执行物理和其他固定帧率更新（如 MonoBehaviour 的 FixedUpdate）的时间间隔（以秒为单位）。
Unity does not adjust fixedDeltaTime based on Time.timeScale. The fixedDeltaTime interval is always relative to the in-game time which Time.timeScale affects.
See Time and Frame Rate Management in the User Manual for more information about how this property relates to the other Time properties.