Wednesday, February 20, 2013

Hubble slow motion

does time pass slower in distant galaxies? if you could observe a planetary system at high redshift and observe the orbits of planets (or any other dynamical system driven by gravity), would the observation be consistent with a lower gravity instead of a slower passage of time? what about atomic processes like chemical reactions or radiative processes?

bonus question: can you show that for a real-valued matrix $A$ the matrix exponential $\exp(A)$ transforms according to
\exp(R^{-1}AR) = R^{-1}\exp(A)R
with any invertible matrix $R$?


  1. bonus question: you can expand the matrix exponential in a series
    = \sum_i\frac{(R^{-1}AR)^i}{i!}
    = R^{-1}\:\:\sum_i\frac{A^i}{i!}\:\:R
    = R^{-1}\exp(A)R
    because of the associativity of the matrix product, $(R^{-1}AR)^i = R^{-1}AR\ldots R^{-1}AR = R^{-1}A\ldots AR = R^{-1}A^iR$.

  2. we see processes in distant galaxies in slow motion because the arrival times of photons gets stretched by the Hubble expansion. in classical planetary systems a rescaling of the gravitational constant is completely degenerate with choosing a different mass of the central body, and the orbit only has to obey Kepler's third law, with a possibly different constant of proportionality. so yes, if we didn't have spectroscopy and could not measure redshifts, one could explain the slowly moving planets by lower gravity.