Can we change the Lebesgue measure by forcing? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-24T06:40:52Z http://mathoverflow.net/feeds/question/88582 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/88582/can-we-change-the-lebesgue-measure-by-forcing Can we change the Lebesgue measure by forcing? Asaf Karagila 2012-02-16T01:01:16Z 2012-02-16T01:21:14Z <p>Suppose $M$ is a model of ZFC, and $\mu^M$ is the Lebesgue measure on $\mathbb R^M$ such that $\mu^M(\mathbb R^M)=1$. It is known that if $r$ is a Cohen real over $M$ and $N=M[r]$ then $\mu^N(\mathbb R^M)=0$.</p> <p>This is a very strong transition from having sets with a full measure being annihilated into nullity. Is it possible that for some[every?] $x\in(0,1)$ there exists $N=M[G]$ a generic extension of $M$ such that $\mu^N(\mathbb R^M)=x$?</p> <p>If the answer is negative in its full generality ($M$ is just any model of ZFC) can we add some assumptions for a positive answer? (e.g. $M\models CH$)</p> <p><em>This is really just idle curiosity which could not be satisfied via Google, references to a possible answer would be just as welcomed as a complete answer</em></p> http://mathoverflow.net/questions/88582/can-we-change-the-lebesgue-measure-by-forcing/88583#88583 Answer by Joel David Hamkins for Can we change the Lebesgue measure by forcing? Joel David Hamkins 2012-02-16T01:15:29Z 2012-02-16T01:21:14Z <p>The answer is no. If $\mathbb{R}^V$ is measurable in a forcing extension $V[G]$ having new reals, then the measure must be $0$. The point is that every new real $x$ in $V[G]$ but not in $V$ is transcendental over $\mathbb{R}^V$, since one cannot add algebraic numbers by forcing. It follows that the translates of $\mathbb{R}^V$ by the powers of $x$ are disjoint. Thus, a Vitali-style argument with wrapped translations of the unit interval of $V$ shows that if $\mathbb{R}^V$ is measurable, it must have measure zero. </p> <p>Gunter Fuchs and I had made this observation in connection with Sebastian's question <a href="http://mathoverflow.net/questions/56990/probabilities-independent-of-zfc" rel="nofollow">Probabilities independent of ZFC?</a>, which is very much related to your question. </p>