It’s often asserted that there’s a contradiction between Special Relativity and our experience of living in the present moment. Einstein made a few statements like that, and it seems many physicists assume that if the theory is correct, this very basic feature of our awareness of the world must somehow be illusory. I don’t believe there’s any such contradiction. And I think it’s worthwhile trying to clear up this misunderstanding, because it leads to confusion about the structure of spacetime. What Relativity tells us is that there is no universal, absolute definition of simultaneity between distant events. There’s no unique way to identify the moment I’m experiencing now with a specific moment in the experience of an observer on Mars. There’s no unique spacelike hypersurface extending throughout the universe, defining a single “present time” that’s simultaneous everywhere. But does that have anything to do with the now I experience? Obviously no one has ever experienced a set of spacelike-separated events. What Relativity tells me about the present time I actually experience is that it consists of local events, and events that are equidistant from me in time and space, i.e. “on my past light-cone.” Simultaneity is not a significant issue, if we’re trying to understand the nature of the physical present moment. The issue of whether two events are simultaneous is the same whether those events are happening now or at any other time. My present moment as I write this, and yours as you read it, are clearly not simultaneous. But that hardly means neither is real, or that there is no such thing as the present moment, for me or for you, or for anything else in the world. It just means our present moments are interconnected in spacetime in a very different way than we’re used to imagining. It makes no more sense to say that my now is “subjective” – only existing “in my head” – than it does to say that my location here in space is subjective. If there’s any illusion involved here, it’s in the classical notion that the whole universe exists “all at once,” and “moves through time” as a single vast object. Relativity tells us, this is not how space and time work. But we still tend to picture the world that way, despite its evident inaccuracy. It’s easy for us to imagine the universe in 4 dimensions of space and time. Whenever we map the trajectory of a moving object onto a diagram with space and time axes, this is the picture we have in mind, with time as a 4th dimension of space. And this picture can be very helpful, even for explaining spacetime relationships in Relativity – e.g. in the light-cone diagram. But it can also be very misleading, since the geometry and even the topology of 4-dimensional space is very different from that of Minkowski spacetime, where space and time coordinates have opposite signs. For example, in Minkowski spacetime, all events “on the light-cone” are contiguous. I think this is an important issue, because the spacetime of Relativity is so often described as a “block universe” in which all the events of past history and everything that will ever happen in the future all co-exist at once. This picture actually made sense in classical physics, but it badly misrepresents how events are physically connected in Minkowski spacetime. So long as we’re satisfied with this picture of the world in 4-space, we’re not rising to the challenge of conceptualizing the world we actually inhabit, where the present moments of different observers are causally interconnected in quite complicated ways. By dismissing the now as an illusion irrelevant to physics, we’re conveniently ignoring what may well be a fundamental aspect of the physical world.