5 ms·

I'm a noob here,and maybe this is not a meaningful question, but why is it a paradox that matter falling into a black hole causes information to be lost? Are t

by prmph 5y ago

I'm a noob here,and maybe this is not a meaningful question, but why is it a paradox that matter falling into a black hole causes information to be lost? Are there not many other physical processes where this happens? For example, if a sound is made, does the universe permanently keep a record of it? I would imagine the waves would die down, until at a certain quantum level, they will stop entirely. Or, if two particles annihilate each other, is it possible from the resulting particles/energy produced to know which particles were involved?

danbruc 5y ago
Yes, the known laws of physics are time-reversible - two distinct states A and B evolve over time but they stay distinct, i.e. they will never evolve into the same state C. If the the universe is also deterministic, then there is also also only one possible way a state can evolve over time. In the examples you mentioned, like sound waves dissipating, the energy just gets spread out over countless particles as heat. If you could measure all the particles in the air and environment precisely enough, you could in theory reconstruct the sound.
- AnimalMuppet 5y ago
  Is that true? Here's a chamber with two equal halves, with a wall between them. I put air in the left half or the right half, and a vacuum in the other half. Then I remove the wall. Once things reach equilibrium, how are you going to determine which half had the air in the beginning? So it looks to me like, even apart from black holes, no, you can't reverse things. Am I missing something here?
  - mywittyname 5y ago
    Equilibrium is local in your example. Globally, the rush of air from one side to the other would result in detectable oscillations whose released energy could be used to determine which chamber was initially empty, assuming one can find and isolate the information.
    - AnimalMuppet 5y ago
      I see. All right, I put oxygen at some pressure on one side, and nitrogen at the same pressure on the other side, and then remove the barrier. Now there are no oscillations, just mixing.
      - danbruc 5y ago
        This doesn't change the situation. Shortly after removing the barrier, some molecules will start moving into the other half. Just reverse all velocities and see all the molecules move back into the half they started in. If you go further forward in time, you will eventually see the first collision. But that doesn't really change the situation, reverse the velocities, see the molecules collide in reverse and then move back into their starting halfs. And this never really changes, particles move and collide and this can be reversed, you just have to do this for very, very many particles and collisions. And this is probably chaotic, the tiniest error in any of the positions or velocities will get massively amplified and give you the wrong initial state. But in principle it could be done. And it has to be done on the microscopic level, macroscopic things like concentrations, temperatures, or pressures by definition do not reflect the microscopic states.
      - SkyBelow 5y ago
        If you know the information about every single molecule of each, you could calculate backwards the probability of where they came from. Eventually you would reach a state where the most likely probability would show all the oxygen on one side and all the nitrogen on the other. This is a purely theoretical calculation, far outside the scope of our ability to calculate. It is one of those, if we turned all known matter into the possible fastest computer, could that even calculate it before the heat death of the universe sorts of questions.
renewiltord 5y ago
Right but they leave a forensic trace, I.e. if you had perfect measurement you would be able to tell which sound was made. Like a slight difference in the pattern of sand or where the grass is, but at a much lower level.
zehaeva 5y ago
A huge part of Quantum Mechanics is Conservation of Information, or better maybe to think of it as a conservation of entropy? Your example of a soundwave is too local in scope, sure the wave will die down, but while the soundwave was propagating it's bumping into other particles and those particles affect other particles, which affect other particles, and on and on and on. If you look locally, sure, eventually you wouldn't be able to tell the wave ever existed, but if you pull back far enough and take the state of the system and rewind it you will eventually reconstitute the wave. Your example with annihilating particles is basically how CERN works, though we do know what we started with in some places. I had started to describe the whole of how Hawking Radiation works, I'm sure you can find that easily elsewhere.
lisper 5y ago
> if a sound is made, does the universe permanently keep a record of it? I would imagine the waves would die down, until at a certain quantum level, they will stop entirely. > Or, if two particles annihilate each other, is it possible from the resulting particles/energy produced to know which particles were involved? These are really great questions! The facile answer is that all known physical processes are reversible in principle, but the actual truth is more complicated than that. For starters, quantum measurement may or may not be reversible depending on which interpretation of QM you subscribe to. On the currently-most-popular account, the Copenhagen interpretation, measurement is by assumption irreversible. There are many reasons to believe that this is not the case, that measurements are in fact reversible (in principle, not in practice) but no one really knows, and no one can know because the process of reversing a measurement is indistinguishable form the normal state of affairs. It might actually be happening all the time. There is no way to know. For more info see: http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar... (And its prequel: http://blog.rongarret.info/2014/09/are-parallel-universes-real.html http://blog.rongarret.info/2014/09/are-parallel-universes-re...) If you want a deeper understanding of all this stuff I recommend: https://www.amazon.com/Philosophy-Physics-Princeton-Foundations-Contemporary/dp/069118352X https://www.amazon.com/Philosophy-Physics-Princeton-Foundati... https://www.amazon.com/Quantum-Mechanics-Experience-David-Albert/dp/0674741137 https://www.amazon.com/Quantum-Mechanics-Experience-David-Al...
vl 5y ago
You can go meta and think about what is knowledge and what is scientific method. Knowledge is inherently axiomatic. In other words there are certain unprovable truths (“axioms”) you agree on as granted and then build your theories from based on certain principles (and this is “scientific method”). In this case “conservation of information” is one of the axioms quantum mechanics is build on. Now, thinkers operating on the fringe of our understanding and advancing our theories have a choice to make: (a) assume it’s true on the black hole event horizon - thus paradox and theories on how to resolve it, (b) assume it’s not true on event horizon. So we don’t know if it’s a paradox at all, we choose to think it’s a paradox because we want our theories to work everywhere. But it’s not necessary correct assumption. When these theories are discussed this is implicitly implied.

by prmph 5y ago

danbruc 5y ago

Yes, the known laws of physics are time-reversible - two distinct states A and B evolve over time but they stay distinct, i.e. they will never evolve into the same state C. If the the universe is also deterministic, then there is also also only one possible way a state can evolve over time. In the examples you mentioned, like sound waves dissipating, the energy just gets spread out over countless particles as heat. If you could measure all the particles in the air and environment precisely enough, you could in theory reconstruct the sound.

AnimalMuppet 5y ago
Is that true? Here's a chamber with two equal halves, with a wall between them. I put air in the left half or the right half, and a vacuum in the other half. Then I remove the wall. Once things reach equilibrium, how are you going to determine which half had the air in the beginning? So it looks to me like, even apart from black holes, no, you can't reverse things. Am I missing something here?
- mywittyname 5y ago
  Equilibrium is local in your example. Globally, the rush of air from one side to the other would result in detectable oscillations whose released energy could be used to determine which chamber was initially empty, assuming one can find and isolate the information.
  - AnimalMuppet 5y ago
    I see. All right, I put oxygen at some pressure on one side, and nitrogen at the same pressure on the other side, and then remove the barrier. Now there are no oscillations, just mixing.
    - danbruc 5y ago
      This doesn't change the situation. Shortly after removing the barrier, some molecules will start moving into the other half. Just reverse all velocities and see all the molecules move back into the half they started in. If you go further forward in time, you will eventually see the first collision. But that doesn't really change the situation, reverse the velocities, see the molecules collide in reverse and then move back into their starting halfs. And this never really changes, particles move and collide and this can be reversed, you just have to do this for very, very many particles and collisions. And this is probably chaotic, the tiniest error in any of the positions or velocities will get massively amplified and give you the wrong initial state. But in principle it could be done. And it has to be done on the microscopic level, macroscopic things like concentrations, temperatures, or pressures by definition do not reflect the microscopic states.
    - SkyBelow 5y ago
      If you know the information about every single molecule of each, you could calculate backwards the probability of where they came from. Eventually you would reach a state where the most likely probability would show all the oxygen on one side and all the nitrogen on the other. This is a purely theoretical calculation, far outside the scope of our ability to calculate. It is one of those, if we turned all known matter into the possible fastest computer, could that even calculate it before the heat death of the universe sorts of questions.

renewiltord 5y ago

Right but they leave a forensic trace, I.e. if you had perfect measurement you would be able to tell which sound was made. Like a slight difference in the pattern of sand or where the grass is, but at a much lower level.

zehaeva 5y ago

A huge part of Quantum Mechanics is Conservation of Information, or better maybe to think of it as a conservation of entropy? Your example of a soundwave is too local in scope, sure the wave will die down, but while the soundwave was propagating it's bumping into other particles and those particles affect other particles, which affect other particles, and on and on and on. If you look locally, sure, eventually you wouldn't be able to tell the wave ever existed, but if you pull back far enough and take the state of the system and rewind it you will eventually reconstitute the wave. Your example with annihilating particles is basically how CERN works, though we do know what we started with in some places. I had started to describe the whole of how Hawking Radiation works, I'm sure you can find that easily elsewhere.

lisper 5y ago

> if a sound is made, does the universe permanently keep a record of it? I would imagine the waves would die down, until at a certain quantum level, they will stop entirely. > Or, if two particles annihilate each other, is it possible from the resulting particles/energy produced to know which particles were involved? These are really great questions! The facile answer is that all known physical processes are reversible in principle, but the actual truth is more complicated than that. For starters, quantum measurement may or may not be reversible depending on which interpretation of QM you subscribe to. On the currently-most-popular account, the Copenhagen interpretation, measurement is by assumption irreversible. There are many reasons to believe that this is not the case, that measurements are in fact reversible (in principle, not in practice) but no one really knows, and no one can know because the process of reversing a measurement is indistinguishable form the normal state of affairs. It might actually be happening all the time. There is no way to know. For more info see: http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar... (And its prequel: http://blog.rongarret.info/2014/09/are-parallel-universes-real.html http://blog.rongarret.info/2014/09/are-parallel-universes-re...) If you want a deeper understanding of all this stuff I recommend: https://www.amazon.com/Philosophy-Physics-Princeton-Foundations-Contemporary/dp/069118352X https://www.amazon.com/Philosophy-Physics-Princeton-Foundati... https://www.amazon.com/Quantum-Mechanics-Experience-David-Albert/dp/0674741137 https://www.amazon.com/Quantum-Mechanics-Experience-David-Al...

vl 5y ago

You can go meta and think about what is knowledge and what is scientific method. Knowledge is inherently axiomatic. In other words there are certain unprovable truths (“axioms”) you agree on as granted and then build your theories from based on certain principles (and this is “scientific method”). In this case “conservation of information” is one of the axioms quantum mechanics is build on. Now, thinkers operating on the fringe of our understanding and advancing our theories have a choice to make: (a) assume it’s true on the black hole event horizon - thus paradox and theories on how to resolve it, (b) assume it’s not true on event horizon. So we don’t know if it’s a paradox at all, we choose to think it’s a paradox because we want our theories to work everywhere. But it’s not necessary correct assumption. When these theories are discussed this is implicitly implied.