Some physicists believe that relativity theory has proved that the past and future exist in a giant four-dimensional block universe. Although his views changed during his career, Einstein, for example, made the following statement in 1952, a few years before he died. He argued that the relativity of simultaneity implies a block universe:
The four-dimensional continuum is now no longer resolvable objectively into slices, all of which contain simultaneous events; “now” loses for the spatially extended world its objective meaning . . .Since there exist in this four-dimensional structure no longer any slices which represent “now” objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four-dimensional existence, instead of, as hitherto, the evolution of a three-dimensional existence. (Einstein, 1952)
Although his language is cautious here, Einstein’s meaning is clear.
There is no physical “evolution” through time, that is, no change or persistence; instead, a static four-dimensional block exists. He says that becoming is not “completely suspended” because there is a residue of change in a block universe; namely, the adjacent, static slices differ slightly, and this creates an illusion of becoming and change.
It should be emphasized first that most physicists regard the entire issue of the block universe as speculative, and simply have no opinion about the matter. For them, it is simply not a scientific question since we cannot experiment directly on the past and future. However, many physicists, such as Einstein, Hermann Weyl and others, thought that relativity theory did prove that our world was a block universe. A number of philosophers have also thought so, although there is naturally disagreement in the details of their views. For example, Bertrand Russell and Hilary Putnam have argued that relativity theory implies some kind of block universe.
Arguments about the block universe all arise from attempts to interpret the special theory of relativity, and all go beyond Einstein’s 1905 theory by adding new premises. In particular, all attempt to say what reality is like if simultaneity is relative. Einstein’s theory, on the other hand, does not mention reality; it merely describes relations between measurements, that is, between appearances. Thus different interpretations of relativity theory will imply different views about the block universe.
As pointed out below, the minority interpretation escapes this strange consequence.
The quotation from Einstein above contains a short but very powerful argument for the block universe. According to his theory, simultaneity is relative. That means that different sets of rulers and clocks, moving relatively to each other, will find that different sets or different “slices” of events are simultaneous. In this sense, to say that two distant events are simultaneous is merely a convention or arbitrary agreement, and has no physical or “objective” meaning. If different clocks were chosen, different events would be simultaneous.
As Einstein interprets it above, this fact already implies that we live in a block universe.
Einstein’s friend and colleague at Princeton, the logician Kurt Gödel, filled in more detail in a 1949 essay. According to him, the relativity of simultaneity seems to lead to:
an unequivocal proof for the view of those philosophers who, like Parmenides, Kant and modern idealists (such as McTaggart), deny the objectivity of change and consider change as an illusion or appearance. The argument runs as follows: Change becomes possible only through a lapse of time. The existence of an objective lapse of time, however, means that reality consists of an infinity of layers of “now” which come into existence succes- sively. But, if simultaneity is relative, reality cannot be split up into such layers in an objectively determined way. Each observer has his own set of “nows” and none can claim the prerogative of representing the objective lapse of time. (Gödel, 1949)
The similarity of this passage to some of Einstein’s writings suggests that he and Gödel had been discussing this issue.
This short argument turns upon the idea that true physical change implies profound differences between the past, present and future.
During change, one and the same object loses some properties and gains others. It also persists through time, moving from one moment, which ceases to exist, into the next moment. But if simultaneity is human choice – mere agreement about which rulers and clocks to use – then there is no real difference between the present and the past or future. These labels, “past”, “present” and “future”, are merely human names that reflect no physical difference in the events they describe.
Thus we have:
Short argument for block universe
- A. If simultaneity is relative, then there is no physical difference between the past, present and future. (P)
- B. Simultaneity is relative. (P)
- C. Therefore, there is no physical difference between the past, present and future. (from A,B)
- D. But, if there is no physical difference between the past, present and future, then there is no true change. (P)
- E. Therefore, there is no true change. (C, D)
The last line means that we live in a block universe. If there is no true change, then any event that ever existed always exists: it cannot change from existent to nonexistent.
By way of analogy, consider a map of Earth showing the equator.
We could travel there and find many physical differences between the northern hemisphere and southern hemisphere on either side of the equator (trade winds in different directions, etc.). Likewise, some claim that relativity theory provides a realistic map of our four- dimensional universe. They insist, however, that unlike the equator, its lines of simultaneity correspond to no physical difference in the universe. We find on the four-dimensional map no objective dividing line between the past and the future, and are supposed to believe that no such line exists in nature.
In other words, a realistic interpretation of the relativity of simultaneity is incompatible with presentism. This doctrine implies that the present “slice” of simultaneous events is the only existent slice. The past slices have ceased to exist, and future slices do not yet exist. According to presentism, change is the passing away of one slice and the emergence of the next. But if Einstein’s theory is a good map of reality, then there is no physical difference between the present slice of events and past or future slices. In particular, the conventional labels “past” and “future” do not imply the physical label “non- existent”.
Much of the literature on the block universe concerns another, related argument that involves three events, and therefore can be called the triangle argument . This argument begins with the premise that, since some distant events coexist with me, at least events simultaneous with me at the present moment exist.
Solipsism is the belief that only I exist. That is, the universe consists of me and nothing else; all other things and space itself are an illusion of some sort in my mind. Surely, however, we deny solipsism. But then some other bodies or events must coexist with me. The only question is which events are the coexistent ones.
The triangle argument is aimed at those who accept coexistence but will not at first agree that past and future events exist now, and resist attempts to drive them to this conclusion. Suppose, the argument begins, that past and future events do not coexist with me at the present moment. Then, since some events do coexist, it must be simultaneous events that coexist. The argument shows, however, that even this modest beginning leads back to the coexistence of the past and the future, and thus to the block universe.
Suppose that there are three events: me-now, me-tomorrow and a distant supernova. Suppose that, according to one set of rulers and clocks, me-now is simultaneous with the distant supernova, but that, according to a different set of rulers and clocks, me-tomorrow is simultaneous with the supernova. Then we have the argument: Triangle argument for block universe
A. If an event exists and it is simultaneous with another event, then the other event also exists. (P)
B. Me-now exists; me-now and the supernova are simultaneous. (P)
C. Therefore, the supernova exists. (from A,B)
D. But, the supernova and me-tomorrow are simultaneous. (P: according to other clocks)
E. Therefore, me-tomorrow exists. (A,C,D)
F. If one event exists and another event exists, then they co-exist. (P)
G. Therefore me-now and me-tomorrow co-exist. (B,E,F) This means that the self I am now and feel to be real (me-now), coexists with myself tomorrow (me-tomorrow), which is just as real.
Of course, since we could have chosen any pair of events far away enough from the supernova, the conclusion means that the present and the future coexist, and thus that the entire future and its past coexist. Thus we live in a block universe.
Clearly the first premise, A, is very suspicious. It moves from a conventional label, “simultaneous”, to an assertion about physical existence. This is precisely the inference that Einstein’s theory is supposed to deny. But the only alternative (short of solipsism) is to concede that some events in the past and future do coexist, and the argument is aimed at those who wish to deny this.
The fourth line of the argument, premise D, has troubled some critics. It implies that two sets of clocks and rulers, and therefore two definitions of simultaneity, are used. In the context of this argument, however, this is legitimate. Briefly put, premise A says that simultan- eity is good evidence for objective existence. Once we know some- thing exists, we are free to use other definitions of simultaneity, and that subjective choice will not affect what objectively exists.
Note that the coexistence does not imply simultaneity. Me-now and me-tomorrow are not simultaneous.
The importance of the triangle argument is that it creates an embarrassing dilemma for interpreters of relativity theory. If they deny solipsism, they must agree that some events coexist. But if they deny that the past and future coexist with the present, then all the coexistent events must be in the present. But this minimalist idea together with the relativity of simultaneity drives them back to the idea that the past and future coexist. For those who interpret relativity, it seems that there is no middle ground between solipsism and the block universe. Any attempt to restrict robust existence to some single slice of the four-dimensional world is the assertion of some privileged or absolute simultaneity, and is profoundly at odds with the mainstream interpretation of relativity theory.
Indeed, one philosopher has argued that relativity theory does imply something very close to solipsism. Howard Stein severely criticized arguments for a block universe, and spelled out in detail which events he believes coexist with me-now. According to his view, only me-now and certain past events coexist. An event in the past coexists with me-now if light from the event could reach me, that is, past events that could have causally influenced me still exist. This appears to be a very strange view. Other people do not exist now, but their past selves may exist and therefore coexist with me. Stein’s view shows that although relativity theory makes good predictions, it appears to be very difficult to spell out what it implies about the nature of reality.
In the end, it is very difficult to interpret the relativity of simultaneity without embracing some form of block universe. If this seems implausible, then there is extra reason to consider the merits of the minority interpretation. According to this, the relativity of simultaneity is mere appearance; in reality, only clocks at rest in the ether show true time and can be used to judge which events are really simultaneous. Thus Lorentz and other defenders of the minority interpretation can naturally say that only the present is real. The minority interpretation is compatible with presentism.