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View Full Version : Can light go in to orbit?

wolrah
2009-Mar-17, 04:54 PM
Just one of those random thoughts that struck me as I was listening to a recent episode of Astronomy Cast.

If I was floating around in space in the general area a black hole and had a laser pointer facing off at exactly a 90 degree angle from the black hole, at some point some perfect distance out would a third party observer see my laser hit me in the back of the head?

Let's ignore the obvious issues of the gravitational effect on me and the need for a handheld laser pointer that's still visible after traveling the distance of the orbit, assuming that "perfect" point isn't far enough in that no outside observer would see anything.

My basic understanding of physics leads me to believe that there is some point around an object of sufficient gravity where light would in fact be capable of orbiting, but there are always interesting things going on when inconceivable levels of gravity or things moving really really fast are involved so I figured it's worth asking.

peter eldergill
2009-Mar-17, 05:00 PM
I think that may be the event horizon, but I'm not sure

Pete

01101001
2009-Mar-17, 05:06 PM
Check out the photon sphere: Falling to the Singularity of the Black Hole (http://casa.colorado.edu/~ajsh/singularity.html)

1.5 Schwarzschild radii from the singularity.

This is the location of the innermost unstable circular orbit. To maintain circular orbit at this radius requires going at the speed of light. In particular light itself can orbit the black hole in circular orbits at this radius. For this reason, this location is sometimes called the photon sphere.

Inside 1.5 Schwarzschild radii, no circular orbits exist, stable or unstable: all free fall orbits fall into the black hole.

grant hutchison
2009-Mar-17, 05:08 PM
It turns out to be at 1.5 times the Schwarzschild radius (the apparent radius of the event horizon).
The region is called the "photon sphere", for obvious reasons. It's unstable, in that photons in other than perfectly circular orbits will end up either falling into the event horizon or escaping.
For rotating black holes the photon sphere separates into two shells, in one of which photons can orbit equatorially in the retrograde direction, and in the other of which they can orbit in the prograde direction. Between those two radii, interesting photon orbits (http://www.physics.nus.edu.sg/~phyteoe/kerr/) can occur.

Grant Hutchison

nauthiz
2009-Mar-17, 05:12 PM
It's theoretically possible as long as the only force acting on the photon is gravity from the massive object. You'd have to put the photon in a perfectly circular orbit in a region called the photon sphere (http://en.wikipedia.org/wiki/Photon_sphere).

The orbit won't be stable if there's anything acting on the photon, including gravity from other objects. That's because photons can only travel at one speed. Since they won't accelerate as they get closer to the object or decelerate as they get further away, they cannot be in an elliptical orbit. If they get out of that photon sphere by even the tiniest amount, they will leave orbit and either fall in or escape.

astromark
2009-Mar-17, 06:13 PM
Thats why I come here... A interesting question and well explained answers,. great.

a1call
2009-Mar-17, 06:57 PM
Thats why I come here... A interesting question and well explained answers,. great.

You are welcome Mark. It's nothing really.:lol:

tommac
2009-Mar-17, 07:58 PM
at just outside the event horizon only light shone at 90 degrees can escape. I think any other angle orbits.

phunk
2009-Mar-17, 08:00 PM
at just outside the event horizon only light shone at 90 degrees can escape. I think any other angle orbits.

It's my understanding that between the event horizon and the photon sphere, any direction other than straight up will end up with an orbit intersecting the event horizon and falling in.

phunk
2009-Mar-17, 08:05 PM
Ah, correction: It's not only straight up unless you're right at the event horizon. The possible exit angles start at 90 degrees just outside the event horizon and increase to 0 degrees at the photon sphere (with 90 being normal to the EH and 0 being tangental).

tommac
2009-Mar-18, 02:26 AM
It's my understanding that between the event horizon and the photon sphere, any direction other than straight up will end up with an orbit intersecting the event horizon and falling in.

I think the angle needed to escape changes. right inside the photon sphere you can have almost any angle other than perpendicular ... at the EH you can only have light that is in the same angle as gravity ( opposite direction of course )

tommac
2009-Mar-18, 02:29 AM
Ah, correction: It's not only straight up unless you're right at the event horizon. The possible exit angles start at 90 degrees just outside the event horizon and increase to 0 degrees at the photon sphere (with 90 being normal to the EH and 0 being tangental).

yes ... was trying to say that in my post. Makes total sense ... as space-time is being sucked into the BH at very near the speed of light only things that are travelling at the speed of light can escape. Any angle would slow the light with respects to the opposite direction of gravity and gravity would win the fight and pull the photon towards the EH.

astromark
2009-Mar-18, 09:40 AM
You are welcome Mark. It's nothing really.:lol:

and you were not part of this... :) I was encouraging wolrah... i may not have said that well...;)... So if I have understood any of this light may only orbit an object that has the gravity force greater or equal to c.

WaxRubiks
2009-Mar-18, 12:13 PM
what if the radiation in the photon shell had the energy/mass-equivalent of a black hole; would the light orbit itself? :D my old bubblestar idea.

wolrah
2009-Apr-02, 04:15 PM
Thanks for the answers guys. Now I have a ton of new terms to google/wiki and go off on random click tangents with next time I get bored at work. Those proton orbits are blowing my mind.

ravens_cry
2009-Apr-03, 09:08 PM
I wonder what it would look like. . .

snowflakeuniverse
2009-Apr-03, 11:51 PM
Hi Wolrah
Interesting “looping” Spherical Orbits.

I hope you get a chance to look at the link that Grant Hutchison gave.
http://www.physics.nus.edu.sg/~phyteoe/kerr/

Angular momentum is one the characteristics that a black hole is not supposed to destroy. Since zero angular momentum would be virtually impossible to exist, all black holes, (if they exist,) would allow photons to orbit in these surprising looping orbital patterns.

Snowflake