Recalling our two observers from different ages: the time in their experiments is shifted by 100 years. If the time when the older observer did the experiment was "t", the time of the modern experiment is "t"+100 years. Both observers discover the same laws of physics. Because light from hydrogen atoms in distant galaxies may reach the earth after having traveled across space for billions of years, in effect one can do such observations covering periods of time almost all the way back to the Big Bang, and they show that the laws of physics have always been the same.