I think a black hole would move through space like stars and galaxies. But then the problem is, what will happen to the event horizon and photon sphere orbiting black hole? If a black hole moves through space does not it allow photons orbiting around it to escape or fall into it, so the black hole should always lose photon sphere?

Same for event horizon, does it move along with blackhole? How can it move? Doesn't it allow some photon at the event horizon to escape like a comet trail?


4 Answers 4


Black holes moves just like all other matter. The photon sphere, the event horizon, just like the inner-most stable circular orbit, etc, etc. all move along with the BH---as they are defined relative to the BH rest-frame, i.e. their relative velocity is zero.

Having the black hole be moving doesn't really effect the of (e.g.) particle trajectories much: it's very rare that a photon becomes trapped (even for a short time) at the photon-sphere, and it would be similar rare for a moving black hole, but the trajectories of the photons would simply be slightly different (in particular they would need have a velocity component in the direction of the black hole's motion. But there's definitely nothing strange or exotic happening here, and based on the fundamental principle of Lorentz invariance (things being the same from different velocity reference frames) nothing exotic is allowed to happen.


Well we don't know yet, However if you consider Hawking Radiation which shows that quantum effects allow black hole to emit black body radiation it can be theorized that Black holes and their event horizons can move.

And if it is correct that black holes are collapsed neutron stars, and being as we know stars move, this is further evidence that black holes do move.

Additionally if indeed galaxies move, and black holes are found in the centers of galaxies even more evidence for consideration.

I don't think you can assume that a Black hole will emit photons just because it moves, it's gravity field will move with it, still holding the photons.

As black holes are most likely moving and we do not observe photon radiation, this would seem to indicate that there is no "photon comet tail" resulting from black hole movement, or we would have seen it by now.

Suppose two black holes are moving towards each other on a collision course and one is larger than the other, you might think that the larger one could pull photons away from the smaller one. Not likely, the collision will happen and still no photons escape, you will just have a larger moving black hole. A feather and a stone fall at the same speed in a vacuum.

Now if a giant black hole is a spining moving collapsed neutron star then, that "comet tail" may be observed as the spiral arms of a galaxy.

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    $\begingroup$ The question has nothing to do with Hawking Radiation, neither do moving event horizons. BH are not always collapsed neutron stars (likely rarely so). There's no "if" in whether galaxies move, or that BH are at their centers. Feather and stone don't have the same gravitational field. Galaxy spiral arms are stars and gas and have nothing to do with BH, which have nothing to do (in general) with spinning NS... $\endgroup$ Commented Feb 23, 2017 at 19:43

Assuming the BH had a stable photon sphere.

I think you may have a point. Specially, for the photons that are orbiting the black hole and are moving at a time in opposite direction to that of the black hole. There must be a light speed lag between actual movement of the singularity, and evolution of the curvature at the edges of the event horizon. So the photons on the rear end of the event horizon are travelling at light speed c (note c is constant), but the EH has moved a little forward, and so they should be outside EH for a moment which should allow them to escape the EH.

Suppose there is a 1 nano second lag between actual movement of the singularity, and evolution of the curvature at the event horizon. The photons are moving as per the EH of singularity 1 nano second ago. Now all of a sudden, (1 nano second later), the EH has moved by (say) 1 micrometer. The photons as per older EH should fall outside EH and thereby escape the EH.

Same can apply to photons on front end of EH and they should fall into the black hole.

I do not know if and how the mathematics accounts for this.

I can understand a north/south satellite around earth because the velocity of the satellite can remain same relative to earth in both directions because, its speed does not have to be constant. But light going north/south around a black hole, its speed has to be same relative to the singularity, at the same time be constant in both the directions, which is hard to grasp.

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    $\begingroup$ This reasoning cannot really work since motion is relative. Two things to consider: 1. Go to the frame where the black hole is at rest. How would you explain this phenomenon in that frame? 2. Start with a black hole at rest, then boost to a frame where it moves. This can be always done, so all black holes should always lose their photon spheres in this manner, which is absurd. $\endgroup$
    – ACuriousMind
    Commented Feb 23, 2017 at 10:24
  • $\begingroup$ @ACuriousMind: can you point to a physical proof of existence of a photonsphere? If we see it, then bh is loosing it, if we do not, how do we know it is there? $\endgroup$
    – kpv
    Commented Feb 23, 2017 at 14:50
  • $\begingroup$ There are lots of issues with this answer, well highlighted by @ACuriousMind. These are special-relativistic issues, not GR---there is no issue with light always traveling at the same velocity. And note that the photon sphere is an unstable equilibrium point and would never be full of eternally-trapped photons in any model; they are there transiently. The photon sphere is seen in popular lensing animations, e.g. 49.media.tumblr.com/c3443b901a2fe91737deab060128f7a7/…, which could eventually be resolved by the event horizon telescope. $\endgroup$ Commented Feb 23, 2017 at 19:39
  • $\begingroup$ @DilithiumMatrix: I can show anything in an animation. Any way, if the photon sphere is not stable, then that answers the question - photon sphere is lost (somehow). I will update my answer so. If the photon sphere is not stable, then, the BH is loosing the photon sphere any way. $\endgroup$
    – kpv
    Commented Feb 23, 2017 at 19:49
  • $\begingroup$ @DilithiumMatrix: I was trying to evaluate constant c from the center of galaxy frame. That frame should see BH moving and light zipping back and forth with respect to BH. For example, if we see moon from sun's reference, it would make a helix where the moon is moving at different speeds when seen back and forth from sun. So, that is fine. But if we replace moon with light, sun should see light moving back and forth with different speeds, still it being constant. I am not questioning the relativity, trying to understand how that situation would be explained. $\endgroup$
    – kpv
    Commented Feb 23, 2017 at 19:55

It is theorized that black holes are only at the center of galaxies. These galaxies are moving relative to each other. By the expansion of space between them, this means that the object at the center relative to the space around it is not moving at all. Hopefully this helps with your question.

  • $\begingroup$ I suggest to start to use the "," character in your sentences. You have a 5 lines long sentence without a single ",". $\endgroup$
    – peterh
    Commented Feb 23, 2017 at 23:32
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    $\begingroup$ Only supermassive black holes are in the center of the galaxy. There are many normal blackhole from dead star. Some even merged with each other $\endgroup$
    – Thaina
    Commented Feb 24, 2017 at 8:16
  • $\begingroup$ @heather thanks for editing my question I tend to ramble in my approach to answering questions. This is an attempt at relieving the formal prose and is an attempt to be more approachable. I now understand this is obviously not the case for this forum. Just one criticism about $\endgroup$
    – 8Mad0Manc8
    Commented Feb 24, 2017 at 19:32
  • $\begingroup$ @heather in continuation. Just one criticism about your edit of my question. I am British we spell the word centre c.e.n.t.r.e and not like our American cousins of which I deduce you are, and as such we use the Queens English lol. $\endgroup$
    – 8Mad0Manc8
    Commented Feb 24, 2017 at 19:44

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