STR and Relative Temporal Order

An immediate reason to hold that temporal order is relative is just because of relative simultaneity. In some frames, two events are simultaneous; in other frames, one of those events comes after the other, i.e., they are temporally ordered. We see this with the example of the trains: e, is simultaneous with e, in the frame of the platform, but temporally ordered in the frame of the train.

Since these events are simultaneous relative to some but not all frames, then they are relatively simultaneous. By the same reasoning, since these events are temporally ordered relative to some but not all frames, then they are relatively simultaneous.

As such, it follows that there is relative temporal order: the order of events such as e, and e, relative to a train speeding through a station.

Does this mean, then, that our sense that the sun’s death is later than the moon’s tidal locking is merely relative? Is this the temporal order that seems to conflict so forcefully with common folk intuitions about time?

Objection: There is Absolute Temporal Order in Str

Not exactly. The events of e( and e, are not causally related. They have a common cause (Cj); they even have a common effect (my seeing them as two bright flashes of light). However, one does not cause the other.

As discussed earlier in this chapter, according to STR, if two events are causally related, then they can be absolutely temporally ordered. They are not forced, given STR, to be ordered in one frame and not in the other. For example, c, is earlier than both e, and e, in both the frame of the train and the platform. Indeed, given STR, this order holds in any other inertial frame. At least, according to STR.

As such, given the moon’s tidal locking is in the causal history of your reading this, and the sun’s death, it is earlier than your reading this, and the sun’s death.

Are these events part of a causal sequence? Plausibly, yes. The moon’s activity at some point in time causes things to happen—events—amongst the activity of the Earth and the sun. Those effects may be miniscule (especially for the sun), but they are still effects. And being effects, they are later than the events on the moon that cause them. Similarly, activity on the Earth and on the sun cause effects on the moon—and, indeed, cause effects on each other. Lastly, many other objects in the universe cause things to happen to the moon, the earth, the sun, and each other.

If this is right, then we might say this: the moon settles into tidal lock, causing an effect that is itself the first cause in a long and complex causal chain of events. This chain interacts with other causal chains, such as from the Earth and sun, or other material bodies. All these chains cause things such as your reading this; your reading this has its own effects, which join in with such causal chains. Until, somewhere far down the lines of these causal chains, there is the effect of the sun dying.

Response: Not All Earlier Events Are Causally Related to Later Events

This account requires that events that are not obviously causally related must be causally related. Otherwise, they are not temporally ordered. For example, perhaps your reading this is part of why the sun dies. But, if it turns out that it is not, does this mean the sun’s death is not after your reading this? Surely, whatever we say of the causal relationship, the temporal order must survive it?

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