For a complexity class $C$ and language $L$, a constructive separation of $L \notin C$ gives an efficient algorithm (also called a refuter) to find counterexamples (bad inputs) for every $C$-algorithm attempting to decide $L$. We study the questions: Which lower bounds can be made constructive? What are the consequences of constructive separations? We build a case that "constructiveness" serves as a dividing line between many weak lower bounds we know how to prove, and strong lower bounds against $P$, $ZPP$, and $BPP$. Put another way, constructiveness is the opposite of a complexity barrier: it is a property we want lower bounds to have. Our results fall into three broad categories. 1. Our first set of results shows that, for many well-known lower bounds against streaming algorithms, one-tape Turing machines, and query complexity, as well as lower bounds for the Minimum Circuit Size Problem, making these lower bounds constructive would imply breakthrough separations ranging from $EXP \neq BPP$ to even $P \neq NP$. 2. Our second set of results shows that for most major open problems in lower bounds against $P$, $ZPP$, and $BPP$, including $P \neq NP$, $P \neq PSPACE$, $P \neq PP$, $ZPP \neq EXP$, and $BPP \neq NEXP$, any proof of the separation would further imply a constructive separation. Our results generalize earlier results for $P \neq NP$ [Gutfreund, Shaltiel, and Ta-Shma, CCC 2005] and $BPP \neq NEXP$ [Dolev, Fandina and Gutfreund, CIAC 2013]. 3. Our third set of results shows that certain complexity separations cannot be made constructive. We observe that for all super-polynomially growing functions $t$, there are no constructive separations for detecting high $t$-time Kolmogorov complexity (a task which is known to be not in $P$) from any complexity class, unconditionally.

翻译：对于复杂度类$C$和语言$L$，$L \notin C$的构建性分离给出了一个高效算法（亦称反证器），该算法能为每个试图判定$L$的$C$-算法找出反例（不良输入）。我们研究了以下问题：哪些下界可以具有构建性？构建性分离能带来什么后果？我们论证了“构建性”可作为区分我们已知如何证明的许多弱下界与针对$P$、$ZPP$和$BPP$的强下界的分界线。换言之，构建性是复杂度障碍的对立面：它是我们希望下界具备的性质。我们的结果分为三大类：1. 第一类结果表明，对于流算法、单带图灵机和查询复杂度的众多著名下界，以及最小电路规模问题的下界，若使这些下界具有构建性，则将推导出从$EXP \neq BPP$到$P \neq NP$等突破性分离。2. 第二类结果表明，对于针对$P$、$ZPP$和$BPP$的大多数主要开放难题（包括$P \neq NP$、$P \neq PSPACE$、$P \neq PP$、$ZPP \neq EXP$和$BPP \neq NEXP$），任何分离的证明都将进一步蕴含构建性分离。我们的结果推广了关于$P \neq NP$ [Gutfreund, Shaltiel, and Ta-Shma, CCC 2005] 和$BPP \neq NEXP$ [Dolev, Fandina and Gutfreund, CIAC 2013] 的早期结论。3. 第三类结果表明，某些复杂度分离无法具有构建性。我们观察到，对于所有超多项式增长的函数$t$，从任何复杂度类中检测高$t$-时间柯尔莫哥洛夫复杂度（一项已知不属于$P$的任务）不存在构建性分离，该结论无条件成立。