Remember Tomorrow?

I can remember yesterday, so why can't I remember tomorrow? This may sound like a pretty silly question, but it's one that physics can't yet answer.

The apparently obvious answer is that yesterday has already happened, whereas tomorrow hasn't yet. But this isn't really an answer at all, because the real point of the question is just to ask, what is the difference between 'already happened' and 'going to happen'? And insist as one may that this difference is obvious, that does not amount to an explanation of exactly what the difference is.

Drifting on the river of time

Where is yesterday when it is gone? We can imagine that each moment simply vanishes as it passes. So perhaps yesterday does not exist at all, and our present memories of it might just as well be legends of an imaginary world. Or we can imagine that the past is just the country that stretches behind us as our awareness drifts steadily forward on the river of time. So perhaps yesterday is still there, and our memories are souvenir sketches of a real landscape to which we will never happen to return.

We can think of the future in the same two ways. Perhaps it doesn't exist at all until it becomes the present. Or perhaps it is all there waiting for us to arrive.

There seems to be some truth in both pictures of the future. Some future events occur as anticipated, while others appear as sudden surprises. Yet we can find the same mixture of definiteness and indefiniteness in the past, as well. Important experiences remain vivid in memory, but obscure details fall into oblivion as soon as they pass. So the distinction between future and past is not as simple as one being real where the other is only potential. The difference between how real the two are seems to be only a difference of degree, not of kind.

The difference of degree is there, however. Memory may be fallible, but it is usually more accurate than prophecy. We have to say that the landscape behind us on the river of time seems more detailed than the terrain still ahead.

Why can't we remember tomorrow? The answer really isn't obvious after all. It has something to do with causality. Perhaps it also has something to do with how some things are more important than others. And that's really the bottom line of what physicists can say, right now, about this startlingly basic question.