[Atomic-S (OP)]

Suppose there exists a room, called Room A. The room has a window that is glazed in a peculiar manner: the "glass" consists of a plane such that electromagnetic radiation cannot propagate beyond it; it is 100% reflective and constitutes, for purposes of the room, a mirror. The window is actually a light switch that is reflective when "off" but transparent when "on". When "on", light can cross from the room into another room (Room B) then on the other side, and vice versa. Otherwise light is blocked both ways. Observer A in Room A with a camera sits in a chair.  T_A denotes time in Room A. At T_A = -2 seconds, Observer A gets up, walks over toward the window.   At T_A = -0.5 seconds, the window turns "on", optically connecting the rooms. At T_A = -0.003 seconds, Observer A clicks the shutter. At T_A = 0, half of the light from the flash has crossed the window into the other room.  At T_A = 0.003 seconds, the shutter closes.  At T_A = 0.5 seconds, the window switches "off".  Then Observer A goes over to a table and lays down the camera.


Meanwhile, in Room B,  Observer B in  with a camera sits in a chair.   Time in Room B will be denoted by T_B.    At T_B = -2 seconds, Observer B gets up, walks over toward the window.   At T_B = -0.5 seconds, the window turns "on", optically connecting the rooms. At T_B  = -0.003 seconds, Observer B clicks the shutter. At T_B = 0, half of the light from the flash has crossed the window into the other room.  At T_B = 0.003 seconds, the shutter closes.  At T_B = 0.5 seconds, the window switches "off".  Then Observer B goes over to a table and lays down the camera.


The rooms are both completely enclosed, and have no light sources other than the camera flashes.


Now a word about the window. Rooms A and B are in different universes, and therefore use different coordinate systems. Of particular importance here is that they do not use the same clock.  The coordinate systems ordinarily would have nothing to do with each  other; however in this case they become connected during the time when the window is "on" (this connection is how the window is turned on). Working from different universes gives us the option of flexibility in how the connection is made. Specifically, the connection is made in such a way that that T_A = -T_B.  Thus, each observer would see the other's clock running backwards.


Question:  What in general  will each observer see, and what will be the state of the photographic film in each camera at various times throughout the experiment?

[Atomic-S (OP)]

Tough problems can sometimes be solved by iteration. A provisional solution is obtained using simpflying assumptions, then that is used as the starting point for a more refined solution, and so on.

Using that method here, the first approximation would be that the window remains closed throughout. Therefore, basically, each observer takes his/her own picture in a mirror.  The camera flashes, the light bounces back, and is reflected, recorded, absorbed, etc. Visibility is limited to the brief interval when light is present.

If, with that in mind, we now look at the case that the window opens as described, when Observer A takes his picture, the light that is returned is (mainly) from Room B, not A; but the situation is otherwise superficially identical  . So we would expect no different outcome, we think. However, one point that has not been made clear in the original problem is whether the contents of Room A and Room B are identical. There is no compelling reason why they need to be, and the assumption that they are creates a complicating feature that limits the generality of the solution. Let us then look at the situation that they are not identical. Observer A wears a red shirt; Observer B wears a blue shirt. Room A has a Victorian chair, B a Danish Modern. A uses a Nikon; B a Minolta.  The walls in Room A are beige; those in B a light blue.

So Observer A, when photographing into Room B, is photographing Room B rather than a reflection of Room A. The implication of that is that the light from A's flash travels into B, where we think it is partly absorbed, partly reflected, and partly returns to A's film (or whatever) to make the picture.  But, in order for the action as seen by Observer B to be likewise from B's perspective, then the view of B's photography from the standpoint of Observer A, whose time is moving in the other direction, the light from B's flash starts (on A's clock) out by extracting itself from A's environment and B's film (resulting in the erasure of B's picture), recollecting itself, returning to B's flash, and recharging B's battery. But part of B's light was involved in some manner with A's camera, inasmuch as both shutters were open during the same short interval. Now we know under normal circumstances, when you take a flash picture of someone while he is taking one of you, both cameras will record te other's flash.  If A's camera does in fact "see" B's flash, we would expect this result. However, everything we have thus far seen regarding B's light is that it proceeds not to the objects in Room A (including A's film), but from them.  Thus, the conclusion is suggested that the light that normally would have recorded B's flash upon A's film, actually departs (by A's clock) A's camera rather than enters it, being generated for this purpose by A's film.  But if that is so, what state must A's film be in at the end (A's clock) of the experiment? One would think that A's film, when developed, would have to somehow look different than it would under the scenario that the window never open.  But what would the difference be?

This question brings up another:  In the iteration thus far, the effects of A's and B's light has been treated as independent phenomena whose results may be simply superimposed linearly.  That proposition, however, remains thus far unproven.  The solution thus far envisions light from B's camera, when viewd by A's clock, to extract itself from the energy in the environment and eventually end back up in B's battery (and vice versa for A.)  But that does not take into account the fact that the environment from which it extracts itself is, at some point, influenced by B's flash.  Does light that extract itself from an environment that is not influenced by any other light, still extract itself the same way from an environment that is?  Do the results of this experiment depend on the relative intensities of the flashes?  What if the intensity of B's flash is, say, .000000001 times the strength of A's flash?