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According to this wiki entry, the PASIV device has a battery life of 200 hours. Multiplied by the time stretching factor of 12 given here, that equates 100 days, or using the factor of 20 for strong sedation, about half a year. No matter how many layers deep you go, when the battery in the previous layer runs out you should be out of dream-time, so how can Cobb claim they'll have up to 10 years in the third layer when the 2nd layer battery wouldn't allow for any more than half a year?

Why don't the batteries run out?

Bonus question: Assuming there's an external power supply or the like, how could Cobb and Mal enter Limbo? (related to this question)

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Zommuter
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  • Half a year times 20 is 10 years, so that seems about right to me (give or take a layer). – Ixrec Jul 19 '15 at 20:28
  • Possible duplicate of http://scifi.stackexchange.com/questions/36021/is-there-an-actual-time-calculation-in-inception?lq=1? – Valorum Jul 19 '15 at 20:30
  • @Richard I'm more concerned with battery life than the maths involved – Zommuter Jul 19 '15 at 20:32
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    You need to make that a lot clearer. Why do you think the battery would run out if it's only running for ten hours? – Valorum Jul 19 '15 at 20:33
  • @Richard The one in "reality" (or not), sure. But in the van layer, the dreamt device would run for 200 hours given the time stretching of 20, which works, but the hotel layer's device would work for 4000 hours, i.e. longer than the battery lifetime – Zommuter Jul 19 '15 at 20:36
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    @Zommuter - Longer than the battery in the real world. What makes you think the PASIV machine in the dream obeys the same rules? – Valorum Jul 19 '15 at 20:42
  • @Richard Good question. Maybe I'm dreaming right now and that's why I didn't question dream physics XD (Unfortunately, it's been a while since my last lucid dream) – Zommuter Jul 19 '15 at 20:42

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As far as the batteries in the PASIV machines in the dream layers are concerned, those are inventions of the dreamers and therefore don't need to obey real-world physics.

They can simply dream them with unlimited batteries in the same way that the Arthur dreamed the impossible 'penrose' staircase.

We actually have two pretty good (in-universe) examples of this;

  • Eames makes a Milkor grenade-launcher appear out of thin air. This launcher actually has a bigger chamber (10 grenade capacity) than its real-world counterpart.

  • The dream-guns used by the team appear to never run out of bullets or need reloading. They seem to just pull the bullets straight out of hammerspace as required.

Obviously, this doesn't apply to the real world PASIV (on the airplane), but that's only got to run for 10 hours in realtime, barely 5% of its charge.

Valorum
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  • "Replacement" sounds like downtime to me, though. And that makes me circle back to my previous question, wondering who'd have replaced 2nd and 3rd layer batteries... – Zommuter Jul 19 '15 at 20:36
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    Yes, but it's the machine on level 2 whose batteries need to last for a long time, not the one on level 0 where the airline hostess is. It would have to be Arthur who changed the batteries, and I don't see him toting around a bag with twenty changes of battery in it. However, as I said in my answer, there's no reason to expect that the dream PASIV device on level 2 has the same battery life as the real device on level 0. – Mike Scott Jul 19 '15 at 20:37
  • Ah, good point. Aw, dream physics and their lack thereof... – Zommuter Jul 19 '15 at 20:42
  • related answer: http://scifi.stackexchange.com/a/1579/769 – Zommuter Jul 19 '15 at 20:48
  • But the speed of perceptions and thoughts in dreams still depends on the physical brain in the real world, so logically there could be limits to how much of a time difference there can be with reality no matter how many dream machines you imagine...I think your deleted answer giving evidence that successive layers ran faster and faster relative to the real world was a better one, and I had thought there was some line to this effect in the final film as well, not just the script draft you posted. – Hypnosifl Jul 19 '15 at 21:14
  • @Hypnosifl - Yes, but that doesn't really answer the question asked. – Valorum Jul 19 '15 at 21:21
  • @Richard -Oh, I missed that the question was asking about fantasy PASIV machines within a dream layer, as opposed to the machines back in the real world. – Hypnosifl Jul 20 '15 at 01:21
  • @hypnosifl - It took me a while (and two answers) to wrap my head around this one :-) – Valorum Jul 20 '15 at 05:42
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If Eames can dream up a bigger and better gun on level 1, then I don't see why Arthur can't dream up a PASIV device with a much longer battery life on level 2. It's a dream, and real-world physics and limitations don't apply. It's only the machine in the real world that is restricted by the manufacturer's specification, and the batteries in that machine only need to last for a few hours.

Mike Scott
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  • Good point, dreams mess up with reality a lot. Though in my personal experience, they screw up electronics pretty badly, even in lucid dreams which the shared dreams resemble a bit, so I wonder how the dreamt devices could work reliably at all... – Zommuter Jul 19 '15 at 20:38
  • @Zommuter That would be a separate question. – Mike Scott Jul 19 '15 at 20:40
  • I wonder if that's worth asking - the answer is probably simply that is suffices if the dreamer fills the gap in "there's a PASIV device with huge battery improvement" no matter how messed up it blinkenlights appear to be – Zommuter Jul 19 '15 at 20:45
  • related answer: http://scifi.stackexchange.com/a/1579/769 – Zommuter Jul 19 '15 at 20:48