How does the "clock" know when to slow down? Screwy question, I know, but the following thought experiment might help to make clear what I'm asking. You and a clock are in a box. You can't see outside the box. You have no external references. Inside the box you appear to be in free fall, i.e. no forces acting on you that you can detect. From a God's eye point of view you and your little box are speeding through an empty part of space at constant speed, and your clock is ticking away at a constant rate. (Not that you can tell that from inside the box, but that's what's going on). Unbeknownst to you as you sit in your little box (possibly watching old Seinfeld re-runs, or making a sandwich), your little box begins to pass within range of a steep gravity well. So now your box starts to fall toward the gravity well. As it does, you start to pick up speed, and as you pick up speed your clock begins to tick more slowly (not that you can tell, since time is slowing for you as well). But the pertinent fact of the matter is that you clock IS starting to tick more slowly. Just because you can't tell it's ticking more slowly doesn't change the fact that it is. So if there's no such thing as an absolute reference frame, how does your clock know enough to start ticking more slowly?