What does E=mc2 really mean, and how are holograms made?

We explore the curious questions that science can answer

Out of the equation: Einstein’s theory of relativity changed the way we see the universe (Granger/Shutterstock)

What does E=mc2 actually mean?

That energy is the same kind of “thing” as mass, except it’s expressed in a different way. (This really is a very profound fact about the universe.) It also means that energy can be turned into mass and mass can be turned into energy.

E stands for energy, the m stands for mass, and c is the speed of light – already a large number which gets even bigger (90,000,000,000, 000,000 J/kg or so) when you square it. That is the exchange rate between mass and energy: the power released in an atomic explosion comes from that conversion. The Hiroshima bomb, with just 10kg of plutonium, only actually converted about 1g into pure energy – but it was enough to wipe out a city.

What are the types of quarks?

Of the six forms of quark, the first two discovered (in the late 1960s) were classified as “up” and “down”. The third was found in a very odd particle (called a K meson) so it was called “strange”. The fourth was named “charm” because someone thought it was a smart name. This was not as bad as the names originally given to the last two: “truth” and “beauty”. Their names were changed to “bottom” and “top”. The top quark was not confirmed found until 1994.

What is Russian multiplication?

Russian or peasant multiplication is multiplication by repeated doubling. For example, to multiply 17 by 13 you double the 17 and halve the 13, and add the doubles that correspond to an odd number in the other column. Like this: 17 x 13 – doubled and halved: 34 x 6 (add 17); doubled and halved: 68 x 3 (add 68); doubled and halved: 136 x 1.

So the answer is 136 + 68 +17 = 221. This may seem a cumbersome way of doing things, but it is a handy way of writing long multiplication in binary.

How does the sulphur dioxide emission of smokeless fuels compare with that of coal?

Generally speaking, smokeless fuels are made with less sulphur than ordinary coal.

This means that they will produce less sulphur dioxide when they are burnt, and so should contribute less to acid rain and similar effects.

Why doesn’t a rocket topple over on take-off when the supports are withdrawn?

The supports are only withdrawn when the jets at the bottom of the rocket are supporting its weight.

It is a question of split-second timing. If you watch a launch countdown carefully, you will see that the rockets are ignited before zero and while the rocket is still held in place. Once the jets have fired up enough to support the rocket – which happens almost instantaneously – the supports are removed.

How is a hologram made?

A hologram is the result of wave interference between two laser beams, which contain the light of a single frequency. One laser beam is shone directly onto a plate covered by a light-sensitive emulsion, like a photographic plate, while another is shone onto an object and then onto the plate.

One beam thus has further to travel than the other. Where they meet, the difference in distance means their wavefronts are out of phase with each other. They “interfere” to create patterns like those formed when two stones are dropped into a pond. These interference patterns are recorded by the plate, and recreated when a light is shone on the plate afterwards. We perceive those recreated patterns as a picture of the object that created the interference.