I've heard/read many times that in the general relativistic description of a black hole an object would pass through the event horizon unharmed (ignoring tidal forces) while quantum mechanics predicts that an object would be thermalised (firewall) and that this presents a paradox. I can't reference any particular papers here but I've watched a lot of Leonard Susskind's stuff on Youtube.

My understanding is that from the point of view of an observer falling into the black hole the field of view gets concentrated in a disk behind them due to the extreme lensing effects and tends towards infinite brightness and blue shift as they fall further towards the black hole.

Surely any object would be destroyed by high energy gamma rays before it reached the event horizon as observed from infinity.

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    $\begingroup$ Time dilation would prevent an observer from seeing what happens. $\endgroup$
    – Jimmy360
    May 30, 2015 at 23:28
  • $\begingroup$ Standard quantum mechanics doesn't predict anything about event horizons because we don't have a version of quantum mechanics that has been tested on a non-flat background. There are different groups of physicists that are trying to develop versions of quantum mechanics that can do that, but neither of those suggestions are standard QM and neither has been verified. Similarly one can argue that we haven't made sufficiently detailed measurements that would back up GR as the correct theory near the event horizon, so it's really Mr. "I don't know" testifying against Mrs. "Maybe...". $\endgroup$
    – CuriousOne
    May 30, 2015 at 23:32
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    $\begingroup$ My understanding is that ignoring tidal forces, in the reference frame, the observer would not experience anything special, It is just a freely fallingframe. However as seen from the outside observer point of view he will simply thermalize at the horizon. This is debatable however, see firewall pradaox. $\endgroup$
    – Prathyush
    May 30, 2015 at 23:54
  • $\begingroup$ Let me rephrase the question: Lets say we have two observers A and B. A is falling into the black hole and B is observing from infinity. A is carrying a torch. B would observe the light from the torch become increasingly red-shifted due to time dilation until they reached the event horizon where the red shift becomes infinite and they disappear from view. Does A see B become increasingly blue shifted until the point where B saw A reach the event horizon and the light from the rest of the universe becomes infinitely blue shifted? Ignore the QM bit. $\endgroup$
    – Ben Booth
    May 30, 2015 at 23:57
  • $\begingroup$ Below the outer horizon the falling observer would, indeed, see the light from the universe come "crashing down on him" if I remember correctly, the problem is... what makes us think that there is anything below that horizon to begin with, let alone that there is a smooth transition for the completely unique case of the freely falling observer and for absolutely nobody else? $\endgroup$
    – CuriousOne
    May 31, 2015 at 0:05

1 Answer 1


I think you are confused about the nature of the firewall paradox and the usual gravitational redshift seen by an observer falling into a black The former does not predict that a falling observer would see infinitely blue-shifted light from the whole universe when they pass through the event horizon.

The scenario is treated in some detail by Taylor & Wheeler ("Exploring Black Holes", Addison, Wesley, Longman - highly recommended) in terms of what a free-falling observer would see on a direct radial trajectory into a non-rotating "Schwarzschild" black hole. Some more details an animations can be found in this set of animations produced by Andrew Hamilton at JILA.

A star situated at exactly 180 degrees from the observer's radial path will always appear in that position as the observer looks back - right down to the singularity. The light will be gravitationally blueshifted by an ever-increasing amount - essentially tending towards an infinite blueshift at the singularity. Nothing special happens as the event horizon is crossed.

For stars at an angle to the radial path, their positions will be distorted such that they appear to move away from the point at which the observer has come from (and are also blue-shifted). In the final moments (it takes less than $1.5\times 10^{-4}$ seconds of proper time to fall from the event horizon to the singularity of a 10 solar mass black hole, but a huge $\sim 60$ seconds for the black hole at the centre of our Galaxy) the light from the external universe will flatten into an intense ring at 90 degrees to the radial direction of motion. So you would end up seeing blackness in front of you, blackness behind and the sky split in two by a dazzling ring of light (almost seems worth it!).

I think the confusion is because the situation would be completely different for a "shell observer" who could somehow lower themselves slowly to just hover (using almost infinite rocket power) just above the event horizon. Yes, they would see the universe shrink to a point overhead and become massively blueshifted.

I am unqualified to write/speculate about the firewall paradox and its association with Hawking radiation, but it appears not to be a generally accepted fact.


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