The ability to manipulate one’s place in time has often been embraced as a key plot element in science fiction. But how realistic is traveling forward or backward in time? The answer might surprise you.

**Traveling back in Time**

Unfortunately for Marty McFly and Doc Brown, traveling back in time is not as simple as strapping a flux capacitor onto a Delorean. In fact, there are a number of physics laws that prevent traveling against the arrow of time entirely. Theoretically, the only way one could travel to an earlier point in time would be to utilize a wormhole. A wormhole is a cosmic “shortcut” between two points in spacetime. Entering on one end would result in exiting the other end faster

than light could get to that point on it’s own, effectively getting to the destination before time does. Unfortunately, the likelihood of finding a stable wormhole capable of allowing anything to pass through it is almost zero, so traveling to the past may never be a possibility.

**Traveling into the Future**

This is where things get interesting. Scientifically, it is quite possible to time travel to the future. This is done utilizing Einstein’s Theory of Relativity, which allows for time to pass at different rates depending upon the environment the observers exist in. This concept is referred to as Time Dilation.

** **

**Relative Velocity Time Dilation**

One postulate of the Theory of Special Relativity states that light will always travel at the same velocity in a vacuum, regardless of the velocity and direction of the source. While on the surface this doesn’t seem particularly interesting, it actually has unexpected consequences on the passage of time.

Imagine two mirrors facing each other, one meter apart. Now imagine a photon of light bouncing back and forth between those two mirrors. At rest, the photon will bounce off each mirror approximately three hundred million times per second, as this is the speed of light.

Now imagine that device on a rocket ship traveling close to the speed of light. Just like throwing a ball up and down on a train, to observer A on the rocket ship, the light looks like it’s bouncing up and down, just like it was at rest. However, to an observer B outside of the ship watching it rocket past, the path of the photon looks like it is traveling on an angled trajectory.

Because the observer B sees the photon travel at a non-vertical trajectory, it actually appears to travel slightly more than one meter between each mirror. But since light MUST always at the speed of light, that extra distance means it takes slightly longer to bounce between the mirrors, according to the observer B. Therefore, the clock looks to be moving at a slower rate than what the observer A sees, as it is bouncing at less than three hundred million times a second.

In essence, the time that passes “feels” the same to both observers, but the one on the rocket ship will actually experience a shorter duration of time relative to the outside observer. This phenomenon can then be used to manipulate time in such a way that a short period of time can pass for someone traveling at an extremely fast rate of speed relative to the Earth. For example, people on a rocket traveling at 87% the speed of light could travel for two year’s worth of Earth time, and only have aged one year.

**Gravitational Time Dilation**

The Theory of General Relativity adds the element of gravity to Einstein’s equations. In Newtonian Physics, gravity and acceleration are essentially analogous when it comes to calculations. Therefore, an observer close to a source of high gravity such as a black hole, will experience time travel at a slower rate relative to an observer further away from the gravitational source.

For instance, if you could shrink all of Jupiter’s mass down to a 5 meter wide sphere and stand next to it, time would travel four times slower for you than an observer in empty space.=

**A Relativistic Tug of War**

One interesting consequence of these two time dilation methodologies is their apparent cancellation in regards to Earth’s satellites. Satellites travel much more quickly around the planet than we do on the surface, which should slow their time relative to ours. However they also are further from Earth’s gravity, which means the surface’s time should be slower. So which wins out?

In fact, it depends upon the distance of the orbiting satellite. At around 3166 km above Earth’s surface, the effects of motion and gravitational time dilation cancels out. Below this distance, such as where the ISS orbits, the relative motion governs the time change. Further out, such as where GPS satellites exist, the difference in gravity is a more significant contributor to the time dilation. In fact, GPS satellites have to be programmed to account for this small but significant difference in time.