Is information conserved in quantum physics?

[Gerinski]

Leonard Susskind says so. But I don't see it.

Yes, the present "here and now" is computed by nature from the events lying in its past light cone + the laws of nature. If we assume a deterministic universe, the present "here and now" contains the information from the events in its past light cone.

But when we introduce quantum randomness and uncertainty, we can not say that anymore. The present "here and now" is computed by the events in its past light cone + the laws of nature, + an element of uncertainty or randomness. The "here and now" can never tell us precisely what the past events were like, because it may have been defined by a random quantum "jump" altering what classical theory would have predicted.

So, in which way can we say that information is conserved when we include quantum mechanics? Surely Susskind is much more clever than I am so I must be wrong in something.

 

[MisterX]

You have touched on one of the great mysteries. Quantum physics describes a precise way to determine the future quantum state. This might tell us in the future that there is an exact probability for each outcome. Information is conserved in the sense that quantum state (that tells us the probabilities) can be advanced forward and backward in time to tell us the probabilities at any other time.

However once something actually happens¹ then the probability for it becomes 1 and the probability for all other outcomes becomes 0. This can imply a loss of information. We explain this by the existence of some element outside the system which has interacted with it.

So with conventional thinking at least, we would conclude that something special seems to happen at the end of the experiment. This quandary is sometimes called the measurement problem or wave-function collapse. This issue has not been fully resolved.

¹ perhaps a problematic notion in light of quantum physics

 

[entropy1]

But when a measurement is made, the future is different, so maybe that new future encodes the new information?