# Center of Black Hole

#### cangus

Has anyone ever heard that the center of a black hole is a point in time rather than a place in space? If so, can someone please explain this to me? A Black hole in in physical reality (space) right? So, how is the center a point in time?

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#### pmb_phy

cangus said:
Has anyone ever heard that the center of a black hole is a point in time rather than a place in space? If so, can someone please explain this to me? A Black hole in in physical reality (space) right? So, how is the center a point in time?
Sorry. Never heard that.

Pete

#### wangyi

The singuliarity is the future for the people droped into the BH, but not another 3-space point with equal time. I think in this sense we say so.

regards
wangyi

#### pervect

Staff Emeritus
cangus said:
Has anyone ever heard that the center of a black hole is a point in time rather than a place in space? If so, can someone please explain this to me? A Black hole in in physical reality (space) right? So, how is the center a point in time?
I haven't heard anyone say that either, and it doesn't make a lot of sene to me.

However, it is true that the Schwarzschild "R" coordinate (the radius) becomes timelike inside the event horizon of a Schwarzschild black hole - while the Schwarzschild 't' coordinate becomes spacelike. Possibly this is what someone was trying to say (not very well) with your original quote.

Note that in spite of the switch of the roles of the Schwarzschild r & t coordinates, there are still three space and one time coordinates inside a black hole, just as there are three space and one time coordiante outside a black hole.

#### George Jones

Staff Emeritus
Gold Member
cangus said:
Has anyone ever heard that the center of a black hole is a point in time rather than a place in space? If so, can someone please explain this to me? A Black hole in in physical reality (space) right? So, how is the center a point in time?
Forget relativity for a moment, and think about the x-y-z coordinates for the physical 3-dimensional space. Fix z = 3 and let let x and y roam over all possible values. This corresponds to a 2-dimensional plane in space - the z = 3 slice.

Now move to the spacetime of special relativity, and consider an inertial reference frame. Set t = 3 and let x, y, and z roam over all possible values. A 3-dimension spacelike hyperplane results that represents, for the chosen reference frame, the single instant in time t = 0. On a standard spacetime diagram, this is represented as a horizontal line because the x and z spatial dimensions are suppressed.

Finally, consider Schwarzschild coordinates for the spacetime of an 'eternal" black hole. As pervect has already noted, inside the event horizon r is a timelike coordinate. A singularity "occurs" at the "centre" r = 0. Since r is a timelike coordinate, the singularity is spacelike. With 2 dimensions supressed, the (future branch of the) singularity r = 0 is represented on: a Kruskal-Szekeres diagram by a spacelike hyperbola; a Penrose diagram by a jagged horizontal line. Very roughly, these lines (actually hypersurfaces) correspond to the "instant in time" r = 0.

OF course, because of of its singular nature, r = 0 is not actually allowed as part of the spacetime manifold.

Regards,
George

#### Dr.Brain

cangus said:
Has anyone ever heard that the center of a black hole is a point in time rather than a place in space? If so, can someone please explain this to me? A Black hole in in physical reality (space) right? So, how is the center a point in time?

Havent heard of something like that but I guess I will like to tell you that near a black hole , space-time coordinates reverse themselves.Like you cannot avoid next thursday in spacetime real universe , near a black hole, you cannot avoid hitting singularity, that is r-----> 0.

Maybe your ststement, though unheard of , might have have to do something with interchange of space-time coordinates , that is the job of 'time always moving forward' is exchanged by 'r--->0'

.BJ.

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