A misconception among many: Why can’t we exceed the speed of light?
By Don Lincoln | Published: 2025-08-29 15:00:00 | Source: Hard Science – Big Think
Sign up for Big Think on Substack
The most surprising and impactful new stories delivered to your inbox every week for free.
This article was first published on Big Think in 2023. It was updated in August 2025.
Einstein’s theory of relativity is one of the most amazing theories ever created. In it, moving clocks tick more slowly than fixed clocks, and rulers shrink. Perhaps the most shocking result of all is that nothing can travel faster than light.
The latter is very disappointing for space enthusiasts, as it dashes their hopes of quickly exploring the universe. Space is vast, with the nearest star located four light-years away. Even a simple radio signal, traveling at the fastest possible speed, would take eight years to make a round trip.
The idea that there is a maximum speed is highly counterintuitive; After all, in your everyday experience, you can make a car go faster by simply pressing harder on the gas or upgrading to a sports car. In rocketry, you can only let the rocket fire longer. Why can’t we move faster than the speed of light?
Cosmic speed limit
If you’ve read anything about Einstein’s theory of special relativity, you’ve probably read that an object’s mass increases as its speed increases. This is kind of a satisfactory and intuitive answer. Larger objects are harder to push, so if the mass of the object becomes heavier, you will have to work harder to move faster. If an object’s mass became infinite near the speed of light, it would require an infinite amount of energy to propel it faster. Voila! The problem has been answered.
Although this answer is satisfactory and intuitive, it is also wrong, at least in detail.
Now before anyone decides to quote me saying that Einstein’s theory of relativity is wrong, don’t do it. Relativity actually states that any object with a mass other than zero cannot move at the speed of light, and even massless objects cannot move faster than light. So, this mass misrepresentation doesn’t help these former interstellar explorers.
No, the problem is not that claiming top speed is wrong; The problem is that the interpretation is wrong. So how do they arise?
Mass versus inertia
This issue arises because we confuse two ideas: mass and inertia. Inertia is actually the property that resists changes in motion. It’s just that inertia and mass are the same at low speeds. But this is not true at high speeds.
It’s easier to see this with equations, so I’ll plot it here, but if you’re not a mathematician, I’ll only use it sparingly. Everyone has seen Einstein’s most famous equation, E = mc², where E is energy, m is mass, and c is the speed of light. If we take it literally, we say that energy equals mass multiplied by a constant. However, this equation is actually a special case. The absolutely correct equation is E = γmc², where γ is the factor that arises essentially in all relativity equations. The factor γ is related to speed, and increases with increasing speed. At zero speed, γ equals one, while when the speed approaches the speed of light, γ approaches infinity. This parameter γ changes, not the mass. The mass is fixed.
Because relativity is difficult for students to understand, physics teachers have created a pedagogical concept called “relative mass.” The relative mass is simply γ times the mass. You can then add relativistic mass to Einstein’s famous equation, and it will look familiar. Relative mass can be used for many other equations taught in introductory physics classes. In essence, replacing mass with relativistic mass makes it easier for students to learn the theory and also gives a nice, intuitive picture of what’s going on, with the nice result that it’s easier for students to accept all the weirdness of relativity. Relative mass is a pet idea for students but not a real idea.
Be careful, pointing this out is not meant to criticize physics teachers. I learned this somewhat misconception myself. Just as a doctor may prescribe a drug that has a side effect, but does more good than harm, physics teachers must balance the value of getting students to embrace the theory of relativity, with the consequences of this misunderstanding being relatively small. Students who study physics will only need to better understand the deeper and correct explanation.
So, what are the consequences of this misconception? Basically, mass, relative mass, what does it matter? It’s important because mass is not just a quantity that resists motion, it is also a quantity that generates gravity. Therefore, many students believe that the gravitational field around a fast-moving object increases. This would make sense if the mass was actually increasing. But it is not.
This universal misconception illustrates a real problem with trying to explain a deep scientific concept using compromises; An enthusiastic thinker will take the compromise as fact and move forward, often arriving at a perfectly reasonable, but wrong, conclusion. A reasonable conclusion follows from what the person learned, but it is wrong because the compromise was not completely accurate. Unfortunately, there is no substitute for deep diving.
So, if you are one of those passionate thinkers who believed that a fast-moving object has greater mass and greater gravitational force, on behalf of physics teachers everywhere, I would like to apologise. Mass does not increase with velocity. Inertia does. The good thing is that many important consequences of the theory of relativity – most importantly the conclusion that nothing can travel faster than light – remain true.
The main message – other than the one that says that although inertia increases with speed, mass stays the same – is that smart people are easy to mislead by simplistic explanations of complex problems. So, if you think you’ve found something that the professional scientific community has overlooked, it’s probably because you’re starting with one of these partial facts.
Sign up for Big Think on Substack
The most surprising and impactful new stories delivered to your inbox every week for free.
ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ
