弦理论，又称弦论，是发展中理论物理学的一支，结合量子力学和广义相对论为万有理论。弦理论用一段段“能量弦线”作最基本单位以说明宇宙里所有微观粒子如电子、质子及夸克都由这一维的“能量线”所组成。

较早时期所创建的粒子学说则是认为所有物质是由零维的点粒子所组成，也是目前广为接受的物理模型，也很成功的解释和预测相当多的物理现象和问题，但是此理论所根据的粒子模型却遇到一些无法解释的问题。比较起来，弦理论的基础是波动模型，因此能够避开前一种理论所遇到的问题。更深的弦理论学说不只是描述弦状物体，还包含了点状、薄膜状物体，更高维度的空间，甚至平行宇宙。值得注意的是，弦理论目前尚未能做出可以实验验证的准确预测，关于这一点，以下内文会说明。

In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. It describes how these strings propagate through space and interact with each other. On distance scales larger than the string scale, a string looks just like an ordinary particle, with its mass, charge, and other properties determined by the vibrational state of the string. In string theory, one of the many vibrational states of the string corresponds to the graviton, a quantum mechanical particle that carries gravitational force. Thus string theory is a theory of quantum gravity.