Got to admit that black holes are one of the most confusing as well as interesting ideas I know of. As for how Smolin defines it I don't know, would be nice to see the citation. But I know that Einstein changed his mind on Black holes. When it comes to a event horizon specifically? Well, I haven't seen any discussion about that one when it comes to Einstein and history, possibly Smolin is correct in arguing that Einstein didn't like that idea, as it could be read as relativity breaks down at a event horizon. But as far as I get it the event horizon has nothing to do with that, presuming that there is a center of 'infinite mass'.

"The simplest answer is that the curvature of space-time is a smoothly changing function of distance up to, and through, the event horizon. There is no indication from the curvature (Riemann's Curvature Tensor, or even Ricci's for that matter) that anything serious is happening just inside the event horizon. Einstein's 'equations' work just fine so long as the local curvature of space-time (the strength of the gravitational field) does not become singular. This does not happen just inside an event horizon, but only happens as you approach the 'r=0' singularity itself.

What all of this means is that the mathematical properties of spacetime that matter (its curvature) change smoothly through the event horizon, much like a ride in a sled down a snow-covered hill. Now, to prove that this is in fact the case will probably not be possible because we can never extract information from inside a black hole withough dying, or never being able to return! "

As for me I would expect the physics inside a Event Horizon to be the same as outside, I do not expect 'time' to become 'space' and 'space' become 'time' for a infalling observer. To the observer the local arrow should be 'as always', his ride toward the singularity's center taking a, for him, measurable time.

What experiment are you referring to? Not the first example over a whole space-times history, right? That's the one I wouldn't know how to define. As for the one with a 'apparent event horizon'. The whole idea of that, is that it for the observer becomes his 'limit of observation'. Meaning that wherever he finds it to be, that also will be the place of 'no return' for his experiments, and as far as I understands it, no more reflections observed, from his frame of reference. That should mean that your mirror will 'disappear' for the observer at that point. And there will also be a 'dimming' of that mirror, due to the redshift of that reflected light as it propagates 'uphill', getting 'stretched out' by the Black holes gravity if described as waves. So it will probably just dim out and disappear as it falls in. To that you can add a time dilation, making it 'slow down' for the observer as it closes in on that event horizon.

As for why a event horizon is needed? It's just a place where all geodesics starts to lead to a same place, relative the observer, or a 'whole SpaceTime'. And that should be the center of its singularity. And light follows those geodesics. A reflection can only be seen if that geodesic leads back to you. A event horizon is defined from the way light, (energy), gravity and mass interacts. But a black hole is truly weird, as it do contain a infinity at some point, if the mathematics are correct. And there is no way to describe what happens there, as far as I know. And it's GR that describes a black hole. Karl Schwarzchild used Einstein's theory of general relativity to define a non spinning black hole 1916.

"An object whose radius is smaller than its Schwarzschild radius is called a black hole. The surface at the Schwarzschild radius acts as an event horizon in a non-rotating body (a rotating black hole operates slightly differently). Neither light nor particles can escape through this surface from the region inside, hence the name "black hole". The Schwarzschild radius of the (currently hypothesized) supermassive black hole at our Galactic Center would be approximately 13.3 million kilometres." from

Schwarzschild radius. =

I wrote ' A event horizon is defined from the way light, (energy), gravity and mass interacts.' But I don't think you need to add 'time' to that. Time is to me locally invariant, its arrow of time always ticking at a same rate for you. And as I define it as equivalent to 'c', also a local invariant, I therefore will assume that this will hold true for accelerations/decelerations too. 'c' must be 'c' at all kinds of 'motion' for the equivalence to hold. And that goes for the inside of a black hole too.