The Speed Debate: Does flying eastward make you older?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

16 April 2006

The concept of speed appears to be troubling many readers, if my mailbox is anything to go by. Dr. Karanja Ngugi and his fiancée have been discussing the experience of time in a plane flying eastwards at the speed of the earths rotation or faster.

The good doctor says: “if the plane kept on flying [eastwards] indefinitely, would the occupants experience time as we know it? Would they … be younger or older depending on the average speed maintained by the plane? I would really appreciate if you could throw in some light.”

This question probably arises because of the fact that eastern regions of the world are ahead in time. For example, Japan is 6 hours ahead of Kenya: when it is 10am here, it is 4pm there. Consequently, the eastern countries get to a new day before the western ones. We see this every New Year: when the Americans, for example, cock their Champaign bottles, it is almost 12 Noon in Kenya.

Therefore, it appears that when you travelled eastwards you get older - at least by some few hours. If a set of twins were born in Nairobi last night at 1am (16th April 2006) and then one of them was flown to Japan, would they celebrate their birthdays at the same time?

The one in Japan would blow his first birthday candle on 16th April 2007. But by that time, it would still be 15th April in Kenya. Does that mean that the twin in Japan is older than the one in Kenya? The reasonable answer is no: but can we prove it?

Suppose that the travelling twin had carried with him a real-time clock. That is, one that shows both date and time. Clearly, this is the clock that would show how old he is. But its reading is only correct if it is not adjusted when the traveller crosses time zones. Thus when he lands in Japan, his clock would still show the time in Nairobi – that is, his “true” time. Therefore he would not be older than the brother he left in Kenya.

But still, would the twin appear to gain a day if he moved around the globe in an eastward direction? Again the answer is no. As soon as he crosses the International Date Line, he will skip back to “yesterday”. Thereby “losing” all the time he had “gained”. Thus if the flight was at the speed of rotation of the Earth, the journey would last 24 hours and, by the time he lands back in Nairobi, it would be the “next” day – in complete agreement with his real-time clock.

Another reader by the same name (Karanja) adds a new angle to this speed debate. He asks: “If an aircraft is moving AWAY from you (the observer) at the speed of light, will you 'see' it? Conversely, if another aircraft is moving TOWARDS you at the speed of light, will you 'see' it BEFORE it arrives? - Twin paradox sort of thing?”

That will be our subject next week.