Pushing our understanding of spacetime to the absolute limit
I'm Alex Lupsasca, a theoretical physicist at Vanderbilt University and Project Scientist for the Black Hole Explorer (BHEX): a space mission to build a telescope bigger than the Earth and take the sharpest images in the history of astronomy.
Why are we doing this? To strengthen—and perhaps revolutionize—our understanding of the laws of physics in domains of nature previously completely beyond our reach. BHEX will conduct the most exquisitely precise test of Einstein's theory of general relativity, a cornerstone of our standard model of physics that offers our best fundamental description of spacetime and is critical to space-based technologies, including GPS. Does our current understanding of the universe still apply in the most extreme conditions that we know of, at the furthest reaches of the observable universe? A perfect confirmation would be stunning, directly showing us some of the weirdest predictions of gravitational theory playing out in reality. By contrast, any deviations would yield our first glimpse into the next, deeper layer of fundamental theory, and possibly open the door to an entirely new realm of technologies.
Gravity is the weakest of all forces, so its detailed effects—and any possible deviations from Einstein’s theory—are minuscule. To give ourselves the best chance to discover new gravitational effects, we must look where gravity is strongest: around a black hole. The supermassive black holes M87* and Sagittarius A* are the biggest black holes in our sky, and therefore the most natural targets for observations. Indeed, M87* is larger than our entire solar system. But, because it is 50 million light years away, it looks smaller from Earth than an apple on the surface of the moon. To see such a tiny feature, we need a telescope of immense scale—a dish with a diameter even larger than the Earth.
How do you create a telescope that surpasses the size of our planet? Imagine shattering the mirror of a giant telescope and then scattering the shards all over the Earth and even beyond, to the far reaches of space. Each shard captures a sliver of light from the target, and when we mathematically piece them back together, we'll have an instrument with a resolving power that surpasses anything we've ever built. As a “hybrid observatory,” BHEX will leverage an existing globe-spanning network of telescopes and also launch a satellite into Earth orbit to effectively create a virtual dish bigger than the globe. The space antenna will downlink observed data at an unprecedented rate of 100 Gbps, using groundbreaking laser communication technologies that will form the backbone of the future space internet.
What do we expect to see? Black holes produce the strongest gravity in the universe. Their boundary—the event horizon—forms an edge of our observable universe: an absolute horizon beyond which we can never peer. But just outside this point of no-return, it is possible for light to orbit around the black hole, probing its warped spacetime geometry, before escaping to our telescopes carrying an imprint of its gravitational field. When it reaches us, this light forms a narrow “photon ring” around the image of the black hole in the sky, a fingerprint of its extreme gravity. In particular, the photon ring encodes all of the black hole’s properties: the size of the ring reveals the black hole mass, and its precise shape encodes its spin, or the rate at which it drags the fabric of spacetime around with it. By measuring this spin, BHEX will deliver the first direct measurement of this spacetime frame-dragging, achieving a breakthrough in black hole astrophysics.
The Black Hole Explorer (BHEX) is our first step towards a new frontier in gravitational physics. Operating a hybrid observatory that observes with both ground-based telescopes and a space satellite, at higher frequencies than ever before, will push the boundaries of what is possible. Tackling such a momentous task will take concerted efforts from scientists, engineers, and philanthropists. I am particularly grateful to tech entrepreneur Fred Ehrsam for his initial seed funding that has enabled crucial development work on the BHEX instruments. With the tools of 21st-century science, the passion of a dedicated team, and the support of those who share our vision, we will trace the fingerprints of the laws of the universe down to the very fabric of spacetime.
This is more than just a telescope. It’s a cathedral of knowledge—a monument to our unrelenting spirit of curiosity and exploration, and a step towards answering the most profound questions of our existence.
We are the Black Hole Explorer. Join us as we embark on a journey to explore black holes: the most mysterious objects in the universe.
