<!DOCTYPE html><HTML lang="en"> <head><meta charset="utf-8"> <title>Horwich (Paul) - On Some Alleged Paradoxes of Time Travel (Theo Todman's Book Collection - Paper Abstracts) </title> <link href="../../TheosStyle.css" rel="stylesheet" type="text/css"><link rel="shortcut icon" href="../../TT_ICO.png" /></head> <BODY> <CENTER> <div id="header"><HR><h1>Theo Todman's Web Page - Paper Abstracts</h1><HR></div><A name="Top"></A> <TABLE class = "Bridge" WIDTH=950> <tr><th><A HREF = "../../PaperSummaries/PaperSummary_17/PaperSummary_17190.htm">On Some Alleged Paradoxes of Time Travel</A></th></tr> <tr><th><A HREF = "../../Authors/H/Author_Horwich (Paul).htm">Horwich (Paul)</a></th></tr> <tr><th>Source: Journal of Philosophy, Vol. 72, No. 14, Time, Cause, and Evidence (Aug. 14, 1975), pp. 432-444</th></tr> <tr><th>Paper - Abstract</th></tr> </TABLE> </CENTER> <P><CENTER><TABLE class = "Bridge" WIDTH=800><tr><td><A HREF = "../../PaperSummaries/PaperSummary_17/PaperSummary_17190.htm">Paper Summary</A></td><td><A HREF = "../../PaperSummaries/PaperSummary_17/PaperCitings_17190.htm">Books / Papers Citing this Paper</A></td><td><A HREF = "../../PaperSummaries/PaperSummary_17/PapersToNotes_17190.htm">Notes Citing this Paper</A></td><td><A HREF="#ColourConventions">Text Colour-Conventions</a></td></tr></TABLE></CENTER></P> <hr><P><FONT COLOR = "0000FF"><U>Introduction</U><FONT COLOR = "800080"><ol type="1"><li>It has been claimed by Kurt Gdel that <a name="1"></a><A HREF="../../Notes/Notes_11/Notes_1133.htm">time travel</A><SUP>1</SUP> is physically possible. Not just that travel into the future is possible  this is a well-known consequence of the Special Theory of Relativity, and of fairly little philosophical interest. But also, more controversially, that travel backwards in time is possible. Gdel s ground for this provocative view is his discovery of certain solutions of the field equations of General Relativity that permit the existence of closed causal chains. A journey back in time would be nothing more than the "backwards" part of such a chain. Thus Gdel is led to the following startling result: "by making a round trip on a rocket ship it is possible in these worlds [i.e., worlds in which his field equation solutions describe the structure of space-time] to travel into any region of the past, present, and future and back again, exactly as it is possible in other worlds to travel to distant parts of space" (560). </li><li>Many people would like to deny this claim. Some would quarrel with the physics involved. They might argue that Gdel s derivation of his solutions was invalid or that those solutions are physically incompatible with various known facts about the universe. Others take the position that <a name="2"></a><A HREF="../../Notes/Notes_11/Notes_1133.htm">time travel</A><SUP>2</SUP>, especially into the past, is conceptually absurd, and a fortiori physically impossible. They might rule out Gdel s solutions in the way that we often reject unacceptable mathematical solutions to physical problems. </li><li>My purpose in this paper is to defend Gdel s claim against the objection that <a name="3"></a><A HREF="../../Notes/Notes_11/Notes_1133.htm">time travel</A><SUP>3</SUP> as he envisions it is impossible since it would engender absurd consequences. I will consider five alleged paradoxes that have been held to refute the possibility of <a name="4"></a><A HREF="../../Notes/Notes_11/Notes_1133.htm">time travel</A><SUP>4</SUP> into the past. All but one of them can, I think, be dealt with fairly quickly. So I shall devote most of the paper to a consideration of the <U><A HREF="#On-Page_Link_P17190_5">fifth</A></U><SUB>5</SUB><a name="On-Page_Return_P17190_5"></A>, which is due to John Earman. </li></ol></FONT><BR><HR><BR><U><B>In-Page Footnotes</U></B><a name="On-Page_Link_P17190_5"></A><BR><BR><U><A HREF="#On-Page_Return_P17190_5"><B>Footnote 5</B></A></U>: <FONT COLOR = "800080">[C]onsider a rocket ship which at some space-time point x can fire a probe which will travel into the past lobe of the null cone at x. Suppose that the rocket is programmed to fire the probe unless a safety switch is on and that the safety switch is turned on if and only if the 'return' of the probe is detected by a sensing device with which the rocket is equipped. Is the probe fired? We find that the answer is that it is fired if and only if it is not fired, which is a contradiction if standard logic holds. </FONT><BR><BR><FONT COLOR = "0000FF"><HR></P><a name="ColourConventions"></a><p><b>Text Colour Conventions (see <A HREF="../../Notes/Notes_10/Notes_1025.htm">disclaimer</a>)</b></p><OL TYPE="1"><LI><FONT COLOR = "0000FF">Blue</FONT>: Text by me; &copy; Theo Todman, 2018</li><LI><FONT COLOR = "800080">Mauve</FONT>: Text by correspondent(s) or other author(s); &copy; the author(s)</li></OL> <BR><HR><BR><CENTER> <TABLE class = "Bridge" WIDTH=950> <TR><TD WIDTH="30%">&copy; Theo Todman, June 2007 - August 2018.</TD> <TD WIDTH="40%">Please address any comments on this page to <A HREF="mailto:theo@theotodman.com">theo@theotodman.com</A>.</TD> <TD WIDTH="30%">File output: <time datetime="2018-08-02T08:40" pubdate>02/08/2018 08:40:33</time> <br><A HREF="../../Notes/Notes_10/Notes_1010.htm">Website Maintenance Dashboard</A></TD></TR> <TD WIDTH="30%"><A HREF="#Top">Return to Top of this Page</A></TD> <TD WIDTH="40%"><A HREF="../../Notes/Notes_11/Notes_1140.htm">Return to Theo Todman's Philosophy Page</A></TD> <TD WIDTH="30%"><A HREF="../../index.htm">Return to Theo Todman's Home Page</A></TD> </TR></TABLE></CENTER><HR> </BODY> </HTML>