Time

From Einstein's perspective, time does not flow: "Does time really flow? If it does, what actually is flowing? And how fast does this time-stuff flow? Does time really have an arrow? Space, for example, does not appear to have an inherent arrow -- to an astronaut in the dark recesses of the cosmos, left and right, back and forth, and up and down, would all be on equal footing -- so where would an arrow of time come from? If there is an arrow of time, is it absolute? Or are there things that can evolve in a direction opposite to the way time's arrow seems to point? "Let's build up to our current understanding by first thinking about these questions in the context of classical physics. ... "From the perspective of sentient beings, the answer seems obvious. As I type these words, I clearly feel time flowing. With every keystroke, each now gives way to the next. As you read these words, you no doubt feel time flowing, too, as your eyes scan from word to word across the page. Yet, as hard as physicists have tried, no one has found any convincing evidence within the laws of physics that supports this intuitive sense that time flows. In fact, a reframing of some of Einstein's insights from special relativity provides evidence that time does not flow. "To understand this, let's return to the loaf-of-bread depiction of space­time [previously] introduced. Recall that the slices making up the loaf are the nows of a given observer; each slice represents space at one moment of time from his or her perspective. The union obtained by plac­ing slice next to slice, in the order in which the observer experiences them, fills out a region of spacetime. If we take this perspective to a logi­cal extreme and imagine that each slice depicts all of space at a given moment of time according to one observer's viewpoint, and if we include every possible slice, from the ancient past to the distant future, the loaf will encompass all of the universe throughout all time -- the whole of spacetime. Every occurrence, regardless of when or where, is represented by some point in the loaf. ...

Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time.

"The perspective should make you scratch your head. The 'outside' perspective of the fig­ure, in which we're looking at the whole universe, all of space at every moment of time, is a fictitious vantage point, one that none of us will ever have. We are all within spacetime. Every experience you or I ever have occurs at some location in space at some moment of time. And since [the loaf-of-bread depiction] is meant to depict all of spacetime, it encompasses the totality of such experiences -- yours, mine, and those of everyone and everything. If you could zoom in and closely examine all the comings and goings on planet earth, you'd be able to see Alexander the Great having a lesson with Aristotle, Leonardo da Vinci laying the final brushstroke on the Mona Lisa, and George Washington crossing the Delaware; as you con­tinued scanning the image from left to right, you'd be able to see your grandmother playing as a little girl, your father celebrating his tenth birth­day, and your own first day at school; looking yet farther to the right in the image, you could see yourself reading this book, the birth of your great­-great-granddaughter, and, a little farther on, her inauguration as Presi­dent. Given the coarse resolution ... you can't actually see these moments, but you can see the (schematic) history of the sun and planet earth, from their birth out of a coalescing gas cloud to the earth's demise when the sun swells into a red giant. It's all there."