My limited understanding is: Photons travel at the speed of light (c). If we could theoretically (for the sake of argument) travel at the speed of light minus 1km/h (c-1km/h). And we turn on a flash light attached to our wing and it flashes both directions straight ahead and backward. Then the light in front of us would travel at 1km/h? How about the one in the back? would it go at the speed of 2c-1km/h relative to us?
Now in the above paragraph I mentioned speed of light minus 1km/h. I'm still unsure how our speed gets measured - relatively to which point in universe? Because from our perspective we would hold still (if we’re not accelerating) and the objects around us would be moving at approximately the speed of light.
If some of what I mentioned above is right than isn’t it just easy to shine flash light in both directions of XYZ axis and measure how quickly it returns back to us? Then the miniscule time differences would show us our movement relative to the center (or non-moving point(s) in universe[some people argue that there's no center) of the universe (or non-moving part of universe)?
Based on this video (time19:20): https://www.youtube.com/watch?v=IJhgZBn-LHg even our Milky Way is moving at 2.1 Million Km/h relative to the first detectable light in universe (approximately 0.2% of the speed of light).
thanks for your input. Paul