I've been wondering why there is so little activity on physics stack exchange compared with math stack exchange. It occurred to me that a contribution to the difference is that we may be closing too many questions as "duplicates".

Using a liberal definition of "duplicate" means that there will be fewer questions available to answer and this reduces the likelihood that someone searching for an answer will be able to locate it.

Let us review official Stack Exchange policy on duplicates. From the blog, my emphasis:

Jeff Atwood, Nov 15, 2010 Dr. Strangedupe: Or, How I Learned to Stop Worrying And Love Duplication

One thing I want to be clear about, though, is that duplication is not necessarily bad. Quite the contrary — some duplication is desirable. There’s often benefit to having multiple subtle variants of a question around, as people tend to ask and search using completely different words, and the better our coverage, the better odds people can find the answer they’re looking for. And isn’t that, really, the whole point of this exercise?


Joel Spolsky, Jan 5, 2011, The Wikipedia of Long Tail Programming Questions

If you’re going to close a user’s question as a duplicate, it has to be a real duplicate. For example, if a user asks, “What does the IP address mean?” it’s OK to close that as a duplicate of a more general question like “What do IP addresses of the form a.b.c.d/e mean?” But it’s not OK to close it as a duplicate of a twenty-seven page guide to netmasks. That’s the moral equivalent of saying “RTFM.” Stack Overflow is not meant to be a library of reference manuals. It’s supposed to contain the same information as a library of reference manuals, in the form of millions of questions and answers. Combined with Google, that gives us the magical power of a library of reference manuals you never have to read! It’s like, you got to the library, and there’s a wizard there at the door, and you ask your question, and, instead of being told to read a book, you just got (are you sitting down?) the actual answer!

That’s why we actually don’t mind having several versions of every question, where there are variations in wording or circumstances. The more chance that someone types a question into Google and finds their exact question already answered, the better a job we’ve done.


Now that we've reviewed official SE policy on duplicate, let's take a look at a question I posted that, as I write, has 3 votes to close based on "duplicate":

Here's the question:

Does throwing a watch into the air cause it to gain or lose time?

Suppose I'm on a non rotating planet. I have two identical, perfect watches. I synchronize them. Then I throw one of them into the air and catch it. Does the one I throw into the air gain or lose time with respect to the one I was holding?

And here's the question proposed as its duplicate:

Maximum time difference between clocks in a gravity field

You blast off in a rocket which has a clock on board, and there's a clock on the ground. The idea is that you have to be back when the clock on the ground says one hour has passed. Now you want it so that when you come back, your clock is as far ahead as possible. According to Einstein, if you go very high, your clock will go faster, because the higher something is in a gravitational field, the faster its clock goes. But if you try to go too high, since you've only got an hour, you have to go so fast to get there that the speed slows your clock down. So you can't go too high. The question is, exactly what program of speed and height should you make so that you get the maximum time on your clock?

This assistant of Einstein worked on it for quite a bit before he realized that the answer is the real motion of matter. If you shoot something up in a normal way, so that the time it takes the shell to go up and come down is an hour, that's the correct motion. It's the fundamental principle of Einstein's gravity--that is, what's called the "proper time" is at a maximum for the actual curve.

The two questions are similar in that they begin on a planet and involve a time piece. But in the first posted the question is "exactly what program of speed and height should you make so that you get the maximum time on your clock?" In the other, the question is: "Does the one I throw into the air gain or lose time with respect to the one I was holding?"

I submit that (under the policy of Stack Exchange) it should be clear and obvious to all that these are not the same question. The first has to do with maximizing a time difference over a path that is not a geodesic (i.e. the rocket ship) as compared to a path that is on a planet's surface (and hence is not a geodesic). The second has to do with the relationship between two paths, one stationary on the planet's surface, the other a geodesic near the planet's surface.

First, it's unfair to expect that a reader not already possessing an understanding of general relativity will see any relationship between these problems. Second, it's unlikely that they will locate the original post in searching for the problem. Third, it's unlikely that they will be able to work out the answer to the problem based on the answers given in the original post.

Finally, I'd like to point out that voting to close a question as duplicate has a tendency to suppress answers to the question. Why bother to write up an answer to a question that is going to be closed as a duplicate? For the question at hand, neither of the two answers give directly tell which watch gains and loses time. The best answer so far provided is: "The "stationary" watch, which is actually accelerated, is following some other path and so must experience a shorter proper time." To someone seeking to understand general relativity this will be confusing at best.

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    $\begingroup$ "The first has to do with maximizing a time difference over a path that is not a geodesic (i.e. the rocket ship)" -> and here's where you've gone wrong. Geodesics are not only included as a motion of that rocket they actually solve the problem ;) And if you'd read the question to the end, it asks why is it that movement along geodesics takes more time. Therefore this question actually asks for more than yours and renders it quite useless. $\endgroup$
    – Marek
    Apr 11, 2011 at 5:56
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    $\begingroup$ the low level of activity is possibly because we are not closing too many low quality questions fast enough. Also physics.SE is much newer than math.SE. Give it some time. $\endgroup$
    – Deepak Vaid
    Apr 11, 2011 at 13:10

3 Answers 3


If it helps at all, I also think some level of duplication in answers (across multiple very similar questions) can be OK as well.

This can often give answerers a chance to show off a different emphasis or some other aspect of an answer, or present it more clearly -- even if it was roughly "the same" as an answer on a duplicate question.

You don't want too much of this, of course, but I believe that repetition is often helpful in learning and clamping down too hard on the "all duplication is bad!" bandwagon is much more harmful to the health of your site than a few great semi-duplicate answers and questions.

Of course

  1. quality should be maintained in any case; a bad duplicate question or answer doesn't help anyone.

  2. nobody should set out with the intent of duplicating work, unless they have strong reason to believe the existing stuff can be substantially improved

I would certainly be generous with the benefit of the doubt here and realize that for most folks, learning == repetition.


It's clear that every problem involving gravity can be solved by understanding gravity. Therefore, we should have only one gravity question. It will be "what is general relativity".

Then we can mark every other question about gravity as a duplicate of "What is general relativity".

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    $\begingroup$ actually, it should be "what is the quantum field theory of general relativity", your question is just a special case :-P $\endgroup$ Apr 13, 2011 at 6:20

Here's an attempt to define exact duplicate questions in physics:

If understanding the answer to question A implies that you will immediately understand the answer to question B and vice versa, then questions A and B are exact duplicates.

Questions with subtle differences could pass this test; I think the questions on watches are an example.

In that case, if question B is closed as an exact duplicate of A, readers who find B through search will be redirected to A, read the answers to A, and, by definition, understand the answer to B.

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    $\begingroup$ I don't agree with "vice versa". It suffices that there is one-way relationship. And as matter of fact, this is actually the case with Joel's IP addresses example and Carl's geodesic example. $\endgroup$
    – Marek
    Apr 11, 2011 at 6:01
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    $\begingroup$ @Marek I included "vice versa" because there can be more and less advanced questions. The more advanced question might not be accessible to many people, in which case I personally wouldn't view the less-advanced question as a repeat. (I don't have an example off the top of my head; I'd have to dig around for one.) $\endgroup$ Apr 11, 2011 at 6:05
  • $\begingroup$ @Mark: that might be justified sometimes (e.g. if someone asks for explanation of some phenomenon specifically for educational purposes) but usually it's not. Moreover the answers under a single question differ greatly in level too. $\endgroup$
    – Marek
    Apr 11, 2011 at 7:31
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    $\begingroup$ Mark, you are basically disagreeing with the OP without explaining why. The OP is actually convincing, but your answer fails to do so for this reason. Joel is saying that two OPs different circumstances that have the same answer should be kept. $\endgroup$
    – Sklivvz
    Apr 11, 2011 at 17:22
  • $\begingroup$ @Sklivvz What do you mean I "didn't say why"? Of course I did. The reason is the answers are the same, so you might as well be redirected. $\endgroup$ Apr 11, 2011 at 17:44
  • $\begingroup$ @Sklivvz: repeating an answer is perhaps beneficial to the site (although I am not sure about this either) but it's certainly not beneficial to my fingers and time schedule to craft longer answers each and every time someone asks for it and doesn't even bother to search. That's why we have close as duplicate option. The question is still there for search engines but you don't need to duplicate (or multiplicate) every answer. Simple redirect to one answer is enough. $\endgroup$
    – Marek
    Apr 11, 2011 at 17:56
  • $\begingroup$ @Sklivvz Why would repeating answers be beneficial? If repeating answers were beneficial, the entire idea of exact duplicates would not exist. The quotes from Jeff and Joel do not say that we should repeat answers. They say we should allow questions with differences. My answer is his an attempt to define when questions have difference. I think Carl's watch example and the specific IP address/general IP address example both pass the test I gave as being "exact duplicates". The IP address / guide to netmasks example does not pass the test. $\endgroup$ Apr 11, 2011 at 18:01
  • $\begingroup$ @mark let me disagree, your proposal and what Jeff and Joel say are two very different things. You propose the bare minimum conditions by which it could be acceptable to close as duplicate. Jeff and Joel suggest raising the bar and leaving subltle variations. It is not apparent that the two clock questions lead to e same answers. $\endgroup$
    – Sklivvz
    Apr 11, 2011 at 18:09
  • $\begingroup$ @Sklivvz: why so hostile? I am discussing this too and showing you some cons (I don't see pros though). And I don't remember ever stating what I just said before, so where exactly am I repeating myself? $\endgroup$
    – Marek
    Apr 11, 2011 at 18:17
  • $\begingroup$ If not hostile then at least everyone's getting a little peeved... besides, this is consuming enough comment space here. This might be a good point to break to the chat room. $\endgroup$
    – David Z
    Apr 11, 2011 at 22:38
  • $\begingroup$ If it was a chat "pub" with virtual drinking games everyone would be there already. $\endgroup$
    – Deepak Vaid
    Apr 12, 2011 at 5:31

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