Hashirama_Senju

To find the speed of light they had to use the measurement of the 'metre' that was not defined by the speed of light, so how can they define the new concept of the 'metre' based on the speed of light if the old 'metre' was measure with the old concept of the metre?

are they

What are the main principles for both theories that everything else branches off?

please make a list of 5 if you can or more would be even better you can link me to it if you want

If there was no matter would space not exist also? and what if space itself is simply gravitational fields or is space nothingness?