In the Preface to his The Great Divorce (London, 1982), CS Lewis refers to an sf story in which a traveller to the past (Lewis' emphasis) was pierced by bullet-like raindrops and unable to bite sandwiches:
"...because, of course, nothing in the past can be altered." (p. 9)
Lewis describes this as a "proper" state of affairs whereas, reading his book in the 1960s, I thought that it was nonsense. I have become more tolerant since then but it remains necessary to clarify our propositions.
"If you have gone into the past, you have made it your present."
-Poul Anderson, "The Year of the Ransom" IN Anderson, Time Patrol (New York, 2006), pp. 641-735 AT 3 November 1885, p. 671.
Anderson is right. Every moment is present to anyone in that moment, future to anyone in earlier moments and past to anyone in later moments. Of course, there is still a discussion about whether a time traveller can change the past, having made it his present. However, the story referenced by Lewis invites us to imagine that past events retain their unchangeable pastness even to someone whose "time travel" has enabled him to perceive them. This is an interesting idea, discussed here, but should be called something other than "time travel."
2. The Immutability of Past EventsPast events, even when visited from the present, remain past and therefore cannot be affected by anyone from the present. Therefore, pastwards time travellers remain invisible, intangible and inaudible when in the past, not just when travelling to or through it. They are like ghosts from the future. Unfortunately, this is time-viewing, not time travel.
-copied from the linked article.