Gravitational lensing paradoxon

In summary, two light rays started at the same time symmetrically, are diffracted due to the gravitational effect of a star and meet at the same place behind the star. This is possible because of a light ray's path on the surface of a light cone, which separates causality regions in general relativity. This is not a paradox, as it is similar to the convergence of two diverging light rays through a lens. Additionally, in the curved spacetime of a gravitational field, the light cone is not a neat cone and can be warped or curved, especially near a black hole.
  • #1
mersecske
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Let us assume that two light rays are started at the same time symmetrically (towards the left and towards the right side of the star) which are diffracted due to the gravitational effect of a star and they meet each other at the same place behind the star. The meeting is the same event, therefore the world-lines of the two light rays cross each other twice. How it is possible? Because a light ray goes on the surface of the light cone, which separates the causality regions in general relativity.
 
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  • #2
mersecske said:
Let us assume that two light rays are started at the same time symmetrically (towards the left and towards the right side of the star) which are diffracted due to the gravitational effect of a star and they meet each other at the same place behind the star. The meeting is the same event, therefore the world-lines of the two light rays cross each other twice. How it is possible? Because a light ray goes on the surface of the light cone, which separates the causality regions in general relativity.

I am not sure why you think this is any more paradoxical than passing two diverging light rays through a suitable glass lens so that they converge on a focal point. Happens all the time in cameras, telescopes, etc.

The diagram of the diverging rays going around a massive body and converging on a point is essentially a space-space diagram, while light cones are normally depicted in space-time diagrams, so there is no conflict here because they are different situations.

In a spacetime diagram of a light cone, with time on the y-axis and distance on the x axis, the physical situation is two rays going in opposite directions parallel to the x axis. The diagram does not depict a physical "light cone" in space.

You should also bear in mind that in the curved spacetime of gravitational field, the spacetime diagram of a light cone is warped or curved, especially near a black hole and is not a neat cone consisting of null paths diverging in straight lines at 90 degrees to each other.
 
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  • #3
kev said:
I am not sure why you think this is any more paradoxical than passing two diverging light rays through a suitable glass lens so that they converge on a focal point. Happens all the time in cameras, telescopes, etc.

Agreed, unless I'm missing something?
 

1. What is the gravitational lensing paradoxon?

The gravitational lensing paradoxon refers to a phenomenon in which light from a distant object is bent by the gravitational pull of a massive object, resulting in multiple images of the same object being seen by an observer. This creates a paradox because the light from the object is being both magnified and distorted by the gravitational lens, making it difficult to determine the true shape and size of the object.

2. How does gravitational lensing occur?

Gravitational lensing occurs when light from a distant object passes through the gravitational field of a massive object, such as a galaxy or a cluster of galaxies. The gravity of the massive object bends the path of the light, causing it to curve and magnify the image of the object. This effect is similar to how a magnifying glass can magnify an image by bending light.

3. What is the significance of gravitational lensing?

Gravitational lensing is significant because it allows scientists to study distant objects that would otherwise be too faint or small to observe directly. By analyzing the distortion and magnification of the lensed images, scientists can gather information about the properties of the massive object and the structure of the universe.

4. Can gravitational lensing be used to study dark matter?

Yes, gravitational lensing can be used to study dark matter, which is a type of matter that does not emit or interact with light. By observing the gravitational lensing effect on light from distant objects, scientists can map the distribution of dark matter in the universe and gain a better understanding of its properties.

5. Are there different types of gravitational lensing?

Yes, there are two main types of gravitational lensing: strong and weak. Strong gravitational lensing occurs when the light from a distant object is significantly magnified and distorted, resulting in multiple images of the object. Weak gravitational lensing, on the other hand, refers to a more subtle distortion of light that can only be detected through statistical analysis of large numbers of objects.

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