Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp 2 bonds formed between the individual carbon atoms. In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 gigapascals (9,100,000 psi).  (For illustration, this translates into the ability to endure tension of a weight equivalent to 6,422 kilograms-force (62,980 N; 14,160 lbf) on a cable with cross-section of 1 square millimetre ( sq in).) Further studies, such as one conducted in 2008, revealed that individual CNT shells have strengths of up to ≈100 gigapascals (15,000,000 psi), which is in agreement with quantum/atomistic models.  Since carbon nanotubes have a low density for a solid of to g/cm 3 ,  its specific strength of up to 48,000 kN·m·kg −1 is the best of known materials, compared to high-carbon steel's 154 kN·m·kg −1 .
Study author Henning Bostelmann from the University of York in the United Kingdom explained that the paper, published last week in Physical Review A , is a mathematical result generalizing this backflow effect to any kind of external force that could act on a particle. But, explained Cadamuro, their math only works for particles in one dimension. That’s as if the people in Halliwell’s example could only walk forward or backward. The paper also doesn’t take into account the specific properties of particles aside from their momentum.