Nega nodded in satisfaction.

     -  If that is true, then why don't we try to have a longer life by moving continuously? After all, not only would the time measured by the clocks slow down, but also the time measured by the heart beats and the body's physical decay, if we take Humans as an example, - concluded Elna.

     - That is an intelligent question, - exclaimed Nega. -It's clear you are thinking carefully about what I've told you. In fact, that is just what happens: the slowing-down that you mention really exists. It has been proved recently, not on Humans but using subnuclear particles: the muons, your friends. But watch out! There is an important "restriction" that keeps us from the "dream of eternal youth".

     - How did they prove this effect on muons? And what is the important restriction? - asked Kate, curiously.

Moving muons are more long-living
     - I'll answer both your questions, but one at a time, - answered Nega.

     - Muons at rest in laboratory decay, that is they spontaneously transform themselves into other particles after roughly two millionths of a second. The poor muons live their brief life "with a pistol pointed at their temple"; after an average time of two millionths of a second from their birth the trigger is pressed and the muons decay into electrons and neutrinos. However, if the same muons travel inside a particle accelerator at a speed close to the speed of light, their average lifetime at rest in laboratory, measured by the scientists,  increases significantly. It seems impossible, but it's true! At 99.5 per cent of the speed of light, the life of the muons increases ten times. The explanation comes from a principle that Humans call "Relativity": the internal clocks of the moving muons are much slower than the stationary ones. And, when the latter (stationary) die, the former (moving ones), for which less time has passed, continue to live for a while. This is very clear and direct proof of the effects of movement on time. Just imagine: If a man could reach 99.5 per cent of the speed of light he would have an average lifetime of 700 years!

    - Fabulous, wonderful! - exclaimed Kate enthusiastically.

The rocket-men live in slow-motion
    - Now we come to the "restriction", - said Nega with her patient but firm voice. - Yes, we have to look at the other side of the coin.

    - The scientists in the laboratory see that the moving muons live longer than those at rest; this is due to the fact that the time of  the moving ones passes slower. Now this slowing-down is applied not only to their clocks, but also to all their activities. For example, if a stationary muon (that we suppose can read!) reads a hundred books in its brief life, then so does a moving muon. Indeed, even if its life seems longer, its reading speed and everything concerning its life slows down in the same way. From the laboratory point of view, the moving muon lives in slow-motion and its lifetime increases. However, what it can quantitatively do during its life doesn't change.

     - The same goes for imaginary ultra-centenarian "rocket-men" flying at 99.5 per cent of the speed of light: from their point of view nothing has changed. Instead, for men remaining on Earth the rocket-men live in slow-motion and their lifecycles last for a large part of their terrestrial stationary time.

A man at the speed of the light?
     - How many things you know, Nega! - said Elna in increasing amazement. - I'd like you to stay with me forever and be my Guide, my supreme Guide!

     - Me too! - Kate rushed to say, sincerely.

Nega, very flattered and embarrassed, could say nothing but - Thank you.

     - I'd like to know something else, - added Kate, who had just had a doubt. - Would it be possible to accelerate a man to a speed close to that of light?

     - Yes.all you have to do is make him fly on a magic carpet! - answered Nega laughing.

(Credit: Adapted from: Settimana Enigmistica)

Then Nega became serious again: - In theory it is possible. However, to accelerate a man (not to mention a space-craft) to a speed of 0.90 per cent of the speed of light, you would need a quantity of energy higher than 13 per cent of the total energy consumed by a terrestrial nation like the United States of America in 1971. On the other hand, the acceleration of particles as electrons at high speed is much easier.