Preliminary note: I have nearly zero economical background, I am a physicist and my question is probably very naive, but I would like a clear and simple explanation.
If I am not wrong, the global economy can be seen as an isolated system (ie we do not have any non-Earth economic exchanges). At any given time $t$, there is a given amount of money $M$ in the world. As worldwide economy is an isolated system, $M$ can vary only if it is created or destroyed within the system. To my knowledge, the only way to create money is by creating a debt at $t_{0}$ and when money is reimbursed at $t_{1}$, the money that was created out of thin air is destroyed. I see no problem when the interest rate is $0\%$, because globally the system stays balanced.
Now, if the interest rate is stricly positive, that means that the amount of money $|m_{1}|$ that will need to be reimbursed at $t_{1}$ is higher than the debt $|m_{0}|$ that was created at $t_{0}$. But the thing is that $dm = |m_{1}|-|m_{0}|$ does not exist in the total amount of money available worldwide at $t_{0}$, $M_{0}$. So for the debt to be reimbursed with the interests, $dm$ will need to be created. And the only way to create money... is by creating a debt... with interest. So the system diverges very rapidly with an ever growing debt.
The other way I can see to make the system stable is by inflation/deflation: if $dm$ exactly corresponds to the inflation/deflation rate, then the system can remain at equilibrium without an ever growing debt. The last way I can see to remain at equilibrium, is by crisis so that $dm$ is never reimbursed.
I would like an explanation in simple words about what is wrong about this naive view.
