The idea of “softness” is generally understood as yielding easily to touch, but this everyday term is not precise enough for physics. To determine the softest state, scientists must move beyond the traditional four states and apply a rigorous definition of resistance. The quest for the ultimate soft matter is fundamentally a search for the state that exhibits the least possible resistance to any form of deformation or flow. This investigation leads into the world of quantum mechanics, where particles behave in highly synchronized ways at temperatures near absolute zero.
Defining Scientific Softness
In physics, softness is quantified by measuring a material’s resistance to external forces, using metrics like viscosity and yield strength. Viscosity describes a fluid’s internal friction, representing its resistance to flow. For a state of matter to be the absolute softest, its viscosity must approach zero, allowing it to flow indefinitely without losing kinetic energy. Yield strength measures the stress required to cause permanent deformation in a solid. An idealized softest solid would have a yield strength of zero, instantly deforming under any applied force.
The Limits of Common States of Matter
Applying these scientific metrics eliminates the four most common states of matter from consideration. Solids possess measurable yield strength and high rigidity, strongly resisting permanent deformation. Liquids exhibit internal friction, or measurable viscosity; even low-viscosity fluids like water dissipate motion due to this resistance. Gases and plasma (an ionized gas) also possess measurable viscosity and internal friction arising from molecular collisions. Since none of the traditional four states achieve truly zero resistance to flow or deformation, scientists must look at more exotic, quantum-driven states.
Zero-Resistance Quantum States
The ultimate candidates for the softest state of matter are Superfluids and Bose-Einstein Condensates (BECs), both existing only at extreme cryogenic temperatures.
Superfluids
A Superfluid, most commonly observed in liquid Helium-4 cooled below 2.17 Kelvin, exhibits a complete absence of viscosity. This zero internal friction means that if set into motion, it will flow forever without dissipating its kinetic energy. This state is a macroscopic manifestation of quantum mechanics, where a significant fraction of the atoms (bosons) enter the same lowest-energy quantum state. This coherent movement prevents the individual, random particle collisions that cause friction in normal fluids.
Bose-Einstein Condensates (BECs)
The Bose-Einstein Condensate is achieved by cooling an extremely dilute gas of bosons to mere billionths of a degree above absolute zero. It represents an even purer form of quantum coherence than a Superfluid. In a BEC, individual atoms merge into a single quantum wave function, acting as one infinitely malleable “super-atom.” This collective, synchronized behavior offers the least possible resistance to changes in its overall shape.
Characteristic Behavior of Superfluids and Condensates
The zero resistance of Superfluids and BECs results in physical phenomena that defy classical intuition and confirm their status as the softest matter. Superfluid Helium-4, for instance, exhibits the “creeping” effect, where a thin, frictionless film of the liquid will spontaneously move up and over the sides of a container to find its own level. This movement is driven by van der Waals forces between the helium and the container wall, and it is only possible because the fluid’s zero viscosity allows it to flow without energy loss against gravity.
The extreme softness of a BEC is demonstrated by its behavior when confined by magnetic fields. When released from the magnetic trap, the condensate expands rapidly, with the atoms maintaining their single quantum state as they deform. Scientists can manipulate the shape of a BEC into highly complex configurations by adjusting the external magnetic potential, showcasing its infinite malleability. The condensate’s ability to smoothly and instantly adapt to the potential landscape, deforming to avoid areas of high energy, is a direct consequence of its quantum coherence and lack of internal resistance.