The core of the debate is contained in the following references.

1. Events

Burt Jordaan wrote:You have not answered where speed comes in for events with different, yet fixed spatial separation in each inertial frame. This (non-existent) "speed" is actually always zero…Faradave » 14 May 2020, 21:35 wrote:I find that confusing. My home and the local convenience store have fixed spatial locations, yet I have a speed (∆x/∆t) going between them. If the store is in my home (∆x=0), my speed would be zero (as you say, a "non-existent speed") since I'm already there from the start.Burt Jordaan wrote:Your home and your local convenience store are not events. Time ticks on for both. Spacetime intervals are defined in terms of events, for which neither space, nor time changes.

2. Speed

Faradave » 14 May 2020, 21:35 wrote:Spacetime events can have different spatial coordinates yet have zero interval (lightlike) separation. An absorber appears on the future light cone of its emitter. If I define interval speed as ∆d/∆t, when ∆d is zero (lightlike), the interval speed is indeed "non-existent" at zero. This also explains the "instantaneous" acceleration (to zero speed) of light quanta.Wkipedia wrote:"In a light-like interval, ... events define a spacetime interval of zero ...the spacetime interval between two events on the world line of something moving at the speed of light is zero"

I construe this to mean the absorption event was already there at the emission event for a light quantum. Since ∆t is relative, the only interval speed (∆d/∆t) that all inertial observers agree on is 0/∆t, making c uniquely invariant. This also explains the instantaneous acceleration of light quanta. Physics offers no other explanation for the existence or invariance of speed limit c.

1. Spacetime events are unique occurrences at a specific place and a specific time. If properly drawn on a Minkowski spacetime diagram (Msd), an event is a single, static point in spacetime. Every conceivable inertial frame drawn on that same Msd, will use that single point as projected onto their respective space- and time axes in, order to establish the unique spatial (x,y,z) and time (t) coordinate of that event for that specific inertial frame.

Since the two inertial frames are drawn with a common origin, the origin is usually taken as a common unique event at (0,0) for convenience, because its simplifies discussions and equations. It is important to emphasize that although the coordinates for an event are different in different inertial frames, it is static in spacetime for all inertial frames. It has happened, has been recorded in arbitrary many inertial frames and that's that.

2. The spacetime interval is a coordinate independent property that can be assigned to two Minkowski spacetime events, originating from the hyperbolic nature of Minkowski spacetime. It is obtained by squaring the time difference and the spatial difference between the two events (in any inertial frame) and then subtracting them. This gives the square of the invariant spacetime interval between the two events, as has been shown plenty of times in the referenced thread and numerous others. It is important to realize that the spacetime interval is not a distance.

When Faradave writes d² = (ct)² – x², it appears as if he intuit it as a distance of some sort, which is unfortunate. There is in fact a property related to spatial distance in SR, called the displacement four-vector, or four-displacement for short, defined as an arrow linking two events in 4-dimensional spacetime. It is patently not the spacetime interval and the spacetime interval is not a distance.

Based on this apparent misconception, Faradave then proceeds by equating the timelike interval (which is zero), to a zero separation between two events, e.g. emission and eventual absorption of a photon can happen 'instantaneously' over a distance. This then leads to the mysterious "interval speed" between two events, which are both static in space and time. Hence my repeated question: "where does speed come in?". One cannot even conceptualize any object to have speed relative to an event.

Apparently, the 'explanation' for the constancy of the speed of light and the "the instantaneous acceleration of light quanta" statement are also linked to this misconception.

The floor is open for discussion.