Recently a paper was published giving the results of a series of observations of a star being torn apart by a supermassive black hole. The event was interesting for many reasons including the fact that it was the nearest such event yet seen and it was noticed early enough so that it was observed in many different ways from the radio to x-rays and these observations followed the evolution of this object for an extended period of time as it both brightened and dimmed.
The popular press published several articles about this event, AT2019qiz, and these observations. In virtually every case they misrepresented several phenomena and added to the already significant confusion most people have regarding, not only black holes, but one of the most widely misunderstood yet seemingly familiar concepts from physics. Tides.
On Tuesday Jan. 4, 2011 David Silverman was on the O'Reilly Factor and the host, displaying an astounding depth of ignorance asserted that tides could not be explained. This resulted in (at least) two interesting results. One, based on the dumbfounded expression on Mr. Silverman's face, is a popular WTF meme.
The other was a large number of "explanations" of tides that appeared to refute this silly statement by O'Reilly. One of which is well characterized by H.L. Mencken oft quoted quip; "neat, plausible, and wrong", delivered by Neil deGrasse Tyson on The Colbert Report. The "mechanism" described would only produce tides once per day. I have no doubt that Dr. Tyson knows what causes the tides. I suspect he did what he thought was the best he could in the time allotted to explain. But none of the explanations actually did a good job. The best one I've seen is far too long for most situations but it has the advantage of actually being correct. I strongly suggest that you follow the link and watch the video now. But understanding tides is only part of the story here so please return.
This relates to black holes because their powerful gravitational fields can produce truly monstrous tidal forces that would tear you apart long before you crossed the event horizon. Steven Hawking popularized a term for this effect when describing what happens when you approach a black hole the mass of the Sun. He wrote that you would be stretched out like spaghetti, or as it is now often called, you would be undergo spaghettification.
Let's define some terms. To keep things simpler I'm going to consider the case of the simplest kind of black hole: One that isn't spinning. The closer you get to a mass the higher the escape velocity. A black hole can be defined as any place where the escape velocity exceeds that speed of light. The formula using mass and distance where this happens in Einstein's General Relativity was first computed by Karl Schwarzschild so it is called the Schwarzschild radius (Rs). The boundary of space that is Rs away from the center of a black hole is called the "event horizon". Black holes have been detected in a large range of masses. At one end are ones that are just a few times the mass of our Sun. In the typical style of astronomical nomenclature these are called "stellar mass black holes". At the other end are the huge behemoths found at the center of most galaxies. These are typically millions to billions times the mass of our Sun and are called "supermassive black holes" (SMBH)
As Hawking pointed out in his book, as the mass of a black hole increases the tidal forces at Rs decrease. For SMBHs the tidal forces at this critical location are too small to notice. Spaghettification does not occur for SMBHs. To be precise I'm only considering things outside of Rs, once closer than that you're doomed.
Let's look at a few actual examples. Consider a one solar mass black hole. At the event horizon a 6ft tall person falling feet first would have their feet accelerated away from their head at about 1 million times the acceleration we experience on the surface of Earth. But it is even worse than this makes it look. Not only are you being stretched out, you would also be crushed from the side. So it is like being extruded through a press, like the way pasta is made. What about large black holes? For a SMBH like the one at the center of our galaxy the effect would be about 1018 times smaller, essentially zero. Going back to the one solar mass case you would need to be about 40*Rs away before the tidal forces are only as large as gravity on Earth.
"Spaghettification" was used in every popular article I saw on AT2019qiz. The first article on this event that I saw posted by a friend on Facebook was this one. It starts very well by pointing out the popular misconception that black holes are cosmic vacuum cleaners. This is true, up to a point. At close distances black holes orbits are not possible. The article then makes up for this by asserting that it is only material that crosses the event horizon that is trapped. This is off by at least a factor of 3. Within 3*Rs no stable circular orbits are possible for anything other than light (and other massless particles) and anything closer will be swallowed up.
It then says "If that object is a star, the process of being shredded (or "spaghettified") by the powerful gravitational forces of a black hole occurs outside the event horizon, and part of the star's original mass is ejected violently outward. This in turn can form a rotating ring of matter (aka an accretion disk) around the black hole that emits powerful X-rays and visible light. Those jets are one way astronomers can indirectly infer the presence of a black hole."
The issue of whether the spaghettification occurs inside or outside of the event horizon is determined by the mass of the black hole, not the mass of the object falling in. So the fact that we are talking about a star doesn't matter. It is true that part of the star will be ejected "violently outward" but this isn't what causes the formation of the accretion disk, it is caused by it. The rest of the excerpt confuses the accretion disk with the jets and where the various emissions come from. It is probably easier to see what a more accurate description would be:
If a star gets too close to a SMBH the tidal forces will disrupt it into stream of hot gas. This is appropriately called a TDE (tidal disruption event) and occurs outside of the event horizon. The strange orbits caused by being so close to the black hole cause the stream to run into itself and flatten out, forming an accretion disk. These interactions are so violent that the disk is hot enough to glow not only in visible light but even in X-rays. In addition accretion disks also create powerful jets that point perpendicular to the disk, although the details of how this works isn't currently well understood. The power of these jets is often used to infer the presence of a black hole.
The preceding paragraph covers the same material as the excerpt from the article without the errors, explains, or at least introduces some of the more interesting physics, and doesn't require any more science knowledge of the reader.
This was an important paper for very technical reasons that are essentially impossible to explain. Wisely, none of the articles I saw tried to do this accurately. Instead they confused the concepts of TDE and spaghettification and got the physics of black holes, accretion disks, and jets wrong all while missing the opportunity this presented to explain these, and other, concepts correctly.
