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Do you believe that entanglement drives the arrow of time?

  1. Feb 21, 2016 #1
    "The idea that entanglement might explain the arrow of time first occurred to Seth Lloyd about 30 years ago, when he was a 23-year-old philosophy graduate student at Cambridge University with a Harvard physics degree. Lloyd realized that quantum uncertainty, and the way it spreads as particles become increasingly entangled, could replace human uncertainty in the old classical proofs as the true source of the arrow of time."

    "In the new story of the arrow of time, it is the loss of information through quantum entanglement, rather than a subjective lack of human knowledge, that drives a cup of coffee into equilibrium with the surrounding room. The room eventually equilibrates with the outside environment, and the environment drifts even more slowly toward equilibrium with the rest of the universe. The giants of 19th century thermodynamics viewed this process as a gradual dispersal of energy that increases the overall entropy, or disorder, of the universe. Today, Lloyd, Popescu and others in their field see the arrow of time differently. In their view, information becomes increasingly diffuse, but it never disappears completely. So, they assert, although entropy increases locally, the overall entropy of the universe stays constant at zero."

    Do you believe what these scientists say in this article in Quanta Magazine?
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  3. Feb 21, 2016 #2


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    If you understand that entropy of a system can be described as the degree of its entanglement with its environment then this is not news. The entropic time arrow is indeed driven by entanglement in this sense. But entanglement is not some mysterious meta-force. It is simply the logical implications of the fact that systems interact. I would say using this to describe time's arrow is not specific to quantum theory it has been made already in the classical idea of entropic time. Quantum simply does the information accounting better.
  4. Feb 21, 2016 #3
    The only problem I have with this is that time would have to be granular for this to work, meaning you would have to say something like "1 picosecond is the smallest fraction of time" but then relativity proves a picosecond in one frame is not the same amount of time as it might be in another... but it could change in discrete steps relating to a quantum source and remain beyond our ability to observe it.
  5. Feb 21, 2016 #4


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    I would say that entanglement is involved in the arrow of time, but not that it causes it. The mechanism of entanglement does not have a preferred direction in time, so for things to become more entangled in the future (as opposed to less entangled) requires that the far past be a special, low-entropy state.
  6. Feb 21, 2016 #5


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    Do you have a proper source for "entanglement might explain the arrow of time" ? This is not generally accepted, and the article is really speculative. Asking folks to speculate on something is itself speculative does not make sense.
  7. Feb 21, 2016 #6


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    Yes - well, haven't read the article itself, so not sure if there are oversimplifications there - but behind the article lies work that is technically solid, and that many others have built on. However, it all still occurs within the Copenhagen interpretation, which itself assumes an arrow of time, so we still need to solve the measurement problem (eg. Bohmian Mechanics, MWI or something else?).

    For an example of related work, see

    Strong and weak thermalization of infinite non-integrable quantum systems
    Mari Carmen Bañuls, J. Ignacio Cirac, Matthew B. Hastings

    I think one can go as far back as von Neumann (!) for related ideas:
    [/PLAIN] [Broken]
    Long-Time Behavior of Macroscopic Quantum Systems: Commentary Accompanying the English Translation of John von Neumann's 1929 Article on the Quantum Ergodic Theorem
    Sheldon Goldstein, Joel L. Lebowitz, Roderich Tumulka, Nino Zanghi

    The approach to thermal equilibrium and "thermodynamic normality" --- An observation based on the works by Goldstein, Lebowitz, Mastrodonato, Tumulka, and Zanghi in 2009, and by von Neumann in 1929
    Hal Tasaki

    It is true that much of this has a classical counterpart. However, there is one trick that is realized in some quantum systems that has no classical counterpart: "It has been proposed, however, that thermalization may happen without any time average in the quantum case [11]; indeed, it is possible that, due to quantum entanglement, starting from fixed, non-thermal initial conditions, the reduced density matrix at a given time t on a given region A, such that A is small compared to the system size, will converge to the thermal expectation value at long times t. Such a phenomenon, which we call “strong thermalization”, cannot occur for a classical system as it relies on the quantum mechanical fact that even if the global density matrix is a pure state, the reduced density matrix may be a mixed state." (from the Banuls et al paper above)

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  8. Feb 21, 2016 #7
    And it may well turn out QM needs modifying, e.g. GRW theory (https://en.wikipedia.org/wiki/Ghirardi–Rimini–Weber_theory) [also discussed in Ghirardi's book 'Sneaking a Look at God's Cards'], or Penrose and his gravity stance.
  9. Feb 21, 2016 #8


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  10. Feb 22, 2016 #9
    Dr Chinese said in his previous posts that entangled photons do not show interference and then he asks why they think entanglement increases entropy.

    Interference is a state of low entropy while lack of interference is a state of high entropy because we can distinguish paths of individual particles.

    Wikipedia says that in the figure caption if photons scatter off an object, they become entangled with it and carry away its quantum nature, leaving it in a mixed state.

    Entropy of the system of entangled particles does not increase as the entangled state is still a pure state, but the local entropy of the particles does does because the individual states become mixed


    "In quantum scattering, the interaction between the scattered photons and the superposed target body will cause them to be entangled, thereby delocalizing the phase coherence from the target body to the whole system, rendering the interference pattern unobservable."

    BUT I wonder what is better for entropy to remain low, entanglement or rather disentanglement?
    Last edited: Feb 22, 2016
  11. Feb 22, 2016 #10
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