Why can’t I go faster than the speed of light?

It doesn't consist of the same photons; the spot in the "new" position consists of photons which have been traveling in a straight line from your flash light at (assuming vacuum) a constant rate of C (and then reflected back to reach your eye again, also at a constant rate of C); they haven't traveled to or from the previous "spot" position at all, much less exceeded the speed of light at any point in the process of reaching the new position.

Right?

You and I survey a building (plot of land, whatever) separately because we want to check each other's work. At then end we look at our data and disagree on every single coordinate except the origin. Ah, you're a crappy surveyor, I conclude. No, you are, you reply.

Then for whatever reason I ask what distance you compute for the distance from the origin to the corner of the desk. 3.183 meters. Huh, exactly same number I compute. Okay, what about the distance from the far corner of the room to the southmost window? 18.45 meters. Me too!

A bit more chatting and it turns out you used true North as the y-positive direction, and I used magnetic North. Opps. Our frames are just rotated in respect to each other.

Now, is it freaky and weird that I say the speaker is 2.78 meters in the y direction, and you say it is 2.619? No, we are just using different frames of reference. There is no 'reality' to any given y direction or coordinate. It is arbitrarily chosen, as our the units (I could use yards instead of meters, and have an entirely different number yet).

OTOH, what is real, and invariant, are distances. That's a physically real thing. hence we always comput the same distance between any two points, despite using different coordinates for our (x,y) tuple. If you want to be mathy about it we say the metric is s^2 = x^2 + y^2. Pythagoras, in other words, in a Euclidean space.

Well, we don't live in space, we live in spacetime, where time is a dimension. When we move at different speeds relative to each other our 4-D coordinate systems are rotated relative to each other. That includes time. So, if you rotate yours relative to mine, travel for awhile (time and space!), well, you will end up with different coordinates for x, y, z, and t. It's no odder than if you and I travel 'North' in your car, but you use true North and I use magnetic we end up in different places on the globe.

In 4D space what is 'real' is not coordinates or time, but events, and what is constant is the interval between events. Just like what is 'real' in 2D Euclid space is not some arbitrary y-direction, but the distance between two objects. Distance is invariant in 2D space, event intervals (space and time) are invariant in 4D Minkowski space (the space we live in absent of gravity).

There's a bit of handwaving in there, but that's pretty much the physics; any 7th grader can do it. The main part that will lead to bad conclusions is that the metric in Minkowski space uses a negative number for time; so s^2 = x^2 - c^2 t^2. That's hyperbolic, so if you use intuition from Euclidean space you may conclude that in some instance distance will contract when it expands, or vice versa.

So, finally, to your Q1, if I travel magnetic North, is the position I reach on the "same" Earth as the one where you use true North? Feels like a weird question that misses the point, right? Same Earth, just a different location than you expected because my frame was rotated wrt yours.

Note that every experiment we have ever carried out bares this out. Accelerate a clock, bring it to Earth, that clock is younger (I'm ignoring general relativity's effects here, but the experiments don't). Measure how long a very short living particles live that are created by other particles crashing into our atmosphere, and they live exactly as much longer as SR would predict. Etc. did that clock "select" a different version of you? No, it just travelled a different path in 4D spacetime than you, and hence ended up at different coordinates. To go deeper into that I'd have to introduce "proper time", but since the ending x,y,z are the same are your x,y,z, can you see that intuitively it must be the t coordinate that changed?

[1] https://www.eftaylor.com/spacetimephysics/ This book is released under CC, free to download and share, and utterly fantastic. All you need is junior high math to master the material.

At our origin, S and E synchronize observe their identical atomic wristwatches coincidentally agree that it is "0". Light-years away, E and L come very close to one another and exchange timestamps showing that coincidentally their identical atomic wristwatches agree. Finally, L and S come very close to one another and compare timestamps from their identical atomic wristwatches. All the wristwatch times are identical to those at the three points in the diagram of the "instant turnaround" version of the twin paradox, we've just turned the travelling twin into two unrelated travellers on different trajectories.

The argument is that this "love triangle" is resolved because E & L are different travellers in uniform motion, so all parties must combine the times acquired in two different reference frames (E's and L's) to compare with the times acquired in S's reference frame. The further argument is that this duplicates the "instant turnaround" version of the twin paradox if we can have the travelling twin change direction without acceleration.

Firstly, we can still solve this with a pseudo-gravitational field popping up at the moment E & L exchange timestamps. It's no more of a coincidence than the identical timestamp when S & E are close.

Secondly, it's not clear that the paradox remains interesting in this case, because there is no expectation that S & L should be the same age when they are close to one another again. They aren't twins. Unless we add in accelerations, there is no way by which S, E, and L could all have been born at close to the same location in spacetime.

Thirdly, it's unclear that there can be an instant turnaround without acceleration. A couple flavours have been explored here and there.

One involves a slingshot around a star to change directions from away to towards the stay-at-home twin. In this picture the travelling twin is always in free-fall. But here we are substituting real gravitation (that of the star) from pseudo-gravitation. We've moved from everywhere-flat Minkowski space -- the spacetime of Special Relativity -- to something closer to Schwarzschild spacetime, which is only asymptotically flat. Moreover, we are using the near region of Schwarzschild to accomplish the slingshot.

Another substitutes the open flat Minkowski space with one in which there is a compact spatial dimension that curls back on it self. A universe with the geometry of a cylinder with infinite height and small circumference, or a torus, or a sphere would do. The cylindrical case has been explored recently : https://doi.org/10.1119/10.0000002 with comparisons to Minkowski space (the spacetime of Special Relativity), §IV (Conclusion) being pithy. Again, I see this as trying to substitute pseudo-geometry with real geometry, an adapted clock-comparison recipe, and a highly privileged frame for the traveller, in order to avoid a non-gravitational acceleration opening the door to a pseudogravitational field arising in the ultrasimplfied and thus strictly Special Relativity problem.

The pseudogravitational field approach comes from Einstein in 1918: https://en.wikisource.org/wiki/Translation:Dialog_about_Obje... which was fun to read.

Finally focusing on the latter part of my comment that I'm self-replying to (mostly for my own benefit), we have only done away with one acceleration by the returning twin. We still have the effects from the behaviour of matter in the expanding universe with which to clock S, E and L, removing the remaining paradox if we somehow contrive to have S, E & L expecting to age similarly. If we are abandoning Special Relativity in order to avoid acceleration by the returning without invoking outright magic, why only do it along one spacelike dimension, or by importing a very finely tuned third traveller?