Challenging 'accepted' theories

Theoretical constructs that were written off as impossible can exist

One of the cardinal rules of science is that every hypothesis is open to experimental testing, and to being discarded or radically modified upon contradictory results. Sometimes a theory is accepted for centuries. For example, Newton’s Laws were accepted until Einstein came along, 250 years later. 

Some things go unchallenged for so long that it is a shock to learn that they require modification. Mass and absolute zero are two things that every school child is taught to be intrinsic: Every object has mass; atoms stop moving at absolute zero.  

Both notions have been challenged in the recent past and now, two-dimensional (2D) magnets, strange objects which were considered only theoretically possible, have been also been discovered. A few years ago, scientists at the University of Munich and the Max Planck Institut cooperated to demonstrate that it was possible under certain circumstances to induce “negative temperatures” of below absolute zero. The details are available at http://science.sciencemag.org/content/339/6115/52.

Some weeks ago, Nature magazine published an article, available at http://www.nature.com/nature/journal/v546/n7657/full/nature22391.html, describing 2D magnetic effects in a single atomic layer. This study was lead-authored by Xiaodong Xu and Pablo Jarillo-Herrero of the University of Washington, Seattle, and the Massachusetts Institute of Technology, respectively. 

Magnetism occurs when electrons spin while aligned in the same direction. Metals display magnetic properties in 3D — in fact, it is considered a property that defines metals. But that magnetism usually disappears when the metal is cut into super-thin 2D mono-layers. Previous experiments have tried to retain magnetic properties at 2D levels by introducing holes and bumps in 2D layer. 

This study describes how a compound, chromium tri-iodide (CrI3), retains magnetic properties at 2D levels without such distortions. 

CrI3 can be reduced to 2D by using scotch tape — literally pulling off layers using adhesive tape. That’s a technique first used to isolate the wonder-material, graphene, and it helped win a Nobel Prize.  

Once down to a one-atom-thick slice of CrI3, the researchers tested its magnetic properties by shining a beam of polarised light. If electrons are spinning in alignment, the reflection of a polarised beam is characteristic. This was visible at the mono-layer. Interestingly, the magnetic property disappears when two mono-layers are used — the two layers must have electrons spinning in opposed alignments. If three mono-layers are used, magnetism returns. Magnetic properties are essential for modern forms of information storage and that’s one of many implications. Other strange properties could come to light if ultrathin layers are combined in other ways. 

Negative mass is considered an even stranger concept. We know that electricity has negative and positive charges. What happens if mass has a negative charge? If you push something with positive mass, it moves away. If you push an object with negative mass it comes towards you.

In gravitational terms, two bodies attract each other with a force proportional to the product of their masses and inversely proportional to the distance between their centres. By Newton’s Law, an object with negative mass should float away from an object with positive mass. 

The mathematical and physical implications get really strange when we consider black holes — what happens to negative mass near such phenomena? In 1914, Saoussen Mbarek and Manu Paranjape, at the Université de Montréal in Canada, wrote a paper hypothesising that negative mass was possible without violation of the laws of physics as we know them. 

Led by Peter Engels, a team of Washington State University physicists claim to have created a fluid with negative mass, which has some strange properties. They have published their findings in Physical Review Letters https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.155301. 

The team cooled rubidium atoms to near absolute zero, using lasers to slow down atomic movements. This created a Bose-Einstein condensate, a state of matter where particles move very slowly and behave like waves. Superfluidity arises, with the condensate flowing without energy loss. 

Now the condensate was hit by a second set of lasers that changed the spins. This changed the condensate into negative mass. It exhibits some of the peculiar properties negative mass objects are supposed to have. The experiments will continue and presumably more data will be generated. The implications are foundational. This confirms that theoretical constructs that were written off as physically impossible can actually exist.