Maybe the strangest thing in all of nature is time.
I say “maybe” because hardly anyone, even scientists who have thought hard about it, has barely a clue what it could be. We know it exists because we live in it, inescapably. Days go by. Summer turns into fall, fall into winter, winter into spring, year after year. We watch time pass before our eyes in the daily, yearly and longer movements of the moon, planets and stars. We’re most intimately aware of time through our own memories of a second or a decade ago, and through the fact that we are carried unstoppably straight toward death.
Time, according to our observation, is a continuous motion forward of some kind. Isaac Newton stated it for mechanical science in the 1660s: “absolute, true, and mathematical time” flows at one exact rate everywhere. For about 250 years this was the best scientific description of how time seems to work. A continuous, measurable flow.
Except, it turns out time is not an absolute flow at all. About 100 years ago, Albert Einstein showed that intervals of time are not the same for everyone. (Not psychologically — that’s a different problem.) But the same interval of time can be actually, physically longer and shorter according to how fast you’re moving in relation to the speed of light or how close you are to a massive object such as a planet.
Light travels about 186,000 miles per second. At or near that speed, time intervals shrink noticeably. Meaning, for example, that a year for a person traveling near the speed of light is shorter than a year for a stationary person. So if one person traveled into space at, say, 80 percent of the speed of light and returned when his very accurate spacecraft clocks measured 12 years, then when he got back, 20 years would have passed for a person who remained stationary. Gravity also affects time. For a person on Earth’s surface, gravity causes time to move more slowly than for a person farther out.
Now, Earth time is not the “real” or “correct” time. Both times are accurate even though they’re different. There is no single absolute flow of time. This has been shown in jet planes and subatomic particles. In 1971 two physics professors flew atomic clocks on jets around the world. In the end, the jet-based clocks differed from synchronized Earth-based clocks by 59 billionths of a second going east and 273 billionths of a second going west. (The differences differed because the Earth’s gravity and rotation also affect our experience of time even though we have no idea it’s happening.) The variations were tiny because the jets were traveling at a tiny fraction of light speed. Likewise, in 1966, some scientists accelerated muons (a species of subatomic particles) to 99.7 percent of light speed and found the speeding muons, which have very precise life spans, lived 12 times as long as muons at rest.
This September a team of scientists reported they set up two atomic clocks on tables differing by about a foot in height and found the clocks detected time differences — due to one table being closer to Earth and its gravity — at a rate of 90 billionths of a second in 79 years. Tiny differences that prove Einstein’s theory of relativity is accurate to the real physical world.
It all points to this amazing fact: Time is not a flow; it is a dimension. Or more accurately: Time is our experience of the fourth dimension.
You can tell time is a dimension because it’s a location, the way the three dimensions of space form a location. I’ll meet you on Main Street (a length) at its intersection with Hammond Street (which runs into Main Street from a width) on the second floor of The Grasshopper Shop building (a height) at 4:20 p.m. (a time). Get any of those four dimensions wrong, and we’ll miss each other. Length unfolds to width, width unfolds to height, and those three somehow unfold to, in Einstein’s phrase, space-time.
For reasons utterly unknown, we experience the fourth dimension in what seem like moments flowing in just one direction, morning to night, summer to fall, birth to death. Physicists call this “the arrow of time.” They think it’s related to the fact that when left to themselves, things tend to come apart rather than assemble, a tendency called by the technical term “entropy.”
No one knows why the universe operates like this. As far as I know, no one has ever visualized the fourth dimension. There are some scientific ideas about what “eternity” means, though. We’ll try to bend our minds around a couple of those next time.
The information given here is distilled from distillations. In other words, the columnist is an amateur, not a professional naturalist, and sought professional help to explain these basic scientific ideas about time (my expertise involving, rather, reading books and nature, and interpreting them). Among many other authorities consulted, the clearest detailed discussion for amateurs on time that I know of is Sean Carroll’s recent book “From Eternity to Here: The Quest for the Ultimate Theory of Time.” Also very helpful is Paul Davies’ “About Time.” Two other reliable interpreters are Dan Falk (“In Search of Time”) and Gary Zukav (“The Dancing Wu Li Masters”).
Read more on the nature of time:
“Trapped in the fourth dimension”
“Time travel: What’s holding us back?”
Amateur Naturalist archive