Kitchen Science: gastrophysics brings the universe into your kitchen

This title will probably come as a surprise to you. Astrophysics concerns enormous scales of space, time and energy; kitchens are relatively small, homely places.

So how can there be a connection between astrophysics and the kitchen?

Have you ever dropped a just-opened plastic bottle of milk or fruit juice on the kitchen bench and had the contents jumped up and hit you in the face? I have. And when it happened, I suddenly realised there was a connection with the physics of a type II supernova explosions.

When a massive star – roughly ten times or more the mass of our sun – comes to the end of its life, it is blown apart in a catastrophic explosion known as a type II supernova.

The explosion is triggered by the sudden collapse of the iron core at the centre of the star. The rest of the star follows and slams into the rebounding core, which creates a shockwave that propagates through the star back to the surface. When the shockwave reaches the surface there is no more star to push on, and so the outer layers of the star are ejected violently into space.

On the kitchen bench, dropping a plastic bottle causes the base to flex and push on the liquid above, creating a shock wave. When the shockwave reaches the top of the bottle there is no more liquid to push on, and so the liquid is ejected up into the air.

Baking the universe

You can also see the evolution of the universe while baking.

Astrophysicists often use the “raisin loaf” analogy to explain the expansion of the universe. The dough represents space and the raisins galaxies.

Imagine sitting on a raisin in the middle of the dough as it bakes in the oven. As the loaf expands we would see every other raisin move away from us. When we look out into the universe (almost) every galaxy is moving away from our home galaxy, the Milky Way.

At home I often make what I call Hubble Damper in honour of Edwin Hubble, who first ascertained that galaxies are moving away from the Earth. He also determined that the farther the galaxy, the faster the recession.

For added realism I add craisins (dried cranberries) rather than raisins. Craisins are red. Galaxies are redshifted.

If you are interested in making your own Hubble Damper, here is the recipe. Mix together:

• 450g self–raising flour
• a pinch of salt
• 80g diced chilled butter
• 185ml water
• 170g craisins.

Bake for 30 minutes at 200℃. Tap the base of the bread; if it sounds hollow, it’s done.

Sunset in a glass

One of the first “science” questions children often ask is: “Why is the sky blue?” And it’s a question dreaded by many parents.

If you have one of those children, here is something you can do in your kitchen to explain why the sky is blue. Get hold of a glass of water – a glass with straight sides is best. Put a drop of milk into the water and stir.

Get hold of a LED torch and shine it through the water. When you look at the beam of light end-on, you will see that it is has a yellow tint. If you can’t see the LED light, dilute the milky water.

The fact that the torch beam has a yellow tint means that blue light has been extracted from the beam, which is what we call scattering.

If you perform the experiment at night, switch off the lights in the kitchen and look at the glass of water from the side. You will notice a blue glow. This is the light scattered by the milk. Congratulations, you’ve just made your own blue sky and sunset in your kitchen.

In the atmosphere, molecules in the air scatter blue light from the sun more than the red wavelengths. The blue light comes into our eyes from all directions, which is why the sky is blue. When the sun is close to the horizon, the sunlight has to pass through a greater thickness of atmosphere and more blue light is scattered making the sun appear red.

After my light bulb moment of dropping the milk bottle I decided to explore other connections between astrophysics and the physics of cooking and kitchen appliances. Over the course of about three years I wrote an eBook book called Gastrophysics, which explores the connections, and I learnt a bit of cooking at the same time.