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My answer to this question was deleted. For anyone who can't see it, the answer was:

I hope somebody will correct me if I'm wrong, but it seems that the simplest way to phrase the main objection is that decoherence doesn't even try to solve anything beyond "for all practical purposes."

Fine: just imagine that we're sufficiently advanced to make it "practical" to track all the degrees of freedom for the system in question. Then it's clear that decoherence is just a fancy word for simple entanglement, and we're back at square one.

I admit it could have been phrased more clearly, but one comment says "If you have a new question, please ask it by clicking the Ask Question button" and another says "This does not provide an answer to the question." I don't think either applies.

It seems to me that it does answer the question. A strong objection to the claim that decoherence solves collapse is that it intends to solve it only "for all practical purposes" (FAPP), which is easily overcome by having enough "practical" ability to track and control all relevant entanglements.

For example, Penrose:

Under normal circumstances, one must regard the density matrix as some kind of approximation to the whole quantum truth. For there is no general principle providing an absolute bar to extracting information from the environment.[...] Accordingly, such descriptions are referred to as FAPP [For All Practical Purposes]

Maybe a future technology could provide means whereby quantum phase relations can be monitored in detail, under circumstances where present-day technology would simply ‘give up’. It would seem that the resort to a density-matrix description is a technology-dependent prescription! With better technology, the state- vector description could be maintained for longer, and the resort to a density matrix put off until things get really hopelessly messy!

Is there a way to contact the members who marked it for deletion and ask for clarification? Is that an appropriate course of action? Is there any sense in asking for undeletion?

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    $\begingroup$ I won't speak for the voters on that action, but your question was flagged and then seen by five separate reviewers (and maybe other who declined to make a call of either kind) and the call was unanimously in favor of deletion. $\endgroup$ – dmckee Feb 14 '18 at 16:31
  • $\begingroup$ @dmckee Yes I saw that, and am a little bit stunned, to be honest. I just updated my question with more detail from Penrose saying the same thing (in clearer language, explicitly calling out the density matrix approach). I'm trying to get a handle on whether my answer was considered merely unclear or bad/wrong in other ways. $\endgroup$ – A_P Feb 14 '18 at 16:37
  • $\begingroup$ I had not voted for delete, but I am typically lenient, compared to the other reviewers. Hopefully one of the delete voters will explain it more clearly. Note also the consensus (all the reviewers voted for delete). $\endgroup$ – peterh Feb 14 '18 at 17:18
  • $\begingroup$ @peterh Is there any way to ask them directly, or an appeals process? Otherwise this is rather disheartening. $\endgroup$ – A_P Feb 14 '18 at 17:28
  • $\begingroup$ @monk Theoretically it is here, on the meta. Typically similar questions are handled in some days. Undeleting the answer requires 3 undelete votes. Capability to vote for undelete requires 20000 rep. It is also possible, that a mod undeletes your answer, but they do this only 1) in very obvious cases 2) or, if there is a clear consensus. $\endgroup$ – peterh Feb 14 '18 at 17:32
  • $\begingroup$ @monk It is possible, that also you have an undelete vote about your own answer, try to do that (in this case, we would need only 2 undel-voters). $\endgroup$ – peterh Feb 14 '18 at 17:46
  • $\begingroup$ @peterh Thanks! Indeed I was able to undelete it. I modified the answer, but without understanding the objections, it's hard to know what to address to avoid re-deletion. I added some comments, but I'm not sure if those are taken into consideration during review. $\endgroup$ – A_P Feb 14 '18 at 17:49
  • $\begingroup$ @monk Write a comment to it, and refer this meta question in a link. Then the reviewers will consider this meta post, before they vote. Low-Quality posts can be reviewed from 2000 rep, but answer deletion requires 20000 rep. Thus, the delete vote in the LQ-review is a special case, when even 2000 rep users can cast answer delete vote. But these delete votes are much weaker (you can see that in a minor difference in the review: users between 2k and 20k voted from "Recommend Deletion", and the only 20k+ reviewer could cast "Delete"). $\endgroup$ – peterh Feb 14 '18 at 18:10
  • $\begingroup$ @monk The "Recommend Deletion" is only an attribute of the review entity, and not a regular "Delete" vote. Thus, your answer was deleted only with 1 regular delete vote. It may be the reason, why was 1 undelete vote enough (what could have been casted also by you, because it seems 20 rep is enough to cast an undel vote for your own post). In short, you had luck. If you mention this meta post in a comment, a next delete will be unlikely, except if there is some non-obvious, really unrepairable and serious problem in your post. $\endgroup$ – peterh Feb 14 '18 at 18:11
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    $\begingroup$ To me, it didn't seem clear if you were answering the question, or just providing commentary. The general wording wasn't very definitive, and seemed conversational. It definitely reads like an answer now. I thought the automatic comment would have mentioned how it appeared to be commentary; because I believe that was the reason I picked. Either way; good job bringing it to meta. That's the best way to try and figure out why the community responded like it did. Also great job fixing the answer. Too many people take that stuff personally; but it isn't meant t be. $\endgroup$ – JMac Feb 14 '18 at 19:48
  • $\begingroup$ Just to check in, yeah, it mostly just looked like commentary. I still don't think it's great or even particularly close to an answer (which would require e.g. a much more solid support that your points are indeed widely raised in the literature as the objection to decoherence as an explanation), but it was on a relatively ambiguous edge to begin with. $\endgroup$ – Emilio Pisanty Feb 14 '18 at 21:10
  • $\begingroup$ Do note the consensus in the review, though ─ six independent people thought that this was too much like commentary to be an answer, so it is something to keep in mind for future posts ─ on something this big, something meatier and better supported is quite preferable. $\endgroup$ – Emilio Pisanty Feb 14 '18 at 21:12
  • $\begingroup$ @JMac Thanks for the clarification! It would be so much nicer if the review page indicated the reason(s) that reviewers rejected it. Otherwise it's impossible to know what (or even whether) to fix. Meta seems useful, but there's no guarantee that the original reviewers will see the question, right? I guess I lucked out here by being able to undelete it and mention you there? $\endgroup$ – A_P Feb 14 '18 at 21:28
  • $\begingroup$ @EmilioPisanty Thank you. Indeed, knowing the reason is very helpful in the learning process. As mentioned above, it appears I lucked out in being able to undelete it and thereby gain a way to contact you. $\endgroup$ – A_P Feb 14 '18 at 21:31
  • $\begingroup$ @monk I'd say in general, you're likely to find at least one person who reviewed your question in the meta. Worst case scenario, someone in the meta should have similar thoughts or be able to think of a reason. $\endgroup$ – JMac Feb 14 '18 at 21:37
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To answer your questions directly:

Is there a way to contact the members who marked it for deletion and ask for clarification? Is that an appropriate course of action?

There are a couple of ways you could try to attract the attention of the people who deleted your post, but our system is designed so you don't have to. Quite a few people have the ability to vote to delete or undelete posts, and if it's really appropriate to undelete your post, it doesn't matter who actually does it, whether the people who originally deleted it or others. So in general, when you think it's appropriate to reverse some sort of community moderation action (like closing a question or deleting a post), don't worry about finding the specific people who took the action in the first place. You only need to bring it to the attention of somebody with the appropriate privileges.

Posting on meta, as you did, is a fine way to do that (although don't make a frequent habit of it). Another option is to check in chat, which gives you less visibility but you may get lucky with a quicker response.

Now, when it comes to asking why your post was deleted in the first place, only the people who actually deleted it can tell you with 100% certainty. But in many cases, other site members can give you a pretty good guess, and again, that's usually good enough.


Is there any sense in asking for undeletion?

Sure. Sometimes mistakes happen and it's totally appropriate to ask for a fix if you think this is the case. But you do have to show that you understand the rules we have surrounding deletion of posts, and you'll have to make a solid argument based on those rules that the reason your post was deleted shouldn't actually apply. This doesn't have to be complicated; in your case something like this is all I'd need to be convinced:

The question asks for "the strongest objections that have been raised against" the acceptance of decoherence, and I addressed that directly by saying "...the simplest way to phrase the main objection is...".

I'm not saying you have to write it like this, or that you have to use direct quotes or anything like that; all I'm saying is that it can be short and direct.

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