Question from a 5 year old: Bell and Black Holes

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Discussion Overview

The discussion revolves around the question of whether entangled particles can retain information if one of them falls into a black hole. It explores concepts of quantum entanglement, black hole physics, and the implications of measurements on entangled particles in the context of black holes. The conversation touches on theoretical aspects, interpretations of quantum mechanics, and the nature of black holes, including the black hole information paradox and Hawking radiation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that the "free" particle would not exhibit any specific trait if its entangled partner falls into a black hole, indicating that the notion of "knowing" is interpretative.
  • It is proposed that the effects of entanglement can only be observed after comparing measurement results from both particles, which is not possible if one particle is inside a black hole.
  • A scenario is presented where an experimenter could potentially measure a particle falling into a black hole and transmit results, but the outcome would remain unknown to those outside the event horizon.
  • Participants discuss the differences between classical and quantum black holes, noting that classical black holes do not radiate, while quantum black holes can emit Hawking radiation, which is entangled with particles inside the black hole.
  • The black hole information paradox is mentioned, with some participants questioning whether information is truly lost or merely appears to be lost due to the nature of entanglement and black hole radiation.
  • There is a discussion about the concept of entanglement being weak in many particles due to the monogamy of entanglement, and the implications of this for understanding quantum states.
  • One participant references the no hiding theorem, suggesting that a pure state falling into a black hole becomes a mixed state through a unitary process.
  • Another participant raises a separate question about time dilation and photons, questioning whether photons experience any time change at all, which leads to further discussion about the nature of proper time for photons.

Areas of Agreement / Disagreement

Participants express various viewpoints on the implications of entanglement and black holes, indicating that multiple competing views remain. The discussion does not reach a consensus on the nature of information retention in black holes or the interpretation of time for photons.

Contextual Notes

Limitations include the dependence on interpretations of quantum mechanics, the unresolved status of the black hole information paradox, and the complexities surrounding the nature of entanglement and measurement in quantum systems.

jshrager
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Seriously, a 5 year old asked me whether entanglement information survives/escapes a black hole. Specifically, he asked me (in only slight paraphrase) whether if one of the particles (headed in different directions) fall into black holes on either end, does the other one know it?
 
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That's some 5 year old! Get him into some advanced physics classes now!

The "free" particle would not evidence any particular trait by having its partner fall into a black hole (as opposed to not). You could say it would "know" it, but that is somewhat a matter of interpretation and semantics. But we wouldn't know it regardless.

Generally speaking, the ordering of measurements on entangled particles is not relevant to the outcome of measurements. No one knows the underlying mechanism past that.
 
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The effects of entanglement can only be seen when we compare the results of measurements on both particles after the fact. Drop one member of the pair into a black hole before you measure it and there's no way for us outside the event horizon to make that comparison.

In principle I suppose we could drop an experimenter along with one particle into the black hole, do our measurement, and then transmit our result to the other guy, and he would be able to make the comparison. Of course we'd never know what he found... But I would expect the results to be no different than comparisons between any other two space-like separated measurement events.
 
If the black hole is big so that the curvature is small at the event horizon, and the black hole is not evaporating, everything will be the same as outside the black hole, except that the guy inside cannot send his results out. However, for a quantum black hole which is evaporating, we don't know the answer. John Preskill has a write-up about why we don't understand entanglement in the evaporating black hole. http://quantumfrontiers.com/2012/12/03/is-alice-burning-the-black-hole-firewall-controversy/
 
Thanks, all, for this interesting discussion.

(Lest you think I'm making up that this came from a 5 year old, here's the relevant snippet of the actual conversation:



You'll accuse me of over-interpretation, and that's fair enough. But, over-interpreted or not, it's an interesting question. :-)
 
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DrChinese said:
That's some 5 year old! Get him into some advanced physics classes now!

Maybe another Terry Tao who used to ask questions like that at 5.

Thanks
Bill
 
At a more basic level than the firewall paradox that Preskill discusses in the blog post I linked to in #4, I should point out that an evaporating black hole is about entangled particles. A classical black hole is truly "black", as it does not radiate and does not have a temperature. However, a black hole in which spacetime is classical but matter is quantum will radiate, and is not "black". The radiation has a thermal spectrum, so the black hole has a temperature. The radiation emitted by the black hole is entangled with particles inside the black hole. This entanglement causes information to appear to be "lost" since we see only the radiated particle, but not its entangled partner inside the black hole. This radiation is known as Hawking radiation. It is one of the big questions as to whether information is truly lost or only appears to be lost, if we also allow spacetime itself to be quantum. This is called the "black hole information paradox". The more modern firewall paradox is a development of the black hole information paradox.
 
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Aren't pretty much all the particles we see entangled all over the place?
 
jshrager said:
Aren't pretty much all the particles we see entangled all over the place?

The entanglement is generally weak, because of monogamy of entanglement. To be "maximally entangled", you can do that with one other particle. The Preskill blog post I linked to in #4 talks about monogamy of entanglement.

BTW, here I am loosely talking as if the wave function is real, and entanglement is real. This is not necessarily the case. The wave function, and entanglement are just tools to calculate the probabilities of measurement results. Only the measurement results are real.
 
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BTW, he also asked me whether, since time dilation increases as you get closer and closer to the speed of light, whether photons, which are actually AT the speed of light, see any time change at all? (Put another way: Is proper time for a photon unchanging?) Alas, I don't have video of that...but anyway, it's a discussion probably more appropriate for a different forum.
 
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"Black holes and time warps"
You will learn for 10 dollars what an observer sees when she falls in a BH.
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  • #13
jshrager said:
BTW, he also asked me whether, since time dilation increases as you get closer and closer to the speed of light, whether photons, which are actually AT the speed of light, see any time change at all? (Put another way: Is proper time for a photon unchanging?) Alas, I don't have video of that...but anyway, it's a discussion probably more appropriate for a different forum.
To elaborate on that question would require a separate thread, even in a different sub forum; however there is no need for a discussion as the short answer is that indeed, if you interpret a photon as a wave packet (in contrast to just a detector "click") then a photon's proper time is "frozen" (and from that immediately follows the impossibility of establishing a co-moving reference system :) ).
 
  • #14
harrylin said:
To elaborate on that question would require a separate thread, even in a different sub forum; however there is no need for a discussion as the short answer is that indeed, if you interpret a photon as a wave packet (in contrast to just a detector "click") then a photon's proper time is "frozen" (and from that immediately follows the impossibility of establishing a co-moving reference system :) ).

There's a FAQ in the relativity subforum: https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/
 

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