# Is there anything that is theoretically impossible but possible in practice? **URL:** \ **Category:** Factual Questions\ **Created:** [November 30, 2010, 1:35am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352 "2010-11-30T01:35:11Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Cagey\_Drifter](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@Cagey\_Drifter](https://boards.straightdope.com/u/Cagey_Drifter)\ **Post date:** [November 30, 2010, 1:35am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/1 "2010-11-30T01:35:11Z")
The maker of this software that [isolates individual notes from audio mixes](http://www.youtube.com/watch?v=jFCjv4_jqAY) claims that his software does something that is theoretically impossible, but apparently possible in practice. I don’t know whether this is true or not, but is there anything else that fits this description? ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [November 30, 2010, 1:46am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/2 "2010-11-30T01:46:18Z")
If a theory is correct, then anything that it says is impossible really is impossible. ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [November 30, 2010, 1:49am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/3 "2010-11-30T01:49:52Z")
If it is possible in practice then the theory needs some work. ---
**Author:** ![cptnred](https://avatars.discourse-cdn.com/v4/letter/c/58956e/32.png) [@cptnred](https://boards.straightdope.com/u/cptnred)\ **Post date:** [November 30, 2010, 1:55am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/4 "2010-11-30T01:55:02Z")
How about that the sky is blue? Given the theory that the sky is green, that is. ---
**Author:** ![Kent\_Clark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kent_clark/32/105_2.png) [@Kent\_Clark](https://boards.straightdope.com/u/Kent_Clark)\ **Post date:** [November 30, 2010, 2:10am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/5 "2010-11-30T02:10:57Z")
The old myth about bumblebees theoretically being unable to fly has been pretty thoroughly [debunked.](http://www.news.cornell.edu/releases/March00/APS_Wang.hrs.html) ---
**Author:** ![penultima\_thule](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/penultima_thule/32/3833_2.png) [@penultima\_thule](https://boards.straightdope.com/u/penultima_thule)\ **Post date:** [November 30, 2010, 2:20am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/6 "2010-11-30T02:20:22Z")
Isn’t that the basis of the scientific principle? 1. Here is our current theory of how xxx works 2. Here is an example that is not possible under current theory 3. Rework theory to incorporate new observations repeat ad inifinitum ---
**Author:** ![KinkiNipponTourist](https://avatars.discourse-cdn.com/v4/letter/k/e9bcb4/32.png) [@KinkiNipponTourist](https://boards.straightdope.com/u/KinkiNipponTourist)\ **Post date:** [November 30, 2010, 2:28am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/7 "2010-11-30T02:28:28Z")
I can tickle myself. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [November 30, 2010, 2:31am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/8 "2010-11-30T02:31:03Z")
> [@kunilou](#): > > The old myth about bumblebees theoretically being unable to fly has been pretty thoroughly [debunked.](http://www.news.cornell.edu/releases/March00/APS_Wang.hrs.html) It wasn’t a myth, they were theoretically unable to fly. It was just that the theory was inadequately informed. ---
**Author:** ![The\_Tao\_s\_Revenge](https://avatars.discourse-cdn.com/v4/letter/t/73ab20/32.png) [@The\_Tao\_s\_Revenge](https://boards.straightdope.com/u/The_Tao_s_Revenge)\ **Post date:** [November 30, 2010, 2:33am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/9 "2010-11-30T02:33:51Z")
Mercury’s orbit is theoretically impossible, according to Newton’s Theory. Where as Relativity is said to explain it perfectly. It’s my understanding that discovering theoretically impossible things is an important part of science. It’s evidence that a theory is incomplete and needs to be revised, or replaced. ---
**Author:** ![Deeg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/deeg/32/2955_2.png) [@Deeg](https://boards.straightdope.com/u/Deeg)\ **Post date:** [November 30, 2010, 2:52am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/10 "2010-11-30T02:52:35Z")
Isn’t the increasing velocity of the expansion of the universe at odds with current theories? (And thus the addition of “dark energy” which nobody knows what it is.) Dunno if that fits the definition of “practice”. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [November 30, 2010, 2:56am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/11 "2010-11-30T02:56:36Z")
That’s a pretty interesting signal processing capability. For anyone who cares, and doesn’t want to watch the YouTube video, the actual quote is “…yet for theoretical reasons, I thought it would be impossible. But the more I pondered the subject, the more I began to see that what doesn’t work in theory can still work in reality.” (the original speaker is speaking German (I think), but the narrator translates his words as those above) ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [November 30, 2010, 2:59am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/12 "2010-11-30T02:59:41Z")
This reminds me of one of the unofficial t-shirts that are sold at the University of Chicago: Front: “That’s all very well in practice…” Back: “… but how does it work in theory?” ---
**Author:** ![Darth\_Panda](https://avatars.discourse-cdn.com/v4/letter/d/ee7513/32.png) [@Darth\_Panda](https://boards.straightdope.com/u/Darth_Panda)\ **Post date:** [November 30, 2010, 3:12am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/13 "2010-11-30T03:12:44Z")
My theories about how stupid people could possibly be are constantly being debunked by human activity. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [November 30, 2010, 3:25am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/14 "2010-11-30T03:25:08Z")
This also happens when something _really is_ impossible to do in general, but most people only care about doing it in specific cases that are a lot simpler than the hardest possible case, or they’re willing to call something that only gets them 80% of the way a ‘solution’. This is one reason a number of programmers adopt programming languages with fairly strict typing rules: It is provably impossible\* to catch every mistake with a type system, regardless of how complex it is, but a number of programmers think that any system that catches certain common errors is worth structuring their software designs around. \*(Solving the Halting Problem leads to a logical contradiction. Therefore, you can’t do it.) ---
**Author:** ![Digital\_is\_the\_new\_Analog](https://avatars.discourse-cdn.com/v4/letter/d/a88e4f/32.png) [@Digital\_is\_the\_new\_Analog](https://boards.straightdope.com/u/Digital_is_the_new_Analog)\ **Post date:** [November 30, 2010, 3:40am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/15 "2010-11-30T03:40:22Z")
I’ve been told that the first step to tacking in a sailboat is to ignore the fact that it’s impossible to sail against the wind, and just do it. Of course, that might just be hearsay. -D/A ---
**Author:** ![godix](https://avatars.discourse-cdn.com/v4/letter/g/278dde/32.png) [@godix](https://boards.straightdope.com/u/godix)\ **Post date:** [November 30, 2010, 3:50am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/16 "2010-11-30T03:50:02Z")
Theory says you can not take the square root of a negative number. In science with real world events, sometimes you need to do exactly that. Thus imaginary numbers were born. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [November 30, 2010, 3:54am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/17 "2010-11-30T03:54:17Z")
There are certain computational problems (I think the traveling salesman problem is an example) that, while not theoretically impossible, are theoretically unfeasible, but methods are known that come very near the optimal solution almost always. I would imagine that you can decide, for most computer programs, whether they will halt or not, but it would be relatively easy to make an undecidable one. Before Wiles, I would have suggested looking for counter-examples to Fermat. Now we know such a prgram will not halt. But I could find a similar problem, say search for counter-examples to the Goldbach conjecture, whose halting problem is (at the current state of knowledge) undecidable. Or, if the conjecture is actually undecidable, then so would this halting problem be. ---
**Author:** ![WarmNPrickly](https://avatars.discourse-cdn.com/v4/letter/w/d78d45/32.png) [@WarmNPrickly](https://boards.straightdope.com/u/WarmNPrickly)\ **Post date:** [November 30, 2010, 4:13am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/18 "2010-11-30T04:13:55Z")
> [@CookingWithGas](#): > > It wasn’t a myth, they were theoretically unable to fly. It was just that the theory was inadequately informed. Technically this may be correct, but it wasn’t as if anyone was surprised the theory was wrong. From what I know, it was started by André Sainte-Laguë, who used the equations for a fixed wing aircraft on a bumblebee. Anyone who took that seriously to begin with, was a fool. I don’t even think André took it seriously. As far as things that don’t correspond to theory, I think in the spectra of metal complexes, there is the spectre of non-allowed transitions. Theoretically some transitions are allowed by symmetry rules, and others are not, yet metallic compounds very often show weak spectral bands for transitions that are “not allowed” Of course, this isn’t a surprise in the real world. There are plenty of ways for electrons to flip symmetry simply because their environment isn’t perfectly symmetric. It’s just a matter of needing a more refined and nuanced theory. I’m not a computational chemist so I have no idea how good they are at predicting these transitions. ---
**Author:** ![Zsofia](https://avatars.discourse-cdn.com/v4/letter/z/7bcc69/32.png) [@Zsofia](https://boards.straightdope.com/u/Zsofia)\ **Post date:** [November 30, 2010, 4:46am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/19 "2010-11-30T04:46:43Z")
There are plenty of problems where we agree to ignore the “actual” complexity - for example, the length of a seashore. ---
**Author:** ![Revtim](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/revtim/32/1042_2.png) [@Revtim](https://boards.straightdope.com/u/Revtim)\ **Post date:** [November 30, 2010, 4:51am UTC](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352/20 "2010-11-30T04:51:06Z")
Universal Health Care. They keep saying it’s impossible, yet somehow lots of countries manage it. [Next page](https://boards.straightdope.com/t/is-there-anything-that-is-theoretically-impossible-but-possible-in-practice/562352.md?page=2)