Black holes tend to defy physics in a number of ways, leaving behind paradoxes and an overall breakdown of known physics in their wake. The same is true regarding the ways that tidal forces interact with black holes from external gravitational sources. This interaction is quantified by what’s known as a “tidal Love number,” and for decades, scientists have known that the tidal Love number for black holes is precisely zero, nada, zilch. This means that a black hole experiences no deformations when it comes into contact with outside gravitational influences. However, a new study published in the journal Physical Review D suggests that under certain conditions, black holes break that “zero” rule, in keeping with their reputation as the supermassive scientific headaches of the universe.
Love numbers were first formulated in 1909 by British mathematician Augustus Edward Hough (A.E.H.) Love, who wanted to understand the tidal deformation experienced by the Earth as it’s pulled on gravitationally by the Moon and the Sun. Today, these Love numbers help scientists explore the internal structure of objects as they shift and stretch due to tidal forces. But black holes—unlike Earth, or even ultra-dense objects like neutron stars—have an atypical Love number (to say the least).
“In general relativity, black holes exhibit a remarkable feature: their tidal Love numbers—associated with the conservative response to static tidal fields—vanish identically,” the authors wrote. “This result stands in sharp contrast with the behavior of other compact objects, which generically have nonzero tidal Love numbers […] the tidal Love numbers are nonzero also for black holes surrounded by matter distributions, for black holes in modified gravity theories and asymptotically nonflat spacetimes, and finally for higher-dimensional black holes.”
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