量子场论与重力结合的历史主轴在于「无穷大」与「负能量幽灵」的数学困境。自1940年代起，重整化技巧依赖以有限常数取代无限微扰，但在重力领域，小尺度涟漪影响反而愈强，令重整化失效。1977年 Stelle 加入两个「曲率平方」项，形成可重整化的「二次重力」，其粒子内容从单一重力子扩增为三种，包括一个标量粒子与一个带负能的幽灵。该幽灵导致负概率与能量无限制产生，使二次重力被视为「病态理论」，其文献引用量仅每年约10至20次。

然而，2010年代后，随超引力与弦论未能实现预期突破、大型强子对撞机未发现超伴粒子，二次重力重新获得关注。2014年 Salvio 与 Strumia 指出该理论的两个额外粒子可解释「层级问题」，即重力与其他三种基本力强度尺度差距过大。此结果提示幽灵或许不是致命缺陷，而是推开能标差距的关键成分。此后，研究团队提出幽灵可在替代性量子规则框架下避免造成实验悖论，暗示负能模式可被限制于仅在方程层面存在。

量子场论方法在重力中的复兴力量正在累积。Buoninfante 与其他学者强调目前没有证据要求弃置量子场论；相反，应重新评估其可行性。二次重力不仅提供唯一的可重整化量子重力候选，也预言新的可测物理效应（尚未实验验证）。若这些负能与因果「超前」现象能在数学上自洽且不在低能实验中引发矛盾，幽灵可能象征更深层的量子重力结构，而非禁令。

Quantum field theory and gravity have long conflicted due to infinities and negative-energy “ghosts.” Since the 1940s, renormalization has tamed quantum fluctuations in other forces, but in gravity small-scale ripples grow more influential, making renormalization fail. In 1977, Stelle added two curvature-squared terms to Einstein’s equation, producing renormalizable “quadratic gravity,” whose particle content expands from the graviton to three modes, including a scalar and a negative-energy ghost. The ghost yields negative probabilities and unbounded energy creation, causing the theory to be labeled “sick” and cited only about 10–20 times per year.

Interest revived in the 2010s as supergravity and string theory failed to produce expected breakthroughs and as the LHC detected no superpartners. In 2014, Salvio and Strumia showed that quadratic gravity’s two extra particles could address the hierarchy problem—the enormous scale gap between gravity and the other forces—suggesting the ghost might not be fatal. Later work by Anselmi showed that alternative quantum rules can avoid standard ghost paradoxes, allowing negative-energy modes to remain confined to equations without generating experimental inconsistencies.

Momentum is now shifting toward reconsidering the quantum field theory approach to gravity. Buoninfante and others argue that no evidence compels abandoning QFT; rather, quadratic gravity may offer the only renormalizable quantum gravity and yields new, testable predictions (not yet experimentally probed). If negative-energy modes and occasional microscopic causal violations can remain mathematically consistent while invisible at low energies, the ghost may signal deeper quantum-gravitational structure rather than pathology.