Relax! There are no black holes in our neighbourhood

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

31 August 2008

Last week, Irungu Thiongo asked about the speed of light in a black hole. Now four readers have pointed out that I never answered the question. So here it is: 299,792,458 metres per second.

This is the value of the speed of light in vacuum and it is a universal invariant constant. That is, it remains the same in all situations and in all places (black holes included). But then reader Wahome adds a new twist to this tale:

He says, “From my understanding, no light can escape a black hole due to the extreme gravitational force. Doesn't that then make the speed of light in a black hole zero? I think it is sort of like asking what the speed of a tethered horse is or a race car that is stuck in mud.”

Not quite, Wahome: a tethered horse can run at high speeds, but in a circle around the central pole. This is the same thing that happens to light in the black hole; it can move in curved paths inside the sphere of influence of the hole (the so-called “event horizon”)…but it still at 299,792,458 metres per second.

That is when talking about the “speed” of light. The “velocity” is a totally different matter. There is a subtle distinction between the two. Both give the distance cover in a unit of time but speed refers to distance along the path travelled while velocity is concerned with the displacement along a straight line in a specific direction from the point of origin.

Think about this way: the average speed of Wahome’s tethered horse if not zero because the animal can move around in a circle. Its velocity however is zero since it never gets away from the tether.

Now, because of their high gravitational forces, black holes are usually very small in size – a few tens of kilometres across compared to the millions of km for ordinary starts. Thus it would be impossible to see one if it were floating alone in space.

However, if there is matter in its vicinity then there then the black hole can be detected. As the matter is pulled into the hole, it gains immense energy, heats up to extremely high temperature (millions of degrees) and emits large amounts of X-rays. This happens before the matter enters the event horizon so the X-rays can escape and be observed from great distances away.

Another way of detecting a black hole is when a nearby star is strapped in orbit around it. The star is then observed to be orbiting about empty space. By measuring the size the orbit and the speed of revolution, astronomers are not only able to determine the location of the black hole but also its mass.

All this may make one worry that one day we will be sucked by a black hole. But have no fear; there is none anywhere near our vicinity. We are still alive, aren’t we?