How to certify that the proper legth of light is zero

[DrHAO]

Homework Statement

I'm a new one on general relativity
there are two prolems:
first how to certify the proper length of light is zero
second ,how to certify proper length of light ,do not change in all inertial frame

Homework Equations

for second question, probally Lorentz transform,

The Attempt at a Solution

for the first
ds²=-c²dt²+dl²
dl/dt = 0
ds=0
I have no idea on Lorentz transform of delta s

 

[phinds]

... how to certify the proper length of light is zero

Proper length is the length of an object in its rest frame. Light doesn't HAVE a rest frame so doesn't have a proper length, unless you're using some different definition.

 

[TJGilb]

Light doesn't HAVE a rest frame

Yeah, from the moment a photon is created, it is instantly and always moving at c.