Another way of consideration toward the reality of spacetime
Author:
Akira Murayama
A system engineer in Japan
Abstract:
According to "Relativity and the Dimensionality of the World"(Vesselin Petkov, 2004), consideration only by relativity of simultaneity may permit relativized version of presentism and the analysis of the twin paradox can show the reality of spacetime completely. As for the idea of "the only view that is consistent with relativity is four-dimensionalism", I agree with it very much, but I think we can show it by only using the relativity of simultaneity.
In that paper, the origin of the two observers' frames of reference are set at common position, but I prefer that an origin is set apart from another origin. For instance, observer B is apart from A, and B is moving to/from A (or A is moving to/from B).
Under this situation, we can consider about the reality of spacetime more simply and vividly.
We can start with evident assumptions as following:
(1) Present A is real. (A can be regarded as I)
(2) Present B that is apart from A is real. (B can be regarded as he or she)
(3) B's velocity is different from A's velocity.
(4) Anything, which exists with real objects simultaneously, is real.
(5) Simultaneity is relative. (This is the most important point)
(1) to (4) will be accepted even by presentists, and (5) was derived from special relativity.
Then, our deduction is following;
(1) Due to relativity of simultaneity, if real B is approaching A, real A for B is a future point of worldline-A than (the first assumed) real A.
(2) Due to the assumption (4), the future point should be judged as real.
Or;
(1') Due to relativity of simultaneity, if real B is leaving A, real A for B is a past point of worldline-A than (the first assumed) real A.
(2') Due to the assumption (4), the past point should be judged as real.
And thus, all points of spacetime should be judged as real. If present I assume present apart others are real, I have to accept that any events of my future and my past are real.
And on this deduction, we can replace the word "real" to another word such as "determined" or so on. We cannot distinguish past and future as any attributes objectively. If we assume the past events are determined, the future events are determined; or if we assume the future events are not determined, the past events are not determined. Such view that the past events are not determined won't be acceptable except for the "Many-Worlds Interpretation" of Hugh Everett. If we assume real events are only on the past light-cone, we may consist with a kind of presentism, but we (?) have to stand for a kind of solipsism. On the other hand, I think the "Many-Worlds Interpretation" means a kind of multi-solipsism because we cannot have any common concrete view of history of the world. Then only acceptable for me and for relativity is the view that all events in the four-dimensional spacetime are real and determined.
Whereas at least for our consciousness, it is also very evident fact that past, present and future are different from each other and time flows. As objectively as I think, time doesn't flow, all events only exist (as H. Weyl said), but in the world there are some objects, which grasp four-dimensional world as changing three-dimensional world (fleeting time) presentation, such objects are called consciousness (i.e. subjects.).
Why does consciousness grasp 4D world as changing 3D? This is a very difficult question. I think we can begin to consider by an awareness that there are two types of velocity. The first type is pure objective velocity that is defined on physics. It should be defined by the ratio to the speed of light in the context of relativity. Such velocities are never affected by any status of sensing subject. We usually use "velocity" as this meaning. But we have to be aware of another type of velocity. The second type is scanning (crawling) velocity of time sensing subject. Time sensing subject is a kind of scanner, which scans four-dimensional spacetime from a starting point of its worldline to ending point. The direction to the starting point is called "past" by consciousness, and the direction to the ending point is called "future". The size of the second type of velocity should be defined by the length of moment that a consciousness feels as an instant. In this interval a subject can sense passing of time but doesn't feel waiting to pass away. If this length is short the consciousness feels as time flows slowly, while this length is long the consciousness feels as time flows rapidly. It is also a sort of objective time length, but it cannot be defined strictly as physical phenomenon due to wavering of consciousness, however it is evident that our length of moment would be neither scale of milliseconds nor scale of hours. It is said that scale of moment is different between smaller animals and bigger animals, moreover talking about extremely fast flying space traveler or falling astronaut into a black-hole, the scale of moment is obviously different.
Upon the argument of J.M.E. McTaggart, subjective (flowing) time is called "A-series" and objective (physical) time is called "B-series". He and many people maintain that we cannot derive "A-series" from "B-series" then "A-series" is essential, (in addition, he proved "A-series" is contradictory in itself, then time does not exist.) But I think we have to find some way to derive "A-time" from "B-time" to be consistent with relativity. Four-dimensionalism and view of time scanning subject will bring us this way.
Nonetheless I know not a few people will refuse such view like four-dimensionalism. Because it might deny our free will. As for this too difficult problem, though I cannot say anything in this short paper, at least I say I won't intend to deny value of freedom, but I'm thinking that we may have to seek some change about concept of freedom.
(Dec. 2005)