Staff Editor
If you ever thought that time doesn’t always flow at the same rate, you’re right. While it may be a trick of perception that the ride home always seems shorter than the ride out, or that the five minutes before quitting hour take two hours to circle the dial, two equally accurate clocks give different readings, even at displacement distances as small as a single millimeter.
White water in the river of time
The shift may be minuscule but the effect is enough to be measured. If time flows like a river, it has white-water rapids and calm peaceful pools.
Just like the flow of water down a mountain, how fast it runs depends on the local gravity. Because gravity slows the flow, clocks on Earth “tick slower than those in space.” That matters to people who need to keep track of where their satellites are at all times.
The latest news is that a group of researchers “measured time passing at different speeds across just one millimeter, the smallest distance yet.” The late great Albert Einstein was the one who first suggested that time itself could be affected by gravity, all the way back in 1915.
It’s built in to the theory of general relativity. Space and time are two sides of the same spacetime coin. “Large masses warp the fabric of spacetime with their immense gravitational influence.”
“This has the effect of making time pass more slowly closer to a large mass like a planet, star, or, in the most extreme example, a black hole. This phenomenon is known as time dilation.”
As Science fiction described decades ago, if someone gets trapped “beyond the blue event horizon,” their partner can spend half a lifetime coming to their rescue. And still get there too late.
Effects hard to notice
For those of us glued to planet Earth, we aren’t likely to notice the time dilation effect. Just because clocks move faster in the mountains than the coastline doesn’t mean you need to change your vacation plans.
“Someone living in a 10th floor apartment will age faster than someone on the first floor, and your head ages faster than your feet.” Across those distances, even to the top of Mt. Everest, the difference isn’t noticeable.
They can, however, “be measured using atomic clocks, which keep time very precisely using the reliable ticks of atoms.”
All those satellites whizzing around above our heads carry atomic clocks which can be compared with similar models on the ground and in moving aircraft. The differences are easy to see and measure. What blew everyone’s mind was the ability to measure a difference without leaving the lab.
Researchers at JILA, which used to be called the Joint Institute for Laboratory Astrophysics, have measured time dilation over just one millimeter. To do it, they “used an atomic clock composed of an ultracold cloud of about 100,000 strontium atoms.” Instead of wheels and gears, the “clock’s ‘ticking’ comes from the atoms switching back and forth between two energy levels, which they do at an extremely reliable frequency.” That frequency may be “reliable” but it changes depending on where it sits above the table.
“Through careful control of these energy states, the team was able to make all the atoms in the cloud tick in perfect unison for 37 seconds, a record length of time.” Then they moved one millimeter. “Sure enough, they detected a difference between the two regions, due to the time dilation. The shift in frequencies was of course tiny, only 0.0000000000000000001, but it was measurable.“
