Man in the Lab Coat reader and friend Dan posed a great question in the comments of my last post: assuming an astronaut was immortal, would they experience an infinite amount of time while falling into a black hole?

The tricky part about this is time dilation. To an outside observer, once the astronaut passes beyond the event horizon she can no longer be observed, and theoretically takes an infinite amount of time to fall in – we’ll never hear from her again. But for the astronaut herself, time proceeds as normal – and it takes her a finite amount of time to fall into the black hole, or at least, to fall far enough past the event horizon that she is spaghettified. I also pointed out that no form of immortality save being unaffected by gravity could save you from this fate – and if you were unaffected by gravity, you wouldn’t be falling in in the first place! So the answer is no.

Note, too, that this whole scenario is only feasible if it’s a supermassive black hole – for smaller black holes, an astronaut would be spaghettified long before crossing the event horizon.

While checking my facts, I came across a fascinating idea – that it’s possible to find a “sweet spot” of acceleration that maximises that finite time before you are destroyed. This study can be found in a paper from last year titled “No Way Back: Maximizing survival time below the Schwarzschild event horizon“, written by two Australian physicists, Geraint F. Lewis and Juliana Kwan from the University of Sydney. A great summary of the article can be found in the Universe Today blog.

Time dilation comes down to the fact that the speed of light kinda slows down to an observer as an object heads toward the “event horizon”. As you get closer and closer to that event horizon gravity pulling on it gets stronger and stronger, it affects light particles as well and eventually the observer will see fewer and fewer photons as the object gets closer and closer to the horizon. Not that the object is emitting any less light, just that the light travelling away from the black hole is taking longer and longer to get to the observer.

So, as this time tends to inifinity, to the observer the object is slowing down in time, while for the object, its life as usual.