- 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.
-
- 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."
-
- © BBC MMV
-
- http://news.bbc.co.uk/2/hi/science/nature/4097258.stm
|
