Gravitational Lensing: Refraction or Something Else?

In summary, the analogy between gravitational lensing and refraction by an ordinary glass lens is only superficial. There is nothing in the mathematical model in GR that corresponds to "density of spacetime", so that part of the analogy does not hold.
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Paige_Turner
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TL;DR Summary
Does 4D spacetime, bent by mass, act like "compressed space" in 3D?
It seems like a strong gravitational field acts like spacetime is denser in some sense. Light passing through a gravitational lens is delayed, just like in a glass lens (which refracts because it's denser than air).
 
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While there are similarities between gravitational lensing and refraction by an ordinary glass lens, they are only similarities and can't be taken too far. There is nothing in the actual mathematical model in GR that corresponds to "density of spacetime", so that part of the analogy does not hold.
 
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Paige_Turner said:
Summary:: Does 4D spacetime, bent by mass, act like "compressed space" in 3D?

It seems like a strong gravitational field acts like spacetime is denser in some sense. Light passing through a gravitational lens is delayed, just like in a glass lens (which refracts because it's denser than air).
I have never seen any paper that made that analogy.
 
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Paige_Turner said:
Summary:: Does 4D spacetime, bent by mass, act like "compressed space" in 3D?

It seems like a strong gravitational field acts like spacetime is denser in some sense. Light passing through a gravitational lens is delayed, just like in a glass lens (which refracts because it's denser than air).
Light follows similar geometry if you only consider a convex (converging) lens (left-side diagrams).

If this analogy really held, you could theoretically find a galaxy or galaxy cluster that is concave in shape. One would naively expect light passing through volume of space with a concave-shaped mass to diverge (right-side diagrams).

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Paige_Turner said:
Summary:: Does 4D spacetime, bent by mass, act like "compressed space" in 3D?

It seems like a strong gravitational field acts like spacetime is denser in some sense. Light passing through a gravitational lens is delayed, just like in a glass lens (which refracts because it's denser than air).
As was stressed before, it is not literally the same to have refraction, which is usually naming the phenomena related to the interaction of the electromagnetic field with matter, i.e., due to scattering of em. waves with charged particles, and empty space is not considered as any kind of matter anymore since Einstein got rid of the aether.

Mathematically in some sense there's an analogy, because to describe "lensing" you can use for both usual refraction as well as the "bending of light" by gravitational fields using geometrical optics, which is the eikonal approximation of Maxwell's equations. It turns out that the light rays as defined by geometrical optics follow from Fermat's principle, and in matter-free space within GR, this leads formally to a geodesic equation for "massless particles". Fermat's principle of course is also a valid description of light propation in matter in the eikonal approximation. In this sense mathematically both effects are a bit analogous.
 

FAQ: Gravitational Lensing: Refraction or Something Else?

1. What is gravitational lensing?

Gravitational lensing is a phenomenon in which the light from a distant object, such as a galaxy or quasar, is bent and distorted by the gravitational pull of a massive object, such as a galaxy cluster or black hole, in between the distant object and the observer.

2. Is gravitational lensing a form of refraction?

No, gravitational lensing is not a form of refraction. Refraction is the bending of light as it passes through a medium with varying density, such as air or water. Gravitational lensing is caused by the bending of light due to the curvature of spacetime caused by massive objects.

3. How does gravitational lensing occur?

Gravitational lensing occurs when the path of light from a distant object is bent by the gravitational pull of a massive object. This bending of light is caused by the warping of spacetime around the massive object, which is predicted by Einstein's theory of general relativity.

4. What are the different types of gravitational lensing?

There are three main types of gravitational lensing: strong lensing, weak lensing, and microlensing. Strong lensing occurs when the light from a distant object is significantly distorted and multiple images of the object are visible. Weak lensing is a more subtle effect, where the shapes of distant galaxies appear slightly distorted due to the gravitational pull of foreground objects. Microlensing occurs when a small, compact object, such as a star or planet, passes in front of a distant object, causing a temporary increase in its brightness.

5. How is gravitational lensing used in astronomy?

Gravitational lensing is a valuable tool in astronomy as it allows us to study objects that would otherwise be too faint or distant to observe. It can also provide information about the distribution of dark matter in the universe, as well as the mass and structure of galaxies and galaxy clusters. Microlensing is also used to search for extrasolar planets by detecting the temporary brightening of a distant star as a planet passes in front of it.

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