I stumbled across this thread today (much to the doom of my productivity) and was very interested in a lot of the ideas that were brought up (surprisingly few of which were critically addressed). I'll organize my thought and make a post as time permits.
Fundamentally I believe that the issue here is that before we discuss
does shared dreaming exist? we must first come to some sort of agreed understanding of
what shared dreaming is, be that a concrete definition or otherwise. Consider the notion of the ego. Equally unprovable as shared dreaming and yet it is scientifically accepted and a useful tool when discussing psychology. Yes people interpret the idea differently but it's been defined to an adequate enough degree that productive discussion can take place.
Furthermore I think that we need to find some sort of common ground between the metaphysical/abstract and the scientific/logically. When discussing the nature of shared dreaming (and, as an extension, dreaming in general) the scientific among us will have to be willing to discuss and toy with ideas that are by no means provable. On the other hand, when discussing potential experiments, those who have experienced shared dreaming will have to take a step back from the abstract and focus on what can actually be measured. Consider how lucid dreaming was proven. Not through elaborate dream recordings of someone transforming a pencil into a dragon. Simple eye movements.
One really interesting idea that was completely ignored, in my opinion, was
flowofmysoul's claim that the easiest way to 'communicate' in a shared dream is through
emotions
(as an aside I'm not sure why we are so quick to dismiss the experiences of those who claim to have SD. We should be picking these peoples' brains for all they have).
While exchanging a password would indeed prove shared dreaming it seems to me that that is more closely tied to telepathic communications (as mentioned previously).
Anyway, what this claim immediately reminded me of was:
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Lovheim's Cube of Emotion.
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The basic idea is that fundamental emotions can be directly correlated to brain chemistry. For example if someone is experiencing fear their brain will produce low amounts of serotonin and noradrenaline and high amounts of dopamine.
This model is fairly new (
2012) and not proven but if it were proven one could attempt to induce an emotion in the person they are sharing a dream with while having that person's monoamine levels monitored.
That's it for now. More to come if this thread livens up again.