But if you ask a scientist who studies time to ex-plain what time is, he or she invariably will turn the question on you: “What do you mean by time?” And already you’ve learned something. You might begin, as I did, by qualifying your statement to mean “time perception,” to distinguish between external time and your internal grasp of it. This dichotomy suggests a hierarchy of truth. Foremost is time as told by one’s wristwatch or the clock on the wall, which we typically think of as “true time” or “the actual time.” Then follows our perception of this time, which is accurate or not depending on how closely it matches the mechanical clock. I’ve come to think that this dichotomy is, if not meaningless, certainly of little help in trying to understand on a human scale where time comes from and where it goes.
But I’m jumping ahead. One of the oldest debates in the scientific literature is whether “time” is something that can be “perceived” at all. Most psychologists and neuroscientists have come around to thinking that it isn’t. Our five senses — taste, touch, smell, sight, and hearing — all involve discrete organs that detect discrete phenomena: sound is what we call it when vibrating air molecules trigger movements of the tympanum in the inner ear; sight is what results when photons of light strike specialized nerve cells at the back of the eye. In contrast,