Personal Milestone: I Finally Get Relativity!

Matt Williams
5 min readJan 19, 2022

In life, it is good to set personal goals. Not just the big and life-changing kind, mind you, I’m talking about the little things that help you to measure your growth as an individual. A few years ago, I set one of these little goals for myself: I wanted to be able to explain Relativity. As Einstein said, “If you can’t explain it to a six-year-old, you don’t understand it yourself.”

The other day, while practicing explaining Relativity, I finally felt the dots connecting. E=mc2, mass-energy equivalence, inertial reference frames, gravitation, spacetime, etc. Basically, I was finally able to explain to myself in a way where what and why came together. While I’m positive my grasp lives up to Einstein’s metric just yet, I’m not sure anyone could explain Relativity to a six-year-old.

Maybe he could, and maybe he did. I really don’t know! In any case, reaching this milestone actually raised a lot of the challenges I face as a science communicator.

Imposter Syndrome

If there is one thing I’ve learned as a science communicator, it’s that Einstein’s metric applies to everything you do. Your one job is to take complex ideas and groundbreaking research and make it interesting and accessible to the general public. Ordinarily, this isn’t too daunting a challenge seeing as how I love this stuff, and it is always of interest to me. As long as you’re interested in the material and willing to learn about it, it’s relatively easy to convey it to others.

But when it comes to theoretical physics and the mathematical and equation side of things, I tend to get a little anxious. In fact, you could say I have a full-blown case of impostor syndrome when it comes to the pure science aspect of what I do. I will often preamble any discussion on astronomy and astrophysics by telling people know that I don’t have a formal education in these fields.

On an intellectual level, this is motivated by a desire to be upfront with people. If you’re going to address a particular subject, you should let others know in advance that you’re not an expert and are not pretending to be. In an academic sense, saying “this is not my field” is simply being honest. On a visceral level, I’m motivated by a sense of insecurity about not having a background in physics, and I’m worried about being caught in a mistake.

On the plus side, this insecurity motivates me to learn more and proceed with a measure of caution. Relativity is not only a cornerstone of modern physics. It’s also one of the toughest to explain. Hence, I picked it (and the ability to explain it in simple terms) as a personal goal. Here’s how it works. Let me know if I reached my goal or not!

Galileo’s Ship

Despite what we tend to be taught, the concept of Relativity did not originate with Einstein. It was Galileo who was credited with coining this term. During the late-16th and early-17th centuries, Galileo was busy educating the public on Copernicus’ Heliocentric Model of the Universe (where the planets revolve around the Sun). In the course of that, he had the unfortunate task of explaining how the Earth could be moving, but those on its surface would be unaware of this fact.

According to established Aristotelian physics, people would feel the motion of the Earth and even be tossed around by its rapid rotation. To illustrate why this wasn’t the case, Galileo illustrated using the clever metaphor of a ship at sea. If the ship was moving at a constant speed, someone on the deck would barely know it was moving. Furthermore, if they dropped a ball over the side, it would appear as if the ball fell straight down. But to a person standing on the shore, the ball would appear to be falling in an arc — moving in the same direction as the ship while it fell.

From this, Galileo showed that motion and velocity are relative to the observer. If you are in a moving (inertial) reference frame — i.e., a ship at sea or planet Earth around the Sun — there is no obvious indication that you’re the one moving. In fact, it would look to us like we’re the center of things, and everything is moving around us. You see where he was going with this? Right! The idea that the Sun, Moon, and planets revolved around Earth was an illusion caused by the relative nature of our reference frame.

Galilean Relativity (or Galilean Invariance) was the name for this concept, and it remains one of his greatest teachable ideas. Galileo also made some pioneering work in gravity, where he demonstrated that objects of different masses fall with the same acceleration towards Earth. According to legend, he did so by dropping weights off of the Leaning Tower of Pisa. Unfortunately, he died before he reached a breakthrough with this particular area of research.

Newtonian Physics

Less than half a century later, Sir Isaac Newton took Galileo’s work and other contemporaneous knowledge about motion, velocity, and gravity and turned them into a single coherent system. These were formalized as Newton’s Laws of Motion (aka. Three Laws) which established that:

  • An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.
  • The acceleration of an object depends on the mass of the object and the amount of force applied (F=ma)
  • Whenever one object exerts a force on another object, the second object exerts an equal and opposite on the first.

These three laws beautifully described three concepts that are central to modern astrophysics: Inertia, Force, and Action-Reaction. These Laws also laid the groundwork for Newton’s theory of Universal Gravitation, which states that all point sources with mass attract each other through gravitational force. Furthermore, this force of attraction is directly dependent on the masses of both objects and inversely proportional to the square of the distance between their centers (Inverse Square Law).

In short, Newton was saying that gravity is universal and can be calculated as a matter of mass and distance. This not only established a theory that applied to everything from a falling apple (Newton’s Apple, another legend) to the orbits of the planets. Another consequence was that it established the idea that time and space were absolute and separate. While Galileo’s work on inertial reference frames still applied, Newton established that there was such a thing as a fixed reference frame.

Okay, this feels like enough for Part I! Stay tuned for how Einstein and modern physics would come along and completely ruin Newton’s day!

Originally published at on January 19, 2022.



Matt Williams

Space/astronomy journalist for Universe Today, SF author, and all around family man!