General relativity is one of the principle theories of modern science, but one that is also hard to grasp at first. The problem is that the effects of relativity are only felt at high energy levels, high speeds, and in large gravitational fields. Since most of us do not live near black holes or regularly travel at speeds near the speed of light, relativity is outside of our standard conception of the universe.
After watching Interstellar I was able to spend some time with my friends discussing relativity and what it means. The concept has been explained countless times in popular science shows and books, but this is my crack at explaining relativity in the easiest way possible.
Let’s start by shaking up our conception of space and time.
We all live in four dimensions. These are the x, y, and z spacial dimensions and one dimension of time. What this means is that I can define where I am in the universe by four numbers: my spacial dimensions (in reference to an object) and where I am in time. Because of this, physicists speak of spacetime as one object. We really should look at space and time as being intricately connected and both part of the underlying fabric of space.
With that in place, let’s talk about light a bit. Light is an interesting object. We know that it travels as both discrete packets (called photons) and as a wave. Light is a form of electromagnetic radiation. It is also incredibly fast, about 186,000 miles per second.
In the late 1800’s, a physicist named James Clerk Maxwell developed a set of equations that gave us the speed of light. This speed seemed to be universal throughout space and time. Albert Einstein came along and found out that light had a universal speed in all reference frames, whether inertial or not. This laid the groundwork for relativity.
Einstein’s work gave us an extraordinary view of how light works.
Imagine that you are flying in a space ship at 100,000 miles per second. You decide to test Einstein’s theory, so you have a light beam shot out of the front of your spaceship. How fast do you think the light would go? Everyday intuition tells us that it should be going at 286,000 miles per second, your velocity plus the velocity of light.
This is an intuitive answer for us. If we are travelling in a car at 60 miles per hour and throw a ball forward with a velocity of 20 miles per hour, the total velocity of the ball is 80 miles per hour. Otherwise it would crash through our window.
Light is weird though. It does not behave like a ball thrown from a car. If you shot a beam of light from your spaceship going at 100,000 miles per second, the speed of light would not be 286,000 miles per second. It would just be 186,000 miles per second, the same speed as if you were sitting at rest. Nothing can travel faster than 186,000 miles per second.
When Einstein figured this out, 20th century physics took off. His theory gave us a better concept of light, but also showed us some odd things that would happen to us as we traveled faster and faster.
Let’s get back to the idea of 4-dimensional spacetime.
If you are sitting still, you have no velocity in any of the spacial directions (this is not completely accurate, since the Earth is moving, the Sun is moving, the galaxy is moving and the universe is expanding. But for simplicity sake, we consider sitting still as sitting still relative to the Earth). All of your “velocity” is forward in time. At the speed of light.
But what happens when we start moving? Say we were riding a spaceship at the velocity of 100,000 miles per second. Now we have a huge, but sublight, velocity. Because of that, the universe compensates, and now time is passing for us at a slower rate. The rate would be the difference between the speed of light and the velocity of our spaceship. For us travelling at these speeds, time would be moving slower relative to somebody standing still. And we would not be travelling faster than the speed of light. (Note that actually finding out the amount of time dilation is a little more complex, and we won’t go into that here).
Here is an example that helped me understand this phenomenon. Imagine that you are driving north in a car at 60 mph on the highway. In order to get to your destination, you have to turn off the highway and start moving in a Northeast direction, but you do not slow down. The magnitude of your velocity is still 60 mph, but now your velocity is split into North-moving and East-moving components. You are no longer moving at 60 mph North, but rather at a lesser velocity because some of your velocity is being used to move you East.
This is the same thing that happens with time. When we are no longer only moving in the time “direction” some of our velocity is used in spacial directions, and the speed that we are “moving” in time is slower.
Sounds kind of crazy right? As exotic as all of this sounds, scientists have actually tested the effects of relativity.
In 1971, two physicists named J.C. Hafele and R.E. Keating tested for time dilation with four Cesium powered atomic clocks. Three clocks were sent on commercial airliners on long flights. When the flights landed, the clocks were compared to the atomic clock at the United States Naval Observatory (which had been stationary), and were found to disagree by small amounts. Since the airliners traveled very slow compared to the speed of light, the differences were minuscule, but they did exist.
Astronauts experience minute time dilation on the International Space Station. The ISS orbits at around 5 miles per second (18,000 mph). Clocks on the ISS have shown discrepancies compared to clocks on the ground on Earth. Astronauts in orbit age slightly less.
If you will remember, Interstellar made a big deal about time dilation, especially on a planet close to black hole. Einstein discovered that large gravitational fields also cause time dilation, and this is known as general relativity. In real life, GPS satellites account for the gravitational field of the Earth on their instruments. Without accounting for relativity, GPS satellites would slowly lose accuracy and eventually become unusable.
This explanation of relativity has been simplified and ignores the fascinating mathematics behind how light works, but hopefully gave you an idea of how this theory changes our conception of the universe.