By Taylor Briggs
From waking up to an alarm in the morning to making it through another full day of school or work, we rely on the consistency of time. Every hour, minute, and second passes just the same, no matter what, right? When the physics of the subject is considered, however, this proposition breaks down entirely, and we find that time is not constant.
Before taking on the counterintuitive nature of time, let’s ground our knowledge in something that is in fact constant: the speed of light. In his Special Theory of Relativity, Albert Einstein proposed that the speed of light is 300 million meters per second relative to anything. In other words, no matter the frame of reference (the position and movement of the observer) it is measured from, light remains at a constant speed of 300 million meters per second. Returning to our conversation about time, we can actually utilize the constant speed of light to explain the strange nature of time.
Relativity of simultaneity, a consequence of Einstein’s theory, introduces the flaw in treating time as constant. This concept shows that time is relative. The idea of simultaneity can be presented with a thought experiment. Imagine observer A is at rest on a train station platform, while observer B is travelling at a constant speed on a train. The train passes between two trees (one tree at the front of the train and one at the back) and observer A stands at the middle of the platform. If both trees are struck by lightning at the same instant, the light from each will reach the eyes of observer A simultaneously. However, since observer B is in motion, moving closer to the tree in front of the train, the light from the front lightning strike will reach observer B before the light from the tree at the back of the train. In other words, the light from the lightning strike at the back has to travel further distance to catch up. Now, what does this have to do with the alleged inconsistency of time presented so far? Well, referring back to “simultaneity,” this experiment proves that it is completely possible for two events (the lightning strikes) to happen at the same time in one reference frame (observer A) and at different times in another reference frame (observer B). Both observers are correct in their findings, and we can deduce that time is not absolute, but it depends on one’s frame of reference. To put it another way, time is relative.
As mentioned previously, a frame of reference is the position and motion of a particular observer. Knowing now that time is relative, we can explore the idea that observed time is affected by motion, just as we have seen in the train example. Time for objects in motion, or moving at some relatively larger speed, is actually slower. This is called time dilation. Vehicles on Earth do not move fast enough for time dilation to become significant. However, if humans could travel at much higher speeds, like we see in science fiction movies, time dilation would have a much greater impact. Someone traveling on a spaceship that moves near the speed of light would age only by a few hours, while a hundred years may have passed on Earth. We can conclude that the inconsistency of time relies heavily on motion.
The time dilation in movies such as Interstellar, however, relies on a different kind of warped time. In the film, there is an extreme amount of time dilation because the planet that the main character travels to, called “Miller’s Planet,” orbits a black hole. Time ticks much more slowly on this planet due to the immense mass of the black hole. Resorting back to Einstein, this is a consequence of his second groundbreaking discovery, the General Theory of Relativity. To briefly summarize, Einstein determined that there is a natural connection between mass, gravity, space, and time — mass generates a curvature in space-time, and gravity is a consequence of that curvature. So, not only does mass affect space, but time also moves slower with increased gravitational force. The extent of time dilation near black holes, which are incredibly dense and have enormous gravitational pulls, will therefore be large. This is why in Interstellar, the characters on Miller’s planet are able to age only one hour for every seven years that pass on Earth. However, it is not just in science fiction that time dilation is worth noting; we are not entirely removed from the effects of time dilation. In fact, the Global Positioning Systems (GPS) on our phones must account for the decreased gravitational pull of the Earth at such a high altitude. The GPS satellites must actually count time slower to make up for time dilation.
In conclusion, the passing of time is not constant, and it depends entirely on an observer’s frame of reference. Within the particular frame of reference, it also depends on both motion and gravity. So, even though we all rely on alarms and schedules, our individual perceptions of time differ very slightly. Just like the observers on the train and the platform, however, we are all correct in our observations. Although our individual perceptions of time are only negligibly different, knowing that we all perceive it slightly differently shows that our perception of time as constant is merely an illusion. In fact, time is not some constant social construct, but a part of nature and of space-time, which is, by definition, relative.