What does E=mc² mean? — Today You Should Know (2024)

The long answer

The equation itself is simple and elegant: E=mc² (or energy = mass x the speed of light squared). But what does it really mean and why does it matter?

What E=mc² means

Here's a breakdown of the equation's components to help you (and me) understand how it works:

What is energy (E)?

Energy, in the most general sense, represents the capacity to do work. In physics, it's talking about the work required to something against a force. There are lots of different forms of energy: kinetic (energy of moving objects), chemical, electrical, nuclear, and thermal, to name a few.

What is mass (m)?

Mass is a measurement of how much stuff is in an object, often measured in grams. Mass can be confused with volume and weight, other physical properties. Let's take a can of soup🥫as an example to explain what mass is and isn't.

If you have two identical cans of soup, but one is empty and the other full, they both have the same volume. This means they take up the same amount of space, but the full one has more mass. Mass is also not weight, since weight is dependent on the amount of gravity applied. The full can of soup has a different weight when weighed on Mars vs. Earth, but it still has the same amount of mass on both planets.

Why do you multiply mass by the speed of light (c)?

Energy travels at a constant speed of 186,000 miles/second – the speed of light.

Why do you have to square the speed of light (c²)?

It has to do with how kinetic energy (or the energy of moving objects) is proportional to mass. When an object moves four times as fast as another object, it has 16 times the energy, not four times the energy.

Okay, so what does E=mc² mean?

To quote Einstein himself, "The mass of an object is a measure of its energy content." Energy and mass were previously seen as entirely different things, but E=mc² demonstrates that mass and energy are interchangeable forms of the same thing.

One of the most mind-blowing things this equation reveals is just how much energy is in matter. You can measure an object by its mass and then plug that into the equation to calculate its energy. And since we're multiplying an object's mass by the speed of light squared, the amount of energy in even a small amount of matter is outrageous. For example, the amount of energy in a penny could theoretically provide enough energy to power the New York City metropolitan area for at least two years.

Why E=mc² matters

Okay now that we have a pretty good understanding of the components of the equation, let's move into why it matters.

What can you do with E=mc²?

In practical terms, this equation opened the doors to nuclear energy. Why? Because E=mc² states that a change in mass requires a change in energy. So if you were to split apart an atom into two parts (nuclear fission) or fuse two atoms together (nuclear fusion), the change in mass would release an enormous amount of energy.

How does E=mc² change the way we view the universe?

E=mc² is kind of hard to wrap your head around because we tend to think of mass as tangible and energy as intangible. But what this famous equation shows us is that mass is the most condensed form of energy, and energy has mass (i.e. it's tangible). It's not that you can convert mass into energy, but that they are two sides of the same coin.

Bonus brain points

Accordingly to E=mc², a hot cup of coffee weighs more than a cold cup. This has been measured using incredibly precise scales.

Check out some other curious questions:


Baird, C. S. (2014, April 1). Light has no mass so it also has no energy according to Einstein, but how can sunlight warm the earth without energy?. Science Questions with Surprising Answers. https://www.wtamu.edu/~cbaird/sq/2014/04/01/light-has-no-mass-so-it-also-has-no-energy-according-to-einstein-but-how-can-sunlight-warm-the-earth-without-energy/

Duke Energy Corporation. (2021, May 27). Fission vs. Fusion – What’s the Difference? Duke Energy | Nuclear Information Center. https://nuclear.duke-energy.com/2021/05/27/fission-vs-fusion-whats-the-difference-6843001

E=MC2: As famous as the man who wrote it. American Museum of Natural History. (n.d.). https://www.amnh.org/exhibitions/einstein/energy/e-mc2

Hathcox, K., Marsch, G., & Ward, D. (2000, May). Einstein was voted the person of the century at the end of 1999. Exactly what does his famous equation E = mc2 mean?. Union University. https://www.uu.edu/dept/physics/scienceguys/2000May.cfm

Lamb, R., & Simón, Y. (2023, September 19). E = mc2: What Does Einstein’s Famous Equation Really Mean? HowStuffWorks Science. https://science.howstuffworks.com/science-vs-myth/everyday-myths/einstein-formula.htm

Northwestern University. (n.d.). What is energy?. Qualitative Reasoning Group. https://www.qrg.northwestern.edu/projects/vss/docs/space-environment/1-what-is-energy.html

PBS Space Time. (2015). The Real Meaning of E=mc2. YouTube. Retrieved October 4, 2023, from https://www.youtube.com/watch?v=Xo232kyTsO0&ab_channel=PBSSpaceTime

Phull, A. (n.d.). E = MC2. Physics of the Universe: Difficult Topics Made Understandable. https://www.physicsoftheuniverse.com/topics_relativity_emc2.html

Public Broadcasting Service. (2005, August). E = mc2 Explained. PBS. https://www.pbs.org/wgbh/nova/einstein/lrk-hand-emc2expl.html

Scharping, N. (2021, April 8). E=mc2: What Does Einstein’s Most Famous Equation Mean? Discover Magazine. https://www.discovermagazine.com/the-sciences/e-mc2-what-does-einsteins-most-famous-equation-mean

Siegel, E. (2022, March 3). The true meaning of Einstein’s most famous equation: E=mc2. Big Think. https://bigthink.com/starts-with-a-bang/meaning-emc2/

University of Wisconsin-Stevens Point. (2023, September 1). Unit 1: What is Energy? Section B. Two Main Forms of Energy. Wisconsin Center for Environmental Education. https://www.uwsp.edu/wcee/wcee/pd/keep-pd/energy-module/unit-1-section-b/

U.S. Nuclear Regulatory Commission. (2020, March 19). What is Matter ?. NRC Web. https://www.nrc.gov/reading-rm/basic-ref/students/science-101/what-is-matter.html

What does E=mc² mean? — Today You Should Know (2024)
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