Fire Engines, Microwaves and Eggs

"Albert, how can you tell the universe is expanding? It’s far too big to see the edges."

Because wherever you look, everything is moving away from each other.

"Yes, but how do they know that?"

In the same way that a blind man can tell whether a fire engine is coming towards him or travelling away from him. The siren from a fire engine that's travelling towards you is slightly higher pitched than the sound of the same siren when it's travelling away from you. An effect that Christian Doppler will discover in Vienna in 1842 towards the end of our journey and is still named after him as the Doppler Effect. It was easy to demonstrate the effect with sound in 1842, but Doppler also predicted that the same principle could apply to light coming from a stars. Rather than becoming lower pitched when moving away, the light is shifted very slightly in colour becoming a little more red. The faster the movement, the bigger the shift. The effect that Doppler predicted in stars is called the Red Shift and was discovered by Edwin Hubble in 1929. From looking at light from distant galaxies it seems that all the galaxies are moving apart. The further away a galaxy is, the faster it seems to be moving.

"So why don't fire engines look redder when they are going away from you."

The Doppler Effect only works with the sirens of fire engines because fire engines can travel at a reasonable fraction of the speed of sound. The speed of sound is only 760 miles per hour. Light travels almost a million times faster at 670 million miles per hour and so a fire engine would have to be going a million times faster than normal to be 'red shifted' in the same way that the siren changes pitch. Galaxies are a lot faster than fire engines and can travel at millions of miles per hour and so that's why their light appears slightly redder than it is.

“So what made scientists believe in the Big Bang?”

One by one, scientists came around to the idea as more clues were found. You know you asked about hearing the Big Bang? Well there are still echoes of the Big Bang out there in the universe, not in the form of sound waves but as faint ripples of microwave signals coming from space.

“Hold on Albert, you’re losing me here. Microwave ovens in space? Poets and priests? Are you sure they put you back together right when they put in this computer?”

I’ve never felt better and trust me this is all true. Microwaves are more than just white boxes to heat food. Microwave ovens are named after the wave, and microwaves are like a small radio wave. Luckily these microwaves from space are so faint that there is no risk of you being cooked, but the pattern of these faint signals gives clues about the start of the universe. This is how science works, you first have theory and idea and then you have to find clues or evidence that support it or disprove it. The Big Bang theory predicted three things. That the universe is expanding. That is should mostly be made of hydrogen and the heat of the explosion should be still detectable as faint traces of microwaves. The first two are true, so if you can find the microwaves and there is no other good explanation for them then your theory is probably correct.

"How long ago was this?"

Like I said before, about 14 billion years ago.

"And there are still microwaves echoing around? I can't believe that."

OK, I admit it's like trying to work out the shape of a stone thrown into a pond fourteen billion years ago by looking at the ripples left. But the ripples are very small and the Big Bang was the biggest splash of all time.

“Is that why you said you were so impressed with the microwave oven back at the start of the journey?”

Of course, isn’t it amazing that the same principle can tell you about how the universe begain and cook popcorn without any obvious source of heat?

“So how does a microwave oven work?”

A microwave is part-way between light and a radio wave. Light can't get through a piece of chicken at all. But a radio wave can pass straight through a chicken and carry on half way around the world. Microwaves are in the middle, they can get most of the way through a chicken but a lot of them will be absorbed. When absorbed they give up their energy and so heat up the food from inside. But there’s another reason why I like a microwaves. The fact they can heat things up was accidentally discovered by Percy Spencer in 1946 when he was experimenting with something called a magnetron. Here's the picture from his patent on the microwave. The first odd thing he noticed was that a chocolate bar in his pocket melted when he was standing next his magnetron. The next thing he tried was popcorn and that started popping too. The day he showed his discovery to someone else in the laboratory using an egg. As they watched the egg it exploded on their faces. It was like recreating the birth of the universe.

“Exploding an egg in a microwave is like the start of the Universe?”

Don’t you see? Lemaitre’s cosmic egg, explosions, egg on my face for not believing in it.

“Hmmm.”

Well next time you cook popcorn you should think about the start of the universe but just don’t try the egg trick. Trust me it explodes and trust me you’ll get in trouble.