What is time?
After taking more physics classes than I can even remember, I still have no clue how to answer that question. I remember sitting at the breakfast table with my roommate –– a philosophy student –– about a year ago. Out of silence, he turned to me and said in a very matter-of-fact tone “You misconceptualize time,” before going back to his coffee and morning paper.
The guy must have thought I was an idiot, because whenever he asked “What is time?” the best response I could muster was “It’s what you measure with a clock.”
A year is one orbit of the Earth about the Sun. A day is one rotation of the Earth about its axis. You keep track of hours and minutes with a set of spring loaded gears or digital circuitry attached to your wrist. And one second is 9,192,631,770 oscillations of an electron between two hyperfine levels in the ground state of a cesium atom. In other words, it comes from an atomic clock.
It therefore seems any practical approach to time involves measuring the duration of some physical process: gears, sand grains, pendulums or electrons. When you do so, you arrive at some surprising results.
Newton saw time as completely divorced from space. It marched forward in a way irrelevant of where you were standing and what you were doing. But that idea proves certifiably false.
Einstein asked us to throw away this picture and instead think about our collective motion through both space and time. In relativity, all objects travel at exactly the speed of light. This velocity can be budgeted between space travel and time travel. If you travel faster through space, you must therefore travel slower through time. Suddenly, folks travelling at different speeds relative to each other would disagree on how much time passes between events.
All this is not just a fun thought experiment. It can be directly observed. In 1971, two scientists placed four atomic clocks on a jet plane, and flew around the world. They recorded the time that transpired aboard the plane, and found all four clocks disagreed with clocks on the Earth’s surface by roughly 275 nanoseconds (for a Westward trip). Not exactly a tragic loss of accuracy, but a measurable effect!
Speed isn’t the only way to slow down your clocks. When you move deep into the gravitational field of a large object, your time also moves slower. Global positioning satellites orbiting Earth are farther out from the center of Earth’s gravitational pull. As a result, their clocks march at a slightly altered pace. If we did not correct for this difference, the GPS systems would fail miserably in triangulating your position. In fact, triangulating is a poor choice of words here, as it implies three unknowns: longitude, latitude and altitude. In fact, time is also an unknown, and it takes at least four satellites to find where you are.
The effect can be more extreme in other parts of the Universe. Physicist Bryan Greene wrote a children’s book about a space traveller named Icarus, who flies too close to a black hole. Icarus was far enough from the beast that he managed to escape but something bizarre happened. When Icarus was close to the hole, his clocks ran very, very slow, as observed by someone back on Earth. What was just a few minutes for Icarus turned out to be many years for everyone back home.
The most profound effect relativity has on time is the loss of “simultaneity.” Imagine Alexander Hamilton and Aaron Burr are standing on a cliff over the Hudson, about to partake in their famous duel. Let’s say their guns fire at exactly the same time –– according to an observer standing nearby. For an interstellar space traveller, travelling close to the speed of light, he might observe Hamilton firing long before Burr. Then again, if the alien were traveling in an opposite direction, the order of firings could be reversed!
The price of gold or U.S. treasuries may fluctuate, and the cost of energy may rise or fall. But time always feels precious and fleeting. I always though it was poignant that Ithaca’s local currency is “the hour.” And even if you travel through time, in your own personal, stationary reference frame, your body’s internal rhythms are marching forward at the exact same pace they always do, aging you.
Whatever time is, what time does is fascinating and enigmatic. As the British science fiction writer and humorist Douglas Adams would say “Time is an illusion. Lunchtime, doubly so.”
Munier Salem '10 is a graduate of Cornell and the founder of The Sun's Science Section.