New Model Solves Grandfather Paradox

goldfishy writes "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 theory, you could go back in time and meet your infant father but you could not kill him." From the article: "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."

Comments

You insensitive clod!

[Anonymous Coward]

I am my own grandfather!

Re:You insensitive clod!

[Fjornir]

Many, many years ago when I was 23
I was married to a Wider who was purty as can be
This Wider had a grown-up daughter who had hair of red
My father fell in love with her and soon they two were wed

This made my dad my son-in-law and changed my very life
For my daughter was my mother cause she was my father's wife
To complicate the matter even though it brought me joy
I soon became the father of a bouncing baby boy

I'm my own grampa,
I'm my own grampa
It sounds funny I know
But it really is so
I'm my own grampa

My little baby then became a brother-in-law to dad
And so became my uncle though it made me very sad
For if he was my uncle then that also made him brother
Of the Wider's grown up daughter who of course was my step-mother

My father's wife then had a son who kept them on the run
And he became my granchild for he was my daughters son
My wife is now my mother's mother and it makes me blue
Because although she is my wife she's my grandmother too

I'm my own grampa,
I'm my own grampa
It sounds funny I know
But it really is so
I'm my own grampa

Oh if my wife is my grandmother then I'm her grandchild
And every time I think of it, it nearly drives me wild
For now I have become strangest case you ever saw
As husband of my own grandmother I'm my own grampa

I'm my own grampa,
I'm my own grampa
It sounds funny I know
But it really is so
I'm my own grampa

Re:You insensitive clod!

[LiquidCoooled]

And I thought compiler dependencies were tough!

Novikov?

[WilliamSChips]

Sounds like the Novikov self-consistency principle to me.

Re:Novikov?

[Fnkmaster]

Yes, it sounds similar.

And this frustratingly vague article makes a meaningless argument.

It tries to use the fact that we observe no disappearing people, or other strange temporal modifications as an argument that such things don't happen, and are thus impossible. But if somebody actually changed the way a wave function collapsed at some time in the past, why on earth would we expect to remember things from the way it was "before" it had been changed, since the change by definition happened in our own past, and thus to us it always occurred the way it now occurs? This isn't a logical argument. And it explains part of the aesthetic appeal of the many-worlds interpretation.

In pure quantum mechanics, time is a special property because wave function collapse via quantum operators (i.e. "observation") is a privileged thing that moves in only one direction. In general relativity, time doesn't have a privileged status. I don't see how you are going to reconcile that basic difference without coming up with a more complete theory (i.e. quantum gravity, GUT, etc.), but then again, my undergrad physics major knowledge is a bit rusty five years later.

That's great!

[cytoman]

This means that you cannot be killed when you go back in time, nor can you kill or destroy anything! That's just perfect!!

Go back in time and be able to observe, only... no ability to interact with anyone either... it should be kinda like ghosts... we go back in time and observe and be like ghosts in the sense that we cannot interact and change anything that has already happened but only observe!

Imagine the possibilities of history classes of the future... maybe there are already a lot of ghosts watching us right now... the future students studying history!!

Re:That's great!

[Lemmy Caution]

Here's an interesting twist: I can change the past in a way that changes the future, if I do so in a way that I don't know now. I could, for example, buy some shares of a stock that I know will rise a hundred-fold in value, but make sure that I don't get the money until after the moment I depart current time. Since I don't know that I don't have an envelope worth millions of dollars hidden, with a mechanism that will inform me of it, say, a week after my time-travel tip, it does not contradict what is known about the present that I become a millionaire a week from now.

No no no!

[jerald_hams]

"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."

That's not clear at all. If I went back in time and killed the baby George W Bush, it's like he would disappear in the middle of a speech. Rather the entire course of history branching from that moment would be changed, so that in the "present" no one would ever know GW had existed.

-Alex

Re:No no no!

[crow]

You mean like when my friend went back in time and killed President Barnes when he was in first grade?

Re:Futurama - Roswell that Ends Well

[SlashThat]

I think a more appropriate quote would be:

Farnsworth: "Don't do anything that affects anything. Unless it turns out you were supposed to do it, in which case for the love of God. Don't not do it!"

One sperm in a million

[Saeger]

The one thing that always bothered me about those time travel movies (besides the ridiculous timetravel part) like "Back To the Future", is that you wouldn't have to go to extremes to prevent your birth. All you would have to do is bump into your Mom or Dad to delay them for 1 second; that slight change in the timeline would guarantee that it would be a different sperm that won the race to impregnate your mom.

Re:One sperm in a million

[imr]

Hmm, how long have you been fantazing about bumping into your Mom so that a different sperm would impregnate her?

Re:One sperm in a million

[H0NGK0NGPH00EY]

I really could have gone without hearing the phrase "impregnate your mom" this evening.

Lame!

[khasim]

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.

Ummm, if you prevented their births, they wouldn't exist to "fade into the ether".

It could be happening all the time and you wouldn't be aware of it (by definition).

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.

Sounds like bad fiction to me.

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.

Sounds like the techno-babble "justification" in the bad fiction.

So, if you know the present, you cannot change it.

And the easiest way to not change it is for time travel to be impossible.

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.

If, for example, you knew a picture would be taken, you could reflect light from your body and appear in that picture, thereby altering the future.

So, travelling back in time, you cannot reflect light, and, by the same token, you cannot absorbe light.

And it just moves up from there for all other physical effects. Nothing touched, no air breathed, no light disturbed, nothing.

So, how would you even know you were in the past?

Re:H'uh?

[daft_one]

"Anyone else having difficulty imagining a scenario where it would be "impossible" to kill somebody?"

*cough* Bin Laden *cough*

Laws of physics are time symmetric

[tylersoze]

Something I don't think a lot of people really grok is that the laws of physics are time symmetric (actually the full symmetry is CPT, charge+parity+time, an electron going back in time would be a positron for example) so the fundamental weirdness is why we perceive time to flow in one direction in the first place. That's why I've always loved Feynman's absorber theory and it's associated spin-off the transactional interpretation of quantum mechanics. Those theories don't discard the so-called "advanced" (that is, backwards in time)solutions and work out how in a universe with appropriate boundary conditions you get an arrow of time. The advanced solutions actually exist but because of the boundary conditions they cancel each other out except where they "count". So according to the theory, when you go to push an electron every other particle in the universe sends waves back in time in response to push back on the electron at the exact instant you push it! The advanced waves only manifest themselves as the normal radiation resistance we observe when accelerating charged particles. The transactional interpretation takes this line of thinking with regards to the collaspe of the wave function. When one particle of a two particle entangled system wave function collapses it sends an advance wave back in time to collapse the wave function of the other particle. So in the EPR experiment there is no instanteous "spooky action at a distance" but travel exactly at the speed of light but in the opposite direction in time.

Sci-fi has two main theories? Try philosophy

[Togra]

Actually, philosophers have had the theories, but time-travel sci-fi is, like most sci-fi, just a futuristic take on one philosophical idea or another.

The "new" model is actually called the "B theory" of time and isn't new at all (although this scientific explanation of it is I guess). The B theory is that every instance in time exists somewhere and it is always "now" in that instance, so there is no real past, present or future. In the B theory if you were to go back in time you would merely fulfil the events that happen in that instance of time, always as the way they were intended.

So if you went back in an attempt to kill the parents of the bully who harassed you in school you would find out that your attempts failed, and that they didn't change your "present" at all. In fact, they would have helped created your present. A good example of this theory in effect is the sci-fi series "Andromeda", which follows the B theory of time in its time-travel episodes. A more well known example is the movie 12 Monkeys.

Star Trek on the other hand follows the multiple futures theory, whereby if you go back in time and change something you actually from that point on move down a different branch of time into an alternate future. The Butterfly Effect is another movie example of this.

The problem with the B theory of time is that it requires a deterministic universe, which is an unpleasant who isn't a materialist (ie. you believe you're made up of more than just matter). Of course the alternate timeline theory also has its own problems in that regard, wherein if you can exist in multiple timelines then which one is really you and where is your soul? If you're a materialist then no worries :p.

My own theory on the matter is that time is nothing more than a human construct. Matter changes, and one change takes place before another, and we measure the order in which these changes occur and call that 'time'.