Since the 2nd law of thermodynamics says the universe's entropy is constantly increasing, then its initial entropy must have been smaller than it is today.

However, no known physical law can reduce net entropy.

Does this mean the initial low entropy state of the universe must be the result of something that is outside the physical laws? Or does it mean there is a physical law that can reduce entropy?

The linked questions sound similar, since they also deal with the low entropy state at the beginning of the universe. But the questions are not asking whether this implies it was caused by something outside our physical laws, which seems the straightforward interpretation.


Saying that entropy increases gives you an arrow of time. We experience time because the entropy is increasing. All the laws in physics are time-reversible except entropy. The answer to how the big-bang occurred is still an unanswered one. Talking about before big-bang is absurd because the concept of time itself becomes obsolete. All we know currently is that the initial entropy of the universe was very low.

We still have to achieve many more milestones before we can answer why.

  • $\begingroup$ Hi. I'm not sure the phrasing in your post is correct: we may see a correlation between the increasing of entropy and the existence of time as change but the relation between the two is not of a cause and effect style. Also, what we may call a law of nature as a reversible process in time, there are systems that do not have time symmetry, both classical and quantum, so I am not sure that we may see the entropy as an increasing parametre as a dictating law for all systems- it depends. Anyway, maybe I did'n understood your post. Thanks. $\endgroup$ Sep 19 '17 at 8:55
  • $\begingroup$ Your answer indicates the big bang in unanswerable within the realm of physical law. $\endgroup$
    – yters
    Sep 19 '17 at 17:20
  • $\begingroup$ @ConstantineBlack you are right in saying that the relation between existence of time and entropy is not cause and effect. What i meant to say is that we perceive time because of the increasing entropy. We take energy from the sun which is a form of low entropy(source) and dump high entropy energy into the surroundings(sink). The conversion between into work is what sustain life. As for violation of time symmetry, can you give me an example which does not include laws of thermodynamics directly or indirectly,i.e something like em waves, gravitation or mechanics? $\endgroup$ Sep 19 '17 at 17:53
  • $\begingroup$ @yters Yes, you are correct in assuming so. After all, we can only speak about what happens after 10^-43 seconds of big bang. Before that, and we need a theory of quantum gravity which we do not have. Even if we do obtain it in the future, people are skeptic if you can actually reach till the big bang point.Maybe we'll reach magnitudes of order lower :) . $\endgroup$ Sep 19 '17 at 17:56

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