Expert: Philip Stahl - 5/5/2006

Question
hello mr. stahl,

i once read that a black hole can move through space. i was wondering if this is true and how it is possible for something without mass to move. so if it's possible for a black hole to move, could you tell me how it could happen?

this question has been bothering me for about a year now, and i finally chose to ask an expert. thanks for your time, and i look forward to hearing from you.

lianne

Answer
Hello.

Unfortunately, the source of your puzzlement arises from a misconception about black holes, e.g. that they lack mass. In fact, a black hole's mass is one of its most important and defining properties.

For example, the mass of a black hole helps to determine the
hole's angular momentum per unit mass, or L/M. This, along with its charge (Q) can then be used to assess its electro-magnetic and gravitational fields.

The so-called, 'Schwarzschild radius' (R) gives the radius at which any given mass M becomes a black hole, meaning that the surface gravity is so strong that not even light can escape.

This is:

R = 2GM/ c^2

where R is the (Schwarzschild) radius, G is the Newtonian gravitational constant, M is the mass, and c is the speed of light. Using this eqn., one finds that the radius needed for the Sun to become a black hole is 3 km (about 2 miles), which is much less than the Sun's actual radius of 700,000 km (437,500 miles). (And don't worry as this could never happen).

What this means is that black holes definitely move through space, certainly under the influence of the local gravity fields or the large scale motion they share with the galaxy. (E.g. all objects in the MIlky Way rotate as if around a large, central mass).

In addition, many black holes are part of "binary star" systems, which means the mass and even the motion can often be deduced from the behavior of the companion star. X-ray emission also is a dead give away, since x-rays will be generated as the companion star's gaseous outer envelope is sucked through the event horizon of a black hole. Cygnus X-1 is probably the best known and cited example of this type of model.

Where you may be having confusion is with the so-called "singularity" at the center of the black hole. In basic terms, this amounts to zero volume and "infinite mass". Up to now, many theoreticians are still at work trying to make sense of this, and how matter can literally be squeezed to the extent it takes up no space.


Even so, it's important to keep in mind that the central singularity is not the same as the black hole itself - which as I noted moves - either in mutual orbit with a (supergiant) companion star, or as part of the large ensemble of objects in the galaxy, or in other ways based on the local fields present.