Wikipedia:Reference desk/Archives/Miscellaneous/2017 February 10

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February 10

Understanding the solution to Loschmidt's paradox

Based on what I've read, the solution to Loschmidt's paradox is that, although the laws of physics may be time-reversible, a boundary condition implies the arrow of time - specifically, the boundary condition that the Big Bang began with low entropy. I don't understand this argument. I don't know how the Big Bang beginning with low entropy implies the second law of thermodynamics. Can someone explain in more detail, perhaps providing mathematical details? 49.184.161.119 (talk) 06:47, 10 February 2017 (UTC)

What this means is that the universe was created in an unlikely state. This state has low entropy compared to later. (although it could be low entropy after the cosmic inflation.) Over time it is changing to a more probable state. Entropy increases. Going forward in time is proceeding from a less likely situation to a more likely one. This is the thermodynamic arrow. Graeme Bartlett (talk) 07:03, 10 February 2017 (UTC)

Mathematical description: Let there be two points in time labelled 1 and 2. The entropy at these two points s₁ s₂. Second law of thermodynamics says that s₂ > s₁. Microscopic physical laws suggest s₂ = s₁. The experience of time is due to this increase in entropy. So going back in time the entropy get smaller and smaller and at the beginning it is the smallest possible. Read the Entropy (arrow of time) page, which also states that "Why did the universe have such low entropy in the past, resulting in the distinction between past and future and the second law of thermodynamics?" is an unsolved problem in physics. Graeme Bartlett (talk) 07:24, 10 February 2017 (UTC)

I understand Loschmidt's paradox; I just don't understand the solution. How does the universe beginning with low entropy imply the fact that entropy nondecreases in an isolated system (the second law of thermodynamics)? 49.184.161.119 (talk) 07:39, 10 February 2017 (UTC)

The Laws of Thermodynamics are not dependent on the initial conditions, as Loschmidt pointed out.[1] Entropy increases in either direction. Loschmidt's explanation was summarised here:

Actually, according to the equations, change is symmetrical whether going forwards or back in time. So entropy tends to increase in either direction, and our current low - entropy universe is a statistical aberration, in the same way that if you toss a coin for long enough eventually heads are going to come up ten times in a row. However, this rule flies in the face of observation, and the explanation is that God (if you will) created a universe with very low entropy and subsequent events are simply reversion to the norm. 2.221.197.64 (talk) 14:40, 20 March 2016 (UTC)

I wish that I understood one word of the above!78.148.42.202 (talk) 03:41, 15 February 2017 (UTC)

Is the Daily Mail really blocked?

I know there's a list somewhere that lists the blocked sites for [new] citations, but i don't recall where. Can someone give me a link for it? I want to check it because i want to know if the daily mail has been added to the list. Thanks in advance. Pancho507 (talk) 14:42, 10 February 2017 (UTC)

There was this RfC. Lugnuts ^{Precious bodily fluids} 14:46, 10 February 2017 (UTC)

That article doesn't actually say that there is a list, and such a list has yet to be properly codified (though one could develop from elaborating FAKENEWS). The DM was just an odd exception because many common editors didn't know that it is regularly unreliable, but it wasn't as obvious a bad source as InfoWars or the Onion. Ian.thomson (talk) 14:52, 10 February 2017 (UTC)

See Wikipedia:Spam blacklist.—Wavelength (talk) 15:59, 10 February 2017 (UTC)

That might be what he was talking about, though Daily Mail doesn't show up on that. I think he's looking for a list of blocked citations rather than blocked external links. Ian.thomson (talk) 01:35, 11 February 2017 (UTC)