How do black holes bend light if light can’t be bent?

Question: How do black holes bend light if light can't be bent?

Asked by anon-202065 to Sophia, Sarah, Meirin, George, Emily, Andy on 12 Mar 2019.
- Sophia Pells answered on 12 Mar 2019:
  
  Nice question! Light can be bent. Whenever light travels through a material that is more dense than where it comes from it gets bent. If you look at a straw in a glass of water sometimes you can see the straw looks bent or not continuous and that is because the light is getting bent when it goes through the water.
  
  Light can also be bent whenever it goes near heavy objects. The object has to be really heavy for us to be able to see the effect. Black holes are really heavy so they can bend light quite a lot. The explanation for it comes from general relativity which says that space and time is like a mesh and heavy objects cause dents in that mesh. If you think of people sat around on a big trampoline they will cause dents in the surface of the trampoline. If you then try and roll a ball from one side of the trampoline to the other, it probably won’t go in a straight line but will be bent to go around the edge of some of these dips in the trampoline surface. Heavy objects bend light in the same way.
  
  The fact that heavy objects bend light is being used to detect planets around other stars in our galaxy. If we can observe that the light from a star is bending, there is probably a planet there that is causing it to bend.
- Andy Buckley answered on 13 Mar 2019:
  
  Light doesn’t think it can be bent, but it can. Like if you’re taking a flight from the UK to Japan or California, you choose the shortest path and head in that direction for 10+ hours without changing direction (weather permitting). You don’t think that you followed a curved path, but then if you look at it on a flat map you’ll see that your path was definitely a curve.
  
  This happened, of course, because the surface of the earth isn’t really flat, so when you followed that surface (at a constant altitude) you were following a curve in any flat projection. Similarly, spacetime isn’t really flat, and it turns out that the way it curves is governed by how much energy is located in different places. Since E = mc^2 and c is a big number, mass is a very concentrated kind of energy that produces a lot of bending: that’s gravity. And light that follows what it thinks is a straight path is being bent by the curvature that the mass/energy produced.
  
  Note that I said space*time*: it’s not just 3D space that gets bent, but time as well. In fact, Einstein’s relativity theories largely treat time like just another direction, that can be bent and rotated into space directions just like they can be among themselves. This takes some getting used to, but it explains a lot.
  
  Great question, thanks!
- Sarah O'Sullivan answered on 14 Mar 2019:
  
  We often talk about light as rays travelling in straight lines which makes it seem like it can’t bend. So idea of light not bending comes more from the words we use to describe it even though refraction is easily demonstrated with a straw in a glass of water and is definitely a result of light bending due to the refractive index of the material

Question: How do black holes bend light if light can't be bent?

Comments