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If a human somehow managed to change their body in such a way that self sustaining was possible and they did not need any outside sources to maintain themselves, does that mean that they cannot grow and or change?

The best example i can think of to compliment this is could a 'Skinny Guy' who changed his body to somehow create everything it needed, internally and did not rely on any outside sources, could he workout like a normal human and perhaps become a 'Buff Guy' or would the form he achieved self-sustainment with be permanent and he could never change due to the body constantly producing everything it needs to maintain its current form?

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    $\begingroup$ As answered already, you can't really do this without magic; physics simply does not allow it. However, it is in theory possible for a human to only require sunlight to live or something like that. In such a case, what you are proposing should work, although to gain muscle mass your self-sustaining human would still need to eat some food for raw materials (they may be able to survive indefinitely without food, but they would still need to eat to grow bigger). $\endgroup$ – trevorKirkby May 16 at 7:03
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Probably not. According to the laws of physics (in this case, the Second Law of Thermodynamics) any exchange of energy leads to the amount of entropy being increased. In order for humans to function, energy needs to be exchanged within the human body. (The usual reaction is ATP -> ADP + P.) The system you're proposing must violate that law. It is not possible within physics, unless magic is added.

The reason why it's not possible is because you also must violate the law of conservation of mass, or in other words, something cannot be created from nothing. (Technically, mass is energy, but the cost to transfer the two is incalculably high.) So, 'Skinny Guy' requires mass to turn into 'Buff Guy'. Most of what he needs is in the air he breathes, carbon, nitrogen, hydrogen, oxygen, but some elements, like iron, phosphorus, sodium, etc. aren't. So, probably not, unless you add more magic.

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If a human somehow managed to change their body in such a way that [...] they did not need any outside sources to maintain themselves

Thermodynamic is a harsh mistress, and tells that in isolate systems entropy can only grow over time.

Life is all about reducing entropy.

Thus a system which is isolated (does not exchange matter and energy with the outside) cannot be live. Your human cannot exist.

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Let's answer your question from another point of view

Let's assume that we have changed the human body such that at that first moment it was capable of being entirely self sustaining. What are a few of the problems?

Weight gain/loss Which happens for more reasons than eating/not-eating too many carbs. As you exercise, fatty tissue is lost and muscle mass increases. Your self-sustaining body could, perhaps, do this perfectly — right up until it needs to add more muscle mass than it has fatty tissue to dissolve/consume. The change in mass from that first moment must come from, or go to, somewhere. The loss is relatively easy to explain, it's the gain that's the problem.

Skin cells Some of the dust in your house comes from you! Dead skin cells that flake off. There's also dead hair, clipped finger/toe nails, lost eye lashes, sweat... the problem here is that they're lost to the self-sustaining system, which means the mass to recreate them is coming only from the internals of the body. It would be a great way to stay thin, but you'd die early due to lack of mass to continue replacing skin cells.

Heat loss Your body is a furnace. Simplistically, it creates heat to stay warm, to move, to breathe, to do pretty much everything. The unfortunate problem with heat (which others have discussed), is that it requires some form of combustion (chemical reaction) that's destructive. You can't even come up with a "perfectly recycled" heat system because the amount of heat generated must change depending on what the body is doing.

Aging We start young and small, add mass as we grow to maturity, then it's a bit of a roller coaster until the end. During that time you work through two sets of teeth, become injured and slowly heal, the length of your bones changes...

And we're going to stop there, because that one word is the fundamental problem: change. As the immortal (and self-sustaining) Data once said:

My legs are exactly eighty-seven point two centimetres in length. They were eighty-seven point two centimetres the day I was created. They will be eighty-seven point two centimetres the day I go off line. My operation depends on specifications that do not change. (Source)

The only way to be 100% self-sustaining is for nothing, absolutely nothing, to change. If your perfect robot climbs a mountain, the energy to do that must come from somewhere. And something was destroyed to provide it.

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No, they wouldn’t be able to add any mass unless they take in an equal or greater amount of mass or energy. E might equal MC^2, but the law of conservation of matter is still a thing that exists.

However, unlike some of the other answers, such a thing would be theoretically possible, just probably not for very long. There are some species of insect that, upon their metamorphoses to their adult forms, lose their mouthpieces, and then die shortly thereafter due to running out of energy - their whole purpose once they become adults is to have sex and lay eggs.

It would be theoretically possible to engineer humans to work similarly, with an adult form that no longer requires sustenance, and their lifespan determined by the amount of stored energy that they possessed when the transition occurs. With sufficiently high tech this might be quite some time - fusion reactors can run a long time on a surprisingly small amount of hydrogen.

However, such humans would not be thermodynamically closed systems- they would still be able to use the air to cool themselves off, and thereby reduce their entropy by increasing the entropy of their environment.

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