[Paleopsych] BBC: New model 'permits time travel'

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New model 'permits time travel'
http://news.bbc.co.uk/go/pr/fr/-/2/hi/science/nature/4097258.stm
    Published: 2005/06/17 10:03:47 GMT

    By Julianna Kettlewell
    BBC News science reporter

    If you went back in time and met your teenage parents, you could not
    split them up and prevent your birth - even if you wanted to, a new
    quantum model has stated.

    Researchers speculate that time travel can occur within a kind of
    feedback loop where backwards movement is possible, but only in a way
    that is "complementary" to the present.

    In other words, you can pop back in time and have a look around, but
    you cannot do anything that will alter the present you left behind.

    The new model, which uses the laws of quantum mechanics, gets rid of
    the famous paradox surrounding time travel.

    Paradox explained

    Although the laws of physics seem to permit temporal gymnastics, the
    concept is laden with uncomfortable contradictions.

    The main headache stems from the idea that if you went back in time
    you could, theoretically, do something to change the present; and that
    possibility messes up the whole theory of time travel.

    Clearly, the present never is changed by mischievous time-travellers:
    people don't suddenly fade into the ether because a rerun of events
    has prevented their births - that much is obvious.

    You go back to kill your father, but you'd arrive after he'd left the
    room, you wouldn't find him, or you'd change your mind
    Professor Dan Greenberger, City University, New York

    So either time travel is not possible, or something is actually acting
    to prevent any backward movement from changing the present.

    For most of us, the former option might seem most likely, but
    Einstein's general theory of relativity leads some physicists to
    suspect the latter.

    According to Einstein, space-time can curve back on itself,
    theoretically allowing travellers to double back and meet younger
    versions of themselves.

    And now a team of physicists from the US and Austria says this
    situation can only be the case if there are physical constraints
    acting to protect the present from changes in the past.

    Weird laws

    The researchers say these constraints exist because of the weird laws
    of quantum mechanics even though, traditionally, they don't account
    for a backwards movement in time.

    Quantum behaviour is governed by probabilities. Before something has
    actually been observed, there are a number of possibilities regarding
    its state. But once its state has been measured those possibilities
    shrink to one - uncertainty is eliminated.

    So, if you know the present, you cannot change it. If, for example,
    you know your father is alive today, the laws of the quantum universe
    state that there is no possibility of him being killed in the past.

    It is as if, in some strange way, the present takes account of all the
    possible routes back into the past and, because your father is
    certainly alive, none of the routes back can possibly lead to his
    death.

    "Quantum mechanics distinguishes between something that might happen
    and something that did happen," Professor Dan Greenberger, of the City
    University of New York, US, told the BBC News website.

    "If we don't know your father is alive right now - if there is only a
    90% chance that he is alive right now, then there is a chance that you
    can go back and kill him.

    "But if you know he is alive, there is no chance you can kill him."

    In other words, even if you take a trip back in time with the specific
    intention of killing your father, so long as you know he is happily
    sitting in his chair when you leave him in the present, you can be
    sure that something will prevent you from murdering him in the past.
    It is as if it has already happened.

    "You go back to kill your father, but you'd arrive after he'd left the
    room, you wouldn't find him, or you'd change your mind," said
    Professor Greenberger.

    "You wouldn't be able to kill him because the very fact that he is
    alive today is going to conspire against you so that you'll never end
    up taking that path leads you to killing him."

    Greenberger and colleague Karl Svozil introduce their quantum
    mechanical model of time travel on the ArXiv e-print service.