Time

If we are part of a 4-D spacetime, why do we experience past and present?

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What we experience depends on what information about the world we receive and when we receive it. We receive information about the world through our senses, as when a ray of light arrives in one of our eyes from an event that occurred sometime in the past. (That light ray may have been launched by an event that occurred just a few millimicroseconds ago, or it might have come instead from an event in the very distant past, as when a ray of light emitted by stars many years ago enter our eyes as we look up at the sky at night. If the star is 100 light years away, then that ray of light was emitted 100 years ago.)

Since we do not receive rays of light today from events that have not happened yet, we do not experience the future (yet!).

Now light travels very quickly, as you know. The various rays of light that are arriving in my eyes right now, having previously bounced off of various objects in the room I'm in, all left those objects at very nearly the same moment. So those rays of light give me information about the states of those objects as they were at very nearly the same moment. which is also very nearly the same moment at which that light arrived in my eyes. We experience that moment as the present. (However, there is also a neurophysiological story about how long "the present" lasts, which I won't get into here.)

If the laws of physics were different, we would not experience past and present in the way that we do. Suppose, for instance, that a ray of light from an object could not enter our eyes all by itself, but instead, it could only enter our eyes as one member of a pair of light rays. In particular, suppose that the laws of physics required that a light ray wait around, cooling its heals somewhere outside of our eyes, until such time as it could enter our eyes accompanied by another ray of light that left that same region of space exactly 10 minutes later or earlier. (These would be peculiar, science-fiction laws of physics, but let's think about them just for fun!) If we could receive visual information about the world only via pairs of light rays, which left the same region of space 10 minutes apart, then presumably, we would not experience past and present as we do. (Or do you think that we would have evolved some mechanism by which to do so?)

By the same token, suppose that light travelled very very slowly. In particular, suppose that the light entering my eyes right now, having last bounced off of the keys of my computer keyboard, left minutes earlier than the light entering my eyes right now that was emitted by my computer screen (which is a little farther away from my eyes than the computer keyboard). What a drag that would be!

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