Does the Past still exist? When is Now? Presentism, growing block universe, and block universe.

Sabine Hossenfelder did a talk at RI recently where she tackled some of the biggest existential questions. One of which was Does the Past still exist? And then she went on to discuss the notion of Now and the relativity of simultaneity. I always find what scientists say such things puzzling. That is what I am going to discuss here.

Here we go...

• First of all, it seems that scientists are redefining the colloquial meaning of now without stating clearly that they are doing so. IMO, by definition, the notion of now encapsulates the notion of in the present. How can the Past exist now? It is basically cheating. It is similar to what Lawrence Krauss did with the definition of Nothing in the book Something from Nothing.

• There are two meanings of the word past in this discussion. One is about the light from a past event that happened at some far distance (some large light length away) reaches us. In this scenario, the light reaching us is showing us that event itself as it happened. For example, when the light from Andromeda galaxy reaches our eye we see what was happening there 2.5 million light years away as it was happening. This is basically the surface of our past light cone. However, there is a more familiar meaning of the word past which means - that we know about that past event because that past event left traces in the environment. We access those traces or memories to remember the past. For example, we have memories of our childhood bike ride to school, or when we excavate a dinosaur fossil, we learn about the dinosaur whose fossil it was. And such a past event may have happened at any time in the past. It is not clear in which sense of the past the question Does the past exist? posed?

• There is similar confusion about the word now. It is said, that when we see the Andromeda galaxy (which is 2.5 million light-years away) now (let us call it subjective or POV now), what we are really seeing is what was happening there 2.5 million light-years ago. But in the next breath, it is said we cannot know what is happening in the Andromeda galaxy right now (global or universal now). So which now is it?

Based on the above it is not clear the question Does the past exist (now)? is being asked about which past and which now?

As we know from the Minkowski space-time geometry, it is divided into the following five parts:

• the interior of the past light cone

• the surface of the past light cone

• the surface of the future light cone

• the interior of the future light cone

• the so-called space-like wedge between the past and future light cones

In this picture, there is a notion of the relativity of simultaneity. Once again in this, the simultaneity is the simultaneity from the POV of various reference frames. Sure that can be relative. However, the global simultaneity still has some meaning which may be imprecise but still has to lie in the so-called space-like wedge between the past and future light cones. If that was not the case, how could a statement like we cannot know what is happening in the Andromeda galaxy right now?