Sociotard Posted March 24, 2007 Report Share Posted March 24, 2007 Did any of you read the New Scientist article about retrocausality? San Fracisco Chronicle reprinted it here. Retrocausality is the idea that Something happening now can cause something to happen in the past. It was an interesting article. Quote Link to comment Share on other sites More sharing options...
BobGreenwade Posted March 24, 2007 Report Share Posted March 24, 2007 Re: Retrocausality Experiment This certainly has interesting implications for the believability of not only time travel, but also precognition. Quote Link to comment Share on other sites More sharing options...
Sociotard Posted March 24, 2007 Author Report Share Posted March 24, 2007 Re: Retrocausality Experiment I find the possibilities of FTL communication to be much more exciting, and possibile. Quote Link to comment Share on other sites More sharing options...
TheRavenIs Posted March 25, 2007 Report Share Posted March 25, 2007 Re: Retrocausality Experiment H. Beam Piper used this idea in he's Paratime novels to rexplain precognition. A person in the now has an event happen and that same person get's the mental image of that event in the past. If you watched ST:TNG. The final show had the idea in it, but the causality was from three different points in the timeline, it created an anti-time wave. Quote Link to comment Share on other sites More sharing options...
Nyrath Posted March 26, 2007 Report Share Posted March 26, 2007 Re: Retrocausality Experiment I find the possibilities of FTL communication to be much more exciting' date=' and possibile.[/quote'] Well, actually, according to Einstein's relativity, FTL communication and cross time communication are the same thing. As is FTL travel and Time travel. If you have one, you have the other. Quote Link to comment Share on other sites More sharing options...
austenandrews Posted March 26, 2007 Report Share Posted March 26, 2007 Re: Retrocausality Experiment I have a feeling this will turn out to be another interesting-but-useless factoid about quantum entanglement. If one receives signals from the future, what happens if you do not then send the signal? Answer: probably nothing, which will invigorate SF and theoretical physics to no end, but will have no meaningful technological application. I've got a spoof story on the backburner positing that information can be sent FTL, but the net amount of information must remain STL. In other words if you receive data FTL, an equal or greater amount of your existing information is destroyed. Which doesn't bode well when they launch an interstellar probe that sends back data via quantum entanglement. Quote Link to comment Share on other sites More sharing options...
Nyrath Posted March 26, 2007 Report Share Posted March 26, 2007 Re: Retrocausality Experiment If one receives signals from the future' date=' what happens if you do not then send the signal? Answer: probably nothing, which will invigorate SF and theoretical physics to no end, but will have no meaningful technological application.[/quote'] Actually, you are stating one of the many ways that time travel can cause a paradox, which is the main reason why most physicists think time travel is impossible. Quote Link to comment Share on other sites More sharing options...
austenandrews Posted March 26, 2007 Report Share Posted March 26, 2007 Re: Retrocausality Experiment Actually' date=' you are stating one of the many ways that time travel can cause a paradox, which is the main reason why most physicists think time travel is impossible.[/quote'] Most "paradox" is only intuitively axiomatic. If the above scenario happens, will they still call it a paradox? Quote Link to comment Share on other sites More sharing options...
Cancer Posted March 26, 2007 Report Share Posted March 26, 2007 Re: Retrocausality Experiment It's worth pointing out that another interpretation of FTL implies existing at infinitely many places at the same time. That makes causality a null concept: it is no longer possible to tell which instance was "first" and caused anything else. And then there's other problems involving conservation laws. Quote Link to comment Share on other sites More sharing options...
austenandrews Posted March 26, 2007 Report Share Posted March 26, 2007 Re: Retrocausality Experiment I confess, I'm unclear how the experiment works. If you measure the near photon, doesn't entanglement force the far photon into the corresponding state? Why is there any belief that it wouldn't? Quote Link to comment Share on other sites More sharing options...
McCoy Posted March 26, 2007 Report Share Posted March 26, 2007 Re: Retrocausality Experiment Actually' date=' you are stating one of the many ways that time travel can cause a paradox, which is the main reason why most physicists think time travel is impossible.[/quote'] I agree, "many," but "most?" The ones I've talked to say the're nothing in current theory forbidding it, and the rule seems to be that everything not forbidden is compulsory. Quote Link to comment Share on other sites More sharing options...
Nyrath Posted March 26, 2007 Report Share Posted March 26, 2007 Re: Retrocausality Experiment I agree' date=' "many," but "most?" The ones I've talked to say the're nothing in current theory forbidding it, and the rule seems to be that everything not forbidden is compulsory.[/quote'] Well, ask your physicists their opinion on violations of causality. Quote Link to comment Share on other sites More sharing options...
Mister E Posted March 27, 2007 Report Share Posted March 27, 2007 Re: Retrocausality Experiment What about the business of taking electrons in the same quantum state, separating them, changing the spin of one, and observing the other change its spin to match (due to positronic retrocausality, if that is a term). This has been measured to occur at superluminal speeds, but I've gleaned that it is not a useful form of communication for some reason, and I don't know why. Does anyone here know why? Does it have something to do with the inability to determine which electron initiated the change in spin? I would have thought that was obvious since the scientists were manipulating the spin themselves... Quote Link to comment Share on other sites More sharing options...
Nyrath Posted March 27, 2007 Report Share Posted March 27, 2007 Re: Retrocausality Experiment What about the business of taking electrons in the same quantum state' date=' separating them, changing the spin of one, and observing the other change its spin to match (due to positronic retrocausality, if that is a term). This has been measured to occur at superluminal speeds, but I've gleaned that it is not a useful form of communication for some reason, and I don't know why. Does anyone here know why? Does it have something to do with the inability to determine which electron initiated the change in spin? I would have thought that was obvious since the scientists were manipulating the spin themselves...[/quote'] You are talking about Bell's Inequality. http://en.wikipedia.org/wiki/Bell%27s_inequality http://www.scifi.com/sfw/issue199/labnotes.html I can go into more detail if you wish, but the practical upshot is [1] Yes, it transmits information FTL and [2] the information is worthless as a communication device. What comes out of the far end of the device is basically a string of seemingly random numbers. If you compare the string with the original input at the near end you will see that FTL communication has happened. But it is impossible to read the information out of the pseudo-random string at the far end without the information from the near end. And sending the information from the near end to the far end by radio or something sort of defeats the purpose of FTL communication. Quote Link to comment Share on other sites More sharing options...
Mister E Posted March 27, 2007 Report Share Posted March 27, 2007 Re: Retrocausality Experiment You are talking about Bell's Inequality. http://en.wikipedia.org/wiki/Bell%27s_inequality http://www.scifi.com/sfw/issue199/labnotes.html I can go into more detail if you wish, but the practical upshot is [1] Yes, it transmits information FTL and [2] the information is worthless as a communication device. What comes out of the far end of the device is basically a string of seemingly random numbers. If you compare the string with the original input at the near end you will see that FTL communication has happened. But it is impossible to read the information out of the pseudo-random string at the far end without the information from the near end. And sending the information from the near end to the far end by radio or something sort of defeats the purpose of FTL communication. Yes, that's was I was talking about; and that explains why it's no good for communication... blast it. Thanks. Quote Link to comment Share on other sites More sharing options...
austenandrews Posted March 27, 2007 Report Share Posted March 27, 2007 Re: Retrocausality Experiment Yes, that's was I was talking about; and that explains why it's no good for communication... blast it. Thanks. And it highlights the question, how is the retrocausality experiment fundamentally different from what we already know about FTL transmissions via entanglement? Long Known: 1) Entangle photons A & B 2) Send transmission from A 3) B displays gobbledegook 4) STL info from A reveals it wasn't gobbledegook but an FTL signal Retrocausality Experiment: 1) Entangle photons A & B 2) B displays gobbledegook 3) Send transmission from A 4) Info from A reveals it wasn't gobbledegook but a signal from the future Relativity says that FTL and time travel are the same thing, so the order of 2 & 3 are irrelevant. Doesn't the experiment just demonstrate that? If we skip step 3 - sending the transmission into the past - there's no paradox because B always displays gobbledegook. This is just another facet of the long-known "spooky action," right? Quote Link to comment Share on other sites More sharing options...
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