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Quantum observation of black hole?

  1. Oct 22, 2015 #1
    [Moderator's note: this discussion has been spun off from another thread.]

    This is probably a silly idea, but I haven't got any reputation to preserve, so I'll mention it anyway.

    My very limited (and probably very wrong) understanding of quantum theory tells me that until something is observed, it hasn't actually happened one way or another, but remains a probability distribution. If there is no way for a person outside the black hole to make any observations which would distinguish the possible cases for the distribution of matter inside the black hole (shell outside the EH, shell a millimetre inside the EH, singularity at the centre, small sphere of matter around the centre etc.), then could it be said that none of those things is actually the case, but rather, just some sort of probability distribution?

    While I'm here and on the topic, I might ask another related question that I've never been able to find the answer to. I understand that, for Schrodinger's cat to stop being alive and dead and go into just one of those states, it has to be observed. But does it have to be observed by the scientist who put him in there, or will some other human do? Will the cat do? (because the cat will know when the poison is released)? Would a severely handicapped person with the same IQ as the cat do? Would a robot do? Or some kind of simple sensing device inside the box that will activate when the cat falls over, but not send any message out of the box?

    Or if the janitor sneaks a look when the scientist is out of the room, could it be that the cat is dead for the janitor, but still alive and dead for the scientist - is the cat's state subjective?
    Last edited by a moderator: Oct 26, 2016
  2. jcsd
  3. Oct 22, 2015 #2


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    My perspective is that the word observation is used in this context because observation is impossible without particle interaction, and anyway one is not interested in an experiment unless something is eventually observed, so to keep ourselves interested we talk about 'observation'. What matters according to discussions on waveform collapse that I have read is particle interaction. Since the cat is composed of multiple particles and they all interact, the cat as a whole would always be predicted to be in a well defined state, as far as I understand the slit experiment and its implications. So the cat's particles are 'observing' each other all the time, if you want to use the word observation, which I agree is the common vernacular.
  4. Oct 22, 2015 #3


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    You've been misled by a (very common, very widely repeated) misconception. Early in the 20th century when the theoretical basis of quantum mechanics was less well understood one of the possibilities that was considered was that observation by a conscious entity was required to get definite results out of the probability distribution (to "collapse the wave function" in the lingo). This idea that something had to be observed to be real was largely abandoned by the 1930s when the theory was properly formalized, but by then the idea had made it into the popular imagination.... and it's like an invasive plant... Once established, it's impossible to kill it off.

    It does not help any that for historical reasons we still use the word "observation", although "interaction" would be more accurate. The interactions among the enormous number of particles around a star completely eliminate the quantum uncertainties even if no one is there to watch.

    Again, you're being misled by common misconceptions in the popular explanations of quantum mechanics. Schrodinger did not suggest his thought experiment about the dead/alive cat in 1935 because he or anyone else thought for a moment that the cat might actually be in that in-between state until it was observed. Instead he was pointing out a problem with the then-current understanding of quantum mechanics: it predicted that dead/alive state, and that couldn't be right. This remained an unsolved problem for several decades, until the discovery of quantum decoherence.

    You can google for "quantum decoherence", but be warned that the math is somewhat daunting. There are also many threads on this topic in the Quantum Physics subforum here, but the best layman-friendly math-free explanation that I know of is the book "Where does the weirdness go?" by David Lindley.
  5. Oct 23, 2015 #4
    Thank you Grinkle and Nugatory. In a few words you have cleared up a misconception I have had for years. It's like someone turned the light on - it all makes better sense sense now, though I will still have to think through the consequences. ( I had a quick look at quantum decoherence, but I think it might be a bit beyond me.)

    I know I should maybe move this to the quantum physics forum, but I would like to ask one more question if I may, as it is vaguely related to the earlier discussions. Suppose a hydrogen atom in inter-galactic space emits a photon and the photon travels through space for a billion years. Before it interacts with another particle, can it be said that the photon is equally likely to be anywhere on a spherical shell with a one-billion-light-year radius around the hydrogen atom?

    If it is, then interaction with a particle at one point on the shell would instantly change the probabilities at all other points, many light years away. Though I guess no information has been transferred because any observers on the rest of the shell wouldn't have known that the photon was coming anyway.

    Also, this is probably invalid reasoning, but, because the photon is travelling at the speed if light, in its frame of reference no time would pass between leaving the original hydrogen atom and arriving at a new one at a time one billion years later according to an outside observer. Does this mean that, in a sense, the photon dematerialises at its source and instantly rematerialises at its destination (a different space-time coordinate) - without spending any time travelling? A bit like going through a worm hole? I would expect, if this were the case though, that it wouldn't matter what was between the source and the destination, but it does seem to - the photon won't arrive anywhere where the geodesic between there and its source passes through an opaque object.

    That was actually three questions, wasn't it?
  6. Oct 24, 2015 #5


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    Pretty much, yes. Quantum mechanics only predicts the results of observations so it would be more precise to say "the photon is equally likely to be detected anywhere on a spherical shell...", but that's a quibble here - you have the right idea.
    Yes, and this was one of the earliest discovered examples of how quantum mechanics is inherently non-local. It bothered Einstein a lot, and it still bothers many people, because even though there's no information transferred and hence no violation of relativity... It's hard to think about this situation without also imagining something moving from the point on the sphere where the photon was found to all the other points on the sphere and forcing the probabilities to change when it arrives. However, there's nothing in the math of quantum mechanics that requires any such thing.
    Yes, and that's why there's no conflict with relativity. It's a digression here, but there is an even more sharp-edged version of this problem. Get hold of a book by Louisa Gilder, "The Age of Entanglement", or try googling for "quantum entanglement, "Bell's Theorem", and the like.

    This line of thinking doesn't work for several reasons.
    - A photon does not have a "frame of reference" (there is no inertial frame in which the photon is at rest) so the time dilation calculations can't be used to draw the conclusion that time doesn't pass for it. There is a FAQ and many discussions over in the relativity subforum.
    - Photons aren't what you think they are. They are not little particles of light moving through space from the source to the destination. You'll find much discussion over in the quantum mechanics section, and I very highly recommend Richard Feynman's book "QED: The strange theory of light and matter".
    Last edited: Oct 24, 2015
  7. Oct 24, 2015 #6
    Thank you Nugatory.
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