The Biggest Myth About Black Holes

Black holes are regions of space where there’s so much mass in such a small volume that there exists an event horizon: a region from within which nothing, not even light, can escape. Yet this doesn’t necessarily mean that black holes suck matter in; they simply gravitate.J. WISE/GEORGIA INSTITUTE OF TECHNOLOGY AND J. REGAN/DUBLIN CITY UNIVERSITY

Black holes are some of the strangest, most wondrous objects in all the Universe. With huge amounts of mass concentrated into an extremely small volume, they inevitably collapse down to singularities, surrounded by event horizons from which nothing can escape. These are the densest objects in the entire Universe. Whenever anything comes too close to one, the forces from the black hole will tear it apart; when any matter, antimatter, or radiation crosses the event horizon, it simply falls down to the central singularity, growing the black hole and adding to its mass.

These properties about black holes are all true. But there’s an associated idea that’s absolute fiction: black holes suck surrounding matter into them. This couldn’t be further from the truth, and completely misrepresents how gravity works. The biggest myth about black holes is that they suck. Here’s the scientific truth.

A black hole is famous for absorbing matter and having an event horizon from which nothing can escape,and for cannibalizing its neighbors. But there isn’t any ‘sucking’ going on to cause it, simply the disruption of matter and an occasional infall of material.X-RAY: NASA/CXC/UNH/D.LIN ET AL, OPTICAL: CFHT, ILLUSTRATION: NASA/CXC/M.WEISS

Both in principle and in practice, there are many different ways to form a black hole. You can have a large, massive star go supernova, where the central core implodes and forms a black hole. You can witness two neutron stars merge, where if they cross a specific mass threshold they’ll result in a newly-formed black hole. Or you could have a large collection of matter — either a supermassive star or a massive cloud of contracting gas — collapse down directly to a black hole.

With enough mass in a concentrated enough volume of space, an event horizon will form around it. From outside the event horizon, you can still escape if you move away from the black hole at the speed of light. Yet if you were located within the event horizon, then even moving at c, the ultimate cosmic speed limit, any path you could take would still lead you towards the central singularity. There is no escape from within the event horizon of a black hole.

Once you cross the threshold to form a black hole, everything inside the event horizon crunches down to a singularity that is, at most, one-dimensional. No three-dimensional structures can survive intact.ASK THE VAN / UIUC PHYSICS DEPARTMENT

For objects outside the black hole, however, there’s still plenty of trouble. Because black holes are such massive objects, when you get close to one, you start to experience significant tidal forces. You might be most familiar with tidal forces from the Moon and how it interacts with Earth.

Sure, on average, you can treat the Moon as a point mass and the Earth as a point mass, separated by the relatively large distance of 380,000 kilometers or so. But in actuality, the Earth isn’t a point, but an object that occupies a real, given volume. Parts of the Earth will be closer to the Moon than others; parts will be farther away. The closer parts will experience a greater gravitational attraction than average; the more distant parts will experience a lesser attraction than average.

From anywhere on the surface of a physical object, there will be a force pulling it in the direction of an external gravitational mass. Different points along that object will experience slightly different forces, resulting in a net tidal force: the differences between the force on the individual points versus the average net force on the entire object.DEPARTMENT OF OCEANOGRAPHY, NAVAL POSTGRADUATE SCHOOL