# In what physical world would you be able to unstir milk from coffee? Time travel is not allowed. **URL:** \ **Category:** Factual Questions\ **Created:** [November 16, 2012, 8:38pm UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236 "2012-11-16T20:38:56Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [November 16, 2012, 8:38pm UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/1 "2012-11-16T20:38:56Z")
See subject. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [November 16, 2012, 8:45pm UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/2 "2012-11-16T20:45:34Z")
OK, so you’ve identified the fact that entropy can be used as an arrow of time. Congratulations. What is the General Question here? ---
**Author:** ![Chief\_Pedant](https://avatars.discourse-cdn.com/v4/letter/c/a6a055/32.png) [@Chief\_Pedant](https://boards.straightdope.com/u/Chief_Pedant)\ **Post date:** [November 16, 2012, 8:52pm UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/3 "2012-11-16T20:52:29Z")
[Here’s a popular video](http://www.youtube.com/watch?v=UpJ-kGII074) that is probably the closest we’ll get in this world. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [November 16, 2012, 8:58pm UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/4 "2012-11-16T20:58:25Z")
I think probably none, since the second law of thermodynamics is not just an arbitrary law, but derives (in this sort of instance anyway) from the statistics of molecular motion: i.e., just from the fact taht stuff is made of particles, and math. Maybe one could conceive of a world where matter was not particulate, but there, how would you stir your coffee at all? ---
**Author:** ![Jas09](https://avatars.discourse-cdn.com/v4/letter/j/d07c76/32.png) [@Jas09](https://boards.straightdope.com/u/Jas09)\ **Post date:** [November 16, 2012, 9:07pm UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/5 "2012-11-16T21:07:44Z")
What do you mean by “unstir”? Couldn’t a centrifuge be used to separate the milk from the coffee? ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [November 16, 2012, 9:33pm UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/6 "2012-11-16T21:33:36Z")
> [@Jas09](#): > > What do you mean by “unstir”? Couldn’t a centrifuge be used to separate the milk from the coffee? I was thinking this or maybe some kind of filtration. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [November 16, 2012, 9:59pm UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/7 "2012-11-16T21:59:12Z")
The water in the milk would become the same as the water in the coffee. Maybe the solids could be centrifuged or filtered out. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [November 16, 2012, 10:15pm UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/8 "2012-11-16T22:15:12Z")
A spontaneous separation would require a physical world in which the separation of milk and coffee is energetically favored for some reason. In our world, this is the case with things like oil and water, but not milk and coffee. Oil and water separate not only because the densities are different, but because they are insoluble in each other; surface tension does the real work. There are lots of ways to force a separation of milk and coffee in our world, but all of these would require energy inputs. Some of them wouldn’t even give you coffee and milk. A centrifuge would give you several milk components and several coffee components, but I suppose you could recombine those to get your coffee and milk back (more or less). ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [November 16, 2012, 10:34pm UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/9 "2012-11-16T22:34:32Z")
Unstirring coffee is just difficult largely because of the chaotic, turbulent nature of stirring, but I don’t think it’s theoretically absolutely impossible. Also, you might find this interesting: [![](https://img.youtube.com/vi/p08_KlTKP50/hqdefault.jpg "Laminar Flow") ](https://www.youtube.com/watch?v=p08_KlTKP50) ---
**Author:** ![moriah](https://avatars.discourse-cdn.com/v4/letter/m/b2d939/32.png) [@moriah](https://boards.straightdope.com/u/moriah)\ **Post date:** [November 16, 2012, 11:44pm UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/10 "2012-11-16T23:44:27Z")
First, the two video links show a thick viscous liquid stretching and unstretching… not the same as mixing and unmixing. Second, even if the arrow of time was reversed, the arrow of entropy doesn’t reverse. Stephen Hawking famously postulated it did in his first book, but has since retracted that. So, there is no known physical universe where unmixing is possible where freedom of motion is allowed. When you disallow freedom by coming up with a sorting scheme to get objects back to their original state… then unmixing could be possible. But not by ever just stirring backwards or ‘inversely.’ ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [November 17, 2012, 12:11am UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/11 "2012-11-17T00:11:18Z")
Scientific American had something like this, a while ago. Basically, you have water that is in a shell between two cylinders. Like, fill a coffee can with water, then put a soup can in the water. Now enlarge the soup can, so it’s _almost_ as big as the coffee can. The water is in the shape of a thin cylindrical shell. Put a dot of ink somewhere in the water. Rotate the inner can. The dot of ink spreads out into a long broad triangle. Now rotate the can back again… The broad smear of ink de-smears back to a dot again. The point is only that in some constrained configurations, mixing of fluids can be (somewhat) reversible. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [November 17, 2012, 12:37am UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/12 "2012-11-17T00:37:25Z")
> [@HoneyBadgerDC](#): > > The water in the milk would become the same as the water in the coffee. Maybe the solids could be centrifuged or filtered out. You have a point. I was checking on the composition of milk and it appears that even if you were to remove all of the “solid” portions (fats and proteins) from milk, you’re still going to have a fluid full of dissolved chemical compounds (minerals, salts, sugars, vitamins, etc) that would be easily distinguishable from water. And coffee has a equivalent chemical composition. I can’t imagine a physical process that would remove the compounds from milk that wouldn’t remove the equivalent compounds from coffee. If you tried to remove everything that was distinguishable as milk from the liquid, you’d also end up removing everything that was distinguishable as coffee. You’d be left with nothing but hot water. So I think we have to explore chemical means. We need something that can distinguish between the milk compounds and the coffee compounds. I’m thinking either some genetically modified enzymes or nanotech devices. ---
**Author:** ![pseudotriton\_ruber\_ruber](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@pseudotriton\_ruber\_ruber](https://boards.straightdope.com/u/pseudotriton_ruber_ruber)\ **Post date:** [November 17, 2012, 12:54am UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/13 "2012-11-17T00:54:22Z")
Why go to all that trouble? Just buy another cup of black coffee, you cheapskate. Jesus. ---
**Author:** ![pseudotriton\_ruber\_ruber](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@pseudotriton\_ruber\_ruber](https://boards.straightdope.com/u/pseudotriton_ruber_ruber)\ **Post date:** [November 17, 2012, 12:55am UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/14 "2012-11-17T00:55:50Z")
hate to add so obvious a note, but I can never tell with the Mods sometimes. I am kidding. Post #13 is a joke. I’m not actually calling anyone a cheapskate. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 17, 2012, 2:30am UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/15 "2012-11-17T02:30:28Z")
> [@](#): > > Quoth **moriah** : > > Second, even if the arrow of time was reversed, the arrow of entropy doesn’t reverse. Stephen Hawking famously postulated it did in his first book, but has since retracted that. The arrow of time _is_ the arrow of entropy. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [November 17, 2012, 2:55am UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/16 "2012-11-17T02:55:02Z")
> [@Chronos](#): > > The arrow of time _is_ the arrow of entropy. Entropy of a closed system is the direction of time. Emphasize closed. You can reduce the entropy of a subsystem within a overall bigger system - but the overall entropy of the system keeps increasing ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [November 17, 2012, 3:57am UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/17 "2012-11-17T03:57:25Z")
> [@Little\_Nemo](#): > > You have a point. I was checking on the composition of milk and it appears that even if you were to remove all of the “solid” portions (fats and proteins) from milk, you’re still going to have a fluid full of dissolved chemical compounds (minerals, salts, sugars, vitamins, etc) that would be easily distinguishable from water. > > And coffee has a equivalent chemical composition. I can’t imagine a physical process that would remove the compounds from milk that wouldn’t remove the equivalent compounds from coffee. If you tried to remove everything that was distinguishable as milk from the liquid, you’d also end up removing everything that was distinguishable as coffee. You’d be left with nothing but hot water. > > So I think we have to explore chemical means. We need something that can distinguish between the milk compounds and the coffee compounds. I’m thinking either some genetically modified enzymes or nanotech devices. I may be wrong, but I do not think the intended question was “Is there any way to separate the milk components from the coffee components once they have been mixed?” The answer to that is clearly yes. Biochemists spend much of their time separating out the individual compounds from complex mixtures of organic chemicals, and they have gotten pretty good at it by now. It is likely to involve a long complicated set of procedures, but it ought to be possible. I think the OP is actually asking is what laws of physics would have to be different in order for the “unstirring” to be possible by relatively simple physical means (comparable in difficulty to regular stirring). As others have been saying, you would have to repeal the second law of thermodynamics, and, as I previously said, _that_ would entail repealing the atomic/molecular theory of matter. With that gone, it does not seem to me that you would have a universe in which stirring and mixing was possible in the first place. ---
**Author:** ![coremelt](https://avatars.discourse-cdn.com/v4/letter/c/4491bb/32.png) [@coremelt](https://boards.straightdope.com/u/coremelt)\ **Post date:** [November 17, 2012, 5:35am UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/18 "2012-11-17T05:35:15Z")
presumably in a world where coffee was a ferrofluid (magnetic) while milk wasn’t (or vice versa) then “unstirring” them would be pretty easy. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [November 17, 2012, 5:50am UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/19 "2012-11-17T05:50:24Z")
> [@coremelt](#): > > presumably in a world where coffee was a ferrofluid (magnetic) while milk wasn’t (or vice versa) then “unstirring” them would be pretty easy. Heck, why not have a world in which they are solids? If we also stipulate that in this world they come in fairly large chunks - lets say about pea sized - you could stir them together and then separate them with a pair of tweezers. ---
**Author:** ![coremelt](https://avatars.discourse-cdn.com/v4/letter/c/4491bb/32.png) [@coremelt](https://boards.straightdope.com/u/coremelt)\ **Post date:** [November 17, 2012, 5:53am UTC](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236/20 "2012-11-17T05:53:58Z")
> [@njtt](#): > > Heck, why not have a world in which they are solids? Funnily enough I have that world, its called “my freezer”. Nonetheless putting a cup of coffee in the freezer until its a solid chunk doesn’t help me separate them. [Next page](https://boards.straightdope.com/t/in-what-physical-world-would-you-be-able-to-unstir-milk-from-coffee-time-travel-is-not-allowed/641236.md?page=2)