Adrian Skov / Oist
Photograph of the black graphite disk floating above a stack of three round magnets
A small graphite disk floats in the air, rotating freely above a ring of magnets in a vacuum chamber. Without physical contact, without energy, it keeps rotating with almost no deceleration, in a way that seems to defy everyday friction — but that follows precise rules of physics.
O pyrolytic graphite It’s a peculiar material. Unlike most substances, it actively repels magnetic fields. Placed on the correct arrangement of magnets, it exerts enough force to levitate at room temperature without any active control or energy source.
In a new study, a team of researchers from the Okinawa Institute of Science and Technology in Japan built a “magnetic trap” with five layers of ring-shaped magnets surrounding two central cylinders.
Each layer alternates north and south poles to create a stable field that keeps a graphite disk floating about 0.82 millimeters from the surface. The small graphite disk Floats in the air, rotating freely, without physical contactwithout energy, and keeps rotating with almost no deceleration
To monitor the disk’s rotation, the authors of , published on Friday in Nature Communications, marked its surface with a white dot of paint and filmed it with a camera specialized in detecting movement.
Even at pressures approaching 5 × 10⁻⁵ pascals, approximately one thousandth of a million of atmospheric pressure, the disk continued to spin with almost no slowdown, says .
At normal air pressure, the rotation of the disk slows down mainly due to collisions with air molecules. Higher pressures mean more collisions and faster deceleration.
The researchers measured the rate of decay of the disk’s rotation at different pressures, from atmospheric levels to near-vacuum conditions. At high pressures, gaseous collisions dominated.
At intermediate intervals, the damping rate scaled linearly with pressure, exactly as theory predicts for molecules that bounce off the surface.
At very low pressures, however, another factor came into play. Even when gas molecules were so scarce that they rarely reached the disk, a small amount of damping persisted below about 0.1 Pascal.
This residual damping was due to imperfect symmetry. When the experimental platform tilted even slightly, a fraction of a degree, gravity displaced the center of mass of the disk from the central axis of the magnetic field.
Once displaced, the rotation of the disc generated small eddy currentsbecause different parts of the disk experienced different magnetic forces as they rotated.
To investigate, the team deliberately tilted the device at various angles, maintaining constant pressure. The damping rate increased dramatically with tilt, rising by an order of magnitude with just half a degree of misalignment.
The researchers confirmed that the damping increases approximately according to a power of the lateral displacement of the disc in relation to perfect symmetry.
Computer simulations confirmed this patternshowing that for displacements greater than 0.05 millimeters from the symmetry axis, the damping followed an almost perfect power law relationship.
Extrapolating backwards, the data strongly indicated that parasitic damping would disappear completely with zero displacementat the point of perfect symmetry.
When researchers levitated the same disk on a different arrangement of magnets — a checkerboard-like pattern instead of the cylindrical arrangement — the disk’s rotation quickly dissipated, despite the intact circular shape.
The lack of rotational symmetry in the checkerboard pattern caused each point on the rotating disk continually experience changing magnetic fields, generating parasitic currents that drained energy.
A rotor that spins almost indefinitely without contact has practical applications. Gyroscopes based on this principle could achieve unprecedented sensitivity in detecting rotations, and could measure rotational movement of the Earth or small angular accelerations in space navigation.
Pressure sensors already use rotating rotors to assess vacuum levels in ultra-high vacuum chambers. Lower damping means greater sensitivity to the few remaining gas molecules that cause deceleration.
The system may also have applications in fundamental physics: Macroscopic rotating objects could be used to test quantum mechanics on large scales or look for subtle effects like vacuum friction — the idea that empty space alone could exert slight drag on rotating objects.
