<s>[INST] Is it true to say that light cannot travel at the speed of light since a perfect vacuum cannot exist? [/INST] Yes it is true. Achieving a perfect vacuum is not possible according to current theoretical physics and this is needed to achieve 299,792,458 m/s which is the speed of light. Therefore, light cannot reach the speed of light in the known universe.</s>\ <s>[INST] How is the currently assigned value for the speed of light actually calculated or measured if it cannot be achieved within the known universe? [/INST] 