It does if you are ray tracing. They are trying to find the source of a theoretical photon that hits the screen at the present time, the source will therefore have been in the past so they need to run time backwards as the photon moves away from the camera.
> But wouldn't we want to see the black hole? One could send a ray through every pixel of a screen, and let them go around the black hole and hit its surroundings. And if the rays are sent back in time, it's as if one received the light emitted by the surrounding objects in the past!
The four-vector contains coordinates of a point in time and space (x,y,z,t).
As a sibling said, light travels forward in time, but also from a light source towards the camera. Raytracing traces rays backwards from the camera (because there are fewer rays to trace this way: just pick the ones that land on the camera and work backwards towards light sources). So backwards in time, trying to figure where the ray originally came from.
Anyway, you need to keep track of the time dimension as light moves (at constant speed) through spacetime in a straight line, but the black hole also stretches the time dimension, affecting what a straight line looks like for an outside observer.
That's my understanding at least, I am not that well versed in relativity.
Edit to make it more interesting: (x, y, z, t) is your position in spacetime, I do not quite understand how you can take its derivative¹ (probably not time-wise), but you can derive the four-velocity, the magnitude of which is constant (c). Intuitively (and probably naïvely) I understand this as: you move trough spacetime at constant speed. Therefore, if your spatial coordinates move faster, your time coordinate moves slower. This gives you back the special relativity results with time that "slows down" for fast-moving objects.
Would the image be very different for a rotating black hole?
Since they probably all rotate in nature.
This is very interesting, but way above my pay grade. Can someone explain how the resulting image reflects light traveling backwards in time?
Can you clarify your question? Light does not travel in time -- that's sort of the main characteristic of things traveling at the speed of light.
It does if you are ray tracing. They are trying to find the source of a theoretical photon that hits the screen at the present time, the source will therefore have been in the past so they need to run time backwards as the photon moves away from the camera.
From TFA:
> But wouldn't we want to see the black hole? One could send a ray through every pixel of a screen, and let them go around the black hole and hit its surroundings. And if the rays are sent back in time, it's as if one received the light emitted by the surrounding objects in the past!
The four-vector contains coordinates of a point in time and space (x,y,z,t).
As a sibling said, light travels forward in time, but also from a light source towards the camera. Raytracing traces rays backwards from the camera (because there are fewer rays to trace this way: just pick the ones that land on the camera and work backwards towards light sources). So backwards in time, trying to figure where the ray originally came from.
Anyway, you need to keep track of the time dimension as light moves (at constant speed) through spacetime in a straight line, but the black hole also stretches the time dimension, affecting what a straight line looks like for an outside observer.
That's my understanding at least, I am not that well versed in relativity.
Edit to make it more interesting: (x, y, z, t) is your position in spacetime, I do not quite understand how you can take its derivative¹ (probably not time-wise), but you can derive the four-velocity, the magnitude of which is constant (c). Intuitively (and probably naïvely) I understand this as: you move trough spacetime at constant speed. Therefore, if your spatial coordinates move faster, your time coordinate moves slower. This gives you back the special relativity results with time that "slows down" for fast-moving objects.
https://en.wikipedia.org/wiki/Four-vector
https://en.wikipedia.org/wiki/Four-velocity
¹: derivated along the "proper time" (time from the perspective of the moving object) according to Wikipedia. Still unsure how to compute that.
Someone should implement this as a mod for RTX No Man Sky
Like 20-30 years ago there was a relativistic raytracer that was like POV-Ray, but with relativistic effects. Maybe named "backlight"?
How does this compare?
Edit: yeah this one: https://web.archive.org/web/20010604001305/http://www.anu.ed...