Science in a Cup of Tea

The Naked Scientists spoke to BROWNIAN MOTION - Philippa Law interviews Dr Lucy Green & Greg Foot
20 March 2005

Interview with 

BROWNIAN MOTION - Philippa Law interviews Dr Lucy Green & Greg Foot


Philippa - This week, I've been finding out how you can see Einstein's science in a cup of tea. Here's Dr Lucy Green from Cardiff University with a quick cookery lesson.

Lucy - The first thing you do would be to fill up your kettle using cold water from the tap. The water is at a low temperature, and the molecules are moving around in your water but they're not moving terribly fast. When you heat it, what you're doing is giving energy to particles in the water. The water molecules then start to move much much faster.

Greg - These particles are far too small for us to see. They actually all travel in straight lines until they hit another one, bounce off, and carry on in a different straight line.

Lucy - When the water boils, you pour the water into your pot of tea with your tea leaves and then give it a stir. Even once the water has settled down, you can still see the tea leaves being jostled around within the water. And that's because the molecules have energy and they're bumping into the tea leaves and making them move.

Greg - Each time a water molecule bangs into a tea leaf, the leaf's going to move slightly in a different direction. This means that you can never predict the way in which it's going to move. Although your cup of tea looks completely still, underneath the surface it's made of all these particles moving rapidly and randomly in all directions.

Lucy - That's something we call Brownian motion.

Philippa - Why's it called Brownian motion?

Greg - Everyone says 'Einstein - Brownian motion'. But Brownian motion was actually discovered by this guy Robert Brown in 1827. That's why we call it Brownian motion. This guy was looking down his microscope at pollen particles in water and saw that they jiggled around a bit. He thought that this meant they were alive. Then what he did was to try some really old pollen and found that it did exactly the same thing. This showed him that it wasn't anything to do with the pollen particles being alive. But no-one could actually explain what caused it until Albert Einstein came along. Einstein said that if atomic theory was right, then using his explanation, it would explain why the pollen particles moved in these jiggly patterns.

Lucy - Einstein really gave us an understanding about what the water is made of. Before Einstein started working, so about 100 years ago, no - one knew that molecules or atoms actually existed, so they didn't know fundamentally what the water was made of.

Philippa - Einstein said that water was made of atoms, but wasn't it Einstein's atomic 'theory'? Doesn't the word theory mean that it hasn't been proven? Does that mean that atoms might not exist?

Lucy - Well we can actually use special instruments now that can see down onto the scale of atoms. Even thought we can see them today, 100 years ago when Einstein was thinking about this problem, he had no way of being able to see an atom because they are so small. So he just had to do a thought experiment. Einstein thought about what he had seen with the tea leaves being bustled around, and he came to the conclusion, which is actually the most logical conclusion, that water is made of molecules that are too small to be seen and that the molecules bump into the tea leaves and make them move.

Philippa - What other kind of things are impacted by Einstein's work with tea?

Lucy - Lots of things! Even in medicine the whole idea of the existence of the atom is used every single day. For example, if you have to go and have a scan made of the inside of your body, the methods that are used involve using atoms. In fact, using atoms that decay and split up because they are so large, you can have small amounts of these atoms injected into your body. Say for example you wanted to study the brain. The brain uses glucose to be able to work. So these atoms will travel up your blood vessels and into your brain. They'll then localize themselves in the area where glucose is being used. So Einstein's work on a cup of tea also helps us understand hw the brain works.

Philippa - Do you reckon Einstein was good at making tea?

Lucy - I think he must have been amazing at making tea! I think he probably had a very logical process and made in hardly any time at all!


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