The Bizarre Behaviour of Negative Mass

"Hokusai's Wavelet", a take on the bouncing droplet in a Getty photograph (see original below). Artwork: NaturPhilosophieObserving Negative Mass at Washington State University

Negative mass has always been theoretically possible, and the concept has finally made it from a mathematical idea on paper to a reality achieved in the lab.  Scientists at Washington State University have created a fluid with negative mass. 

Everyday physical objects have a certain, positive mass and move in a certain, usual, expected way.  They move forward.  Because mass accelerates in the direction of the force.


Matter and Negative Mass

A very arty photograph of a droplet rebounding from a liquid surface.
A falling droplet rebounding from a liquid surface provides a physical representation of the bizarre behaviour of matter. Image: Getty

Hypothetically-speaking, however, matter can have a negative mass in the same sense that an electric charge can be either negative or positive.

But most people rarely think in other terms than what we observe in our everyday world.


Newton’s 2nd Law

Indeed, we normally think of a force in terms of Newton’s Second Law of Motion, in which a force is equal to the mass of an object times its acceleration, or

F = ma


This Classical Mechanics concept has previously been explored in the case of ion propulsion.

So we normally only see the positive aspects of a force.


Washington State University physicists have created a fluid with negative mass.  Which is exactly what it sounds like…  Push it, and unlike every other physical object in the World that we know, it does not accelerate in the direction it was pushed.

It accelerates backwards.


Push it… and it moves back towards you!


Bizarre Behaviour of a Bose-Einstein Condensate

A series of photographs and diagrams from the experiments on SOC.
Emergent phenomena from spin–orbit coupling (SOC) at surfaces and interfaces  Source: Niffenegger et al. 2015 / Purdue University

Rarely created in laboratory conditions, the phenomenon can be used to explore some of the more challenging astrophysical and cosmological concepts.

The research paper titled ‘Negative-Mass Hydrodynamics in a Spin-Orbit–Coupled Bose-Einstein Condensate‘ was published in the journal Physical Review Letters.

The conditions for negative mass can be created with what is known as a Bose-Einstein condensate, by cooling rubidium atoms to just a hair above absolute zero.


Bose-Einstein Condensate

The Bose-Einstein state of matter was created by two scientists, Cornell and Weiman, in 1995.  The word condensate refers to the way gas molecules come together and condense to a liquid.

In this state, predicted by Satyendra Nath Bose and Albert Einstein in the mid-1920s, the molecules get denser, as they are packed closer together.  The particles thus move extremely slowly and, following the principles of quantum mechanics, behave like waves.

The particles of a Bose-Einstein condensate also synchronise and move in unison as what is known as a superfluid, which flows without losing energy.


Engineering Negative Mass with a Spin-Orbit Coupling


Superfluidity is the characteristic property of a fluid with zero viscosity, which therefore flows without loss of kinetic energy.  When stirred, a superfluid forms cellular vortices that continue to rotate indefinitely.  The state occurs in two isotopes of helium (helium-3 and helium-4) when they are liquefied by cooling it to cryogenic temperatures.  It is also a property of various other exotic states of matter theorised to exist in astrophysics, high-energy physics, and theories of quantum gravity.

In 1938, Nobel laureate Pyotr Kapitsa, and physicists John Allen and Don Misener discovered that helium-4 became a new kind of fluid (or superfluid), at temperatures less than 2.17 K – the lambda point.


Spin-Orbit Coupling

A schematic illustration of the connection between the presence of strong SOC at material surfaces and interfaces, and the resulting emergence of new interactions and electronic states.
Emergent phenomena from spin–orbit coupling (SOC) at surfaces and interfaces.  A schematic illustration of the connection between the presence of strong SOC at material surfaces and interfaces (inner ellipse) and the resulting emergence of new interactions and electronic states (middle ellipse), such as Dzyaloshinskii–Moriya interaction, Rashba interfaces and topological surface states. These emergent phenomena can in turn be used to generate new 2D spintronics effects (outer ellipse), such as spin–charge conversion, the photogalvanic effect, enhanced SOC in 2D materials, such as graphene, magnetic skyrmions and chiral domain walls, which have direct device applications (periphery). FM, ferromagnet; NM, non-magnetic material. Source: Soumyanarayanan et al. 2016

A negative effective mass can be realised in quantum systems by engineering what physicists call a dispersion relation.

A powerful method for this is provided by spin-orbit coupling (SOC), currently at the centre of intense research efforts.

Spin–orbit coupling (SOC) describes the relativistic interaction between the spin and momentum degrees of freedom of electrons, and is central to the rich phenomena observed in condensed matter systems.

In recent years, new phases of matter have emerged from the interplay between SOC and low dimensionality, such as chiral spin textures and spin-polarised surface and interface states.

These low-dimensional SOC-based realisations are typically robust and can be exploited at room temperature.


Trapping and Slowing Rubidium Atoms

Researchers, led by Peter Engels, created these conditions by using lasers to slow the rubidium particles, making them colder, and allowing hot, high energy particles to escape like steam, cooling the material further.


What the… Rubidium?!!


A close-up photograph showing the texture of a rubidium sample.
A sample of Rubidium metal. Source: Science Magazine

Rubidium Rb is a soft, silvery-white metallic element of the alkali metal group, with an atomic mass of 85.4678.

Elemental rubidium is highly reactive, with properties similar to those of other alkali metals, including rapid oxidation in air: it can ignite in the air and reacts violently with water.

On Earth, natural rubidium comprises two isotopes: 72% is the stable isotope, 85Rb, and 28% is the slightly radioactive 87Rb, with a half-life of 49 billion years – over three times longer than the estimated age of the Universe.

Rubidium compounds have various chemical and electronic applications.  Rubidium metal is easily vaporised.  It also has a convenient spectral absorption range, making it a frequent target for laser manipulation of atoms.


The lasers trapped the atoms as if they were in a bowl measuring less than a hundred microns across.  At this point, the rubidium superfluid has a regular mass.

Breaking the bowl allows the rubidium to rush out, expanding as the rubidium atom in the centre push outward.

To create negative mass, the researchers applied a second set of lasers that kicked the atoms back and forth and changed their spin.  When the rubidium rushes out fast enough, it behaves as if it has negative mass.


Gross-Pitaevskii Simulation

The technique used by the Washington State University researchers avoids some of the underlying defects encountered in previous attempts to understand negative mass.  The experimental findings can be reproduced by a single-band Gross-Pitaevskii simulation, demonstrating that the emerging features – shock waves, soliton trains, self-trapping – originate from a modified dispersion.

Although more research will need to be done in order to confirm the conclusions of the experiment, the possible creation of stuff with negative mass has some definitely mind-bending consequences.  It would create ambiguity as to whether attraction should refer to force or the oppositely oriented acceleration for negative mass.

Just a little conundrum to wrap your mind around…

Such a heightened control of a Bose-Einstein condensate gives researchers a tool to shed new light on related phenomena in optical lattices, where the underlying periodic structure often complicates interpretation, and engineer experiments to study analogous physics in astrophysics, like neutron stars, and cosmological phenomena, like black holes and dark energy, where experiments are impossible.