Energy, Mass, the Velocity of Light and Cake

Einstein’s Relativity is a little over a century old, and is still our best description of space and time. But trying to explain this unintuitive theory of distorted space and time, without using mathematics, has always been a challenge. I recently referred back to one of Einstein’s early papers on relativity, dated September 1905, to try and find a clear and concise way to explain the derivation of one of the most important equations in physics, E=mc2.. The first thing that hits you about this paper is its brevity, not quite two and a half short pages. So I certainly had a concise explanation in my hands, but was it going to be clear. The title did not instill confidence

“Does the inertia of a body depend upon its energy-content?”

With Einstein framing the title as a question, and not a statement, you are left thinking that this man, who was happy to distort space and time, was having a hard time with this concept. In fact he wrote to a friend saying “I cannot know whether the dear Lord doesn’t laugh about this and has played trick on me” when he first came up with his derivation. So after having read many texts explaining E=mc2 I was looking for new inspiration. And indeed it came. Not immediately after reading the paper, but 3am the next morning in a bolt of clarity that woke me, and kept me awake for fear I would lose it. It was clear to me that any explanation I was to give needed a simple prop. It had to be a cake. A cake would provide me with all the qualities I needed to transport my class from the familiar to the wonderful realm of abstract higher physics. Consuming the cake after would be the proverbial icing. Perfect!

The essence of the explanation is that a stationary cake, if it were to radiate light uniformly, remains stationary. Yet when the same thing is viewed from a moving vantage point the cake has kinetic energy, and some of that energy reveals itself in the radiated light. For the cake to radiate kinetic energy away without changing speed means the light must be carrying away inertia.
Now if the reason for my rapture at this realisation is lost on you, do not be disappointed.
I would love to be able to give you a clearer explanation of where E=mc2 came from. Sadly I cannot in these short paragraphs. The full explanation that I gave my class came after exposing them to 3 evenings of relativity at the observatory. But I encourage you to download Einstein’s paper, which is freely available, to experience the work of a genius. And I invite you to come along to the observatory and have some cake with me.

Paul Payne conducts evening adult education courses at Sydney Observatory.
Understanding Relativity, 6 Tuesday evenings, commences 8th March
Astronomical Concepts, 8 Thursday evenings, commencing 25th February


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