253. Video of the day: first time machine
ASTRO — By Dmitry Podolsky on February 12, 2009 at 4:05 pm
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Dmitry Podolsky has got his PhD from Landau Institute for Theoretical Physics. He currently works as postdoc at Case Western Reserve University. He is also one of the editors of NEQNET.
Discovery Channel program (for children really – but featuring Ronald Mallett from the U. of Connecticut) related to the problem N1 in my list. NEQNET would like to state officially that time travel is impossible 
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Hi, I am a time traveler and decided to return back to 2009 because an object identical to myself has written an incorrect statement about the impossibility of time travel.
So I need to fix it. However, I must avoid the other Dmitry because we have to obey Fermi-Dirac statistics at all times.
Dear all,
Interestingly, the other Dmitry seems to be Czech, I wonder though why I would go to live to Prague in the future
Probably, future president Motl has offered me the place of the Minister of Science and Education in his government.
Cheers,
Dmitry.
Your attraction to high government chairs is hard to hide, Dmitry! Maybe you should work on it – you would be very good, I guess.
Concerning the topic, I agree with you, at least in the macroscopic realm. But would you kindly give more technical details about it?
There exist all kinds of solutions to GR with closed time-like curves, and some of them seem to have a good past without CTCs. So doesn’t it mean it should be possible to create such loops?
I have no idea what would happen with slice-wise evolution to guarantee consistency of the spacetime, so I tend to think it’s extremely difficult. But can you prove that all those solutions are unphysical? And what is the mechanism or principle that locally prevents the spacetime from evolving in a way that creates CTCs?
There are all kinds of open questions for me. Incidentally, I just went 1 year into the past to discuss time travel in February 13, 2008:
http://motls.blogspot.com/2008.....myths.html
I presented the discussion to be a reply to a press release by two rather well-known Russian physicists, which I also inserted into 2008, of course.
Thanks for the link, it is a fascinating reading!
No, thanks, I am too naive for politics
I have only one naive argument involving QFT: CTCs with small proper lengths (but not as small as Planckian) would mean effectively tachyons – since you can effectively send information between two points with a speed larger than a speed of light. Tachyons would in turn mean strong instability of vacuum (like string tachyon). Impossibility of microscopic CTCs makes macroscopic CTCs (wormholes for example) even less likely.
Susskind had a paper about impossibility of macroscopic wormholes two years ago.
Cheers,
Dmitry.
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