- #36
Garth
Science Advisor
Gold Member
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The question is "How do we measure mass, length and time across cosmological distances?"MeJennifer said:It may be a convention fo some but that does not make it any way true.
The issue is simple and directly related to the principle of equivalence in general relativity.
The assumption that a photon changes due to the curvature of space-time is a direct violation of the principle of equivalence.
So "stretching photons" is, as Pauli could have said: "not even wrong".
To be able to make cosmological measurements we need something that stays constant when transported across the universe as a standard, therefore a conservation principle is absolutely essential, not just a vague "assumption", but the question is which principle?
As we cannot prove that anything remains constant over cosmological space-time all we can do is define a conservation principle and test to see whether it is concordant and internally consistent.
At the heart of GR is the conservation of energy-momentum, which leads to the atom, atomic 'regular' clocks and 'rigid' steel rulers, being the standard by which to measure the universe. GR has been tested, and is still being tested and so far has not be falsified. Perhaps Saturday will tell!
Garth
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