The question in question: What parameters control the amount of thermal energy an object must possess for it to be detectable in space?'

It was not a homework question, and I do not know enough physics to be able to make it more descriptive/specific. Can you help?

  • $\begingroup$ I actually don't agree that it is homework-and-exercises like in nature. I know it's phrased like you are looking for a number for temperature or something, but I think that's just because you are unclear on what we actually measure when we see a distant object. I've voted to reopen it. $\endgroup$ – tpg2114 Feb 7 '15 at 22:48
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    $\begingroup$ The question has since been re-opened. $\endgroup$ – HDE 226868 Feb 8 '15 at 15:26

Currently the conceptual basis of the question is wrong at a level that poses a real problem.

(And there are vocabulary issues that confuse things: say "thermal energy" not "heat energy", please. Heat is a property of interactions not of objects.)

You asked detectability as a function of something that is not directly (or even closely) related to detectability. To be sure, a good answer would start by setting you straight on that , but it still leaves the question of which of the several ways to proceed from where you started to a answerable question should be taken.

I was not among the close-voters, but I would have chosen "unclear what you're asking" if I had evaluated the question.

The main step to fix it would be to figure out what you really want to ask about. Possibilities include

  • Temperature for blackbody objects (are you assuming a size?)
  • Total radiated power assuming isotropic radiation
  • Radiated intensity along our line of site (for non-isotropic cases)
  • $\begingroup$ Thanks. That's true that I was confused between "heat" and "energy" stored. I was kind of thinking about it as "how hot should an object be to be detectable", and the correct translation would have been "how much thermal energy should an object have". $\endgroup$ – user89 Feb 7 '15 at 22:57
  • $\begingroup$ However, I am really happy with the answer I received, that cleared me up on those confusions. $\endgroup$ – user89 Feb 7 '15 at 22:57
  • $\begingroup$ If the question is "how hot" then you want to ask about temperature. But to give you an answer people will still need to know about the size of the object (and it's emissivity in theory). $\endgroup$ – dmckee --- ex-moderator kitten Feb 7 '15 at 22:58
  • $\begingroup$ So, I should be expected to be clear on all of those concepts before I can ask a question on detectability? Is this MathOverflow, just for physics? I have mainly studied biology, math, and classical mechanics, so I don't really know statistical mechanics. $\endgroup$ – user89 Feb 7 '15 at 23:00
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    $\begingroup$ If you get close enough people will generally make the right guess and get you squared away. That said, Physics SE is for "active researchers, academics and students of physics and astronomy". That's not as stiff a barrier as you find on Math Overflow, but it's stiffer than, say, Mathematics. $\endgroup$ – dmckee --- ex-moderator kitten Feb 7 '15 at 23:05
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    $\begingroup$ How does one define student of physics? I am learning physics, and actively apply it in my research area, plus I am proficient at least in one field of physics. Also, having a look at the hot posts (physics.stackexchange.com/?tab=month), most seem to be at the level of undergraduate students, not higher. $\endgroup$ – user89 Feb 7 '15 at 23:09
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    $\begingroup$ Also, I have edited my question, and I wonder if its appropriate enough to be reopened. $\endgroup$ – user89 Feb 7 '15 at 23:11
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    $\begingroup$ I don't have the meta-link handy, but "student" definitely includes undergrads. And even interested amateurs (though personally I'm looking for more than yet another "I read this pop-sci book can you explain what it said"). We're not going to check your credentials, we just want you to use the language and conventions of physics. $\endgroup$ – dmckee --- ex-moderator kitten Feb 7 '15 at 23:15

I closed the question because you're just asking us to do a calculation for you. That kind of question is not what this site is for. Simply asking us to calculate something is one big marker for what we call "homework-like" questions, even though not all calculation requests are actually homework questions.

If you had shown some effort to do the calculation yourself and identified the step at which you got stuck, and asked about the reason you weren't able to proceed past that step rather than just asking us to do the calculation for you, then I would not have closed the question.

Granted, this wasn't the clearest case, but I stand by my belief that the question is not appropriate for the site as is. And none of the edits you've made (as I write this) change the body of the question, so there is no reason my decision would be any different for the current version of the question as opposed to its original form.

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    $\begingroup$ I didn't want a calculation, as much as understand what parameters are of concern when it comes to detectability. Sorry, but this was not a homework question at all. $\endgroup$ – user89 Feb 7 '15 at 23:53
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    $\begingroup$ @user89 okay, and in that case the question you wanted to ask sounds like a good one (or at least an okay one) for the site and I probably would not have voted to close it. But I believe the question you actually posted was not quite the question you wanted to ask. I think it would improve it significantly if you edit the question to clarify that you want to understand what parameters are involved. $\endgroup$ – David Z Feb 8 '15 at 7:24
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    $\begingroup$ I changed the title of the question again, to reflect your suggestion. What do you think of it now? $\endgroup$ – user89 Feb 8 '15 at 17:06
  • $\begingroup$ @user89 any changes you make to the title are irrelevant for determining whether it's on topic, so I think the same thing I thought before. You would need to edit the body of the question to change my opinion of it. $\endgroup$ – David Z Feb 8 '15 at 18:06
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    $\begingroup$ I changed the body of the question now as well, although I wasn't too sure how to add more to it than the title contains. So, I simply clarify that I consider both the object, and the sensor, to be in space and thus unaffected by complications such as a planetary atmosphere. $\endgroup$ – user89 Feb 8 '15 at 18:41
  • $\begingroup$ @user89 As I said, changing the question to ask what parameters are of concern, rather than asking for a calculation, would help it a lot. $\endgroup$ – David Z Feb 9 '15 at 1:05
  • $\begingroup$ That's what I changed it to: "What parameters control the amount of thermal energy a space object must possess for it to be detectable by a sensor also in space (i.e. one that does not have to deal with interference from a planet's atmosphere)?" $\endgroup$ – user89 Feb 9 '15 at 1:48

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