Speed of light is the same everywhere

Suppose you are driving at 110km/h and then you approach another car doing 100km/h. You would probably get into the overtaking lane when you are about one car-length behind, pass the slow car and return to your lane at one car-length in front.

Since you are only 10km/h faster, the overtaking process would be equivalent to passing a stationary car while doing 10km/h. The total distance you would travel in this instance would be three car-lengths. This is about 15m since an average car is approximately 5m long.

At 10km/h, how long would it take to travel 15m? The answer is 15m divided by 10km/h. But before pulling out the calculator, we must recognise that it is not right to divide km by metres.

So we first convert 10km/h to 10,000m/h. Doing the calculation now, gives 0.0015h or 0.09 minutes, or 5.4 seconds.

This is exactly the same duration that would elapse when a car travelling at 110km/h overtakes another doing 100km/h. The only difference is the distance required. At 110km/h, the overtaking car will travel a distance of 165m in the 5.4 seconds.

Real speed

Now suppose there was a housefly in your car when you are driving 110km/h. If it flew from the back to the front of the car at, say, 5km/h, what would be its “real” speed?

That is, what would be the speed of the fly as measured by some one standing on the side of the road? The answer is 5km/h plus the 110km/h of the car. That is, 115km/h. It is faster than the car!

But light does not behave in the same manner as the fly in the car. Last week we noted that the speed of light is exactly 1,079,252,848.8km/h.

If you are seated at the back of a car that is moving at 110km/h and you lit a torch forward, the beam would emerge at 1,079,252,848.8km/h. This is the speed you would measure when inside the car.

What about a person standing by the roadside? Would he measure 1,079,252,848.8km/h + 110km/h = 1,079,252,958.8km/h like in the case of the fly? The surprising answer is no!

Whether you measure it from inside the moving car or from outside, you always get the same answer: 1,079,252,848.8km/h!

Furthermore, if the person standing by the roadside lit a torch along the direction of your motion and you measured its speed from inside the car, you would still get 1,079,252,848.8km/h: not the 1,079,252,848.8km/h – 110km/h = 1,079,250,738.8km/h expected.

In other words, the speed of light is not only the fastest speed in nature, but it is also an absolute constant and does not depend on the motion of the source or that of the person observing it.