How is light affected by gravity?
How is light affected by gravity?
𝗜𝗻𝗱𝗲𝘅:
⏲ 0:00 Photons
⏲ 0:18 How we know that gravity affects light
⏲ 2:00 Newton vs Einstein
⏲ 2:30 What is gravity in general theory of relativity
⏲ 2:40 Spacetime's curvatures
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(https://www.amazon.com/Weirdest-Maths...)
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📄𝐓𝐞𝐱𝐭:
Light consists of photons, which can be thought of as tiny packets of energy. Photons have energy but they don’t have mass. So how can they be affected by gravity? First, let’s talk about how we know that gravity affects light.
In 1979, astronomers discovered two quasars that were so similar they were called the twin quasar. The problem was, they were too much alike. After their discovery, astronomers studied them closely and found them to be identical. More studies revealed that actually there was only one quasar, but we can see 2 copies of it! How is that possible? The answer is that there’s a galaxy lying between the quasar and Earth. This galaxy, named YGKOW G1, bends the quasar’s light so that we see 2 copies of the same object. The existence of such a phenomenon, known as a gravitational lens, had already been conjectured by theoreticians such as Einstein, Zwicky, Khvolson and Link. The twin quasar was the first gravitationally lensed object to be identified. After its discovery, scientists found many more examples of gravitational lenses. They’ve been able to use the phenomenon to search for exoplanets, a topic we talk about in another video.
Black holes bend light too. In the extreme case where light crosses a black hole’s event horizon, it can’t escape because of the enormous gravitational pull. We can even observe the bending of light by gravity on Earth. If we point a laser beam at a wall and measure precisely the heights of the source and the laser light on the wall, we can see that their vertical positions aren’t the same. This is because Earth’s gravity acts on the laser beam and deflects it. So we know for sure that light is bent by gravity.
According to Newton, gravity is a force that acts between any two objects that have mass. But if one of the objects is massless, this would make the gravitational force zero. So we can’t explain why light is bent by gravity using Newtonian physics. Newton's law has since been superseded by Einstein's general theory of relativity, although it continues to be used as an excellent approximation of the effects of gravity for most applications. In Einstein's general theory of relativity, gravity is seen not as an invisible force but rather as a consequence of the curvature of spacetime.
In the absence of a gravitating object, light travels in straight lines. But massive objects, such as stars or entire galaxies, cause the spacetime around them to curve. Light will always follow the shortest available path through spacetime, so if spacetime is curved so will be the paths of light beams passing through it. The bending of light has nothing to do with whether light has mass or not. It’s simply a result of the curvature of spacetime caused by massive objects.
#light #gravity #bend