Friday, March 20, 2015

Introduction to Time Travel: Let’s Travel into the Future!

We live in a world with three spatial dimensions: width, height and depth. One might think that is enough to accurately pinpoint a location. But let’s think about this more carefully. Let’s say my friend is hosting a party. Excited to be social, I ask him where it will be located. He says, “Of course! It’s on the corner of 34th Street and 4th Avenue in NYC.” Visualizing NYC as a two-dimensional grid, I now know where the party will be located in the x and y direction. He continues and says, “Oh yeah, I live on the third floor. Apartment number 36149. See ya later.” Perfect- now I know exactly where the party is. Thinking the z-axis as height, I have pinpointed an exact point in space where the party will be. I have all the information to go to his party…or do I? Although I know exactly where this party will be held, I have no idea when it is to happen. Tomorrow? Next week? In three years? Without a time, the location of the party is just a concept. Just like how a line is physically impossible as it has no thickness, a three-dimensional object (or place) cannot exist if it not passing through time. So time is our fourth dimension. Now the question is if we can manipulate objects within the three spatial dimensions, can we also manipulate our fourth dimension, time?

There have been countless books having characters go into a time machine and travel back/forward through time. There have been countless movies with time travel as the main plot device. There have been countless theories on time travel, from the absolutely absurd ones to ones take make sense in the realms of physics. Let’s take a look at one plausible way of traveling into the future: traveling at high speeds.

Einstein’s special theory of relativity is one of the most commonly used ideas to prove that theoretically one can time travel. This theory is based on two ideas: (1) the laws of physics are the same for everyone, regardless where they are in space, and (2) the speed of light is constant for every person that is in uniform motion. Einstein’s theory leads us to think that if there was one person on Earth and one person in a spaceship that could travel very near the speed of light (like 99.0006% the speed of light), the person on the spaceship could travel out into space, come back, and find he/she is much younger than the person on Earth. To look at this more in depth, Einstein developed a thought experiment by using light clocks. These clocks are made up of two mirrors with light reflecting in between them. With two mirrors that are three feet apart and assuming that light travels at 300,000 kilometers/second, it takes one photon three nanoseconds to get from one mirror to another. That is one “tick” for the clock. Using the spaceship example again, if a light clock was put into a transparent spaceship, the observer on the ground will see something peculiar. As the spaceship moves, the light bouncing in-between the mirrors will look like it’s going in a zigzag pattern. The light will actually travel more distance between the two mirrors than three feet because to the observer on the ground, the light from the mirror has already shifted to the right due to the speed of the spaceship (the diagram illustrates this). If distance has grown, but the speed of light is still constant, then that means time must have increased. The person on the ground will see the ticks of the light clock to be longer than three nanoseconds.

Now instead of a light clock and its ticks, let’s look instead at a person’s heartbeat. The same concept applies; the person on the ground will see (or hear) the heartbeat of the astronaut in the spaceship to be much slower. Biologically, this means the person on the ground sees the person in the spaceship to be aging slower. Conversely, after traveling through space at the speed of light, the person on the spaceship will come back to Earth noticing he is a lot younger relative to everything on Earth. Thus, the person in the spaceship has time traveled into the future.


J. Richard Gott. Time Travel in Einstein’s Universe
- Eric Lee