Institute of Astronomy

Why is a black hole called a 'hole'?

Published on 07/01/2014 
Question: 

At the greatest risk of appearing dumb....I really would love it if you could help me get my mind around a black hole...before I go any further..it's the hole description I cannot conceive.

I am aware of what a black 'hole' is..I just don't understand why it is a black hole.

Let me expand:

A black 'hole' is formed when a star collapses on it self, which I understand, how ever it is the gravitational force that causes the black hole to appear which I don't understand.

Again let me expand:

If I was stood anywhere on earth, australia for example, would not the gravitational pull on me and everything be exactly the same as it would on me in england?

So when a neutron star collapses, would the gravitational  pull be equal in ALL directions? and if so..if this process continued to the creation of a black 'hole' surely the end result would be a 'SPHERE'  not a black 'hole'

And if..lets say a space craft approached a black 'hole' in earthly simulations it shows a swirling vortex which it is drawn into... you can only enter a black hole from one direction...yet with a sphere and in my mind..because of the way the black 'hole is created..the black 'sphere' would draw the same craft into it regardless of what direction it approached the 'hole' from?

Am I dumb or has my assumption any founding at all? this question has baffled me for years..I even asked a scientist on a radio programme who I think must of misinterpreted my question regarding this.

I would be grateful if you can point out where my theory fails and correct me.

That is not a dumb question at all, in fact I dare say you may have a better intuition than many film makers.

The 'hole' description is perhaps unfortunate in some ways, since as you rightly picked up on it naturally conjures up images of a swirling plug-hole which material can only fall into from one direction.  The idea of a 'black hole' was first suggested back in the 18th century by John Michell, though at the time he called them 'dark stars'.  If you increase mass of an object then the speed at which you have to travel to escape it's surface increases, so Jupiter has a larger escape velocity than Earth for example.  John Michell hypothesised that if a star could be massive enough then it might have an escape velocity greater than the speed of light, such that it would appear dark, hence 'dark star'.

The name 'black hole' dates from the 1960s when work on general relativity showed that massive enough objects will collapse down to an infinitesimal point (a singularity) due to the effects of gravity.  At some distance from this singularity the escape velocity is equal to the speed of light, and since relativity says that nothing can travel faster than light, once anything passes within this distance (called the event horizon), it cannot escape.  The 'hole' label is a partly result of this idea, that nothing can escape from it, it is simply sucked in and can never get out, like a bottomless pit.  The description of it as a 'hole' also references the existence of a singularity at the centre of the event horizon, since at the singularity the laws of physics break down and so it is in some ways a 'hole in space' that we can't describe.

However, as you have surmised while the name 'black hole' does make some sense, it is indeed a spherical hole.  It doesn't matter what direction material approaches the black hole from, once it passes the event horizon it will be unable to escape.

The 'swirling vortex' effect that is often used to indicate the presence of a black hole is partly the result of the fact that it is rather difficult to represent a spherical piece of absolute nothingness.  Particularly against the black background of space. Now that modern computer graphics are better we can show the presence of a black hole by the distorting effect it has on light which passes near to the event horizon, but does not cross it. Wikipedia in fact has an animation on their page about black holes showing just that.  The 'swirling vortex' effect is also partly a result of misunderstanding an accretion disc around a black hole.  Most material that approaches a black hole won't do so head on, but from an angle and will, at least initially, get trapped in orbit around it rather than falling straight in.  When there is a lot of material approaching the black hole the material will all collide with each other and find a mutual direction of rotation, forming a disc.  That disc, viewed face-on, looks rather like the swirling plug-hole, though material then falls in from the inner edge of the disc rather than from the 'top' or 'bottom'.

Page last updated: 7 January 2014 at 12:35