Timeline for Why is the information paradox restricted to black holes?
Current License: CC BY-SA 4.0
24 events
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| Dec 29, 2018 at 9:26 | comment | added | Luaan | @JoL You solved the momentum problem - the momentum of the system is still zero, as before the collision. But you completely ignored the energy problem - and that's where your missing entropy went. The two balls aren't the whole system - you also need to account for all the photons that left with what used to be the kinetic energy of the two balls. If we assume the collision was perfect (no lasting deformation, chipping etc.), all of the kinetic energy is eventually dissipated as photons. Add it all up, and no information (in the quantum sense) is lost. | |
| Dec 28, 2018 at 16:21 | comment | added | JoL | @IbrahimAbdelFaruk-Shaik I was going to say that if that mechanism was a previous collision with other billard balls, then how would you know that that collision happened without prior knowledge of the variables in question? However, after remembering what Dan Yand said of looking at the microscopic effects, I guess you're right that you can figure out these variables by looking at the microscopic effects of that mechanism, whatever it would be. | |
| Dec 28, 2018 at 8:03 | comment | added | Hans Hinterseher | @JoL: My comment concerned the fact that the initial positions of the balls and their trajectories can be retraced, for instance by the radiation emitted from the balls and by the mechanism that started them. | |
| Dec 28, 2018 at 0:06 | comment | added | Chiral Anomaly | @JoL You're on the right track. Somewhere there should be enough info to reverse the macro effect. We are basically discussing the statistical-mechanics foundation of the second law of thermo. Suppose the balls stick together when they collide. Where does the energy go? Energy is conserved, so it must go somewhere. The waves may die out because their energy becomes rearranged into "practically random" molecular motions (heat) and maybe eventually infrared radiation. Same reason sound waves always die out. You're asking good questions! But these comment boxes have so... little... room... | |
| Dec 27, 2018 at 23:59 | comment | added | JoL | @DanYand Staying with the same example model, since they're in space, there is no atmosphere (or at least I meant to imply that they're in a vacuum). Are you saying that microscopically inside the balls there should be enough information to reverse the macroscopic effect? Like waves of movement across the molecular structure from the collision? Do such waves not die out? Do they simply become infinitely small? I guess you can get the time of impact from there, and the velocity of both before impact. Positions at t=0 would then be obtained from the velocity. Hope I understood right. Thanks! :D | |
| Dec 27, 2018 at 22:05 | comment | added | Chiral Anomaly | @D.Halsey Your concern seems to refer to the effect of measurement: "the same quantum experiment repeated multiple times with the same initial states gives different final states". This gets into the (unsolved) measurement problem, which is why I wrote "...the preceding answer neglects to consider the issue of measurement." The black hole info paradox is equally paradoxical whether or not we try to account for this. I think this whole circle of questions is one of the most interesting parts of physics -- interesting because we don't know the answers, and because even the problem isn't obvious. | |
| Dec 27, 2018 at 21:53 | comment | added | Chiral Anomaly | @JoL Macroscopic irreversibility does not imply microscopic irreversibility. Two states that are initially easily distinguishable may evolve into states that are practically impossible to distinguish, but they remain microscopically distinguishable (according to both classical and quantum physics), provided we take into account everything that happens when the billiard balls collide, including all of the detailed molecule-level changes in the atmosphere due to the emitted sound and heat. If we don't account for those things, then sure, physics is obviously irreversible in practice. | |
| Dec 27, 2018 at 16:03 | comment | added | JoL | @IbrahimAbdelFaruk-Shaik The real question should be, given that the balls are together in space now, is it possible to reverse the physics and know what happened before? The impossibility of knowing who, if anyone, pushed the balls is the whole point. There are infinite possibilities on how long they might have been like that without moving. There are infinite possibilities on the possible collisions that would have led to them being like that. Because of that, it would seem that some trivial physics proves irreversible. | |
| Dec 27, 2018 at 10:24 | comment | added | Hans Hinterseher | @JoL: Very good example! But who pushed the billard balls? | |
| Dec 27, 2018 at 1:21 | comment | added | Rob | I appreciate the expanded answer, but not the example: "... all of the physically possible outcomes in Scenario 1 are different than all of the physically possible outcomes in Scenario 2. ..." --- You are artificially limiting the actual scope of the experiment to make it supposedly a true outcome. IRL: You and your pen, along with each individual paper, in an experiment repeated twice (also impossible) have an infinitesimally minute chance of the same outcome, theoretically. It is akin to saying black holes are "black" (EM is not emitted, from the poles). A different example please | |
| Dec 27, 2018 at 1:18 | comment | added | JoL | "the laws of physics as we understand them today are reversible in principle" -- If two billiard balls in space move towards each other at the same, constant speed, they should stop on collision regardless of the initial distance and speed. Once stopped, they can also stay together for an indefinite amount of time. Pick a random pause, and stop time. How can the physics be reversed? Any length of final pause, travel speed, and initial distance would be valid. That information should be lost in principle, no? | |
| Dec 27, 2018 at 0:29 | comment | added | D. Halsey | Expanding on my confusion: It seems that the reversibility should apply to the evolution of the probabilities associated with the states, rather than to the states themselves. | |
| Dec 27, 2018 at 0:12 | comment | added | D. Halsey | "It is still reversible in principle, though, in the sense that distinct initial states produce distinct final states" I'm confused how this can be true, when the same quantum experiment repeated multiple times with the same initial states gives different final states. | |
| Dec 27, 2018 at 0:04 | history | edited | Chiral Anomaly | CC BY-SA 4.0 |
Added content from comments, by request
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| Dec 26, 2018 at 21:53 | comment | added | Pedro A | @DanYand Ibrahim's comment and your reply are very enlightening, can you please add those to your answer itself? (In SE in general this is a good practice since comments aren't guaranteed to stay forever) | |
| Dec 26, 2018 at 4:23 | history | edited | Chiral Anomaly | CC BY-SA 4.0 |
Removed a copy-paste error from one sentence
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| S Dec 26, 2018 at 0:48 | history | suggested | Turnip | CC BY-SA 4.0 |
Correcting “an” to “and” in multiple places.
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| Dec 26, 2018 at 0:33 | review | Suggested edits | |||
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| Dec 25, 2018 at 21:41 | comment | added | Hans Hinterseher | Most helpful for me is your last comment. | |
| Dec 25, 2018 at 21:40 | vote | accept | Hans Hinterseher | ||
| Dec 25, 2018 at 19:28 | comment | added | Chiral Anomaly | @IbrahimAbdelFaruk-Shaik Your comment raises a very good point. When we say that the laws of physics as we currently know them are "reversible", we are ignoring the infamous measurement problem of quantum physics. Since we don't know how to resolve that, either, I suppose we should remain open to the possibility that the black hole information paradox and the measurement problem might be related in some yet-undiscovered way; but such a connection is not currently clear. | |
| Dec 25, 2018 at 19:23 | comment | added | Hans Hinterseher | I am sceptic against "in principle". When trying to measure the tiny differences between the ashes of 12345 and ABCDE there are necessarily position and momentum of many particles involved. Measurement of one of them could prevent measurement of another one. | |
| Dec 25, 2018 at 18:57 | history | edited | Chiral Anomaly | CC BY-SA 4.0 |
Replaced a non-word
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| Dec 25, 2018 at 18:52 | history | answered | Chiral Anomaly | CC BY-SA 4.0 |