Want to grow younger? Fly at one billion kilometres per hour

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

30 April 2006

Last week’s article investigated what we would expect to see if we were watching an aircraft that was flying at the speed of light. The reasoning however, did not tie up properly – as Michael Oduor puts it “I don’t understand why Einstein was troubled [by the image disappearing from the mirror inside a ship moving at the speed of light]”. So let us recapitulate:

Suppose you are inside an aircraft, facing forward and holding a mirror. If the craft accelerated to the speed of light, you might expect that the light from you face would never catch up with the mirror and therefore you image would disappear. However, there is a problem with that expectation.

It is a well-established fact of nature that if you are inside a steadily moving system, there is no experiment that you can do to determine if you are in motion – unless you observe outside the system. This is the reason why we cannot feel the movement of the Earth. We are only able to tell that the planet is moving when we look at the sun and the stars.

The problem with the disappearing image inside a speed-of-light aircraft is that it provides an experiment to determine your state of motion. This goes against the fundamental principle of nature. Thus Einstein concluded that the image wouldn’t disappear.

The meaning of that conclusion is that the speed of light is an absolute constant. That is, it does not depend on the velocity of the observer or the source. If you recall the stone throwing experiment, the observed speed of the stone depended on whether you are on the pick-up or on the ground. That is not the case with light – the two observers will measure the same value (299,792.458km per second).

This fact has actually been proven experimentally. The Earth moves around the sun at about 30km/s, thus we might expect speed of light measured in the “forward” direction to be different from that obtained in the reverse direction. However, both turn out to be the same – right down to the last decimal place.

So, what is the implication of all this? Speed is the distance travelled divided by the time take. If the speed of light does not depend on the motion of the observer, then it means that the perception of distance (or length) and/or time must change when this speed is reached.

It turns out that when speeds are very high (comparable to that of light), time dilates and length contracts. The “clock” in a fast moving craft will be slower than one that is stationary. The word “clock” here does not just mean the mechanical and electronic gadgets that we use to tell time. It encompasses everything that changes with time, including physiological changes in a living organism.

Consequently, if you took a one-year journey in a craft flying at, say 85 percent of the speed of light (about one billion km/h!), you would return to earth to find that two years have passed! Due to your high speed, your time would dilate to double that of earth. When one minute elapses at home, your time will advance by only 30 seconds. So if you want to slow down your aging, just take a trip in a very high-speed aircraft.