# Why can't perpetual motion machines work in space? **URL:** \ **Category:** Factual Questions\ **Created:** [August 11, 2002, 11:35pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447 "2002-08-11T23:35:11Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [November 28, 2012, 11:54am UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/21 "2012-11-28T11:54:50Z")
> [@FilthytheBear](#): > > What if say we build a ramp and put a magnet at the top. At the base of the ramp we have what looks like a roller with an arm sticking out the side. Lets say the magnet pulls the roller up the ramp and the arm moves past and ticks over a cog, (a dynamo) then the roller drops down a trap before the magnet and rolls down a slide to appear back at the bottom of the ramp to then start the process over again. If the magnet was just the right strength, surely it would be able to pull the roller up the ramp while allowing it to fall down the trap, especially if you build the base of the ramp from mumetal or something that will shield from the magnetic field to an extent. You can get a magnet with just the right strength to pull the roller up the ramp while allowing it to fall down the trap, but not one that doesn’t also prevent the roller from going down your slide. And you can’t shield the magnetic field while leaving openings for the roller to move. ---
**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 28, 2012, 1:06pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/22 "2012-11-28T13:06:14Z")
> [@naita](#): > > You can get a magnet with just the right strength to pull the roller up the ramp while allowing it to fall down the trap, but not one that doesn’t also prevent the roller from going down your slide. Correct - the strength of magnetic fields is inversely proportional to the cube of the distance from the magnet - so if it’s powerful enough to lift the magnet up the ramp, it’s too poweful to let go at the top. ---
**Author:** ![The\_Other\_Waldo\_Pepper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_other_waldo_pepper/32/12370_2.png) [@The\_Other\_Waldo\_Pepper](https://boards.straightdope.com/u/The_Other_Waldo_Pepper)\ **Post date:** [November 28, 2012, 1:40pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/23 "2012-11-28T13:40:16Z")
> [@Mangetout](#): > > Correct - the strength of magnetic fields is inversely proportional to the cube of the distance from the magnet - so if it’s powerful enough to lift the magnet up the ramp, it’s too poweful to let go at the top. Could you use an electromagnet to pull the roller up the ramp, with the roller helpfully clicking an “off” button as it nears the top? ---
**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 28, 2012, 1:56pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/24 "2012-11-28T13:56:04Z")
> [@](#): > > Could you use an electromagnet to pull the roller up the ramp, with the roller helpfully clicking an “off” button as it nears the top? No reason you couldn’t. But then you need some other power supply for your electromagnet. ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [November 28, 2012, 2:34pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/25 "2012-11-28T14:34:56Z")
And I am certain (before even running any numbers) that the power requirement for that electromagnet would be higher than the energy you could get out of the roller. ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [November 28, 2012, 4:43pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/26 "2012-11-28T16:43:13Z")
> [@mangeorge](#): > > I think every youngster ponders this. My solution was simple. Take an electric motor, connect the shafts and wires, and start spinning. Pretend the losses don’t exist. > Welcome to here, **Alietta**. 🙂 > Peace, > mangeorge Very eloquent solution. My [spherical cow](http://en.wikipedia.org/wiki/Spherical_cow) concurs. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [November 28, 2012, 4:47pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/27 "2012-11-28T16:47:21Z")
The roller machine - using gravity or magnets or whatever - has been invented hundreds of times. It works until it stops (which could be the very first trial). If you want to read why it can’t work. If you want to read every possible variation of attempts at perpetual motion. If you want to see lovingly detailed drawings of 2000 years of perpetual motion machines. If you want to know anything at all on the subject, take a look at [Perpetual Motion: The History of an Obsession by Arthur W.J.G. Ord-Hume](http://www.amazon.com/Perpetual-Motion-Obsession-Arthur-Ord-Hume/dp/1931882517/ref=sr_1_13?s=books&ie=UTF8&qid=1354120933&sr=1-13&keywords=perpetual+motion+machine) That gives a later but cheap to buy edition. He actually wrote it several decades ago. Everything else I’ve read on perpetual motion blatantly steals from this book. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [November 28, 2012, 4:51pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/28 "2012-11-28T16:51:15Z")
Now if you’re talking about (wiki): a more obscure category is a perpetual motion machine of the third kind, usually (but not always)[5] defined as one that completely eliminates friction and other dissipative forces, to maintain motion forever (due to its mass inertia). Third in this case refers solely to the position in the above classification scheme, not the third law of thermodynamics. Although it is impossible to make such a machine,[6][7] as dissipation can never be 100% eliminated in a mechanical system, it is nevertheless possible to get very close to this ideal (see examples in the Low Friction section). Such a machine would not serve as a source of energy but would have utility as a perpetual energy storage device. Then yes, that’s theoretically possible in space, or at least a machine that would run for centuries. Wouldn’t do anything. ---
**Author:** ![The\_Other\_Waldo\_Pepper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_other_waldo_pepper/32/12370_2.png) [@The\_Other\_Waldo\_Pepper](https://boards.straightdope.com/u/The_Other_Waldo_Pepper)\ **Post date:** [November 28, 2012, 5:11pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/29 "2012-11-28T17:11:44Z")
> [@DrDeth](#): > > Such a machine would not serve as a source of energy but would have utility as a perpetual energy storage device. > > Then yes, that’s theoretically possible in space, or at least a machine that would run for centuries. Wouldn’t do anything. Hmm. The moon circling the Earth presumably counts, and IIRC it’s doing something to the tides… ---
**Author:** ![black\_rabbit](https://avatars.discourse-cdn.com/v4/letter/b/f19dbf/32.png) [@black\_rabbit](https://boards.straightdope.com/u/black_rabbit)\ **Post date:** [November 28, 2012, 5:18pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/30 "2012-11-28T17:18:05Z")
What if instead of a ramp, you use a treadmill, and instead of a ball, you use an airplane? ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [November 28, 2012, 5:18pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/31 "2012-11-28T17:18:14Z")
> [@The\_Other\_Waldo\_Pepper](#): > > Hmm. The moon circling the Earth presumably counts, and IIRC it’s doing something to the tides… Which is why the Moon’s orbit is not 100% stable. [Wikipedia](https://en.wikipedia.org/wiki/Orbit_of_the_Moon#Tidal_evolution)says: > [@](#): > > The Moon is gradually receding from the Earth into a higher orbit, and calculations[9][10] suggest that this would continue for about fifty billion years. By that time, the Earth and Moon would become caught up in what is called a “spin–orbit resonance” or “tidal locking” in which the Moon will circle the Earth in about 47 days (currently 27 days), and both Moon and Earth would rotate around their axes in the same time, always facing each other with the same side. ---
**Author:** ![robert\_columbia](https://avatars.discourse-cdn.com/v4/letter/r/e79b87/32.png) [@robert\_columbia](https://boards.straightdope.com/u/robert_columbia)\ **Post date:** [November 28, 2012, 6:28pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/32 "2012-11-28T18:28:10Z")
> [@Sam\_Stone](#): > > The key thing is that it can’t do any work. If work is done, either on other particles through friction, or through heating by spinning in a field or whatever, then it must eventually stop. This. In theory, a fully weightless object in space that is currently spinning should spin forever. In reality, no object exists in full isolation from the rest of the universe, and can and will experience forces from elsewhere. True, these forces might not be significant enough to visibly affect the object’s behavior this afternoon or even in your lifetime, but as the eons pass it will eventually have a significant effect. Second, the moment you try to extract some of the energy, for example by trying to use the motion to drive an electric generator, power an elevator, or generate heat to cook a meal, you will convert some of the energy the spinning object has and it will slow down or otherwise be affected. Effectively, by drawing energy from the system you literally _take energy_ from the system and that energy is no longer there (it went into your burger). ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [November 28, 2012, 6:31pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/33 "2012-11-28T18:31:44Z")
> [@DrDeth](#): > > Then yes, that’s theoretically possible in space, or at least a machine that would run for centuries. Wouldn’t do anything. And that’s the beauty of it! ---
**Author:** ![user\_hostile](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/user_hostile/32/474_2.png) [@user\_hostile](https://boards.straightdope.com/u/user_hostile)\ **Post date:** [November 28, 2012, 6:37pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/34 "2012-11-28T18:37:40Z")
What about superconducting loop? Not that they provide any “free” energy, but presuming an ideal location with no outside interference, would the current keep traversing forever? Or, are there subtle things that happen at the quantum level the will cause the current to wane over time? ---
**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 29, 2012, 1:42am UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/35 "2012-11-29T01:42:47Z")
“Superconducting” is fine, but “loop” is a problem. In order for it to be a loop, the wire has to curve, and if the wire is curved, the charges accelerate, and if the charges accelerate, they radiate away energy. They’ll lose energy very slowly, if the loop is very large, but they’ll still lose it eventually. ---
**Author:** ![Flyer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/flyer/32/7298_2.png) [@Flyer](https://boards.straightdope.com/u/Flyer)\ **Post date:** [November 29, 2012, 6:46am UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/36 "2012-11-29T06:46:50Z")
It’s very easy to build a perpetual motion machine–as long as it’s battery powered. 😃 ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [November 29, 2012, 9:10am UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/37 "2012-11-29T09:10:43Z")
> [@Mangetout](#): > > Correct - the strength of magnetic fields is inversely proportional to the cube of the distance from the magnet - so if it’s powerful enough to lift the magnet up the ramp, it’s too powerful to let go at the top. I don’t think this is correct. You are assuming the force needed to roll a ball up a ramp is the same as the force needed to stop a ball dropping vertically, and that patently isn’t so. We’ll ignore friction for the moment, and let’s imagine a ramp the lowest end of which we shall call point A. Your magnet you fix at a point being distance D up the ramp. You have an iron ball upon which gravity exerts a vertical force W. The magnet exerts an attractive force on the ball, which we shall call M. The force required to cause the ball to roll up the ramp we will call R. I think you will agree that due to the mechanical advantage offered by the ramp (at any angle other than vertical) R must be less than W. You set your ramp angle such that at distance D the force M is only just greater than R so that the ball will (only just) roll up the ramp. Such an angle must exist, since as the ramp angle decreases from vertical toward horizontal, the component of force W that the magnet has to overcome to cause the ball to go up the ramp reduces towards zero. Now, as the ball goes up the ramp force M will increase as you say. At the point where M approaches but does not exceed W, you cut your drop slot. When the ball reaches the slot it will fall because now that the ball is exposed to gravity without the ramp R is no longer relevant, and M can no longer use the mechanical advantage of the ramp to assist it in overcoming W, and so W will overcome M and the ball will fall. You could then have a return ramp leading away from the slot, down and then out back to point A. Obviously for the return ramp to end up back at point A, and given that the slot must allow the ball to fall vertically initially, to get away from the magnet the latter parts of the return ramp would have to be a shallower angle than the up ramp. The ball will have gained potential energy by being lifted up by the magnet. The problem however is that as the ball moves down and out via the slot and the return ramp it will be fighting the pull of the magnet. And the work required to fight that fight will precisely equal the potential energy gained by going up the ramp. In a totally frictionless environment, the ball might go round and round forever, but in reality it would fall down the slot but then slow and not make it back out to Point A because, due to friction losses, the speed gained by falling down the slot would not be enough to overcome the pull of the magnet as the ball rolled out on the return ramp. And if you made the ball do any work (such as pushing a lever) then all the more so. In essence, a simpler model of the same thing is this: anchor a mega magnet on a perfectly flat frictionless plane. Around the mega magnet, place a perfectly elastic bumper. Put an iron ball on the plane within the magnetic field. It will roll in, and hit the bumper. Despite the (necessarily) stronger magnetic field near the bumper, the kinetic energy of the rolling ball will permit it to bounce away from the magnet (just as the potential energy of the ball up the ramp permits it to get away from the magnet). The iron ball will just bounce out a way, then back in to the bumper endlessly. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [November 29, 2012, 9:30am UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/38 "2012-11-29T09:30:12Z")
Gah, misplaced comma. The last sentence in the fourth last para should read: "Obviously for the return ramp to end up back at point A, and given that the slot must allow the ball to fall vertically initially to get away from the magnet, the latter parts of the return ramp would have to be a shallower angle than the up ramp ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [November 29, 2012, 9:40am UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/39 "2012-11-29T09:40:49Z")
Even a rare earth magnet, suspended in a perfect vacuum, centered and spinning in place surrounded by—but not touching—a coiled spool of copper wire would impart an electromagnetic force on the coiled wire (i.e. moving electrons along the wire, producing electric current), and eventually resistance will slow the magnet down to a stop. The laws of thermodynamics are a bitch. As they say: _Zeroth: You must play the game. First: You can’t win. Second: You can’t break even, except on a very cold day. Third: It doesn’t get that cold, even in Space._ ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [November 29, 2012, 4:21pm UTC](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447/40 "2012-11-29T16:21:23Z")
What about [wormholes](http://en.wikipedia.org/wiki/Wormhole)? Suppose you constructed a wormhole, such that the exit is a few metres above the entrance. Pour some water into the entrance. It comes out of the exit, falls a few metres, and goes back into the entrance, comes out of the exit, &c. As the water falls, it can turn a turbine, thus creating energy from nowhere. I know this is \*slightly \*beyond current technological ability to create, but is there anything wrong in principle? [Previous page](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447.md?page=1) [Next page](https://boards.straightdope.com/t/why-cant-perpetual-motion-machines-work-in-space/123447.md?page=3)