of 1905 and 1915, respectively. In them, Einstein elaborated on and explained curiosities and paradoxes noted in the preceding decades by George FitzGerald and Hendrik Lorentz. In a nutshell, a totally unexpected revelation emerged: Even if time is an actual entity, it cannot be a constant like lightspeed or gravity. It flows at different rates. The presence of a gravitational field retards the passage of time, as does rapid motion.
We’re intuitively ignorant of this because we all attended a high school where everybody hung out in the same gravitational field—and never, even in our wildest teenage years, sped our car in a joyride faster than an eight-millionth of the speed of light. Because one must go 87 percent of lightspeed to feel time slow by half its normal rate, we’ve never even come close to directly experiencing time’s fickleness—a function of our still-sluggish ground vehicles rather than any personal wisdom.
Astronauts do better. Orbiting at one-twenty-six-thousandth the speed of light, they can actually gauge the amount by which their time runs slow, using sensitive clocks—which brings up a seldom discussed puzzle. Though they move faster, astronauts have also traveled away from Earth’s surface into a weaker gravity, which has the opposite effect, speeding their passage of time. Turns out, their high-speed factor prevails. They age less quickly than people on the ground. They’d have to be eight times higher than the International Space Station’s orbit, or two thousand miles above Earth’s surface, before the weaker gravity there exactly balanced their now slower orbital speed to let them age at the same rate as those back home. Still farther away, timepieces on the Moon tick faster than those at mission control in Houston—even if nobody compensated Apollo crews with early Social Security benefits.
These time distortions aren’t subtle, nor are they merely of academic interest. Those GPS satellites simply wouldn’t work if continual compensations weren’t added for various time-warping effects. Since receiving precise time signals from each satellite lies at the very heart of that navigation system, anything that throws off the instruments’ or receivers’ time passage will blow the whole thing.
Are you a truly nerdy, geeky person who cares about such technological or physics details? If so, consider the many wrinkles in how time seems to flow, all introduced by the very technology designed to measure it:
Wrinkle one: Satellites travel at 8,700 miles per hour, slowing their clocks.
Wrinkle two: They’re distant from Earth in a reduced gravitational field, which accelerates their time relative to Earth’s surface.
Wrinkle three: GPS users on the Earth’s surface are located at various distances from Earth’s center (at Denver’s high altitude versus low-altitude Miami, say), producing a variety of time-passage rates.
Wrinkle four: The difference in Earth’s rotation speed at separate ground-based locations produces inconsistencies in their agreement about the passage of time, which is called the Sagnac effect.
Wrinkle five: Time runs slower for all earthly observers (as compared to any future lunar colonists) because of our planet’s 1,040-mph equatorial spin. (The speed decreases the farther one is from the equator.)
Wrinkle six: Satellites’ time passage continually changes because their slightly elliptical orbits make them speed up and slow down, plus they zoom through irregularities in Earth’s gravitational field due to things like our planet’s equatorial bulge.
All told, six separate Einsteinian time distortions affect receivers’ clocks; half of these also distort the satellites’ clocks. They must all be accurately and continuously corrected. Any inconsistencies would ruin the system’s accuracy, big time.
And always remember: We’re not talking about the warping of an actual entity called time. We’re noticing only that events unfold at more leisurely rates, or more hurriedly, than they did before, relative to others. This remains a central point. A hawk flaps its wings slowly, whereas a hummingbird’s wings beat furiously. Sure, we could bring our concepts of time into the discussion, yet we needn’t do so. The event is one thing. How we categorize or measure it is another.
For those who may imagine that such “time warps” are only a mind game, a mere theory, the fact is, Einstein’s time dilation even causes death. When cosmic rays (highly energetic particles striking our atmosphere) collide with molecules in the upper layer of air, they break atoms apart like a cue ball smashing a stack of billiards. The resulting rain of subatomic particles includes some that can be lethal to humans if they strike the wrong bit of genetic material. These muons dash through our bodies constantly, causing some of the spontaneous natural cancers that have always plagued our species. Over 200 of these penetrate each of our bodies every second—more if you live higher up, like in dangerous Denver again. The point is, muons, intermediate in mass between protons and electrons, exist for just 2 microseconds before decaying into harmless by-products. And a few microseconds is not long enough for them to make it all the way to Earth’s surface and into our cells, even though they travel a hefty fraction of the speed of light.
Muons should decay so quickly after being created thirty-five miles up, they ought not be able to reach us. They should never arrive here. They should not cause us any trouble. But they do. What we count as a few microseconds becomes a longer period of time to the muons. Long enough to live on and on. Their time has slowed because of their high speed. To us observing it, the muon’s life has been extended—and ours perhaps shortened. Yet from the particle’s perspective, time passes normally.
There are places in the universe where only a single second of events pass while a million years’ worth of activities simultaneously elapses here on Earth. Yet both feel a normal passage of time.
So observers in different places experience out-of-sync sequences. If the rate of the passage of events depends on factors like the local gravity and one’s speed, how can there be a stable commodity called time?
Exploring this, physicists look to see if time is critical, or even has existence, in their physics equations—or whether what has been spoken of as time is merely the fact of change, long represented by the capital Greek letter delta: ∆. Doing so, they find that Newton’s laws, Einstein’s equations in all his theories, and even those of the quantum theory that came later, are all time symmetrical. Time simply plays no role. There is no forward movement of time. Many in the physical sciences thus declared time to be nonexistent.
As Craig Callender wrote in 2010, in Scientific American2:
The present moment feels special. It is real. However much you may remember the past or anticipate the future, you live in the present. Of course, the moment during which you read that sentence is no longer happening. This one is. In other words, it feels as though time flows, in the sense that the present is constantly updating itself. We have a deep intuition that the future is open until it becomes present and that the past is fixed. As time flows, this structure of fixed past, immediate present and open future gets carried forward in time. This structure is built into our language, thought and behavior. How we live our lives hangs on it.
Yet as natural as this way of thinking is, you will not find it reflected in science. The equations of physics do not tell us which events are occurring right now—they are like a map without the “you are here” symbol. The present moment does not exist in them, and therefore neither does the flow of time. Additionally, Albert Einstein’s theories of relativity suggest not only that there is no single special present but also that all moments are equally real.
Philosophers generally agreed. After all, the past is just a selective memory; your recollections of an event are different from mine. Both memories are simply
