causes easily. If you play a film backwards it is easy to tell because causes precede events. This is not true for simple events like billiard balls scattering off each other, but it is for complex systems, like balls of two colours put into a box and shaken together. If the two colours appear to separate, you are looking at it backwards (ever see the Red Dwarf episode on this subject? Hilarious!). There is even a new theory that time does not exist as a dimension, but is a property of causality just like temperature is related to energy (New Scientist a few weeks ago).

3. Time for living organisms is even more problematical. Cold-blooded animals actually respond to the product of time and temperature (above a certain base temperature), not time itself. We humans sense the passing of time because our brains operate as a loop, and each iteration takes a certain time. Normally this loop operates about 40 times a second (which is why TV scans must be faster than this), but it is quicker in children or when you are in a dangerous situation, like a car crash. At such times, time seems to go slower. Conversely, as you age the loop slows and time seems to go quicker.

4. So what happens to subjective time when you die? Well, if time is sensed by the operation of the loop in our brains, then the last loop lasts forever. Ergo, eternal life, at least subjectively. And it seems that as the brain dies, you experience euphoria as the various neurotransmitters are released from the dying nerve cells, so that eternal life will be euphoric, or at least I hope so!