Equations · Physics
Mass–energy equivalence
Einstein's statement that mass is a form of energy: the rest energy of a body is its mass times the speed of light squared.
E = mc²
Aethel says
E equals m c squared
What it says
Because the speed of light is so large, a tiny mass corresponds to an enormous energy: one gram is about 90 trillion joules.
It accounts for the energy released in nuclear fission and fusion, where the products weigh slightly less than what went in.
Symbol by symbol
| Symbol | Say it | Means |
|---|---|---|
| E | E | the rest energy |
| m | m | the mass |
| c | c | the speed of light in vacuum, exactly 299,792,458 m/s |
Where you meet it
Special relativity and nuclear physics.
Questions
How do you say Mass–energy equivalence out loud?
E = mc² is read: “E equals m c squared.” That is exactly what Aethel says when it meets the formula in a book.
What does E mean in Mass–energy equivalence?
E (said “E”) is the rest energy.
What does m mean in Mass–energy equivalence?
m (said “m”) is the mass.
Can I listen to the equations in my own textbook like this?
Yes. Upload a PDF, EPUB or text file and Aethel reads it aloud with every equation spoken as maths, the way it reads this one, and highlights each sentence as it goes. You can start free.