Superconductors: Floating trains, perfect electricity and a quantum leap

The strange case of a floating magnet

The other great signature of superconductivity is something even stranger: the complete expulsion of magnetic fields, known as the Meissner effect. When a material turns superconducting, it pushes magnetic fields out of its interior. 

Set a small magnet above a chilled superconducting disk and it can float in mid-air, or even lock firmly into place, as if held by invisible rails. This is one of the clearest proofs that the material has truly entered a superconducting state.

This effect often confuses newcomers, because superconductors are also used to generate extremely powerful magnetic fields, e.g., in MRI machines or particle accelerators. The two facts are not contradictory. The superconductor itself contains no magnetic field inside, thanks to the Meissner effect, but a coil of superconducting wire can carry enormous currents without loss. 

A visual representation of Meissner Effect.

Those currents create intense magnetic fields outside the wire. Inside: field-free. Around it: some of the strongest magnets ever built.

How scientists verify a true superconductor

Because superconductivity promises so much, i.e., perfect efficiency, no resistive losses, levitating trains, it has attracted a long history of false alarms. So scientists insist on three clear signatures. 

First, the electrical resistance must drop to exactly zero, demonstrated by passing current and detecting no voltage whatsoever. Second, when cooled in a magnetic field, the material must expel that field, a behaviour that shows the transition is genuine and not just an ordinary metal becoming very cold. Third, magnetic levitation experiments, where a magnet rises or locks in place above the sample, provide a vivid and reliable demonstration of superconducting behaviour. 

Only when all three tests converge do physicists declare a material to be a true superconductor.

Understanding the physics—and a Nobel Prize

For decades after Onnes’s discovery, no one knew why superconductivity occurred. Even Einstein attempted an explanation and failed. The puzzle was so deep that some doubted it would ever be solved. 

Then, in 1957, John Bardeen, Leon Cooper, and Robert Schrieffer produced what became known as BCS theory. They showed that electrons pair up into “Cooper pairs,” forming a single quantum wave spread throughout the entire material. Because this wave moves without disruption, the electrons never collide with atoms and never lose energy. The explanation was so elegant and transformative that the three physicists received the 1972 Nobel Prize in Physics.

One of the most remarkable aspects of superconductivity is that it makes quantum mechanics, normally the realm of atoms, visible at scales you can hold in your hand. Reflecting on early superconducting circuits, physicist John Clarke recalled, “We could watch quantum behaviour unfold in a device large enough to solder.” It was a moment when quantum physics escaped the microscopic world and became something you could build on a workbench.

Frogs that float and magnets that never weaken

Superconductors have produced memorable demonstrations. In 1997, researchers at Radboud University in the Netherlands famously levitated a frog using magnetic fields made possible by superconducting magnets – a delightful proof that diamagnetism, given strong enough fields, works on anything, even living creatures. 

In early MRI prototypes, engineers found that once they started a circulating current in a superconducting loop, it persisted without any power source, seemingly forever. It was tangible proof of resistance falling to truly zero.

False claims and scientific discipline

Because the stakes are high, the field has seen its share of premature announcements. Claims of room-temperature superconductivity have surfaced repeatedly, only to collapse under scrutiny. In 2023, a Korean group claimed a material called LK-99 was superconducting in everyday conditions. Within days, labs worldwide showed that the supposed levitation came from ordinary magnetic impurities. 

Earlier claims involving hydrogen-rich materials under enormous pressure have also faltered when independent groups attempted replication. These episodes underscore why strict experimental tests – zero resistance, Meissner expulsion, reproducible levitation – remain the gold standard.

Practical uses of superconductors

Even without room-temperature superconductors, the technology is deeply woven into modern life. 

➡️ MRI machines rely on superconducting coils to produce stable, powerful magnetic fields. 

➡️ Maglev high-speed trains float above their tracks using the Meissner effect and accelerate with almost no friction. 

➡️ Particle accelerators like the Large Hadron Collider steer particle beams with superconducting magnets cooled to near absolute zero. 

➡️ Quantum computers use superconducting circuits as qubits, taking advantage of their extraordinary coherence. 

➡️ And fusion reactors, such as ITER, rely on superconductors to confine plasma hotter than the Sun.

A future waiting to be unlocked

A material that becomes superconducting at room temperature would transform civilisation. Power grids could transmit electricity with zero losses, dramatically reducing global energy waste. 

Motors, generators, desalination plants, and industrial machinery would leap in efficiency. Transportation, from trains to cargo ships, could be reimagined. The impact would rival or exceed the invention of electricity itself.

The late Nobel laureate K. Alex Müller, who helped discover high-temperature superconductors, captured the spirit of the field when he said during his Nobel lecture, “It was as if nature were waiting for us to look in the right place.” 

Superconductors remain one of the rare scientific frontiers where the underlying physics is profound, the practical stakes are immense, and a single breakthrough could reshape the world.

Shravan Hanasoge is an astrophysicist at the Tata Institute of Fundamental Research.