On Friday, October 16, 2020 at 6:19:14 AM UTC-5 Bruce wrote: > On Fri, Oct 16, 2020 at 5:49 PM Jason Resch <[email protected]> wrote: > >> On Thu, Oct 15, 2020 at 6:07 PM Bruce Kellett <[email protected]> wrote: >> >>> On Fri, Oct 16, 2020 at 9:51 AM Jason Resch <[email protected]> wrote: >>> >>>> I noticed that Victor Stenger's position on entropy, as described here: >>>> https://arxiv.org/pdf/1202.4359.pdf on page 7, appears to be the same >>>> as described by the cosmologist David Layzer in a 1975 issue of >>>> Scientific >>>> American: >>>> https://static.scientificamerican.com/sciam/assets/media/pdf/2008-05-21_1975-carroll-story.pdf >>>> >>>> The basic idea, which is described graphically here: >>>> https://www.informationphilosopher.com/solutions/scientists/layzer/arrow_of_time.html >>>> >>>> It is a counter-argument to the commonly expressed idea that the >>>> universe began in a low entropy state. Rather, it explains how the >>>> expansion of the universe increases the state of maximum possible entropy. >>>> If the universe expands more quickly than an equilibrium can be reached, >>>> then there is room for complexity (information / negative entropy) to >>>> increase. >>>> >>>> Why is it that the "low entropy" myth is so persistent, and this >>>> alternate explanation is so little known? Some physicists, such as Penrose >>>> are still looking for alternate explanations for the special low entropy >>>> state. What fraction of physicists are aware of Stenger's/Layzer's view? >>>> Does it appear in any physics textbooks? Has it been refuted? >>>> >>> >>> It is refuted by the idea of unitary evolution in QM. Unitary evolution >>> means that everything is reversible, If new microstates are created as the >>> universe expands, then this expansion cannot be reversed: the creation of >>> such microstates gives an absolute arrow of time. This is generally >>> rejected, because physicists tend to believe in unitary dynamics. If >>> dynamics are not unitary, then the universe is not governed by the >>> Schrodinger equation, and arguments for the multiverse collapse. >>> >> >> I understand unitarity for a fixed physical system with certain finite >> boundaries. But how does that work for the case of an expanding universe? >> If you define the wave function for the observable universe at time 1, what >> is the wave function for time 2? Doesn't the number of possible states in >> time 2 not increase beyond what it was in time 1, given new information has >> entered the system from the cosmological horizon? >> > > If there is a unitary operator that takes the wave function at time 1 to > time 2, the the evolution is unitary and reversible. Horizons play no part > in this. > >> Also, I think we can borrow a lesson from quantum computing to shed some >> light on the problem of irreversibility and entropy. Quantum computers need >> to use reversible logic gates to prevent premature decoherence. Reversible >> circuits generate garbage (ancilla) bits as a result of the continued >> operation of the computation. (see >> https://en.wikipedia.org/wiki/Ancilla_bit and >> https://quantumcomputing.stackexchange.com/questions/1185/why-is-it-important-to-eliminate-the-garbage-qubits >> ). >> >> If we extend this analogy to the universe, can we envision the rise of >> complexity/macroscopic order in a similar way to the locally growing order >> of a reversible computation, which must generate waste heat >> ("garbage/ancilla bits") leading to global rise in entropy? So long as >> there are enough places to dump these ancilla bits (such as into the low >> temperature, non-equalized environment), then there is space for growth of >> local order through the process of reversible computations. >> > > The quantum process of generating the ancilla bits is unitary, hence > reversible. If these bits are treated as garbage and thrown away, then the > result is irreversibility. No new space for bits is created. > > Bruce >
A Turing machine replaces symbols on a tape. This is erasure, which is irreversible. A quantum computer is governed by a Hamiltonian, which in classical mechanics defines the energy surface of constant energy. Hence for a Turing machine to be reversible requires ancillary tapes or registers that send "erased information" into some record that can be accessed in a reversible way. A quantum computer, in principle a quantized Turing machine requires the same. These are the trash qubits, which are required in order to maintain the fidelity of the wave function. LC -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/fd025e25-c375-4d4a-8d15-7517acf23a79n%40googlegroups.com.
