Can Black Holes Bend Light and Act as a Time Machine?

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Discussion Overview

The discussion revolves around the behavior of light in the vicinity of black holes, particularly focusing on whether black holes can bend light and the implications of such bending, including the concept of time travel. Participants explore theoretical aspects of general relativity, the nature of light, and the dynamics of black holes and their photon spheres.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that black holes can bend light due to their immense gravitational fields, suggesting that light could potentially curve around a black hole like a slingshot.
  • Others explain the concept of the photon sphere, where light can theoretically orbit a black hole, but emphasize that these orbits are unstable and not all photons will follow such paths.
  • A participant notes that while photons can wind around a black hole multiple times, the resulting images would be faint and mixed with other light sources.
  • There is a discussion about the nature of light and gravity, with some participants questioning why light, which has no mass, is affected by a black hole's gravity.
  • Some participants clarify that light follows geodesics in spacetime and that curvature is related to energy-momentum rather than mass.
  • Questions arise regarding the specific radius of the photon sphere and the underlying reasons for its location at 3M, with references to general relativity and null geodesics.
  • One participant mentions the challenge of understanding the mathematical derivations related to the photon sphere found in general relativity literature.

Areas of Agreement / Disagreement

Participants express a range of views on the behavior of light near black holes, with some agreeing on the existence of the photon sphere while others debate the implications and mechanics of light's interaction with gravity. The discussion remains unresolved regarding the specifics of how light behaves in these extreme conditions.

Contextual Notes

There are limitations in understanding the mathematical foundations of the photon sphere and the nature of light's interaction with gravity, as well as the assumptions made about the behavior of light in strong gravitational fields.

Who May Find This Useful

This discussion may be of interest to those studying general relativity, astrophysics, or anyone curious about the nature of black holes and their effects on light and spacetime.

PhantomOeo
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The reason I heard that black holes are black is because any light that trying to leave the black hole's gravity gets sucked in by the massive amounts of gravity.

So this implies that light can curve or even reverse direction if enough gravity is present. If this is the case then what stops a black hole from acting as a slingshot? I am asking if a black hole could cause light coming from Earth to bend around it (like an asteroid slingshoting around a planet) coming back to earth? To us it would look like a earth-like planet on the other side of the galaxy or universe but in reality we would be looking at Earth millions of years in the past (i.e. no signs of civilization or anything else that would indicate that it is Earth not another inhabitable planet)
 
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Welcome to PF!

Hi PhantomOeo! Welcome to PF! :smile:

A black hole has a "photon sphere" of radius 3M (where 2M is the Schwarzschild radius, of the event horizon), at which photons (light) can orbit the black hole for ever.

A photon which just misses the photon sphere will go in an ellipse, and the more it just misses it, the more eccentric the ellipse.

In a limiting case, yes, a photon could more or less go back the way it came :smile: …

but very few photons would do that,

and there'd be no focussing (unless you could place a "mask" near the black hole), so they'd be swamped by all the other photons from paths of different eccentricity. :wink:
 


tiny-tim said:
A photon which just misses the photon sphere will go in an ellipse, and the more it just misses it, the more eccentric the ellipse.

The only closed orbits orbits around spherical black holes for both massive particles and massless photons are circles. The only bound orbits for photons are circles, while bound, non-closed orbits exist for massive particles.

If gravity is fairly weak, the bound, non-closed orbits of massive particles are almost closed ellipses, but nothing like this exists for the trajectories of photons. The photon sphere circular orbits are at an unstable equilibrium of the effective potential, and a small perturbation will either start the photons spiraling into the black hole or out to infinity.
 
Hi George! :smile:

Yes, you're right … I'm being stupid. :redface:

The photon sphere circular orbits are unstable, and there are no other closed orbits.

I should have said that photons going near enough to the black hole can wind round the black hole many times before coming out again …

a black hole would act as a lens giving rise to n ring-shaped images of a background star, each ring corresponding to light which had been bent 1,2,3,..n times around, for some positive integer n (which depends on the distance beyond the black hole) …

and it would act as a mirror giving rise to n ring-shaped images of an observer, in the same way.

(all amazingly faint, of course, and jumbled up with images of everything else :smile:)

Thanks for the help! :smile:​
 
If light doesn't have mass, why does the Black Hole bend it?

The accretion disc is matter trapped in the Black Hole's gravity, slowly going into the event horizon. If light is trapped along with matter in the accretion disc, shouldn't light contain some kind of mass?

I might be missing something here, but it is a question that has been on my mind for a long time.
 
Hi Division! :smile:
Division said:
If light doesn't have mass, why does the Black Hole bend it?

Mass in energy, energy is mass, so light has mass.

Light doesn't have rest-mass. :wink:
The accretion disc is matter trapped in the Black Hole's gravity, slowly going into the event horizon. If light is trapped along with matter in the accretion disc, shouldn't light contain some kind of mass?

Light is not trapped in the accretion disc.

And the accretion disc is not "trapped in the Black Hole's gravity" any more than the Earth is trapped in the Sun's gravity. Orbits in the accretion disc are stable … the accretion disc itself however generates (and therefore loses) energy, probably through ordinary or magnetic viscosity (see http://en.wikipedia.org/wiki/Accretion_disc), causing it to gradually spiral in.
 
The light follows geodesics in space-time, and massive objects (well anything that has energy) curves space-time.

Curvature is proportional to the energy-momentum tensor, not mass.
 
So why is the photon sphere at 3M? What causes the circular orbit to be there? If photons behaved like massy particles it'd be at M, assuming Newtonian gravity applied to an object traveling at c.
 
the difference is that light follows null-geodesics
 
  • #10
malawi_glenn said:
the difference is that light follows null-geodesics

And? How did they compute that?
 
  • #11
qraal said:
And? How did they compute that?

what? that light has null geodesics or where the photon sphere is located around Schwarzschild BH?
 
  • #12
malawi_glenn said:
what? that light has null geodesics or where the photon sphere is located around Schwarzschild BH?

Sorry I wasn't clearer. I meant the radius of the photon sphere - why is the geodesic wrapped in a circle at 3M?
 
  • #13
Oddly enough Wikipedia has a decent entry on the Photon Sphere and actually derives it...

http://en.wikipedia.org/wiki/Photon_sphere"

...though it's all in GR tensor notation that I'm really not sure how to translate. At least the equations themselves, extracted from the tensor, aren't inscrutable.
 
Last edited by a moderator:
  • #14
I hope that question will come on my GR exam next friday lol
 
  • #15
how do something overcome the huge amount of gravity of black hole?
 

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