system
Given the following post, respond as if you're commenting on Reddit:
human
If the speed of light is static, can we use it to determine the speed of other things?

This question obviously stems from ignorance of the topic at hand and may be worded improperly, but I'll do my best:

Since the speed of light is constant, could we theoretically then just "point" light in various directions, measure its speed relative to us, and use that to determine how fast we are actually moving through the universe?

Example: I'm moving forward at 1mph, I launch a projectile forward that moves at a constant 2mph irrespective to me, therefore it appears to me to be moving at 1mph. I then launch it directly backwards, and it therefore appears to be moving 3mph away from me.  Knowing the speed of the object I can then calculate my actual speed - 1mph.

Or, is this question completely moot because we already know exactly how fast we're moving through the universe? Or is it moot for some other reason?
gpt
When we say the speed of light is constant, what we mean is that light travels in the same speed in *every* frame. So, no matter how fast you're moving with respect to other things, if you shine a light ray in any direction you will see it travel at the speed of light in that direction. At first you might think this would lead to contradictions, because observers in other references frames should *also* see the light ray you fired travel at the speed of light in any direction. But it turns out in special relativity that's just the way things work; different observers don't agree on the relative velocity of moving things, and this is very counter intuitive for us.

Directly, the answer to your question is "no". There is no way to determine how fast things are "really moving" in the universe, and special relativity tells us that no such concept exists.