The fluid, which defies everyday laws of motion, is a rare achievement and provides a platform to study an otherwise hypothetical form of matter. Scientists have created a fluid that exhibits the bizarre property of “negative mass” in an experiment that appears to defy the everyday laws of motion. Push an object, and Newton’s laws dictate that it will accelerate in the direction in which it was shoved.
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“That’s what most things that we’re used to do,” said Michael Forbes, a physicist at Washington State University and co-author of the paper, which shows that normal intuitions do not always apply to physics experiments. “With negative mass, if you push something, it accelerates toward you.”
Negative mass has previously cropped up in speculative theories, including those suggesting the existence of wormholes, a form of cosmological shortcut between two points in the universe. Just as electric charge can be either positive or negative, matter could, hypothetically, have either positive or negative mass.
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For an object with negative mass, Newton’s second law of motion, in which a force is equal to the mass of an object multiplied by its acceleration (F=ma), would be experienced in reverse. Theoretically, this sounds straightforward, but picturing how this behaviour would work in the real world is bewildering, even for experts.
“It’s very counterintuitive and weird,” said Jon Butterworth, a physicist at University College London, who was not involved in the latest work.
For instance, you might expect a ball with negative mass to be repelled from the Earth’s surface, but theory predicts that it would behave just like ordinary matter and fall downwards. No fundamental particles with negative mass have ever been discovered, meaning that there have never been any experimental insights into how they might behave, if, indeed, they exist. The latest study provides a new platform to study this hypothetical form of matter by showing that under certain precise conditions, normal particles can be made to behave as though they had negative mass.
“It provides another environment to study a fundamental phenomenon that is very peculiar,” said Forbes.
The experiment, published in Physical Review Letters, created negative mass by cooling rubidium atoms near absolute zero to form a Bose-Einstein condensate. In this state, particles move very slowly and behave like waves due to quantum mechanics. This type of matter is known for odd characteristics like superfluidity, where a liquid can climb up jar sides and spill over.
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The cooling was achieved by using lasers to slow the particles until they were confined in a laser trap, less than 100 microns across. Breaking the trap causes the rubidium atoms to rush out, expanding in a spherical formation. However, when researchers applied a second set of lasers that kicked the atoms back and forth within the trap, they started to behave as though they had negative mass on exiting the trap.
“Once you push, it accelerates backwards,” said Forbes, who acted as a theorist analysing the system. “It looks like the rubidium hits an invisible wall.”
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Martin McCall, a professor of theoretical optics at Imperial College London, described the paper as a neat demonstration of a system exhibiting “effective negative mass”, something that has only rarely been created before in laboratory conditions, prompting excitement within the scientific community.
This concept of negative mass, which allows for intriguing theoretical possibilities, raises questions about its implications for physics as a whole. But does it tell us anything about the possibility of cosmological wormholes or how hypothetical exotic particles might behave?
McCall expressed his skepticism on this front: “Personally, I doubt it,” said McCall, reflecting a cautious perspective that suggests while the findings are fascinating, they may not directly lead us closer to unraveling the mysteries of the universe or the fundamental nature of spacetime itself.
His viewpoint emphasizes the distinction between experimental physics and the broader, more speculative realms of theoretical cosmology, where many ideas remain untested and open to interpretation.
