Negative Mass: Scientists Create Fluid That Moves Forward When You Push It Back

Milky Way
The Milky Way seen from the White Desert outside Cairo. Amr Dalsh/Retuers

Updated | Physicists in the U.S. have created a fluid with “negative mass”—meaning that when you push it away, it accelerates towards you. That means researchers can now use this bizarre phenomenon will now be used to study some of the universe’s biggest mysteries, including dark energy and black holes.

Negative mass is a hypothetical concept that says matter can exist with a mass opposite to normal matter. Instead of having a positive weight—1kg, for example—negative mass would weigh minus 1kg. This works in the same way that an electric charge can be either positive or negative.

In 2014, a team of Canadian cosmologists announced negative matter could indeed exist in the universe without violating our laws of physics. They said as long as negative mass was being produced in a certain way, it could exist according to Einstein’s theory of general relativity.

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Now, Michael Forbes and his team from Washington State University have managed to create negative mass in a laboratory. Their results are published in the journal Physical Review Letters.

Newton’s Second Law of Motion says that if you push something with a given force it will move away at a given speed—depending on the object’s mass, the force and friction. “That’s what most things that we’re used to do,” Forbes explains in a statement. “With negative mass, if you push something, it accelerates toward you.”

To create these conditions, the team used rubidium atoms that had been cooled down to just above absolute zero using lasers. In this state of matter—known as a Bose-Einstein condensate—particles move very slowly and start behaving like waves. The particles move in unison and flow without losing energy, becoming what is known as a superfluid. At this point, the cooled rubidium has a regular mass.

To create the negative mass, the team shot the rubidium with another set of lasers that pushed the atoms back and forth and changed the way they spun. When the “bowl” holding the rubidium they had created smashed, it appeared to have a negative mass.  

“Once you push, it accelerates backwards,” Forbes says. “It looks like the rubidium hits an invisible wall. What’s a first here is the exquisite control we have over the nature of this negative mass, without any other complications.”

In an email interview with Newsweek, Forbes says there are still aspects of their experiment they do not understand: “Although we have a microscopic theory that works quite well to explain the experiment, it was a challenge to try to figure out how to explain the qualitative features that we see. For example, we see the formation of a shock-wave when the expanding cloud starts experiencing negative mass and accelerates in the wrong direction: The atoms pile up and the resulting bump moves a constant speed, but we do not yet have a solid understanding of what sets this speed.”

Nevertheless, scientists can now use this work to come up with experiments to test theories relating to extreme astrophysics, such as dark energy and black holes. For example, scientists have previously suggested that dark matter and dark energy should be considered a negative mass.

“One of the reasons these cold atom systems are interesting is that they allow us to simulate other systems where it is difficult or impossible to do experiments,” Forbes says. “For example, I am interested in understanding what happens in neutron stars—some of the densest objects in the universe—but these are so far away that we cannot visit them [nor would we want to as we would be ripped apart!] and the matter is so dense, we cannot make it on Earth.

“It turns out, however, that the physics inside the neutron stars can be quite accurately simulated in cold atom experiments. Thus, we are able to test our theories on the cold atom systems where we can compare with experiment, and then apply the results to neutron stars.”

Forbes continues: “In the case of dark matter, one proposal is a yet-to-be-seen particle called an axion. Depending on properties of these axions, it is possible that they might actually cool down when they fall into galaxies, and form a superfluid on a galactic scale that is very similar to the superfluids we study in the lab. From cold atom experiment and theory we know that when you rotate such a fluid, you form a lattice of vortices with a very particular structure. A similar structure in a dark matter superfluid might have observable consequences on the structure of galaxies that would allow us to detect its presence.”

This story has been updated to include further comment from Forbes.