Ever since the speed of light became the universal speed limit, there has been the counter argument: if I’m taveling at almost the speed of light, and I turn on my flashlight do the photons emit at less than light speed? I know this argument is almost as viable as the airplane on the treamill one, but it escapes me why that’s the case.

I guess my question is, is the speed limit relative to a starting point? or some other fixed point in space? or am I really not thinking about it in the right way?

The speed limit is the only absolute thing in a universe where everything else is relative. If you drive by me in your car at almost the speed of light, we’ll both measure the photons your flash-light emits as moving at the speed of light. To me that will be because your time is slowed down by special relativity, to you that will be because you have an equal claim that I’m the one moving at almost the speed of light and that I’m the one whose clock is running slow.

In your frame of reference, they appear to be traveling at “the speed of light” - 186,000 mi/sec. In a stationary frame of reference, they also appear to be traveling at the same speed. Which is why the speed of light is invariant.

The point is that the statement, “if A sees B moving at velocity U, and B sees C moving at velocity V, then A sees C moving at velocity U + V”, is true by every intuition we have, but nevertheless is wrong. In actuality, A sees C moving at (U + V)/(1 + UV/cc).

It’s not exactly a “counter argument” (it would require decades of experimental proof to gain that status, and there is none, given that it’s not true).

It’s a question, though, for sure, and a common one.

The answer is that - and this has been proven time and time again both mathematically and in practice for over a hundred years; there’s no dispute that this is the case - the speed of light is the same for all observers whatever speed they’re moving at. This is known as “relativity” and, yes, it’s counter-intuitive, like much of our universe is. It “gets around” that by doing wacky things like actually slowing time down and stuff.

The speed of light is the same relative to all observers.

Time passes at different rates and distances are warped for observers moving with a nonzero velocity with respect to each other such that they both observe a given photon move at exactly the speed of light through a vacuum.

You can’t actually get to C, because it would require an infinite amount of energy to accelerate you to C, and there isn’t an infinite amount of energy. If you are at .99999999 C, you will still see your light traveling away from you at C.

I recommend One, Two, Three, Infinity by George Gamow. Written 60 years ago, fun and easy to read. Explains relativity to a child (a pretty sophisticated child).

Hasn’t lost its relevance, as long as the speed of light hasn’t changed.