A lot of things time isn't

kw: book reviews, nonfiction, physics, time

Just over two years ago I reviewed an earlier book by Carlo Rovelli, Seven Lessons On Physics. That was a fun and interesting read, so I was happy to find a new book of his, The Order of Time. It is equally fun and interesting.

Let's first get to the bottom line: Nobody yet knows just what time IS. So of necessity, a lot of this book is about things that time is NOT. Understanding how time works, and perhaps to approach knowing what it is, constitutes Dr. Rovelli's life's work.

Now that we can make comparatively affordable (cheaper than an automobile) instruments that "measure time" to incredible accuracy, it is possible to obtain two of them, place one on the tabletop, and the other on the floor…Then, within perhaps a half hour, the one on the tabletop will show a slight positive difference in "what time it is", to the nearest picosecond or so. You can then switch their places. After a while, the one that is now on the tabletop will have caught up with the other, that is now running slower, and then will pass it by, so to speak. Why? The force of gravity is just a tiny bit greater at floor level than at tabletop level, and gravity slows time.

You can also take one of the "clocks" for a joyride, and when you return, it will have recorded the passage of a little less time than the one that remained behind. Motion slows time.

While we are at it, the equivalence of gravity and acceleration that underpins the General Theory of Relativity by Einstein indicates that acceleration also slows time. It's a bit harder to measure, since we don't have a simple way to divorce acceleration from velocity of motion. Even driving around with a hyper-precise clock, we don't know how to distinguish the change in what it measures that is due to the acceleration from that which is due to the speed(s) traveled.

The fact that time flows differently because of one's velocity, and one's position relative to a large mass, results in the necessity for the satellites used in the GPS system to correct the time their very precise clocks record. Otherwise, in just a day, your navigation device would be misplacing your calculated location by a few kilometers. These corrections, for both the speed of the satellite and its elevation (~22,000 km) above Earth, are sufficiently accurate that your device can know its location within a 5-to-10-meter radius, and a military-grade (and much more costly) device can determine its location within a centimeter or so. The radio signals that the satellites send travel about a foot (0.3m) in a nanosecond. Centimeter-level precision implies time accuracy of ~10 picoseconds, or trillionths of a second.

Back to the book. We have a number of practical and customary definitions of time, that allow us to go about our day-to-day work. Part I of the book draws us to realize that, to a physicist, time is changeable. Physics equations that include time and rate terms work in either direction. There is no "past", "future", or "present". Even the notion of Entropy, which is a physicist's description of the direction of time, is actually based on the "blurring" of our perceptions.

We may think we have pretty sharp vision. Indeed, since our visual cortex is nearly as large as the entire brain of a chimpanzee, our general vision is better than any other animal's (the very acute vision of a hawk is in a very small part of the bird's visual field). But how sharp is sharp? Human visual acuity ranges from 1/20 to 1/60 of a degree of arc. That means, if you hold something in your hand at "reading distance" of about 16 inches (40 cm), you'll be able to distinguish features on its surface as small as 1/60 inch, or perhaps 1/200 inch apart (0.4 to 0.13 mm). But we know we need a microscope to see "small stuff" like pollen grains (10-100 times smaller) or bacteria (even smaller). Special microscopes are needed to "see" atoms, which are smaller than a millionth of a millimeter.

So, we cannot see the molecules moving in a glass of water, but their motion gives the water its temperature. What we measure as a temperature of, say 20°C or 68°F, represents a certain average velocity of the molecules in the water. We don't see that. But the basic concept of entropy can be considered this way: moving energy through a system tends to make it less ordered. Thus, ice is very ordered, because the water molecules are in fixed relationships to one another. When ice melts, the molecules come "unglued" and can move about. Even a small glass of water contains, not just millions or billions, but billions of trillions of them. So there are a lot of ways for those molecules to be arranged, but they all look the same to us. In the glass full of ice, there was only one arrangement. We can't distinguish the motion; to us the glass of water just sits there. That is the blurring of our perception.

In the second part of the book the author describes a world without time; where there are no "things", just events. Not only do you and I not understand this, neither does he. Some very smart scientists have developed mathematical formulas that describe events with no time element. That doesn't mean we have any way to experience utter timelessness.

In Part III, he claims that "Time is Ignorance" (a chapter title). We can say that "Time is nature's way of keeping everything from happening at once", or "Time is a dimension" (as Relativity states). Whatever time "really is", we have our perceptions, which include a flow and direction of time, because that's what we need to survive. We evolved to perceive successions of events as time. We are pretty far from really knowing much more than that.