Intergalactic Travel at No Cost

It is commonly thought by “down to Earth” scientists that dark matter has no impact on our daily life. However, if dark matter had been composed of asteroid-mass primordial black holes, some of them could have collided with the Sun and sunk to its center. Stephen Hawking suggested in 1971 that accretion onto a primordial black hole could power the Sun in addition to nuclear fusion. His proposal could have resolved the deficit in observed Solar neutrinos, pointed out by John Bahcall. John was generous enough to take a gamble and offer me a postdoctoral fellowship in astrophysics, even though I did not know how the Sun shines. By now, the solar neutrino problem was explained as the transformation of electron-neutrinos to other flavors when they travel through the solar interior.

The nature of the interaction between black holes and stars changes qualitatively as we dial up the black hole mass. Let us start moving up in mass from asteroid-mass to stellar-mass black holes.

The gravitational waves detected by LIGO-Virgo-KAGRA confirmed that black holes result from the death of massive stars. If a stellar-mass black hole is accompanied by a companion star at a sufficiently tight orbit, the black hole accretes gas from the envelope of the companion star and appears as an X-ray binary.

The interaction with a companion star changes dramatically around supermassive black holes at galactic nuclei. Radio images from the Event Horizon Telescope confirmed the existence of supermassive black holes, like the Milky-Way’s 4 million solar mass black hole, Sagittarius A*, abbreviated as Sgr A*. When a star like the Sun arrives at a distance of ten times the scale of the event horizon of Sgr A*, it gets ripped apart by Sgr A*’s gravitational tide and converted to a spaghetti-like stream of gas that wraps around and eventually feeds the black hole. Together with my student, Betty Hu, I studied in a recent paper another process that could destroy stars and feed a supermassive black hole: star-star collision. Near a black hole, stars move at a significant fraction of the speed of light and their collisions would produce a flare similar to a supernova. In the aftermath of a collision, the central black hole will be fed with stellar debris and could shine brightly for a few years.

The event horizon of the black hole scales with mass and defines the size of its mouth as it feasts on stars. For a mass above a hundred million solar masses, the tidal force at the horizon would be smaller than the self-gravity of a Sun-like star. As a result, the most massive black holes in the Universe swallow stars whole. The eventual tidal disruption is hidden from view behind the horizon. Just as in Las Vegas: “What happens inside the horizon, stays inside the horizon.”

The most massive black holes in the Universe have a mass of ten billion solar masses and reside at the centers of clusters of galaxies. Their horizon is as big as the Kuiper belt in the Solar system at a hundred times the Earth-Sun separation. Even though the black hole mouth could have contained all planets, the gravitational tide would unbind them from the Sun prior to its arrival to the mouth.

If the Solar system was on a head-on collision course with a supermassive black hole, all planets would have been torn apart from the Sun before it gets inside the horizon. This is good news for survival of civilizations on habitable Earth-like planets around Sun-like stars. They would avoid death near the black hole singularity as their home planet would get ejected into space at a percent of the speed of light. A decade ago, I wrote a paper with my student then, Idan Ginsburg, where we calculated the statistics of such hypervelocity planets.

This fate could alter the space aspirations of technological civilizations like ours. As he contemplates settling on Mars, Elon Musk often stresses the importance of us becoming a multi-planet species to avoid a single-point catastrophe on Earth. But frankly, if we were to reside on a planet getting close to the horizon of the 7 billion solar mass black holes in the giant galaxy M87, I would advocate staying put on our planet in order to enjoy the exhilarating ride in our future.

In 2009 I wrote with my former postdoc, Avery Broderick, the first detailed paper suggesting that we image the tiny ring of light around black hole in M87 at millimeter wavelengths, 44 million times smaller than the angular size of the Sun, using a radio interferometer of the size of the Earth. This task was accomplished a decade later by the Event Horizon Telescope and first presented in the conference room of the Black Hole Initiative at Harvard University, for which I served as the founding director.

If we had resided on a planet approaching the M87 horizon, then the ring of light of that black hole would have occupied a large portion of our sky, bigger than the Sun.

Long after the gravitational slingshot ejection from a Solar-like system, the Earth’s surface would freeze in the absence of heat supply from the Sun or M87. But radioactive heat would keep subsurface layers at a habitable temperature, as I discussed in a paper on subsurface life with my former postdoc, Manasvi Lingam.

When the Earth-like planet gets tossed to a percent of the speed of light, its inhabitants could survive in a sealed underground base, maintaining atmospheric conditions and powered by electricity and heat from artificial nuclear reactors. Subsurface agriculture would be feasible using the large reservoirs of water in Earth’s rocks, which is comparable to the amount of water in the oceans.

These intergalactic spaceships created by nature traverse a hundred million light years during the age of the Universe. They offer a ride through a large volume of the Universe at no cost. Any subsurface civilization would want to get data from remote telescopes on the frozen surface of its planet, and view the surrounding Universe as it travels through it.

To all intergalactic travelers out there: Bon Voyage!

ABOUT THE AUTHOR

Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, is scheduled for publication in August 2023.

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Avi Loeb

Avi Loeb is the Frank B. Baird Jr Professor of Science and Institute director at Harvard University and is the bestselling author of “Extraterrestrial”.