That Time the Way That It Is and Was
The Time Machine, Metro-Goldwyn-Mayer, 1960
In order for something to change, it needs to be one way, and then some other way. In ordinary cases of change things change when they are one way at one time, and some other way at some other time. Suppose that on Monday at 2 pm Annie’s mass is 21.2 kgs (Annie is a dog). Suppose she wants to change her mass. She won’t do this by trying to make her mass different at Monday at 2 pm. Instead, she will try and make her mass at times later than Monday at 2 pm, less, or more, than 21.2 kgs. Suppose, though, that instead of changing the properties of an object, like Annie, we want to change the properties of a time itself. Then that time needs to go from being one way, to being some other way. But how can it do that? A time can’t go from being one way at one time, to another way at another time, because no time exists at more than one time. At least, no time exists at more than one time in the same temporal dimension.
If we were to introduce a second temporal dimension – sometimes called meta-time – then a single time in one temporal dimension could exist at multiple times in a second temporal dimension. If we posit a second temporal dimension then changing the past amounts to making some time one way at one location in meta-time, and some other way at some other location in meta-time. We can now at least make sense of how a time could change. The “first way” the time is (the pre-change way) is how that time is at one time in meta-time. The “second way” the time is (the post-change way) is the way the time is at some other time in meta-time.
The problem is that most philosophers think there is no meta-time, and most think that even if there were, this still wouldn’t really be changing the past. That’s because the time, as it was before the change, still exists in meta-time. All that has happened is that another version of that time has been created in meta-time, at which different events occur. But the original events, at the original time in meta-time still exist. If one wanted to change the past in order to make it the case that those events never happened, then one has failed. All one has done is made it the case that in another version of that time, those events didn’t happen. So if erasing certain past events and replacing them with new events, is what changing the past would amount to, then even were there a second temporal dimension, we still could not change the past.
So far this makes it sound as if philosophers are a bunch of science fiction spoil sports. Not so! Although philosophers typically agree that we cannot change the past, most think it possible to causally affect the past. What’s the difference? Changing is altering a time from being one way, the first time around, to being some other way, the second time around (hence “the second time around fallacy”). Causally affecting a time is making a time the way that, in fact, it is. If anyone travels to the past they will causally affect the time to which they travel: they will breathe air, tread on bugs, talk to people, and so on. But nothing the time traveller does changes the past: instead, her actions are part of what makes that time the way that it is (and was).