In 1905 Einstein showed that space and time are two parts of a unity: spacetime. In our ordinary life, however, we treat space and time differently, measuring one in meters and the other in seconds. They look so distinct in our experience that it seems obvious to measure them in different ways.
If, however, we accept the Einstein’s conclusion about spacetime, we must deal with space and time on an equal footing. So let’s do it!
How much space is 1 hour of space? What about 2 meters of time? Even if hard to grasp in the beginning, these questions are completely reasonable. There is, in fact, a conversion factor between space and time that physicists use to call c and it’s approximately equal to 299,792,458 meters per second.
Wait a minute, is that the speed of light?
Yes. Actually, there’s no particular reason to call it the speed of light, it may be called the speed of graviton as well. As a matter of fact, it’s the speed at which travels every massless particle.
Keeping in mind this new idea we can see that 1 hour of space is the space that light travels in 1 hour (the space traveled by light in one year is probably more familiar to us and we call it one light-year). Similarly 2 meters of time is the time that takes light to travel 2 meters.
This however doesn’t resolve our whole problem, since we’re still left with human-invented units like seconds and meters upon which the universe surely can’t be founded on. To create a description that works independently of the units we must, finally, use the same units for space and time. In this way we have that light travels at 1 meter per meter (that is, each meter of time light travels 1 meter of space), or 1 second per second, or 1 inch per inch etc. In this description the units cancels out and the factor c, the speed of light, has simply value 1. It becomes evident, then, as just a unitless factor of conversion between space and time.
- Spacetime Physics by J.A. Wheeler & E.F. Taylor