# Fate of helium moving toward a singularity

On the molecular level, if an atom, consider helium, were to fall into a black hole, what would be the ultimate fate of its electrons, protons and neutrons?

• FYI, google exactly: "magnetic field around a blackhole" for some recent awesome articles relating to this. – Fattie Oct 16 '16 at 22:22
• And be sure to check out the awesome second paragraph here! en.wikipedia.org/wiki/Magnetar#Magnetic_field – Fattie Oct 16 '16 at 22:25
• As we have no theory of a singularity - the clue is in the name - this question about the "ultimate fate" of anything is essentially metaphysical. – adrianmcmenamin Oct 18 '16 at 16:45
• A lot depends on the black hole. If it's large, and "quiet" (ie not much else is in close orbit, or falling into it at the same time) then the answer from @JamesK is spot on. If it's "busy" the area near the event horizon will be full or radiation and the atom will surely be ionized, and might even be smashed apart. If it's small, then tidal forces near the event horizon may be enough to disrupt it, and there is also interaction with Hawking radiation to think about. – Steve Linton May 8 '18 at 9:53

I'll take the reference of an observer falling with the helium atom. At the Event horizon, the tidal force across a helium atom is still minute, $10^{-29}N$ much less than the Coulomb forces between the electrons and nucleus, still less than the strong forces inside the nucleus. So the atom will fall through the event horizon without damage. (The region around the black hole can be a pretty violent place, with lots of hard X rays from the superheated matter, which would likely have ionised the atom, but I'll suppose it somehow manages to avoid this)

As it falls past the event horizon it is doomed. Think of the singularity less of a place in the middle of the black hole, and more as a point in time. It is inevitable our He atom will reach that point. As it approaches it the tidal forces will grow larger, eventually sufficient pull the atom apart. To describe what happens now would require a good description of quantum gravity, which we don't have.

The is no physics that describes what happens "after" the singularity. Think of the singularity as a point in time, but for the atom it is also the end of time for the atom. It is possible that something beyond our current understanding prevents a singularity from forming, but that is speculation.

This is explained diagramatically in a Penrose diagram

• The tidal force at the event horizon depends on the mass of the black hole – Rob Jeffries Jan 21 '19 at 6:38
• @RobJeffries Yeah, a tiny BH will start shredding things way before they reach the event horizon. But the answer as given is valid for stellar mass black holes, which are probably the most likely a random He atom would encounter. – Florin Andrei Jan 21 '19 at 11:26

The answer from @JamesK is pretty much all you need to know on this topic.

Since you mention the word "singularity" in the title, let me add this - the singularity is not a thing. I know how pop sci articles talk about it as if it was a thing, but it's just a mathematical abstraction.

At the center of the black hole - we don't really know what happens. The equations from GR (general relativity) basically blow up, we get divisions by zero and whatnot. This in mathematics is called a "singularity". But in the physical reality that simply means our theory is inadequate.

We will have to wait for a future theory to properly understand the center of BH. For now, the GR equations are just hitting a wall. GR predicts infinite densities and zero diameters and so on - this is virtually certain to be wrong. But as of now there is no other theory we can use to figure this out.

So don't say it's falling towards the "singularity". Say it falls toward the center, whatever that may be.

Make no mistake, your helium atom is doomed. Its particles will get shredded down to whatever scale things exist at the center. Maybe down to the Planck length, but we don't know for sure.