3 ms·

Is information ever lost? Not a rhetorical question, I genuinely want to know. Once the vibration due to a sound become smaller than a certain level, do quantum

by prmph 8y ago

Is information ever lost? Not a rhetorical question, I genuinely want to know. Once the vibration due to a sound become smaller than a certain level, do quantum effects make it disappear entirely

kurthr 8y ago
Not just quantum effects (although they can dominate especially at low temperatures or superfluid/conducting states), but thermodynamic effects. Once the energy falls below ~kT/2 in a dissipative system (quadratic hamiltonian terms) you've lost the information.
TekMol 8y ago
I find that hard to imagine. Let's take the information 'The universe exists'. How can that information get lost? Wouldn't the universe have to disappear for that to happen? I have never heard about a model of reality where that is a possibility.
- gbear605 8y ago
  I think by information they mean more concrete things. For instance, imagine a bird landing on a branch. If someone is around to see it, they know that if happened. If no one is around though, once the bird flies away and the branch stops shaking, the information that a bird had landed on the branch is gone.
archgoon 8y ago
> Is information ever lost? From a practical perspective, yes; but I get the feeling you're not asking that ;). Let's say that we know everything possible about the universe at a time t (in some reference frame). Can I reconstruct what happened at any time in the past? If I can, then it seems reasonable to say that 'no information has been lost' (going the other way suggests that no information gets created either). A lot of Physicists seem to think the answer is yes, we can construct the past if we actually knew everything. The evolution of the wavefunction is unitary, so you should be able to run the universe backwards to find out what happened (unless wavefunction collapse is real, in which case, we're losing information all the time). Note that in order to do this, you need to know the entire wave function; which isn't measurable. However, under our current understanding of black holes, the answer seems to be "Looks like it?" https://en.wikipedia.org/wiki/Black_hole_information_paradox https://en.wikipedia.org/wiki/Black_hole_information_paradox Since we don't know how to resolve General Relativity with Quantum Mechanics, we can't really say with confidence whether the universe can 'be played backwards' or not.

by prmph 8y ago

Is information ever lost? Not a rhetorical question, I genuinely want to know. Once the vibration due to a sound become smaller than a certain level, do quantum effects make it disappear entirely

kurthr 8y ago

Not just quantum effects (although they can dominate especially at low temperatures or superfluid/conducting states), but thermodynamic effects. Once the energy falls below ~kT/2 in a dissipative system (quadratic hamiltonian terms) you've lost the information.

TekMol 8y ago

I find that hard to imagine. Let's take the information 'The universe exists'. How can that information get lost? Wouldn't the universe have to disappear for that to happen? I have never heard about a model of reality where that is a possibility.

gbear605 8y ago
I think by information they mean more concrete things. For instance, imagine a bird landing on a branch. If someone is around to see it, they know that if happened. If no one is around though, once the bird flies away and the branch stops shaking, the information that a bird had landed on the branch is gone.

archgoon 8y ago

> Is information ever lost? From a practical perspective, yes; but I get the feeling you're not asking that ;). Let's say that we know everything possible about the universe at a time t (in some reference frame). Can I reconstruct what happened at any time in the past? If I can, then it seems reasonable to say that 'no information has been lost' (going the other way suggests that no information gets created either). A lot of Physicists seem to think the answer is yes, we can construct the past if we actually knew everything. The evolution of the wavefunction is unitary, so you should be able to run the universe backwards to find out what happened (unless wavefunction collapse is real, in which case, we're losing information all the time). Note that in order to do this, you need to know the entire wave function; which isn't measurable. However, under our current understanding of black holes, the answer seems to be "Looks like it?" https://en.wikipedia.org/wiki/Black_hole_information_paradox https://en.wikipedia.org/wiki/Black_hole_information_paradox Since we don't know how to resolve General Relativity with Quantum Mechanics, we can't really say with confidence whether the universe can 'be played backwards' or not.