One of the amazing things about a black hole, and the thing that makes it very hard to get our heads around the whole idea, is that black holes force us to confront the fact that time is not what we think it is.

We tend to think of time as being universal, the same everywhere and for everyone. We feel like there is a moving “now”, with the past “behind us” and the future not yet formed… and therefore we assume that “now” is a universal thing. But all of that is wrong! Oh, it is true in our everyday experience. But black holes really turn the knobs up to 11, and force us to let go of these local and naïve notions.

I am sure that some answers will point out that, yes, an object’s time seems to stop (as seen by a distant observer) as it approaches the black hole — but that you can’t see that object because it’s light is equally red-shifted to infinite wavelengths. That’s true. But not the whole truth.

Other answers will point out that the ‘freezing in time’ is just what you see, but the object that is falling in experiences the fall into the black hole, and in quite a short amount of time. That’s also true. But not the whole truth.

Because in some important sense there isn’t a whole truth. The time near a black hole and the time for that distant observer are increasingly disconnected, and at the event horizon the time for the infalling observer and the time for the distant observer stop having any connection at all. It is meaningless to ask what is “inside” the black hole at this time, because “this time” has no relation to what is inside! This is the reason it that the event horizon is an event horizon! There are two partial views (the in falling observer and the distant observer) but those views cannot be stitched together in any simple way.

Let’s try this out. We’ll start with a very large black hole so that tidal forces near the event horizon are fairly small — for a less massive black hole the tidal forces are paradoxically greater, and we don’t want to be ripped apart. At least not right away (later all bets are off). You stand way over there where it’s safe. Hold my beer…

OK, so now I jump into this black hole (don’t try this at home!) This sounds drastic, but in fact I might not even notice crossing the event horizon — our current physics says there is no discontinuity for an infalling observer. Side note: some people think this is quite wrong and I would in fact be incinerated at the moment of crossing… it actually doesn’t matter for this answer, oddly enough!

The key point is that my time feels perfectly normal to me, and I pass that point of no return in all too finite a time.

From outside, though, any information from me is more and more delayed when you see it. As I approach the event horizon my clocks slow and grind toward a stop without every quite getting there. Messages I send you come further and further apart, and are dr.a..w…n…..n…….o…….u……….t more and more. The radio signals I send have lower and lower frequency (keeping pace with my clocks) and any visible light is shifted to the red, infrared, microwave and below…

From your perspective I don’t exactly ‘become stationary’… what I am is literally ‘smeared out’ across time. So you aren’t seeing a still image of me falling in… you are seeing a dim (and dimmer) slow (and slower) slice of my fall.

If, somehow, you had something magical like a transporter you conceivably “beam me out” years later (if you somehow could lock onto me across that time distortion…), or if there was some way that I could remain near the black hole without falling in, then I could emerge from the black hole’s vicinity years later, having passed through much less of my time than you have of yours. In that sense time is indeed slowed down (a la the movie Interstellar if you have seen that).

But from the viewpoint of someone near the black hole time is passing perfectly normally and they are not stationary at all. I crossed the event horizon in a short time. You saw me slow down and ‘smear out’ across time, and that I never cross the horizon at all.

And I, the person falling in, and you, the person watching from a distance, are both right… but only because we are passing through different time.

<an attempt to show the light cones (which map how time and space are structured) near the event horizon of a black hole… I’m afraid I have lost the source for this!>

Most of us struggle with that! We want to know what is ‘really’ happening. We want one description and really have a hard time with the idea that two very different descriptions of what is happening can both be correct. But this seems to be the case, and (again oddly) the event horizon is no where near the expected limits of our physics! We are pretty confident that our description of event horizons is solid. This is (subject to the limits of my attempt at explanation) how the world is put together.

It’s just, as Haldane famously remarked — “the universe is not only queerer than we suppose, it is queerer than we can suppose.” Black holes force us to confront that head on.