This guest post is authored by Janna Levin (@jannalevin), the Tow Professor of Physics and Astronomy at Barnard College of Columbia University. Janna has contributed to an understanding of black holes, the cosmology of extra dimensions, and gravitational waves in the shape of spacetime. Janna is also director of sciences at Pioneer Works, a cultural center dedicated to experimentation, education, and production across disciplines, as well as Pioneer Works’ virtual home, The Broadcast.
Janna’s books include How the Universe Got Its Spots, the novel A Madman Dreams of Turing Machines, which won the PEN/Bingham Prize, and Black Hole Blues and Other Songs from Outer Space, the inside story on the discovery of the century: the sound of spacetime ringing from the collision of two black holes over a billion years ago. In 2012, she was the recipient of a Guggenheim Fellowship, a grant awarded to those “who have demonstrated exceptional capacity for productive scholarship.”
Enter Janna . . .
Light Falls Too
You should appreciate the hazards of encountering a black hole unawares. A black hole is invisible in the absence of any tracers, just darkness against darkness. You may well not realize the threat before your fate is secured. You must carry a powerful light source to reveal in backlight the clandestine black hole, an unilluminated disk, an absence in a bright world.
Light lives in space too and has to follow some path. If you shine a flashlight from your couch, you don’t notice the light beam falling toward the Earth. The lines appear straight. But they’re not perfectly straight. The comparative inflexibility of light’s route through space is attributable to its intrinsic speed. Light only travels at one speed, the speed of light. The free-fall curves for light—launched as light must be, at the cosmic speed limit—are straighter than the curves of slower objects. So the bend that the Earth’s gravity imparts to the path of light is more subtle, straighter, and more difficult to detect.
The bending of light offered the first test of general relativity. On the twenty-ninth of May, 1919, the Moon eclipsed the Sun to allow a thin ray of light from the Hyades star cluster to fall into Arthur Eddington’s telescopes. With the blinding solar rays occulted, the faint image of Hyades could be collected. But at the time of the eclipse, Hyades was positioned directly behind the Sun from the Earth’s perspective. If light traveled along straight lines, none of the luminosity from Hyades should have made it to the Earth. The cluster would spray its rays in all directions, and those headed toward the Earth would fall into the Sun. If light traveled along the curved paths predicted by relativity, then the Sun would act like a lens and bend an image of Hyades our way.
The Sun was eclipsed in totality only from the point of view of a swatch of the Earth that cut through Brazil at dawn, across the Atlantic Ocean as the Earth turned, and fell off Africa by dusk. From any given vantage point between sunrise and sunset, totality would last less than seven minutes. Barely six months after the end of the First World War, on the small island of Principe off the coast of Africa, Eddington and his team waited out heavy rains and cloud cover until the Sun broke through, the eclipse already under way. Although behind the Sun, the star cluster was visible to their telescopes (glossing over some ambiguities in the data), proving that light did not travel on straight lines around our star, but rather curved ones that deflected the light toward Africa.
Eddington presented his analysis of the resultant photographic plates from the eclipse expedition in Principe, in conjunction with those from his team near the equator in Brazil, and his announcement made headlines, catapulting Einstein’s fame in the English-speaking world. Eddington became the conduit that transmitted these discoveries, discoveries that transcended political animosity and nationalism stirred up by the war. Eddington, unlike some of his compatriots, had no urge to denigrate Einstein’s accomplishments, though he was a citizen of a country so recently a bitter enemy. Einstein was born in Germany, Eddington in England.
Out of the shadow of the war into the shadow of the Moon, they were citizens of the same Earth, and the theory of relativity was heralded as one of humanity’s greatest achievements.
Eddington measured the slight deflection of light’s path around the Sun. Outside every black hole, there is a curve in space so sharp that light can fall around the hole in a circular orbit. You could jet-pack to the location of light’s circular orbit and hover there. You will need to fire engines to resist your fall into the hole. Once there, you could shine a light on your face. Your face will reflect some of the light (if we didn’t reflect light, we’d be invisible) and send the image of your face in a reverse orbit toward the back of your head. You could wait a few tenths of a millisecond for that light to reflect off the back of your head then circle back about and land in your eyes. You could watch your own back.
When you get near enough to a black hole, the curves are sharper and the free fall faster. It becomes harder to stop your fall. You will need a considerable payload of fuel to accelerate you to sufficient speeds. To escape from the Earth, a rocket launched from Cape Canaveral must reach more than 11 kilometers per second.
The escape velocity from the Moon is about 2.5 kilometers per second, the Moon being less substantial than the Earth. The escape velocity from the Sun is more than 600 kilometers per second, the speed of the plumes of plasma able to blow off the hot stellar atmosphere into the solar system in the form of ionized winds.
The nearer you approach a black hole, the faster you need to drive your spacecraft to resist falling. In the emptiness that surrounds a black hole, you can get closer and closer to the center until you hit a proximity outside the hole at which no amount of jet thrusters is sufficient to reverse your decline. You would need to reach a velocity greater than the speed of light to escape, a speed of almost 300,000 kilometers per second. And since nothing can travel faster than the speed of light, you would fail to escape. Extrication impossible, you plunge irrevocably.
That special location where the escape velocity climbs to the speed of light defines the infamous event horizon: “The region beyond which not even light can escape.” I’m using unattributed quotes because everyone seems to use that exact sentence. We all say it at some point in our lives: “The region beyond which not even light can escape.” I said almost these exact words at the opening of this guide.
Light shining from the precise location of the event horizon seems to hover there, traveling at its fixed speed and still unable to escape, a fish swimming against an insurmountable waterfall of space. Knock the light slightly and it loses the battle, falling down the hole.
The event horizon is the extent of the black hole’s shadow. Anything that crosses the event horizon is forever lost to the outside. From the exterior, the black hole is dark. It is black. It’s a black hole. If you were to fall through that shadow, you’d find no physical, material surface. You’d just plummet in the dark emptiness, the moment of transition unspectacular, as unspectacular as stepping into the shadow of a tree.
The essence of a black hole is the event horizon, the severe demarcation between events and their causal relationships. Events interior to the horizon can have no effect on the outside, can tolerate no transmission of any kind to the exterior of the horizon, although the reverse is not the case. Happenings outside the horizon can be transmitted to the interior. The world beyond the black hole can have consequences for the world within. The black hole is thoroughly opaque from the outside, utterly transparent from the inside.
Should the choice arise, you are advised to fall into as big a black hole as possible. If you are very small compared to the black hole, you will hardly notice that the horizon is curved, like you hardly notice that the Earth’s surface is curved. If you stood on a basketball, the curvature under your feet is more noticeable than the curvature of the entire Earth. The bigger the black hole, the less you’ll notice. Your feet and your head could more or less fall in unison. You’d find yourself compromised if the hole was so small that your feet fall across the horizon along their own path that diverges from that of your head. Your connective tissue would have to resist the fall of your feet away from your crown and eventually the ligaments would fail and rip and the consequences would be pretty gruesome. But traverse the event horizon of a big black hole, no problem.
So you could survive the fall across the shadow, although you’d never find your way out again. And once inside the black hole you will eventually get pulverized, a caution we’ll detail later.
If you fall close to the center of the Sun, you could in principle escape the gravitational pull, but you’d be incinerated in the nuclear furnace of the core, a scenario that concludes with equally emphatic finality. At least you can get a million kilometers closer to the center of a black hole and live, in principle.
Your ruination inside a black hole the mass of the Sun typically takes less than microseconds. You could prolong your life expectancy to as much as a year in a black hole trillions of times more massive. You should aim for a bigger black hole if you intend to survive long enough to reflect on your experience. You may not want to prolong your demise if you cannot tolerate the existential dread, the gnawing suspicions, the cogent predictions about your unstoppable departure from this adventure.
If you find yourself approaching an utterly dark shadow, visible only in contrast to a bright background of light, beware. Avoid at all costs. Maintain a safe distance. If you get too close you will need all the fuel in the universe to escape, and that will not be enough. It will be hard to assess if the shadow is small and nearby or big and far away. If you do cross the shadow, you will be the first human to be dismembered by spacetime and obliterated near the infamous singularity, prophesized to be an actual hole in the center of the black hole, a tear in the fabric of spacetime that leads nowhere.
Take solace in your observations, but they will be yours alone. None of your transmissions will make it out of the hole to home. All documentation of your demise will follow you into oblivion.
Did you enjoy this chapter? Check out the rest of the book here.
Excerpted from Black Hole Survival Guide by Janna Levin. Copyright © 2020 by Janna Levin. Excerpted by permission of Alfred A. Knopf, a division of Penguin Random House, LLC. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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