# Q & A: Light absorption by a black hole

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Most recent answer: 08/08/2011
Q:
My assumption is that light has mass and the path it takes in space can be effected by a black hole. How massive does a black hole has to be in order for light to orbit around the black hole. And if there is such a thing as light orbiting around black holes, how long would it take for the accumulation of light orbiting a black hole to actually effect the black hole's mass. I am assuming that since light moves in every direction, coming from trillions of stars, that over time light would accumulate around the black hole.
- Panos (age 29)
Lockport, IL, 60441
A:
If a photon is headed toward a black hole and its impact trajectory is within the Schwartzchild radius  (see  ) then it will be absorbed and contribute to the black hole's mass otherwise it will just be bent a bit and go off happily back toward infinity, or wherever photons go.  In the very early times after the big bang the density of photons was much bigger than the density of ordinary matter (radiation density vs. matter density).  After 60 to 100 thousand years the expansion of the universe diluted the radiation so that now the matter density is much greater than the equivalent photon density.   So the likelihood of matter being sucked in to the black hole is much larger than that of photons and hence contribute much more to the mass accumulation rate.

LeeH

(published on 09/10/2009)

## Follow-Up #1: light orbiting black hole

Q:
Thanks for the quick reply. If I am understanding the answer correctly you are saying that a black hole can either suck light into it or just bent the path of light as it passes by, but it can never be massive enough to actually make light orbit around the black hole (not falling in but also not escaping... like a comet around the sun or suns around the black hole in the center of our galaxy). I thought that the same way we can calculate the trajectory and speed that would make a comet orbit around a star of mass X, maybe we could calculate the mass X of a black hole that would make light orbit (since we know the speed of light and its mass). Am I thinking of this in the wrong way comparing light to a comet and a black hole to a star? Again thank you for the response.
- Panos (age 29)
Lockport, IL
A:
I believe you're right that exactly at the event horizon, light can orbit a black hole, at least in a classical picture. It's not quite like ordinary massive objects orbiting something, including a black hole, however. A planet can orbit the Sun at a range of different distances, although of course with different orbital speeds, as described by Kepler. Since light has a fixed speed, there's only one distance at which it can orbit. (We're considering a spherically symmetric hole, for simplicity.) Anything closer, and it's sucked in. Farther out, and it escapes. Since actual light obeys the quantum uncertainty relations, it can't actually be perfectly located at the horizon, but always has some spread. So weird things happen, beyond what I understand. One of Lenny Susskind's books discusses this sort of thing.

Mike W.

(published on 09/11/2009)

## Follow-Up #2: light around black holes

Q:
Thank you Mike and Lee. I will try to find the book from Lenny Susskind since it sounds interesting. I do not know how plausible a symmetrical/spherical black hole would be, but if there was such a thing do you think that over time it could concentrate orbiting light around it. In other words could someone traveling through the event horizon of the black hole be hit by huge amounts of energy due to orbiting light. I promise this to be my last question on the subject. Time to read some physics books before I can open my mouth (type on my keyboard) again. Again thank you for your replies.
- Panos (age 29)
Lockport, IL
A:
I think most real black holes are close to spherically symmetrical. Certainly the spherically symmetrical solution is the simplest and most often described. I don't think that symmetry is important to understanding whether light can be found in orbit at the event horizon.
I'm not an expert on this, but I don't think the picture of lots of light orbiting the black hole is realistic. The reason, to put it in a slightly different way, is that the orbit is unstable. Tilting in a little means falling in, tilting out means escaping. So for any particular distance the fraction of directions which would orbit is zero. However, a light wave has an intrinsic spread of directions which gets larger the narrower the beam is. So I don't think you can keep light in orbit.

Mike W.

(published on 09/15/2009)

## Follow-Up #3: expanding space

Q:
Mike, We have visible matter,which accumulated into a largest ever black hole which blew up into space and expanding ever since into SPACE. There is also vacuum in space which I presume is lack of matter in that particular space. Now my question is for matter to expand space is required. where did this space come from? Is this space infinite? Is the Universe a vacuum cleaner because of which matter is being sucked into .... or rather expanding universe
- Prabhakar (age 47)
Hyderabad,Andhra Pradesh,India
A:
Let me start with the question I understand best, whether space is infinite. We don't know.

Where did the space come from? We can trace things back to near the Big Bang. The space descended from other space. There's no "conservation of space" law. In fact, given the choice of valid coordinate systems, there's only a conventional, convenient answer to the question of whether space is growing.

I'm not sure about the meaning of the vacuum cleaner question.

Mike W.

(published on 07/28/2011)

## Follow-Up #4: cosmic issues

Q:
Mike, I did not understand your answer at all. My question basically was where did the matter which resulted in `big Bang` come from, What was there before the `Big Bang` and for matter to expand we need `Space`. What is the origin of `space`. This also raises the question why light has a finite speed, but why gravity is unlimited. seems to be a mismatch. finally, why all this exists at all? By `vacuum cleaner` I meant since space is a `void` it follows that it is a vacuum and matter rushes in to fill this vacuum which probably we are seeing the universe as expanding from our perch `the Earth.` Am I right? Prabhakar
- Prabhakar (age 47)
Hyderabad,Andhra Pradesh,India
A:
We know nothing at all about why things in general exist.

We know enough about the universe to rule out the idea that there's a big empty space, with matter rushing out to fill it. On a large scale, the space that exists has a fairly uniform density of matter. So there's no "vacuum cleaner" effect going on.

The question of what was before the Big Bang is being actively investigated. One type of idea involves a periodic colliding set of universes. If that is correct, some evidence should show up in features of the Cosmic Microwave Background, currently being measured.

The Big Bang did not result from matter, in the ordinary sense of the word. Instead, matter is a leftover from the Big Bang. We don't understand well why an imbalance between matter and antimatter developed after the Big Bang.

We don't know much about the ultimate origins of space. We do know that now it obeys the laws of General Relativity. Those say that if there's a background energy in space (whose origin is also not well understood) then the space expands faster and faster, if you use distance units in which atoms, etc. have fixed sizes.

Mike W.

(published on 08/08/2011)