What makes you think that owning a physical object is "a natural property of the physical world"?
Owning a physical object isn't like gravity, mass, or distance. 'Ownership' isn't something that exists 'out there', enforced by the physical world. The physical world doesn't know or care about your relation to items you consider your property.
What does this scenario suggest:
Suppose that you own a piece of land. That is, you consider that physical object, the parcel of land, to be your property. Now suppose that someone else shares a similar view. Namely, they also consider that land to be their property. How is this conflict resolved? How do we determine the real owner of the property?
Well, we don't consult the physical world. The natural properties of the physical world have nothing to tell us about any ownership claim on the property. Typically, in the modern industrial world, the the validity of the ownership claims are adjudicated by a court of the land.
Now suppose that (after a trial that follows the laws of the land fairly) the court decides in favor of the other claimant; The court rules that you are not the owner of the land, the other person is. Do you still own this land? If you disagree (and exhaust all appeals without success) and remain on the land, because, in your mind, this is really your land, what will happen next? Will you be able to keep this property which you know is yours? And after the government branch (the court) assigns the police or another governmental agency to evict you from the land, and you are now elsewhere because you have been forcefully removed from your land by the government, is this land still your property? Or is it now the property of the other claimant, who is living on the land with the full support of the government, and perhaps thanks to the assistance of governmental force?
The 'ownership' of this physical object appears to be a social convention, perhaps not all that different from the social convention we live by that says that the government issued piece of paper that has a five dollar bill printed on can be traded for five times the value of a similar piece of paper that has a one dollar bill printed on it. As others in this thread have said, the notion of property appears to be a construct that we live by. Some perhaps unknowingly. More than that, it is a government regulated and enforced construct. One might even say that it is a 'monopoly' of sorts - the government has granted you sole ownership of the property, to do with it as you see fit. And if that ownership is contested or violated, the government will lend you its bodies of force in order to enforce your ownership claim.
It could be disputed that calling this kind of property ownership a 'monopoly' is not accurate - that 'monopoly' typically refers to a 'supplier' of 'commodities' rather than an 'owner' of 'property'. Investigating those ideas further would no doubt provide further insight into the similarities and differences between these uses of the term 'monopoly'. Regardless, what is clear at this point is that the two uses of 'monopoly' share a useful family resemblance and, in particular, the statement that "private property is a government enforced monopoly" appears to be closer to the reality than the statement that "Owning a physical object is a natural property of the physical world around us."
