|You know it makes sense|
The TNG episode "Parallels" is justly famous as a Star Trek classic. It is based on the concept of the Many Worlds Interpretation, by which the universe continually splits into different versions. Worf starts drifting uncontrollably between realities, which seem to diverge further and further from the life he knew. See the page on this site about "Parallels" for more discussion.
After you've seen it a few times, you start to wonder, how consistent are the progressive changes? Is Worf in fact getting to realities that are "further" from his own? One way of thinking about the various changes is to consider when the world Worf is in would have diverged from the world in which he was before all these changes started. Looking at the the notable changes we can consider (i) the changed circumstance, (ii) what must have happened differently, and (iii) how far back this must have been.
In general, there is a fairly good level of consistency. That is, in most cases each change implies a divergence further back in the past. Consider the first change: at the start of the party, the cake is chocolate. But then it is yellow. This implies a different decision by whoever ordered the cake. Given that the cake merely needs an order to a food replicator, this decision need not be more than a few minutes in the past. (It could be more, of course.) Also, we are told that Captain Picard is absent because he is needed on the bridge, but then he is present. The presence of Captain Picard implies that he was not, in fact, needed on the bridge—I am guessing something like an hour for the different events. As you will see from the table, by and large the implied times get steadily further back. There are a number of minor changes from which no conclusions can be drawn. For example at one point Data has blue eyes. This could have been a different choice by his creator Dr Soong, but it could also be Data experimenting, as with his beard.
It is eventually established that the changes take place when Geordi's visor is in close proximity to Worf. In general it appears that there is one shift on each such occasion (which may of course change multiple things), but on the occasion when Geordi comes to Worf's quarters, it seems that he remains close enough to trigger a rapid sequence of changes.
Worf's relationship with Troi seems to involve a definite divergence several years ago. However, if you watch the episode carefully you will notie that Troi's manner toward Worf has subtly shifted toward greater closeness quite early. This doesn't really fit the strict logic of the chronology of divergence, but is artistically important in conveying the sense of moving toward a progressively closer relationship.
There are two cases which don't fit very well, though. The first is the changes about the Argus Array. The Enterprise has been sent to investigate why the Array isn't working. First, they discover that the Cardassians have taken it over for espionage. But after a change, the problem was just due to a technical fault. This implies that in the first case the Cardassians had decided on an operation—this is presumably at least a few weeks in the past, possibly much longer. The change is that they decided against the operation, or never considered it. But in that case, the fact that the Argus Array stopped working at exactly the same time in the second reality is a pure coincidence. In gneeral, the episode is saying, among other things, "If this had gone differently you would have this different situation." But here, the change is not connected to what Worf is now observing. There is a dramatic reason, however: while it would be logical for the Enterprise not to be at the Array at all, that would introduce a dramatic discontinuity too early. At this stage we are following Worf in what seems to be a continuous sequence, but in which he is being told that his memories of the past are wrong.
The other is the reality where Nurse Ogawa is a doctor (though she then goes back to being a nurse). How far back does the change have to be for this? With the present system of medical practice, quite a long time, since nursing and medicine are different professions. Alyssa Ogawa would, at present, have had to choose medicine over a decade before "Parallels" just to be qualified, and we can assume she would have a bit of experience before being posted to a starship. Shall we say 12 years? But it's possible that the medical professions work differently in the 24th century and Nurse Ogawa was able to shift to being a doctor during the course of her service on the Enterprise, in which case it might be only a few years. Christine Chappell, the Nurse in TOS, is a doctor in the Star Trek films, though this was never explained. (Some writers described this as a "promotion", which shows a certain ignorance of the role of nurses.)
Of the two, it's the Argus Array issue that I noticed when I watched the episode. The logical problem stands out too much. But Nurse/Dr Ogawa is arguably the one that is hardest to fit into the chronological pattern.
The Bajorans having overpowered the Cardassian Empire is hard to date.
Two things toward the end. The first is one of the most memorable moments. When multiple Enterprises start appearing in the "rift", one of them is from a reality where the Borg have won. A wild-eyed Riker pleads: "We won't go back. You don't know what it's like in our universe! The Federation's gone, the Borg is [sic] everywhere! We're one of the last ships left. Please, you've got to help us!" In the background Worf, presumably one of a limited number of survivors, can be seen jumping between panels trying to keep things working—suggesting that all these realities are linked by Worf in his state of "quantum flux" (see below). We can't really date this because it's not clear if the general rule of Worf drifting further and further applies. The most obvious interpretation is that it diverged when the Borg attacked four years ago, but it's quite possible it was earlier or later depending on what the Borg did.
The other is that right at the end, when Worf is back in his original reality, there is no birthday party. (Troi knew he wouldn't like it and persuaded Riker.) This implies that when Worf arrived onboard, there had already been a change. This is consistent with Data's explanation that Worf's shuttle had crossed a "quantum fissure" putting him into "quantum flux". Geordi's visor periodically re-triggered it but the first change was in the shuttle. This divergence could be anything from a week to a day.
Overall, then, my conclusion is that the changes are (with a few exceptions) in the order of implied time passed, and it seems the writers probably planned it fairly deliberately. So, here is the table.
|Change to:||Divergence:||Time from divergence to present:|
|Geordi arrives at party|
|Cake is yellow||Cake-making||10 min (given replicator)|
|Picard is present at party||Bridge duties||One hour?|
|Best guess: a couple of hours|
|Worf meets Geordi in Engineering, discussing Argus Array|
|Worf had concussion||At bat'leth tournament||A few days|
|Argus Array was not taken over by Cardassians||Decision by the Cardassians, but possibly also fault in Array||Cardassian decision on operation presumably at least a few weeks, probably longer|
|Best guess: a couple of months|
|Geordi comes to Worf's quarters where he is discussing the discontinuities with Troi|
|Changes to Worf's quarters, Troi's clothes etc.||Unknown||Unknown|
|Geordi still in Worf's quarters|
|Battle with Cardassians||Unknown||Unknown|
|Worf married to Troi||Troi and Worf form relationship||3 years|
|Alyssa Ogawa is a doctor rather than nurse||Depends on medical training system||12 years min if training different from the start; less if change during TNG|
|Best guess: 3+ years|
|Geordi's visor is activated (Engineering)|
|Riker is Captain||Picard killed in conflict with Borg||4 years|
|Worf is First Officer||Picard killed in conflict with Borg; possibly earlier since Worf promoted over Data||4+ years|
|Worf and Troi have children||Conception of oldest child; relationship starts earlier||4 years|
|Alexander does not exist||Alexander not conceived||Alexander is conceived in "The Emissary" season 2, so at least 5 years|
|Dr Ogawa is nurse again||Depends on medical training system||12 years min if training different from the start; less if change during TNG|
|Wesley is Lieutenant||Enters Academy earlier?||Unknown|
|Bajorans are aggressive||Bajorans overpowered Cardassian Empire||Unclear. The Cardassians took over Bajor 40 years ago ("Ensign Ro") so somewhere between 40 and 5 years?|
|Best guess: 6+ years|
|Borg have won (this may not fit the pattern as it is one of the ships appearing in the rift)||Federation loses conflict with Borg||Unknown but probably 4 years|
|Also we learn at the end that the surprise party does not take place in this (original) reality.||Troi persuaded Riker against||Half a day?|
 Inconsistencies are not flaws unless they bother you. In cases like this it is fun to work out the implications, though. [Return]
 When Worf refers to her as a doctor, Dr Crusher says "Doctor Ogawa"? as if it were not so much obviously wrong as laughable. Gates McFadden plays these distinctive (and not always attractive) traits of Dr Crusher very well, but the script usually doesn't give her much opportunity. [Return]
 This means, incidentally, that the scene near the start where Worf invites Troi to be Alexander's "soh-chim" (i.e. guardian in the event Worf dies) has not taken place in the reality Worf has returned to. On the other hand, since Worf has returned to the moment he started all this, none of what happened in the episode has taken place yet anyway. It seems likely that Worf repeats the offer, unless his possible new interest in a romantic relationship has changed the situation, but I don't think we ever learn this definitely. [Return]