Two nicely-endowed identical twins - lovely girls - decided to strip relativity of its mystery by resolving once and for always the riddle of the "twin paradox." They began by spending considerably on a spacious user-friendly ion blaster equipped with exercise room, bathroom, makeup room, and other amenities so that the life style of the traveling twin could remain equivalent to that of her sister left behind. In addition, they spent even more to instrument themselves to the hilt - medical equipment costs being what they were in the US at the time. This involved specially developed brassieres with sensitive nonintrusive transducers in the left cups that could detect each heartbeat and powerful transmitters to broadcast each coded beep to the ends of the universe. In addition each maintained a receiver antenna for her own and the other coded beeps with a readout of the cumulative heart beats of both twins. When the instrumentation was so well implemented that it no longer itched and could not be seen under a silk gown, they were satisfied.
Perhaps they were operating under false assumptions. For they had come to believe that without mishap or sickness identical twins should have identical numbers of heart beats in their lifetimes and that on the average they would have the same number of heartbeats each year. They tested this hypothesis for a couple of years early in their lives and found that whereas Dierdre had 31,600,029 beats between their 16th and 17th birthdays, Alana had 31,558,371. But then Deirdre had had her first fling somewhat later than Alana and they were gratified that between their 17th and 18th birthdays, Deirdre had 31,579,181 and Alana had 31,579,219. So the idea seemed to work fairly well as biological clocks go. By then the preparations of the ion rocket were completed and so, being inhibited by God's not playing dice, they decided to draw straws. Deirdre drew the short straw so she would have to stay home and watch. They reset their counters to zero, fastened their bras, and with no more adieu Alana was off!
Deirdre watched with some alarm as her own counter ticked along at its usual rate while Alana's crept along, slowing ever so methodically so that at the end of one year it read only 27,932,420 and during the second year it registered only 23,684,400 more ticks. Deirdre was happy that during the third (and final) year of the outward bound leg of Alana's mission she had 23,693,767 beats. At this point Deirdre's readout said 94,737,600 whereas Alana's read only 75,310,587. For the next year Deirdre worried because the number of heartbeats from her beloved sister did not increase as dramatically as she had hoped. But eventually it began picking up and by the end of the fourth year Deirdre was worrying about whether Alana's heart could hold up under the stress of the increasing toll of heartbeats.
The spaceship was sighted at an extreme distance some five and a half years after blast off and the sisters became ecstatic at the prospects of giggling together once again as they had when they were both young. Once they were in voice contact, they no longer watched their readouts as they had so assiduously before. Upon touchdown Alana stepped through the hatch opening she beamed and said, "One tiny step for me and a giant one for womankind!" Whereupon the sisters embraced with giggles enough to make up for years of loneliness. Their beepers raced.
Luckily a couple of thoughtful - though somewhat insensitive - male geek scientists who had become fascinated with the story (and instrumentation, to say nothing of the attractive girls) ripped the bras off the women to stop the beeping and read the meters at this historic point. The bewildered men looked from the now bare-breasted women, back to their readouts, and back again, over and over again in excitement. They shook themselves and looked again. Finally, in total disarray and confusion one of the men asked the other, "Does this mean there's more to life than just so many heart beats?"
The other thought for a while and said finally, "I think it means that if life, or time, or whatever you want to call it is measured as a number of significant events such as heartbeats, then covariance must apply and that quantity must be preserved across reference frames - but damn those twins are beautiful, aren't they? I think the younger one wants me!" he added with a wink.
The twins held their breasts modestly and looked at the men and back at each other in utter disbelief and amazement. Rapidly their biological clocks pheromonally re-synchronized and began pulsing in unison.
"I've been away a long time," Alana said wistfully.
"But not as long as you've been gone," Deirdre stated as a final scientific wrap-up to the just-completed experiment, and then with much more enthusiasm she asked, "Which one do you want?"