In one of the labs on the sixth floor of the Webster Hall at University of Washington, physicists have managed a very rare feat: they’ve made a fluid with negative mass.

In the case of objects with negative mass, these accelerate towards you if you push them.

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When scientists say something has negative mass, they don’t mean it literally. What they mean is that when an object with a negative mass is being pushed, it moves in the opposite direction, towards you, instead of the direction of the force. That’s totally unrelatable to most people without a PhD in physics but this can happen, as the team from the University of Washington elegantly demonstrated. And yes, Newton’s Second Law of Motion is still in place.

“Newton’s laws dictate that objects accelerate in proportion to the applied force. An object’s mass is generally positive, and the acceleration is thus in the same direction as the force. In some systems, however, one finds that objects can accelerate against the applied force, realizing a negative effective mass,” the researchers wrote in their paper published in the Physical Review Letters.

Michael Forbes, an assistant professor of physics and astronomy at the University of Washington, and colleagues, cooled rubidium atoms almost to absolute zero (−273.15 °C; −459.67 °F). At those temperatures, the gas is in an exotic state of matter called Bose–Einstein condensate (BEC). BEC was first predicted in the 1920s by Albert Einstein and Indian physicist Satyendra Bose but it wasn’t until very late in 1995 that scientists were able to produce the necessary conditions for this extreme state of matter to occur. At room temperature, atoms are incredibly fast and behave akin to billiard balls, bouncing off each other when they interact. As you lower the temperature (remember temperature reflects atomic agitation or vibration), atoms and molecules move slower. Eventually, once you get to about 0.000001 degrees above absolute zero, atoms become so densely packed they behave like one super atom, acting in unison. This is the domain of quantum mechanics so prepared for a lot of weirdness.

Essentially, BEC particles behave like waves, moving in unison as a superfluid which is a fluid that flows without losing energy. Superfluids are so ‘out of this world’ that they can escape containers by themselves, climbing walls. Because a superfluid loses all viscosity, it can not be contained and thus can even leak through a glass beaker, as shown below (superfluid helium).

To create the necessary conditions for the superfluid to form, the rubidium atoms were cooled by lasers. That sounds counter-intuitive given lasers usually heat stuff but scientists have been using lasers for cooling purposes for some years. The kind of lasers used for cooling fire at a specific angle and frequency. Typically multiple lasers are used. As a result of this clever tweaking photons actually end up snatching energy from its target instead of releasing it, and it’s all done by literally pushing the atoms. More about this in this amazing demonstration performed by Veritasium. 

Since the rubidium atoms are cooled just a hair above absolute zero, they’re almost still. The area the almost motionless atoms are confined to is akin to a bowl measuring less than a hundred microns across. When a second set of lasers was fired on this ‘bowl’, the atoms were kicked back and forth changing the way the spin (spin-orbit coupling). When the rubidium atoms are allowed to flow out of the bowl fast enough, the superfluid behaves as it had negative mass.

“Once you push, it accelerates backwards,” said Forbes in a statement. “It looks like the rubidium hits an invisible wall.”

This isn’t the first time scientists have observed negative mass in action but the technique designed by the WSU researchers avoids some of the underlying defects encountered in previous attempts to understand negative mass. The same technique could be used to study analogous physics in astrophysics, like neutron stars, but also black holes and dark energy.

“What’s a first here is the exquisite control we have over the nature of this negative mass, without any other complications,” says Forbes. “It provides another environment to study a fundamental phenomenon that is very peculiar.”