Could Eru change the very laws of nature if he wanted to?

[Inziladun]

The passage from the letter quoted by Morthoron seems to me to reflect the idea that Men in Middle-earth did not, in fact know about the Straight Road, or the means by which Elves found it.
However, another quote from the same letter says:

The 'immortals' who were permitted to leave Middle-earth and seek Aman - the undying lands of Valinor and Eressëa....set sail in ships specially made and hallowed for this voyage....They only set out after sundown; but if any keen-eyed observer from that shore had watched one of these ships he might have seen that it never became hull-down but dwindled only by distance until it vanished in the twilight; it followed the straight road to the true West and not the bent road of the earth's surface.

Clearly, a special circumstance beyond "nature" in Arda had removed the Undying Lands from the physical plane. That much is certain, though the specifics of the situation would indeed logically have been a matter of dim legend to the majority in Middle-earth.

[jallanite]

Originally Posted by Inziladun

Clearly, a special circumstance beyond "nature" in Arda had removed the Undying Lands from the physical plane. That much is certain, though the specifics of the situation would indeed logically have been a matter of dim legend to the majority in Middle-earth.

It is not certain that removing the undying lands from the physical plane was “a special circumstance beyond ‘nature’” unless we have set forth a complete set of rules that comprehend all of nature. Tolkien does not provide any such complete set of rules for the world he writes about and we have none for the real universe either. What we have are apparent norms, by which for example people cannot fly by their own power.

That at one period anyone could reach the undying lands by sailing west and after the destruction of Númenor only Elves can, for the most part, is simply data about Tolkien’s fictional world, and indicates nothing about whether the accessibility of the Undying Lands was either natural or unnatural.

See https://en.wikipedia.org/wiki/Miracle which points out differences in the understanding of miracles, some seeing them as acts beyond the laws of nature and others seeing them as acts within the laws of nature, and many dubious that any so-called miracles ever occurred.

Now whether an other-dimensional gateway (to use science-fictional terminology) in the mid-Atlantic to allow access to the Undying Lands was supposed to be natural, using laws of nature not known to us, or is supposed to be unnatural, a breaking of the laws of the universe, is not stated in Tolkien’s text.

Tolkien in originally writing The Book of Lost Tales imagined a simple flat-earth cosmology. This appeared in the original Hobbit (in 1937) which stated in chapter 8:

In the Wide World the Wood-elves lingered in the twilight before the raising of the Sun and Moon; and afterwards they wandered in the forests that grew beneath the sunrise.

In 1966 Tolkien changed this to a version in which the cosmology fits our universe with no late “raising of the Sun and Moon”:

In the Wide World the Wood-elves lingered in the twilight of our Sun and Moon, but loved best the stars; and they wandered in the great forests that grew tall in lands that are now lost.

The late “raising of the Sun and Moon” now becomes only a feature of the incorrect supposed Mannish Silmarillion text.