Just when you think you know how the world works, there is some new surprise. For me, today the surprise was discovering that with a strong enough magnet, scientists can levitate an ordinary strawberry. Really? Below is a video showing this in action. This video would be possible in a freefall lab on an airplane or, of course, in space, but the claim is that this was done here on earth using something called diamagnetic levitation:
Also levitated was a living frog, which seemed to survive the experience with no damage, acccording to one site (see story below.) I’m interested in the fate of that frog because I had a full body 3 Tesla MRI a few years ago and now I’m having a mysterous systemic connective tissue breakdown causing all of my joints to crack and pop. My mystery medical condition is probably unrelated to the 3T MRI I had done out of medical curiosity, but it does make me wonder. Fields of the order of 1 and 10T, under the right conditions, are calculated as sufficient to cause levitation of some objects we would not ordinarily consider to be magnetic.
… a frog hovering inside a magnet (not on board a spacecraft) [is] somewhat counterintuitive and will probably take many people (even physicists) by surprise.
This is the first observation of magnetic levitation of living organisms as well as the first images of diamagnetics levitated in a normal, room-temperature environment… In fact, it is possible to levitate magnetically every material and every living creature on the earth due to the always present molecular magnetism. The molecular magnetism is very weak (millions times weaker than ferromagnetism) and usually remains unnoticed in everyday life, thereby producing the wrong impression that materials around us are mainly nonmagnetic. But they are all magnetic. It is just that magnetic fields required to levitate all these “nonmagnetic” materials have to be approximately 100 times larger than for the case of, say, superconductors.
Whether an object will or will not levitate in a magnetic field B is defined by the balance between the magnetic force F = M∇B and gravity mg = ρV g where ρ is the material density, V is the volume and g = 9.8m/s2. The magnetic moment M = (χ/ µ0)VB so that F = (χ/µ0)BV∇B = (χ/2µ0)V∇B2. Therefore, the vertical field gradient ∇B2 required for levitation has to be larger than 2µ0ρg/χ. Molecular susceptibilities χ are typically 10-5 for diamagnetics and 10-3 for paramagnetic materials and, since ρ is most often a few g/cm3, their magnetic levitation requires field gradients ~1000 and 10 T2/m, respectively. Taking l = 10cm as a typical size of high-field magnets and ∇B2 ~ B2/l as an estimate, we find that fields of the order of 1 and 10T are sufficient to cause levitation of para- and diamagnetics. This result should not come as a surprise because, as we know, magnetic fields of less than 0.1T can levitate a superconductor (χ= -1) and, from the formulas above, the magnetic force increases as B2.
Got lost? Read the Simple explanation
The water and the frog are but two examples of magnetic levitation. We have observed plenty of other materials floating in magnetic field – from simple metals (Bi and Sb), liquids (propanol, acetone and liquid nitrogen) and various polymers to everyday things such as various plants and living creatures (frogs, fish and a mouse). We hope that our photographs will help many – particularly, non-physicists – to appreciate the importance of magnetism in the world around us. For instance, it is not always necessary to organize a space mission to study the effects of microgravity– some experiments, e.g. plants or crystal growth, can be performed inside a magnet instead. Importantly, the ability to levitate does not depend on the amount of material involved, V, and high-field magnets can be made to accommodate large objects, animals or even man. In the case of living organisms, no adverse effects of strong static magnetic fields are known – after all, our frog levitated in fields comparable to those used in commercial in-vivo imaging systems (currently up to 10T). The small frog looked comfortable inside the magnet and, afterwards, happily joined its fellow frogs in a biology department.
There is one important aspect in which the diamagnetic levitation differs from any other known way of levitating or floating things. In the case of diamagnetic levitation, the gravitational force is compensated on the level of individual atoms and molecules. This is, in fact, as close as we can – probably ever – approach the science-fiction antigravity machine.
What is diamagnetic levitation?
Many common materials such as water, wood, plants, animals, diamonds, fingers, etc. are usually considered to be non-magnetic but in fact, they are very weakly diamagnetic. Diamagnets repel, and are repelled by a strong magnetic field. The electrons in a diamagnetic material rearrange their orbits slightly creating small persistent currents which oppose the external magnetic field. Two of the strongest diamagnetic materials are graphite and bismuth.
The forces created by diamagnetism are extremely weak, millions of times smaller than the forces between magnets and such common ferromagnetic materials as iron. However, in certain carefully arranged situations, the influence of diamagnetic materials can produce startling effects such as levitation.
It was proved in 1842 that it is impossible to stably levitate any static array of magnets by any arrangement of fixed magnets and gravity. However, the addition of diamagnetic materials makes such levitation possible. The July 22 Nature paper, Magnetic Levitation at your fingertips, describes two configurations where diamagnetic materials are used to stabilize the levitation of a magnet in the field of a fixed lifting magnet.
Why did they levitate frogs?
Frogs are convenient not only because they have a high water content, which is a good diamagnetic material, but also because they fit easily inside a tube-shaped Bitter electromagnet. Bitter electromagnets use a very large electric current to create an extremely strong magnetic field which magnetises the frog in such a way that its magnetisation is in the opposite direction to the applied field. This means that the magnetised frog is pushed up from a region of high magnetic field into one of lower field, and levitates.
When was diamagnetic levitation discovered?
Diamagnetic levitation was first demonstrated as long ago as in 1939 when small beads of graphite and bismuth were levitated in an electromagnet (for historic details, read Physics Today (pdf, 689 kB)). It took scientists another 50 years to rediscover levitation when physicists from Grenoble lifted several organic materials by the diamagnetic force. They were not aware of the earlier experiment. Although Grenoble’s research was published in Nature, a few scientists noticed it.
When we, in our turn, rediscovered levitation being unaware of the previous experiments, we were amazed to find out that 90% of our colleagues did not believe that we were not joking that water can levitate.
How powerful are the magnets used?
One person commented that “A 16 tesla field is enough to levitate a frog.” The lab in the Neatherlands has several systems that are over 30 T. Here is a 33 Tesla magnet used by the ru.nl lab where I found the information on magnetic levitation. An even more powerful 38 T system is listed but is not pictured.
The laboratory is equipped with several resistive, super-conductive and hybrid magnet sites.
Continuous fields up to 37.5 tesla.
It looks like a robot, doesn’t it?
What is the most powerful magnet we know?
One in 10 neutron stars becomes a magnetar, a star with a magnetic field of about a quadrillion Gauss. In Teslas, this 100 billion Teslas, or 100 Gigateslas. (Convert between Teslas and Gauss here.) Magnetars are the most magnetic objects in the universe.
A magnetar’s magnetic field is about a quadrillion (1015) Gauss. In comparison, the Earth’s magnetic field is uneven, ranging from 0.25 to 0.65 Gauss. A 15 Tesla Magnetic Resonance Image (MRI) machine generates a magnetic field of 150,000 Gauss. A standard neodymium magnet’s field is about 12,500 Gauss. One Gauss is defined as one maxwell per square centimeter.
Are magnetars able to impact the earth?
If one was close enough and experienced a starquake, it could blow away our ozone layer and destroy life on earth. Luckily, the nearest magnetar is located 9,000 light years from our planet in the constellation Carina.
Light takes time to reach us and fifty thousand years after a starquake occurred on the surface of SGR 1806-20, the radiation from the resulting explosion reached Earth on December 27, 2004 (GRB 041227). The burst had an absolute magnitude around −29 making it the brightest event known sighted on this planet from an origin outside our Solar System. The magnetar released more energy in one-tenth of a second (1.0×1040J) than our Sun releases in 150,000 years.
On December 27, 2007, a magnetar designated “SGR 1806-20” released a burst of x-rays more powerful than any ever recorded. SGR 1806-20 is about 50,000 light years away. If it had been only 10,000 light years away, it probably would have destroyed the earth’s ozone layer and caused mass extinctions. As it was, the burst was powerful enough to interfere with the planet’s atmosphere, as it gave off about 10,000 trillion, trillion, trillion watts of power.
A similar blast within 3 parsecs (10 light years) of Earth would destroy the ozone layer and be similar in effect to a 12-kiloton nuclear blast at 7.5 kilometers. The nearest known magnetar to Earth is 1E 1048.1-5937, located 9,000 light-years away in the constellation Carina.
How strong could we make a magnet?
There seems to be a practical limit of about 100 Tesla which is a million Gauss.
The strongest human-made magnetic fields are about a few hundred thousand gauss. In fact, we can’t make magnetic fields stronger than a million gauss or so without our machines just breaking down from the stress.
his amazing magnet, located at the MagLab’s Pulsed Field Facility inside the Los Alamos National Laboratory in New Mexico, produces the highest non-destructive field in the world.
The magnet produces a whopping 100 tesla. There are higher-field magnets out there; the problem is, they explode directly after the very brief experiments they are used for because they are not strong enough to withstand the forces created by such a powerful magnetic field.
This magnet is called a multi-shot because it can be used over and over again. In fact, researchers can pulse the magnet as frequently as once an hour.
Just when you think you know how the world works, there is some new surprise. Perhaps that’s exactly how the world works. 😉
Thanks for learning about this strange universe with me. I hope you enjoyed this post.