Putnam claims that future events are already "real." This is a "tenseless" view of the future, like that of J. J. C. Smart's block universe of special relativity.

Putnam discusses the related problems of the truth of "future contingents" and Aristotle's famous "sea-battle." Also see Diodorus Cronus's Master Argument.

Putnam's argument depends on paradoxes (the most famous being the twin paradox) that are the results of moving observers having different ideas about what events in their view correspond to "now." They have different "planes of simultaneity." "Now" means they have synchronized their clocks according to Einstein's famous procedure.

A moving observer B thinks some event in his plane of simultaneous events is in B's future. At the same (cosmic time) moment A thinks that B is in his (A's) plane of simultaneity.

But, despite Putnam, this is not a transitive relation. Just because A sees B as his "now" and B sees an event in A's future as B's "now" does not make the event in A's future "now" for A.

Putnam says

(1) All (and only) things that exist now are real.

Future things (which do not already exist) are not real (on this view); although, of course they will be real when the appropriate time has come to be the present time. Similarly, past things (which have ceased to exist) are not real, although they were real in the past.

If we assume classical physics and take the relation R to be the relation of simultaneity, then, on the view (1), it is true that all and only the things that stand in the relation R to me-now are real.

We now discover something really remarkable. Namely, on every natural choice of the relation R, it turns out that future things (or events) are already real!"

you-now and I-now are at the same place now, but moving with relative velocities which are very large (relative to the speed of light, which I take to be = 1). Thus, our world-lines look as shown in figure 1 (I have also drawn our "lightcone" for the purpose of the later discussion):

It is well known that, as a consequence of Special Relativity, there are events which lie in "the future" according to my coordinate system and which lie in the "present" of you-now according to your coordinate system. Since these things stand in the relation R to you-now, and you-now are real, and it was assumed that all and only the things that stand in the relation R to me-now are real, the principle III requires that I call these future things and events real! (But, actually, I now have a contradiction: for these future things do not stand in the relation R to me-now, and so my assumption that all and only the things that stand in this relation R to me-now are real was already inconsistent with the principle that There Are No Privileged Observers.)

The difficulty is obvious: what the principle that There Are No Privileged Observers requires is simply that the relation R be transitive; i.e., that it have the property that from xRy and yRz it follow that xRz. Simultaneity-in-my-coordinate-system has this property, since if x is simultaneous with y in my coordinate system, and y is simultaneous with z in my coordinate system, then x is also simultaneous with z in my coordinate system; but simultaneity-in-my-coordinate system is not admissible as a choice of R, because it depends on the coordinate system. And the relation "x is simultaneous with y in the coordinate system of x" (which is essentially the relation we just considered), while admissible, is not transitive, since, if I-now am simultaneous with you-now in the coordinate system of me-now, and you-now are simultaneous with event X in the coordinate system of you-now, it does not follow that I-now am simultaneous with event X in the coordinate system of me-now.

Now then, if we combine the fact that the relation R is required by iii to be transitive with our desire to preserve the following principle, which is one-half of (1):

(2) All things that exist now are real.

-then we quickly see that future things must be real.

For, if the relation R satisfies (2)-and I take (2) to mean (at least when I assert it) that all things that exist now according to my coordinate system are real-and you-now are as in figure 1, then you-now must stand in the relation R to me-now, since you exist both now and here. But, if the relation R always holds between all the events that are on some one "simultaneity line" in my coordinate system and meat- the-appropriate-time, then (since the laws of nature are invariant under Lorentz transformation, by the principle of Special Relativity), the relation R must also hold between all the events on some one "simultaneity-line" in any observer's coordinate system and that-observer- at-the-appropriate-time. Hence, all the events that are simultaneous with you-now in your coordinate system must also bear the relation R to you-now. Let event X be one such event which is "in the future" according to my coordinate system (if our velocities are as shown in figure 1, then such an event X must always exist). Then, since the event X bears the relation R to you-now, and you-now bear the relation R to me-now, the event X bears the relation R to me-now. But we chose R to be such that all and only those events which bear R to me-now are real. So the event X, which is a future event according to my coordinate system, is already real!

I conclude that the problem of the reality and the determinateness of future events is now solved. Moreover, it is solved by physics and not by philosophy. We have learned that we live in a four-dimensional and not a three-dimensional world, and that space and time or, better, space-like separations and time-like separations - are just two aspects of a single four-dimensional continuum with a peculiar metric which sometimes permits distance (y, x) = 0 even when x ≠ y. Indeed, I do not believe that there are any longer any philosophical problems about Time; there is only the physical problem of determining the exact physical geometry of the four-dimensional continuum that we inhabit.

In this paper I have talked only about the relativistic aspects of the problem of physical time: there is, of course, also the problem of thermodynamics, and whether the Second Law does or does not explain the existence of "irreversible" processes (the so-called "problem of the direction of time"), and the problem of the existence or nonexistence of true irreversibilities in quantum mechanics, which, I gather, is currently under hot discussion. I have not talked about these problems.