Light Bending Effect in General Relativity: Understanding Gravitational Lensing

[Spin_Network]

In GR the bending of light around a Large Mass produces a Gravitational Lensing effect.

I have a gedanken, I have a light source that has a finite 'ridgid' beam, a L.A.S.E.R beam. I point the beam into the night sky, at a location to the far West Horizon. I now proceed to arc the beam across the sky, to the farthest location in the East Sky Horizon.

Q)Does the swift motion of my arm cause the Beam's ridgidity to produce a drag "bending" effect?, similar to such as here:http://www.maths.soton.ac.uk/staff/Andersson/gwastro/sld016.htm

In the image at the left, the spacetime lines which all 'bend' inwards towards the BH. For light traveling along these lines, the light would be bent inwards, a Gravitational Bending of Light.

Now in my above L.A.S.E.R light gedanken, I would not see any bending by the beam because of my proximity to the source, UNTIL I move my arm across the sky! I would definately see a Drag-Effect in the beam.

I am always aligned with the beam, as I am directly at source. But surely another observer some distance away, close to the Beams directional path, will observe a "bending" of the beam? For instance, if the image above is used purely for "effect", choose any line upon the grid, move away from the Blackhole, it's curveture would become more rigid, the lines would automatically strighten out, gradually one's path is fixed towards the Light Source.

Ok, but if the Blackhole is in a high transit, if it was traveling at speed across space, then as with my Laser Beam, its effect of light bending would produce a lot of curveture in not just the Local Space, but Space far..far away.

Can this effect real within GR?..can observers outside the line of sight, of both source, me and laser, and Blackhole, conclude that there is a geniune warping of Light in any given intervening Space?

 

[JesseM]

The beam is not really a single rigid object, but just a series of photons, so the beam can move much faster than light without any individual photon exceeding the speed of light, meaning there's no need for any drag effect. See this page, for example.

 

[R0man]

Yes, the swift motion of your arm in your gedanken experiment would indeed cause a bending or "drag" effect on the laser beam, similar to how a gravitational field can bend light in general relativity. This is because, in both cases, the curvature of spacetime is influencing the path of the light.

In general relativity, the presence of a large mass, such as a black hole, creates a curvature in spacetime that can bend the path of light passing by. This is known as gravitational lensing, and it is a well-established phenomenon that has been observed in many astronomical observations.

In your gedanken experiment, the movement of your arm would create a similar curvature in spacetime around the laser beam, causing it to bend as it travels across the sky. This effect may be more noticeable at larger distances away from the source, as the curvature of spacetime would be stronger.

As for observers outside the line of sight, they may indeed observe a warping of light in the intervening space. This is because the gravitational lensing effect can cause light from a distant source to be bent towards the observer, creating a distorted or magnified image of the source. This has been observed in many astronomical observations, such as the famous "Einstein Cross" where the light from a distant galaxy is bent into four distinct images by a closer galaxy acting as a gravitational lens.

So in conclusion, the bending of light in general relativity is not limited to just the presence of a large mass like a black hole, but can also be caused by the curvature of spacetime due to other factors, such as the motion of objects. This effect is a fundamental aspect of general relativity and has been observed in various astronomical observations, providing further evidence for the validity of this theory.