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All organisms on Earth produce waste. Whether an organism's respiration has a byproduct, or it just doesn't process all food it consumes, it will always put some amount of matter into the environment after consumption.

Would this have to be true for an alien? i.e.

Could an alien organism choose and process food so efficiently that only excess energy left its body? Is this biologically possible?

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    $\begingroup$ it's not physically possible unless it's an entirely closed system. $\endgroup$
    – ratchet freak
    Commented Oct 17, 2016 at 10:36
  • $\begingroup$ @ratchetfreak do we not have an equation that, under the right circumstances, could explain this? (e=mc^2) $\endgroup$
    – Zxyrra
    Commented Oct 17, 2016 at 10:40
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    $\begingroup$ But you can only use it without waste if you work with matter / antimatter reactor. Not feasible in living being. $\endgroup$
    – Mołot
    Commented Oct 17, 2016 at 10:55
  • $\begingroup$ Fungi never poops, but I think that's about about as close as your gonna get with a biological creature $\endgroup$
    – Amoeba
    Commented May 2, 2018 at 4:48

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It's not about choice!

Evolution "chooses" the minimal cost of living. Waste disposal is costly. So on Earth organisms that create less waste are preferred, "chosen" already.

Why "no waste" is not plausible?

  1. Feeding and breathing. To get energy we turn substances with high chemical potential and turn them into substances with low potential. You need to discard used ones. Unless you can recharge using direct energy like light. Sadly, all ways to biologically turn light into energy are not really high power. Not enough watts from human skin area to power up one human. So you would need to handwave it, or came up with "fueling" unknown in nature.

  2. Feeding and growing. Only way to have no waste is to only allow precisely what you need in. You'll need to have food that matches composition of your aliens. Exactly. Possible? Maybe. Believable? No.

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  • $\begingroup$ Even charging via direct energy like light typically has waste. Remember, the waste product from photosynthesis is Oxygen. $\endgroup$
    – Azuaron
    Commented Oct 17, 2016 at 14:56
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    $\begingroup$ @Azuaron yes, I know, but we are talking about hypothetical aliens, so this might, in theory be untrue for them. And oxygen created with photosynthesis may be used later to "burn" sugars for energy. Hard to balance it perfectly, but not totally impossible. Only very, very unlikely. $\endgroup$
    – Mołot
    Commented Oct 17, 2016 at 15:11
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Not really possible, but it depends how much you are willing to bend what you're calling an organism.

Hypothetically you can have an organic creature that feeds off electricity, charges it's system the same way batteries do and release no waste in that way. But you're not going to get that in a pure organic world unless you somehow a highly charged atmosphere and even then it's questionable. And even then you're then looking at a whole host of adaptations to make that work for a naturally evolved creature.

But if you're open to more of a robotic organism which might arise from man creating them, then sure, but I doubt that's what you mean. And even then you're not looking at just energy we also create by-products such as sweat, which aren't wastes, to keep our bodies lubricated and such which all organism whether robotic or not would have to do.

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