
Ask HN: Why do we believe that quantum information cannot be destroyed? - AnimalMuppet
Is there experimental evidence to support this idea?  (I assume no, at least not directly, but correct me if I&#x27;m wrong.)<p>Also, if one subscribes to the many-worlds interpretation, then isn&#x27;t the creation of another world the same as the loss of information from <i>this</i> world (that is, the other world takes a different eigenstate of the waveform, and therefore this world is left with just one eigenstate)?
======
gus_massa
If we ignore wavefunction collapse ...

The time evolution operator is unitary, so it is in particular inversible. So
there is a one to one correspondence between the posible current states of the
system/universe and the posible states of the universe one minute later. So
all the information that is available now, must be preserved one minute later
because the transformation can be (theoretically) reversed. If some
information disappear, you could not reverse the changes due to time
evolution.

The experimental evidence shows that the equations for the time evolution are
correct (up to the current experimental precision), and these equations have a
form that is unitary. I'm not sure that it is a direct proof, but it's quite
strong evidence.

And about wavefunction collapse or the multiuniverse alternative ...

<personal opinion warning [1]> Nobody like neither of the options but they
make predictions that agree with the experiments. If we assume that humans,
dogs and measurement equipment are not magic then we could explain the
apparent wavefunction collapse using a normal time evolution. IIRC the current
betting favor decoherence, where the equipment somehow has an apparent effect
of the wavefunction collapse, but the information that is apparently lost is
spread in many small parts that evolve slightly different and after some time
it's imposible to experimentally collect all the parts and reverse the
transformation (in spite it is theoretically posible to collect all the parts
and reverse the transformation)[2]. Nobody is sure, and it is an active
research topic. </personal opinion warning>

[1] I took enough quantum mechanic courses in the university to give a good
answer, but I'm not expert in (quantum) measurement theory.

[2] It's slightly similar to how a drop of ink in a glass of water spread
after some time by diffusion. It's theoretical posible to reverse the system
and get the original result, but in practice it's imposible. But the equations
of both process are very different. I hate analogies that have very different
underlaying equations because they are misleading sooner or later. So take
this as a bad analogy and don't draw too many conclusions from it.

