Time is Relative

Occam’s Razor tells us that the simplest idea is always the best. In many situations, this is true. There is no need to needlessly complicate. The more complications you create, the more explanations you need to find.

This rule does not apply in the realm of time. We perceive time to move in a linear fashion, but this is not the case at all. Einstein discovered almost 100 years ago that in every situation, time is relative. Via special and general relativity, he showed the world that the simplest explanation was not the correct one. Time isn’t linear at all. Time is relative.

What does that even mean? It means that time isn’t uniform for all objects in the universe. In fact, the way time progresses depends on the relation (hence, relativity) between two entities. The ideas behind special and general relativity are incredibly complex, but there are really fascinating implications that we can all understand. Special relativity tells us that light travels at the same speed in all reference frames. In simple terms, no matter how fast you are moving, the light around you is moving at the constant speed of 300,000 km/s. Why is this weird? Imagine running next to a train moving at 60mph. The faster you go, the slower the train seems to go. Once you’re running at 60mph, it is as if the train is not moving at all. However, light is like a train that keeps moving at the same speed as you no matter how fast you run. If you’re running at 100 mph, the train will still seem to be moving at 60mph. Trippy, I know.

There are a few consequences of this observation. In special relativity, there exists the idea of time dilation, where time will pass differently based on a velocity difference between two objects. Imagine you are on a spaceship traveling at a high speed, and you have a clock on board. From your perspective, time is passing normally on your clock. However, if you were to look out the window at the Earth, you would see that time is zipping by quicker. For example, if you were traveling at 90% of the speed of light, and you stayed on the spaceship for one year, when you returned to Earth, you would find that 100 years have passed. Another phenomenon, called length contraction, occurs when an object is traveling toward you. If a meter stick is traveling towards you at 90% of the speed of light, it will appear to be less than half of a meter. As it approaches the speed of light, it will continue to shrink towards zero.

Note how I didn’t mention a minimum velocity it takes for these phenomena to occur. That means they occur in everyday life, you just don’t notice it. At human speeds, they are practically nonexistent. At 60mph, the time dilation factor for an entire year is on the scale of 10 trillionths of a second. And that’s for a year. For a second, it’s on the scale of femtoseconds. That’s 1 / 10^15 of a second! So realistically speaking, you could live your life as if special relativity didn’t exist. But, we’ll see later that it actually does affect your everyday life, just in ways that you don’t think about.

What is general relativity? Einstein theorized that gravity is a result of the curvature of spacetime, where larger objects would distort the “fabric of spacetime” even more, resulting in larger gravitational effects (Einstein must have been insane). What the hell does that even mean? I don’t know, but I can do my best to describe it. Essentially, spacetime is a 4D way of describing physical space– the fourth dimension being time. Heavy objects will warp this spacetime in the fourth dimension, similar to the way a large bowling ball will bend a stretched-out bedsheet. And somehow, objects will follow the curvature, bending in the direction of the object that created it. With this theory, even massless “objects” like light (photons) will be influenced by gravity, which Newtonian gravity does not explain. The details don’t matter too much, but there are some bizarre consequences.

Time dilation also exists in general relativity, but in a different form, aptly termed gravitational time dilation. It predicts that the passage of time can be affected by the strength of a gravitational field. In a strong gravitational field, time will appear to pass more slowly, as compared to a weaker gravitational field or the absence of a gravitational field. This effect occurs because the fabric of spacetime is curved by the presence of a massive body, and the closer an object is to the massive body, the more pronounced the curvature of spacetime will be. As a result, a clock that is located closer to a massive body will be measured to tick slower than a clock that is farther away. So, the more massive planet or the closer you are to it, the slower time will tick. Near the singularity of a black hole, time will essentially cease to exist.

This probably just sounds like a bunch of gibberish at this point. Can we even measure these effects? The problematic answer is yes. Humans are so brilliant that we can even measure these effects on our home planet. We’ve seen the bending of light near the Sun (called gravitational lensing), measured time dilation using atomic clocks that we flew on planes, and corrected time delays on spacecraft. For example, it is critical for GPS to have accurate timing in order to synchronize with devices on our planet. GPS satellites are affected by both general and special relativity. Since they are moving fast relative to the surface, special relativity predicts a time delay of 7 microseconds per day. Since they are also further from Earth’s gravitational center (higher potential energy), general relativity predicts that their time gets ahead by 45 microseconds a day. Overall, GPS clocks tick about 38 microseconds faster every single day¹. This can add up over time and cause problems, so the engineers make sure that the GPS ticks a bit slower to match clocks on Earth. Brilliant.

At the end of the day, special and general relativity are just theories. What are theories? Theories are not facts. They are systems of thoughts that are constructed to explain reality. There is a very good chance that these theories are not 100% accurate or even true at all. Newton’s theory of gravity is incomplete, yet physicists and engineers use his equations every single day because they explain reality very accurately most of the time. Even if general relativity and special relativity are “false”, they have accurately predicted many real-life situations, just as how Newtonian gravity predicts most real-life situations. We even know general relativity doesn’t mesh well with quantum effects. But, not all theories have to be true in every situation. They just have to be useful in the situations where they apply. And unless we can find something better, they are the best we can do today. And if you think you can do better, then maybe you are the next Einstein.