The ocean bulges toward the Moon in a tidal hump. But some objects do the opposite — instead of stretching matter, the tide presses a dent into it. These are boson stars: invisible clumps of boson particles that can all collapse into a single quantum state. Normal matter is propped up by heat; boson stars are held together by quantum pressure.
For them, gravity is spacetime curvature, and they warp it mightily. In fact, such objects might be what dark matter is made of. Scientists are hunting for them in the data from gravitational waves observatories.
In a recent study, physicists looked at boson stars that mix two types of field — the ground state and an excited state. They calculated how such a star would respond to a tidal tug from a nearby body. The key is to compute their Love numbers, a measure of squishiness. And that’s where things got weird: the electric Love numbers start out positive, meaning the star reaches toward its neighbor. But once the parameters cross a threshold, the numbers abruptly flip sign. From then on, instead of stretching the star, the tidal force compresses it along the line of attraction. The star puffs out sideways, forming not a bulge but a depression.
The switch happens when the frequency of the excited field becomes high enough. Magnetic Love numbers are always negative and are smaller in absolute value than the electric ones. In the future, as the sensitivity of gravitational waves detectors improves, such subtleties will let us tell apart an invisible companion in a binary system. A black hole, a neutron star, or a boson star with an “anti-tidal” response: each will leave its own signature. Then the ocean metaphor will become an observational fact: a tidal bulge points to ordinary matter, while a dent reveals a star made of pure quantum stuff.
If the Moon caused tides on Earth like those on the boson stars studied, the oceans wouldn’t rise — instead, the water level would drop at the point nearest the Moon.