# Computer + entropy thought experiment **URL:** \ **Category:** Great Debates\ **Created:** [September 12, 2008, 11:50pm UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365 "2008-09-12T23:50:50Z")\ **Posts on this page:** 15\ **Page:** 2
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [September 13, 2008, 11:51pm UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/21 "2008-09-13T23:51:33Z")
How many digits of pi would the computer need to drive Redjac out? ---
**Author:** ![Indistinguishable](https://avatars.discourse-cdn.com/v4/letter/i/90ced4/32.png) [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)\ **Post date:** [September 13, 2008, 11:54pm UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/22 "2008-09-13T23:54:03Z")
Let me put it another way: I don’t think there’s any entropic difference between a computer running a program to calculate the billionth prime and a plain-vanilla heat generator doing no computation. ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [September 14, 2008, 12:14am UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/23 "2008-09-14T00:14:01Z")
> [@The\_Tao\_s\_Revenge](#): > > Where on earth did I say I was adding energy from the outside? You mean the fans or whatever this supposed computer uses to keep from over heating? If so that’s an interesting assumption you made. > > The assumption I made was what ever kept it from over heating (for example fans) would be part of the computer’s total load. Sort of like how a car’s radiator runs off energy produced from the engine and not a really long extension cord you drag down the highway. Fans keep it from overheating by moving hot air out and cold air in. If there’s no cold air to grab from anywhere, there’s no cooling. Your suggestion implies that cold is air is coming in from somewhere–which is adding energy to the system from outside. ---
**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:** [September 15, 2008, 4:25am UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/24 "2008-09-15T04:25:20Z")
Just in case I’m not being clear, I do **not** believe the thought experiment is violating the 2nd Law. > [@Sage\_Rat](#): > > You can’t recycle heat without a temperature difference. I’ve wrestled with this and for a while I agreed but now I’m not sure. Say the computer is suspended in a vacuum (in the box). The CPU eventually gets so hot that it radiates the heat away as photons. The inside of the box is coated with 100%-efficient photoelectric cells and pumps the electricity back into the computer. Wouldn’t this be a way to reclaim the heat? (This would assume that energy is pumped into the box until the CPU gets hot enough to create an equilibrium between the energy used and the energy radiated.) > [@Indistinguishable](#): > > Let me put it another way: I don’t think there’s any entropic difference between a computer running a program to calculate the billionth prime and a plain-vanilla heat generator doing no computation. I hesitantly disagree. It seems like the computer is doing work but I admit I can’t quantify why I think so. If you are correct then my theoretical computer-in-a-box can run for a million years (assuming there is a way to recycle 100% of the energy) and I have a hard time believing that. ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [September 15, 2008, 4:26am UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/25 "2008-09-15T04:26:38Z")
> [@Deeg](#): > > Just in case I’m not being clear, I do **not** believe the thought experiment is violating the 2nd Law. Your thought experiment is violating the second law. Heat cannot be recycled with 100% efficiency. Period. ---
**Author:** ![Grumman](https://avatars.discourse-cdn.com/v4/letter/g/43a26b/32.png) [@Grumman](https://boards.straightdope.com/u/Grumman)\ **Post date:** [September 15, 2008, 4:42am UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/26 "2008-09-15T04:42:37Z")
> [@Deeg](#): > > If you are correct then my theoretical computer-in-a-box can run for a million years (assuming there is a way to recycle 100% of the energy) and I have a hard time believing that. Exactly. **Assuming there is a way to recycle 100% of the energy**. If you could build a break-the-laws-of-thermodynamics machine (a machine that can recycle 100% of its lost heat), then it is going to break the laws of thermodynamics (by running forever without the input of energy). ---
**Author:** ![Indistinguishable](https://avatars.discourse-cdn.com/v4/letter/i/90ced4/32.png) [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)\ **Post date:** [September 15, 2008, 4:47am UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/27 "2008-09-15T04:47:42Z")
> [@Deeg](#): > > If you are correct then my theoretical computer-in-a-box can run for a million years (assuming there is a way to recycle 100% of the energy) and I have a hard time believing that. If you have no difficulty believing that a bit-flipper or a direct electricity-to-heat machine can run for a million years (under the counterfactual assumption of 100% efficiency recycling of heat), why such difficulty believing that a prime calculator can do the same? ---
**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:** [September 15, 2008, 4:53am UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/28 "2008-09-15T04:53:24Z")
The 2nd Law says heat cannot be completely converted into _work_. If calculating primes is not work then theoretically the computer could continue running. Right? ---
**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:** [September 15, 2008, 5:30am UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/29 "2008-09-15T05:30:06Z")
> [@Sage\_Rat](#): > > Fans keep it from overheating by moving hot air out and cold air in. If there’s no cold air to grab from anywhere, there’s no cooling. Your suggestion implies that cold is air is coming in from somewhere–which is adding energy to the system from outside. That would be true if you had no heat source. The CPU is a heatsource. After air travels over it, it’s hotter then the air that hasn’t traveled over it. There’s your temp difference. ---
**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:** [September 15, 2008, 5:40am UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/30 "2008-09-15T05:40:06Z")
Strike that last post I don’t know what I was thinking with the not needing refrigeration track. That said dividing up the box into sections would work with refrigeration. Hot air into one chamber, cold into another. It still wouldn’t work because of the 2nd law, but before the power ran out it would still be possible to have warmer and cooler sections without outside intervention ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [September 15, 2008, 5:51am UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/31 "2008-09-15T05:51:59Z")
> [@The\_Tao\_s\_Revenge](#): > > That said dividing up the box into sections would work with refrigeration. Hot air into one chamber, cold into another. It doesn’t work like that. You’re using energy to split the air into hot and cold. The same hot/cold then needs to power both the refrigerator plus the computer, both of which are adding heat into the system. With heat **building** , you have to consecutively use more energy to cool it, which means even more energy use. Since the idea is for the device to be self-supporting, “more energy use” is outside of the realm of possibility. Energy exists as the differential between states, like hot and cold, high and low, or positive and negative charge. The OP is trying to posit a circumstance in which the energy of the differential being released not only powers returning the differential, but also does work along the way. That’s asking for more energy than there even was to begin with. ---
**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:** [September 15, 2008, 6:03am UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/32 "2008-09-15T06:03:27Z")
> [@Deeg](#): > > The 2nd Law says heat cannot be completely converted into _work_. If calculating primes is not work then theoretically the computer could continue running. Right? Computer runs on electron movement. That’s work. ---
**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:** [September 15, 2008, 6:10am UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/33 "2008-09-15T06:10:55Z")
> [@Sage\_Rat](#): > > It doesn’t work like that. You’re using energy to split the air into hot and cold. The same hot/cold then needs to power both the refrigerator plus the computer, both of which are adding heat into the system. With heat **building** , you have to consecutively use more energy to cool it, which means even more energy use. Since the idea is for the device to be self-supporting, “more energy use” is outside of the realm of possibility. > > Energy exists as the differential between states, like hot and cold, high and low, or positive and negative charge. The OP is trying to posit a circumstance in which the energy of the differential being released not only powers returning the differential, but also does work along the way. That’s asking for more energy than there even was to begin with. I never said op’s system was feasible. I likened it to plugging a power strip into it’s self. I called it magic up thread too. My only point is it is possible to have warmer and cooler regions in the box. ---
**Author:** ![Superfluous\_Parentheses](https://avatars.discourse-cdn.com/v4/letter/s/8edcca/32.png) [@Superfluous\_Parentheses](https://boards.straightdope.com/u/Superfluous_Parentheses)\ **Post date:** [September 15, 2008, 4:58pm UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/34 "2008-09-15T16:58:25Z")
> [@The\_Tao\_s\_Revenge](#): > > Computer runs on electron movement. That’s work. I think this related to what I consider the more interesting part of the question: if you have a running computer (disregard all moving parts, assuming it’s just doing random calculations); what happens to the energy you put in it? Is all the energy consumed converted to heat, or is some part of it lost anywhere else? If **all** of the consumed energy/electricity is turned to heat, then the answer to the OP would be: it would run forever (assuming some impossible way of turning the heat back to energy 100% efficiently). ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [September 15, 2008, 5:45pm UTC](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365/35 "2008-09-15T17:45:25Z")
> [@Superfluous\_Parentheses](#): > > Is all the energy consumed converted to heat, or is some part of it lost anywhere else? I suppose some minuscule fraction might escape as light or vibration, but the heat generated by computation is enough of a problem that an entire cooling industry grew up alongside the advent of computers. Not only is the energy converted to heat, it’s converted so readily to heat that chips must be cooled or they destroy themselves. [Previous page](https://boards.straightdope.com/t/computer-entropy-thought-experiment/463365.md?page=1)