It turns out Doctor Who was right: “Timey-wimey” is, indeed, a bit “wibbly-wobbly.”
But before we explore the concept of time, we need to first explore the concept of gravity. We experience gravity two-dimensionally: up and down. However, gravity is not two-dimensional. If you were to drop a bowling ball off of a tower, it would fall down to the ground. If you were to drop a bowling ball off a tower in New York City at the same moment your Aussie friend dropped a bowling ball off a tower in Perth, both bowling balls would fall “down”—but they’d also be traveling in opposite directions. They’d be falling “down” as well as toward one another.
Earth’s gravity pulls all objects—from all directions—toward the center of the planet. We define “up” and “down” based on that gravitational pull. If you were to jump into a big underground drill and travel to the center of the Earth, you’d reach a point where there was no more “down”—only “up.” “Down” doesn’t exist at that point—at least in terms of the Earth.
Mass creates gravitational force. Therefore, all objects have gravity. The more mass an object has, the more gravitational force it exerts. Marbles have a little. I have some and would like less. Earth has the perfect amount, I’d argue. And Jupiter has a pantload. If you were to stand on the surface of Jupiter (a concept with which every geek reading this just started arguing), you would be pulled toward the center of the planet 154 percent harder than you are here on Earth. You’d weigh 254 percent of what you weigh here. You’d be crushed—not only emotionally, but physically as well.
Now, let’s take a step off the surface of these planets and explore what’s going on in the space around them. Our moon is held in orbit by the Earth’s gravitational pull. The Earth circles the Sun because of the Sun’s massive gravitational force. That’s all basic stuff we learned in fourth grade. Here’s where it gets wiblby-wobbly: Gravity also tugs at time and space.
In his 1905 special theory of relativity, Einstein asserted that space and time were not two separate phenomenons or entities. Instead, space (height, width, length) and time (forward? backward?) were mixed together in one continuum—woven together in one fabric. This single continuum of time and space has come to be called “spacetime”—the physical manifestation of all space and all time. I think of it as a soup. Spacetime is the broth and we’re the chunky bits floating about.
Think of spacetime soup this way: If you’re standing alone on top of a hill waiting for your significant other to wander up, you can point in an infinite number of directions: up, down, left, right, in front of you, behind you, slightly up-left, kinda down right, and so on. You can see and define the three physical dimensions of our universe. But if you stood perfectly still, you’d still move forward through time. At a point, your significant other would appear. And you would have traveled—while standing still on a hill—forward through time to that point.
In 1916, Einstein updated his special theory of relativity to account for gravity. Einstein’s first theory of relativity defined time as part of the physical world. (By “physical world” I mean all the aspects of the universe that exhibit as matter or energy—you know, that whole E=mc2 mass-energy equivalence thing.) This new theory of general relativity postulated that if time was, in fact, part of the physical world then gravity should affect time as well as space.
Scientists have confirmed this to be true several times since Einstein first put forth the idea. Using highly technical experiments and incredibly-sensitive instruments, people much smarter than I have been able to prove that gravitational force alters the speed of time. The most accessible example and proof of this phenomenon, I think, comes to us from the clocks aboard our nation’s GPS satellites.
There are a few dozen GPS satellites floating high above the Earth. Each satellite carries an atomic clock that, when on Earth, is perfectly precise and in sync with Earth time. However, when lifted to the less dense gravity of the upper atmosphere, the satellites’ atomic clocks speed up. Were an observer to fly up to one of these satellites and watch the on-board atomic clock, he would see no difference in the length of a second. It would still be that familiar tick, tick, tick of Earth seconds. At that level of gravity, he, too, would be moving faster through time and would therefore see one second to be one plain old second. But, from here on the Earth’s surface and from within our denser gravitational field, we can see that the seconds pass a little more quickly on the satellites.
Time is slowed by heavy gravity. Just as it’s easier to swim through outer space, than it is through the atmosphere, than it is through water, than it is through rock, time moves more quickly through less dense gravity. Time passes more slowly on Jupiter than it does here on Earth. And, as the impatient clocks on the Mars rover prove, time passes more quickly on Mars due to its lighter gravitational pull.
Now, here’s where it gets really interesting. “Gravity” is the same as “gravitational force”—or G-force—or acceleration. Therefore, as Einstein explained in his famous “twin thought experiment,” objects in motion experience greater gravitational force (G-force) and therefore travel more slowly through time than objects that are standing still. Therefore, Einstein said (and physicists have since proven), if two 20-something twins with identical heart rates were separated—one stayed put on Earth, and one traveled away from Earth at light speed for six months and then back for six months—the traveling twin would be one year older, and the stationary twin would be in a nursing home gumming Jell-O.
Time for the traveling twin was slowed by the gravitational force created by traveling at light speed, and time rushed forward—though rather boringly—for the twin who was left behind. And, most interestingly, throughout the whole experiment, each twin experienced their own heart rates as nothing but normal the entire time.
The implications of this discovery and still-relatively-new-model-of-thinking are staggering. First, and most importantly, move your bed to your basement and your evil twin’s bed to the attic you’ll get a longer night’s sleep and outlive the thieving bastard. Secondly, if time is affected by gravitational force, and we can create gravitational force through acceleration—whether in a rocket or a merry-go-round—we can control time.
If we can control time, can we now claim to be time-travelers? And if so, where’s my TARDIS?>
Image via (cc) flickr user Kevin Krejci
