There seems to be some confusion over the term “block universe.” Let me try to clarify what that means.
We all experience Now as unique from past experiences or experiences that are yet to happen (I won’t qualify all such statements with “I assume”). We tend to believe that we only exist (are only “real”) Now, i.e., my past self no longer exists and my future self has yet to exist. I assume there are other things that coexist with me at each Now, e.g., my twin brother who lives 600 miles away has a Now of his own and his Now coexists with my Now. When we turned 50 our 49 year-old selves no longer existed and our 51 year-old selves were yet to exist. We use this reasoning out of necessity nearly every day. If I want to call my brother I first ask myself, “What is my brother doing Now?“ because I don’t want to call if I believe he is sleeping or at work, for example. Not only is it perfectly reasonable to assume the validity of a shared set of Nows with other people, but it is absolutely necessary in dealing with objects at large distances. If I work for NASA and I’m in charge of a probe circling Jupiter 40 light-min away, I need to know where it is at any given Now, so I know where it will be 40 min from that Now when it receives my next orbital command. When I talk to engineers in Australia who will take over command of that spacecraft in 12 hours, we all share the same notion of our set of overlapping Nows, albeit with different time stamps for our different time zones. Of course, it could be that our probe has been struck by a meteor and it doesn’t exist where I think it does at some Now — I won’t know it has been struck for at least 40 min. But, that’s simply a result of the finite speed of light that carries information to me from the probe. That is, it’s a fact about my knowledge of the probe at some Now, not about whether or not the probe actually coexists with me (is “equally real”) at any Now. The probe doesn’t experience Now, but it’s meaningful to think about it as only existing (as only being “real”) at each Now. If everyone agrees on a shared collection of Nows, that would constitute a global “Now slice“ of spacetime. That is Presentism and that accords nicely with our everyday experience.
Unfortunately, that intuitive notion cannot hold if everyone measures the same speed of light regardless of their motion relative to the source, which is an empirical fact. That empirical fact is itself very counterintuitive. If we are at rest with respect to each other and I throw a ball away from me and towards you at 5 m/s relative to me, then you measure the speed of the ball to be 5 m/s relative to you. If I’m in a car moving at 30 m/s towards you and I throw a ball away from me and towards you at 5 m/s relative to me (and the car), then you measure the speed of the ball to be 35 m/s relative to you. That’s intuitive. But, if I do the same with a flashlight, you always measure the speed of the light from my flashlight to moving towards you at c. Even if I’m moving towards you at 0.5c, in which case intuition tells us you should measure the light from my flashlight to be moving towards you at 1.5c, you will still measure its speed to be c.
As a consequence of that fact, observers moving relative to each other will no longer agree on what events are simultaneous (are “equally real”), i.e., on what events occupy a “Now slice.” Thus, if everyone continues to attribute “existence” only to those events in spacetime that share their Now slice at any given moment of time in their existence, e.g., my attributing existence to the version of my twin brother that is the exact same age as me, then there is some frame of reference in which my Now coexists with a younger (or older) version of my brother. Thus, that special feeling of Now that I experience along my worldline in spacetime exists equally everywhere along my worldline as far as other observers are concerned. Here is 9-
min video showing the relativity of Now slices (aka the relativity of simultaneity) that results from the light postulate. That is the block universe and it violates our everyday experience.
Of course, that doesn’t entail that our dynamical experience of time is ”an illusion” as the video implies. You can rather believe that the block universe is simply a tool for coordinating, explaining, and predicting our experiences. See Section 5 of this
paper by Mermin, for example. That’s an “interpretative” aspect of block universe.
