Eli5 If space is curved in many locations because of big objects and stuff, how do we know where an observed light originates then?


If it passes in/by those gravitation fields, it must (I guess) also change direction at least slightly?

In: 43

You are indeed correct. It’s called gravitational lensing. We use it for calculating distances and estimate the position of unobservable stars

Space is (mostly) flat. Ish. We think.

Anyway, it doesn’t bend all that much except in very close proximity to really massive objects, and the gravitational lensing we observe can be corrected for.

For example; relativity was actually proven by precise measurements of gravitational lensing of background stars caused by our own Sun.

Imagine that you are standing in a large field (this would be planet earth) and you have a friend way out in the distant part of the field shining a flashlight back at you (this is the star in this example). When you point to this flashlight, it’s in a specific direction that everyone can agree on. So how much of an impact does this location change mean when the flashlight is moved slightly to the side about 5 feet? Not much…because of the distance between the observer and the flashlight, the relative change in position is negligible given the distances involved. From your perspective the light is still <points with finger> over there.

First off, you are correct about it changing the perceived locations of objects. However, the amount light is bent (or really, the curvature of spacetime since that’s what light follows) is incredibly small on interplanetary scales, much less universal scales.

But to answer your question? Astronomers are very clever. And also computers exist. We have models that can compensate for microlensing effects, and have indeed documented the shifts in perceived locations of things before. If you’ve heard of an Einstein Ring, that’s what this is. We are able to reconstruct the image and location of the lensed object fairly well.

Additionally, there’s a very famous and interesting case of this being done to confirm some aspects about spacetime geometry. There was a supernova (iirc a type 2), and we captured it in our telescopes and from historical observations. And we saw it three times. Some astronomers calculated that we should see it again, a few years later, and calculated where exactly on the sky we should see it. They did this by calculating the path light would take from the original event to get to us. Then Hubble focused on that part of the sky during the scheduled time and wouldn’t you know it, we saw it again. Precisely in the window they calculated.

Because we can (generally speaking) detect the *sources* of those gravitational fields.

Take a star, for instance. The star is also a source of light. So we can see the star, and thus know to account for its impact on any light that passed near it, allowing us to correct for the bend it put on the light.

Other things like “dead” stars or black holes are harder to detect, but generally not *impossible*, and as long as we can detect it and get an idea of its mass, we can account for the effect of its gravity on light.