Registered: Aug 2000
Location: In a Blue, Blue State
Posts: 13077

New model 'permits time travel'
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.

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."

http://news.bbc.co.uk/2/hi/science/nature/4097258.stm

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Damn. I guess I can't go back to yesterday and use today's winning Lotto numbers.

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Registered: Feb 2004
Location: in colorado somewhere!
Posts: 26135

The only way this is true is if you go back in time,you don't materalize into a person.
The only way to only observe is to be a ghost.
You can't even eat in the past.
Even that may affect the future.
So right now the old man behind me only I could see is really me. And he can't tell me anything cause it will affect the whole balance of life.
Anything we do affects the future.
That's beyond crazy.
Cause we have like crazy powers.

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Registered: Jul 2000
Location: Montreal
Posts: 2992

Damn that Quantum Butterfly!

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"Please pardon my levity, I don't see how to take death seriously. It seems absurd." R A Wilson (R.I.P)

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Registered: Aug 2000
Location: Los Angeles, CA
Posts: 15130

Try to think along these lines... You can't go back and change things because anything you'd do in the past will have already happened by the time you left.

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Registered: Jan 2003
Location: mediocre apartment
Posts: 2634

No, it's all about ghosts. Didn't you read LF's post?!

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Registered: Mar 2004
Location: USA
Posts: 24752

Dueling gorillas.

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Registered: Dec 2002
Location:
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This whole circular argument is predicated upon the foundational argument that we do not observe the past to change, therefore it cannot and does not change.

First, the fact that to you, no such change is visible does not extend that limitation to all potential observers. An actor in such a time looping scenario might know full well the past has been altered, while to all others the past appears tro have remained immutable.

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Registered: Feb 2004
Location: in colorado somewhere!
Posts: 26135

So what if the argument has no foundation?
What if it's just a guess?
What if everything we know as tangible is in fact not?
What if we are actually living in the past now?
Who's to say we came from the future and as we travelled back our minds erased?
What if were really not on earth?
What if there's no universe?
That would suck.

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Registered: Sep 2000
Location: atlanta
Posts: 6398

you're all dorks.
why don't you argue about something worthwhile, like why Klingons are better than humans?

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Registered: Jun 2002
Location: Just to the right of the moon
Posts: 3141

If I go back in time I'll probably materialise as a toilet brush

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Registered: Jan 2002
Location: Shreveport, LA
Posts: 3834

quote:

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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

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Registered: Apr 2004
Location: Den
Posts: 11291

You could also read this.

quote:

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1687: Isaac Newton

The universe has one absolute clock:
• Time and space are independant of the observer.
• Time's arrow points forward; events move ahead from the now.

1905: Albert Einstein

every observer has his or her own (accurate) clock:
• The universe exists in a space-time manifold.
• Everyone's "now" is different.
• Acceleration affects time.

2003: Peter Lynds

There is no clock; "time" is an illusion
• Time has no indivisible unit.
• There is no "now," only sequences of events.

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Registered: Mar 2005
Location: Snoville, USA
Posts: 119

Are you saying Back to the Future wasn't based on a true story? Michael J. Fox got to change shit around when he went! Damn, it's always the celebrities that get away with everything. ha ha ha lol

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Registered: Jan 2002
Location: Shreveport, LA
Posts: 3834

quote:

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Originally posted by Coincidence
You could also read this.

quote:

---

1687: Isaac Newton

The universe has one absolute clock:
• Time and space are independant of the observer.
• Time's arrow points forward; events move ahead from the now.

1905: Albert Einstein

every observer has his or her own (accurate) clock:
• The universe exists in a space-time manifold.
• Everyone's "now" is different.
• Acceleration affects time.

2003: Peter Lynds

There is no clock; "time" is an illusion
• Time has no indivisible unit.
• There is no "now," only sequences of events.

---

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Registered: Apr 2004
Location: Den
Posts: 11291

Yeah I always thought that sounded right too. Or rather, since I tried psilocybine.

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Registered: Jul 2000
Location: aggressivly happy
Posts: 9130

i thought this was all allready explained in the hitch hikers guide?

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