popcorn.
One of life’s small hassles is getting ketchup out of the bottle. What does physics tell us about it—and why doesn’t banging the bottom of the bottle help?
It’s a great disappointment to me that ketchup bottles are now made out of plastic rather than glass because now you can squeeze them and you don’t get to appreciate this nice bit of physics. We’ve all been in a pub or a diner, where you have a glass bottle of ketchup. You turn it upside-down, shake it, hit the bottom, and the ketchup doesn’t come out. Then it all comes out at once! [Laughs]
That’s not random. It happens because ketchup has got this weird property known as shear thinning. What that means is, it’s really viscous until you force it to move a little bit. When you’re shaking the bottle, the ketchup can’t go anywhere, so it stays thick. Once you hit it hard enough that it has to go somewhere, then it becomes runny, so a whole load of ketchup comes out at once.
The trick is to hold the bottle at a slight angle and tap the neck of the bottle, because then you’re making it runny where you need it to be runny. Everything further up the bottle stays nice and thick. Instead of having ketchup splodge everywhere, you can control the amount that comes out.
You’re British so tea and toast play an inordinate role in your life. There’s lots of physics in them, isn’t there?
The best thing about physics for me is that the same pattern explains lots of different things. There’s no reason why you have to see that pattern in a posh lab wearing a lab coat with some expensive bit of kit. You can see those patterns in your cup of tea. When you stir milk into your tea, the white milk doesn’t just merge with the dark tea. It swirls around. Then we look up into the sky at the big, rotating storms that come past at our latitudes, and there are also two fluids mixing together, via swirling. Even though looking at your teacup is one of the most mundane things in daily life, what you see there applies in other places.
It’s the same with toast. You shouldn’t worry about the universe until you understand your toaster because there’s a lot of fundamental physics in your toaster. [Laughs] One is, they glow. It sounds obvious. It’s hot so it glows, right? But that dull red glow that gets brighter and brighter as it gets hotter comes from one of the most fundamental laws of physics. That law says that anything that has a temperature above absolute zero is glowing. The type of light it gives out depends on how hot it is. So, the hotter it gets, the shorter the wavelengths.
The other interesting thing in a toaster is an electromagnet. When you push the lever down on the side of the toaster and the toast stays down inside the toaster, the thing holding it down is an electromagnet. The lever connects up a little circuit and sends a current round in a spin. And when a current is going round in circles, the laws of physics say that it will generate a magnetic field. When the toast is done, the current is switched off, the magnetic field goes away, and your toast pops up. You’ve got two of the most essential foundations of physics. And they are both in your toaster!
You say, “The fabric of our civilization is woven together with electromagnetic threads.” Why is electromagnetism so important?
Each of us has three life support systems: We have a body, a planet, and a civilization. Our civilization is keeping us alive now in just the same way that the other two do. We are absolutely dependent on it. And the thing that connects our civilization together is electromagnetism.
There are two forms: visible light—signals we can see from a long way away—which has connected humans for a long time. And now there are the signals we can’t see: radio signals, Wi-Fi signals, mobile phone signals. They’re making a web of electromagnetic waves that we walk through. We don’t see them directly but we use radios, TV, and the Internet, which all depend on them.
One hundred years ago that didn’t exist! Earth was dark in the radio wave and microwave wavelengths. Today, we’ve lit it up with this web of light. L