How to certify that the proper legth of light is zero

AI Thread Summary
The discussion centers on understanding why the proper length of light is zero in the context of general relativity. It is established that proper length is defined for objects in their rest frame, and since light does not have a rest frame, it cannot possess a proper length. The concept of light traveling at the speed of light (c) from the moment of its creation is emphasized. Additionally, the Lorentz transformation is mentioned as a potential tool for analyzing the second question regarding the invariance of light's proper length across inertial frames. Overall, the consensus is that light's nature prevents it from having a proper length.
DrHAO
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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
 
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DrHAO said:
... 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.
 
phinds said:
Light doesn't HAVE a rest frame

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

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