Many of us are familiar with the concept of time dilation, though we may not know it by that name. Through the worlds of science fiction, we often read about the relativistic effects of traveling at or near the “cosmic speed limit” known as speed of light. Many of you may not know, however, that time dilation actually occurs with respect to gravitational pull as well. You may also not know that the effects of time dilation have been observed in experimental settings using atomic clocks. In fact, the astronauts living and working onboard the International Space Station (ISS) get to experience both types of time dilation.
So what is time dilation? Essentially, it is an effect predicted by the theory of relativity in which two objects moving relative to each other, or situated differently with respect to the pull of gravitational masses, experience an observable difference of time between them. An atomic clock can actually be measured to tick at a different rate when compared to a second observer’s own atomic clock. You may think that perhaps this is related to a mechanical difference between the clocks, or from the fact that signals take time to transfer back and forth, or even the fact that light itself takes time to reflect back and show the observers what they see. This is not the case. It is a natural phenomenon in our universe, explained by mathematics and tested through experimentation.
In relative velocity time dilation, a person approaching the speed of light would experience time at a slower rate than those observing the traveler. In other words, the faster the relative velocity, the greater the magnitude of time dilation. One could hop onboard a starship, travel for a few weeks, and arrive back at Earth to find that thirty years had passed by. In a way it is like time travel, except you are limited to a one way trip.
In gravitational time dilation, observable time actually speeds up the further you get from the source of gravity. A person at sea level would experience slower time than someone at the peak of Mount Everest. Gravitational time dilation is also the direct cause of gravitational redshift. Redshift is the process by which electromagnetic radiation (light, radio waves, microwaves, gamma waves, etc) originating from, or passing through, a high gravitational field is reduced in frequency when observed in a region of a weaker gravitational field. There also exists a corresponding blueshift when moving in the other direction.
Since time slows down with increased gravity and increased velocity, this means that astronauts on the ISS experience two opposing effects of time dilation. They are further away from the Earth, so their time speeds up. They are also traveling at high velocity (17,000 mph (~27 500 km/h)), so their relative time is slowing down. The effects of the relative velocity time dilation is actually stronger than the effects of the gravitational time dilation, so when the astronauts return to Earth after their 6 month stay, they have aged less than the folks in mission control that stayed on Earth. The difference is about 0.007 seconds.
As you can see, this phenomenon can present a bit of a challenge for the science fiction author. One must either choose to accept this as fact and apply them to the story, or come up with a way around them. For sufficiently high speeds the effect is dramatic, and space travellers could leave on a light-speed mission, and return to Earth billions of years in the future. In the Planet of the Apes and the Ender’s Game series, the authors chose to accept this limit as fact and apply it. In other works of science fiction, such as Dune, Star Trek and Star Wars, they found ways around it.
In Star Trek, warp drive is a concept that uses a bubble of “normal time” to surround the spacecraft and allow them to get around the relativistic impacts of their faster than light travel, and continue to be able to interact with objects in “normal space”. Star Wars uses “hyperspace,” which is an alternative region of space coexisting with our own. Entering hyperspace requires some sort of shield to protect the craft, and traveling through it allows the people to move from point to point faster than the speed of light, but they cannot interact with objects in “normal space.”. Dune uses a concept that folds space at the quantum level and enables travelers to move from point to point instantaneously.
In my own writing I plan to take an approach closer to that of Star Wars, since multiple dimensional layers will be somewhat important to the plot. The idea of 11 dimensional space in M-Theory caught my attention, and so I began to wonder if travel through one of these different dimensions might make it possible to travel through time or space without the impacts of relativistic time dilation. Travelers in my universe will shift into one of these dimensions assuming an effect similar to that of Hyperspace, where one slips put of Normal space into this alternate dimension in which they cannot communicate with, or interact with, normal space. The exception of course would be gravity, since this seems to be one of the only constants throughout the multi-dimensional universe. Gravity can also permeate the multi-verse, and has an effect on time travel, the idea being that the graviton particle is the most fundamental piece of life, the universe, and everything. (And you thought it was 42 ;) )
Of course there are many holes and mathematical improbabilities associated with faster than light travel, but I find a universe where I can travel from place to place without having to worry about when I leave and arrive relative to my speed to be a lot more fun to write about. I don’t want to believe that it is impossible for me to explore faster than the speed of light. Of course there are also holes and issues with existing theories of relativistic time dilation, and the interactions of the microverse with the macroverse, and no matter how many times I hear people say something is impossible, I would rather think of it as improbable. To say that we as humans have learned all that there is to know, and that we truly understand the universe we live in is a farce. There is always something more to do and something more to learn. If it can be imagined, it can in all probability be achieved.