# As a black hole passes through spacetime, does it affect it?

Nothing in the universe stands still. It's always on its way from somewhere to somewhere. A black hole follows a trajectory through the universe, moving from one locus in spacetime to another locus.

Deep inside each black hole is a point that we've never been able to characterize.

My question is, does the singularity, and possibly the powerful gravitational fields around the singularity have any measurable effect on the area of spacetime it passed through? One imagines an undeformed area of spacetime. After some time, a black hole passes through this spacetime, then moves on. Does the spacetime it passed through simply go back to being everyday, garden variety undeformed spacetime?

No, a black hole, or any other gravitationally attracting object for that matter, does not change spacetime permanently, but only affects spacetime around it. The easiest way to see this, is to view the BH in its own frame of reference. Here, the BH is stationary, and spacetime around it is described by the Schwarzschild metric. If a BH came by you and left a physical imprint on spacetime after it (like a wake), then from the BH's frame that would mean that you would fly by it, and it would start producing a wake in the direction that you moved. You don't have such an effect on BHs.

Wrong scenario of observer and black hole passing each other, as viewed from the observer's (left) and the black hole's (right) reference frames.

Nothing in the universe stands still.

This is not quite correct.

Every object is still (i.e. at rest) in it's own reference frame.

A black hole follows a trajectory through the universe, moving from one locus in spacetime to another locus.

Except that the black hole is at rest in it's own frame of reference.

Deep inside each black hole is a point that we've never been able to characterize.

We do characterize it as a singularity.

My question is, does the singularity, and possibly the powerful gravitational fields around the singularity have any measurable effect on the area of spacetime it passed through ?

The singularity is a property of the space-time distortion inside a black hole, i.e. a result of the gravitational field of the black hole.

The gravitational field has effects on space-time both inside and outside the event horizon. The event horizon is what defines the black hole's extent.

One imagines an undeformed area of spacetime. After some time, a black hole passes through this spacetime, then moves on. Does the spacetime it passed through simply go back to being everyday, garden variety undeformed spacetime?

It becomes more and more distorted and then less and less distorted. As the range of the gravitational field is infinite, it never actually is undistorted.

There is no permanent effect on space-time (e.g. nothing like a track "goughed" through space-time).

However, as the space-time is distorted there is a kind of permanent effect in the sense that e.g. an observer could see an object passing through such a gravitational field as experiencing a time dilation relative to the (distant) observer. That would be permanent in the sense that the difference in the time observer and object calculate is not going to be undone. The rate at which time changes will return to normal, but not the accumulated time they each record, which will remain different (in the absence of the observer passing through a similar distortion in space-time).

• I'll have to consider your assertion about everything being still in its own reference frame. The problem of course lies with the nature of spacetime, something we talk about freely but can't yet characterize. My question lies in areas of spacetime that were once occuppied by a black hole, but now are a large enough remove from the black hole it is no longer influenced in any way. If, as you assert, an object is standing still in its own reference frame, does this mean that the spacetime it occuppies is also standing still? – Howard Miller Mar 7 '17 at 21:55
• @HowardMiller spacetime isn't "stuff" that things move through, it's a coordinate system. You're thinking of it as a material that can be permanently deformed, but it can't be. – Asher Mar 14 '17 at 3:29
• @Asher so, what is en.wikipedia.org/wiki/Frame-dragging dragging? – Howard Miller Mar 14 '17 at 21:30
• @HowardMiller frame dragging is an effect that happens near a mass while the mass is still nearby, like with all gravitational effects. It doesn't happen after a mass has come and gone; you won't be flying through empty space away from all massive objects and be effected by frame dragging. – Asher Mar 14 '17 at 23:26
• You are still thinking in terms of space-time as a substance. It simply isn't like that (and that's what the experimental evidence says - see Aether theories for more). – StephenG Mar 15 '17 at 0:09