Re: [Phys-l] Absolute four-momentum of massless particles

[John Denker <jsd@av8n.com>]

On 10/01/2010 10:07 PM, carmelo@pacific.net.sg wrote:
> We have defined time using speed of light. We have also defined  
> distance using speed of light. Our experiments are based on speed of  
> light.
> In a sense, we cannot define time without space, and we cannot define  
> space without time.

That's going too far.  We can measure space in lots of ways, 
including prosaic rulers.  We can measure time in lots of 
ways, including prosaic wristwatches based on mechanical 
mass-on-a-spring oscillators.

The atomic clocks and the wavelength-based length standards
were adopted as standards because they are conveniently
reproducible *and* are consistent with all the other ways 
of measuring time and space.  If they were ever found to be
not consistent, they would be de-adopted instantly.

To the extent that we know the structure of spacetime, we 
know it because we have measured it, not because we assumed 
it, and not because it had to be the way it is.

The existence of superstring theory indicates that any
attempt to "define" space and time is very premature, and
that many of the conventional assumptions about space and
time remain open to serious question.