Avoiding The Grandfather Paradox

"Among the many intriguing concepts in Einstein’s relativity theories is the idea of closed timelike curves (CTCs), which are paths in spacetime that return to their starting points. As such, CTCs offer the possibility of traveling back in time. But, as many science fiction films have addressed, time travel is full of potential paradoxes. Perhaps the most notable of these is the grandfather paradox, in which a time traveler goes back in time and kills her grandfather, preventing her own birth.

In a new study, a team of researchers has proposed a new theory of CTCs that can resolve the grandfather paradox, and they also perform an experiment showing how such a scheme works. The researchers, led by Seth Lloyd from MIT, along with scientists from the Scuola Normale Superiore in Pisa, Italy; the University of Pavia in Pavia, Italy; the Tokyo Institute of Technology; and the University of Toronto, have published their study in a recent issue of Physical Review Letters. The concepts in the study are similar to an earlier study by some of the same authors that was posted at arXiv.org last year.

“Einstein's theory of general relativity supports closed timelike curves,” Lloyd [explained]. “For decades researchers have argued over how to treat such objects quantum mechanically. We believe that our theory is the correct theory of such objects. Moreover, our theory shows how time travel might be accomplished even in the absence of general relativistic closed timelike curves...”

(for complete article including illustrative graph of "quantum gun" experiment, see PhysOrg.com)

More information: Seth Lloyd, et al. “Closed Timelike Curves via Postselection: Theory and Experimental Test of Consistency.” Physical Review Letters 106, 040403 (2011). DOI:10.1103/PhysRevLett.106.040403 (abstract only, subscription required for complete access)

Addendum- PDF of the original incarnation of the Lloyd Team paper prior to 1/11 revision: "Closed Timelike Curves Via Post-Selection: Theory and Experimental Demonstration"  Download Lloyd Team Paper 5:12:10.

Further thoughts re: resolution of the grandfather paradox based on Richard Feynman's multiple histories theory: http://www.physicsforums.com/archive/index.php/t-162.html

 

-Maureen Lang

Comments

[Jake]

Maureen,

Thanks for the article. But as a physicist, I do smile at many of these theories and bench tests. I do not discount them, just smile. I can't wait to see the peer review on this report. Heck we are still fighting over perpetual motion machines and the second law of thermodynamics...

But as most physicists will tell you. If its impossible its more than likely probable.

[FB Ali]

Several years ago I read a book that dealt, in passing, with the paradoxes of time travel, including something similar to this one. It gave a rather simple explanation that avoids such paradoxes. Not being a physicist, I don’t know how valid it was (or still is), but it seemed to make sense.

The solution was based on the concept of spacetime consisting of moments, each forever frozen in place (though these 'nows' would be viewed differently by observers in different states of motion). Thus, moments just ‘are’, static and unchanging, an eternal and immutable feature of spacetime. So, one cannot go back in time and change an event that has already happened.

The author does recognize certain problems posed by the quantum mechanics concept of spacetime becoming unavoidably wavy and bumpy on extremely short scales. But, he believes the paradox can be resolved there by the Many Worlds interpretation (i.e, travelling back in time you could never visit the world in which your grandfather and your parents caused you to be born).

[walrus]

Readers might like to visit this website and learn about Mayers Minkoski DeSitter Riemann model of the universe.

Time is not an arrow. The direction of time is orthoginal to Three Dimensional space - ie orthogonal to curved space time. That is the implication of Minkowskis Four vector that Einstein called "superflous erudition".

In the MDR model if it is extended, I suspect that when photons interact but have different time vectors, something like what is demonstrated happens.

http://jaypritzker.org/

[Maureen Lang]

Anyone care to weigh in on whether the "quantum gun" experiment delineated in the article also solves Deutsch's unproven theorem paradox?

"We want to perform the so-called `unproved theorem paradox' experiment, in which the time traveler sees an elegant proof of a theorem in a book,” [Dr. Seth] Lloyd said. “She goes back in time and shows the proof to a mathematician, who includes the proof in the book that he is writing. Of course, the book is the same book from which the time traveler took the proof in the first place. Where did the proof come from? Our theory has a specific prediction/retrodiction for this paradox*, which we wish to test experimentally.”
http://www.physorg.com/news/2011-03-grandfather-paradox.html

*Been trying to track down particulars of the Lloyd team's prediction/retrodiction re: Deutsch paradox, but so far no luck. Any comments/links on that would be appreciated.

[LeaNder]

Maureen, in context I can't help but read: "unproven theorem paradox" as slightly ironic and as a description of their own design.

They do not want to solve Deutsch's paradox but use a different strategy to avoid it. Deutsch used the parallel universe, to avoid the grandfather paradox, these guys use a different trick. They post-select self-consistent object quantums, only these that do not produce the paradox, or as your quote shows go back without changing the past.

They are ultimately aiming at quantum computation:

http://www.physorg.com/news146398685.html

“Admittedly, how nature manages to actually do the work of finding the fixed point (and thereby making the universe causally consistent) remains a great mystery,” he said. “But the point is, this is the sort of thing nature would presumably need to do if CTCs [closed timelike curves] existed.”

[Maureen Lang]

Jake, FB Ali, walrus, LeaNder, & all readers,

I've been forwarded access to the original 5/10 version of the paper by the Lloyd team, "Closed Timelike Curves Via Post-Selection: Theory and Experimental Demonstration."- have added a link to the pdf in the main post above. LeaNder, you in particular might like to have a look @ page 2's description of the a & b experiment circuits- a) grandfather paradox b) unproved theorem paradox.

[LeaNder]

Thanks Maureen, I didn't realize the paper was available via Google scholars.

There are of cause still too many unknowns, apart from the missing grasp of the higher mathematical formulae. Entropy in information science? Entangled state? Projection in this context? Pre-selection and last but surely not least CPT symmetry or the CPT theorem (projection?) and the Arrow of time (entropy). Or if I may ask more dumbly: were exactly and how does "past" enter their system? Or Deutsch's for that matter.

Now, with full acknowledgement of the above informational lacunae, I'll attempt an preliminary attempt at interpretation. If I had the time I would focus on all quotes containing "entangle" and then move on to the many questions above.

They suggested a solution to Deutsch unproven theorem paradox by eliminating Deutsch's maximum entropy postulate?

For the moment I discovered a more workaday solution to my information technology problem of posting in spite of the fact that Firefox tells me: We are sorry we cannot accept this data.

But thanks for alerting us to the original paper.