After decades’ worth of mystery, it feels as if physicists are finally closing in on the nature of black holes, thanks to Nobel-winning breakthroughs like the first detections of black hole mergers at the Laser Interferometer Gravitational-wave Observatory.
But Columbia University physicist Brian Greene warns that those matter-gobbling monsters may have a few surprises in them yet.
“To watch the history of this subject unfold from a purely theoretical idea to one that now is driving observational tests is enormously exciting,” Greene told GeekWire.
Greene has plumbed the mysteries of black holes in several books on cosmic subjects, including “Icarus at the Edge of Time,” a children’s book that was turned into a Philip Glass performance piece.
Black holes were at center stage again — figuratively speaking, that is — at this week’s World Science Festival in New York. Greene is one of the festival’s founders, and serves as a host and impresario.
Among the speakers were UCLA astronomer Andrea Ghez, who has been tracking stars to map the gravitational effects of the supermassive black hole at the center of our own Milky Way galaxy; and Northwestern University’s Vicky Kalogera, part of the LIGO gravitational-wave team.
LIGO produced the best evidence yet for the existence of black holes and their nature, thanks to the bursts of gravitational waves they throw off when they smash together. Such observations are opening a new era in astrophysics — and could open a new window on how black holes work.
Black holes are concentrations of matter so dense that nothing, not even light, can escape their grip. They can be detected only by their gravitational effect, or by the high-energy emissions given off by the matter that swirls into them.
A project known as the Event Horizon Telescope aims to track radio emissions from the edge of our galaxy’s central black hole, using a global network of radio telescopes. Early findings from the effort were published in the Astrophysical Journal a week ago.
Greene said future results from LIGO and the Event Horizon Telescope could shed light (so to speak) on the big questions that surround black holes. For example, what happens to the stuff that falls into a black hole? Depending on which perspective you take, you get different answers — which leads to a conundrum known as the firewall paradox.
“There are some interesting works that suggest that … there should be an echo effect in the gravitational waves that is generated by the firewall’s presence, or not generated if it’s absent,” Greene said. If the echoes are detected, that could force a rethinking of Albert Einstein’s general theory of relativity.
Greene said string theory, which has been considered a candidate for “the theory of everything,” suggests an alternate concept for black holes. They could be “fuzzballs” made up of tangled cosmic strings, with no sharply defined event horizon.
“If the fuzzball paradigm is right, then the whole notion of a black hole gets turned on its head,” Greene said. “It’s not something that is a place of no return. It’s not something that is far more exotic than stars, or planets. … That would be a sea change. That would be a completely new way of thinking of black holes.”
The mysteries of black holes and string theory are wound together like balls of cosmic yarn.
“The deep motivator of string theory is to understand the quantum nature of space and time. That really is the ultimate prize. And black holes really are our theoretical lab over the past 20 years to birth our ideas on the quantum nature of space and time as far as we can,” Greene explained.
“If new observations can winnow down the theories, we’re going to be homing in on the fundamental nature of the spacetime continuum,” he said.
It might not take all that long for efforts like LIGO and the Event Horizon Telescope to do the winnowing.
“I would say that in the next five, 10, 15 years, we may be able to speak of the ingredients of space and time with some degree of confidence,” Greene said. “And what an amazing moment that would be.”
The World Science Festival continues at various locations in New York through Sunday.
For more visuals about black holes, check out this 360-degree dive into a black hole from PBS’ “What the Physics?!” series on YouTube. The video series also offers a recipe for a black hole star cake and this “Choose Your Own Adventure” black hole tour: