In the case of black holes, lights are pulled into by great gravitational force exerted by the dying stars (which forms into a black hole). If light has no mass, how is light affected by gravity?
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Gravity is associated with mass. Mass bends spacetime. Light just goes in a straight line through the bent spacetime, and that sends it in what looks like a curve. Just like a “straight line” on the surface of the Earth is a great circle.
Have you ever looked at the [flight path of a plane on a flat map](https://previews.123rf.com/images/metelsky/metelsky1911/metelsky191100072/134427430-plane-routes-over-world-map-with-markers-or-map-pointers-travel-by-airplane-concept-flight-path.jpg)? Notice how they all look curved?
At first that might not make sense, because we’re always told that the shortest path between two points is a straight line, so why don’t the planes just fly in a straight line? The reason, of course, is that the Earth is not actually flat: it’s a three-dimensional surface and the map we’re looking at is just a projection of that 3D surface onto two dimensions.
When you look at the flight paths on a globe, you’ll see that the planes are actually taking the shortest possible path *along the curved surface of the globe*.
This is the important thing: what looks like a curved path in two dimensions may actually be a “straight line” (meaning the shortest path) in three dimensions if the 3D surface that the path is drwn on is curved.
According to Einstein, space-time is a four dimensional surface, and when we look at paths in space, we are looking at a 3D projection of the 4D paths that the objects are actually following. And just like the “straight” 3D flight paths that were projected onto a 2D map, when you project a “straight” 4D path onto a 3D surface, it may look curved even though it’s not.
Now light is special: light always follows a “straight” line, just like the airplanes do (the airlines want to spend as little as possible on fuel, so they always take the shortest path).
When there is no matter around, the surface of space-time is flat: there’s no curvature to it in 4D. So the path that light follows when projected into 3D looks like a straight line.
BUT, Einstein says when there’s matter present, the 4D surface of space-time curves, sort of like the surface of a globe. And even though space-time is curved, light still follows a straight line path across the surface. BUT, the path is a straight line **in 4D**. When you project the path into 3D, then it will look curved. But the reason it looks curved is because the path is actually following a curved 4D surface.
TLDR: gravity affects the path of light because it bends space-time.
Imagine you’re driving on a road. The road turns to the left. And so, you go left. Gravity turned the road, the light was simply driving on it.
as many have already pointed out that light is indeed traveling straight-it’s just space that’s bent.
a neat analogy i was once told goes as follows.
imagine riding a bike and steer straight ahead. you will travel straight.
now the same scenario but you start leaning to the left. you are still steering straight ahead but bc you are leaning to the left the straight line turns into a left curve! (light = bike; leaning to the left = influence of bent spacetime)
hope that helps:)
gravity curves space itself.
imagine light is a spongey cylinder rolling along. when it passes a source of gravity, the end of the sponge closer gets compressed by the gravity more than the end farther away. The cylinder is now tapered, and when it rolls once, the part with less gravity has larger diameter and larger circumference and rolls farther.
the cylinder has now turned towards the source of gravity.
ELI5 version: race cars can go anywhere but if you watch a race, they just keep going in circles. Circles aren’t a limitation on the car, it’s a limitation on the road.
Light is a car and the road is space. Gravity makes space curved, and light just follows the track.
Technically nothing is affected directly by gravity. Gravity interacts with spacetime and curves it and thats what light and matter interact with and alters their perceived path.
Gravity distorts spacetime, meaning it’s not the mass attracting other mass, it’s the mass deforming space, through which the light has to travel.
Think of it like a car on a road: if the road is straight, the car goes straight. If the road is curved, the car follows the curve (hopefully).
Gravity doesn’t affect the car, it affects the road the car travels on.
Imagine you have a super-duper bouncy ball, like a superball. It’s so bouncy that it can even bounce off the ceiling. Now, think about the floor as space, and the ball as a beam of light. Normally, the ball goes in a straight line.
But what if the floor wasn’t flat? What if it was curved like a big slide? If you roll the ball on the curved slide, it won’t go straight; it will follow the curve of the slide, right? Well, light does something similar when it travels near something very, very heavy, like a super heavy star.
These super heavy things, like black holes, make space act like a curved slide. So when the light goes near them, it doesn’t go straight anymore; it follows the curve of space just like the bouncy ball on the slide. That’s why we say light gets “pulled” by gravity. It’s not because light is heavy; it’s because space itself is curved by the heavy things, and light follows the curve.
You can kind of think of mass as “at rest” since speed affects mass. Light has no “at rest” so the concept of mass doesn’t really even make sense.
Light does have momentum though, which is classically mass times velocity which implies mass can’t be zero. But Einstein made it all weird (ie. more correct) with relativity, so it’s mostly two not quite compatible systems conflicting in our heads.
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