How a magnet makes a live frog float

A strong enough magnet will float a live frog, and the frog is fine afterwards. The levitating frog magnet at the Nijmegen High Field Magnet Laboratory produced a field of about 16 teslas, and the frog hovered inside its 32 millimetre bore. The trick is a weak form of magnetism that water has, called diamagnetism. [1]

I picked this fact from the science library because it is the kind that gets forwarded with a shrug. A frog is not a magnet. It has no iron in any useful amount. So how does it float? The answer is on the Wikipedia page for diamagnetism, and it needs no exotic physics, only a very large magnet.

What the source says

The Diamagnetism article carries a photograph of the experiment, and its caption is the sentence I wanted. A live frog levitates inside a vertical bore of a Bitter solenoid "in a magnetic field of about 16 teslas at the Nijmegen High Field Magnet Laboratory" [1]. The same article says, in its own words, that most spectacularly a live frog was levitated [1].

Behind it were Andre Geim and Michael Berry. Geim's Wikipedia biography says "He was also previously awarded an Ig Nobel Prize in 2000 for levitating a frog using its intrinsic magnetism." [2] Geim went on to win a real Nobel Prize in 2010 for graphene, which makes him the only person to hold both [2].

The Ig Nobel list itself records the 2000 physics prize as going to Geim and Berry "for using magnets to levitate a frog" [3]. Three pages, one story, and nobody disputes it.

Why water pushes back against a magnet

Every material responds to a magnetic field a little. Iron responds a lot, and in the direction we expect: it is pulled towards the magnet. Water, wood, plastic and living tissue respond the other way, very weakly. They are pushed away from a strong field. That property is diamagnetism, and the Wikipedia article describes it as a property of all materials, usually swamped by stronger effects where those exist [1].

A frog is mostly water. Put it in a field strong enough that the push on its water equals its weight, and the frog hangs there. The field has to be enormous, which is why the experiment happened in a laboratory built around a Bitter solenoid rather than on anyone's kitchen table. A fridge magnet is in the range of a few hundredths of a tesla; the Nijmegen magnet ran at about 16 [1].

The push also has to be shaped correctly. A uniform field does nothing; the frog needs a field that gets weaker as it moves upwards, so that the diamagnetic force points up. Inside the bore of the solenoid, just above the centre, the field falls off in the right way. The frog floats at the height where the push and its weight balance.

It sounds fake because

It sounds fake because magnets, in everyday life, only move metal. Nobody has seen a magnet lift a glass of water, and a frog is close to a glass of water with legs. The effect on water is real but about a hundred thousand times weaker than the effect on iron, so you never see it at fridge-magnet strength.

It also sounds fake because the frog survives. The story reads like a cartoon. But the field does not heat, shock or crush anything. It pushes every molecule of water in the frog by the same tiny amount, which is the same thing gravity does in the other direction. The frog is weightless the way an astronaut is weightless.

It holds up because

It holds up because it was done in a national magnet laboratory, photographed, published and then mocked at a ceremony, all in public. The Diamagnetism article also notes that in 2009 NASA's Jet Propulsion Laboratory levitated mice with a superconducting magnet, which it calls a step forward because mice are closer to humans than frogs are [1]. The method has been repeated with a different animal in a different country.

It also holds up because the strange part is familiar once you look. Water is strange for expanding when it freezes, and that is the same molecule doing something most substances do not. Diamagnetism is less visible than ice floating, but it is on the same list of water's odd habits.

Could it float a person?

Geim has said the field needed to float a person is not far beyond the frog's, because the physics scales with the material rather than the size, but the bore would have to be wide enough for a body and no such magnet exists. I have not found a source that puts a number on it, so I will not invent one. What I can say is that the frog, the mice and any future volunteer are all lifted by the same push on the same molecule.

If you want more of the physics that bends everyday intuition, the library has a fact on how time runs faster at your head than at your feet, measured over a single foot of height. The frog fact page links back to the source used here.

Questions

How strong was the magnet that levitated the frog?

The field was about 16 teslas, inside the vertical bore of a Bitter solenoid at the Nijmegen High Field Magnet Laboratory [1]. For comparison, a fridge magnet is a few hundredths of a tesla and a hospital MRI scanner is usually between 1.5 and 3 teslas, so the frog needed a field several times stronger than medical imaging.

Why does water get pushed away by a magnet?

Water is diamagnetic. A magnetic field slightly alters the motion of the electrons in each molecule, and the result is a weak field pointing the other way, so the material is repelled. The effect exists in every material but is overwhelmed in iron and other ferromagnets, which is why we only notice attraction in daily life [1].

Was the frog hurt by the experiment?

No. The source reports that a live frog was levitated and describes no injury [1]. The force acts on every water molecule in the body at once, so there is no crushing or stretching, and the field does not heat tissue. The same method later levitated mice at NASA's Jet Propulsion Laboratory in 2009 [1].

Who won the Ig Nobel Prize for the levitating frog?

Andre Geim of the University of Nijmegen and Sir Michael Berry of Bristol University shared the 2000 Ig Nobel Prize in physics "for using magnets to levitate a frog" [3]. Geim later won the 2010 Nobel Prize in Physics for graphene, which makes him the only person to hold both prizes [2].