Can an aircraft moving at the speed of light be seen?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

23 April 2006

A reader who gave only one name (Karanja) added an interesting angle to the speed debate when he asked: “If an aircraft is moving AWAY from you 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?”

To understand where Mr. Karanja is coming from, let us consider a different question: why would we expect to see an aircraft that is moving away from us at the speed of light? The answer is; to be able to see anything, light must travel from the object to our eyes. But if the object is flying away at the speed of light, would the light ever leave the rear surface?

Suppose you were riding on the back of a pick-up truck that is moving forward at, say, 30km/h. If you threw a stone vertically upwards, would it be left behind by the vehicle? Well, no: the stone is already moving at the speed of the pick-up. Therefore it will fall back to the spot from where it was thrown, that is, into you hands.

Now suppose that that your friend was watching the stone from the side of the road. He would see the stone moving in the same direction as the vehicle at 30km/h. However, from your point of view, the stone and the pick-up appear motionless. You only become aware of the vehicle’s motion when you look outside.

If you now throw the stone backwards at 30km/h you will see it moving at that speed but from the roadside, your friend would see the stone dropping vertically downwards with no horizontal motion. If you don’t believe that, think of it this way: The stone was initially moving forward at 30km/h. Then it was thrown backwards at 30km/h. The net result is zero kilometres per hour (30 minus 30), that is, motionless.

Would the same thing happen to light emitted from the back end of an aircraft moving at the speed of light? If yes, then we would never see the craft because the light would be motionless and, therefore, never reach our eyes.

This is a issue that troubled Albert Einstein some 100 years ago. Einstein’s concern was this: It is a well-known fact that if you are in a steadily moving vehicle (aeroplane, ship, very smooth car, etc), you cannot tell whether you are in motion unless you “look” outside. This is a fundamental principle of nature.

Suppose light behaves like the stone on the pick-up. If you were inside an aircraft facing forward and looking at yourself in a mirror, what would happen when the craft accelerated to the speed of light? Light from you face would never catch up with the mirror. Therefore, your image would disappear. But that gives you an experiment that determines your state of motion without looking outside. It goes against the fundamental principle. Einstein therefore held that your image should not disappear from the mirror. That is, that the speed of light is not affected by the motion of the source.

With that in mind, we can now answer Mr Karanja’s questions: If the aircraft is moving away at the speed of light we will still be able to see it. If it is approaching us at the speed of light, we will see it before it arrives. However, this reasoning has serious implications on how we perceive time and distance. That will be our subject next week.