What is the exact speed of light in a vacuum?

Answer 1

The current internationally accepted number is: 299,792,458 meters per second.

Note. While the number above is correct, it is not 'the current internationally accepted number'. The number is now correct by definition. Physicists world-wide have agreed that c is 299 792 458 m s-1.. Physical constants are usually quoted with 'error bars', which indicate the accuracy of measurement. Here there are no error bars - what you see is what you get, and what physics will get ever after. The second is now defined by an atomic clock, and the meter is defined by reference to c and the second. If there were to be a tiny error that had crept in, the meter is what would change.