# How Things Work: Time

I?m not quite sure how to say this, but I?ll try to break it to you easy: We have no idea how time works.

The quantization of time was man?s first scientific idea, and while the accuracy of our measurements of this magic quantity have improved, our understanding of it has not. I can tell you how a cesium clock works, but I can?t tell you how time works. The best I can do is describe it.

Here?s the official version: ?The International System of Units (SI) has defined the second as the period equal to 9,192,631,770 cycles of the radiation which corresponds to the transition between two energy levels of the ground state of the cesium-133 atom.? Wow. To my remaining readers: I promise I won?t do that again.

Time has boggled our minds ever since the first sundial was built. It has been studied by all the great minds of history, and still no one has a clue. In physics, time is the fourth dimension. Physicists talk about the ?fabric of spacetime,? a canvas on which they can paint their causality laws and predict effects. Physics has made great leaps in investigating the properties of time.

Einstein?s theory of special relativity tells us that time does not move at a constant rate, but is dependent on one?s speed through space. The faster you go, the slower time moves for you. As you approach the speed of light, time will move slower and slower. Of course, we can?t tell if time stops at the speed of light, because no one can ever go that fast. Consider the International Space Station, circling the earth at 7.7 km/s. Time actually moves a tiny bit more slowly for these astronauts than it does for us.

Yet there is another situation in which time can be warped. Einstein?s theory of general relativity says that time moves slower in gravitational fields. Again consider the space station, orbiting 400 km above us. Since the force of Earth?s gravity is very small, time actually moves faster for the astronauts.

So, on one hand, time moves faster for the ISS because it is going so fast. On the other hand, time moves more slowly because there is little gravity. The question is: Which effect wins out? Beyond a certain orbit (9500 km), the gravitational effect is dominant. However, the ISS actually orbits only 407 kilometers up, so the ISS clock does run slow: about 0.0000000014 percent slower than a clock on Earth.

Perhaps the most important question is: Can I go back in time and replay last night?s poker game? Some ?glass-half-empty? people point out that if time travel is truly possible, then where are the time travelers? There are a variety of answers. Perhaps Earth will be destroyed by an asteroid before technology advances to the point where we can travel through time. Perhaps it?s too expensive to travel back this far in time. Perhaps the CIA already has all the time travelers locked up in Area 51. My personal favorite: Maybe we broke all the flux capacitors.