{"id":1221,"date":"2006-04-05T15:24:40","date_gmt":"2006-04-05T22:24:40","guid":{"rendered":"http:\/\/dabacon.org\/pontiff\/?p=1221"},"modified":"2006-04-05T15:24:40","modified_gmt":"2006-04-05T22:24:40","slug":"1221","status":"publish","type":"post","link":"https:\/\/dabacon.org\/pontiff\/2006\/04\/05\/1221\/","title":{"rendered":"Forward To the Past"},"content":{"rendered":"

Daniel sends me this<\/a> Physorg article titled: “Professor Predicts Human Time Travel This Century.” (Rule of thumb: never believe any prediction whose time span streches beyond the retirement age of the person making the prediction!) The physicist in this title is University of Connecticut’s Ronald Mallett and the prediction is based, in some ways, on his paper <\/a>, “The gravitational field of a circulating light beam,” Foundations of Physics 33, 1307 (2003). In this paper the Mallett solves Einstein’s equation for an infinitely long cylinder of rotating light and finds that this solution contains closed timelike curves. A similar construction of an infinite cylinder of rotating dust also produces closed timelike curves (this solution actually predates Godel’s universe historically, although the closed time like curves were not pointed out until after Godel constructed his crazy universe.) I’m always skeptical about these solutions as it is generally the case that there is something unphysical about these solutions. In the above papers case this appears to be the fact the solution is not assymptotically flat. It is also not clear that the solution is robust to perturbations or will be valid for a cyllinder of finite length (although apparently for the dust case the finite length solution has closed time like curves.) But what is fascinating to me is how simple it sometimes appears to be to make general relativistic systems which have closed time like curves. Okay, simple is perhaps not the best word, and of course the real question is whether it is possible to make solutions which don’t have physically bad properties. For some excited crazy optimism along these lines, check out gr-qc\/0211051<\/a>.
\nOf course in any popular science article about time travel, the question of grandfather violence comes up. Interestingly Mallett deals with the issues of causality for systems with closed time like curves a la David Deutsch:<\/p>\n

\n\u201cThe Grandfather Paradox [where you go back in time and kill your grandfather] is not an issue,\u201d said Mallett. \u201cIn a sense, time travel means that you\u2019re traveling both in time and into other universes. If you go back into the past, you\u2019ll go into another universe. As soon as you arrive at the past, you\u2019re making a choice and there\u2019ll be a split. Our universe will not be affected by what you do in your visit to the past.\u201d\n<\/p><\/blockquote>\n

Which makes me think that this process should not be called “time travel” but should be called rewind, as in “pushing rewind on the VCR and start recording a new program over the old one.” When I was growing up I used to wonder what made us think that if we traveled back in time whether there would actually be anything back there. What a bummer to build a time machine, take a trip into the past, and find that you are the only thing in existence in this past!
\nFinally, the article ends in a very sad manner:<\/p>\n

\nIn light of this causal \u201csafety,\u201d it\u2019s kind of ironic that what prompted Mallett as a child to investigate time travel was a desire to change the past in hopes of a different future. When he was 10 years old, his father died of a heart attack at age 33. After reading The Time Machine by H.G. Wells, Mallett was determined to find a way to go back and warn his father about the dangers of smoking.\n<\/p><\/blockquote>\n

Which should convince you that physicists too are only human<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"

Daniel sends me this Physorg article titled: “Professor Predicts Human Time Travel This Century.” (Rule of thumb: never believe any prediction whose time span streches beyond the retirement age of the person making the prediction!) The physicist in this title is University of Connecticut’s Ronald Mallett and the prediction is based, in some ways, on … <\/p>\n

Continue reading “Forward To the Past”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[53],"tags":[],"class_list":["post-1221","post","type-post","status-publish","format-standard","hentry","category-physics"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/dabacon.org\/pontiff\/wp-json\/wp\/v2\/posts\/1221","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/dabacon.org\/pontiff\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/dabacon.org\/pontiff\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/dabacon.org\/pontiff\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/dabacon.org\/pontiff\/wp-json\/wp\/v2\/comments?post=1221"}],"version-history":[{"count":0,"href":"https:\/\/dabacon.org\/pontiff\/wp-json\/wp\/v2\/posts\/1221\/revisions"}],"wp:attachment":[{"href":"https:\/\/dabacon.org\/pontiff\/wp-json\/wp\/v2\/media?parent=1221"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/dabacon.org\/pontiff\/wp-json\/wp\/v2\/categories?post=1221"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/dabacon.org\/pontiff\/wp-json\/wp\/v2\/tags?post=1221"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}