Speed of light vs standing still

I’m having a hard time with relativity, please help me out. If I was traveling at 9.999…% the SOL and I passed you and you are standing stationary and turned on a laser beam running parallel to me, would I be able to see the very tip of you laser while looking back and would it be receding away from me?

Your description of the set-up is a little confusing, so I’m going to restate what I think you’re asking:

You’re traveling at nearly the speed of light relative to me. As you zip past me, I turn on a laser pointed in the same direction you’re traveling. Your question is, what do you see?

It will appear to you that the laser beam is shooting out ahead of you at the speed of light. Of course, the only way you can “see” this is if the beam hits something that bounces some of the light back to your eye, but any observations you make will be consistent with the beam traveling at the speed of light relative to YOU. Of course, from your perspective it seems like the beam is traveling at almost twice the speed of light relative to ME, since I’m quickly receding in the distance behind you at 99.9999… % of the speed of light.

It will NOT appear to you as though you’re “keeping pace” with the laser beam. (Although that’s what it will look like to me … .)

Right. Just to elaborate one point, since nobody’s speeding up or slowing down or changing direction, there’s no way to say which one of us is standing still and which one is moving.

So, from your point of view, you’re just sitting there and I come whizzing past at 99.999% lightspeed. Then I turn on a laser just as we go past. The light takes off at light speed in whatever direction the laser was pointing, while I continue on my merry way at 99.999% lightspeed.

Things would actually appear much stranger due to relativistic effects. You travelling at 0.99c and I would not agree about the time the laser was turned on. I would think I turned it on just as you passed, but you would not think so.

Moreover, your view of me and the “stationary” world would be oddly distorted, compared to what I saw:




Actually, as long as you turned on the laser right when we passed, we would both agree on that. If, however, you turned it on a little before or after I passed, then we would disagree on how much before or after it was.

Also, the color of the laser would be significantly redshifted, such that if it was visible originally, it’d appear to be well into the infrared to me. But the speed would still appear the same.

People noticed that, while you can measure how fast light travels, you always measure the same value. This is true regardless of how you move. Starting with this you can derive Einstein’s special relativity, and with pretty basic math, too. It isn’t the speed of light per se that is so important, but a constant value that is woven into existence, so to speak, that has units of distance over time. It’s called “c” and it appears in other places you might not anticipate, like being the square root of the conversion factor between mass and energy. Light travels at this velocity, but so do many other kinds of radiation.

I want to emphasize this. The most fundamental realization about relativity is that the speed of light is the same to everybody, no matter their location or speed, and this was already measured and known before Einstein came up with his theories. Once you fix the SoL to a single speed, all kinds of weirness can be concluded from that, but the SoL being fixed is an observed fact.