Potential time travel paradox discounted by quantum mechanics.

[goofball]

link

[ QUOTE ]

New model 'permits time travel'
By Julianna Kettlewell
BBC News science reporter

Tardis, BBC
The concept of time-travel is laden with uncomfortable paradoxes
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."

[/ QUOTE ]

[drudman]

Nonsensical.

[brassnuts]

Oooooor...

It's impossible to travel backward in time.

[goofball]

Damn. You must have already gotten your Ph.D in physics and your Nobel. I didn't know that time travel had been proven impossible, where was your research published, I can't believe I missed it.

[brassnuts]

Don't get so defensive. I was just kidding around. A serious response:

To me, the paradox still exists in comprehending the first time that you go back in time. Is an initial journey even rational? Had the feedback loop been created prior to this journey?

Also, I do agree with the article that while being back in time, you couldn't change anything. Disregarding the paradox I stated above, everything you do would have already happened in the past you came from. Anything that you affect while being back in time would be part of this "feedback loop." Your actions don't have to be as egregious as going back to kill your father before you're born.

[Victor]

has any one seen the simpsons when homer travels back in time?

he is told not to touch anything as it could affect the future. but he touches something and it rains donuts. that one was funny.

[drudman]

The question of whether or not you can change things is moot. There isn't really even a paradox involved.

Many-worlds Theory

[Jeff W]

[ QUOTE ]
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.

[/ QUOTE ]

How does this make it past the editors? That article is a joke.

[hbaromega]

Traveling back in time is theoretical. This is just a theory to explain how the "killing your own grandfather " paradox can be avoided. Neither this or the many worlds theory have been tested.

[PairTheBoard]

[ QUOTE ]
Traveling back in time is theoretical. This is just a theory to explain how the "killing your own grandfather " paradox can be avoided. Neither this or the many worlds theory have been tested.

[/ QUOTE ]

Funny how Fathers and Grandfathers are chosen as default for lineage when they are in reality somewhat problematic unless dna testing was done to prove who the father was. Meanwhile, the absolutely certain lineage by way of Mother and Maternal Grandmother is payed no mind.

PairTheBoard