One possible way to explain the arrow of time is to say that the entropy is increasing simply because the entropy in the beginning was at unsustainably low levels and is therefore bound to increase. The way that the entropy got so low in the beginning can, in turn, be explained by postulating special conditions that prevailed at that time---such as cosmic inflation, high density, extreme temperature, or others. Without going into the details of such explanations, there seems to be a general objection that could be raised to counter any explanation in that direction. The general objection is that space-time homogeneity should rule out special conditions anywhere in space-time, including the very onset of time itself.

Therefore, my question is this. How do we reconcile the principle of space-time homogeneity with the intuition that time can only have one origin?

  • maybe you have a different definition for space time homogeneity? have a look at this question and answers physics.stackexchange.com/q/83393 – anna v Oct 2 at 13:02
  • You are probably mixing spacial homogenity with space-time homogenity. – MBN Oct 2 at 13:06
  • Welcome to Physics! Minor comment to the post (v3): since your revision history is public, there's no need to annotate your post with that information. If you find yourself making another substantial edit, you might remove the last line. – rob Oct 2 at 13:54
  • The way that the entropy got so low in the beginning can, in turn, be explained by postulating special conditions that prevailed at that time---such as cosmic inflation, high density, extreme temperature, or others. No, this is wrong. You may wish to search to see if we already have a question on this topic. – Ben Crowell Oct 2 at 15:23
  • It should be noted that in accepted cosmological models homogeneity and isotropy of space hold, but homogeneity of time doesn't, because of expansion. E.g., many cosmological parameters, like $a$ (scale factor), $H$ (Hubble constant), $\Omega_{\rm m}$ (cold matter density) do depend on cosmological time. As a consequence energy is not conserved. – Elio Fabri Oct 4 at 9:55

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