In this blog post, we’ll explore Einstein’s theory of relativity in an easy and interesting way, focusing on the time dilation phenomenon featured in the movie Interstellar.
The movie ‘Interstellar’ depicts humanity’s search for habitable planets in space to overcome Earth’s food shortage crisis. While its stunning visuals and the transcendent love between father and daughter contributed to its box office success, the most crucial element was its grounded scientific content. Unlike other sci-fi films, ‘Interstellar’ was crafted following scientific principles, allowing audiences to learn about science through entertainment and imagine the possibility of such events occurring in reality. So, let’s take a closer look at the scientific content within ‘Interstellar’ that sparked interest in physics even among those who previously had none.
The most significant event in the film’s plot is when the protagonist, Cooper, stays too long near a star with strong gravity, causing an enormous amount of time to pass on Earth. While only a few dozen minutes passed on the planet Cooper stayed on, decades had elapsed on Earth. Seeing his son and daughter aged beyond recognition in an instant, Cooper’s anguish moved the film’s audience to tears. The scene where his daughter Murphy, on the brink of death from old age, dramatically reunites with her father Cooper is arguably the most emotionally powerful moment. Here, many people struggled to accept the fact that time flows at different rates on Earth and on other planets in space, and they were curious about the principle behind it. This is precisely what Einstein’s theory of relativity explains. The theory of relativity is divided into special relativity and general relativity; special relativity deals with inertial reference frames, while general relativity deals with accelerated reference frames.
An inertial coordinate system is one where no external forces act on an object, thus conserving momentum. Consequently, the object either remains at rest or moves at constant velocity. Newton’s First Law of Motion, the law of inertia, holds only in such cases, hence the term inertial coordinate system. The law of inertia states that an object in motion will continue in its state of motion unless acted upon by an external force, and an object at rest will remain at rest unless acted upon by an external force. For example, when a bus suddenly starts moving forward, our bodies resist the motion and we lean backward. Conversely, when a bus moving forward suddenly stops, our bodies resist the change and we lean forward. In this inertial frame of reference, an object moving forward appears faster to someone moving backward than to someone stationary. However, Einstein realized that the speed of light remains constant regardless of whether observed by someone moving backward or someone stationary. Based on this, he established the theory that all motion is relative. This means absolute space and absolute time do not exist; time and space are defined by the observer. It is not that light travels 300,000 km in one second, but rather that the time it takes for light to travel 300,000 km is one second. When a spacecraft is moving very fast, if light is emitted perpendicular to the spacecraft’s direction of motion, the distance the light travels appears greater to an observer stationary outside the spacecraft than to an observer inside the spacecraft. Since the absolute quantity is the speed of light, not time, an event that occurs within one second for someone inside the spacecraft will appear to take longer than one second to someone stationary outside the spacecraft. This leads to the conclusion that, from the perspective of a stationary observer, time passes more slowly for an object moving at a constant velocity than for a stationary object.
An accelerated coordinate system, unlike an inertial coordinate system, is one where an external force acts upon an object, causing its velocity to change. In this accelerated coordinate system, a force called inertial force exists. When undergoing accelerated motion, an object tends to lean to one side. An observer in an inertial coordinate system can understand this through the laws of inertia, but an observer in the same accelerated coordinate system, unaware of the cause of this force, refers to it as inertial force. Examples include the phenomenon of grab handles tilting backward when a bus starts and accelerates, or the sensation of increased weight when an elevator accelerates upward. However, if an observer is inside a sealed box within an accelerated coordinate system, that observer cannot distinguish whether the force acting on themselves and objects is due to gravity or an inertial force. This means dropping a ball on the Earth’s surface and releasing a ball while accelerating upward in weightless space appear to be the same phenomenon. The inability to distinguish between inertial force and gravity is called the equivalence principle.
General relativity explains gravity as a curvature of spacetime. It describes the gravitational pull as objects drawing closer to each other due to the curvature of space, much like a ball rolling down a sloped surface. Similarly, the slowing of time in regions of strong gravity can be explained as time being warped by gravity.
Gravity causes time to stretch. The stronger the gravity, the greater the slowing of time. In fact, even on Earth, differences in altitude cause variations in gravitational strength, leading to differences in the rate at which time passes. However, these differences are so extremely small that they are imperceptible. This is precisely why, in the movie ‘Interstellar’, a few tens of minutes on a star with strong gravity equate to several decades on Earth.
Beyond the theory of relativity driving the main narrative, the film featured other scientific concepts like parallel universes and black holes, allowing many people to encounter these ideas. Through ‘Interstellar,’ I felt how a single medium can spark interest in so many people. If more films or other entertainment media emerge that handle science in an accessible and engaging way, like ‘Interstellar,’ specialized fields like physics could gain significant public interest.
