It's true! We'll explain how and why grapes can be shoved around with a magnet — at least under the right circumstances — and why the phenomenon behind it is helping physicists levitate things.
Top image from the YouTube video "Magnetic Grapes" by Mark Lorch.
Here's a quick, easy experiment. Get two grapes and spear them on either end of a straw or a chopstick — anything that isn't magnetic. Balance the straw-grape dumbbell on your finger, or something else equally non-magnetic. (If you want extra stability, skewer the straw with a paperclip and drive the paperclip into something soft to make a good pivot.) The contraption should be able to spin around like a weather vane. Now, get a rare earth magnet, which might be the tough part. Rare earth magnets are made from rare-earth elements, and they make very strong permanent magnets. If you have a regular magnet that's strong, it should do as well.
Get one pole of the magnet close to one of the grapes. You'll see the grape push away from the magnet, gently spinning the entire structure. Now turn the magnet, and present the other pole to the grape. Again, the grape will be repelled. Whatever the pole, whichever the grape, the magnet will push the grape away without ever touching it.
So are grapes the world's first magnetic fruit? No, but the water inside them is diamagnetic. Diamagnetic materials are repelled by both poles of a magnet. As you can see when doing the experiment, the force is incredibly weak. Diagmagnetism is roughly 100,000 times weaker than ferromagnetism, which is the type of magnetism that causes metal to stick to a regular magnet. Its weakness is why we very rarely notice diamagnetism in action — it takes just the right conditions, when the grapes are on a light pivot, are filled with water, and are exposed to a strong magnetic field, for diamagnetism to be visible to the eye.
Under even better conditions, diamagnetism can be very useful. Just this week researchers at The University of Nottingham used the diamagnetism of wax to simulate something we've never been able to experimentally simulate before.
When a meteor strikes the Earth, the heat and force send molten rock droplets spinning into the air, distorting as they move. Their trip through the air cools them until they harden. When they come back down to Earth solidified, they are known as splash form tektites. They could probably tell us a lot about the meteors that hit the Earth, if we could understand the exact conditions that caused them to form.
Melting down rock and spinning it through the air at precise speeds and with precise spins is practically impossible. Before now, scientists used computer models to simulate the shapes these tektites would make as they moved through the air. With diamagnetism, simulation isn't necessary. Wax is soft, easy to manage, and diamagnetic. Using magnets to repel melted wax, scientists can spin it through the air and see the shapes it makes. We can now make our own splash form tektites. And we can probably celebrate the achievement with some grapes.
Tektite Image: University of Nottingham.