Riding into a black hole is a fun mental exercise. General relativity predicts* that an observer who falls freely into a large black hole will have no means with which she can establish whether she is at the event horizon, which is the radius below which there is no escape possible. I explained this more detailed in Black Hole Duality: Not noticing crashing with light speed. Nevertheless, many popular descriptions leave the feeling as if the event horizon is also locally, to the one who falls freely through it and his close surroundings that fall with her, a recognizable place. This can be misleading in interesting ways and a thought experiment involving a train will clarify that such popular science descriptions must always be taken with great care.

Imagine you and your friends sitting on a very long train. Every wagon has one friend leaning out of his window. There are perhaps a few hundred such wagons before you and a hundred still hanging behind the wagon in which you happen to travel. The train is passing a station sign. You and your friends previously agreed to raise your arms when you are exactly next to the sign. And so, while riding the train, you watch the sign approach from very far away and whenever a friend passes it, you see him or her lifting their arms, one after the other until it is your turn.

Now say the station sign was removed but you all calculated the time at which each would pass that sign, or say you all know for some other reason where the proper location of the sign is anyway (hole in the ground perhaps). Now you cannot see the sign (and also not its hole in the ground from further away), but you still see how there is wagon after wagon having a friend raise his or her arms, until it is the one in front of you, and then you raise your arms, and then when you turn around, you see the one in the wagon behind you raise her arms, and so on.

Now ride this train into a very large black hole. Each friend has to raise his or her arms when they cross the event horizon. What will you see?

It may seem plausible that the situation is similar: One friend after the other is seen to raise their hands. But what you will see is quite disturbing if you are unprepared for it:

All your friends in front of you raise their arms all “at the same time” **together with you** while those behind you do not yet raise their arms (making you appear to be special, although all friends are in pretty much the same situation)!

Why is that? Well, the light that is emitted backwards at the event horizon will stay right there at the event horizon. In some sense, space “is rushing into the black hole” at the speed of light at the event horizon, thus, since the outwards emitted light is traveling inside that "in-rushing" space with the velocity of light, the two movements compensate each other and the light just stays at the same radius coordinate. Whatever light is emitted backwards at the event horizon will be received exactly at the event horizon – it must be so because the light never goes anywhere else.

Therefore, the light that any of your friends emitted/reflected while raising their arms when passing the event horizon will stay at the event horizon until your eyes pick it up when you are at the event horizon. This holds for all the friends that fall in before you, so just when you raise your arms, you see them all raising their arms, every one in front of you as if in unison. It looks like *as if the event horizon is everywhere along the train in front of you*, but not behind you, or as if the event horizon is rather a moment in time than a location (and these statements are true in some sense, but it is difficult to describe such properly without sounding nuts, so I won’t attempt it).

I recommend thinking about it in order to weaken rigid concepts about time. Also,it helps to see that a lot of popular descriptions of black holes are wrong. If you and your friends fall into a large black hole, you will see the whole train in front of you and all its lights and perhaps hear the whistle. Black holes are only black if nothing else fell inside for a while.

One interesting issue is the following: According to what I wrote, even just a few meters in front of the event horizon, you would still *see* the wagons in front of you as if nothing happened to them, although they are many meters in front of you and thus *behind* the event horizon. They do not even look distorted or let alone shrunk to fit in between you and the horizon. That is not yet a problem (just think about where the light you see was emitted), but it becomes a problem with extremely long trains. My description cannot hold for arbitrarily long trains.

What will you see if the train’s length is comparable to the black hole's radius? This question touches on the fact that the singularity** is not so much* inside *of a black hole as it is the only possible *future* that can be had in the black hole. However long the train, you will never see any wagons in front of you being destroyed. Already at the event horizon, you saw your friends all only having come as far as the event horizon. This “delay” or "distortion" becomes worse inside the black hole. From your point of view, you will always be the first one of all your friends to hit the singularity! Quantum solipsism is preempted by relativity theory!

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-------------------------------------* If the unification of quantum theory with gravity will only modify general relativity close to the curvature singularity** of the black hole, not already at its event horizon***. Though this is not necessarily so and the issue is still an open question today, one aspect is certain: Especially if the event horizon is fully described by relativity, it should for that very reason not be presented as something tangible.

** A singularity is something inside a model that tells us that the model does not properly describe the physics. General relativity has singularities because it does not include quantum mechanics. So, "singularity" in the above article is basically the middle of the black hole; I do not claim there are true singularities.

*** We assume that general relativity does not break down as a description already close to the event horizons. This may be wrong and in any case we need to consider very large black holes. The maximum free fall time between event horizon and singularity in a massive hole like Sagittarius A* would only be 20.05 seconds!

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