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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."
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.
Power to the Peaceful
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.
It's still hard to grok what this "prevention" means to the time traveller. If you go back, are you physically prevented from firing the gun or will the gun misfire? Or if you make a change, does the timeline establish a new universe with the old one running along merrily as a parallel universe.
When we use our senses, we only see things in the typical 4D realm, so is it possible that all those other postulated dimensions (to 11) give the degrees of freedom to allow bifurcations in the timeline? Geez this is confusing.
What this sounds like to me: If you have directly or indirectly observed something occur, then there is no uncertaincy of it occuring. If you haven't directly seen something happen or been informed that it happened, then it may not have happened. Why do we need scientists to tell us this?
Nope. I can prove it to you.
Lets say that at t = 0 your father is alive. And you go back to t = -10 to kill him. Let's say, further, that you kill him. So at t = -10 your father is dead. Then at t = 0 your dad is dead. This is a contradiction by hypothesis. The logic here is valid, so some premise must fail.
So it is logically impossible to kill your own father, given a relatively naive understanding of causation and fatherhood. A more nuanced understanding of causation and space-time might include things like "branching universes" and the like. Which is perfectly fine. But then there's the philoshical issue whether the person killed is actually your father or "merely" your "parallel universe father."
After all, I am strangely colored.
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.
Just because you have an influence on the past doesn't mean your influence didn't shape time into the way you remembered it.
For this to work, there would have to be no beginning and no end. In other words, no free will.
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Then you should read "Up the Line" by Robert Silverberg. That's exactly the subject matter -- guy goes back in time and does a maternal ancestor. Repeatedly and in a manner you'll probably sympathize with. Quite entertaining and very, very well done as you would expect of Silverberg.
I've fallen off your lawn, and I can't get up.
Doesn't work. At some point you would have had to decide to go to the past or not. What you are saying is that the past already contains whatever decision you made. If so that would mean you had no real choice. There has to be a beginning in order to have free will. If the past already contains all of our decisions of the present then we couldn't have really decided anything.
You are also thinking of the future as something undetermined but the past as determined and unchangable. Our present is the past of the future. For the past to be unchangable our present would also have to be unchangable. Again, we are back to no free will.
The whole point of time travel presenting us a paradox is because we think we have free will. If there is no possibility of a paradox then we have no free will.
The NSA: The only part of the US government that actually listens.
> 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.
IIRC it has been proven that no time machine could take you back before the time when the machine was created, so unless someone has already created on and kept it secret we shouldn't be seeing tamper effects or visitors from the future anyway.
Sheesh, evil *and* a jerk. -- Jade
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.
:p.
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
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'.
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.
In nonrelativistic QM you mean. In the Dirac and Klein-Gordon equations, time is treated identically with the other dimensions, ie, the Dirac and Klein-Gordon equations are Lorentz invariant. So, yes, time-reversal is something that must be dealt with in relativistic QM. Unfortunately, these equations only describe a few very restricted situations, so they are not as generally applicable as the Schroedinger equation. Also, they only include special relativity, not general relativity, which seems to be where the time-travel is coming from in the as you said frustratingly vague article.
More bad science in 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.
This is a common misconception. Prior to an observation, any system has a perfectly well defined state: its wave-function. This state, however, may or may not determine various properties of the system. In fact, for a given wave-function, or state, most properties (ie position, velocity, kinetic energy, etc) are restricted to a certain set of eigenvalues, and the wave-function merely determines the probability that a measurement of that property will yield any particular eigenvalue. Immediately after the measurement, the system will be in a state such that that particular property is exactly determined (ie the wave-function will have changed so that the probability of measuring that value for that property is 1, and the probability of measuring any other eigenvalue for that property is 0). This is called the collapse of the wave-function. However, other properties, some of which may have been uniquely determined by the previous wave-function, some of which were not, are now not uniquely determined.
In other words, what the article said was precisely wrong. Any system always has exactly one state which it is in, and after a measurement (or observation), whatever was measured is no longer uncertain, but most other properties are still uncertain.
IANA Physicist, yet. I have just finished my junior level undergrad physics courses, and am currently working for the summer at the Fermi National Accelerator Laboratory. Just to establish credentials.
SIGSEGV caught, terminating
wait... not that kind of sig.
Ahhh, someone who's willing to ask. :) Thanks. :)
While trying to avoid sounding like a complete lunitic, I'll try to explain the best I can. I'm no expert in paranormal events, so some of my phrasing may be a bit off. I was corrected on my experience of "Deja Vu" a few months ago, being told it's really "Precognition".
I was told when I was a kid that if I live through an event a second time, where I'm sure it couldn't have happened before, it was a "Deja Vu". This was clarified by someone more into paranormal phenomena to be a precognition. A deja vu is where you've lived it once, and you're living it again, even though you probably weren't able to have lived it once before. Use the example from the Matrix, where he sees a black cat run by, and then turns to see the same black can run by the same way again.
My precognitions usually happen years before the real event. They come in dreams. Usually they're very clear events, as viewed from my own eyes. Hollywood never portrays them like that, usually to show the stars involved in the scenes.
The most notable one was a conversation I had with 4 complete strangers. I went to a city I hadn't been to before, with a new friend. We met 4 of his friends there, and through the evening, we were having a conversation. We ended up in a library, and for 10 seconds through the conversation, I knew exactly what everyone was to say. At the point where I was suppose to say something, I didn't say a word. By not saying my part, the next person to speak didn't say anything, because I had changed the chain of events. They continued talking, it was just that it changed subtly.
Another changed event was visiting a strange house as a child. We went to a house, and I asked to play downstairs. I named very specific details of the downstairs of the house, because I **KNEW** I had been there before. They corrected me in that I had never been there before, because they had just moved in, but my details of the basement were absolutely correct, including an item of furnature which was left there by the previous owner. I may have changed this event by mentioning it too early, or it may have been changed by someone else changing plans.
My precognitions come more frequently when "something" is going to happen. The precognitions never have anything to do with the event that is going to happen, they're just like warnings that it will happen. The event isn't necessarly important to me, about half the time they are. I may get precognitions several times a day when the event is coming close. After the event, they can completely go away for a while. Sometimes it's weeks, sometimes it's years.
I can usually remember when the dreams are from. Usually they don't make sense at all, because of the time that I have the vision. I pass them off as a weird dream, until it really happens.
For example, I was dating this really nice girl. I dated her for years. While I was dating her, I had this dream. I was with this other girl in my car. This was a nice car, which at the time I didn't have anything like. I was at a particular intersection in a city I had never been in. I was messing with the air conditioning controls, because this girlfriend had changed something while I was driving. She was also talking on a cell phone to my ex-girlfriend (the girl I was dating when I had the dream), and their conversation was exactly from the dream.
When I had the precognition, I didn't see the girlfriend in the dream, because I was looking out at the traffic, which was a very specific part of the precognition. I didn't know what she'd look like, and I didn't realize what it was until it all happened. That dream was about 4 years previous to the event.
I spoke with someone who does remote viewing professionally. He's described some of his work. One that he told me about was an event that he was asked to view where something important to the investigat
Serious? Seriousness is well above my pay grade.
Why do we need scientists to tell us this?
/. saying "I could have told you that, who's paying these guys?" And all it really is, is that there was actual good scientific research going on, and the press got ahold of the simplest sound bite they could and presented THAT. There's usually a lot more than meets the eye.
Because this is a horribly simplified and popularized version of actual scientific work. It happens all the time: you hear "Study says: People would be happier if they chose jobs they liked." Everyone posts to
IIRC it has been proven that no time machine could take you back before the time when the machine was created, so unless someone has already created on and kept it secret we shouldn't be seeing tamper effects or visitors from the future anyway.
1. Never use the word "proven" around scientists. They'll kick your butt for it.
2. The theory of which you refer to is only applicable to using stable wormholes for time travel.
3. Stable wormholes are a thought experiment and have not been shown to exist. (In fact, it now seems unlikely. Which may add credence to this theory. i.e. Time travel is too unpredictable to provide a mechanism through which history may be damaged.)
4. Never use the word "proven" around scientists. They'll kick your butt for it.
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It all falls out of the many-worlds interpretation of quantum mechanics; the closed time-like loops would not "really" be closed and paradoxes could not happen, but you could meet many copies of yourself. Or find out what might have happened had you made a different decision.
The reason I've always understood is that the only theoretical model for (backward) time travel thus far involves temporally dialating (which is in itself, forward time manipulation) one end of a wormhole by sending it around a big circle at near-c. You then step back through the wormhole, back in time across the temporally dialated wormhole, and come out... a few feet away, where you then walk over to the same end you walked through last time and step back across time again. But at some point the point in time you step back across will be when the other end of the wormhole is still on it's journey through space and thus you CAN'T just walk over to the stationary end and do it again. Thus, you can never travel back before the time machine was built, because there needs to be the temporally dialated wormholes (the time machine) to come out of. It's not temporal teleportation.
Of course, even that model may be incorrect, and timetravel may be utterly impossible (unless by some other strange means).
Personally, after that thread I just linked, I'm leaning in favor of impossible. If it were possible though, I definitely go with the many-worlds interpretation. (Hell, I already go with the many-worlds intrepretation just of quantum physics. Wave collapse my ass).
-Forrest Cameranesi, Geek of all Trades
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