[Paleopsych] BBC: New model 'permits time travel'
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Wed Jun 22 19:43:29 UTC 2005
New model 'permits time travel'
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.
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.
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
"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
"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.
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