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Albert Einstein revolutionized how we think about time and space more than a century ago. Instead of Newton's fixed time, the quantity read by a clock becomes a dimension in its own right on equal footing with space. Early on in its inception, Einstein's theory of general relativity produced many tests for its validity. One such test was that of gravitational lensing: the bending of light as it passes near a massive object creating a magnified image or multiple copies of one. In this article, we explore gravitational lensing and its recent observation with a six-copy image.

Gravitational Lensing

To truly understand gravitational lensing, we must first look towards a simpler case. A similar phenomenon already occurs for everyday objects, namely that of a glass. When light passes through a glass, it encounters some resistance because of the material of the object. This causes light to slow down as it passes through the medium, and this leaves a signature by changing the trajectory of light as it exits. This process is called refraction, and the index of refraction of the glass lens dictates to what extent light is slowed down in this process.

If we imagine a system of multiple lenses and mirrors, one can get more interesting effects. If we have a mirror and shine light through a glass lens, the light will now have two paths to go: either it reflects off the mirror, or it gets refracted. The multiple paths of light due to this effect create two images or a copy of the image the light is reflecting. This relies on the potential of different trajectories of light as it passes through the lens. There are even some lenses that independently provide two different paths for the light to exit, sharing a property called birefringence.

Gravitational lensing can be understood by analogy to the above process, where instead of the index of refraction of the glass lens determining the slowing down of light, it is the gravitational pull of a massive object like a star or a planet. The gravitational field of such an object may not necessarily be uniform, and so light gets slowed down at different speeds as it passes by it. The main difference between the usual refraction and gravitational lensing is that in the latter, there is no physical contact between light and the massive body. It is solely mediated by the gravitational field.

The Einstein Zigzag

One can now imagine a gravitational field sourced by a quasar, and let light travel accross it. The trajectory of light will become distorted and presumably split into multiple paths. When viewed from Earth, it is as though the light traveled in multiple ways to reach the same point. The Einstein Zigzag is precisely an example of this situation, where light appears to have traveled in six paths leaving six copies of the star in its wake.

While this is independently interesting as an experimental verification of general relativity, it has profound implications for science as a whole. Among these exciting avenues are:

1. Dark matter distribution: Since gravitational lensing is mediated by the invisible gravitational pull of massive objects, it is highly likely that light is also influenced by dark matter in the universe. This means that observing more Einstein images like rings and zigzags will allow us to map out the distribution of dark matter in the 海角社区 we live in.
2. Distant cosmic objects: Gravitational lensing also leads to the magnification of the objects that are being copied. This means that it allows us to close the gap between the Earth and distant objects to better understand their structure and makeup.
3. Hidden objects: Following from the previous point, gravitational lensing also opens up the possibility of discovering hidden objects that we have not yet detected so far. These objects can include exotic black holes, faint galaxies, and rogue planets.